Galaxy discovered lacking Dark Matter Recently a Galaxy was discovered that contained no Dark Matter. I was wondering what explanation could be offered for this ? This Galaxy also contains only 1% as many Stars as our Milky Way does . Is there be a connection between these two things, lack of many Stars and lack of Dark Matter ?

The paper describing this finding is van Dokkum et al. (2018). In a galaxy, the ratio of the stellar mass $M_\star$ to dark matter mass $M_\mathrm{DM}$ is normally very small, increasing with mass until Milky Way-sized galaxies where it reaches $\sim1/30$, then decreasing again.
But the dwarf galaxy NGC1052–DF2, seems to contain little or no dark matter. Galaxies of that mass ($M_\star\sim2\times10^8\,M_\odot$) typically has several hundred times more DM than stars.

How the galaxy was formed is not known, but the authors speculate on a few different mechanisms that all have to do with the fact that gas, in contrast to DM, may cool and condense and thus form clouds of very high $M_\mathrm{gas}/M_\mathrm{DM}$ ratios:

NGC1052–DF2 is located neat an large elliptical galaxy (NGC 1052) which could have gone through a merging event, tidally stripping a chunk of gas from one of the mergers. This is consistent with NGC1052–DF2 having a large velocity wrt. the elliptical.
NGC1052–DF2 could have formed from low-metallicity gas that was swept up in quasar winds (as described in Natarajan et al. 1998).
Lastly, NGC1052–DF2 could have formed from the fragmentation of gas accreting onto the elliptical, possibly aided by shocks.

EDIT (thanks to @WayfaringStranger): A fourth possibility is that the authors have misinterpreted the data. Shortly after the paper was put out, several other papers criticized the statistical methods used by van Dokkum et al. to infer their result (Martin et al. 2018;
Famaey et al. 2018;
Laporte et al. 2018).
van Dokkum wrote in a very long reply on his blog how at least the first of these papers actually confirm, rather than refute, his results. I am not enough of a statistician to comment on who's right and who's wrong, but note that there is currently an ongoing debate on the Facebook group astrostatistics.

You also ask whether the lack of DM is related to its "lack of stars". I wouldn't say that NGC1052–DF2 "lacks stars", anymore than the Milky Way "has too many stars" — it's just a small galaxy. But in general, the smaller a galaxy is, the larger the scatter in the $M_\star/M_\mathrm{DM}$ ratio is. Small galaxies, or small clumps of gas, have shallow gravitational potentials, so a small galaxy can more easily get stripped of gas, and a small chunk of gas can more easily escape a galaxy, or accretion stream, without attracting dark matter. In contrast, if would be very hard to conceive a very large galaxy having a considerably different $M_\star/M_\mathrm{DM}$ ratio, and indeed the scatter for massive galaxies is less than a factor of two (e.g. More et al. 2010).

Bullet cluster We've already seen an example of a large galaxy stripping dark matter from a smaller one.


How do rogue planets orbit around stars in other planetary systems? I got some interesting answers for What would happen if a rogue planet hit one of the planets in our Solar System?

But I have seen some documentaries that state that rogue planets from other planetary systems (which are ejected from the planetary systems in which they formed) have become part of the other planetary systems and orbit around those new stars as planets.

For example, please check this link:

  The latest research suggests that sometimes, these rogue, nomadic worlds can find a new home by being captured into orbit around other stars.

How is this possible? Stars' gravity can pull the rogue planet into them. How do rogue planets escape this and orbit around the new stars. This seems to be a bit mysterious.

Capture of any object is never common. A rogue planet that passes a star will be accelerated by the star's gravity, and provided that it doesn't hit anything will pass the star in a hyperbolic path.

For capture to occur, the rogue planet has to lose momentum, and there are a few ways in which this can happen. The most general way is for the rogue planet to interact with one of the stars existing planets. If the rogue planet passes close one of the star's orbiting planets, it can interact gravitationally, and transfer momentum to the planet, and slow down in the process. This is rather like the reverse of a gravitational slingshot.

It is also possible that a double planet can be captured, if one of the bodies transfers momentum to the other. One of the double planet will escape, the other is captured by the star's gravity.

Capture of rogue planets is very rare. There is no evidence that it has occurred in our solar system (never in 4.7 billion years is a big Never) though some have speculated that Sedna might be a captured object. But we know that captures like this are possible, as Jupiter and Saturn have a collection of moons, some of which seem to have been captured from the asteroid belt.

There are a few ways in which this can happen.

One of them is by a supernova explosion. A star explodes pushing out the planets who were once orbiting this into free space, going too fast for recapture by other stars. Some planets closer to the star would obviously be decimated but the outer ones could survive the blast(as said here by UserLTK).

Another is that a space object with enough velocity and mass to knock out a planet from its orbit into space. Such as if the originally solar system has too much planets, planet to planet interaction could send one if not both out of the system.

A rare occasion for this way of separating planets by their host star is by a black hole. There is current 16 (as far as I could remember) stars travelling at hyper speeds through the galaxy because it came too close to the black hole at the center of the milky way, then spinning outwards at incredible speeds and then the velocity finally comes over the blackhole's gravitational pull and shoots out. The speeds in which they come out is about 7-10 million miles per hour and the top speed achievable is at 30 million miles per hour. That's about crossing earth's diameter in about 10 seconds.

As LocalFluff said and UserLTK's contribution to it in the comment above, that's one of the ways too for a planet to be ejected out of the system. Link for that: http://blogs.discovermagazine.com/badastronomy/2011/11/16/did-jupiter-toss-a-giant-planet-out-of-the-solar-system/#.Vicx5aQx5mo

Here's some good sites for answering your question:

http://www.space.com/15308-rogue-alien-planets-billions-stars.html
http://www.space.com/15023-warp-speed-planets-light-speed.html


What is the degree of reflection of light from sun to the moon? As Moon reflects the light from the sun it must have an ANGLE OF INCIDENCE and ANGLE OF REFLECTION...
What Is the angle of reflection then?

Basically, the sunlight is reflected from the Moon at different angles. It works the same as on Earth. For instance, during the sunrise & sunset, the sunlight is reflected at a bit above 0 degrees angle, whereas at the noon in the tropical areas (in general) where Sun passes the zenith, the sunlight is reflected at 90 degrees. On Moon and other planets, it works exactly the same! This is how the angle of reflection works. Obviously, because we have a rough surface built with different rocks, we can't treat the celestial body as a mirror. However, the phenomenon is analog, as each of them has its own albedo - ability to reflect the light. Lunar albedo is 10%.
https://en.wikipedia.org/wiki/Albedo#:~:text=Albedo%20(%2F%C3%A6l%CB%88b,that%20reflects%20all%20incident%20radiation.
https://scienceworld.wolfram.com/physics/AngleofReflection.html#:~:text=The%20angle%20of%20reflection%20of,same%20point%20as%20the%20ray.

Obviously, it's worth mentioning that as the angle of light reflection is smaller, then the light beam is weaker (less intensive). It arises out of the inverse square law:
https://en.wikipedia.org/wiki/Inverse-square_law

which can be explained by the image above.
The angle of incidence is always equal to the angle of reflection, but it's perpendicular to the surface exactly, where the beam hits this surface.

http://www.mkrgeo-blog.com/the-aspect-of-light-reflection-in-the-long-distance-observations/
https://en.wikipedia.org/wiki/Angle_of_incidence_(optics)
Otherworldly, the angle of incidence is related to the plane of incidence. The plane of incidence is the "place" where an observer can see the reflected solar beam on the surface.
Along with the angle of incidence, we can mention the light glitters produced when our angle of reflection is low enough.


What would happen to black holes during the Big Rip? If the universe expands fast enough to rip atoms apart, what will happen to black holes? Would they rip apart too? What would that be like? Would they expand along with the universe's expansion, or would their gravitational force be able to resist it?

The universe won't expand fast enough to rip atoms apart, but if dark energy turns out to be a fact it will expand fast enough to avoid gravitational collapse. Black holes won't be ripped apart, they will just carry on as normal, as will the galaxies they belong to. But as space expands the galaxies will move further and further apart, going their own separate ways with not enough gravity to keep them together, eventually fading away into the black nothingness of space. Further research may mean that dark energy will have to be revised or reinterpreted, so at the present time a collapsing, blue shifted universe can't be completely ruled out.


What exactly is it that is being magnified 50 times in this gravitational lensing observation? In the Los Angeles Times news item Scientists get a rare view of a type Ia supernova magnified 50 times what exactly is magnified 50 times?

This supernova is really very far away. Is it somehow imaged by the gravitational lensing and being resolved — the actual supernova itself?

edit: I believe this is it, but it's behind a paywall (will confirm in due course): http://science.sciencemag.org/content/356/6335/291

above: From the Los Angeles Times. Photo credit: Joel Johansson

above: "This schematic image represents how light from a distant galaxy is distorted by the gravitational effects of a nearer foreground galaxy, which acts like a lens and makes the distant source appear distorted, but brighter, forming characteristic rings of light, known as Einstein rings. An analysis of the distortion has revealed that some of the distant star-forming galaxies are as bright as 40 trillion Suns, and have been magnified by the gravitational lens by up to 22 times. (Credit: ALMA ESO/NRAO/NAOJ, L. Calçada (ESO), Y. Hezaveh et al., edited and modified by Joel Johansson)" From the Los Angeles Times.

Reading through the full article, I see that the authors write

  we estimated the lensing amplification to be $\mu\sim52$.

This refers to the magnification factor. Essentially, it describes the solid angle of the image, as related to the solid angle of the source1:
$$\mu\equiv\frac{\theta}{\beta}\frac{d\theta}{d\beta}$$
Here's a diagram of the magnitudes of the angles:

Image courtesy of Wikipedia user Falcorian under the Creative Commons Attribution-Share Alike 3.0 Unported license.

It appears to just be a regular photo. It could've been taken by Hubble, and someone just magnified the photo. It doesn't appear to be gravitational lensing. If it was you would be able to see space being bent a bit in the photo, read here


Why can't ptolemaic Venus be fully lit? In wikipedia's article on the phases of Venus it says:

The Ptolemaic system would never allow Venus to be fully lit from the perspective of the Earth, as this would require it to be on the far side of the Sun, which is impossible if Venus's orbit in its entirety is between the Earth and the Sun,

But the moon's orbit "in its entirety is between the Earth and Sun", and is "fully lit from the perspective of the Earth" once a month, so how can we conclude that Venus revolves around the Sun?

The Moon orbits the Earth in such a way that it sometimes can be on the opposite side of the Earth as seen from the Sun. In other words, it’s possible to have Sun-Earth-Moon along an imaginary straight line, in that order.
However, by Ptolemy’s time, it had already been noticed that Venus is always seen close to the Sun (maximum angular distance about 47°). So it can never get opposite to the Sun as seen from Earth.
Ptolemy didn’t know why (the reason is that Venus orbits the Sun, not the Earth, which was inconceivable to Ptolemy), but his system made it so that Venus (and Mercury, for that matter) were orbiting the Earth closer to it than the Sun, and in such a way as to always be near it.
So imagine moving your two hands at arm’s length, but one closer to your face than the other. Imagine the distant one is the Sun. Then your closest hand would never get completely lit. Unless you put it behind your back, where Venus (and Mercury) never go.
So that’s why in the Ptolemaic system, Venus (or Mercury) would never get fully lit.

The moon's orbit is most certainly not "in its entirety between the Earth and the Sun"! A little more than half the time, it is further away from the Sun than the Earth is. As you will realise if you give it a little thought.


Would we be able to feel the acceleration of a planet with a highly eccentric orbit? Even though our Earth moves on average 67,000 miles per hour around the Sun, we cannot feel its movement since its speed is fairly constant. More precisely, its change in speed is very low and our bodies cannot detect that low of acceleration.

What if instead a hypothetical human observer were placed on a planet with a very eccentric orbit, say 0.99? Would that human be able to feel the planet's acceleration as it approached its host star, and as it receded from its host star? And if a human observer would not be able to feel any acceleration even at e=0.99, what is the minimum eccentricity, if any, that a planet must have in order for a hypothetical human observer to feel the effects of its acceleration and deceleration?

No.

While you're on that planet, you are also in the same orbit around the star as the planet is. As long as you're in the same orbit as your vehicle (planet, spacecraft), you don't experience any acceleration relative to the vehicle. Whether you're on the planet, or you're just tagging along on the same orbit, is the same thing.

Similar question: in a satellite in a very eccentric orbit around the Earth, is any break in microgravity experienced in various parts of the orbit? Same answer - no. The satellite and everything in it follow the same orbit, so microgravity conditions are maintained.

Now things would be different if you had a very eccentric orbit around a source of much more intense gravity, like a neutron star or a black hole. At the lowest point in the orbit, you could experience tidal forces: the things closer to center experience stronger gravity compared to things further away from center.

But for this to be experienced within a small spacecraft, or within the limits of one human body, the gradient of the field would have to be very strong indeed; the orbit would have to reach very low, very close to the neutron star or black hole.

However, what you would experience would not be orbital "acceleration", but just tidal force. Even so, in extreme cases the tidal force could be strong enough to destroy the ship or kill the passengers.

See the short story "Neutron Star" by Larry Niven for a literary depiction (fairly accurate physically) of this phenomenon. Unfortunately, the story is spoiled by the explanation above, but hopefully learning about gravity compensates for spoiling the story. :)

First I think you have miss understood some basic principals. Although the Earth's speed (a scalar quantity) is nearly constant its velocity is not, acceleration is a change in velocity (a vector quantity). Due to its orbital motion the acceleration of the Earth is constantly changing.

As for the actual question, the acceleration would be so small compared to that of gravity (between the person and the planet) that the effect will not be noticeable.


What do the fusing 'onion layers' of a pre-supernova star look like to scale? I'm sure we've all seen the diagrams of various layers of element fusion from hydrogen to silicon in a star that's just about to go supernova.

(Picture from courses.lumenlearning.com)

I suspect these pictures grossly exaggerate the relative radii at which these fusing layers occur for the sake of readability. What would a more accurate to-scale picture of these fusing layers look like?

Now, I assume the answer will vary significantly based on the mass of the star, and I suspect in certain mass regimes some layers will not be fusing at all. I can think of a few other variables that may affect the answer too.

So in CYA fashion, I'll leave it up to the answerer to identify specific illustrative or interesting cases, as I'm not after a specific answer but rather a general feel for how large some of the layers are in comparison to the others.

Pre- supernova models often characterise the compactness of the core using a "compactness parameter" defined as
$$ \upsilon = \frac{(M/M_{\odot})}{R(M)/1000\ {\rm km}},$$
where $M$ is usually chosen to be $2.5M_{\odot}$ and $R(M)$ is the radius within which $M$ is contained.

Pre supernova models by Farmer et al. (2016) show that the central $2.5M_{\odot}$ of a massive star included the Carbon-burning core in a $15M_{\odot}$ (initial mass) star, but only contained the oxygen-burning core in a more massive star (models are presented up to 30$M_{\odot}$. i.e. Your answer is going to be mass and composition dependent (these models are for a solar metallicity initial composition).

In the $15M_{\odot}$ model (with mass-loss), $\upsilon \sim 0.08$ at core collapse, which means the carbon burning core would have been contained within 31,000 km. The $30M_{\odot}$ model is more compact with $\upsilon= 0.58$, indicating that the oxygen-burning core would have been contained within 4,300 km.

At collapse the iron core might have a mass of 1.4-1.8$M_{\odot}$, will be supported by electron degeneracy pressure and should have a size a bit smaller than a typical carbon white dwarf (radius of a few thousand km).

You can compare these sizes with the size of the entire red supergiant star, which might have a radius of a few au (e.g. Betelgeuse).

The shell-burning regions would be found out to slightly larger radii than this I think, but these numbers are a reasonable estimate. Looking in detail at the models I don't think that the final stages of pre- supernova evolution look anything like the strictly stratified onion-shell picture that is seen all over the internet.

Answer seems to be : In Red Super Giats the star can have 300 - 1000 the diameter of the sun and its core where nucleosynthesis occurs is only the diameter of the earth.
This would explain why after 13.8 Billion years still 98% of the universe is made of hydrogen and helium

See image on page 42 of this slide show https://slideplayer.com/slide/13959320/


Why is the track of the subgiant stage almost horizontal on the HR diagram? It is stated that:

After the main sequence, as fusion weakens or stops in the core, outward radiation weakens. The helium core contracts and heats up. Gravitational energy is converted to thermal energy again!
The star will appear to cool slowly and will undergo a modest increase in luminosity. During this phase, the path the star will follow in the HR diagram is almost horizontal to the right of its position on the Main Sequence. Stars in this phase are usually referred to as subgiants.

But I still can't understand.

I will replace the previous answer to focus on the "subgiant" branch prior to red giant, rather than the pre-main-sequence or the "horizontal branch" of core helium fusion.  Those are other times that the luminosity is constant, but this question is about the subgiant branch, which I missed before.

The reason the luminosity is nearly constant on the subgiant branch is related to the "mass-luminosity relation" of pre- and main-sequence stars.  It is due to radiative diffusion and how it leads to a luminosity that depends only on mass, for a given composition.  If you compare to pre-main-sequence tracks, you should find that the subgiants more or less retrace that prior evolution, just with a somewhat higher luminosity because many of the electrons have been swallowed into neutrons, reducing the opacity and increasing the all-important rate of radiative diffusion.  It's essentially just a helium-dominated mass-luminosity relation, instead of hydrogen-dominated, as the radius rises due to the details of how the interior is evolving.

The reason the luminosity eventually rises steeply on the red giant branch is that as the degenerate core starts to build in mass, it starts to control the temperature of the fusing region, and this significantly changes the internal structure in ways that get into explaining red giants.

From blackbody radiation ($L = c r^2 T^4$) the horizontal evolution implies increasing radius and decreasing temperature.

This happens because of the depletion of H at the core:

Depletion of H at core -> core contraction -> increasing core temperature + starting H fusion in the envelope (in HR diagram, the star goes up) -> to maintain the equilibrium from the injection of energy of H fusion in the envelope, envelope expands and cools (moving horizontally to the right in HR diagram) -> ...


How small can a spiral galaxy be? The smallest observed spiral galaxies I can find are NGC 2976 and NGC 4605, both with a diameter of 20 kly, but I don't have anything like an exhaustive source to search. I've also found a mention that the Small Magellanic Cloud (7 kly diameter) might have had more spiral structure before it was distorted by the Milky Way.
From looking at these galaxies, spiral structure seems to be less clear the smaller the galaxy is, which makes me think there might not be a clear lower limit. Still, I'm curious how small a galaxy with recognizable spiral structure could be, either based on observations or theory.

The first Google-hits suggest that Segue 2 is the smallest spiral galaxy, but that is actually incorrect since it is the smallest spherical galaxy yet discovered. It has a radius of roughly 34 parsec or 110 light years. The original publication on that topic dates back to 2013, but it was again in the news in April 2020.
Then, there are lecture notes on a course Astronomy 626 Galaxies which shows the following diagram:

Also, the lecture notes claim:

Typical values for the scale length [of spiral glaxies] are: $1 {\rm kpc} < h_R < 10 {\rm kpc}$

I agree that this does not answer your question fully, but I got the impression that the lower bound should be around $1 {\rm kpc}$.
References

https://www.nasa.gov/press/2014/september/hubble-helps-find-smallest-known-galaxy-containing-a-supermassive-black-hole
https://www.treehugger.com/just-how-big-is-the-smallest-galaxy-in-the-universe-4862842
http://www.sci-news.com/astronomy/article01147-segue-2-dwarf-galaxy.html
Evan N. Kirby et. al: Segue 2: The least massive galaxy
https://physics.stackexchange.com/questions/148418/


Ratios of magnitudes instead of differences The color index in the photometric system is defined by the differences in magnitudes between two wavelength filters, which gives the ratio of intensities. For example, to determine the B-V index, you'd need the magnitudes in the B band (which is centered on $\lambda = 445$nm) and in the V band, centered on $\lambda = 551$nm.

But what about other operations? For instance, would there be any significance in taking, say, the ratio $B/V$ or $B\times V$?

Consider for example these bright stars (V and B-V values from Yale Bright Star Catalog).
Near V=0, B/V is ill-behaved, and B*V is near 0.

Name       B     V     B-V    B/V   B*V
Arcturus  1.19 -0.04  +1.23 -29.7  -0.05
Vega      0.03  0.03   0.00   1.00  0.00
Capella   0.88  0.08  +0.80  11.0   0.07
Rigel     0.09  0.12  -0.03   0.75  0.01

Now imagine that each of those stars were 10 times as far away.
Neglecting interstellar extinction, this would decrease the apparent brightness by a factor of 100 and increase the apparent magnitude by 5.0.
As B and V increase together, B/V approaches 1 and B*V approaches V^2, telling us nothing meaningful about a given star.

Name       B     V     B-V   B/V    B*V
Arcturus  6.19  4.96  +1.23  1.25  30.7
Vega      5.03  5.03   0.00  1.00  25.3
Capella   5.88  5.08  +0.80  1.16  29.9
Rigel     5.09  5.12  -0.03  0.99  26.1

B-V, on the other hand, has stood the test of time as a distance-independent metric of a star's color and surface temperature.

The magnitude is logarithmic, so taking the difference between two values is effectively the same as taking the ratio between the two unlogged values.


Jupiter FM - What are practical and inexpensive ways for the amateur detection of signals from Jupiter, especially of the transit of her moons? What modifications to a standard AM/FM or shortwave radio are needed in order to be able to detect radio-wave signals emitted from Jupiter?

Would it be possible to detect the transit of the major moons and even minor planets (maybe even Ceres and Mars) using this method?

@Arne is right in his answer about two things, that the most suitable frequency for Jovian amateur radio is 20.1 MHz, and that this is a 15 meter wavelength. However, the antenna can actually be half the wavelength, and amateur radio astronomers have had good results listening to all kinds of Jovian sounds, including detecting occultations of its many moons as they produce change in frequency due to the Doppler shift when passing in front of it, amplification due to echo of Jupiter's own radio wave signature when close beside it from the observer's vantage point, and other effects causing radio wave frequency and amplitude changes with a simple dual dipole array that might look something like this:

        The Jove dual dipole array. Dipoles are suspended between PVC masts. Signals from the dipoles go to a power combiner and        then to the receiver. Source: Amateur Radio Astronomy Projects — Radio Signals from Jupiter (PDF)

Now this dual dipole antenna is more or less a simple two-piece of roughly half the wavelength of coaxial cable, stripped of insulation at the dipole ends and run dipoles parallel to each other at roughly 6.1 meters (20 ft) apart suspended over PVC masts. That is perfectly acceptable for nearly any amateur radio astronomer in both space and the price of the parts needed. There are also ways to help oneself with a lot smaller antenna, which I'll mention a bit later. Let's first clear the frequency range discussion with this really informative quote from Radio Receiver for Jupiter webpage (based on NASA's Radio JOVE project):

  The peak of Jupiter signals occur around 10 MHz. Still this frequency
  is not that suitable, as it is very near to Ionospheric cutoff. The
  best suitable frequencies are in between 18 to 22 MHz, as the chances
  of getting emissions are more. In practice 18.7 MHz, 20.1 MHz, 22.3
  MHz are common. The frequencies above 30 MHz are not suitable because
  of less strength. The frequency of 20.1 MHz is used for this project,
  as the probability of getting emission is high. So the entire receiver
  is designed considering 20.1 MHz as the operating frequency.

Antenna is of course merely one part of the story. Second thing that is required is the receiver. NASA is sponsoring Radio JOVE project with an assembly manual for roughly 100 electronic components and pieces of hardware worth RJ1.1 Receiver (Radio Jove 1.1 Receiver) that can be almost completely assembled out of parts ordered from Radio Shack (it even includes RS part numbers). I'll add a photograph and a few other links, and you'll have to be on your own then:

   Self-assembled Radio Jove Receiver's front plate with two turning knobs for volume and tuning. Source: KB0LQJ Observatory

Some relevant links for building Radio Jove (or Jupiter FM, if you like, it will be your own receiver so name it as you wish), starting with already mentioned ones:

Amateur Radio Astronomy Projects — Radio Signals from Jupiter (PDF)

  Ten years ago a group of (mostly)
  University of Florida graduates working at
  NASA conceived an educational outreach
  program known as Radio Jove. The idea was
  to build an inexpensive radio telescope kit
  suitable for detecting signals from Jupiter.
  The Jove receiver (Figure 2) is a simple
  direct-conversion design operating over a
  few hundred kilohertz range centered at
  20.1 MHz.

NASA's Radio JOVE project (PDF)

  The site is currently down due to US government shutdown, so here's a Google cached version that sadly doesn't come with in-document images

KB0LQJ Observatory Amateur Radio Astronomy — Radio Jove observations

  For my home observatory, I started with the Radio Jove receiver from NASA’s Radio Jove Project. This was a fairly easy-to-build kit with great directions, not just for the receiver but also the antenna setup and installation.  Unfortunately, I don’t have enough space to put up a phased antenna array.  Also, I’m bordered by power lines on the north and south sides of my property.   Also, since I’m in an urban area, I knew that I’d pick up a lot of noise.   Not to worry.   Solar observations are also quite interesting, and since the Sun is such a good signal source (especially so far this year), I opted for an antenna in my attic.

Radio Receiver for Jupiter

  This website contains the Technical details of the antenna and receiver used for receiving natural radio emissions from Jupiter at 20.1MHz. The antenna and receiver discussed in this site is based on the design given by NASA Radio Jove Program.
  Natural radio emissions from Jupiter or from Sun are detected by using a dual dipole array as an antenna and with a sensitive receiver. The RF voltage developed at the antenna terminals is amplified with the RF amplifier and converted in to audio frequencies using a mixer. The audio signal thus generated is recorded on PC through a sound card in ‘wav’ format. Also a strip-chart recorder software is available to generate the strip-chart of data coming through sound card.

So there are indeed many ways to build your own antenna and receiver out of affordable and easily obtainable electronic parts, and some of these websites listed above will help you through the process of self-assembly, even providing some tricks how to do it easier, like for example assembling a smaller, in-house antenna array.

Now, one other thing that all these websites mention is also using various PC software that lets you analyze through computer's sound card received Jovian radio sounds, but since there's many different and free solutions out there, including NASA's own that runs on Windows PCs, I'll let you discover those on your own. Here's one page listing many links, to get you started. More computer savvy of you might even write your own software for this purpose, this is Stack Exchange afterall.

And if anyone is wondering what such self-assembled radio receivers and antennas are capable of, here's a link to a collection of various Jovian sounds in the radio wavelengths on Astrosurf.com, and another one as a collection of solely amateur radio astronomy sounds of Jupiter and its moons (scroll a bit down to the list of recordings). And here's a short description of various sound types one might hear:

Chorus (quote source: The University of Iowa Department of Physics and Astronomy)

  Chorus consists of brief, rising-frequency tones that sound like the chorus of birds singing at sunrise, hence the name "chorus" or "dawn chorus." Chorus at Earth is generated by electrons in Earth's Van Allen radiation belts. Once generated, the chorus waves affect the motions of the electrons through a process called a wave-particle interaction. Wave-particle interactions disturb the trajectories of the radiation belt electrons and cause the electrons to hit the upper atmosphere.

Decametric noise storms (quote source: Radio-Jupiter Central on RadioSky.com)

  The emissions we can hear are often referred to as decametric noise storms, because the waves are tens of meters long. Okay, it is possible to hear Jupiter from 15 to 38 MHz, but what are the optimal frequencies?  The consensus seems to be that 18 MHz up to about 28 MHz is a good place to listen.  A good rule would be to pick the lowest frequency in this range which was not being hindered by ionospheric refraction.

Whistlers (quote and image source: Wikipedia):

  A whistler is a very low frequency or VLF electromagnetic (radio) wave generated by lightning.1 Frequencies of terrestrial whistlers are 1 kHz to 30 kHz, with a maximum amplitude usually at 3 kHz to 5 kHz. Although they are electromagnetic waves, they occur at audio frequencies, and can be converted to audio using a suitable receiver. They are produced by lightning strokes (mostly intracloud and return-path) where the impulse travels along the Earth's magnetic field lines from one hemisphere to the other.

Auroral Radio Emissions  (quote source: Wikipedia)

  Auroral radio emissions from giant planets with plasma sources such as Jupiter's volcanic moon Io could be detected with radio telescopes.

And so on. The last two on the list are probably a bit of a stretch for amateur astronomers, with auroral radio emissions average frequency range at 100 to 500 kHz, and whistlers cutoff at usually around 30 kHz, and both requiring too big antennae, but I wouldn't bet against it being possible with smaller antennae either, although bow frequency cutoffs might prevent identification of actual aural events. But there are many other sounds to listen to both from Jupiter, as well as it's many moons, mostly though the bigger and closer ones.

Good luck tuning in to your own Jupiter FM and happy hunting for rare radio frequency events!

At a minimum, you'll need a highly directional antenna appropriate for the frequencies in question (I do not know what they are).  The radio would also have to able to detect the frequencies that you expect to hear signals on.


What's the likelihood of the existence of unknown elements in the Solar System? What's the chance that there might be undiscovered chemical elements in the Solar System - either on planets or around the Sun or on asteroids of the Oort-cloud?

As far as elements (e.g. on the periodic table) go, I would say the odds are very slim.  We already discovered or produced all the elements of the Periodic Table up to atomic number 112 at least.  As the number increases, the half lives of the elements generally decreases, and is very short for elements above 102. If this trend holds true as the number increases, practically all the "undiscovered" elements should have turned into the lower known atomic number elements.

However, there is hope.  There is a theorized "island of stability" where a narrow range of yet to be discovered high atomic number elements may be stable: http://en.wikipedia.org/wiki/Island_of_stability
I would say there is a slight chance this element could be discovered in the solar system.

It is definitely possible, but in a very hot and active part of the universe. In order to discover these elements, there would have to be a lot of waiting for these elements to form. Our solar system isn't active enough, and a nebula would be the best place to look.


Why more matter still left after collision with anti-matter? Question implies all, why is that? Why we exist even after equally creation of matter and anti-matter, any possible exception?

I presume you're asking about the mystery of the missing antimatter. This features extensively in the popular and scientific media. For example the CERN webpage on the matter-antimatter asymmetry problem is fairly typical. It says the Big Bang should have created equal amounts of matter and antimatter, but everything we see today is made almost entirely of matter. It goes on to say “something must have happened to tip the balance. One of the greatest challenges in physics is to figure out what happened to the antimatter”.

I think it's something of a "clickbait" mystery I'm afraid, to try to curry favour with the paying public. Why do I think that? Because there’s baryon asymmetry, where we see lots of protons but hardly any antiprotons, and there’s also lepton asymmetry where we see lots of electrons but hardly any positrons. And they match. Take a look at positronium. It’s “a system consisting of an electron and its anti-particle, a positron, bound together into an exotic atom”. The electron is matter, and the positron is antimatter. Positronium however is neither matter nor antimatter. It’s both. It’s an exotic atom. Now see page 314 of photon and electron collisions with atoms and molecules edited by Philip Burke and Charles Joachain. It says “the positronium atom thus created is in essence a very light hydrogen atom”. So positronium is both matter and antimatter, and it’s like light hydrogen.

The clear inference from that is hydrogen is also a combination of matter and antimatter. We say the electron is matter, and the proton too. But why? Surely since the proton is more like the positron than the electron, we ought to say the proton is the antimatter. Like this:

That would mean antimatter is a lot more common than people say. The proton mass is 1836 times the electron mass. Which would mean that weight for weight, 99.95% of everything around you is made of antimatter. Where’s all the antimatter gone? Nowhere. Because it’s hiding in plain sight. Because truth be told and by and large, you are made of it.


If we could travel far enough would we be able to see a constelation in reverse? I am enjoying Orion high in the sky this winter as always. I wondered if I could travel beyond it, say 2730 light years* and turn around, would I see Orion in reverse? If so, what of the neighboring constellations. I know brightnesses would be different but would I recognize the shape?

Happy new year to all.

I chose this number because Alnilam is 1360 light years away so I doubled the distance.

Source: https://www.space.com/3380-constellations.html

If the stars are all similar distance from us, like the Pleiades, then yes, you could find a place from where the stars would appear in a similar pattern and brightness, but the pattern would be a mirror image.

You can't do it when the stars are significantly different distances because they wouldn't look the same.   Most constellations are a range of distances, which wouldn't look like a mirror image from any distance.

Take Canis Major for example.   Sirius is the brightest star in the sky but in large part because it's only 8 light years away.   The 2nd brightest star in that constellation, Epsilon Canis Majoris is about 430 light years away.   At that distance, Sirius might not even be visible to the human eye.  It certainly wouldn't be constellation worthy.

Pack orion in a cuboid of custom dimensions & make sure maximum number of stars touching the faces of cuboid from inside.
Make a normal (perpendicular line from the face) reaching earth and then extend the line to other side of the cuboid such that cuboids centre is the centre for the line.
Be careful going to another end ;)


Is normal matter always accompanied by dark matter and vice versa? Is normal matter always accompanied by dark matter and dark matter by normal matter?

Was ever observed effect of gravity lensing without normal matter?

We lack the precision to say that there aren't regions where there is matter without dark matter or vice-versa. But what is clear is that the ratio of dark matter to normal matter, which is (or needs to be) around 5 on average to explain the flatness of the universe, varies by orders of magnitude from place to place.

The reason for this is that matter interacts with matter in a completely different way to that in which dark matter interacts (weakly) with normal matter or itself by any other means than gravity. Gravitational interactions between normal/dark matter are not dissipative.

By that I mean that the sum of gravitational and kinetic energy is preserved. When normal matter interacts with itself there are normally dissipative consequences. For example, interacting matter may become hot and radiate away energy.

That is why the normal matter in our Milky Way galaxy is concentrated into a plane. Dissipative interactions during the gravitational collapse of our Galaxy, combined with the requirement for conservation of angular momentum result in disc formation. The same is not true for the dark matter associated with the Milky Way, which is expected to have a much more spherical distribution.

Likewise, when we look at the required density profiles of normal and dark matter in order to explain the kinematics of stars and gas in our Milky Way, we see that the normal matter is much more centrally concentrated than is the dark matter - hence we talk about a "dark halo". Again, dissipative interactions in normal matter are the reason.

Another famous example of the separation between dark and normal matter is the bullet cluster. The image below shows where the hot gas is (normal matter, observed by an X-ray telescope) compared to where it is inferred that the dark matter is (shown by contours) on the basis of gravitational lensing. This pair of galaxy clusters have recently interacted, passing through each other. The hot, intracluster gas has been stripped out of the pair and shock-heated, so that it appears concentrated between the two galaxy clusters. The dark matter content of each cluster has sailed on oblivious and remains concentrated around the visible cluster galaxies.

Yet another example would be dwarf spheroidal galaxies, these are relatively small galaxies that are nevertheless almost totally dominated by dark matter - they have mass-to-light ratios of $\sim$1000, compared with a "normal" galaxy, where this ratio might be $\sim$ 10.

There are also examples of dark galaxies. These may not be galaxies made of non-baryonic dark matter, but dark matter in the sense that for whatever reason, these galaxies do not contain many bright stars. Thus although they may be massive, they do not emit much light and contain a lot of "inert", non-starforming gas.

Mass attracts mass, and so dark matter will attract normal matter and vice versa. However, people have looked for "dark lenses", so far without success.

Examples:

https://arxiv.org/abs/astro-ph/0307381
https://arxiv.org/abs/astro-ph/0203061 ("dark" only in X-ray?)
https://arxiv.org/abs/astro-ph/9804136 (mostly "not a lens" rather than a dark one)
https://arxiv.org/abs/astro-ph/0202360 discusses what a dark lens should look like

On the other side of the question, normal matter has a chance to cool and thus clump together more densely than is possible for dark matter. Hence, small-scale dense environments are dominated by normal matter.


Where on Earth could dinosaurs feel the impact of their extinction asteroid? Did the possible Chicxulub asteroid impact around 66 million years ago that made the dinosaurs and many other species extinct, cause the Earth to wobble? How large was the area, where they could "feel" the impact's consequences?

The whole Earth "rippled" after the Chicxulub impact. By ripple, I specifically mean surface displacement.  Current models indicate that it rippled enough that most of the dinosaurs and other species with brains certainly would have felt the ripple.
How much did the Earth ripple? Well, that depends on the location. Meschede et al. [2011] in their paper Antipodal focusing of seismic waves due to large meteorite impacts on Earth, visualize a computer model of the surface wave displacement due to the Chicxulub impact in the following figure:

Note that the displacement doesn't get much smaller than 1 centimeter, which is certainly large enough to feel over the order of tens of minutes.  As another point of interest is that their model shows large displacement near the antipodal point, which some geoscientists correlate to the increased volcanic activity at that time in the Deccan Traps. Also note that these are just the seismic waves.  We know from geological evidence at least from the Southeastern United States, that the resulting tsunami was much bigger.
As far as a "wobble", or change in angular momentum, the Chicxulub impact was too small to have a noticeable effect for the dinosaurs.  JamesK has a nice example here: If an Asteroid was to strike the Earth, would it affect the Earth's rotation?.

The impact energy wasn't so large (the asteroid had just 10–15 km of diameter – compare that to 12700 km = Earth's diameter), so the asteroid didn't cause Earth to wobble (of course, only a little – if I jump, I cause Earth to wobble : )
Of course, it was felt everywhere on Earth. The dinosaurs were spread all over the world, but they more or less got extinct (if not directly, then indirectly).

Edit: There are two downvotes (I don't care about the reputation much, but there is a thing we need to agree on), so I suppose you wanted the sound amplitude, but note that the definition of "being felt" can be very broad. The dinosaurs must have felt the impact, otherwise they would still be scaring us (we might not have even evolved). For this, I advise you the link @Jean-MariePrival gave in a comment above. @ConnorGarcia 's answer doesn't really answer the question about sound, but rather about the earthquake (and even not that rigorous as said in @leftaroundabout 's comment). Where are the differences?


Would time go by infinitely fast when crossing the event horizon of a black hole? If you were to fall into a black hole, my understanding is that from your reference point, time would speed up (looking out to the rest of the universe), approaching infinity when approaching the event horizon.  If this is correct, would you see the whole universe's future "life" flash before your eyes as you fall in, assuming you could somehow withstand the tremendous forces, and assuming black holes don't evaporate?  If it is correct that black holes evaporate due to Hawking radiation, would you be "transported" forward in time to where the black hole fully evaporates?

This is considering the "alternate" frame of reference from my question: Does matter accumulate just outside the event horizon of a black hole?  In that question, I thought about what happens to matter falling into a black hole from the perspective of someone observing from the outside (e.g. as seen from earth).  Here I am considering the perspective of the thing falling into a black hole.

This also takes into account ideas discussed in: Why does time get slow near a black hole?

Note: This answer to another question provides some insight here too (refer to the last part of the answer): https://astronomy.stackexchange.com/a/3713/1386

(I will assume a Schwarzschild black hole for simplicity, but much of the following is morally the same for other black holes.)

  If you were to fall into a black hole, my understanding is that from your reference point, time would speed up (looking out to the rest of the universe), approaching infinity when approaching the event horizon.

In Schwarzschild coordinates,
$$\mathrm{d}\tau^2 = \left(1-\frac{2m}{r}\right)\mathrm{d}t^2 - \left(1-\frac{2m}{r}\right)^{-1}\mathrm{d}r^2 - r^2\,\mathrm{d}\Omega^2\text{,}$$
the gravitational redshift $\sqrt{1-\frac{2m}{r}}$ describes the time dilation of a stationary observer at a given Schwarzschild radial coordinate $r$, compared to a stationary observer at infinity. You can check this easily: plug in $\mathrm{d}r = \mathrm{d}\Omega = 0$, the condition that neither the radial nor the angular coordinates are changing (i.e. stationary observer), and solve for $\mathrm{d}\tau/\mathrm{d}t$.

The conclusion is that if you have the rocket power to hover arbitrarily close to the horizon, you will be able to see arbitrarily far into the history of the universe over your lifetime. However, that doesn't actually cover what happens to an observer that crosses the horizon. In that case, $\mathrm{d}r\not=0$, and the coefficient of $\mathrm{d}r^2$ above becomes undefined at the horizon: as in the other question, the Schwarzschild coordinate chart simply fails to cover the horizon and so is ill-suited for talking about situations cross the horizon.

But that's a fault of the coordinate chart, not of spacetime. There are other coordinate charts that are better adapted to questions like that. For example, the two Eddington-Finkelstein charts are best suited for incoming and outgoing light rays, respectively, and the Gullstrand-Painlevé chart is adapted to a freely falling observer starting from rest at infinity.

  If this is correct, would you see the whole universe's future "life" flash before your eyes as you fall in, assuming you could somehow withstand the tremendous forces, and assuming black holes don't evaporate?

No. I think this is best seen from the Penrose diagram of Schwarzschild spacetime:

Light rays run diagonally. In blue is an example infalling trajectory, not necessarily freely falling. Note the two events where it crosses the horizon and where it reaches the singularity. Shown in red are inward light rays that intersect those events. Thus, the events that the infalling observer can see of the external universe consist of the region between those light rays and the horizon. The events occurring after that won't be seen because the the observer will have already reached the singularity by then.

Now suppose the observer tries a different trajectory after crossing the horizon, accelerating outward as much as possible in order to see more of the future history of the external universe. This will only work up to a point: the best the observer can do is hug the outgoing light ray (diagonally from lower-left to upper-right) as much as possible... but since the observer is not actually allowed to go at the speed of light, seeing all of the future of history will be impossible. The best the observer can do is to meet the singularity a bit more on the right of the diagram.

By the way, since the light ray worldlines have zero proper time, trying to do that will actually shorten the the observer's lifespan. If you're in a Schwarzschild black hole, you would live longer if you don't struggle to get out.

The above is for an eternal, non-evaporating black hole, as that's what you're asking about here. (The 'antihorizon' is there because the full Schwarzschild spacetime is actually an eternal black hole and its mirror image, a white hole in a mirror 'anti-verse', which not shown on this diagram. That's unphysical, but not relevant to the situation we're considering here.)

  If it is correct that black holes evaporate due to Hawking radiation, would you be "transported" forward in time to where the black hole fully evaporates?

An evaporating black hole is morally the same as above: only an ideal light ray can reach the point when the black hole fully evaporates; everyone else gets the singularity. (Since this ideal light ray right along the horizon would be infinitely redshifted, arguably not even that.) You can repeat the above reasoning on its Penrose diagram yourself:

Addendum:

  I have thought a bit about this, and does this solution take into account the relativistic time effects near the horizon of the black hole (e.g. is my understanding correct that the observer would observe time in the universe passing approaching infinitely fast when approaching the event horizon)?

How much time dilation happens depends entirely on what coordinates we're talking about (more generally, which frame field). What a given observer will actually see, however, is completely independent of choice of coordinates. In particular, Penrose diagrams illustrate the light cone structure of the given spacetime, and what an observer can in principle see depends entirely on what light rays intersect the observer's wordline. So yes, it's taken into account by default.

If you're actually falling in it, no, your understanding is mistaken, for reasons explained above. For additional motivation, flip the question around: what does the very distant stationary observer see of the infalling object? On the above Penrose diagram, outwardly directed light rays are diagonal, from lower-left to upper-right. Draw some some outward light rays from the blue infalling worldline. You will see that no matter how far into the far future (up on the diagram) you pick an event outside the black hole to be, you can connect that event with an outward light ray originating from the blue infalling worldline before it crosses the horizon. The conclusion would be that an observer that stays outside the black hole would be able to see the infalling object arbitrarily far into the future. No matter how much time passes for someone who stays out of the black hole, the image of the infalling object would still be visible as it was before it crossed the horizon. (In principle at least; in practice it will get too faint to see after a while.)

Thus, the usual result of "infinite gravitational time dilation makes the image of the infalling object hover forever near the horizon" is also straightforwardly deducible from the diagram, and so is completely consistent with the infalling object being able to see a finite part into the future of the external universe. Perhaps it is best to emphasize that the situation is not actually symmetric: what the external observer sees of the infalling object is not some straightforward flip-around of what the infalling object sees of the external universe. The black hole itself breaks that symmetry.

You are correct, the universe you were leaving would seem to speed up and ultimately the whole history of the universe would occur as you cross the critical circumference and onwards to the point like singularity. Galaxy mergers would take place, other black holes would coalesce with yours and so on. The singularity you finally reach would in fact be one and the same with the singularity at the end of the universe. There is only one.

Regarding Hawking radiation, from the viewpoint of an outside observer, who would see you frozen in time (or at least the constituent matter and energy in your body) at or around the critical circumference, this may not actually take place.  If matter is frozen by time dilation, there will be no quantum fluctuations taking place and nothing falling through the critical circumference into the black hole, therefore no Hawking radiation taking place. Looked at from the viewpoint of an infalling observer it will take a very short time to cross the critical circumference, therefore the significance of quantum fluctuations in this period of time would seem to be small.


Does the universe have an edge? I've heard the universe is just an empty cavity. Does the universe have any edge or stop or is it just an infinite empty cavity?

As far as theories I've heard of, we can't see the edge of the universe (if truly exists) because the farther you look out into space through a telescope, the farther back in time you are actually looking. For example, if you are looking at a star 20 light-years away... you're actually currently observing what was happening on that star 20 years ago... the theory is that there is only so far you can look out into the universe because there is only so much history post Big Bang.

Image above: What is the furthest we can see? In 2003, NASA's WMAP satellite took images of the most distant part of the universe observable from Earth. The image shows the furthest we can see using any form of light. The patterns show clumps of matter that eventually formed into galaxies of stars. Credit: NASA/WMAP Science Team

Dark energy is driving the accelerated expansion of our universe.

Is it accelerating into itself?

Or is it accelerating into nothing, An outer void?

If we could see the furthest away accelerating object that would be the edge of the Universe.  Thus there is an edge to the Universe.

Remember, just because we can't look and see today doesn't mean it doesn't exist.


What is gravity really? What is gravity?  I want to know more than it being simply the "mysterious force" that attracts things to earth.  Is it a particle, a wave, or something else entirely?

I can attempt to address the second part of your initial question (*"Is it a particle, a wave,...?") Einstein's theory of general relativity states that mass and energy bend space-time. Space-time, in turn, tells matter how to move (John Wheeler put this more elegantly).

This concept is completely different from the theories of the other three fundamental forces (electromagnetism, and the strong and weak nuclear forces). In these quantum theories, forces are mediated by particles called gauge bosons. Electromagnetism is carried by photons, the strong nuclear force by gluons, and the weak nuclear force by W+, W-, and Z bosons. There have been many attempts to find a quantum theory of gravity - that is, to use quantum principles to construct a field theory of gravity. In these theories, gravity would indeed be mediated by a particle, dubbed the graviton. String theory is one example of these theories; the physics community is divided about it.

You probably have heard other terms tossed around that embody interesting concepts. A gravitational wave is essentially a ripple in space-time emitted by an object or system of objects. There are strict restrictions over what sort of objects can emit these waves; binary neutron stars are one consistently cited example. These waves should not be confused with the aforesaid hypothesized gravitons; while gravitational waves carry energy, they do not "mediate" gravity. They should also not be confused with the unrelated gravity waves.

So, basically, the predominant view in the physics community is that general relativity is the best description of gravity; at the moment, gravity is considered to be the bending of space-time, so it is indeed "something else entirely." Many theories of quantum gravity, including string theory, however, attempt to create particles called gravitons as force-carrying bosons. If evidence is found relating to these theories, then we may well learn whether or not gravitons do exist. One more thing about waves: Because of the quantum concept of wave-particle duality, any particle can be described as a wave, which has a wave function. So if gravity is carried by a particle, then it is also due carried by a wave!

This is one of the great remaining mysteries of the universe.  I have an idea on this that one can derive known equations for gravity, as described below (this has not been proven, but I believe it is very likely to be the case).  Other people have thought of this as well, but it is not yet the mainstream view.  In fact, someone wrote a paper on it, discussed here: http://www.angelfire.com/pq/spaceflow/part2.html
My gravity idea (simplified):
This recent YouTube video by ScienceClic English actually illustrates my idea very well:

I believe gravity is the flow of space itself into mass.  By space, I am referring to the 3 dimensions of space that we observe in the universe.  I believe all mass constantly "absorbs" the space around it, bringing everything around closer to itself.  To conceptualize this, imagine mass being a vacuum cleaner, sucking up the air, and the dust that is in the air all around it.  This has the greatest effect on the air that is nearest to the vacuum, and I believe mass behaves likewise with space.  Since the linked video is explaining Relativity, perhaps my idea is actually an explanation of Relativity.
Several things this explains:

This explains why objects with more mass have more gravity (e.g. each bit of mass is absorbing a small amount of space, so has gravity).  The more mass you have, the more gravity you have.

This explains why gravity is strong when near an object (e.g. when close to earth), and decreases inversely proportional to square distance from the object.  If you imagine an expanding "spherical wave" of space moving inward, surface area of the sphere increases in relation to the square of the distance.  However, the effect is dispersed over this increased surface area, hence it decreases with the square of the distance from the object.  A fairly simple (but 2D vs. 3D) analogy is how when you throw a stone into water, the ripple will be strong at first, near the entry point, and then the ripple disperses as it expands into the surrounding pond.  A good 3D analogy is how sound (or light) expands spherically, and is more faint the further you are away from the source.

The orbit of "binary" systems, including the dynamics between the earth and moon.  The earth is more massive than the moon, and is absorbing more space than the moon.  Therefore, it is moving the moon toward it fairly quickly (but the moon has enough momentum that it remains in orbit vs. falling toward earth).  However, the moon is also pulling on the earth, so the earth is also "falling toward the moon" a little.  Again, the earth has enough momentum not to "fall into the moon".  This results in mostly the moon orbiting around the earth, but the earth is also "slightly" orbiting around the moon, which would be observed as a slight wobble.

This explains why large mass objects and small mass objects are affected the same by gravity.  Since it is space itself that is moving, whatever is in that space will be moved accordingly.  If it is an anvil, it will be moved just the same as a feather.  Therefore (if it weren't for other effects such as air resistance), a feather would fall to earth the same speed as an anvil.  In practice, air resistance makes the feather small much slower (but repeat the experiment on the moon, and they should hit at the same time since it has no air).

This even explains why things without mass (e.g. light) are also affected by gravity.  Since space itself is moving, light moves along with it.  This would explain gravitational lensing, and why light cannot escape a black hole.

Second part of my idea (more speculative):
I also believe antimatter somehow constantly ejects space back into the universe.  In the video, imagine the reverse happening (e.g. if you could play it in reverse).  I believe when matter and antimatter form, they are somehow "linked" so that space absorbed by matter is ejected by the corresponding antimatter particle.  This would give antimatter a "negative" gravity effect.  I believe there is equal amounts of matter and antimatter in the universe, and the antimatter is in a diffuse cloud spread throughout the universe, where matter is not present.  I believe this accounts for dark energy - the accelerating expansion of the universe.


Would we have more than 8 minutes of light, if the sun "went out"? The common theory is, that if the Sun "shut down", we would see the light for eight more minutes (the time that it takes the photons to reach the Earth).

However recently I have read that photons need around 100 000 years to reach the Earth, since the reactions are happening at Sun's core, and gamma rays can't leave the Sun without interacting with other particles, unlike neutrinos for example.

Is that theory correct? If the Sun's core "shut down", would we still receive photons (light) for another 100 000 years, with only neutrinos disappearing immediately?

If nuclear fusion were to suddenly stop in the centre of the Sun, then the only clear signature we would have of this is the lack of detectable neutrinos received at Earth, starting about 8 minutes after the reactions ceased. The Sun however would continue to shine for tens of millions of years at roughly its current luminosity.

The power source is not "stored" photons. The Sun itself would simply resume the slow gravitational contraction that was halted about 4.5 billion years ago when nuclear reaction rates at the centre were able to increase sufficiently to supply the radiative losses from the surface of the Sun.

The characteristic timescale for the contraction is about
$$\tau_{\rm KH} = \frac{GM^2}{RL},$$
which is 30 million years. i.e. The Sun has enough gravitational potential energy to supply its current luminosity for tens of millions of years.

While this is happening, the Sun would approximately maintain its current luminosity, but decrease in radius, meaning that its surface temperature would increase.

Once the Sun had contracted to a few times the size of Jupiter (so about 30% of its current radius), the contraction would begin to slow, because the electrons in the core become degenerate and the pressure increases with density by more than expected for a perfect gas. The slowing contraction decreases the rate of potential energy release and hence the solar luminosity. The contraction continues at a slow rate until the Sun becomes a hot "hydrogen white dwarf" a few times the size of the Earth, which then cools to a glowing cinder, with no further contraction, over billions of years (see What would the Sun be like if nuclear reactions could not proceed via quantum tunneling? for some more details).

Even if you were to not allow the Sun to contract, it would take some time to radiate it's thermal energy. This timescale is approximately
$$\tau_{\rm therm} \simeq \frac{3k_B T M}{m_H L},$$
which assumes the Sun is a perfect gas of protons plus electrons, with an average temperature $T$. If we take $T =10^7$ K and the current solar luminosity, then $\tau_{\rm therm}=$ 40 million years.

On the other hand, if your scenario is just that light from the Sun stops being emitted, then of course it goes dark on Earth about 8 minutes later.

The "common theory" you're reading is not about the processes that produce light in stars, it's just intended as a demonstration of the speed of light through space. When it talks about the Sun "shutting down", it's not talking about the nuclear fusion processes stopping, it means that the Sun as a whole stops shining. I'm not a physicist, but I don't think there's anything that would actually cause this to occur at a sudden time, such that we could measure the 8 minute time difference for the last photons to reach Earth.

It's just a thought experiment, using a simplified description of an impossible event, to make a point about some other process. In this model, the Sun either gives off light or it doesn't, and we're measuring the time since it changed that state; we're not concerned with what the photons do before they're emitted from the Sun (any more than measuring time that light travels from a light bulb cares about how the electricity that powered the bulb was produced -- we could measure that all the way back to the dinosaurs dying and eventually turning into fossil fuels that were used in the power plant, or even further back to the solar energy that powered life on earth).


Are there equal number of planets, stars, galaxies etc in observable universe spinning in both directions? Just because we observed that our milky way galaxy is spinning in a certain direction therefore we assume it is applicable to all other galaxies, I am curious to find out if hypothetically most of the celestial objects such as natural satellites, planets, stars even galaxies within our observable universe are spinning/rotating in the same direction as the milky way what kind of implications can we say about the condition in the early universe? or is it just an coincident?

Angular momentum of the entire universe remains constant. Assuming that the big bang was the only  thing present initially, it wouldn't  posses any angular momentum. Now, if there are unequal number of planets, galaxies, stars,etc. in the observable universe spinning in both directions, there'd be some net angular  momentum which would violate the conservation of angular momentum. Hence, in my opinion, there are equal number of celestial bodies spinning in both directions.

Galaxies are oriented in different directions, if we consider the orientation to be relative to their central axis of rotation. Then when we consider things like galaxies and their "direction" of spin, we tend to orient them so that "up" is the direction that, when we look at them from "above", they rotate in an anticlockwise direction - the same as observing Earth's rotation looking down from the North Pole. We tend to use a "right-hand rule" for this rotation, so that when the direction of rotation matches the curl of the fingers on the right hand, the thumb points in the "up" direction. What this means is that every galaxy rotates the same way.

On the smaller scale what we see is that things in systems tend to rotate in the same direction - looking "down from above" on our Solar System we see that the planets all revolve in the same direction around the Sun (anticlockwise). The vast majority of smaller bodies also revolve in this same direction. The Sun rotates in this same direction. And the planets themselves tend to rotate in the same direction (exceptions being Venus and Uranus). This is understandable as the accretion disc would have been rotating during Solar System formation.

So yes, for the most part at least, as far as we have been able to determine, everything spins the same way within its own environment.


Would things be exactly the same if the Sun was orbiting the Earth and the Earth was in the center of the Solar System? Would things be the same if the Sun was orbiting Earth and the Earth was in the center of the Solar System, or would there be any big difference?

It seems like why people before thought the Earth was the center of the Solar System since it didn't conflict with their experience.

It depends on what you mean by "in the center" or "switch places."  If you physically moved the Earth to where the Sun is, and the Sun to where the Earth is, and gave each the speed that the other had while keeping the same reference frame, this would do tremendous violence to all the other planets in the solar system (though the Earth would continue to orbit more or less the same).

However, if what you are talking about is simply the language of what you call the center, then you are talking about changing to a different set of coordinates where the Earth is at the stationary center of your coordinate system.  Doing that presents no problems, and nothing is different except the language you are using to talk about what is going on.  One of the great things Einstein did was to reframe the laws of physics in a form that was independent of coordinates, allowing us to forever separate what the laws say is happening from the language we use to talk about what is happening (i.e., our chosen coordinates).  So it all comes down to whether you think "being in the center" is a real thing, or just the language of some coordinate system.  If the former, then no, things would not be at all the same, if the latter, then yes, they'd be exactly the same, if you are careful to use coordinate-independent laws like general relativity.

So, to answer your question let's go to 322 BCC where Earth was at the center and all stars and planets revolves around it. But planet used to show unusual behavior. They use to retrograde (https://en.m.wikipedia.org/wiki/Apparent_retrograde_motion).

So, in retrogration, unlike stars, motion the planets seems to move forward then backward and again forward. To understand this go to link above.

Later to explain retrogration of planets let's go to 3rd century of BC where in the Hipparchian and Ptolemic Astronomy they used concept of epicycle to explain retrogration of planets.

(https://en.m.wikipedia.org/wiki/Deferent_and_epicycle)

So, in epicycle, there is one orbit where some invisible thing is revolving, now on this invisible thing, imagine another orbit. It is like orbit of moon in our present solar system, but we can't see earth, and in this model the orbit of moon is similar to orbit of planets. (See link for better explanation)

Now, with this we can explain retrogration motion of planets. But it lack precision. So what we can do is, introduce epicycle within an epicycle, and epicycle within it... And so on, and use FFT (Fast Fourier Transformation) to decide radius of orbits of various epicycles to match observation data.

Now you have a system where Earth is at center, stars move around it. So, do the Sun. And planets moves in FFT epicycles.


If the earth stopped spinning what would happen to the moon A question was posed at work today, "if the earth stopped spinning what would happen to the moon"? Ignoring any effects on the Earth, what actually happens to the Moon? Does it continue to orbit the now non-rotating Earth, or does it fly off into space like a massive asteroid, or more likely some other thing that I can't conceive of?

Note, the Earth stops spinning, not orbiting the sun.

For additional curiosity, what would be the differences if this were an extremely gradual slowing vs an almost instantaneous stoppage?

Earth's current rotation is faster (~1 day) than the Moon's orbital period around Earth (~28 days). This leads to tidal acceleration -- the tidal bulges raised by the Moon rotate ahead, pulling on (and being pulled by) the Moon. Over a long period, this converts Earth's rotational momentum into Lunar orbital momentum. The Earth's rotation slows, and the Moon's orbit expands. Eventually, if nothing else interfered, Earth and Moon would become tidally locked, with the length of a day and the length of a month equal.

If Earth weren't rotating, this effect would happen in reverse. Tidal acceleration (deceleration?) would transform Lunar orbital momentum into Earth rotational momentum. The Earth would slowly start to rotate again, and the Moon's orbit would contract, again until the two were tidally locked.

I'm not in a position to run the numbers, but it should be possible to estimate the final configuration -- how close the Moon would be, and how long the day/month would be.

There is not the slightest chance of the Earth stopping spinning, but if it did there would be hardly any effect on the moon. It would continue to orbit just as it has always done. You ask about a very gradual slowing ,and what the result of that would be. We are on very firm ground here, because there has already been a gradual slowing and we know exactly what the results have been.

The tidal interaction of the moon in its orbit around the Earth has considerably slowed the Earth's rotation over the last 4 billion years, and this rotational energy was captured by the moon and has boosted it into a higher orbit. The moon is still moving away from us at the rate of several centimetres per year. Presumably, if the Earth stopped rotating there would be no rotational energy for the moon to capture, but that is not going to happen.


Does the Sun rotate? The planets rotate as an after effect of their creation, the dust clouds that compressed span as they did so and the inertia has kept it rotating ever since.

It's fairly easy to prove that planetary bodies are rotating just by watching their features move across their respective horizons.

This seems less easy to discern for amateur astronomers  in the case of the sun though.

Does the Sun also rotate as a by-product of it's creation? What evidence is there to support this? Does the sun have any discerning features that make it evident it is rotating?

Yes.

It does not rotate uniformly though, different portions have a different angular velocity (as a body made of plasma, it can get away with this).

Measuring this in theory is pretty easy, we just need to track the motion of the sunspots. This isn't as simple as calculating the changes in relative positions of the sunspots, though, as the Earth is rotating and revolving, which makes the calculations harder. This measurement can be done using the celestial sphere (the field of stars that we see) as a "fixed" reference point and seeing how the Earth and the sunspots move relative to that.

Almost everything in the universe rotates/revolves, at least a little bit, because angular momentum is hard to get rid of. It can be transferred from body to body, but for a body to end up with zero angular momentum, it needs to meet another body with the exact same angular momentum and collide with it in a particular way. Given that this is pretty rare, all celestial bodies rotate.

In addition to that, a non rotating body that is revolving will eventually start spinning due to tidal forces.

So, far all the answers have shed light on the experimental reasons. But you can say it with full confidence without any astronomical observations that the Sun rotates.

The logic is that any closed system have a center of mass that is unaltered unless acted upon by some external force. So, given that Earth rotates around Sun then the system forming Earth and Sun must have a center of mass which is not changing. Therefore Sun must rotate to counter the change in center of mass caused by the rotation of Earth. And so it does with any other system belonging to our solar system.


What's the fastest moving object in the universe? We know that nothing can have proper velocities larger than the speed of light in vacuum. But are there any objects in space that get close to it? Any comets, or other objects thrown by gravity or supernova explosions that were hurled to incredible speeds?

The answer to this is surprising:

We are.

And many (if not all) other galaxies.

And they move faster than light.

See, the universe is expanding, at an accelerating rate. The fabric of spacetime itself stretches out, so that galaxies seem to move away from each other. The interesting thing is that relativity does not forbid these from moving away faster than light. While local space is flat and the local speed of light must be upheld, this need not hold at a global scale, so it is possible to have frames which move away from each other faster than $c$. Indeed, there are some galaxies that are moving away from us faster than light (the only reason we see them is that they used to be closer and moving at a slower speed). Any pair of galaxies that are 4200 Mpc away from each other (that is, with a redshift of 1.4), are moving away from each other faster than light in each other's frames (numbers stolen from the linked page).

Since the only consistent way to talk about motion is relative, one can say that we are moving away from other galaxies faster than light, since the reverse is also true. This can put galaxies in the bucket of the fastest moving objects in the universe. As for which is the fastest, I don't know, we would have to find a pair of galaxies which are the farthest apart (distance measured in the frame of the galaxy, of course), but since the universe is probably more than what we observe1, we can't pinpoint the pair of galaxies for which this is true.

For those who think that it is cheating2 to short-circuit the question with space expansion, there are other objects that go faster than light (they are not the fastest objects in the universe though), and these can be found on good 'ol Earth.

Electrons:

In nuclear reactor cooling pools3, we have a phenomenon known as Cerenkov radiation. Basically, emitted beta particles move faster than the speed of light in water. This creates an effect of similar origin as the sonic boom, where strong light emanates from the medium.

Saywhat? You think I'm cheating again2 by putting everything relative to the speed of light in a medium?

Alright, fine. Here are some fast objects that don't require space expansion to be fast, nor do they involve any trickery of semantics where the medium in which they are being measured is not mentioned. Many have already been mentioned by astromax.

Tachyons:These are particles which go faster than $c$ — this does not violate relativity as long as they never decelerate to subluminal speeds. However, there isn't much (any?) experimental evidence for these. A lot of BSM models do predict their existence, though. So there's still some cheating here, on to bradyonic matter:
Gluons: These are massless, and though they don't occur freely (except possibly in glueballs, though these most probably have mass) they do travel at $c$. But these can't move at any other speed, so again, this is slightly cheating. On to fermionic matter:
Neutrinos: Now these are viable candidates. The electron neutrino is known to have very, very little mass (we have an upper bound for it, which gives ), and as a result it can easily attain very high speeds. Put it in a gravitational field, and it goes even faster. If you want macroscopic objects, however:
Stuff spiraling around spinning black holes: Black holes have a strong gravitational field, and when rotating, they can impart angular momentum (lots of it) to nearby objects like accretion disks. Objects close to a black hole are accelerated to pretty high speeds. In fact, if an object is within the ergosphere, it moves faster than light from the point of view of certain frames of reference.
Stuff falling into black holes: From the faraway frame, an object speeds up and approaches the speed of light as it approaches the horizon of a black hole. Arbitrarily large speeds bounded by $c$ can be attained here.
Black hole plasma jets: Jets being flung out of black holes can get pretty fast relative to each other.

1. Due to cosmic expansion, there can be galaxies that are no longer visible to us. Some galaxies may never have been visible to us, if we start watching from when galaxies started forming.

2. I, for one, agree with you.

3. And other places where you have massive particles being emitted really fast into  a medium

I think there was an ahem ACTUAL EXPERIMENT DONE ahem where two electrons were fired in the exact opposite direction and the negative charge changed to positive with same said change happening at the same time to the other electron.
This experiment if valid and true would mean something is travelling faster than the speed of light.


Why don't we have 2 Summers and 2 Winters? Due to Earth's elliptical orbit, its distance from Sun varies by almost 5 million Kilometers (147 million Kilometers at closest point & 152 million Kilometers at farthest point, i.e. almost 3% of the average distance).

As evident from the fact that that Venus has hotter environment than Mars due to their respective distances from the sun.

Why then Earth does not observe two winters (at farthest points) and two summers (at closest points)?

Additional Note:
I know that Earth's seasonal climate change is caused by its 23 degrees tilt that causes the sunlight density variations for the hemispheres.

But to me this 5 million Km distance seems more relevant than the 23 degrees tilt.

There are a few incorrect assumptions in your post, so it is difficult to answer as asked. But I can address the misconceptions.

1. The seasons are not caused by our distance from the sun
The seasons are caused by the 23.5° tilt in Earth's axis. When the Northern Hemisphere is tilted towards the sun (summer), the Southern Hemisphere is simultaneously tilted away from the sun (winter). So the seasonal temperature difference has little to do with the Earth's position in its elliptical orbit. Without this tilt, there would be no seasons and the temperature day to day across the globe would be relatively uniform.

2. Even the GLOBAL temperature is NOT consistent with our change in distance
As a matter of fact, the average temperature of the Earth globally is hottest when it is the furthest from the sun — hotter by about 2.3°C (ref). That's because there is a lot more landmass in the Northern Hemisphere facing the sun (when Earth is farthest away in its orbit). So even though there is less intensity of sunlight, the land is able to be heated up much faster than the vast oceans which have to be heated at perihelion.

This distance-temperature inconsistency isn't unique to the Earth. Look at the average temperature of the other inner planets as we move away from the sun:

Mercury (167°C)
Venus (460°C)  ← farther, but hotter than Mercury?
Earth (14.0°C)
Mars (-60°C)

Venus is actually warmer than Mercury because of the thick carbon dioxide atmosphere causing runaway global warming. So it isn't simply the distance from the sun that determines the average temperature of a planet.

3. There's only ONE aphelion/perihelion
The closest point of the Earth's orbit (perihelion) and the farthest (aphelion) only happens once per year; not twice. That is because the elliptical orbit of the Earth is such so the sun is at one of the foci, not the center (as illustrated below).

Note that the size of the bodies and the eccentricity of the orbits are greatly exagerated here.

We DO have 2 summers and two winters - and 2 springs and autumns too.  The varying distance between the sun and earth does not cause the seasons.  They are caused by the tilt of the Earth towards the sun in summer and away from the sun in winter.

The northern and southern hemispheres both have the same seasons, but at opposite times of the year.
For example, when it is winter in the United States, it is summer in Argentina and vice versa.

Since the seasons in the northern and southern hemispheres occur at opposite times of the year, it means that there are 2 of each season or 8 distinct seasons each year, including 2 separate summers and 2 separate winters.

Perhaps this link might help:
http://neoprogrammics.com/equinoxes_and_solstices/


Why doesn't Earth's axis change during the year? My understanding is that the Earth's axis points in the same direction in space during its entire orbit around the sun. And this is what causes our seasons.

My question is why doesn't the axis follow the orbital path (kind of like a car driving around a circle)?

I imagine the Moon plays a part in this.

As a secondary question, do the other planets in our solar system orbit the same as Earth (its axis points in the same direction in space for its entire orbit)?

My understanding is that the Earth's axis points in the same direction in space during its entire orbit around the sun. And this is what causes our seasons.

The second statement is correct. The axial tilt is the primary driver of the seasons.

The first statement is not exactly correct. There is a small but persistent change in the orientation of the Earth's axis. The change over the course of a year is small. The change over the course of 13,000 years is large. In 13,000 years the Earth's axis will be tilted by about 47 degrees compared to the current orientation. In yet another 13,000 years the Earth will be oriented very similarly to how it is oriented now. I'll write more about this later.

  My question is why doesn't the axis follow the orbital path (kind of like a car driving around a circle)?

That is not a good mental model. The only contact between your car and the ground are the bottoms of the four wheels on your car. The forces at these four small contact areas are what change your car's orientation and change your car's velocity vector. This means that the torque on your car as you turn the steering wheel and the centripetal force on your car that makes your care negotiate a turn are intimately connected. Turn the steering wheel gently and the heavy front end of your car follows suit. Assuming a gentle turn, the light rear end of you car soon follows suit. If you turn too hard, your car starts to skid instead of turn. Turn even harder yet and you risk having your car flip and roll. The motion of your car depends on that intimate coupling between force and torque.

Gravitation doesn't work like that. It instead acts on each and every bit of the Earth, all together, all at once, all the time. There is a slight variation in the gravitationally-induced acceleration across the Earth due to variations in distance to the Moon (or Sun), but these variations are small. The net gravitational force on the Earth is very much decoupled from the small net gravity gradient torque on the Earth.

The principal result of the gravity gradient torque exerted by the Moon and the Sun on the Earth is a very slow but large precession of the Earth's rotation axis (for more, read this wikipedia article on axial precession). This precession makes the Earth's rotation axis rotate about the normal to the ecliptic at a rate of one revolution per 25772 years. There are lesser effects collectively called nutations and polar motion that result from gravity gradient torque. These lesser effects are much, much smaller in magnitude but have much, much shorter periods. The largest of these lesser effects results from the 18.61 year cyclical variations in the lunar node. Even this largest component of nutation is small, two orders of magnitude smaller than the large but slow precession.

If the Earth's axis is always pointing toward Polaris then the Earth's axis has to continually change throughout year over a 186,000,000 mile orbit distance change.

If the tilt in December is 23.4 degrees then, March would be less tilt and an added tilt toward the sun, June would be the smallest Earth tilt, October would be less tilt and and added tilt toward the sun. Without this Earth tilt change Polaris would only be aligned one day a year in December.

You can easily prove this yourself.


Why don't small stars end up as a black holes? I have recently done some research into black holes, and realized that big stars form black holes, whilst smaller ones don't. Is this because the gravity isn't strong enough for it to fall in on itself, or something else?

The actual answer has nothing at all to do with the temperature.

Even low-mass stars would form black holes if they ran out of nuclear fuel to burn, and simply cooled whilst being supported by "standard" gas pressure in their centres.

That is because that gas pressure would be proportional to the temperature, but the star is able to cool so it would need to shrink and use gravitational energy to stay hot, but of course eventually it would disappear inside its event horizon and become a black hole.

The real reason this does not occur is electron degeneracy pressure. This is a quantum mechanical effect, related to the Pauli exclusion principle, that does not allow two electrons to occupy the same quantum state. As the gas gets squashed in the core, the electrons are forced to fill higher and higher momentum states in order to "avoid" each other. Because these electrons have large momentum they also exert a large pressure - degeneracy pressure. This degeneracy pressure does not depend on temperature and so the star (it is a white dwarf when a low-mass star arrives at this stage) can cool and remain in hydrostatic equilibrium without shrinking any further. Thus it avoids becoming a black hole (or even a neutron star) and simply cools at nearly constant radius.

Gravitational collapse occurs when an object's internal pressure is insufficient to resist the object's own gravity. In the cases of stars, it normally usually happens because of one of these 2 reasons:

The star has too little "fuel" left to maintain its temperature
The star that would have been stable receives extra matter in a way that does not raise its core temperature.

In either of these cases, the star's temperature is no longer high enough to prevent it from collapsing under its own weight (gravity). The collapse may be stopped by various factors condensing the matter into a denser state. The result is one of the various types of compact star.

As said above, it needs a lot of mass to actually collapse into either a black hole, or implode as a supernova, and smaller stars simply don't have this.

Wikipedia


What type of planetary-mass object would Planet Nine be? Given that Planet Nine is hypothesized to be 10-20 times the mass of the Earth orbiting the sun some 200-2500 AU out, what kind of planetary-mass object would it be?

Even given that it has apparently "influenced" several Trans-Neptunian objects, since its orbit is so large and so slow (approximately 25,000 years) it would take an awful long time to clear out its own orbit (especially given that we do not know how long it has been in that orbit), so it seems like it would not qualify as an actual planet under the rules that re-classified Pluto.

Since it orbits our sun, it wouldn't be an exoplanet nor a rogue planet.  And it sure seems too big to be called a dwarf planet.

A captured planet seems closest in other attributes, but it probably was not either a capture nor a re-capture, but more likely a partial ejection.

So what classification would it fall into?

It is a planet - either a gas giant core, a mini-Neptune, or a super-Earth.

First off, Mike Brown has stated outright

  “It is a planet—there’s virtually no doubt,” he said. “What we now call planets are objects that can gravitationally dominate their neighborhood. Pluto is a slave to the gravitational influence of Neptune. By area, Planet Nine dominates more of the solar system than any other known planet—it’s only because of this that we can infer its existence. And because of this we’re pretty sure it’s not a small object: it’s at least ten times more massive than Earth and five thousand times more massive than Pluto. In many ways, you could argue that this is more of a planet than anything else in the solar system.”

I wrote an answer to How was the hypothetical ninth planet kicked so far out of the Solar System? that discussed the possibility that the ninth planet is the hypothetical 5th gas giant proposed by Nesvorný (2011). The details can be found in that answer, but basically, Brown & Batygin's reference to previous papers by Morbidelli et al. suggest that they think that this is what remains of an ice giant, like Uranus and Neptune, that formed near Saturn and Jupiter but was ejected in a series of gravitational interactions that led it to the Kuiper Belt and beyond.

That said, Batygin's estimates place the ejection of this planet way before previous models of a 5th gas giant predict, so either we accept models with a  6th gas giant (also an ice giant) and say that both of these were ejected independently of one another - not impossible - or we go for some other option. The bottom line is that this is a planet, if we go with the above assumptions.

Two other guesses have been made based on the planet's mass, estimated by Brown & Batygin at about 10 Earth masses. One is that it is a very massive super-Earth, hanging around the proposed upper mass limit for those objects. This implies that the planet is rocky, but does not imply anything else. The other guess is that it is a mini-Neptune, with a predominantly gaseous composition but at about the same mass. The super-Earth hypothesis arose in part because of claims a long time ago about the ejection of a 5th terrestrial planet.

I've used the term "planet" throughout this entire answer, and that's because this object may be one - and most likely was in the past. Let's look at the IAU's three major criteria to be a planet:

It orbits the Sun. This is definitely true, if it exists. Also note that if this is a captured rogue planet, as some have proposed, then we can still call it a "planet", because it now orbits the Sun.
It is in hydrostatic equilibrium. This is most likely true, and is true if any of the three models I suggested above are correct.
It clears its neighborhood. This is a bit iffy. It clearly influences objects around it, but we don't know much about those influences besides observations of the six Trans-Neptunian Objects (TNOs) Batygin & Brown studied. That said, an object with a mass of 10 Earth masses will most likely clear the neighborhood around its orbit if it has a stable orbit, which should be the case here. See the quote at the start of this answer for more information.

It would either be a super-Earth or an ice giant like Uranus and Neptune. If it is a planet. In September 2019 astrophysicists suggested that it actually may be a primordial black hole. Such black holes are extremely tiny and if it is one, it would have a diameter of about 2 inches (5 cm). It would reportedly also have an accretion disc likely. And it could have own planets of course.

And please forget the 2006 definition of a planet which became selected by 4% of the IAU; according to it, there would be no planet at all.