Solar System Planets: Order of the 8 (or 9) Planets Solar System Planets: Order of the 8 (or 9) Planets By Robert Roy Britt | January 22, 2016 12:35pm ET MORE The planets of the solar system as depicted by a NASA computer illustration. Orbits and sizes are not shown to scale. Credit: NASA Ever since the discovery of Pluto in 1930, kids grew up learning about the nine planets of our solar system. That all changed starting in the late 1990s, when astronomers began to argue about whether Pluto was a planet. In a highly controversial decision , the International Astronomical Union ultimately decided in 2006 to call Pluto a “dwarf planet,” reducing the list of “real planets” in our solar system to eight. However, astronomers are now hunting for another planet in our solar system, a true ninth planet , after evidence of its existence was unveiled on Jan. 20, 2016. The so-called "Planet Nine," as scientists are calling it, is about 10 times the mass of Earth and 5,000 times the mass of Pluto.

[ Solar System Pictures: A Photo Tour ] If you insist on including Pluto , then that world would come after Neptune on the list; Pluto is truly way out there, and on a wildly tilted, elliptical orbit (two of the several reasons it got demoted). Interestingly, Pluto used to be the eighth planet, actually. More on that below. Terrestrial planets The inner four worlds are called “ terrestrial planets ,” because, like Earth, their surfaces are all rocky. Pluto, too, has a solid surface (and a very frozen one) but has never been grouped with the four terrestrials. Jovian planets The four large outer worlds — Jupiter, Saturn, Uranus, and Neptune — are known as the “Jovian planets” (meaning “Jupiter-like”) because they are all huge compared to the terrestrial planets, and because they are gaseous in nature rather than having rocky surfaces (though some or all of them may have solid cores, astronomers say).

According to NASA , "two of the outer planets beyond the orbit of Mars — Jupiter and Saturn — are known as gas giants; the more distant Uranus and Neptune are called ice giants." This is because, while the first two are dominated by gas, while the last two have more ice. All four contain mostly hydrogen and helium. Dwarf planets The IAU definition of a full-fledged planet goes like this: A body that circles the sun without being some other object's satellite, is large enough to be rounded by its own gravity (but not so big that it begins to undergo nuclear fusion, like a star) and has "cleared its neighborhood" of most other orbiting bodies. Yeah, that’s a mouthful. The problem for Pluto, besides its small size and offbeat orbit, is that it shares its space with lots of other objects in the Kuiper Belt , beyond Neptune. Still, the demotion of Pluto remains controversial . The IAU planet definition puts other small, round worlds in the dwarf planet category, including the Kuiper Belt objects Eris , Haumea , and Makemake . Also now a dwarf planet is Ceres , a round object in the Asteroid Belt between Mars and Jupiter.

Ceres was actually considered a planet when discovered in 1801 and then later deemed to be an asteroid. Some astronomers like to consider Ceres as a 10th planet (not to be confused with Nibiru or Planet X ), but that line of thinking opens up the possibility of there being 13 planets, with more bound to be discovered. The planets Below is a brief overview of the eight primary planets in our solar system , in order from the inner solar system outward: Mercury The closest planet to the sun, Mercury is only a bit larger than Earth's moon. Its day side is scorched by the sun and can reach 840 degrees Fahrenheit (450 Celsius), but on the night side, temperatures drop to hundreds of degrees below freezing. Mercury has virtually no atmosphere to absorb meteor impacts, so its surface is pockmarked with craters, just like the moon. Over its four-year mission, NASA's MESSENGER spacecraft has revealed views of the planet that have challenged astronomers' expectations.

Discovery: Known to the ancients and visible to the naked eye Named for: Messenger of the Roman gods Diameter: 3,031 miles (4,878 km) Orbit: 88 Earth days Venus' southern hemisphere, as seen in the ultraviolet. Credit: ESA Venus The second planet from the sun, Venus is terribly hot, even hotter than Mercury. The atmosphere is toxic. The pressure at the surface would crush and kill you. Scientists describe Venus’ situation as a runaway greenhouse effect. Its size and structure are similar to Earth, Venus' thick, toxic atmosphere traps heat in a runaway "greenhouse effect." Oddly, Venus spins slowly in the opposite direction of most planets. The Greeks believed Venus was two different objects — one in the morning sky and another in the evening. Because it is often brighter than any other object in the sky — except for the sun and moon — Venus has generated many UFO reports.

Discovery: Known to the ancients and visible to the naked eye Named for: Roman goddess of love and beauty Diameter: 7,521 miles (12,104 km) Orbit: 225 Earth days An image of the Earth taken by the Russian weather satellite Elektro-L No.1. Credit: NTsOMZ Earth The third planet from the sun, Earth is a waterworld, with two-thirds of the planet covered by ocean. It’s the only world known to harbor life. Earth’s atmosphere is rich in life-sustaining nitrogen and oxygen. Earth's surface rotates about its axis at 1,532 feet per second (467 meters per second) — slightly more than 1,000 mph (1,600 kph) — at the equator. The planet zips around the sun at more than 18 miles per second (29 km per second).

Diameter: 7,926 miles (12,760 km) Orbit: 365.24 days Day: 23 hours, 56 minutes Related: Mars researchers are focusing both Earth-based and planet orbiting sensors to better understand sources of methane on the red planet. Image Credit: Space Telescope Science Institute Mars The fourth planet from the sun, is a cold, dusty place. The dust, an iron oxide, gives the planet its reddish cast. Mars shares similarities with Earth: It is rocky, has mountains and valleys, and storm systems ranging from localized tornado-like dust devils to planet-engulfing dust storms. It snows on Mars. And Mars harbors water ice. Scientists think it was once wet and warm, though today it’s cold and desert-like. Mars' atmosphere is too thin for liquid water to exist on the surface for any length of time. Scientists think ancient Mars would have had the conditions to support life, and there is hope that signs of past life — possibly even present biology — may exist on the Red Planet.

Discovery: Known to the ancients and visible to the naked eye Named for: Roman god of war Diameter: 4,217 miles (6,787 km) Orbit: 687 Earth days Day: Just more than one Earth day (24 hours, 37 minutes) Related: Close-up of Jupiter's Great Red Spot as seen by a Voyager spacecraft. Credit: NASA/JPL-Caltech Jupiter The fifth planet from the sun, Jupiter is huge and is the most massive planet in our solar system. It’s a mostly gaseous world, mostly hydrogen and helium. Its swirling clouds are colorful due to different types of trace gases. A big feature is the Great Red Spot, a giant storm which has raged for hundreds of years. Jupiter has a strong magnetic field, and with dozens of moons, it looks a bit like a miniature solar system.

Discovery: Known to the ancients and visible to the naked eye Named for: Ruler of the Roman gods Diameter: 86,881 miles (139,822 km) Orbit: 11.9 Earth years The shadow of Saturn's moon Mimas dips onto the planet's rings and straddles the Cassini Division in this natural color image taken as Saturn approaches its August 2009 equinox. Credit: NASA/JPL/Space Science Institute Saturn The sixth planet from the sun is known most for its rings . When Galileo Galilei first studied Saturn in the early 1600s, he thought it was an object with three parts. Not knowing he was seeing a planet with rings, the stumped astronomer entered a small drawing — a symbol with one large circle and two smaller ones — in his notebook, as a noun in a sentence describing his discovery. More than 40 years later, Christiaan Huygens proposed that they were rings. The rings are made of ice and rock. Scientists are not yet sure how they formed. The gaseous planet is mostly hydrogen and helium. It has numerous moons .

Discovery: Known to the ancients and visible to the naked eye Named for: Roman god of agriculture Diameter: 74,900 miles (120,500 km) Orbit: 29.5 Earth years Day: About 10.5 Earth hours Related: Near-infrared views of Uranus reveal its otherwise faint ring system, highlighting the extent to which the planet is tilted. Credit: Lawrence Sromovsky, (Univ. Wisconsin-Madison), Keck Observatory Uranus The seventh planet from the sun, Uranus is an oddball. It’s the only giant planet whose equator is nearly at right angles to its orbit — it basically orbits on its side. Astronomers think the planet collided with some other planet-size object long ago, causing the tilt. The tilt causes extreme seasons that last 20-plus years, and the sun beats down on one pole or the other for 84 Earth-years. Uranus is about the same size as Neptune. Methane in the atmosphere gives Uranus its blue-green tint. It has numerous moons and faint rings.

Discovery: 1781 by William Herschel (was thought previously to be a star) Named for: Personification of heaven in ancient myth Diameter: 31,763 miles (51,120 km) Orbit: 84 Earth years Neptune’s winds travel at more than 1,500 mph, and are the fastest planetary winds in the solar system. Credit: NASA/JPL Neptune The eighth planet from the sun, Neptune is known for strong winds — sometimes faster than the speed of sound. Neptune is far out and cold. The planet is more than 30 times as far from the sun as Earth. It has a rocky core. Neptune was the first planet to be predicted to exist by using math, before it was detected. Irregularities in the orbit of Uranus led French astronomer Alexis Bouvard to suggest some other might be exerting a gravitational tug. German astronomer Johann Galle used calculations to help find Neptune in a telescope. Neptune is about 17 times as massive as Earth. Discovery: 1846 Pluto and its moons orbit the sun near the edge of our solar system.

Learn all about Pluto's weirdly eccentric orbit, four moons and more in this Space.com infographic . Credit: SPACE.com/Karl Tate Pluto (Dwarf Planet) Once the ninth planet from the sun, Pluto is unlike other planets in many respects. It is smaller than Earth's moon. Its orbit carries it inside the orbit of Neptune and then way out beyond that orbit. From 1979 until early 1999, Pluto had actually been the eighth planet from the sun. Then, on Feb. 11, 1999, it crossed Neptune's path and once again became the solar system's most distant planet — until it was demoted to dwarf planet status. Pluto will stay beyond Neptune for 228 years. Pluto’s orbit is tilted to the main plane of the solar system — where the other planets orbit — by 17.1 degrees. It’s a cold, rocky world with only a very ephemeral atmosphere. NASA's New Horizons mission performed history's first flyby of the Pluto system on July 14, 2015.

[Related: New Horizons' Pluto Flyby: Latest News, Images and Video ] Discovery: 1930 by Clyde Tombaugh Named for: Roman god of the underworld, Hades Diameter: 1,430 miles (2,301 km) Orbit: 248 Earth years NASA Solar System Exploration: Dwarf Planets Planet Nine Planet Nine orbits the sun at a distance that is 20 times farther out than the orbit of Neptune. (The orbit of Neptune is 2.7 billion miles from the sun at its closest point.) The strange world's orbit is about 600 times farther from the sun than the Earth's orbit is from the star. Scientists have not actually seen Planet Nine directly . Its existence was inferred by its gravitational effects on other objects in the Kuiper Belt, a region at the fringe of the solar system that is home to icy objects left over from the birth of the sun and planets. Planet Closest to the Sun - Universe Today Universe Today Planet Closest to the Sun Article Updated: 24 Dec , 2015 by Jerry Coffey [/caption]Mercury, the planet closest to the Sun, is a study in extremes and offers many surprises.

The extremes of the planet have made it an understudied body in our Solar System, though the MESSENGER mission is trying to change that as you are reading this article. In addition to being the planet closest to the Sun, Mercury is also the smallest by mass. If you ignore the former planet Pluto, it is also the smallest by surface area, as well. The planet has the most eccentric orbit: at perihelion it is 46,001,200 km from the Sun and at aphelion it is 69,816,900 km. The planet’s short orbital period(87.969 Earth days) and slight axial tilt combine to make the day on Mercury(116 Earth days) longer than the year. The average temperature on the planet is 442.5°K. Because of the planet’s thin atmosphere there is a wide temperature range, 100°K to 700°K. The temperature at the equator can be as much as 300°K more than the temperature at the poles.

Despite its proximity to our central star, the poles of the planet are thought to have water ice hidden within impact craters. Claims for water ice are substantiated by observations by the 70 m Goldstone telescope and the Very Large Array. There are areas of very high radar reflection at the pole areas so, since water is highly reflective of radar, astronomers believe that water ice is the most likely cause of this reflection. Due to its size and average temperatures, the planet’s gravity can not retain a significant atmosphere over a long period. It does have a negligible surface-bounded exosphere that is dominated by hydrogen, helium, oxygen, sodium, calcium, and potassium. Atoms are continuously being lost and replenished from this exosphere. Hydrogen and helium atoms are thought to derive from the solar wind that buffets the planet. These elements diffuse into Mercury’s magnetosphere before escaping back into space. Radioactive decay within the crust is a source of helium, sodium, and potassium. Mercury has been explored by two mission: Mariner 10 and MESSENGER. Mariner 10 was able to map 40-45% of Mercury’s surface through more than 2,800 photos.

It revealed a more or less moon-like surface, a slight atmosphere, a magnetic field, and a large iron rich core. MESSENGER was launched in August of 2004. After a 31/2 year flight, it made its first flyby in January 2008 and arrived in orbit on March 18, 2011. So far, the probe has discovered large amounts of water in the exosphere, evidence of past volcanic activity, and evidence of a liquid planetary core. As the MESSENGER mission continues, the closest planet to the Sun should continue to reveal more surprises for the scientists at NASA. It appears a new age of discovery has begun for Mercury. We have an extensive section just on Mercury on Universe Today. And did you know there’s a spacecraft visiting Mercury called MESSENGER? You can read news about this mission here . Here’s a link to NASA’s Solar System Exploration Guide on Mercury. We have recorded an episode of Astronomy Cast just about the Sun called The Sun, Spots and All .

References: Order of the Planets From The Sun – Planet Closest Distance to the Sun – PlanetFacts.org The Moon Order of the Planets From The Sun The solar system is a group of celestial bodies orbiting around the dwarf star called the Sun. The main members of the solar system are eight major planets, and their satellites. Other interesting members are comets, meteors and meteorites and dwarf planets. The order of the planets from the Sun from nearest to the farthest is Mercury, Venus, earth, Mars, Jupiter, Saturn Uranus, and Neptune. 1. The nearest planet to the Sun is Mercury. Though it is very near to the Sun, it is not the hottest planet at all. The smallest among the inner planets has actually temperature of 450 degrees Celsius during day time while it drops to a very frigid negative 170 degrees Celsius at night. It is named after the Roman messenger of the Gods as it orbits the Sun once every 88 days. 2. The second planet from the Sun is named after the Roman Goddess of Beauty, Venus. True to its name, this beautiful planet is also called the evening star.

It is also the hottest planet in the solar system due to clouds of toxic gas with temperatures over 460 degrees Celsius. It rotates backward and makes one complete revolution around the Sun in 225 days. 3. The third planet from the Sun is Earth, the beautiful blue planet blessed with a diversity of living organisms of varying shapes and sizes. It is the only planet known to sustain life. It makes a complete revolution around the Sun for 365 days. It has one satellite called the Moon. 4. Mars is the fourth planet from the Sun. It is named after the Roman God of War. It is usually called the red planet. A Martian year is equivalent to 687 Earth days. It is usually the third brightest object of the night sky after Earth’s moon and Venus. The possibility of life on Mars is still a subject of probes and studies. By the way, Mars has similar seasons as here on Earth. 5. The biggest planet, Jupiter is the fifth planet from the Sun. Although it has the greatest volume among the planets, it has the lowest density. It is separated from the inner planets by the asteroid belt where many other interesting heavenly bodies orbit around the Sun. 6.

Saturn is the second of the outer planets and is the sixth in order from the Sun. Many astronomers tag it as the most beautiful planet because of its prominent rings made up of ice, dust, gas, and other particles. 7. Uranus is the third of the outer planets, the seventh in order from the Sun. This planet has a very low density and is made up of a large proportion of gas. It is also referred to as an ice giant because a decent percentage of its gases are trapped in ice form. It has faint rings. 8. Neptune is the fourth among the outer planets and the farthest planet away from the Sun. It is also made up of frozen gases like its neighboring planet Uranus. A great percentage of these gases are methane making it appear as a blue planet. What was once recognized as the ninth solar planet, Pluto, was just recently reclassified as a dwarf planet. It was placed in this category along with Eris, Vespa, and Ceres. Awesomeninja 18hunter235 StarChild: The planet Mercury Mercury: Planet Closest to the Sun Guess what?

Due to Mercury's rotation and highly elliptical orbit , the Sun appears to rise briefly, set, and rise again before it travels westward across the sky. At sunset, the Sun appears to set, rise again briefly, and then set again. Mercury is only about one-third the size of the Earth. It is smaller than any other planet. Mercury is very close to the Sun and has no substantial atmosphere . These factors contribute to the fact that the surface of Mercury has the greatest temperature range of any planet or natural satellite in our solar system . The surface temperature on the side of Mercury closest to the Sun reaches 427 degrees Celsius , a temperature hot enough to melt tin. On the side facing away from the Sun, or the night side, the temperature drops to -183 degrees Celsius. Scientists have detected a magnetic field surrounding Mercury, though it is not as strong as the field around the Earth. Scientists theorize that Mercury's field is due to an iron-bearing core or possibly to the solar winds . Mercury's atmosphere is very thin and is composed of helium and sodium.

The surface of Mercury has been shaped by three processes: impact cratering where large objects struck the surface resulting in crater formation, volcanism where lava flooded the surface, and tectonic activity where the planet's crust moved in order to adjust to the planetary cooling and contracting. Mercury does not have any naturally occurring satellites. Mercury A Question What is the difference in the temperature of Mercury's surface when it is facing towards and away from the Sun? Planet Mercury: Facts About the Planet Closest to the Sun Planet Mercury: Facts About the Planet Closest to the Sun By Charles Q. Choi, Space.com Contributor | November 30, 2016 08:44pm ET MORE This view is one of the first from the MESSENGER probe's Oct. 6, 2008 flyby of Mercury. Credit: NASA/JHUAPL/CIW Mercury is the closest planet to the sun. As such, it circles the sun faster than all the other planets, which is why Romans named it after their swift-footed messenger god. The Sumerians also knew of Mercury since at least 5,000 years ago. It was often associated with Nabu, the god of writing .

Mercury was also given separate names for its appearance as both a morning star and as an evening star. Greek astronomers knew, however, that the two names referred to the same body. Heraclitus believed that both Mercury and Venus orbited the sun, not Earth. [ Latest Photos: Mercury Seen by NASA's Messenger Probe ] Mercury's physical characteristics Because the planet is so close to the sun, Mercury's surface temperature can reach a scorching 840 degrees Fahrenheit (450 degrees Celsius). However, since this world doesn't have a real atmosphere to entrap any heat, at night temperatures can plummet to minus 275 F (minus 170 C), a temperature swing of more than 1,100 degrees F (600 degree C), the greatest in the solar system. Mercury is the smallest planet — it is only slightly larger than Earth's moon. Since it has no significant atmosphere to stop impacts, the planet is pockmarked with craters.

About 4 billion years ago, an asteroid roughly 60 miles (100 kilometers) wide struck Mercury with an impact equal to 1 trillion 1-megaton bombs, creating a vast impact crater roughly 960 miles (1,550 km) wide. Known as the Caloris Basin, this crater could hold the entire state of Texas. Another large impact may have helped create the planet's odd spin . As close to the sun as Mercury is, in 2012, NASA's MESSENGER spacecraft discovered water ice in the craters around its north pole, where regions may be permanently shaded from the heat of the sun. The southern pole may also contain icy pockets, but MESSENGER's orbit did not allowed scientists to probe the area. Comets or meteorites may have delivered ice there, or water vapor may have outgassed from the planet's interior and frozen out at the poles. [Related: First Photos of Water Ice on Mercury Captured by NASA Spacecraft ] As if Mercury isn't small enough, it not only shrank in its past but is continuing to shrink today . The tiny planet is made up of a single continental plate over a cooling iron core.

As the core cools, it solidifies, reducing the planet's volume and causing it to shrink. The process crumpled the surface, creating lobe-shaped scarps or cliffs, some hundreds of miles long and soaring up to a mile high. In the past, the surface was constantly reshaped by volcanic activity , some fairly recently in the planet's past. "The young age of the small scarps means that Mercury joins Earth as a tectonically active planet with new faults likely forming today as Mercury's interior continues to cool and the planet contracts," Tom Watters, Smithsonian senior scientist at the National Air and Space Museum in Washington, D.C., said in a statement . Mercury is the second densest planet after Earth, with a huge metallic core roughly 2,200 to 2,400 miles (3,600 to 3,800 km) wide, or about 75 percent of the planet's diameter. In comparison, Mercury's outer shell is only 300 to 400 miles (500 to 600 km) thick. The combination of its massive core and abundance of volatile elements has left scientists puzzled for years.

A completely unexpected discovery Mariner 10 made was that Mercury possessed a magnetic field. Planets theoretically generate magnetic fields only if they spin quickly and possess a molten core. But Mercury takes 59 days to rotate and is so small — just roughly one-third Earth's size — that its core should have cooled off long ago. "We had figured out how the Earth works, and Mercury is another terrestrial, rocky planet with an iron core, so we thought it would work the same way," Christopher Russell, a professor at the University of California, Los Angeles, said in a statement . An unusual interior could help to explain the differences in Mercury's magnetic field when compared to Earth. Observations from MESSENGER revealed that the planet's magnetic field is approximately three times stronger at its northern hemisphere than at its southern. Russell co-authored a model that suggests that Mercury's iron core may be turning from liquid to solid at the core's outer boundary rather than the inner. "It's like a snow storm in which the snow formed at the top of the cloud and middle of the cloud and the bottom of the cloud too," said Russell.

"Our study of Mercury's magnetic field indicates iron is snowing throughout this fluid that is powering Mercury's magnetic field." The discovery in 2007 by Earth-based radar observations that Mercury's core may still be molten could help explain its magnetism, though the solar wind may play a role in dampening the planet's magnetic field. Although Mercury's magnetic field is just 1 percent the strength of Earth's, it is very active. The magnetic field in the solar wind — the charged particles streaming off the sun — periodically touches upon Mercury's field, creating powerful magnetic tornadoes that channel the fast, hot plasma of the solar wind down to the planet's surface. Instead of a substantial atmosphere, Mercury possesses an ultra-thin "exosphere" made up of atoms blasted off its surface by solar radiation, the solar wind and micrometeoroid impacts. These quickly escape into space, forming a tail of particles. Mercury's orbital characteristics Mercury speeds around the sun every 88 Earth days, traveling through space at nearly 112,000 mph (180,000 km/h), faster than any other planet.

Its oval-shaped orbit is highly elliptical, taking Mercury as close as 29 million miles (47 million km) and as far as 43 million miles (70 million km) from the sun. If one could stand on Mercury when it is nearest to the sun, it would appear more than three times as large as it does when viewed from Earth. Oddly, due to Mercury's highly elliptical orbit and the 59 Earth-days or so it takes to rotate on its axis, when on the scorching surface of the planet, the sun appears to rise briefly, set, and rise again before it travels westward across the sky. At sunset, the sun appears to set, rise again briefly, and then set again. Mercury is the closest planet to the sun and has a thin atmosphere, no air pressure and an extremely high temperature. Take a look inside the planet. Credit: Karl Tate, SPACE.com Which of the Planets is Closest to the Sun? - Universe Today Universe Today Which of the Planets is Closest to the Sun? Article Updated: 24 Dec , 2015 by Fraser Cain [/caption] Were you wondering which of the planets is closest to the Sun?

The answer is planet Mercury. At an average distance of only 57.9 million km, Mercury orbits closer to the Sun than any other planet. Just to give you a comparison, Venus orbits the Sun at an average distance of 108.2 million km. And Earth orbits the Sun at an average distance of 149.6 million km. So you can see that Mercury is much closer, taking only 88 days to complete an orbit around the Sun. So, Mercury is the closest planet to the Sun. And then Venus is the second closest planet to the Sun, and Earth is the third closest planet to the Sun. The distance of Mercury to the Sun is actually an average. Mercury follows an elliptical orbit around the Sun. Sometimes it’s much closer, and other times it’s much further. At its closest point, called perihelion, Mercury is only 46 million km from the Sun. And then at its most distant point in orbit, called aphelion, Mercury gets out to 69.8 million km from the Sun. And just for comparison, the planet furthest from the Sun is Neptune, with an average distance of 4.5 billion km from the Sun.

If you’d like more information on Mercury, check out NASA’s Solar System Exploration Guide , and here’s a link to NASA’s MESSENGER Misson Page . We’ve also recorded an entire episode of Astronomy Cast all about Mercury. Listen here, Episode 49: Mercury . Proxima b: Closest rocky planet to our solar system found - CNN.com READ: Hate your job? NASA wants you to work on Mars Read More Proxima Centauri coexists with a binary star in Alpha Centauri, a well-studied star system that serves as a neighbor to our sun. Proxima b is a mere 4.2 light-years away from our solar system, or 266,000 times the distance between the Earth and the sun, which are 92.96 million miles apart. Previous rocky exoplanet discoveries, like those orbiting ultracool red dwarf star TRAPPIST-1 , were previously described as "close" at 40 light-years away.

"It's not only the closest terrestrial planet found, it's probably the closest planet outside our solar system that will ever be found because there is no star closer to the solar system than this one," said lead study author Guillem Anglada-Escudé. "The only thing you can hope to find between that is Planet Nine , but that would (require) a solar system object or a brown dwarf that hasn't been discovered," researcher Pedro Amado added. Here is what we know about the planet, as well as the questions that researchers hope to be able to answer. Photos: Weird and wonderful planets beyond our solar system Artist's conception of the binary system with three giant planets discovered, where one star hosts two planets and the other hosts the third. The system represents the smallest-separation binary in which both stars host planets that has ever been observed. Hide Caption 17 of 17 Meet Proxima b Proxima b is a rocky, terrestrial planet with a surface -- unlike a gas giant, such as Jupiter -- that is 1.3 times the size of Earth and orbits its star every 11.2 days.

It is in a close orbit of Proxima Centauri: only 5% of the distance between the Earth and the sun. They are even closer together than Mercury and the sun. But because its star is much cooler and fainter than our sun, Proxima b has a temperature that is suitable for liquid water to exist on the surface without evaporating. Researchers estimate that if the planet has an atmosphere, which could be assumed but isn't known, it may be between 86 to 104 degrees Fahrenheit (31 to 40 degrees Celsius) on the surface. Without an atmosphere, it could be -22 to -40 degrees Fahrenheit (-30 to -40 degrees Celsius). To put that in perspective, Earth would be -4 degrees (-20 degrees Celsius) if it didn't have an atmosphere, Reiners said. Given the proximity to its star, Proxima b is also subject to less pleasant factors like ultraviolet and X-ray flares that are 100 times the intensity of what Earth receives from the sun. In the paper, researchers estimated it to be 400 times the intensity, but recent research has caused them to create a new estimate, they said.

If there is life on the planet, it would be affected by this radiation, but it is pure speculation as to what kind of effect. This infographic compares the orbit of the planet around Proxima Centauri (Proxima b) with the same region of the Solar System. What took so long? If the Alpha Centauri system is well-studied and Proxima Centauri is our sun's cozy star neighbor in the universe, why did it take so long to find Proxima b? It comes down to an understanding of the star this planet orbits, as well as how data collection has evolved during the last 16 years. Proxima Centauri is a low-mass red dwarf star, known as an M-class dwarf, that happens to be close to the bright binary star Alpha Centauri AB, which outshines its cool stepbrother, so to speak. All of these stars are within the faint Centaurus constellation, which can't be seen with the unaided eye. Taking a closer look at new Earth-like planets for the first time M-class dwarves are not well understood in comparison with other types of stars, Reiners said.

Because of that, researchers don't know much about the history of these stars or their radiation in the early days. "But within the field of exoplants, [researchers] have recently realized that looking for planets around M dwarves is what is going to be the most spectacular, because you can find these plants in the liquid water zone more easily than other stars," Reiners said. NASA's K2 mission finds more than 100 new planets Because it's an active star, Proxima Centauri can behave in varied ways that mimic the presence of a planet, according to the study. Researchers wanted to observe it for a long period of time, so for the first half of this year, telescopes around the world were pointed at Proxima Centauri. The researchers looked for a "Doppler wobble," or back and forth wobble of Proxima Centauri that would be caused by the gravitational pull of a planet in orbit. This was combined with research, data and published studies of Proxima Centauri dating to 2000. "The significance of the detection went sky high," Anglada-Escudé said. "Statistically, there was no doubt.

We have found a planet around Proxima Centauri." What we don't know Exoplanet hunter seeks life on other worlds The research around Proxima b will continue, and the researchers have more questions they want to answer. They don't know whether there is water on the surface or if the planet has an atmosphere, although both are likely. They also don't know whether, like Earth, the planet has a protective magnetic field to help with some of the radiation it receives. Perhaps one of the biggest questions includes the history of the star and the planet. How did they form? "What happened during the formation?" Reiners asked. "Was the star more active than the sun is today, and where during that phase was Proxima b located?" This would indicate whether the plant was rich with water in its early days or started out dry, as well as whether there was any high-energy radiation that could have blasted away an atmosphere during formation of the planet. A view of the southern skies over the ESO 3.6-meter telescope at La Silla Observatory in Chile with images of the stars Proxima Centauri (lower right) and the double star Alpha Centauri AB (lower left).

There is also some debate over whether this planet is Earth-like, which comes with some connotations. Depending on its formation, perhaps it could be like Venus. Learning the answers to these questions about formation are possible with research. The habitability of a planet like Proxima b is also "a matter of intense debate," according to the study, due to arguments against it: tidal locking, strong stellar magnetic fields, strong flares, and high ultraviolet and X-ray fluxes. But, as they point out, none of those has been proved definitive, either. Growing excitement Researchers have long looked to Alpha Centauri for study. Now, they want us to go there. Programs like Mission Centaur intend to design and build a space mission with a small telescope to point at the star system. It would look for exoplanets by imaging or other techniques that could find more of them around these three stars. Given how long it took us to confirm Proxima b and the fact that the researchers encountered a puzzling extra signal in some of their data and models, it's entirely possible that there are more planets to be found.

It is also the target of the Starshot project , which aims to create and send ultra-fast light-driven nanocraft that would reach the system 20 years after launch and beam home images. This is on the list of Breakthrough initiatives, an effort whose board includes Stephen Hawking and Mark Zuckerberg. Join the conversation See the latest news and share your comments with CNN Health on Facebook and Twitter . Because Proxima b exists outside our solar system, it doesn't change our well-known roster of planets (and we know some of you are still rather upset over Pluto). But it does add to the field of exoplanet research that's underway, some of which hopes to identify Earth-like planets that future telescopes, such as the James Webb Space Telescope and the European Extremely Large Telescope, can use for observation. Many researchers hope that we can actually image these planets in the future, getting an idea of their atmospheric makeup and surface composition, and strive to answer the question of "Are we alone in the universe?" "We know that there are terrestrial planets around stars. The excitement is because it's the nearest one, and we expect to characterize it and maybe visit in a couple of centuries," Anglada-Escudé said.

Question: Which planet is closest to the sun?
Answer: Mercury