Query: It seems scientists have noticed recently that quite a few planets in our solar system seem to be heating up a bit, including Pluto."

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Exoplanet <=SEP=> An exoplanet ( -LSB- ˈɛk.səʊˌplæn.ɪt -RSB- , -LSB- ˌɛk.soʊˈplæn.ɪt -RSB- ) or extrasolar planet is a planet outside of our solar system that orbits a star . The first scientific detection of an exoplanet was in 1988 . However , the first confirmed detection came in 1992 ; since then , and as of , there have been exoplanets , in planetary systems and multiple planetary systems , confirmed detections .   HARPS ( since 2004 ) has discovered about a hundred exoplanets while the Kepler space telescope ( since 2009 ) has found more than two thousand . Kepler has also detected a few thousand candidate planets , of which about 11 % may be false positives .  On average , there is at least one planet per star , with a percentage having multiple planets .  About 1 in 5 Sun-like starsFor the purpose of this 1 in 5 statistic , `` Sun-like '' means G-type star . Data for Sun-like stars was not available so this statistic is an extrapolation from data about K-type stars have an `` Earth-sized `` For the purpose of this 1 in 5 statistic , Earth-sized means 1 -- 2 Earth radii planet in the habitable zone.For the purpose of this 1 in 5 statistic , `` habitable zone '' means the region with 0.25 to 4 times Earth 's stellar flux ( corresponding to 0.5 -- 2 AU for the Sun ) . Assuming there are 200 billion stars in the Milky Way , About 1/4 of stars are GK Sun-like stars . The number of stars in the galaxy is not accurately known , but assuming 200 billion stars in total , the Milky Way would have about 50 billion Sun-like ( GK ) stars , of which about 1 in 5 ( 22 % ) or 11 billion would be Earth-sized in the habitable zone . Including red dwarfs would increase this to 40 billion . one can hypothesize that there are 11 billion potentially habitable Earth-sized planets in the Milky Way , rising to 40 billion if planets orbiting the numerous red dwarfs are included .   The least massive planet known is Draugr ( also known as PSR B1257 +12 A or PSR B1257 +12 b ) , which is about twice the mass of the Moon . The most massive planet listed on the NASA Exoplanet Archive is DENIS-P J082303.1-491201 b , about 29 times the mass of Jupiter , although according to some definitions of a planet , it is too massive to be a planet and may be a brown dwarf instead . There are planets that are so near to their star that they take only a few hours to orbit and there are others so far away that they take thousands of years to orbit . Some are so far out that it is difficult to tell whether they are gravitationally bound to the star . Almost all of the planets detected so far are within the Milky Way , but there have also been a few possible detections of extragalactic planets . The nearest exoplanet is Proxima Centauri b , located 4.2 light-years ( 1.3 parsecs ) from Earth and orbiting Proxima Centauri , the closest star to the Sun .   The discovery of exoplanets has intensified interest in the search for extraterrestrial life . There is special interest in planets that orbit in a star 's habitable zone , where it is possible for liquid water , a prerequisite for life on Earth , to exist on the surface . The study of planetary habitability also considers a wide range of other factors in determining the suitability of a planet for hosting life .   Besides exoplanets , there are also rogue planets , which do not orbit any star and which tend to be considered separately , especially if they are gas giants , in which case they are often counted , like WISE 0855 − 0714 , as sub-brown dwarfs . The rogue planets in the Milky Way possibly number in the billions ( or more ) .

Solar System <=SEP=> The Solar SystemCapitalization of the name varies . The International Astronomical Union , the authoritative body regarding astronomical nomenclature , specifies capitalizing the names of all individual astronomical objects , but uses mixed `` Solar System '' and `` solar system '' in their naming guidelines document . The name is commonly rendered in lower case ( `` solar system '' ) , as , for example , in the Oxford English Dictionary and Merriam-Webster 's 11th Collegiate Dictionary . is the gravitationally bound system comprising the Sun and the objects that orbit it , either directly or indirectly.The natural satellites ( moons ) orbiting the Solar System 's planets are an example of the latter . Of those objects that orbit the Sun directly , the largest eight are the planets , Historically , several other bodies were once considered planets , including , from its discovery in 1930 until 2006 , Pluto . See Former planets . with the remainder being significantly smaller objects , such as dwarf planets and small Solar System bodies . Of the objects that orbit the Sun indirectly , the moons , two are larger than the smallest planet , Mercury.The two moons larger than Mercury are Ganymede , which orbits Jupiter , and Titan , which orbits Saturn . Although bigger than Mercury , both moons have less than half the mass of Mercury .   The Solar System formed 4.6 billion years ago from the gravitational collapse of a giant interstellar molecular cloud . The vast majority of the system 's mass is in the Sun , with the majority of the remaining mass contained in Jupiter . The four smaller inner planets , Mercury , Venus , Earth and Mars , are terrestrial planets , being primarily composed of rock and metal . The four outer planets are giant planets , being substantially more massive than the terrestrials . The two largest , Jupiter and Saturn , are gas giants , being composed mainly of hydrogen and helium ; the two outermost planets , Uranus and Neptune , are ice giants , being composed mostly of substances with relatively high melting points compared with hydrogen and helium , called volatiles , such as water , ammonia and methane . All planets have almost circular orbits that lie within a nearly flat disc called the ecliptic .   The Solar System also contains smaller objects . The asteroid belt , which lies between the orbits of Mars and Jupiter , mostly contains objects composed , like the terrestrial planets , of rock and metal . Beyond Neptune 's orbit lie the Kuiper belt and scattered disc , which are populations of trans-Neptunian objects composed mostly of ices , and beyond them a newly discovered population of sednoids . Within these populations are several dozen to possibly tens of thousands of objects large enough that they have been rounded by their own gravity . Such objects are categorized as dwarf planets . Identified dwarf planets include the asteroid Ceres and the trans-Neptunian objects Pluto and Eris . In addition to these two regions , various other small-body populations , including comets , centaurs and interplanetary dust clouds , freely travel between regions . Six of the planets , at least four of the dwarf planets , and many of the smaller bodies are orbited by natural satellites , See List of natural satellites of the Solar System for the full list of natural satellites of the eight planets and first five dwarf planets usually termed `` moons '' after the Moon . Each of the outer planets is encircled by planetary rings of dust and other small objects .   The solar wind , a stream of charged particles flowing outwards from the Sun , creates a bubble-like region in the interstellar medium known as the heliosphere . The heliopause is the point at which pressure from the solar wind is equal to the opposing pressure of the interstellar medium ; it extends out to the edge of the scattered disc . The Oort cloud , which is thought to be the source for long-period comets , may also exist at a distance roughly a thousand times further than the heliosphere . The Solar System is located in the Orion Arm , 26,000 light-years from the center of the Milky Way .

Comet <=SEP=> A comet is an icy small Solar System body that , when passing close to the Sun , warms and begins to release gasses , a process called outgassing . This produces a visible atmosphere or coma , and sometimes also a tail . These phenomena are due to the effects of solar radiation and the solar wind acting upon the nucleus of the comet . Comet nuclei range from a few hundred metres to tens of kilometres across and are composed of loose collections of ice , dust , and small rocky particles . The coma may be up to 15 times the Earth 's diameter , while the tail may stretch one astronomical unit . If sufficiently bright , a comet may be seen from the Earth without the aid of a telescope and may subtend an arc of 30 ° ( 60 Moons ) across the sky . Comets have been observed and recorded since ancient times by many cultures .   Comets usually have highly eccentric elliptical orbits , and they have a wide range of orbital periods , ranging from several years to potentially several millions of years . Short-period comets originate in the Kuiper belt or its associated scattered disc , which lie beyond the orbit of Neptune . Long-period comets are thought to originate in the Oort cloud , a spherical cloud of icy bodies extending from outside the Kuiper belt to halfway to the nearest star . Long-period comets are set in motion towards the Sun from the Oort cloud by gravitational perturbations caused by passing stars and the galactic tide . Hyperbolic comets may pass once through the inner Solar System before being flung to interstellar space . The appearance of a comet is called an apparition .   Comets are distinguished from asteroids by the presence of an extended , gravitationally unbound atmosphere surrounding their central nucleus . This atmosphere has parts termed the coma ( the central part immediately surrounding the nucleus ) and the tail ( a typically linear section consisting of dust or gas blown out from the coma by the Sun 's light pressure or outstreaming solar wind plasma ) . However , extinct comets that have passed close to the Sun many times have lost nearly all of their volatile ices and dust and may come to resemble small asteroids . Asteroids are thought to have a different origin from comets , having formed inside the orbit of Jupiter rather than in the outer Solar System . The discovery of main-belt comets and active centaur minor planets has blurred the distinction between asteroids and comets .   there are 5,253 known comets , a number that is steadily increasing as they are discovered . However , this represents only a tiny fraction of the total potential comet population , as the reservoir of comet-like bodies in the outer Solar System ( in the Oort cloud ) is estimated to be one trillion . Roughly one comet per year is visible to the naked eye , though many of those are faint and unspectacular . Particularly bright examples are called `` Great Comets '' . Comets have been visited by unmanned probes such as the European Space Agency 's Rosetta , which became the first ever to land a robotic spacecraft on a comet , and NASA 's Deep Impact , which blasted a crater on Comet Tempel 1 to study its interior .

Pluto <=SEP=> Pluto ( minor-planet designation : 134340 Pluto ) is a dwarf planet in the Kuiper belt , a ring of bodies beyond Neptune . It was the first Kuiper belt object to be discovered .   Pluto was discovered by Clyde Tombaugh in 1930 and was originally considered to be the ninth planet from the Sun . After 1992 , its planethood was questioned following the discovery of several objects of similar size in the Kuiper belt . In 2005 , Eris , a dwarf planet in the scattered disc which is 27 % more massive than Pluto , was discovered . This led the International Astronomical Union ( IAU ) to define the term `` planet '' formally in 2006 , during their 26th General Assembly . That definition excluded Pluto and reclassified it as a dwarf planet .   Pluto is the largest and second-most-massive known dwarf planet in the Solar System and the ninth-largest and tenth-most-massive known object directly orbiting the Sun . It is the largest known trans-Neptunian object by volume but is less massive than Eris . Like other Kuiper belt objects , Pluto is primarily made of ice and rock and is relatively small -- about one-sixth the mass of the Moon and one-third its volume . It has a moderately eccentric and inclined orbit during which it ranges from 30 to 49 astronomical units or AU ( 4.4 -- 7.4 billion km ) from the Sun . This means that Pluto periodically comes closer to the Sun than Neptune , but a stable orbital resonance with Neptune prevents them from colliding . Light from the Sun takes about 5.5 hours to reach Pluto at its average distance ( 39.5 AU ) .   Pluto has five known moons : Charon ( the largest , with a diameter just over half that of Pluto ) , Styx , Nix , Kerberos , and Hydra . Pluto and Charon are sometimes considered a binary system because the barycenter of their orbits does not lie within either body .   On July 14 , 2015 , the New Horizons spacecraft became the first spacecraft to fly by Pluto . During its brief flyby , New Horizons made detailed measurements and observations of Pluto and its moons . In September 2016 , astronomers announced that the reddish-brown cap of the north pole of Charon is composed of tholins , organic macromolecules that may be ingredients for the emergence of life , and produced from methane , nitrogen and other gases released from the atmosphere of Pluto and transferred about 19,000 km to the orbiting moon .