EXTRA SOLAR PLANTES

                                                                                                                                                                                                                                        EXTRA SOLAR PLANTES                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                           An exoplanet or extrasolar planet is a planet that does not orbit the Sun and instead orbits a different star, stellar remnant, or brown dwarf. More than 1800 exoplanets have been discovered (1885 planets in 1184 planetary systems including 477multiple planetary systems as of 27 January 2015). 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 Kepler space telescope has also detected a few thousand candidate planets, of which about 11% may be false positives. There is at least one planet on average per star.Around 1 in 5 Sun-like stars have an "Earth-sized"habitable zone,with the nearest expected to be within 12 light-years distance from Earth. Assuming 200 billion stars in the Milky Way,that would be 11 billion potentially habitable Earth-sized planets in the Milky Way, rising to 40 billion if red dwarfs are included. The rogue planets in the Milky Way possibly number in the trillions.

                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                               However, the IAU's working definition is not universally accepted. One alternate suggestion is that planets should be distinguished from brown dwarfs on the basis of formation. It is widely believed that giant planets form through core accretion, and that process may sometimes produce planets with masses above the deuterium fusion threshold;massive planets of that sort may have already been observed. Brown dwarfs form like stars from the direct collapse of clouds of gas and this formation mechanism also produces objects that are below the 13 M_Jup limit and can be as low as 1 M_Jup. Objects in this mass range that orbit their stars with wide separations of hundreds or thousands of AU and have large star/object mass ratios likely formed as brown dwarfs; their atmospheres would likely have a composition more similar to their host star than accretion-formed planets which would contain increased abundances of heavier elements. Most directly imaged planets as of April 2014 are massive and have wide orbits so probably represent the low-mass end of brown dwarf formation.

Also, the 13 Jupiter-mass cutoff does not have precise physical significance. Deuterium fusion can occur in some objects with mass below that cutoff. The amount of deuterium fused depends to some extent on the composition of the object. The Extrasolar Planets Encyclopaedia includes objects up to 25 Jupiter masses, saying, "The fact that there is no special feature around 13 M_Jup in the observed mass spectrum reinforces the choice to forget this mass limit,". The Exoplanet Data Explorer includes objects up to 24 Jupiter masses with the advisory: "The 13 Jupiter-mass distinction by the IAU Working Group is physically unmotivated for planets with rocky cores, and observationally problematic due to the sin i ambiguity."NASA Exoplanet Archive includes objects with a mass (or minimum mass) equal to or less than 30 Jupiter masses Another criterion for separating planets and brown dwarfs, rather than deuterium burning, formation process or location, is whether the core pressure is dominated by coulomb pressureor electron degeneracy pressure.