“According to the team, the definition of a planet should be based on its intrinsic properties, rather than ones that can change, such as the dynamics of a planet’s orbit.”
This is especially true in the case of “rouge planets”. Planets that form in a solar system but then somehow get kicked out, possibly by the gravitational pull of a passing star.
Such objects have been ejected from the planetary system in which they formed or have never been gravitationally bound to any star or brown dwarf.[1][2][3]
The Milky Way alone may have billions of rogue planets.[4]
Some planetary-mass objects may have formed in a similar way to stars, and the International Astronomical Union has proposed that those objects be called sub-brown dwarfs.[5]
A possible example is Cha 110913-773444, which might have been ejected and become a rogue planet, or otherwise formed on its own to become a sub-brown dwarf.[6]
Astronomers have used the Herschel Space Observatory and the Very Large Telescope to observe a very young free-floating planetary-mass object, OTS 44, and demonstrate that the processes characterizing the canonical star-like mode of formation apply to isolated objects down to a few Jupiter masses.
Herschel far-infrared observations have shown that OTS 44 is surrounded by a disk of at least 10 Earth masses and thus could eventually form a mini planetary system.[7]
Spectroscopic observations of OTS 44 with the SINFONI spectrograph at the Very Large Telescope have revealed that the disk is actively accreting matter, in a similar way to young stars.[7]
In December 2013, a candidate exomoon of a rogue planet was announced.[8]
Observation:
Astrophysicist Takahiro Sumi of Osaka University in Japan and colleagues, who form the Microlensing Observations in Astrophysics and the Optical Gravitational Lensing Experiment collaborations, published their study of microlensing in 2011.
They observed 50 million stars in the Milky Way using the 1.8-meter MOA-II telescope at New Zealand’s Mount John Observatory and the 1.3-meter University of Warsaw telescope at Chile’s Las Campanas Observatory.
They found 474 incidents of microlensing, ten of which were brief enough to be planets of around Jupiter’s size with no associated star in the immediate vicinity.
The researchers estimated from their observations that there are nearly two Jupiter-mass rogue planets for every star in the Milky Way.[9][10][11]
Other estimates suggest a much larger number, up to 100,000 times more rogue planets than stars in the Milky Way.[12]
A 2017 study by Przemek Mroz of Warsaw University Observatory and colleagues, with six times larger statistics than the 2011 study, indicates an upper limit on Jupiter-mass free-floating or wide-orbit planets of 0.25 planets per main-sequence star in the Milky Way.[13]
Nearby rogue planet candidates include WISE [....] at a distance of 7.27+/-0.13 light-years.[14]