A rogue planet, also termed a free-floating planet (FFP) or an isolated planetary-mass object (iPMO), is an interstellar object of planetary mass which is not gravitationally bound to any star or brown dwarf. Rogue planets may originate from planetary systems in which they are formed and later ejected, or they can also form on their own, outside a planetary system. The Milky Way alone may have billions to trillions of rogue planets, a range the upcoming Nancy Grace Roman Space Telescope is expected to refine. The odds of a rogue planet entering the solar system, much less posing a direct threat to life on Earth, are vanishingly small: celestial mechanics professor Cassidy Ward has estimated the odds of a rogue planet entering the solar system in the next 1,000 years to be one in a billion. Some planetary-mass objects may have formed in a way similar to how stars form, and the International Astronomical Union has proposed that such objects be called sub-brown dwarfs. A possible example is Cha 110913−773444, which may have either been ejected and become a rogue planet or formed on its own to become a sub-brown dwarf.
Terminology The two first discovery papers use the names isolated planetary-mass objects (iPMOs) and free-floating planets (FFPs). Most astronomical papers use one of these terms. Rogue planet is more often used for microlensing studies, which also often use FFP. A press release intended for the public might use an alternative name. Press releases regarding the discoveries of at least 70 FFPs in 2021, for example, used the terms rogue planet, starless planet, wandering planet and free-floating planet.
Discovery Isolated planetary-mass objects (iPMO) were first discovered in 2000 by the UK team P. W. Lucas & P. F. Roche with UKIRT in the Orion Nebula. In the same year the Spanish team María Rosa Zapatero Osorio et al. discovered iPMOs with Keck spectroscopy in the σ Orionis cluster. The spectroscopy of the objects in the Orion Nebula was published in 2001. Both European teams are now recognized for their quasi-simultaneous discoveries. In 1999 the Japanese team Yumiko Oasa et al. discovered objects in Chamaeleon I that were spectroscopically confirmed years later in 2004 by the US team Kevin Luhman et al.
Observation
There are two techniques to discover free-floating planets: direct imaging and microlensing.
Microlensing 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 by using the 1.8-metre (5 ft 11 in) MOA-II telescope at New Zealand's Mount John Observatory and the 1.3-metre (4 ft 3 in) 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. One study suggested a much larger number, up to 100,000 times more rogue planets than stars in the Milky Way, though this study encompassed hypothetical objects much smaller than Jupiter. A 2017 study by Przemek Mróz 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. In September 2020, astronomers using microlensing techniques reported the detection, for the first time, of an Earth-mass rogue planet (named OGLE-2016-BLG-1928) unbound to any star and free floating in the Milky Way galaxy.
Direct imaging
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