thumb|300px|This video shows an artist's impression of the free-floating planet (or brown dwarf) [[CFBDSIR 2149−0403|CFBDSIR J214947.2-040308.9. ]]
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. One celestial mechanics professor 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) wandering planet
Discovery
Isolated planetary-mass objects (iPMO) were first discovered in 2000 by the UK team Lucas & Roche with UKIRT in the Orion Nebula. In the same year the Spanish team Zapatero Osorio et al. discovered iPMOs with Keck spectroscopy in the σ Orionis cluster. that were spectroscopically confirmed years later in 2004 by the US team Luhman et al.
Observation
thumb|115 potential rogue planets in the region between Upper Scorpius and Ophiuchus (2021)
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 MOA-II telescope at New Zealand's Mount John Observatory and the 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
thumb|The cold planetary-mass object WISE J0830+2837 (marked orange object) observed with the [[Spitzer Space Telescope. It has a temperature of .]]
Microlensing planets can only be studied by microlensing events, making the characterization of such planet difficult. Astronomers therefore turn to isolated planetary-mass objects (iPMO) that were found via the direct imaging method. To determine a mass of a brown dwarf or iPMO one needs for example the luminosity and the age of an object. Determining the age of a low-mass object has proven to be difficult. It is no surprise that the vast majority of iPMOs are found inside young nearby star-forming regions of which astronomers know their age. These objects are younger than 200Myrs, are massive (>5) Nearby rogue planet candidates of spectral type Y include WISE 0855−0714 at a distance of . If this sample of Y-dwarfs can be characterized with more accurate measurements or if a way to better characterize their ages can be found, the number of old and cold iPMOs will likely increase significantly.
The first iPMOs were discovered in the early 2000s via direct imaging inside young star-forming regions. None of the iPMOs found inside young star-forming regions show a high velocity compared to their star-forming region. For old iPMOs the cold WISE J0830+2837 one alternative scenario explains this object as an ejected exoplanet due to its high V<sub>tan</sub> of about 200 km/s, but its color suggests it is an old metal-poor brown dwarf. Most astronomers studying massive iPMOs believe that they represent the low-mass end of the star-formation process. Spectroscopic observations of OTS 44 with the SINFONI spectrograph at the Very Large Telescope have revealed that the disk is actively accreting matter, similar to the disks of young stars. WISE 1828+2650, WISE 0146+4234, WISE J0336−0143 (could also be a brown dwarf and a planetary-mass object (BD+PMO) binary), NIRISS-NGC1333-12 and several objects discovered by Zhang et al. If they formed like stars, then there must be an unknown "extra ingredient" to allow them to form. If they formed like planets and were later ejected, then it has to be explained why these binaries did not break apart during the ejection process. Future measurements with JWST might resolve if these objects formed as ejected planets or as stars. Kevin Luhman reanalysed the NIRCam data and found that most JuMBOs did not appear in his sample of substellar objects. Moreover, the color was consistent with reddened background sources or low signal-to-noise sources. He considers only JuMBO 29 as a good candidate for a binary planetary-mass system.
Total number of known iPMOs
There are likely hundreds Follow-up observations with spectroscopy from the Subaru Telescope and Gran Telescopio Canarias showed that the contamination of this sample is quite low (≤6%). The 16 young objects had a mass between 3 and 14, confirming that they are indeed planetary-mass objects.
Formation like a star
Objects with a mass of at least one Jupiter mass were thought to be able to form via collapse and fragmentation of molecular clouds from models in 2001. Pre-JWST observations have shown that objects below 3–5 are unlikely to form on their own. Sometimes young iPMOs are still surrounded by a disk that could form exomoons. Due to the tight orbit of this type of exomoon around their host planet, they have a high chance of 10–15% to be transiting.
Disks
Some very young star-forming regions, typically younger than five million years, sometimes contain isolated planetary-mass objects with infrared excess and signs of accretion. Most well known is the iPMO OTS 44 discovered to have a disk and being located in Chamaeleon I. Chamaeleon I and II have other candidate iPMOs with disks. Sigma Orionis cluster, Orion Nebula, NGC 1333 and IC 348. A large survey of disks around brown dwarfs and iPMOs with ALMA found that these disks are not massive enough to form earth-mass planets. There is still the possibility that the disks already have formed planets. Recent studies of the nearby planetary-mass object 2MASS J11151597+1937266 found that this nearby iPMO is surrounded by a disk. It shows signs of accretion from the disk and also infrared excess. In May 2025 researchers using JWST found that the disk around Cha 1107−7626 contains hydrocarbons. Cha 1107−7626 (6–10) is one of the lowest-mass objects with a dusty disk. Additional JWST spectroscopy did show that silicates and hydrocarbons are a common feature in disks of planetary-mass objects. The disks showed strong evidence of grain growth and crystallization, similar to what is seen in disks around brown dwarfs and stars. This showed that these disks are capable to form rocky companions.
Formation like a planet
Ejected planets are predicted to be mostly low-mass (<30 Figure 1 Ma et al.) and their mean mass depends on the mass of their host star. Simulations by Ma et al. predicted that exomoons can be scattered by planet-planet interactions and become ejected exomoons. Higher mass (0.3–1) ejected FFP are predicted to be possible, but they are also predicted to be rare. Another suggested scenario is the ejection of planets in a tilted circumbinary orbit. Interactions with the central binary and the planets with each other can lead to the ejection of the lower-mass planet in the system. Although the effectiveness of this mechanism depends on the encounter geometry, which is not well constrained yet both observationally and theoretically.
Formation via encounters between young circumstellar disks
Encounters between young circumstellar disks, which are marginally gravitationally stable, can produce elongated tidal bridges that collapse locally to form iPMOs. These iPMOs host expansive disks similar to observations,
Other scenarios
If a stellar or brown dwarf embryo experiences a halted accretion, it could remain low-mass enough to become a planetary-mass object. Such a halted accretion could occur if the embryo is ejected or if its circumstellar disk experiences photoevaporation near O-stars. Objects that formed via the ejected embryo scenario would have smaller or no disk and the fraction of binaries decreases for such objects. It could also be that free-floating planetary-mass objects form from a combination of scenarios.
Warmth
thumb|200px|Artist's conception of a [[Jupiter-size rogue planet]]
Interstellar planets generate little heat and are not heated by a star. However, in 1998, David J. Stevenson theorized that some planet-sized objects adrift in interstellar space might sustain a thick atmosphere that would not freeze out. He proposed that these atmospheres would be preserved by the pressure-induced far-infrared radiation opacity of a thick hydrogen-containing atmosphere.
During planetary-system formation, several small protoplanetary bodies may be ejected from the system. An ejected body would receive less of the stellar-generated ultraviolet light that can strip away the lighter elements of its atmosphere. Even an Earth-sized body would have enough gravity to prevent the escape of the hydrogen and helium in its atmosphere. Around five percent of Earth-sized ejected planets with Moon-sized natural satellites would retain their satellites after ejection. A large satellite would be a source of significant geological tidal heating.
List
The table below lists rogue planets, confirmed or suspected, that have been discovered. It is yet unknown whether these planets were ejected from orbiting a star or else formed on their own as sub-brown dwarfs. Whether exceptionally low-mass rogue planets (such as OGLE-2012-BLG-1323 and KMT-2019-BLG-2073) are even capable of being formed on their own is currently unknown.
Discovered via direct imaging
These objects were discovered with the direct imaging method. Many were discovered in young star-clusters or stellar associations and a few old are known (such as WISE 0855−0714). List is sorted after discovery year.
{| class="wikitable sortable" style="margin:1em auto; text-align: center;"
! Exoplanet
! Mass
()
! Age
(Myr)
! data-sort-type="number" | Distance
(ly)
!Spectral type
! Status
!Stellar assoc. membership
! Discovery
|-
| OTS 44 || || 0.5–3 || 554
|M9.5|| Likely a low-mass brown dwarf
|Chamaeleon I|| 1998
|-
| S Ori 52 || || 1–5 || data-sort-value="1150" |1,150
| || Age and mass uncertain; may be a foreground brown dwarf
|σ Orionis cluster|| 2000
|-
|Proplyd 061-401 || || 1 || 1,344
|L4–L5
|Candidate, 15 candidates in total from this work
|Orion nebula
|2001
|-
| S Ori 70 || || 3|| 1150
|T6||interloper?
|-
| SIMP J013656.5+093347 || || 200~ || 20–22
|T2.5|| Candidate
|Carina-Near moving group|| 2006
|-
| Cha 1107−7626 || || 1–5 || data-sort-value="620" |620 || L0–L1|| Confirmed
|Chamaeleon I|| 2008
|-
| UGPS J072227.51−054031.2 ||
|-
| WISE 1828+2650
|
|