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This video shows an artist's impression of the free-floating planet 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.[1][2][3][4]

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.[5][6] The odds of a rogue planet entering the solar system, much less posing a direct threat to life on Earth are slim to none with the odds being about one in one trillion within the next 1,000 years.[7]

Some planetary-mass objects may have formed in a similar way to stars, and the International Astronomical Union has proposed that such objects be called sub-brown dwarfs.[8] A possible example is Cha 110913−773444, which may either have been ejected and become a rogue planet or formed on its own to become a sub-brown dwarf.[9]

Terminology

[edit]

The two first discovery papers use the names isolated planetary-mass objects (iPMO)[10] and free-floating planets (FFP).[11] Most astronomical papers use one of these terms.[12][13][14] The term rogue planet is more often used for microlensing studies, which also often uses the term FFP.[15][16] A press release intended for the public might use an alternative name. The discovery of at least 70 FFPs in 2021, for example, used the terms rogue planet,[17] starless planet,[18] wandering planet[19] and free-floating planet[20] in different press releases.

Discovery

[edit]

Isolated planetary-mass objects (iPMO) were first discovered in 2000 by the UK team Lucas & Roche with UKIRT in the Orion Nebula.[11] In the same year the Spanish team Zapatero Osorio et al. discovered iPMOs with Keck spectroscopy in the σ Orionis cluster.[10] The spectroscopy of the objects in the Orion Nebula was published in 2001.[21] Both European teams are now recognized for their quasi-simultaneous discoveries.[22] In 1999 the Japanese team Oasa et al. discovered objects in Chamaeleon I[23] that were spectroscopically confirmed years later in 2004 by the US team Luhman et al.[24]

Observation

[edit]
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

[edit]

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.[25][26][27] 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.[28] 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.[29]

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.[16][30][31]

Direct imaging

[edit]
The cold planetary-mass object WISE J0830+2837 (marked orange object) observed with the Spitzer Space Telescope. It has a temperature of 300-350 K (27-77°C; 80-170 °F).

Microlensing planets can only be studied by the microlensing event, which makes the characterization of the 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.[32] 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 200 Myrs, are massive (>5 MJ)[4] and belong to the L- and T-dwarfs.[33][34] There is however a small growing sample of cold and old Y-dwarfs that have estimated masses of 8-20 MJ.[35] Nearby rogue planet candidates of spectral type Y include WISE 0855−0714 at a distance of 7.27±0.13 light-years.[36] 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.[37][10][21] These iPMOs found via direct imaging formed probably like stars (sometimes called sub-brown dwarf). There might be iPMOs that form like a planet, which are then ejected. These objects will however be kinematically different from their natal star-forming region, should not be surrounded by a circumstellar disk and have high metallicity.[22] 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[38] shows a Vtan of about 100 km/s, which is high, but still consistent with formation in our galaxy. For WISE 1534–1043[39] one alternative scenario explains this object as an ejected exoplanet due to its high Vtan 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.[22]

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 large satellite system.[40] 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.[40]

Binaries

[edit]
2MASS J1119–1137AB, the first planetary-mass binary discovered, located in the TW Hydrae association
JuMBO 29, a candidate 12.5+3 MJ binary, separated by 135 AU, located in the Orion Nebula

The first discovery of a resolved planetary-mass binary was 2MASS J1119–1137AB. There are however other binaries known, such as 2MASS J1553022+153236AB,[41][42] 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[43] and several objects discovered by Zhang et al.[42]

In the Orion Nebula a population of 40 wide binaries and 2 triple systems were discovered. The discovery was surprising for two reasons: the trend of binaries of brown dwarfs predicted a decrease of distance between low mass objects with decreasing mass. It was also predicted that the binary fraction decreases with mass. These binaries were named Jupiter-mass Binary Objects (JuMBOs); they make up at least 9% of the iPMOs and have a separation smaller than 340 AU.[44] It is unclear how these JuMBOs formed, but an extensive study argued that they formed in situ, like stars.[45] 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.[44] 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.[46]

Total number of known iPMOs

[edit]

There are likely hundreds[47][44] of known candidate iPMOs, over a hundred[48][49][50] objects with spectra and a small but growing number of candidates discovered via microlensing. Some large surveys include:

As of December 2021, the largest-ever group of rogue planets was discovered, numbering at least 70 and up to 170 depending on the assumed age. They are found in the OB association between Upper Scorpius and Ophiuchus with masses between 4 and 13 MJ and age around 3 to 10 million years, and were most likely formed by either gravitational collapse of gas clouds, or formation in a protoplanetary disk followed by ejection due to dynamical instabilities.[47][17][51][19] 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 MJ, confirming that they are indeed planetary-mass objects.[50]

In October 2023, an even larger group of 540 planetary-mass object candidates was discovered in the Trapezium Cluster and inner Orion Nebula with JWST. The objects have a mass between 13 and 0.6 MJ. A surprising number of these objects formed wide binaries, which was not predicted.[44]

Formation

[edit]

There are in general two scenarios that can lead to the formation of an isolated planetary-mass object (iPMO). It can form like a planet around a star and is then ejected, or it forms like a low-mass star or brown dwarf in isolation. This can influence its composition and motion.[22]

Recent research indicates that rogue planets may form both through direct gravitational collapse within stellar nurseries and through ejection from their natal planetary systems, later interacting with established systems and influencing their orbital architectures and overall demographics. Many of these objects likely originated within planetary systems before being dynamically expelled, while others may have formed in isolation. Besides altering system stability during close encounters or possible capture events, rogue planets can also deliver volatiles that enhance prebiotic chemistry and create conditions conducive to increased biological diversity. These combined formation, dynamical, biochemical, and ecological effects play a significant role in shaping the distribution and evolution of exoplanetary systems.[52]

Formation like a star

[edit]
Main article: Sub-brown dwarf

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.[53] Pre-JWST observations have shown that objects below 3-5 MJ are unlikely to form on their own.[4] Observations in 2023 in the Trapezium Cluster with JWST have shown that objects as massive as 0.6 MJ might form on their own, not requiring a steep cut-off mass.[44] A particular type of globule, called globulettes, are thought to be birthplaces for brown dwarfs and planetary-mass objects. Globulettes are found in the Rosette Nebula and IC 1805.[54] 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.[55]

Disks

[edit]

Some very young star-forming regions, typically younger than 5 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.[56][57][33] Other star-forming regions with iPMOs with disks or accretion are Lupus I,[57] Rho Ophiuchi Cloud Complex,[58] Sigma Orionis cluster,[59] Orion Nebula,[60] Taurus,[58][61] NGC 1333[62] and IC 348.[63] 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.[58] Studies of red dwarfs have shown that some have gas-rich disks at a relative old age. These disks were dubbed Peter Pan Disks and this trend could continue into the planetary-mass regime. One Peter Pan disk is the 45 Myr old brown dwarf 2MASS J02265658-5327032 with a mass of about 13.7 MJ, which is close to the planetary-mass regime.[64] 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.[65] In May 2025 researchers using JWST found that the disk around Cha 1107−7626 contains hydrocarbons. Cha 1107−7626 (6-10 MJ) is one of the lowest-mass objects with a dusty disk.[66] 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.[67]

Formation like a planet

[edit]

Ejected planets are predicted to be mostly low-mass (<30 M🜨 Figure 1 Ma et al.)[68] and their mean mass depends on the mass of their host star. Simulations by Ma et al.[68] did show that 17.5% of 1 M☉ stars eject a total of 16.8 M🜨 per star with a typical (median) mass of 0.8 M🜨 for an individual free-floating planet (FFP). For lower mass red dwarfs with a mass of 0.3 M☉ 12% of stars eject a total of 5.1 M🜨 per star with a typical mass of 0.3 M🜨 for an individual FFP.

Hong et al.[69] predicted that exomoons can be scattered by planet-planet interactions and become ejected exomoons. Higher mass (0.3-1 MJ) ejected FFP are predicted to be possible, but they are also predicted to be rare.[68] Ejection of a planet can occur via planet-planet scatter or due a stellar flyby. Another possibility is the ejection of a fragment of a disk that then forms into a planetary-mass object.[70] 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.[71][72] 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

[edit]

Encounters between young circumstellar disks, which are marginally gravitationally stable, can produce elongated tidal bridges that collapse locally to form iPMOs.[73] These iPMOs host expansive disks similar to observations,[60] which the ejected planet hyperthesis can hardly explain. They also have a high multiplicity fraction in their formation, as suggested by iPMOs in the Trapezium cluster.[44] Although the effectiveness of this mechanism depends on the encounter geometry, which is not well constrained yet both observationally and theoretically.[74]

Other scenarios

[edit]

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.[70]


Fate

[edit]

Most isolated planetary-mass objects will float in interstellar space forever.

Some iPMOs will have a close encounter with a planetary system. This rare encounter can have three outcomes: The iPMO will remain unbound, it could be weakly bound to the star, or it could "kick out" the exoplanet, replacing it. Simulations have shown that the vast majority of these encounters result in a capture event with the iPMO being weakly bound with a low gravitational binding energy and an elongated highly eccentric orbit. These orbits are not stable and 90% of these objects gain energy due to planet-planet encounters and are ejected back into interstellar space. Only 1% of all stars will experience this temporary capture.[75]

Warmth

[edit]
Artist's conception of a Jupiter-size rogue planet

Interstellar planets generate little heat and are not heated by a star.[76] 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.[77]

During planetary-system formation, several small protoplanetary bodies may be ejected from the system.[78] 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.[77] In an Earth-sized object the geothermal energy from residual core radioisotope decay could maintain a surface temperature above the melting point of water,[77] allowing liquid-water oceans to exist. These planets are likely to remain geologically active for long periods. If they have geodynamo-created protective magnetospheres and sea floor volcanism, hydrothermal vents could provide energy for life.[77] These bodies would be difficult to detect because of their weak thermal microwave radiation emissions, although reflected solar radiation and far-infrared thermal emissions may be detectable from an object that is less than 1,000 astronomical units from Earth.[79] 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.[80]

List

[edit]

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

[edit]

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.

Exoplanet Mass

(MJ)

Age

(Myr)

Distance

(ly)

Spectral type Status Stellar assoc. membership Discovery
OTS 44 ~11.5 0.5–3 554 M9.5 Likely a low-mass brown dwarf[37] Chamaeleon I 1998
S Ori 52 2–8 1–5 1,150 Age and mass uncertain; may be a foreground brown dwarf σ Orionis cluster 2000[10]
Proplyd 061-401 ~11 1 1,344 L4–L5 Candidate, 15 candidates in total from this work Orion nebula 2001[21]
S Ori 70 3 3 1150 T6 interloper?[22] σ Orionis cluster 2002
Cha 110913-773444 5–15 2~ 529 >M9.5 Confirmed Chamaeleon I 2004[81]
SIMP J013656.5+093347 11-13 200~ 20–22 T2.5 Candidate Carina-Near moving group 2006[82][83]
UGPS J072227.51−054031.2 0.66–16.02[84][85] 1000 – 5000 13 T9 Mass uncertain none 2010
M10-4450 2–3 1 325 T Candidate rho Ophiuchi cloud 2010[86]
WISE 1828+2650 3–6 or 0.5–20[87] 2–4 or 0.1–10[87] 47 >Y2 candidate, could be binary none 2011
CFBDSIR 2149−0403 4–7 110–130 117–143 T7 Candidate AB Doradus moving group 2012[88]
SONYC-NGC1333-36 ~6 1 978 L3 candidate, NGC 1333 has two other objects with masses below 15 MJ NGC 1333 2012[89]
SSTc2d J183037.2+011837 2–4 3 848–1354 T? Candidate, also called ID 4 Serpens Core cluster[90] (in the Serpens Cloud) 2012[12]
PSO J318.5−22 6.24–7.60[84][85] 21–27 72.32 L7 Confirmed; also known as 2MASS J21140802-2251358 Beta Pictoris Moving group 2013[14][91]
2MASS J2208+2921 11–13 21–27 115 L3γ Candidate; radial velocity needed Beta Pictoris Moving group 2014[92]
WISE J1741-4642 4–21 23–130 L7pec Candidate Beta Pictoris or AB Doradus moving group 2014[93]
WISE 0855−0714 3–10 >1,000 7.1 Y4 Age uncertain, but old due to solar vicinity object;[94] candidate even for an old age of 12 Gyrs (age of the universe is 13.8 Gyrs). Closest known probable rogue planet none 2014[95]
2MASS J12074836–3900043 ~15[96] 7–13 200 L1 Candidate; distance needed TW Hydrae association[97] 2014[98]
SIMP J2154–1055 9–11 30–50 63 L4β Age questioned[99] Argus association 2014[100]
SDSS J111010.01+011613.1 10.83–11.73[84][85] 110–130 63 T5.5 Confirmed[84] AB Doradus moving group 2015[34]
2MASS J11193254–1137466 AB 4–8 7–13 ~90 L7 Binary candidate, one of the components has a candidate exomoon or variable atmosphere[55] TW Hydrae Association 2016[101]
WISEA 1147 5–13 7–13 ~100 L7 Candidate TW Hydrae Association 2016[13]
USco J155150.2-213457 8–10 6.907-10 104 L6 Candidate, low gravity Upper Scorpius association 2016[102]
Proplyd 133–353 <13 0.5–1 1,344 M9.5 Candidate with a photoevaporating disk Orion Nebula 2016[60]
Cha J11110675-7636030 3–6 1–3 520–550 M9–L2 Candidate, but could be surrounded by a disk, which could make it a sub-brown dwarf; other candidates from this work Chamaeleon I 2017[33]
PSO J077.1+24 6 1–2 470 L2 Candidate, work also published another candidate in Taurus Taurus Molecular Cloud 2017[103]
2MASS J1115+1937 6+8
−4 		5–45 147 L2γ has an accretion disk Field, possibly ejected 2017
Calar 25 11–12 120 435 Confirmed Pleiades 2018[104]
2MASS J1324+6358 10.7–11.8 ~150 ~33 T2 unusually red and unlikely binary; robust candidate[84][85] AB Doradus moving group 2007, 2018[105]
WISE J0830+2837 4-13 >1,000 31.3-42.7 >Y1 Age uncertain, but old because of high velocity (high Vtan is indicative of an old stellar population), Candidate if younger than 10 Gyrs none 2020[38]
2MASS J0718-6415 3 ± 1 16–28 30.5 T5 Candidate member of the BPMG. Extremely short rotation period of 1.08 hours, comparable to the brown dwarf 2MASS J0348-6022.[106][107] Beta Pictoris moving group 2021
DANCe J16081299-2304316 3.1–6.3 3–10 104 L6 One of at least 70 candidates published in this work, spectrum similar to HR 8799c Upper Scorpius association 2021[47][50]
WISE J2255−3118 2.15–2.59 24 ~45 T8 very red, candidate[84][85] confirmed?[108] Beta Pictoris moving group 2011,2021[49]
WISE J024124.73-365328.0 4.64–5.30 45 ~61 T7 candidate[84][85] Argus association 2012, 2021[49]
2MASS J0013−1143 7.29–8.25 45 ~82 T4 binary candidate or composite atmosphere, candidate[84][85] Argus association 2017, 2021[49]
SDSS J020742.48+000056.2 7.11–8.61 45 ~112 T4.5 candidate[84][85] Argus association 2002, 2021[49]
2MASSI J0453264-175154 12.68–12.98 24 ~99 L2.5β low gravity, candidate[84][85] Beta Pictoris moving group 2003, 2023[84][85]
CWISE J0506+0738 7 ± 2 22 104 L8γ–T0γ Candidate member of the BPMG. Extreme red near-infrared colors.[109] Beta Pictoris moving group 2023

Discovered via microlensing

[edit]

These objects were discovered via microlensing. Rogue planets discovered via microlensing can only be studied by the lensing event. Some of them could also be exoplanets in a wide orbit around an unseen star.[110]

Exoplanet Mass (MJ) Mass (M🜨) Distance (ly) Status Discovery
KMT-2023-BLG-2669 0.025–0.25 8–80 candidate; distance needed 2024 [111]
OGLE-2012-BLG-1323 0.0072–0.072 2.3–23 candidate; distance needed 2017[112][113][114][115]
OGLE-2017-BLG-0560 1.9–20 604–3,256 candidate; distance needed 2017[113][114][115]
MOA-2015-BLG-337L 9.85 3,130 23,156 may be a binary brown dwarf instead 2018[116][117]
KMT-2019-BLG-2073 0.19 59 candidate; distance needed 2020[118]
OGLE-2016-BLG-1928 0.001-0.006 0.3–2 30,000–180,000 candidate 2020[110]
OGLE-2019-BLG-0551 0.0242-0.3 7.69–95 Poorly characterized[119] 2020[119]
VVV-2012-BLG-0472L 10.5 3,337 3,200 2022[120]
MOA-9y-770L 0.07 22.3+42.2
−17.4 		22,700 2023[121]
MOA-9y-5919L 0.0012 or 0.0024 0.37+1.11
−0.27 or 0.75+1.23
−0.46 		14,700 or 19,300 2023[121]
OGLE-2017-BLG-1170L 3.06+1.34
−1.16 		24,700 candidate 2019[122]
1.85+0.79
−0.70

Discovered via transit

[edit]
Exoplanet Mass

(MJ)

Age

(Myr)

Distance

(ly)

Spectral type Status Stellar assoc. membership Discovery
J1407b <6 <451 Candidate ALMA detection; although the object's brightness and proximity is consistent with it being the same object that eclipsed the star V1400 Centauri in 2007, follow-up observations by ALMA are needed to confirm whether it is moving, let alone in the right direction.[123] none 2012, 2020[123]

See also

[edit]

In fiction

[edit]

References

[edit]

Bibliography

[edit]
[edit]
Revisions and contributorsEdit on WikipediaRead on Wikipedia

Rogue planet

View on Grokipedia
from Grokipedia
A rogue planet, also known as a free-floating planet or orphan planet, is a that does not any star and instead drifts independently through , untethered from any gravitational bond to a stellar host. These objects range in size from Earth-like worlds to gas giants larger than , and they emit no reflected , making them inherently dark and challenging to observe. Rogue planets are thought to originate primarily from dynamical instabilities in young planetary systems, where gravitational interactions—such as close encounters between planets or with a companion star—eject them from their natal orbits around a host star. Formation mechanisms include ejection during the early, chaotic phases of planet assembly, which occurs within the first 10 million years after a star's birth, or rarer direct formation as isolated protoplanetary disks in molecular clouds that fail to capture a central star. Some models also suggest contributions from the disruption of binary star systems or the stripping of planets during stellar flybys in dense clusters. Detection of rogue planets relies on indirect methods due to their faintness and lack of stellar illumination; the primary technique is gravitational microlensing, where a rogue planet passing in front of a distant background star temporarily amplifies the star's light as its gravity bends spacetime. Direct imaging in infrared wavelengths has also proven effective for young, warmer rogue planets in nearby star-forming regions, revealing their thermal glow from residual formation heat. Notable discoveries include rogue planetary-mass objects identified in 2000 via direct imaging in the Orion Nebula and a 2021 survey in the Upper Scorpius association that identified 70 to 170 free-floating planetary-mass objects, mostly Jupiter-sized, using the Subaru Telescope and other instruments, with recent additions from JWST in 2024. Estimates indicate that rogue planets may vastly outnumber star-bound planets in the , with recent microlensing surveys suggesting their total abundance could be up to 20 times that of stars across a wide mass range down to masses, potentially totaling trillions galaxy-wide. This high prevalence implies that ejections are a common outcome of planet formation, reshaping our understanding of evolution and the initial mass function of substellar objects. Future missions like 's , set to launch by May 2027, are expected to detect hundreds of -mass rogues, providing deeper insights into their mass distribution and origins.

Terminology and Definition

Terminology

The concept of planets drifting through interstellar space without orbiting a star was first formally proposed in a 1999 paper by David J. Stevenson, who explored the possibility of such objects sustaining life through retained heat and atmospheres, referring to them simply as "planets in interstellar space." Early popular descriptions in the early adopted terms like "orphaned planets" to evoke these ejected worlds separated from their stellar hosts, as seen in discussions of observations in young clusters. Scientific evolved with the first discoveries of such objects. In 2000, astronomers identified young objects in the σ Orionis star cluster with masses below the threshold, terming them "isolated planetary-mass objects" (iPMOs) to emphasize their detachment from any parent star and planetary-scale masses, typically 3–8 times that of . This terminology highlighted their isolation within star-forming regions, distinguishing them from bound companions. Subsequent confirmation of similar objects in the in 2001 led to the widespread adoption of "free-floating planets" (FFPs), a term that captured their unbound, nomadic nature while aligning with planetary formation origins. The evocative phrase "rogue planet" draws from and is commonly used in popular astronomy to describe these interstellar wanderers, often interchangeably with FFPs and iPMOs. Notable examples in science fiction include H.G. Wells' short story "The Star" (1897), depicting a rogue star entering the solar system and causing apocalyptic events on Earth; the novel When Worlds Collide (1933, with a 1951 film adaptation) by Philip Wylie and Edwin Balmer, depicting a rogue star and its planet on a collision course with Earth; the 1936 serial Flash Gordon, featuring the rogue planet Mongo controlled by Emperor Ming on a collision course with Earth; Fritz Leiber's short story "A Pail of Air" (1951), in which Earth is pulled into interstellar space by a dark star; the British television series Space: 1999 (1975–1977), where a nuclear explosion ejects the Moon from the Solar System; and the film Melancholia (2011) directed by Lars von Trier, featuring a rogue planet on a collision course with Earth. The concept has also been popularized in pseudoscience through ideas like Hercolubus, Nibiru, and Planet X, which are often depicted as rogue planets or large celestial bodies on collision courses with Earth in doomsday scenarios. Key distinctions in arise from observational and physical criteria. Rogue planets, FFPs, and iPMOs specifically denote objects completely unbound to any star, in contrast to wide-orbit planets, which remain gravitationally tied to a host but at extreme separations exceeding 10,000 AU, where detection challenges blur boundaries. Further differentiation separates true planetary-mass objects (PMOs) from sub-brown dwarfs: the former form via planetary processes and have masses below the deuterium-burning limit of approximately 13 masses, preventing sustained fusion and classifying them as planets rather than failed stars, while objects above this threshold are deemed regardless of isolation. This mass boundary, established through models of substellar evolution, underscores formation history over mere isolation in defining PMOs.

Definition and Characteristics

A rogue planet, also referred to as a free-floating planet or isolated , is an interstellar body of that orbits no star, , or other central object. According to the (IAU) working of an , amended in 2018, such objects qualify as exoplanets if their true masses fall below the limiting mass for thermonuclear fusion of , calculated at approximately 13 masses for solar-metallicity compositions. This criterion ensures rogue planets are distinguished from , which exceed this mass threshold and can sustain limited deuterium burning. Proposed updates to the IAU definition in 2024 aim to more explicitly include exoplanets and free-floating objects, though as of 2025, the 2018 version remains in effect. Rogue planets span a mass range from approximately 1 to 13 Jupiter masses, encompassing subtypes from terrestrial worlds and super-Earths to gas giant analogs. Lacking any stellar companion, they experience no external and depend entirely on internal heat sources—primarily residual gravitational contraction from formation and radioactive decay within their interiors—for maintaining temperatures above . For massive, gas giant-like rogue planets, physical sizes resemble those of bound gas giants, with equatorial radii on the order of 71,492 km, as exemplified by , and compositions dominated by and envelopes, often comprising over 90% of their mass, potentially overlying rocky or icy cores formed during accretion. Lower-mass rogue planets, such as Earth-like objects, would have smaller radii and primarily rocky or icy compositions. Boundary cases arise in the overlap with sub-brown dwarfs, where the IAU 2018 definition prioritizes planetary formation mechanisms—such as core accretion or disk instability—over alone to classify objects in the 5–13 regime, avoiding conflation with failed stars.

Formation Mechanisms

Ejection from Planetary Systems

The primary mechanism for the formation of rogue planets is their dynamical ejection from host planetary systems through gravitational interactions. In multi-planet systems, planets can experience close encounters that lead to chaotic scattering, where mutual gravitational perturbations alter orbits dramatically, often resulting in one or more planets being accelerated to escape velocities and cast into . This process is particularly prevalent during the early evolution of planetary systems, when the dissipation of the reduces damping forces, allowing instabilities to develop. A classic example of such instability is illustrated by the Nice model of solar system formation, which describes how the giant planets underwent a phase of orbital reconfiguration approximately 4 billion years ago due to interactions with a massive disk. Simulations within this framework suggest that the solar system originally hosted five giant planets, with one being ejected during the instability to account for the current orbital architecture of , Saturn, , and . This ejection scenario not only explains the excitation of the outer planets' eccentricities but also highlights how giant planets can scatter smaller bodies or additional companions outward. External perturbations, such as stellar flybys in young, dense star clusters, can further contribute to ejections by temporarily disrupting planetary orbits, increasing the likelihood of escape for outer or low-mass planets. These ejections predominantly occur on short timescales, peaking within the first 100 million years of a system's age, as planetary formation and migration finalize and dynamical instabilities manifest before the system stabilizes. During this period, inward disk migration can crowd planets into overlapping orbits, heightening the probability of events that culminate in ejection. Numerical simulations of multi-planet system demonstrate that ejection rates vary with system parameters, but overall, 5-20% of planets formed in the may end up as rogues, with recent microlensing surveys suggesting abundances up to 20 times the number of stars for Earth-mass objects. For instance, models incorporating planet-disk interactions show that resonant configurations formed via migration often destabilize post-disk dispersal, leading to efficient ejection of super-Earths and ice giants.

Star-like Formation Processes

Some rogue planets, particularly those classified as planetary-mass objects (PMOs) with masses between approximately 1 and 13 masses, may form through processes analogous to , involving the of fragments into low-mass clumps that do not accumulate sufficient material to sustain fusion. In this mechanism, interstellar , composed primarily of molecular and , undergo fragmentation driven by gravitational instabilities, leading to the formation of dense cores. These cores collapse under their own , accreting gas directly from the surrounding to build mass, much like protostars, but halting at planetary scales due to insufficient initial cloud mass or external factors that truncate accretion. Turbulence within the plays a crucial role by generating density perturbations that promote fragmentation into multiple low-mass clumps, while influence the collapse by providing support against gravity, potentially delaying or preventing full stellar-mass accumulation in smaller fragments and allowing for the survival of PMO-scale objects. These fields can channel gas flows and suppress excessive fragmentation, ensuring that some clumps remain isolated and evolve independently without merging into higher-mass bodies. Unlike traditional planetary formation, which occurs within a orbiting a central through core accretion or disk instability, star-like PMO formation involves no parent ; instead, these objects accrete material directly from their natal cloud environment, often developing their own circumplanetary disks during the process. Observational evidence for this formation pathway includes the isolated PMO OTS 44, discovered in the I star-forming region in 1998, which exhibits a substantial circumstellar disk and ongoing accretion indicative of star-like growth rather than ejection from a . Spectroscopic and photometric data reveal accretion rates comparable to those of young low-mass stars, with the disk extending to about 100 AU and containing roughly 30 masses of material, supporting direct collapse from a cloud fragment. More recent evidence comes from 2025 observations of the rogue planet Cha 1107-7626 in the region, which showed a growth spurt accreting gas and dust at a record rate of 6 billion tons per second in August 2025, rates comparable to forming stars and reinforcing the direct collapse mechanism. Theoretical support comes from hydrodynamical simulations of formation, such as those conducted in 2012, which demonstrate that 1-5% of PMOs can arise via this isolated cloud collapse mechanism, complementing ejection as a primary origin for most rogues.

Disk Interactions and Alternative Scenarios

In dense star clusters, close stellar flybys can significantly disrupt protoplanetary disks, leading to the stripping and ejection of forming planets as rogues. These encounters exert gravitational torques that truncate disks and destabilize planetary orbits, particularly for outer planets, resulting in their hyperbolic ejection from the host system. Simulations indicate that such dynamical interactions in young clusters can produce a rogue planet fraction of 1-10% relative to bound planets, depending on cluster density and substructure. Alternative formation pathways for rogue planets include the rare capture of interstellar wanderers by passing stars. During close encounters in the galactic field, a rogue planet can be temporarily or permanently bound to a new host if its velocity relative to the star is sufficiently low, with estimates suggesting that up to several billion in the may harbor such captured objects. However, the probability remains low due to the high relative speeds typical of interstellar objects. Disruption of systems also contributes to rogue planet production, especially in cases of disk misalignment. In misaligned circumbinary configurations, the of protoplanetary disks induces eccentric orbits and instabilities, ejecting planets even from wide separations that would be in aligned systems. This mechanism may account for a substantial portion of observed free-floaters, as binary stars comprise about half of all stellar systems. Hybrid formation processes in dense clusters involve direct collisions between circumstellar disks during stellar flybys, compressing gas and dust into isolated planetary-mass objects that are promptly ejected as rogues. These violent interactions create tidal bridges and knots of material that collapse independently, bypassing traditional disk accretion around a single star. Hydrodynamic simulations from 2023 on disk warping and misalignment in binaries show enhanced ejection efficiencies through resonant torques that amplify instabilities over timescales of 10^5 years, while 2025 high-resolution simulations further demonstrate that close disk encounters in young clusters can directly form PMOs via material compression, contributing significantly to the rogue population in stellar birth environments.

Detection and Observation

Microlensing Surveys

Gravitational microlensing serves as the primary detection method for rogue planets, leveraging the temporary gravitational lensing effect produced when such an isolated object passes in front of a more distant background . The rogue planet acts as a lens, bending and amplifying the starlight according to , resulting in a detectable increase in the source's apparent . For a Jupiter-mass rogue planet situated approximately 8 kiloparsecs from , the peak magnification under favorable alignments can reach factors of several times the baseline , depending on the impact parameter and source size. The timescale of the event—spanning hours for Earth-mass objects and up to several days for Jupiter-mass ones—offers a key diagnostic for estimating the lens , as shorter durations indicate lower masses due to the smaller lensing cross-section. The angular scale of the lensing effect is defined by the Einstein , given by θE=4GMc2DS−DLDLDS,\theta_E = \sqrt{\frac{4 G M}{c^2} \frac{D_S - D_L}{D_L D_S}},
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