Rogue planet
A rogue planet, also termed a free-floating planet or an isolated planetary-mass object, 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 slim to none with the odds being about one in one trillion within the next 1,000 years.
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. 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.
Terminology
The two first discovery papers use the names isolated planetary-mass objects and free-floating planets. Most astronomical papers use one of these terms. The term rogue planet is more often used for microlensing studies, which also often uses the term FFP. 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, starless planet, wandering planet and free-floating planet in different press releases.Discovery
Isolated planetary-mass objects 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. 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 Oasa et al. discovered objects in Chamaeleon I that were spectroscopically confirmed years later in 2004 by the US team 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 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 unbound to any star and free floating in the Milky Way galaxy.
Direct imaging
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 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 200 Myrs, are massive and belong to the L- and T-dwarfs. There is however a small growing sample of cold and old Y-dwarfs that have estimated masses of 8-20. 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. These iPMOs found via direct imaging formed probably like stars. 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. 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 shows a Vtan of about 100 km/s, which is high, but still consistent with formation in our galaxy. For WISE 1534–1043 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.
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. 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.
Binaries
The first discovery of a resolved planetary-mass binary was 2MASS J1119–1137AB. There are however other binaries known, such as 2MASS J1553022+153236AB, WISE 1828+2650, WISE 0146+4234, WISE J0336−0143, NIRISS-NGC1333-12 and several objects discovered by Zhang et al.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 ; they make up at least 9% of the iPMOs and have a separation smaller than 340 AU. It is unclear how these JuMBOs formed, but an extensive study argued that they formed in situ, like stars. 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 of known candidate iPMOs, over a hundred 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 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. Follow-up observations with spectroscopy from the Subaru Telescope and Gran Telescopio Canarias showed that the contamination of this sample is quite low. The 16 young objects had a mass between 3 and 14, confirming that they are indeed planetary-mass objects.
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. A surprising number of these objects formed wide binaries, which was not predicted.
Formation
There are in general two scenarios that can lead to the formation of an isolated planetary-mass object. 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.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.