Rogue planets

A rogue planet—also called a free-floating planet (FFP) or an isolated planetary-mass object (iPMO)—is a body with a mass similar to a planet that travels through interstellar space without being gravitationally tied to any star or brown dwarf. Such objects can come from two different origins: they may form within a planetary system and later get flung out, or they can form independently, outside any system. In the Milky Way, the total number of rogue planets could range from billions to trillions; the upcoming Nancy Grace Roman Space Telescope is expected to narrow down that estimate.

The terms used to describe these objects vary. The first two discovery papers introduced the names "isolated planetary-mass objects" (iPMOs) and "free-floating planets" (FFPs), and most astronomical studies adopt one of these. "Rogue planet" appears more often in microlensing research, which also frequently uses "FFP." For public-facing announcements, alternative names are sometimes chosen. For instance, press releases about the 2021 discoveries of at least 70 FFPs referred to them as rogue planets, starless planets, wandering planets, and free-floating planets.

The first iPMOs were found in 2000. The UK team of P. W. Lucas and P. F. Roche, using UKIRT, detected them in the Orion Nebula. That same year, a Spanish team led by María Rosa Zapatero Osorio identified iPMOs via Keck spectroscopy in the σ Orionis cluster. The spectroscopy of the Orion Nebula objects was published in 2001. Both European groups are now credited with the quasi-simultaneous discoveries. In 1999, a Japanese team headed by Yumiko Oasa spotted objects in Chamaeleon I, but spectroscopic confirmation did not come until 2004, from the US team of Kevin Luhman and colleagues.

Two main techniques are used to detect free-floating planets: direct imaging and microlensing.

In microlensing, a 2011 study by Takahiro Sumi of Osaka University and collaborators from the Microlensing Observations in Astrophysics and the Optical Gravitational Lensing Experiment collaborations monitored 50 million stars in the Milky Way. They used the 1.8-meter MOA-II telescope at New Zealand's Mount John Observatory and the 1.3-meter University of Warsaw telescope in Chile. Out of 474 microlensing events detected, ten were brief enough to indicate planets roughly Jupiter's size with no nearby star. From these data, the researchers estimated that nearly two Jupiter-mass rogue planets exist for every star in the Milky Way. Another study suggested a far higher number—up to 100,000 times more rogue planets than stars—but that work included hypothetical objects much smaller than Jupiter. A 2017 analysis by Przemek Mróz of Warsaw University Observatory and colleagues, using six times more statistics than the 2011 study, found an upper limit of 0.25 Jupiter-mass free-floating or wide-orbit planets per main-sequence star. In September 2020, astronomers using microlensing reported the first detection of an Earth-mass rogue planet, designated OGLE-2016-BLG-1928, floating unbound in the Milky Way.

Direct imaging offers a way to characterize these objects, since microlensing planets can only be studied during lensing events. To determine the mass of a brown dwarf or iPMO, astronomers need its luminosity and age—but age is notoriously hard to pin down for low-mass objects. Most known iPMOs are found in young, nearby star-forming regions whose ages are known; these objects are under 200 million years old, more massive than 5 Jupiter masses, and belong to the L and T dwarf classes. A small but growing sample of cold, old Y-dwarfs have estimated masses between 8 and 20 Jupiter masses. One nearby candidate, WISE 0855−0714, lies about 7.27 light-years away. If more accurate measurements or better age estimates become available for these Y-dwarfs, the count of old, cold iPMOs is expected to rise significantly.

The first iPMOs, discovered in the early 2000s via direct imaging in young star-forming regions, likely formed like stars (sometimes called sub-brown dwarfs). There may also be iPMOs that formed as planets and were later ejected. Such objects would differ kinematically from their natal star-forming region, would lack a circumstellar disk, and would have high metallicity. None of the iPMOs found in young star-forming regions show a high velocity relative to their region. For older iPMOs, WISE J0830+2837 has a tangential velocity of about 100 km/s—high, but still consistent with formation within the galaxy. WISE 1534–1043 has a tangential velocity of roughly 200 km/s, leading some to suggest it is an ejected exoplanet, though its color indicates it is an old, metal-poor brown dwarf. Most astronomers studying massive iPMOs consider them to be the low-mass end of the star-formation process.