Rogue planet

A rogue planet, also called a free-floating planet (FFP) or an isolated planetary-mass object (iPMO), is an interstellar object of planetary mass that does not orbit any star or brown dwarf. These planets move through space alone, not like Earth which orbits the Sun.

Rogue planets may start in planetary systems where they form and are later pushed out. They can also form on their own, away from any planetary system. The Milky Way might have billions to trillions of rogue planets. Scientists hope to learn more with the new Nancy Grace Roman Space Telescope.

The chance of a rogue planet coming close to our solar system or harming life on Earth is very small. Experts think it is very unlikely to happen in the next 1,000 years.

Some planetary-mass objects may form like stars. The International Astronomical Union has suggested calling these objects sub-brown dwarfs. One example is Cha 110913−773444, which might have been pushed out of its system or formed on its own.

Terminology

Scientists use different names for planets that move through space without orbiting a star. Some call them isolated planetary-mass objects (iPMOs), while others say free-floating planets (FFPs). The term rogue planet is often used when scientists search for these planets using a special method called microlensing. Sometimes, news reports might use other names like starless planet or wandering planet. For example, in 2021, scientists found about 70 of these planets and used many different terms to describe them.

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Discovery

Isolated planets were first found in 2000 by a team from the United Kingdom using the UKIRT telescope in the Orion Nebula. Later that year, a team from Spain found similar planets using the Keck telescope in the σ Orionis cluster. These discoveries were important because they showed planets could exist far from any star. More discoveries followed, with a team from Japan finding planets in Chamaeleon I in 1999. These were confirmed later by a team from the United States in 2004.

Observation

There are two main ways scientists look for rogue planets: direct imaging and microlensing.

Microlensing finds rogue planets by watching how their gravity bends light from faraway stars. In 2011, scientists saw some stars and found signs of Jupiter-sized rogue planets. They think there might be more rogue planets than stars in the Milky Way. In 2020, they found an Earth-sized rogue planet floating alone in our galaxy.

Direct imaging looks for young rogue planets where stars are born. These young planets are often found near groups of new stars. Scientists have found many of these objects, and some may have formed like planets and then been thrown out into space.

Formation

There are two main ways a rogue planet can form. One way is that it starts forming around a star but then gets pushed out into space. Another way is that it forms on its own, similar to how a small star might form, without needing a star nearby.

Some rogue planets may form when small stars or groups of stars interact and push them out. Others might have started orbiting a star before being thrown out. These planets can change how other planets move and may bring materials that help life develop in other places. Their formation and movement help shape systems of planets around stars.

Main article: Sub-brown dwarf

Fate

Most isolated planets float in space forever. Sometimes, one might come close to a planetary system. This can happen in a few ways: the planet might stay free, connect loosely to the star, or push another planet away and take its place. Most of the time, these close calls mean the planet orbits the star in a stretched-out, unstable path. However, about 90% of these planets later get enough energy from other planets and are sent back out into space. Only about 1% of stars might briefly capture a planet this way.

Warmth

Interstellar planets do not get warmth from stars and make very little heat. But a theory from 1998 says some of these planets may keep a thick atmosphere that stays warm because of special properties of hydrogen.

When small planets are pushed out of their solar systems, they get less ultraviolet light. This helps them hold onto their atmospheres. Even a planet about the size of Earth could keep its atmosphere and maybe stay warm enough to have oceans. These planets might stay active for a very long time. If they have magnetic fields and volcanic activity under their oceans, they might be able to support life. Finding these planets is hard because they give off very weak signals, but we might see them if they are close to Earth. Some of these planets could keep their moons after leaving their systems. The moons might help the planets stay warm through tidal heating.

Main article: Planetary-mass object

List

The table below lists rogue planets that scientists think they have found. We do not yet know if these planets were thrown out of a solar system or if they formed all by themselves, far away from any star. Some very small rogue planets, like OGLE-2012-BLG-1323 and KMT-2019-BLG-2073, might be able to form on their own, but we are not sure.

These planets have been found using different methods. Some were spotted directly using telescopes, often in groups of young stars called star-clusters or stellar associations. Others were found using a method called microlensing, where the gravity of the planet bends light from stars behind it.