Rogue planet

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. These planets wander through space on their own, not orbiting a star like Earth orbits the Sun.

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 number that scientists hope to learn more about with the upcoming Nancy Grace Roman Space Telescope.

The chances of a rogue planet entering our solar system, or posing any danger to life on Earth, are extremely small. Experts estimate the odds of this happening in the next 1,000 years to be about one in one trillion.

Some planetary-mass objects may form in a way similar to stars. The International Astronomical Union has suggested calling these objects sub-brown dwarfs. One example is Cha 110913−773444, which might have either been ejected from its system or formed independently as a sub-brown dwarf.

Terminology

Scientists have used different names to describe planets that float through space without orbiting a star. Some call them isolated planetary-mass objects (iPMOs), while others use free-floating planets (FFPs). The term rogue planet is often used in studies that look for these planets using a special method called microlensing. Sometimes, press releases for the public might use other names like starless planet or wandering planet. For example, in 2021, when scientists found about 70 of these planets, they used several different terms to describe them.

press release

Discovery

Isolated planetary-mass objects were first discovered in 2000 by a team from the United Kingdom using the UKIRT telescope in the Orion Nebula. Later that same year, a team from Spain found similar objects using the Keck telescope in the σ Orionis cluster. These discoveries were important because they showed that planets could exist far from any star. More discoveries followed, with a team from Japan finding objects in Chamaeleon I in 1999, which 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 helps find rogue planets by watching how their gravity bends light from distant stars. In 2011, scientists observed many stars and found some tiny events that suggested the presence of Jupiter-sized rogue planets. They think there might be many 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 in areas where stars are born. These young planets are usually found in groups of new stars and can be studied because their age is known. Scientists have found many of these objects, and some might 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 forms like a planet 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. Recent studies show that rogue planets can form in different places and through different processes.

Some rogue planets may form when small stars or groups of stars interact in ways that push them out. Others might have started their lives orbiting a star before being thrown out. These planets can affect the way other planets move and may even bring materials that help life develop in other places. Their formation and movement play an important role in shaping systems of planets around stars.

Main article: Sub-brown dwarf

Fate

Most isolated planetary-mass objects will float in interstellar space forever. Occasionally, an iPMO might come close to a planetary system. This rare event can lead to a few different results: the iPMO might stay free, become weakly attached to the star, or push out another planet and take its place. Studies show that most of these close calls result in the iPMO being weakly bound to the star with a stretched-out, unstable path around it. However, about 90% of these objects later get enough energy from interactions between planets and are sent back out into space. Only about 1% of stars might briefly capture an iPMO this way.

Warmth

Interstellar planets generate little heat and are not warmed by a star. However, a theory from 1998 suggests that some of these drifting planets might keep a thick atmosphere that stays warm due to special properties of hydrogen.

When small planets are thrown out of their solar systems, they receive less ultraviolet light, which helps them hold onto their atmospheres. Even a planet about the size of Earth could keep its atmosphere and might stay warm enough to have oceans. These planets could stay active for a very long time, and if they have magnetic fields and volcanic activity under the oceans, they might support life. Detecting these planets is hard because they give off very weak signals, but we might spot them if they are close enough to Earth. Some of these planets could also keep their moons after being ejected from their systems, which would help them stay warm through tidal heating.

Main article: Planetary-mass object

List

The table below lists rogue planets that scientists think have been 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.