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. It is not tied to any star or brown dwarf by gravity. These planets move through space alone, not orbiting a star like the planets in our solar system.

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

The chance of a rogue planet coming into our solar system or harming life on Earth is very small. Experts think it would happen about once in a trillion times over the next 1,000 years. So we do not need to worry about rogue planets visiting us soon.

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

Terminology

Scientists have different names for planets that float alone in space, not tied to any star. Some call them isolated planetary-mass objects (iPMOs) or free-floating planets (FFPs). The term rogue planet is often used when studying these planets. You might also hear them called starless planets or wandering planets, especially in news stories. In 2021, scientists found about 70 of these lonely planets.

Discovery

Isolated planets, called isolated planetary-mass objects, were first found in 2000. A team from the United Kingdom used a telescope called UKIRT to spot them in the Orion Nebula. Around the same time, a team from Spain used another telescope, Keck, to find similar objects in a group of stars called the σ Orionis cluster. In 1999, a team from Japan found objects in Chamaeleon I, which were confirmed later in 2004 by a team from the United States.

Observation

There are two main ways scientists look for rogue planets. One way is called microlensing. In 2011, scientists watched many stars and saw some get dimmer for a short time. This happens when a rogue planet passes in front of a star, like a magnifying glass. They think there may be more rogue planets than stars in our galaxy.

Another way to find rogue planets is by taking pictures of them. Scientists look at young areas where stars are born because these areas help them learn the age of objects. Some of these objects might be rogue planets that were thrown out of their solar systems. Scientists have found many of these objects. Some even have companions, forming pairs or groups. These discoveries help us learn more about rogue planets.

Main article: Microlensing Observations in Astrophysics

Formation

There are two main ways a rogue planet can form. One way is that it starts forming like a normal planet around a star but then gets pushed away into space. The other way is that it forms on its own, similar to how very small stars form, without needing a star nearby.

Recent studies show that rogue planets can form either by being thrown out of their original solar systems or by forming alone in clouds of gas and dust. Many of these planets probably began in solar systems before being pushed out, while others might form completely by themselves.

Fate

Most isolated planets float through space forever, never getting close to another star or planet. Very rarely, a planet might pass near a planetary system. When this happens, a few things can happen: the planet might stay free, it might get pulled a little by a star, or it might push another planet away. Usually, these planets get pulled just enough to travel a very long, stretched-out path around the star. But these paths don't last, and most of these planets get pushed back out into space again. Only a very small number of stars might hold onto one of these wandering planets for a short time.

Warmth

Interstellar planets do not get much heat and are not warmed by a star. In 1998, a scientist named David J. Stevenson thought that some of these drifting planets might keep a thick atmosphere that does not freeze. He believed this could happen because of the special properties of a thick atmosphere that contains hydrogen.

When planets are thrown out of their solar systems, they receive less ultraviolet light. This helps keep their atmospheres intact. Even a planet about the size of Earth could hold onto gases like hydrogen and helium. The heat from deep inside the planet could keep its surface warm enough for liquid water to exist. These planets might stay active for a very long time. If they have strong magnetic fields and volcanic activity under the oceans, they might provide energy that could support life. However, these planets are hard to spot because they give off very little heat. We might detect them if they are close enough to Earth, using light reflected from the Sun or heat they give off in a special type of light called far-infrared.

List

The table below shows 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.

These planets were found using different methods. Some were spotted directly using telescopes. Others were discovered using a method called microlensing, where the gravity of the planet bends light from stars behind it, acting like a magnifying glass. These planets can only be studied during the short time when this bending of light happens.