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 the planets in our solar system.

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 new telescopes like the upcoming Nancy Grace Roman Space Telescope.

The chance of a rogue planet entering our solar system, or posing any danger to life on Earth, is extremely small. Experts estimate the odds of this happening in the next 1,000 years to be about one in one trillion, so we don’t need to worry about rogue planets visiting us anytime soon.

Some of these planetary-mass objects might have formed in a way similar to stars. The International Astronomical Union has suggested calling such objects sub-brown dwarfs. One possible example is Cha 110913−773444, which could either have been thrown out of 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 using a special method called microlensing. You might also hear them called starless planets or wandering planets, especially in news stories. For example, in 2021, scientists found about 70 of these lonely planets and used several different names to describe them.

Discovery

Isolated planets, called isolated planetary-mass objects, were first found in the year 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, a group of scientists watched many stars in our galaxy and saw some stars get dimmer for a short time. This dimming can happen when a rogue planet passes in front of a star, acting like a magnifying glass. They found that there might be many more rogue planets than stars in our galaxy.

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

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. Another 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. These planets can change how other planets move and might even help create conditions that support early life by bringing in materials from space.

Fate

Most isolated planets drift through space forever, never coming close to another star or planet. Very rarely, one might pass close to a planetary system. When this happens, there are a few possible outcomes: the planet might stay free, it might get weakly pulled by a star, or it might push another planet away. Studies show that usually, these planets get pulled in just enough to have a very long, stretched-out path around the star. However, these paths are not lasting, and most of these planets end up getting pushed back out into space again. Only a tiny fraction of stars might briefly hold onto one of these wandering planets.

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, which 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. About five percent of Earth-sized planets that were thrown out might still keep moons around them, which could also help heat the planet from inside.

List

The table below shows 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 yet.

These planets were found using different methods. Some were spotted directly using telescopes, often in groups of young stars or in areas where stars are born. A few older rogue planets have also been found, like WISE 0855−0714. 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. Some of them might actually be planets orbiting a star that we cannot see.