An interstellar object is an object such as an asteroid, a comet or rogue planet—but not a star—in interstellar space, the space between the stars in a galaxy. An interstellar object is not subject to the gravitational pull of a star. The term can also be applied to an object that temporarily passes close to a star. It is then sometimes called an "interstellar interloper". Astronomers estimate that several interstellar objects pass inside Earth's orbit each year, and that as many 10,000 may pass inside the orbit of Neptune on any given day.

Updated 23rd June 2026

Brown dwarfs

There are some objects in space that have the characteristics of both stars and planets—but are neither. Known as brown dwarfs, they are bigger than the largest planets (specifically, between 13 and 80 times the mass of Jupiter), but smaller than the smallest stars. Brown dwarfs form in the same way that stars do: out of clouds of interstellar gas. But they never sustain nuclear reactions in their cores. Instead, they resemble enormous gas giant planets—except unlike them they do not have rocky cores or icy mantles.

Rogue planets

A rogue planet, also known as a free-floating planet (FFP) or an isolated planetary-mass object (iPMO), is an interstellar object with the mass of a planet, but which is not subject to the gravitational pull of any star. Rogue planets may originate in planetary systems before being ejected. They may also form outside a planetary system in the same way stars do, but just never got big enough for nuclear fusion to start. The Milky Way Galaxy alone may contain billions, or even trillions, of rogue planets.
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JuMBOs

Observations made by the James Webb Space Telescope reveal the presence of a number of free-floating, hot, gassy, Jupiter-sized objects in the Orion Nebula. The objects appear to be planet-like in their composition, but they are not in orbit around a parent star, so are not technically planets. On the other hand, they are too small to be stars, since it is not thought possible for objects the size of Jupiter to form through the process that gives rise to stars inside the clouds of dust and gas found in a nebula. The smallest stars are about 80 Jupiter masses; below that, the core is not dense enough to fuse hydrogen. The objects are called Jupiter-Mass Binary Objects ("JuMBOs"), reflecting the fact that, out of the hundreds identified, around 40 pairs (binaries) can be observed.
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ʻOumuamua

ʻOumuamua, discovered by Canadian astronomer Robert Weryk using a telescope at Haleakalā Observatory, Hawaii, in October 2017, is the first interstellar object detected passing through the Solar System. When it was first observed, the "interstellar interloper" ʻOumuamua was about 33 million kilometres (21 million miles) from Earth and already heading away from the Sun.

Most astronomers believe ʻOumuamua to be a natural object, but do not know exactly what it is. Although it showed no signs of having a coma (a surrounding cloud of dust and gas) some experts think it could be an interstellar comet, or the remnant of one. Other observers have suggested that it could be a product of extraterrestrial technology, but there is no evidence for this. Its name comes from Hawaiian word ʻoumuamua, meaning "scout". This reflects the way the object could be seen, in legend, as a messenger sent from the past to reach out to humanity.

Comet 3I/ATLAS

3I/ATLAS was the third confirmed interstellar object to pass through the Solar System, after 1I/ʻOumuamua (2017) and 2I/Borisov (2019, see Q-facts). Discovered on 1st July 2025 by the Asteroid Terrestrial-impact Last Alert System (ATLAS) station, the comet passed by Earth at a distance of 270 million kilometres (168 million miles) as it looped past the Sun between the orbits of our planet and that of Mars. Travelling at 250,000 km/h (around 150,000 mph), it came closest to the Sun on 29th October 2025, when it lay at a distance of 203 million kilometres (126 million miles).
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According to a study published in June 2026, 3I/ATLAS contained 10 times more deuterium, a "heavy" form of hydrogen, than Solar System comets. This suggested to astronomers that it formed in a very cold environment, possibly up to 12 billion years ago. That would make it around three times as old as the Solar System. Since its origin, 3I/ATLAS was probably flung beyond the gravitational pull of any star, spending billions of years hurtling about space until it was drawn towards our own Sun.