Interstellar object
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What is an interstellar object?
An interstellar object is any object in space that isn't bound to a star and has entered our solar system from interstellar space.
Three interstellar objects have been observed:
* ‘Oumuamua (discovered in 2017)
* Comet Borisov (discovered in 2019)
* Comet 3I/ATLAS (discovered in 2025)Comet 3I/ATLAS has the highest velocity relative to the Sun at 58 km per second, and may be the oldest comet known at 11 billion years old.
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How are interstellar objects different from objects in our solar system?
Interstellar objects are distinct from objects within our solar system, with key differences in composition and origin.
Comet Borisov, an interstellar object, shared many properties with solar system comets but had a much higher abundance of carbon monoxide. Another interstellar object, `Oumuamua, displayed an unusual cigarlike or pancakelike shape and may have been composed of nitrogen ice.
Objects in the solar system, such as planets and asteroids, orbit the Sun in a prograde direction within the ecliptic plane, whereas comets from the Oort cloud have random inclinations. Interstellar objects originate from outside the solar system, possibly ejected from other planetary systems during their early formation stages.
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Where do interstellar objects come from?
Interstellar objects originate from planetary systems early in their history, where giant planets scatter material as they migrate inward. Some of this scattered material attains high enough velocity to escape the planetary system, becoming interstellar objects. This process ejects comets and pieces of large, Pluto-like objects from their systems due to collisions within the Kuiper belt region.
Comet 3I/ATLAS, the third interstellar object discovered, may have originated from the Milky Way's thick disk. Its high velocity suggests that it comes from a population of old stars, making it potentially the oldest comet known, with an age of approximately 11 billion years.
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How do scientists detect interstellar objects entering our solar system?
Scientists detect interstellar objects using various methods. Telescopes gather radiant energy, focusing it on detectors calibrated for sensitivity and spectral response. Precise pointing and timing correlate observations from different instruments across wavelengths. Analyzing the radiation emitted helps identify the processes at work.
Objects from interstellar space, like ‘Oumuamua and Comet Borisov, have been observed passing through our solar system. Comet Borisov, discovered by Russian amateur astronomer Gennady Borisov, displayed a coma and tail, with an orbit indicating it originated outside the solar system. The Asteroid Terrestrial-impact Last Alert System (ATLAS) discovered Comet 3I/ATLAS, which had the highest velocity of the three known interstellar objects (ISOs).
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What are some famous examples of interstellar objects?
Three interstellar objects have been observed: ‘Oumuamua, Comet Borisov, and Comet 3I/ATLAS. 'Oumuamua, discovered in 2017, has an unusual cigarlike or pancakelike shape and rotates every 7.34 hours. Comet Borisov, or 2I Borisov, was discovered in 2019 and has properties similar to solar system comets, but with much higher carbon monoxide gas abundances. Comet 3I/ATLAS, discovered in 2025, may be the oldest comet known at 11 billion years old and may have originated from the Milky Way's thick disk. Additionally, a meteor was detected in 2014 entering Earth's atmosphere near Papua New Guinea and is believed to be of interstellar origin.
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Why is the study of interstellar objects important for understanding space?
The study of interstellar objects is important because they offer insights into the composition of other star systems and the processes of planet formation beyond our solar system. Interstellar objects may provide information about the conditions in other planetary systems, including the building blocks of planets and the potential for habitability. Studying these objects helps to understand the diversity of planetary systems and the prevalence of different elements in the cosmos. Analysis of interstellar objects can also reveal how common or unique our solar system is compared to others.
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Interstellar object, an object that has entered the solar system from interstellar space. Three such objects have been observed: ‘Oumuamua (discovered in 2017), Comet Borisov (discovered in 2019), and Comet 3I/ATLAS (discovered in 2025).
‘Oumuamua
‘Oumuamua was discovered on October 19, 2017, by the Pan-STARRS 1 telescope in Hawaii. The object was confirmed to be on a hyperbolic orbit—that is, an orbit in which it would not return to pass by the Sun again. The object was given the name ‘Oumuamua, Hawaiian for “reach out for first,” and given the designation 1I/2017 U1, the “1I” denoting it was the first known interstellar object.
‘Oumuamua was not considered a comet because no dust was observed being released. As it went away from the Sun, it accelerated faster than it would have under the influence of gravity alone, suggesting that there was some cometlike release of gases from underneath its surface. However, observations with the Spitzer Space Telescope did not show any release of typical cometary gases, such as carbon dioxide and carbon monoxide. One possible explanation is that ‘Oumuamua is composed of nitrogen ice and is a piece of a body like Pluto.
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‘Oumuamua rotates every 7.34 hours. It has an unusual cigarlike or pancakelike shape with a length of several hundred metres. ‘Oumuamua’s incoming velocity was close to the mean for stars near the Sun, so ‘Oumuamua’s journey through the solar system was possibly its first close encounter with a star, and, thus, ‘Oumuamua could likely be a young object, only a few hundred million years old. The detection of ‘Oumuamua has been interpreted to show that interstellar objects routinely pass through the solar system, with a population estimate of at least one such object like ‘Oumuamua inside Earth’s orbit at any time and with about 10,000 in the solar system.
Comets Borisov and ATLAS
The second interstellar object, Comet Borisov (also called 2I Borisov), was discovered by Russian amateur astronomer Gennady Borisov on August 30, 2019. Observations revealed a cometary coma and tail as well as an orbit showing an origin from outside the solar system. In many of its properties the comet was similar to those of the solar system; however, it had much higher abundances of carbon monoxide gas than any other previously observed comet.
The third interstellar object, Comet 3I/ATLAS, was discovered by the robotic survey project Asteroid Terrestrial-impact Last Alert System (ATLAS) on July 1, 2025. ATLAS had a velocity relative to the Sun of 58 km per second, the highest of the three known ISOs. ATLAS’s high velocity suggest it comes from the Milky Way’s thick disk, a population of old stars, thus making ATLAS possibly the oldest comet known, with an age as much as 11 billion years.
One meteor likely of interstellar origin has also been detected. In 2022 the U.S. Department of Defense announced that a review of classified data indicated that on January 8, 2014, a meteor about 1 metre (3 feet) across entered Earth’s atmosphere near Papua New Guinea with a velocity consistent with being an interstellar object.
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Planetary systems early in their history undergo a phase in which giant planets that had formed far from their sun migrate inward. As they travel through the system, they scatter material, some of which attains a velocity high enough that it can escape the planetary system altogether and become an interstellar object. Some of this material would be comets such as Borisov, and planetary migration would cause collisions within the Kuiper belt region, in which pieces of large Pluto-like objects would be ejected from their systems.