Interstellar object

An interstellar object is an astronomical body located in interstellar space that is not gravitationally bound to a star. This means the object does not follow a closed, repeating loop around a host star. Instead, it travels through the vast gaps between stars. These objects can include various types of matter, such as asteroids, comets, and even rogue planets. While stars and stellar remnants are not classified this way, smaller objects like planetesimals often become interstellar travelers. These visitors are significant because they may carry material from distant exoplanetary systems into our own neighborhood.

How does an object become an interstellar traveler? Most interstellar objects were once part of a stable solar system. They were originally bound to a host star in an elliptical orbit. However, various gravitational processes can cause them to become unbound. One common method involves a gravitational interaction with a massive third body, such as a large planet. For example, if an object passes close to a giant planet like Jupiter, the planet's gravity can accelerate it. This boost provides the object with enough energy to reach escape velocity. Once it reaches this speed, its path changes from an elliptical orbit to a hyperbolic trajectory.

Astronomers identify these visitors in our Solar System as interstellar interlopers. They distinguish them by observing a significant hyperbolic excess velocity. This term describes the speed an object maintains even as it moves far away from a star. Unlike objects bound to the Sun, which follow elliptical paths, interlopers follow strongly hyperbolic trajectories. This movement proves they did not originate within our Solar System. Because of this, the International Astronomical Union (IAU) uses a specific naming system. They assign "I" numbers to these objects, such as 1I or 2I, to denote their status as confirmed interstellar interlopers.

We have officially confirmed three major interstellar interlopers passing through our system. The first was 1I/ʻOumuamua, discovered on October 19, 2017, by the Pan-STARRS telescope. Its name comes from a Hawaiian word meaning "a messenger from afar arriving first." This object had a very high eccentricity of 1.199, meaning its path was extremely stretched. The second was 2I/Borisov, discovered on August 30, 2019, by Gennadiy Borisov. This object appeared more like a typical comet than ʻOumuamua did. The third was 3I/ATLAS, discovered on July 1, 2025, near the orbit of Jupiter. This object set a record with an eccentricity of 6.14 and an interstellar velocity of approximately 58 km/s.

These objects are much more common than they might seem. Astronomers estimate that several interstellar objects pass inside the orbit of Earth every year. On any given day, roughly 10,000 such objects may be passing inside the orbit of Neptune. Researchers have also studied the density of these objects in deep space. One 1986 paper suggested a limit of 10 trillion comets per cubic parsec. More recent analyses have set different upper limits. Following the discovery of ʻOumuamua, scientists predicted a steady population of about 100-meter scale objects inside Neptune's orbit. These objects have a residence time of approximately 10 years.

Interstellar visitors come in a massive range of sizes. They span from kilometer-wide asteroids and comets down to tiny submicron particles. This spectrum includes interstellar dust and meteoroids that carry chemical information from their parent systems. However, detecting the smallest particles is difficult. Smallest dust particles are often filtered out of the Solar System by electromagnetic forces. Meanwhile, the largest objects are so sparse that spacecraft detectors struggle to gather enough statistics. Identifying interstellar meteoroids in Earth's atmosphere is also a major challenge. It requires extremely accurate measurements to ensure a near-parabolic orbit is not mistaken for a hyperbolic one.

Studying these interlopers connects our Solar System to the broader galaxy. They provide a way to study the composition of other star systems without traveling there. For instance, 2I/Borisov showed a surface composition similar to comets in our own Oort Cloud. There is even evidence that some objects might be captured. Research suggests the asteroid 514107 Kaʻepaokaʻāwela may have been an interstellar object captured 4.5 billion years ago. Other simulations show that Jupiter is massive enough to capture an interstellar comet once every sixty million years. These objects serve as a material link between worlds, potentially exchanging matter and even spores across the cosmos.