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Extreme & Unusual Objects

Interstellar Objects

Interstellar objects are visitors from other star systems, passing through ours on paths that never bring them back.

Only a few have ever been identified, and each has challenged expectations.

  1. Observe

    A faint object moving too fast on a path that does not curve back.

  2. Measure

    Positions over several nights to compute orbit shape.

  3. Model

    Orbit fitting; comparisons with comets and asteroids.

  4. Explain

    Matter from other star systems can pass through ours.

What is it?

An interstellar object formed around another star and was ejected into interstellar space. If it passes through the Solar System, its speed and path show it is not bound to the Sun: its orbit is a hyperbola rather than an ellipse.

They are designated with an 'I', as in 1I, 2I and 3I. Astronomers expect them to be more common than we have seen, but they are faint, fast and visible only briefly.

How does it happen?

  1. Stage 1
    Object forms around another star
  2. Stage 2
    It is ejected into interstellar space
  3. Stage 3
    It travels for millions of years
  4. Stage 4
    It passes through our Solar System

What do we actually observe?

ʻOumuamua (1I)
Discovered in October 2017 by Pan-STARRS. It showed no visible coma, an unusual shape and a small, unexplained push away from the Sun. Explanations have been debated; the scientific consensus is a natural origin.
2I/Borisov
Discovered in August 2019 by Gennady Borisov. It looked like an ordinary comet with a coma and tail, and its gas was rich in carbon monoxide.
3I/ATLAS
Discovered in July 2025 by the ATLAS survey, the third confirmed interstellar object.

How do scientists detect it?

  • Sky surveys. Pan-STARRS, ATLAS and Catalina catch fast movers. The Vera C. Rubin Observatory is expected to find more.
  • Follow-up telescopes. Determine orbit, size and composition.

Why does it matter?

  • They are direct samples of material from other planetary systems.
  • Their properties test ideas about how planets and comets form.

The SpaceWeatherTracker connection

Like comets, they interact with the Sun's radiation and wind, which drives their gas and dust tails.

Related phenomena

Primary family: Extreme & Unusual Objects

Interstellar Objects: common questions

How do we know an object is interstellar?

Its orbit is hyperbolic: it moves too fast to be bound to the Sun and does not loop back.

Was ʻOumuamua an alien spacecraft?

There is no evidence for that. Most scientists favor natural explanations, though what caused its small acceleration is still debated.

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