---
title: "Introduction to Interstellar Objects"
date: "2026-10-13"
description: "A short introduction to unbound asteroids and comets from outside our Solar System."
draft: false
---

Young star systems shed a tremendous amount of material in the form of asteroids, comets, and occasionally even planets! When a star forms, the surrounding gas and dust form a proto-planetary disk. This debris slowly increases in size as gas and dust as small grains collide and stick growing into planetesimals. Planetesimals are large enough that their own gravity pulls in more material whereas grains and smaller structures rely solely on random collisions to increase their material. Planets form through this process since larger planetesimals experience runaway growth as larger planetesimals grow faster than smaller ones. Some planetesimals add to the system's planets size but most do not. Planets eject through close gravitational encounters that impart kinetic energy into planetesimals, kicking them out of the system. The planetesimals kicked out are then unbound from the system, spending most of their life between stars.

We discovered the first interstellar object, ʻOumuamua[^oumuamua], in 2017 in the Pan-STARRS survey. Although interstellar objects[^name] were first predict by Oort in 1951, they remained undiscovered until ʻOumuamua. Since 2017, we have only discovered two more interstellar objects, 2I/Borisov, and 3I/ATLAS. ʻOumuamua was an asteroid, with rocky body with a light curve indicating a 'cigar-like' shape. In contrast 2I/Borisov and 3I/Atlas were cometary, with large comas that obscured the body beneath.

[^oumuamua]: ʻOumuamua means "a messenger from afar arriving first" ([NASA](https://science.nasa.gov/solar-system/comets/oumuamua/)). Don't forget the ʻokina, the Hawaiian glottal stop, at the start of ʻOumuamua!

[^name]: Oort did not use the name Interstellar Objects when referring to these objects.