Centaurs — Asteroids in the Outer Solar System

by Carson

Apart from the main asteroid belt and the Kuiper belt, do you know there is a reservoir of asteroids among the outer planets? This population is called the centaurs. Let’s learn about them in this article.

What is a Centaur?

Centaurs are asteroids that orbit the Sun in planet-crossing orbits in the outer Solar System. They are very similar to near-Earth asteroids. While NEOs cross the orbits of Mercury, Venus, Earth, and Mars, centaurs cross the orbits of Jupiter, Saturn, Uranus, and Neptune.

Specifically, according to the JPL small-body database, a Centaur is defined as an asteroid with a semi-major axis of 5.5 to 30.1 astronomical units. The lower limit gets close to the Jupiter trojans, while the upper limit approaches the trans-Neptunian objects.

Where Did They Come From?

Centaurs are very dynamically unstable. Often, they have close approaches with the massive outer planets, which can modify the orbits in any way you can imagine. It could be ejected out of the Solar System or fall into the near-Earth population at any time. So there must be dynamical pathways that supply the centaurs back into the region to see so many of these asteroids today. But what do they look like?

Well, it’s precisely these destabilizing close approaches that bring them into this transient region. Many of these objects are icy, so most of them probably came from the Kuiper belt, whose objects are also icy (i.e., rich in volatiles). As their orbit migrated slowly in the region, it eventually came close to Neptune, which introduced them to the region of the outer gas giants.

It could also happen the other way around. Main-belt asteroids may get little gravitational kicks from Jupiter, modifying their orbits from far away. This progresses until it comes close to Jupiter, which happens to push this object to an orbit among the outer planets. In fact, we have already observed one in action. In 1963, comet 39P/Oterma approached Jupiter to within 0.1 astronomical units, turning it from a quasi-Hilda object with a 3:2 resonance with Jupiter into a centaur traveling among the outer Solar System.

The orbit of 39P/Oterma from 1955 to 2000 projected on the plane of the ecliptic, showing how it changed due to a close approach of Jupiter.
Data source: JPL Horizons
Blue: Earth; Red: Mars; Brown: Jupiter; Magenta; 39P/Oterma; Yellow: Saturn

How Can Centaurs Become Comets?

As you know, comets release volatiles and form comae and tails when they’re close to the Sun. But at the freezing temperatures of the outer Solar System, many centaurs are still classified as comets. This includes 95P/Chiron, 166P/NEAT, and 174P/Echeclus. How can this happen?

Well, these comets emit substances that ordinary comets don’t. For example, typical Jupiter-family comets emit water vapor as the water on the surface sublimates. But centaur comets emit even more volatile substances with really low boiling points, such as carbon monoxide.

Another way for them to become comets is through orbital perturbations. Again, when the object has a close encounter with a planet, it could change its orbit in every way imaginable, including sending it to the inner Solar System. This happened with comet 81P/Wild 2, which was visited by the Stardust spacecraft. It was a centaur until it came close to Jupiter in 1973, which turned it into a short-period comet.

The orbit of 81P/Wild 2 from 1920 to 1985 projected on the plane of the ecliptic, showing how it changed due to a close approach of Jupiter.
Note that 81P did not have a hyperbolic orbit to begin with, but its closed orbit before the Jupiter approach was so large that the graph could not plot its entirety without risking losing details on the post-approach orbit.
Data source: JPL Horizons
Blue: Earth; Red: Mars; Brown: Jupiter; Magenta; 81P/Wild 2; Yellow: Saturn


Centaurs are intriguing objects because most of them are fresh immigrants from the Kuiper Belt. By studying their composition at a much closer distance, we can learn much about what the region beyond Neptune looked like when the objects there formed. Unfortunately, like Kuiper Belt objects, they are also very distant, and no spacecraft has ever flown past any of them so far. If you found something that we should have included in this article, please comment below to help us improve the article.


  1. Laskar, J., Gastineau, M. (2009.). “Existence of collisional trajectories of Mercury, Mars and Venus with the Earth”. Retrieved January 8, 2023, from https://www.academia.edu/3476045/Existence_of_collisional_trajectories_of_Mercury_Mars_and_Venus_with_the_Earth

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