Sciency Words: Trojans

Today’s post is part of a special series that first appeared on Planet Pailly. Every week, we take a look at a new and interesting scientific term to help us all expand our scientific vocabularies together. Today’s sciency word is:


Trojan asteroids are asteroids that share their orbits with a planet. This may not seem like a particularly safe arrangement for the asteroids (or the planet), but so long as the asteroids are positioned just right, their orbits will remain stable.

The asteroid must be located near something called a Lagrange point, specifically the L4 or L5 points. These are points in the orbital plane where the distance to the planet equals the distance to its host star. The combined gravitational pulls of the planet and star will cause the asteroid to circle round and round the Lagrange point in a bizarre, corkscrew-like orbital path.

The first known Trojans were discovered near Jupiter very early in the 20th Century. The initial plan was to name them all after characters from the Trojan War, as told in Homer’s Iliad; however, it turned out that there were way, way more Trojan asteroids than named characters in that particular story.

We now know that Jupiter has over 6,000 Trojans, about 4,000 orbiting ahead of it and another 2,000 orbiting behind. Most of the other planets in the Solar System have Trojans too.

Neptune has a dozen confirmed Trojans, according to the IAU’s Minor Planet Center. Mars has four, which were probably captured from the asteroid belt. Earth and Uranus each have one. And Saturn… Saturn has none. No Trojans. Probably because Jupiter stole them all.


Aww, cheer up, Saturn! You have something way cooler than Trojan asteroids: Trojan moons!

Saturn is the only planet where moons are known to share orbits with each other. Tethys, Telesto, and Calypso orbit together, with Telesto near Tethys’s L4 point and Calypso near the L5 point. Dione, Helene, andPolydeuces make a similar set, with Helene and Polydeuces near Dione’s L4 and L5 points, respectively.

Trojan asteroids are interesting; Trojan moons moreso. But what would be really fascinating, should we ever discover them, are Trojan planets. Somewhere out there, could there be terrestrial worlds hovering near the L4 or L5 points of gas giants? Could these worlds support life? What sort of civilization might develop there, and what strange sights would they see in the night sky?

Written by James Pailly.

To read all the articles in the ‘Sciency Words‘ series, visit the Planet Pailly blog.

  • John H Reiher Jr

    Huh, 2010 TK7 would make for a good first asteroid mission. It’s the same distance from the sun as the earth and it’s in a location that should be easy to get to… I hope.

  • Paulo R. Mendes

    It is a shame that Earth is not s Trojan world. 🙁

  • DT Krippene

    Has me wondering how brave and bold represents the erratic, tidal locked orbit of a “Trojan” asteroid. Strange how titles stick. Fun sciency word, James. Thanks.

  • Leonardo Faria

    How on earth or outer space come that sharing an orbit with a planet — or whatever with whom(what)ever — earns you the title of Trojan?

    • It sounds cool! What title do you sugget?

      • Leonardo Faria

        I didn’t mean the title of the article, which is fine, but properly the honorable status and rank of Trojan for a piece of rock that happens to be — not for its merit — in the same orbit of a planet. I think horsefly asteroids would be more appropriate, and we could even compromise on Trojan Horsefly, to keep to the homeric narrative.

        • DT Krippene

          Now that’s funny, and a tad more accurate.

        • James Pailly

          Horsefly asteroid does kind of make sense. If we ever had the chance to change the names of these objects, I could go with that.