Sciency Words: Trojans

Sciency Words MATH

Sciency Words is a special series here on Planet Pailly celebrating the rich and colorful world of science and science-related terminology. Today, we’re looking at the term:


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.

Sp07 Sad Saturn

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, and Polydeuces 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?

These are questions best answered by science fiction writers. At least for now.

2 thoughts on “Sciency Words: Trojans

  1. Technically, under the IAU’s definition of “planet”, Trojan planets would not be planets, since they wouldn’t have cleared their orbit of similar sized bodies. (Whether or not to tell the IAU to go stick themselves is a matter of personal philosophy.)

    Although it’d be an interesting conundrum if a Mars sized body was in a Trojan relationship with a Jupiter sized one. (Apologies for writing a sentence that sounds vaguely dirty.)

    Liked by 1 person

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