Move Over, Moons — Some Planets Also Have Trojans

Our solar system seems straightforward enough: There are eight planets that travel in elliptical orbits around the sun and a few handfuls of moons that orbit those planets. But have you ever wondered why each of the planets gets an orbital path all to themselves? It turns out that not all of them do. Some of the solar system planets share an orbit with celestial hangers-on called Trojans. NASA already has a mission planned to check them out.

Cover Me!

Trojans are asteroids that were captured by the gravitational dynamics of larger bodies nearby. In all, five of the planets in our solar system have at least one Trojan: Uranus has one, Neptune has 13, and Mars has seven. Earth has one, too. But as in most things, Jupiter outguns them all with somewhere around one million.

While they get their names from Greek mythology, we prefer to compare Trojans to a presidential motorcade. The president — in this case, let’s say it’s Jupiter — rides in a limousine along a prearranged route at the center of the motorcade, closely flanked by cars that contain other important officials. Driving in front of the limo are Secret Service personnel and police officers, and driving behind are SWAT teams and press vans. In the same way, Jupiter travels along an orbit flanked by its moons, preceded by the L4 Trojans (aka the “Trojan node”), and trailed by the L5 Trojans (aka the “Greek node”).

L4 and L5 are named for their Lagrange points, specific spots in space where the combined gravitational forces of large bodies — in this case, Jupiter and the Sun — create an area of equilibrium that allows for a stable orbit. Every pair of bodies has five Lagrange points, some of which are more stable than others. Astronomers here on Earth use Lagrange points L1 and L2 to park spacecraft, though the instability of these points means craft must constantly tweak their orbits to stay put.

Lagrange points of a planet (blue) orbiting a star. L4 and L5 are the places where Trojans can survive. The other points (L1, L2, and L3) are not stable.

But L4 and L5 are very stable, and that’s why so many asteroids and other cosmic debris call those Lagrange points home — not only in Jupiter’s orbit, but also the orbits of Earth, Mars, Neptune, and Uranus. (MercuryVenus, and Saturn are affected by various types of gravitational interference that keeps Trojan swarms from forming). Objects at the L4 and L5 points share an orbit with their chosen planet about 60 degrees ahead or behind.

A Mission to History

So far, all we know about the Trojans in our solar system, and the ones that flank Jupiter in particular, has come from what we can see with Earth-based telescopes. But NASA has already planned a mission to do more than that: Scheduled to launch in 2021, the Lucy mission will take a 12-year journey to one asteroid in the main belt and six Trojans around Jupiter.

The fact that it gets its name from the fossil skeleton of an early human ancestor is no accident — scientists believe the Trojans may be remnants of the primordial material that formed our solar system, so just as the fossil Lucy rewrote our understanding of humanity’s origins, the Lucy mission may rewrite our understanding of the solar system’s origins. Not only will Lucy be the first space mission to study these Jovian groupies, but it’s also going to set a record for most destinations on independent orbits visited in one trip. It’s estimated time of arrival should be around 2027, so sit tight, Trojans. We can’t wait to meet you!

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