Co-orbiting worlds, also known as Trojan horses, have yet to be found outside the solar system. It’s a strange thing, a new study suggests, given that we’re constantly seeing them closer to home.
Our solar system is full of Trojan horses; asteroids orbit the sun along the same path as a larger planet (like Jupiter) about 60 degrees ahead and behind the planet. But it could be that tidal forces are altering the orbits of larger worlds, like an exoplanet.
While exoplanets with Trojan orbits can form, the study by the SETI (Search for Extraterrestrial Intelligence) Institute suggests that larger worlds with short-period orbits are thrown off course due to gravitational forces of the neighboring star. Once ejected, the Trojan collides with the star or a larger planet.
Related: Earth has an additional companion, a Trojan asteroid that will hang around for 4,000 years
The study has implications for our current catalog of 5,000 exoplanets, which primarily includes worlds close to their parent stars because they are easier to detect (either because they are easier to see when moving on a star from our perspective, or because their gravitational tug is a bit stronger on the star.)
In other words, the Trojans could be there, but not yet detectable with current techniques. Alternatively, Trojans can be removed from their systems quite quickly relative to the relative age of the universe, making them harder to spot.
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Earthlings experience tidal friction due to the interaction between Earth and the Moon, SETI pointed out in a press release. The tides slow the rotation of the Earth and move the Moon away from the Earth.
The new study generalizes this friction to theoretical more than two-body exoplanetary systems, assuming a star, a gas giant planet, and a rocky planet moving around Lagrange points, or gravitationally stable zones between multiple worlds.
The analysis suggests that the star’s tides and the giant planet’s tides working together on the Earth-sized planet would make its orbit unstable. Oscillations in the small planet’s orbit, the statement said, “would go from oval to banana-shaped and eventually move out of the shared orbit, colliding with the giant star or planet.”
Although tidal forces can eject Trojans before we can observe them, some may be in newer systems and can be observed. There might also be more insight into Trojan behavior from an in-flight NASA mission. Lucy set off last October to closely observe the Trojan asteroids, which is a first.
A research-based study was available as a pre-trial on Icarus on May 21. It is directed by Anthony Dobrovolskis, researcher at the SETI Institute.