Objects that share the same orbit are common in the solar system. But we have never seen co-orbital exoplanets. Why?

“Where are all the Trojans” is a valid question in both the study of ancient history and the study of exoplanets. Trojan bodies, which share orbital paths with other larger planets, are widespread in our solar system – most obviously in the Trojan asteroids that follow Jupiter on its orbital path. However, they appear to be absent from any star systems found with exoplanets. Now, a team of researchers from NASA’s SETI Institute and Ames Research Center believe they’ve found a reason.

According to Anthony Dobrovolski of SETI and Jack Lissauer of Ames, an extreme version of the tides could be the cause. While most people think that tides are simply a reason for water moving in and out of shorelines everywhere, there is another effect that is much harder to discern. The friction of all that water moving back and forth across the Earth’s surface actually slows the rotation of the planet. In turn, this slower rotation allows the Moon to glide further and further.

Of course, speed and distances are almost imperceptible – about 1.78 milliseconds over a century and 3.78 cm per year, to be precise. But expand that to the billions of years it takes for a planet to form, and you’re talking about pretty big changes. This idea inspired the team to think about how much tidal forces might play a role in shaping planetary orbits – and what else they might have.

UT video explaining Lagrange points.

Turns out that could be a pretty big role. The researchers developed a model that places an Earth-sized planet at the L4 or L5 Lagrange points of a giant planet or at its equilateral point. They noticed that the tides caused by the three-body system caused what looked like harmless original oscillations in the orbit of the Earth-sized planet to eventually spin out of control, eventually throwing the planet itself against its giant neighbor or the star itself.

Since the current technology we have only allows us to see planets roughly the size of Earth, this model would show how unlikely a planet of this magnitude is to form in the trajectory. orbit of a much larger planet. However, there is always a chance that a smaller planet, or even a collection of asteroids such as the Trojans and the Greeks, could maintain the same orbit as a larger planet without being disturbed by the forces of tide.

A graphical representation of the orbit that a potential Trojan exoplanet takes when under the influence of tidal forces.
Credit – SETI Institute

If such objects exist, it will only be a matter of time before we find them, with humanity’s ever-increasing ability to detect exoplanets. When we do, the new model will also help inform astronomers about the internal composition of such a co-orbital planet. That might come in handy sooner rather than later, as there’s already a mission on the way to the Trojan asteroids. Lucy, which launched in October, could help characterize what co-orbital objects look like in our own solar system – and therefore be able to help characterize what they might look like in other star systems. Either way, the search for Trojan horses will remain a question with very interdisciplinary roots.

Learn more:
SETI Institute – Why haven’t we discovered co-orbital exoplanets? Could the tides offer a possible answer?
Dobrovolskis & Lissauer – Do the tides destabilize the Trojan exoplanets?
Space.com – Do Trojan exoplanets exist?
UT – Earth’s first Trojan asteroid discovered
UT – Jupiter’s Trojan asteroids offer surprises even before NASA’s Lucy mission has a chance to visit them.

Main picture:
Artist’s rendering of a moon orbiting a giant blue planet.
Credit – Claudioventrella

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