Objects that share the same orbit are common in the solar system. But we’ve never seen common orbiting exoplanets before. why?

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

According to Anthony Dobrovolsky of SETI and Jack Lissauer of Ames, an extreme version of the tides may be to blame. While most people think that tides are just a cause of water going in and out of beaches everywhere, there is another effect that is hard to discern. The friction caused by all that water moving back and forth across the Earth’s surface is slowing the planet’s rotation. In turn, this slower rotation of the Moon allows it to slide farther and farther.

Sure, speeds and distances are almost imperceptible – about 1.78 milliseconds over a century and 3.78 centimeters per year, to be exact. But extend those billions of years it takes to form a planet, and you’re talking about some very important changes. This idea inspired the team to think about how much tidal forces might play a role in shaping the planets’ orbits — and what hangers they might have.

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Video explaining Lagrange points.

It turns out that it can be a very important role. The researchers developed a model that placed an Earth-sized planet at either the L4 or L5 Lagrangian points of a giant planet, or an equilateral point within it. They observed that the tides caused by the three-body system caused what appeared to be original, harmless oscillations in the orbit of an Earth-sized planet eventually spiraling out of control, eventually pushing the planet itself into either its giant neighbor or the star itself.

Given that our current technology only allows us to display planets down to roughly the size of Earth, this model would show how likely a planet of that size would form in the orbital path of a much larger planet. However, there is still a chance that a smaller planet, or even a group of asteroids like the Trojans and Greeks, will be able to maintain the same orbit as a larger planet without being affected by tidal forces.

Diagram of the possible orbit of the exoplanet Trojan when it is under the influence of tidal forces.
Credit – SETI Institute

If there are any of these objects, it will only be a matter of time before we find them, with humanity’s ever-increasing ability to discover exoplanets. When we do, the new model will also help inform astronomers of the internal makeup of any common orbital planet. That may come in handy sooner rather than later, as there is already a mission en route to the Trojan asteroids. Lucy, launched in October, could help determine what common orbital objects in our solar system look like — and thus be able to help discern what they might look like in other star systems. Either way, finding Trojans will remain an issue with interdisciplinary roots.

Learn more:
SETI Institute – Why haven’t we discovered co-orbiting exoplanets? Could tides offer a possible answer?
Dobrovolskis & Lissauer – Do tides destabilize the outer planets of Troy?
Space.com – Are there Trojan exoplanets?
Utah – The discovery of the first Trojan asteroid on Earth
Utah – Trojan asteroids are giving Jupiter surprises even before NASA’s Lucy mission has a chance to visit.

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