Can We Find Exomoons?

We haven’t detected an exomoon. Yet.

If an exoplanet is “a planet orbiting another star in our galaxy,” according to Kepler mission co-investigator Natalie Batalha, then an exomoon is a moon orbiting a planet orbiting another star in our galaxy.

Got it?

But while Kepler has discovered thousands of exoplanets, it hasn’t found any definitive signs of moons orbiting them. No worries. When astronomers lack the real thing, they build models. (Not like model airplanes or model homes—computer models that simulate reality based on the laws of physics.)

Last year, I wrote about UC Santa Cruz grad student Michael Nayak and his work modeling the potential for exomoons in the crowded Kepler-32 system. This week, Amy Barr of the Planetary Science Institute has two papers based on her models on exomoon size and formation. And according to her, perhaps there are exomoons large enough for Kepler to detect.

In the first paper, Barr used hydrodynamical simulations to determine how much material is launched into orbit by the collision of two rocky exoplanets—super-Earths. Similar simulations have been used to study the origin of Earth’s moon. “These outcomes are broadly similar to the Moon-forming impact, but when two super-Earths collide, the disk is much hotter and more massive,” says Barr. The simulations were performed in collaboration with Megan Bruck Syal of Lawrence Livermore National Laboratory.

“Our results are the first to demonstrate the masses of the moons that could form in the varied set of impact conditions possible within exoplanetary systems,” Barr explains. “Most importantly, we have shown that it is possible to form exomoons with masses above the theoretical detection limits of the ongoing Hunt for Exomoons with Kepler survey, moons of more than a tenth of an Earth mass.” So that’s good news—the computer models produce theoretical exomoons we can actually detect!

The second paper describes how large exomoons could form by co-accretion around growing gas giant planets, or by processes that did not occur in our solar system. Perhaps there’s more than one way to skin a moon. “Some of the old theories about the formation of Earth’s moon, for example, fission, could operate in other solar systems,” Barr says. “With new observatories coming online soon, this is a good time to revisit some of the old ideas, and see if we might be able to predict how common exomoons might be, and what it would take to detect them.” So even more good news—exomoons may form in many different ways, which means there could be more of them to find!

Why bother with exomoons? Remember what Nayak said in our interview last year: “Moons have a large impact on the geology and orbital stability of the planets they orbit.” In other words, an exomoon could influence an exoplanet’s habitability. If the first step to finding life outside our own solar system is finding exoplanets, then exomoons could be an important next step.

Image: Figure 2 of Barr & Bruck Syal (2017)