NASA's Kepler observatory should be able to detect planetary moons - yet to be discovered - formed by far-away planetary collisions outside our solar system, research by Amy Barr of the Planetary Science Institute shows. The Kepler spacecraft has discovered thousands of exoplanets, but has not yet detected definitive signs of moons - exomoons - orbiting them.

A pair of papers authored by Barr describes how exomoons large enough to be detected by Kepler could form. Barr's paper, "Formation of Massive Rocky Exomoons by Giant Impact" appearing in the Monthly Notices of the Royal Astronomical Society, looks at the formation of moons via giant impacts around rocky extrasolar planets.

"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," said Barr, a senior scientist at PSI.

"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."

This research used hydrodynamical simulations to determine how much material is launched into orbit by the collision of two rocky exoplanets.

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," said Barr. The simulations were performed in collaboration with Megan Bruck Syal of Lawrence Livermore National Laboratory.

A second paper, "Formation of Exomoons: A Solar System Perspective" appearing in Astronomical Review, describes how large exomoons could form by co-accretion around growing gas giant planets, or by processes that did not operate in our solar system.

In this paper, Barr describes what is known about the formation of planetary moons in our solar system, and how those theories might apply to the formation of exomoons.

"Some of the old theories about the formation of Earth's Moon, for example, fission, could operate in other solar systems," said Barr. "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."