Super-Earth Smackdowns May Explain Diverse Worlds

Earth's moon formation — An artist's image of the violent collision that formed Earth's moon. Such collisions could explain the surprising differences in more massive super-Earths and super-Neptunes.

(Image credit: NASA)

Catastrophic collisions may explain differences in giant rocky planets around other stars.

A new study suggests that the heat generated by material smashing into a planet plays an important role in removing some or all of a planet's atmosphere. A wide variety of sizes for these deadly asteroids would explain differences seen in the more massive rocky worlds.

NASA's planet-hunting Kepler Space Telescope revealed a surprising number of worlds with sizes that fall between Earth and Neptune in relatively short orbits. By calculating the densities of the planets, astronomers learned that many of them seem to boast massive hydrogen-helium atmospheres. However, these atmospheres seem to come in many different flavors, suggesting something was happening to the worlds after planetary formation. [Tales from the Exoplanet Archive: How NASA Keeps Track of Alien Worlds]

"Giant impacts are very effective at reducing or removing the hydrogen or helium envelope," John Biersteker told his colleagues last month at the 233rd semiannual meeting of the American Astronomical Society in Seattle. Biersteker, a graduate student at the Massachusetts Institute of Technology, studied how impacts by rocky debris affected the atmosphere of young planets. He found that impacts could create many different types of worlds.

After a star is born, the dust- and gas-filled ring left behind begins the planetary formation process. Once gravity pulls together enough pieces to create a core, the newborn planetesimal begins to collect hydrogen and helium from the leftover gas, building its primal atmosphere. Planets that are very close to their stars can be affected by stellar radiation, which heats the top layers of the gas so that it escapes into space, leaving a thinner atmosphere.

Eventually, the gas in the disk is either piled onto planets or swept away by the star, leaving behind only dust and rock. Mixed among the small and medium-size debris are the failed cores that weren't able to grow large enough to attract an atmosphere. When these massive objects slam into planets, the collision can blow the planet's atmosphere off into space. One such collision helped to form Earth's moon.

A planet's youth may also work against it when it comes to holding onto an atmosphere. When a planet is young — only a few tens of millions of years old — its core is still fully molten, with no crust to insulate it from the atmosphere. Since a hotter atmosphere is harder to hold onto, it doesn't take much of an impact to strip off some of the atmosphere. An object crashing into a planet raises the energy, in the form of heat, allowing more of the atmosphere to be blown away.

Nola Taylor Tillman is a contributing writer for Live Science and Space.com. She loves all things space and astronomy-related, and enjoys the opportunity to learn more. She has a Bachelor’s degree in English and Astrophysics from Agnes Scott college and served as an intern at Sky & Telescope magazine. In her free time, she homeschools her four children.

Planetary beatdown