An artist’s concept of super-Earth planet 55 Cancri e, which races around its host star once every 18 hours. New research led by Penn State astronomers improves our understanding of how large super-Earth planets with small, quick orbits form.
UNIVERSITY PARK, Pa. — The galaxy is littered with planetary systems vastly different from ours. In the solar system, the planet closest to the Sun — Mercury, with an orbit of 88 days — is also the smallest. But NASA's Kepler spacecraft has discovered thousands of systems full of very large planets — called super-Earths — in very small orbits that zip around their host star several times every 10 days.
Now, researchers may have a better understanding how such planets formed.
A team of Penn State-led astronomers found that as planets form out of the chaotic churn of gravitational, hydrodynamic — or, drag — and magnetic forces and collisions within the dusty, gaseous protoplanetary disk that surrounds a star as a planetary system starts to form, the orbits of these planets eventually get in synch, causing them to slide — follow-the-leader style — toward the star. The team’s computer simulations resulted in planetary systems with properties that match up with those of actual planetary systems observed by the Kepler space telescope. Both simulations and observations show large, rocky super-Earths orbiting very close to their host stars, according to Daniel Carrera, assistant research professor of astronomy in Penn State’s Eberly College of Science.
Carrera said the simulation is a step toward understanding why super-Earths gather so close to their host stars. The simulations may also shed light on why super-Earths are often located so close to their host star, where there doesn’t seem to be enough solid material in the protoplanetary disk to form a planet, let alone a big planet, according to the researchers, who report their findings in the Monthly Notices of the Royal Astronomical Society.
“When stars are very young, they are surrounded by a disc that is mostly gas with some dust — and that dust grows into the planets, like the Earth and these super-Earths,” said Carrera. “But the particular puzzle for us is that this disc doesn’t go the all way to the star — there's a cavity there. And yet we see these planets closer to the star than the edge of that disc.”