Collision super-lands. The figure shows an image in the middle of a hydrodynamic simulation of high-speed frontal collision between two planets of 10 terrestrial masses. The temperature scale is represented by four colors: gray, orange, yellow and red, where gray and red respectively correspond to the lowest and highest temperature. These collisions lead to the expulsion of a large amount of silicate matter in the mantle and to the formation of a "surviving" planet with a high iron content and therefore a high density, similar to the super-earth Kepler-107c. Credits: Zoë M. Leinhardt and Thomas Denman, University of Bristol
There are currently about 2000 confirmed exoplanets with radii less than about three Earth-radii, and measurements of their densities reveal an astonishing diversity. Some have densities lower than Neptune which is made mostly of volatiles (materials less dense than metal and rock), but Neptune has almost four times the Earth's radius), while others appear to have rock-like densities, as high as the Earth's or higher. Such a wide range of compositions may be the product of the different initial conditions in the planet-formation process, or it could be because something dramatic happens to the planet to alter its initial properties as it evolves.
In a new paper in Nature Astronomy, Istituto Nazionale Di Astrofisica (INAF) astronomers Aldo S. Bonomo and Mario Damasso, along with a large team of colleagues including Goddard Scientist Eric Lopez, report that a giant collision must have occurred in the exoplanetary system Kepler-107. While there is some observational evidence for the collisional process in our own solar system, so far there has been no unambiguous finding in support of the impact scenario among exoplanets.
Of the four known planets in Kepler-107, the two innermost ones have nearly identical radii of 1.536 and 1.597 Earth-radii, respectively, (the uncertainty of each is only about 0.2%). Their periods are also similar at 3.18 and 4.90 days, meaning that they orbit relatively nearby each other. Using the HARPS-N spectrograph at the Telescopio Nazionale Galileo in La Palma, the team determined the planet masses, and hence their densities. The observations are surprising -- their densities are very different: 5.3 and 12.65 grams per cubic centimeter, respectively. For comparison, water's density is 1 gram per cubic centimeter and the Earth's is 5.5 grams per cubic centimeter. The fact that one of the planets has a density more than twice the other cannot be easily explained by stellar radiation effects which should have affected them both in the same way. Moreover, it is the outer one that is denser than the inner one. The astronomers argue instead that a giant impact on one planet, Kepler-107c (the denser planet), stripped off part of its initial silicate mantle, leaving it dominated by its dense iron core. They support this hypothesis with theoretical calculations, including planetary structure and evolution models from Dr. Lopez at NASA's Goddard Space Flight Center.