Formation of super-Earths via pebble accretion onto planetesimals
Sammanfattning: Data from NASA's Kepler space telescope, which searches for exoplanets via the transit method, produced 1108 new planetary candidates in 2013 with a total of 91% being smaller than Neptune in size. These were mostly super-Earths, terrestial planets between Earth and Neptune in size, with orbits around 10 days. In order for any theory of planet formation to be valid it must be able to account for the existence of these super-Earths. Current models put the transition from planetesimal to planet to be the result of planetesimal collisions. In only the past few years the theory of pebble accretion onto planetesimals has emerged. It centers around the idea that the accretion of pebbles onto planetesimals in the protoplanetary disk is a large part of planet formation, able to rapidly speed the process up. In this thesis we investigate whether the pebble accretion theory can account for the super-Earths in Kepler's data. This is done using a statistical code called PAOPAP. We simulate the accretion of mm-cm sized pebbles onto already existing planetesimals and investigate what effect different sized annuli and the amount of pebbles has on the final mass of the planets produced in the code. We find that while wider annuli make no discernible pattern in the final mass of the planets, increasing the amount of mass in pebbles for a 0.2 AU annulus allows us to create planets with masses up to ~8 Earth masses or ~2 Earth radii. The reason the annulus width does not determine mass is because the planets become isolated at a certain point, having accreted all nearby pebbles, giving them an isolation mass. We also vary the size of the pebbles being accreted to show that larger pebbles only brings about a faster growth process but with the same final mass in a simulation. Lastly we show a selection of the largest planetesimals in each simulation to give a demonstration of oligarchic growth of planets over time. In the end, we are able to show that the large population of super-Earths found by the Kepler satellite can be explained by the theory of pebble accretion.
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