Accretion discs and planet formation around young stars

Abstract

Among the extrasolar planets discovered so far, the most abundant are the close-in super-Earths. These are planets with sizes between that of the Earth and Neptune, and orbits typically smaller than Mercury's. In this thesis, I study the innermost regions of accretion discs surrounding young stars, and if and how close-in super-Earths can form at such short orbital periods.

I start by discussing a simple model of the inner disc structure coupled to a detailed prescription of disc accretion due to the magneto-rotational instability (MRI). I use the inferred structure of the gas to show that the MRI leads to accumulation of dust in the inner disc, as necessary for the formation of solid planet cores. Next, assuming that solid cores do form in the inner disc, I investigate the accretion and evolution of planetary atmospheres. I show that, despite the MRI-accreting inner disc being gas-poor, the predicted planet atmospheres are at least as large as observed. Finally, I present an improved model of the inner disc that accounts for disc heating due to accretion and stellar irradiation, vertical energy transport, dust opacities, and dust effects on disc ionization. The optically-thick inner disc is weakly affected by stellar irradiation, and also convectively unstable. Dust controls the ionization state of the inner disc, and thus the onset of the MRI. I show that sustained dust accumulation can occur in the inner disc, without suppressing the MRI. If planets form in the inner disc, larger gas accretion rates (and thus earlier times in the disc lifetime) are favoured.

The work in this thesis advances our knowledge of the planet-forming environment at short orbital distances and supports the hypothesis that super-Earths could form near their present orbits. This work also identifies impediments to planet formation in the inner disc which require further study.