Abstract
Recent {\em Kepler} observations revealed an unexpected abundance of "hot" Earth-size to Neptune-size planets in the inner $0.02-0.2$ AU from their parent stars. We propose that these smaller planets are the remnants of massive giant planets that migrated inward quicker than they could contract. We show that such disruptions naturally occur in the framework of the Tidal Downsizing hypothesis for planet formation. We find that the characteristic planet-star separation at which such "hot disruptions" occur is $R \approx 0.03-0.2$ AU. This result is independent of the planet's embryo mass but is dependent on the accretion rate in the disc. At high accretion rates, $\dot M \simgt 10^{-6}\msun$ yr$^{-1}$, the embryo is unable to contract quickly enough and is disrupted. At late times, when the accretion rate drops to $\dot M \simlt 10^{-8} \msun$ yr$^{-1}$, the embryos migrate sufficiently slow to not be disrupted. These "late arrivals" may explain the well known population of hot jupiters. If type I migration regime is inefficient, then our model predicts a pile-up of planets at $R\sim 0.1$ AU as the migration rate suddenly switches from the type II to type I in that region.
Highlights
Standard protoplanetary disc models (Chiang & Goldreich 1997) show that the inner ∼0.1 au region is too hot to allow for the existence of small solid particles there
We believe that low-density initial conditions are more relevant to the problem at hand because (i) our embryos are not isolated, irradiation and tidal heating effects are present; (ii) the embryo may be rapidly rotating before the second collapse (Boley et al 2010; Nayakshin 2011a) and specific angular momentum conservation could slow down the contraction of the planets after H2 dissociation; and (iii) Nayakshin (2011b) finds that the accretion luminosity of the solid core inside the giant embryo may be significant and may even unbind the gas envelope if the opacity is large enough
This extra heating source inside the embryo is likely to keep it hotter. Keeping in mind these present uncertainties in the properties of the young massive gas giants, we consider below a simple model with a range of initial second embryo radii that we believe roughly brackets the possible outcomes of a planet migrating very close to its parent star
Summary
Standard protoplanetary disc models (Chiang & Goldreich 1997) show that the inner ∼0.1 au region is too hot to allow for the existence of small solid particles there. The key importance of the radial migration of the earliest gas condensations formed in the massive protoplanetary discs It was pointed out by Boley et al (2010) and Nayakshin (2010a) that this migration-and-disruption sequence yields an unexplored way of forming terrestrial-like planets. In the TD hypothesis for planet formation, as we show below, this final step is not automatically successful – planets continuing to migrate rapidly towards their parent stars may still be disrupted at R ∼ 0.1 au We suggest this process as a way of forming the hot Super Earths observed by the Kepler mission (Borucki et al 2011). In analogy to the star formation literature, we refer to the GEs that are mainly molecular, and the embryo’s temperature Te < T2nd, as the ‘first GEs’; those where H2 is disassociated are termed ‘second GEs’ instead
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