* Astronomy

Blobrana · L

Hot Neptunes and super-Earths

Blobrana · L

Title: Formation of hot Neptunes by evaporation of hot Jupiters
Authors: Gwenaël Boué, Pedro Figueira, Alexandre C.M. Correia, Nuno C. Santos

Hot Jupiters are subject to intense energetic irradiations from their stars. It has been shown that this can lead to significant atmospheric mass-loss and create a population of smaller mass planets. Here, we analyse whether the observed hot Neptunes can be the outcome of the partial evaporation of hot Jupiters. The orbital evolution of a planet undergoing evaporation is derived analytically in a very general way. Analytical results are then compared with the period distribution of the two classes of inner exoplanets: Jupiter-mass planets and Neptune-mass planets. We show that hot Jupiters and hot Neptunes have a very distinct period distribution, with a probability lower than 0.0001 that they were derived from the same parent distribution. This difference can be perfectly explained by the presented migration mechanism if hot Neptunes are partially evaporated hot Jupiters, where matter is ejected from the hottest region of the planet surface. Hot Neptunes and lower-mass planets are thus likely to be partially evaporated hot Jupiters.

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Blobrana · L

Birthplace of 'hot Neptunes' revealed

Hot Neptunes are modestly giant planets that resemble their namesake but orbit close to their stars. But the puzzle is why we see so many hot Neptunes elsewhere but none in our solar system.
The conventional view is that these worlds formed in cold regions far from their stars and then migrated inwards. Now Brad Hansen at the University of California, Los Angeles, and Norm Murray of the Canadian Institute for Theoretical Astrophysics in Toronto say hot Neptunes may have arisen right where they are.
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Blobrana · L

Title: Migration then assembly: Formation of Neptune mass planets inside 1~AU
Authors: Brad M. S. Hansen (UCLA), Norm Murray (CITA)

We demonstrate that the observed distribution of 'Hot Neptune'/'Super-Earth' systems is well reproduced by a model in which planet assembly occurs in situ, with no significant migration. This is achieved only if the amount of mass in rocky material is 50--100 M_{\oplus} interior to 1 AU, so significant radial migration of material is likely still required, but it must occur earlier than the final assembly stages. The model not only reproduces the general distribution of mass versus period, but also the detailed statistics of multiple planet systems in the sample.
We furthermore demonstrate that cores of this size are also likely to meet the criterion to gravitationally capture gas from the nebula, although accretion is rapidly limited by the opening of gaps in the gas disk. If the mass growth is limited by this tidal truncation, then the scenario sketched here naturally produces Neptune-mass objects with substantial components of both rock and gas, as is observed.
The quantitative expectations of this scenario are that most planets in the 'Hot Neptune/Super-Earth' class inhabit multiple-planet systems, and that they divide naturally into gas-rich (Hot Neptune) and gas-poor (Super-Earth) classes at fixed period. The dividing mass ranges from 3 M_{\oplus} at 10 day orbital periods to 10 M_{\oplus} at 100 day orbital periods. For orbital periods < 10 days, the division is less clear because a gas atmosphere may be significantly eroded by stellar radiation.

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Blobrana · L

Title: On the formation of hot Neptunes and super-Earths
Authors: D.S. McNeil, R.P. Nelson

The discovery of short-period Neptune-mass objects, now including the remarkable system HD69830 (Lovis et al. 2006) with three Neptune analogues, raises difficult questions about current formation models which may require a global treatment of the protoplanetary disc. Several formation scenarios have been proposed, where most combine the canonical oligarchic picture of core accretion with type I migration (e.g. Terquem & Papaloizou 2007) and planetary atmosphere physics (e.g. Alibert et al. 2006). To date, published studies have considered only a small number of progenitors at late times. This leaves unaddressed important questions about the global viability of the models. We seek to determine whether the most natural model -- namely, taking the canonical oligarchic picture of core accretion and introducing type I migration -- can succeed in forming objects of 10 Earth masses and more in the innermost parts of the disc. This problem is investigated using both traditional semianalytic methods for modelling oligarchic growth as well as a new parallel multi-zone N-body code designed specifically for treating planetary formation problems with large dynamic range (McNeil & Nelson 2009). We find that it is extremely difficult for oligarchic tidal migration models to reproduce the observed distribution. Even under many variations of the typical parameters, we form no objects of mass greater than 8 Earth masses. By comparison, it is relatively straightforward to form icy super-Earths. We conclude that either the initial conditions of the protoplanetary discs in short-period Neptune systems were substantially different from the standard disc models we used, or there is important physics yet to be understood.

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