59 research outputs found
Super-Earth Atmospheres: Self-Consistent Gas Accretion and Retention
Some recently discovered short-period Earth to Neptune sized exoplanets
(super Earths) have low observed mean densities which can only be explained by
voluminous gaseous atmospheres. Here, we study the conditions allowing the
accretion and retention of such atmospheres. We self-consistently couple the
nebular gas accretion onto rocky cores and the subsequent evolution of gas
envelopes following the dispersal of the protoplanetary disk. Specifically, we
address mass-loss due to both photo-evaporation and cooling of the planet. We
find that planets shed their outer layers (dozens of percents in mass)
following the disk's dispersal (even without photo-evaporation), and their
atmospheres shrink in a few Myr to a thickness comparable to the radius of the
underlying rocky core. At this stage, atmospheres containing less particles
than the core (equivalently, lighter than a few % of the planet's mass) can be
blown away by heat coming from the cooling core, while heavier atmospheres cool
and contract on a timescale of Gyr at most. By relating the mass-loss timescale
to the accretion time, we analytically identify a Goldilocks region in the
mass-temperature plane in which low-density super Earths can be found: planets
have to be massive and cold enough to accrete and retain their atmospheres,
while not too massive or cold, such that they do not enter runaway accretion
and become gas giants (Jupiters). We compare our results to the observed
super-Earth population and find that low-density planets are indeed
concentrated in the theoretically allowed region. Our analytical and intuitive
model can be used to investigate possible super-Earth formation scenarios.Comment: Updated (refereed) versio
Blackbody Radiation From Isolated Neptunes
Recent analyses of the orbits of some Kuiper Belt objects hypothesize the presence of an undiscovered Neptune-size planet at a very large separation from the Sun. The energy budget of Neptunes on such distant orbits is dominated by the internal heat released by their cooling rather than solar irradiation (making them effectively "isolated"). The blackbody radiation that these planets emit as they cool may provide the means for their detection. Here we use an analytical toy model to study the cooling and radiation of isolated Neptunes. This model can translate a detection (or a null detection) to a constraint on the size and composition of the hypothesized "Planet Nine". Specifically, the thick gas atmosphere of Neptune-like planets serves as an insulating blanket which slows down their cooling. Therefore, a measurement of the blackbody temperature, Teff∼50K, at which a Neptune emits can be used to estimate the mass of its atmosphere, Matm. Explicitly, we find the relation Teff∝M1/12atm. Despite this weak relation, a measurement of the flux at the Wien tail can constrain the atmospheric mass, at least to within a factor of a few, and provide useful limits to possible formation scenarios of these planets. Finally, we constrain the size and composition of Planet Nine by combining our model with the null results of recent all-sky surveys.Astronom
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