144 research outputs found
Impact of photoevaporative mass loss on masses and radii of water-rich sub/super-Earths
Recent progress in transit photometry opened a new window to the interior of
super-Earths. From measured radii and masses, we can infer planetary internal
compositions. It has been recently revealed that super-Earths are diverse in
composition. Such a diversity is thought to arise from diversity in volatile
content. The stability of the volatile components is to be examined, because
hot super-Earths undergo photo-evaporative mass loss. While several studies
investigated the impact of photo-evaporative mass loss on hydrogen-helium
envelopes, there are few studies as to the impact on water-vapor envelopes. To
obtain theoretical prediction to future observations, we also investigate the
relationships among masses, radii, and semimajor axes of water-rich
sub/super-Earths that have undergone photo-evaporative mass loss. We simulate
the interior structure and evolution of sub/super-Earths that consist of a
rocky core surrounded by a water envelope, including mass loss due to the
stellar XUV-driven energy-limited hydrodynamic escape. We find that the
photo-evaporative mass loss has a significant impact on the evolution of hot
sub/super-Earths. We then derive the threshold planetary mass and radius below
which the planet loses its water envelope completely as a function of the
initial water content, and find that there are minimums of the threshold mass
and radius. We constrain the domain in the parameter space of planetary mass,
radius, and semimajor axis in which sub/super-Earths never retain water
envelopes in 1-10 Gyr. This would provide an essential piece of information for
understanding the origin of close-in low-mass planets. The current
uncertainties in stellar XUV flux and its heating efficiency, however, prevent
us from deriving robust conclusions. Nevertheless, it seems to be a robust
conclusion that Kepler planet candidates contain a significant number of rocky
sub/super-Earths.Comment: 13 pages, 14 figures, accepted for publication in Astronomy &
Astrophysic
Constraints on the mass of a habitable planet with water of nebular origin
From an astrobiological point of view, special attention has been paid to the
probability of habitable planets in extrasolar systems. The purpose of this
study is to constrain a possible range of the mass of a terrestrial planet that
can get water. We focus on the process of water production through oxidation of
the atmospheric hydrogen--the nebular gas having been attracted
gravitationally--by oxide available at the planetary surface. For the water
production to work well on a planet, a sufficient amount of hydrogen and enough
high temperature to melt the planetary surface are needed. We have simulated
the structure of the atmosphere that connects with the protoplanetary nebula
for wide ranges of heat flux, opacity, and density of the nebular gas. We have
found both requirements are fulfilled for an Earth-mass planet for wide ranges
of the parameters. We have also found the surface temperature of planets of <=
0.3 Earth masses is lower than the melting temperature of silicate (~ 1500K).
On the other hand, a planet of more than several Earth masses becomes a gas
giant planet through runaway accretion of the nebular gas.Comment: 25 pages, 8 figures, to appear in the 01 September 2006 issue of Ap
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