Results from Kepler and HARPS (High Accuracy Radial-Velocity Planetary Search) have shown that most Sun-like stars harbour close-in super-Earths. These planets have sizes between 2 to 5 Earth radii and orbital periods of less than 100 days, hence the term close-in super-Earths. The existence of such planets around most Sun-like stars suggests that the dominant mode of planet formation may not have occurred for our Solar System since it has no planet interior to Mercury’s 88 day orbit.
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The population of close-in super-Earths
is characterised by orbital periods ranging from days to weeks, mass ratios on
the order of 1/10,000th to 1/100,000th the mass of the parent star and nearly
circular orbits that are co-planar to within a few degrees for the known
multi-planetary systems. Such characteristics resemble the satellite systems of
our Solar System’s giant planets - Jupiter, Saturn and Uranus. This could mean
that the formation process of close-in super-Earths may be more akin to the
formation of satellite systems around the giant planets. Based on the
characteristics of close-in super-Earths around Sun-like stars, red dwarf stars
and brown dwarfs are correspondingly expected to be accompanied by close-in
super-Earths and Earths. For red dwarf stars, many of these close-in
super-Earths and Earths can be situated within the circumstellar habitable zone
where a planet with sufficient atmospheric pressure can maintain liquid water
on its surface.
Close-in super-Earths around Sun-like
stars can form in situ from circumstellar disks of solids and gas extending
interior to 0.5 AU and inward orbital migration is not required. The need for
inward orbital migration was partly motivated by the minimum mass solar nebula
(MMSN) which contains too little material inward of 0.5 AU to form close-in
super-Earths. Since close-in super-Earths are the norm and our Solar System is probably
the exception, the MMSN may not be the right approach to explain the formation
of these worlds.
Instead, a minimum mass extrasolar
nebula (MMEN) computed based on the super-Earths detected by Kepler is used to
explain the formation of close-in super-Earths. With the MMEN, there exists
sufficient material inward of 0.5 AU to form close-in super-Earths in the
observed abundance around Sun-like stars. These planets form quickly, with a
formation timescale that is orders of magnitude less than the circumstellar
disk lifetime. Close-in super-Earths are expected to remain where they form
because the largest velocity dispersion they can attain by mutual planet-planet
scattering is much less than the escape velocity from the star.
Reference:
E. Chiang and G. Laughlin (2012), “The
Minimum-Mass Extrasolar Nebula: In-Situ Formation of Close-In Super-Earths”, arXiv:1211.1673v1
[astro-ph.EP]