PLANETARY POPULATIONS IN THE MASS-PERIOD DIAGRAM: A STATISTICAL TREATMENT OF EXOPLANET FORMATION AND THE ROLE OF PLANET TRAPS
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abstract
The rapid growth in the number of known exoplanets has revealed the existence
of several distinct planetary populations in the observed mass-period diagram.
Two of the most surprising are, (1) the concentration of gas giants around 1AU
and (2) the accumulation of a large number of low-mass planets with tight
orbits, also known as super-Earths and hot Neptunes. We have recently shown
that protoplanetary disks have multiple planet traps that are characterized by
orbital radii in the disks and halt rapid type I planetary migration. By
coupling planet traps with the standard core accretion scenario, we showed that
one can account for the positions of planets in the mass-period diagram. In
this paper, we demonstrate quantitatively that most gas giants formed at planet
traps tend to end up around 1 AU with most of these being contributed by dead
zones and ice lines. In addition, we show that a large fraction of super-Earths
and hot Neptunes are formed as "failed" cores of gas giants - this population
being constituted by comparable contributions from dead zone and heat
transition traps. Our results are based on the evolution of forming planets in
an ensemble of disks where we vary only the lifetimes of disks as well as their
mass accretion rates onto the host star. We show that a statistical treatment
of the evolution of a large population of planetary cores initially caught in
planet traps accounts for the existence of three distinct exoplantary
populations - the hot Jupiters, the more massive planets at roughly orbital
radii around 1 AU orbital, and the short period SuperEarths and hot Neptunes.
There are very few evolutionary tracks that feed into the large orbital radii
characteristic of the imaged Jovian planet, which agrees with recent surveys.
Finally, we find that low-mass planets in tight orbits become the dominant
planetary population for low mass stars ($M_* \le 0.7 M_{\odot}$).
