The Signature of the Ice Line and Modest Type I Migration in the Observed Exoplanet Mass-Semimajor Axis Distribution

TL;DR

This paper examines how the ice line influences planet formation and migration, showing that reduced Type I migration rates and ice line effects better match observed exoplanet distributions, predicting observable features for future surveys.

Contribution

It demonstrates that incorporating ice line effects and suppressed Type I migration rates aligns models with observed exoplanet distributions, highlighting the importance of these factors in planet formation theories.

Findings

Observed semimajor axis distribution is inconsistent with full linear theory of Type I migration.

Suppressed Type I migration and ice line effects produce models compatible with observations.

Predicted features include a short-period rocky planet population and a mass-semimajor axis 'desert'.

Abstract

Existing exoplanet radial velocity surveys are complete in the planetary mass-semimajor axis (Mp-a) plane over the range 0.1 AU < a < 2.0 AU where Mp >~ 100 M_Earth. We marginalize over mass in this complete domain of parameter space and demonstrate that the observed semimajor axis distribution is inconsistent with models of planet formation that use the full Type I migration rate derived from a linear theory and that do not include the effect of the ice line on the disk surface density profile. However, the efficiency of Type I migration can be suppressed by both nonlinear feedback and the barriers introduced by local maxima in the disk pressure distribution, and we confirm that the synthesized Mp-a distribution is compatible with the observed data if we account for both retention of protoplanetary embryos near the ice line and an order-of-magnitude reduction in the efficiency of Type I migration. The validity of these assumption can be checked because they also predict a population of short-period rocky planets with a range of masses comparable to that of the Earth as well as a "desert" in the Mp-a distribution centered around Mp ~ 30-50 M_Earth and a < 1 AU. We show that the expected "desert" in the Mp-a plane will be discernible by a radial velocity survey with 1 m/s precision and n ~ 700 radial velocity observations of program stars.