An extreme test case for planet formation: a close-in Neptune orbiting a very low mass star

Theories of planet formation predict planets as massive as Neptune to be rare around very low-mass stars. Using near-infrared radial velocity observations, we report the discovery of a Neptune-mass planet orbiting the nearby very low mass star LHS 3154 which has a mass of 0.11 Solar masses. The planet’s orbital period is 3.7 days and its minimum mass is 13.2 M_Earth, giving it the largest known planet-to-star mass ratio among short-period planets orbiting very low mass stars. Both the core accretion and gravitational instability theories of planet formation struggle to account for the system. In this talk, we place the surprisingly massive planet in context of other known planetary systems orbiting very low mass stars, and we present planet formation simulations showing that planets such as LHS 3154b are only formed if the protoplanetary dust disk mass is substantially higher than typically observed around very low mass stars.