An extreme test case for planet formation: a close-in Neptune orbiting an ultracool star

TL;DR

This paper reports the discovery of a Neptune-mass planet orbiting an ultracool star, challenging existing planet formation theories which predict such massive close-in planets should be very rare around low-mass stars.

Contribution

It presents the first known close-in Neptune-mass planet around an ultracool star, highlighting a significant challenge to current planet formation models.

Findings

Discovery of a Neptune-mass planet orbiting an ultracool star.

The planet has the largest known planet-to-star mass ratio among similar systems.

Current formation theories struggle to explain this system.

Abstract

In current theories of planet formation, close-orbiting planets as massive as Neptune are expected to be very rare around low-mass stars. We report the discovery of a Neptune-mass planet orbiting the `ultracool' star LHS 3154, which is nine times less massive than the Sun. The planet's orbital period is 3.7 days and its minimum mass is 13.2 Earth masses, giving it the largest known planet-to-star mass ratio among short-period planets ($<$\,100 days) orbiting ultracool stars. Both the core accretion and gravitational instability theories for planet formation struggle to account for this system. In the core-accretion scenario, in particular, the dust mass of the protoplanetary disk would need to be an order of magnitude higher than typically seen in protoplanetary disk observations of ultracool stars.