The discovery of a Neptune-mass exoplanet orbiting the very low-mass M dwarf star LHS 3154 challenges theoretical models of planet formation, according to a new study. The planet, which has a mass at least 13 times that of Earth, tightly orbits a star 9 times less massive than the Sun, demonstrating that small stars can sometimes host larger planets than was previously thought. Planets form in the dense circumstellar discs of gas and dust that surround newborn stars. The amount of material in these structures determines how massive the planets that form within them can grow. Because the mass of dust contained in protoplanetary discs closely scales with the mass of the host star, theories of planet formation predict that red dwarf stars (M dwarfs – the least massive type of star) shouldn't host close-orbiting exoplanets with masses greater than that of Neptune (17 Earth masses). Although massive planet candidates have been detected around a few very low-mass dwarfs, they have all exhibited very wide orbital periods. Using precise radial velocity observations in the near-infrared from the Habitable-zone Planet Finder (HPF) spectrograph, Guõmundur Stefánsson and colleagues report the detection of a massive exoplanet on a close orbit around LHS 3154, a very low-mass M dwarf star. Observations of LHS 3154 revealed Doppler shifts indicating the presence of an almost Neptune-mass exoplanet with an orbital period of 3.7 days. According to the authors, current theories of planet formation, including core-accretion and gravitational instability mechanisms, struggle to explain how such a massive planet formed around LHS 3154. Stefánsson et al. also performed planet formation simulations to show that the amount of dust in the protoplanetary disc that formed the planet would need to be at least 10 times greater than is typically observed in the protoplanetary discs surrounding low-mass stars. In a related Perspective, Frédéric Masset discusses how the LHS 3154 system challenges planet formation theory.
For reporters interested in trends, a September 2019 study by Morales et al. presented the detection of GJ3512b, another high-mass planet orbiting a very low-mass M dwarf star, but with a much longer orbital period.
