Astronomers are calling into question everything they thought they knew about planet formation after making an unexpected discovery – an exoplanet that shouldn’t exist. The planet, named LHS 3154b, is about the mass of Neptune and more than 13 times more massive than Earth. It was found orbiting an ultracool M dwarf star called LHS 3154, which is nine times less massive than our Sun. According to a new study published in the journal Science, the planet completes one orbit around the star every 3.7 Earth days, making it the highest-mass planet known to orbit one of the coldest and lowest mass stars in the universe.
This discovery challenges the current understanding of planet formation, as the amount of material present in the disks around stars determines the mass of the planets that form around them. Small, M dwarf stars are typically orbited by small, rocky planets, rather than gas giant planets. Therefore, the planet-forming disk around the low-mass star LHS 3154 was not expected to have enough mass to form a planet like LHS 3154b.
The researchers estimate that the amount of dust in the planet-forming disk around the star would have to be at least 10 times greater than that usually found in disks around low-mass stars. This finding provides an extreme test case for all existing planet formation theories, prompting scientists to re-examine their understanding of how planets and stars form.
The planet was discovered 51 light years from the Sun, using the Habitable Zone Planet Finder (HPF) installed on the Hobby-Eberly telescope at the McDonald Observatory in Texas. The HPF was designed to detect planets orbiting within the habitable zone of small, cool stars, and it was able to detect the wobble created by the gravitational pull between the planet and the ultracool star.
The discovery of the LHS 3154b will help scientists better understand how the most common stars in the galaxy form planets, and to find those planets. Despite being a rare find, this discovery will prove invaluable in advancing planetary science.