A PLANET TOO BIG FOR ITS STAR

In the annals of planet hunting, LHS 3154 b is a head-scratcher: a giant exoplanet tightly orbiting a star so tiny, it’s hard to understand how the star could have birthed it. Reported Nov. 30 in Science, the planet tips the scales at 13.2 times the mass of Earth or more, putting it roughly on par with Neptune’s 17.1 Earth masses. Yet, its host star has just 11 percent the mass of the Sun.

“Previously, it was just thought that, ‘Oh, no, there’s no way the lowest-mass stars can actually form this type of planet,’” says Guðmundur Stefánsson, an astrophysicist at Princeton University in New Jersey and the paper’s first author.

Stefánsson and his colleagues found the planet with the 10-meter Hobby-Eberly Telescope at McDonald Observatory in Texas by detecting a periodic shift every 3.7 days in the spectrum of light coming from the red dwarf LHS 3154. Follow-up analysis of data from NASA’s Transiting Exoplanet Survey Satellite (TESS) and the European Space Agency’s Gaia satellite confirmed that the tug of a close-in, unseen planet was causing the star to wobble.

LHS 3154 b weighs in at 0.35 percent of the mass of its host star. Among planets with orbits less than 10 days, that’s more than twice the ratio of any other known system. (For comparison, Jupiter is less than 0.1 percent the Sun’s mass.)

Because planets form alongside their stars from the same disk of material, astronomers have long assumed that low-mass stars also tend to form lowmass planets in their protoplanetary disks, as solid bits and pieces clump together. This left the team at a loss to explain LHS 3154 b with computer models of planet formation. “We were really struggling — like, we said, ‘OK, how can we actually form this type of planet?’ ” says Stefánsson.

But by increasing the amount of dust in the protoplanetary disk by a factor of 10 compared to observations, the simulations could produce close-in planets with the girth of LHS 3154 b. This suggests that there is more material available to nascent planetary systems than astronomers have suspected.

PENN STATE UNIVERSITY PLANETARY MACROSOMIA. The light of the red dwarf LHS 3154 scatters off the Neptune-sized planet LHS 3154 b in this artist’s concept.