[PLS] Zoë de Beurs (MIT)

“CALM Your Stars: Revealing Small Planets and Measuring Their Masses”

Understanding the diversity of small exoplanets requires determining their compositions, explaining how they form and evolve, and measuring how common they are. Planet masses, combined with radii, help distinguish predominantly rocky worlds from planets with substantial gaseous envelopes. Radial velocity (RV) observations constrain these masses by tracking the small stellar motions induced by orbiting planets. However, starspots, bright magnetic regions, and convection at the stellar surface can hide or mimic planetary signals, limiting both the characterization of individual worlds and our census of planetary systems.

By accounting for this stellar variability, we have enabled precise mass measurements of small planets, including K2-167 b, a sub-Neptune at the upper edge of the radius valley—the relative scarcity of planets with sizes between super-Earths and sub-Neptunes. Together with its radius, this mass constrains the planet’s bulk composition and helps test models for how these two populations form and evolve. Across our previous applications, our approaches reduced RV variability by 41% for the Sun and by up to 60% for more active stars, improving our ability to recover the weak signals of small planets.

We now extend this work to a large-scale planet search using my CALM periodogram method, which searches for repeating planetary signals while simultaneously modeling stellar variability traced by changes in spectral line shapes. We applied CALM to hundreds of stars observed with the HARPS spectrograph, ranging from stars hotter than the Sun to cool red dwarfs, and identified several new planet candidates with RV semi-amplitudes as small as 30 cm/s. Confirming and characterizing these candidates would expand the sample of low-mass planets available for testing planet formation and evolution models. More broadly, this work supports efforts to build a more complete census of small planets, determine the occurrence of terrestrial worlds, and ultimately characterize Earth-mass planets in habitable zones. The accompanying models of stellar rotation and magnetic cycles also provide a foundation for investigating how convection contributes to stellar RV variability.

Planetary Lunch Seminar —

Colloquia topics span the range of research interests of the department’s planetary sciences research program, and the talks are intended to appeal to any graduate students, postdocs, research scientists, and faculty with a background in planetary science. Speakers include members of the MIT community and visitors.

Contact: planetary-org@mit.edu