Presentation #212.08 in the session “Giant Planets, Exoplanets and Systems”.
The Kepler Space Telescope revolutionized the search for extrasolar planets by discovering thousands of planets with sizes ranging from a fraction of an Earth radius to several Jupiter radii. However, a vast majority of discovered planets orbit stars with masses less than 1.4 times that of the Sun. Kepler focused on these low-mass stars because they are small and slow-rotating, which makes it easier to detect and characterize their transiting planets. By monitoring bright stars across nearly the entire sky, the Transiting Exoplanet Survey Satellite (TESS) allows us to search for transiting planets around a significantly higher number of more massive stars, such as A-type stars. Only a few planets have been found around this type of star, all of which are larger than Jupiter. Uncovering smaller planets, such as those the size of Neptune, would shed new light on how planets form and evolve around hotter, more massive stars.
Reaching masses and effective temperatures up to twice those of the Sun, A-type stars subject close-in planets to vastly different environments than their low-mass counterparts, and are therefore an untapped testbed for planet evolution theories. For instance, A-type stars lack a convective interior and therefore lack the ability to spin down via magnetic braking, causing them to rotate significantly faster than the typical planet-hosting star. Because many planet formation and evolution theories are intimately tied to the rotation rate of the host star, this property should lead to discernible features in the planet occurrence rate. In addition, because these hot stars have blackbody-like spectra that extend into the near ultraviolet, they bombard their planets with high-enery photons that can erode their atmospheres via photoevaporation. Photoevaporation is believed to occur around lower mass stars only during the first 100 million years of their lifetimes, but would persist for the entire 1 billion year main sequence lifetime of an A-type star. This would have a profound impact on the occurrence rate of close-in planets, especially Neptune-size planets whose atmospheres are more weakly gravitationally bound than those of Jupiter-size planets.
We discuss the ongoing search for Neptune-size planets around A-type stars with TESS as well as the challenges associated with validating planet candidates of this type. In addition, we detail how we plan to calculate the occurrence rate of this class of planet and interpret our results in the context of planet formation and evolution.