A close-in giant planet escapes engulfment by its star
Marc Hon, Daniel Huber, Nicholas Z. Rui, Jim Fuller, Dimitri Veras, James S. Kuszlewicz, O. Kochukhov, Amalie Stokholm, Jakob Rørsted Mosumgaard, Mutlu Yıldız, Zeynep Çelik Orhan, Sibel Örtel, Chen Jiang, Daniel Hey, Howard Isaacson, Jingwen Zhang, Mathieu Vrard, Keivan G. Stassun, B. J. Shappee, Jamie Tayar, Zachary R. Claytor, Corey Beard, T. R. Bedding, Casey L. Brinkman, T. L. Campante, W. J. Chaplin, Ashley Chontos, Steven Giacalone, Rae Holcomb, Andrew W. Howard, Jack Lubin, Mason G. MacDougall, Benjamin T. Montet, Joseph M. Akana Murphy, J. M. Joel Ong, Daria Pidhorodetska, Alex S. Polanski, Malena Rice, Dennis Stello, Dakotah Tyler, Judah Van Zandt, Lauren M. Weiss
Abstract
When main-sequence stars expand into red giants, they are expected to engulf close-in planets1–5. Until now, the absence of planets with short orbital periods around post-expansion, core-helium-burning red giants6–8 has been interpreted as evidence that short-period planets around Sun-like stars do not survive the giant expansion phase of their host stars9. Here we present the discovery that the giant planet 8 Ursae Minoris b10 orbits a core-helium-burning red giant. At a distance of only 0.5 au from its host star, the planet would have been engulfed by its host star, which is predicted by standard single-star evolution to have previously expanded to a radius of 0.7 au. Given the brief lifetime of helium-burning giants, the nearly circular orbit of the planet is challenging to reconcile with scenarios in which the planet survives by having a distant orbit initially. Instead, the planet may have avoided engulfment through a stellar merger that either altered the evolution of the host star or produced 8 Ursae Minoris b as a second-generation planet11. This system shows that core-helium-burning red giants can harbour close planets and provides evidence for the role of non-canonical stellar evolution in the extended survival of late-stage exoplanetary systems. The giant planet 8 Ursae Minoris b seems to have avoided engulfment by its giant host star through a stellar merger that either affected the evolution of the host star or produced 8 Ursae Minoris b as a second-generation planet.