Life after death: Astronomers discover a mysterious planet that shouldn’t exist

Death-defying planet illustration

The Jupiter-like planet Halla survived the expansion of its sun Baekdus into a red giant, a process that should have engulfed it, according to astronomers at the University of Hawaii. The surprising survival suggests theories of planetary evolution, including potential origins of binary stars or Halla which is a newly formed second generation planet. Credit: WM Keck Observatory/Adam Makarenko

Astronomers at the University of Hawaii’s Institute of Astronomy have discovered a planet that survived what should have been a cataclysmic event caused by its sun.

When our Sun reaches the end of its life, it will expand to 100 times its current size, enveloping the Earth. Many planets in other solar systems face a similar fate as their host stars age. But not all hope is lost, as astronomers from the University ofHawaiiInstitute of Astronomy (If a) made the remarkable discovery of a planet’s survival after what should have been certain death at the hands of its sun. The study was published June 28 in the journal Nature.

THE[{” attribute=””>Jupiter-like planet 8UMib, officially named Halla, orbits the red giant star Baekdu (8UMi) at only half the distance separating the Earth and the Sun. Using two Maunakea Observatories onHawaiiIslandW. M. Keck Observatory andCanada-France-HawaiiTelescope (CFHT)a team of astronomers led by Marc Hon, aNASAHubble FellowatIfA, discovered that Halla persists despite the normally perilous evolution of Baekdu. Using observations of Baekdus stellar oscillations fromNASAsTransiting Exoplanet Survey Satellite, they found that the star is burning helium in its core, signaling that it had already expanded enormously into a red giant star once before.

Planet Survives Star

The planet Halla may have once orbited two stars that interacted with one another by mass transfer as depicted. The eventual merger between the stars allowed Halla to escape engulfment and persist around a helium-burning giant star. Credit: W. M. Keck Observatory/Adam Makarenko

The star would have inflated up to 1.5 times the planets orbital distanceengulfing the planet in the processbefore shrinking to its current size at only one-tenth of that distance.

Planetary engulfment has catastrophic consequences for either the planet or the star itselfor both, said Hon, the lead author of the study. The fact that Halla has managed to persist in the immediate vicinity of a giant star that would have otherwise engulfed it highlights the planet as an extraordinary survivor.

Maunakea observatories confirm survivor

The planet Halla was discovered in 2015 by a team of astronomers from Korea using the radial velocity method, which measures the periodic movement of a star due to the gravitational tug of the orbiting planet. Following the discovery that the star must at one time have been larger than the planets orbit, theIfAteam conducted additional observations from 2021 to 2022 using Keck Observatorys High-Resolution Echelle Spectrometer andCFHTs ESPaDOnS instrument. These new data confirmed the planets 93-day, nearly circular orbit had remained stable for more than a decade and that the back-and-forth motion must be due to a planet.

Death Defying Planet

Portrayed is the violent merger between two stars that may have formed the helium-burning giant star Baekdu. The merger debris forms a disk from which the planet Halla formed, enabling the planets unlikely survival around the star. Credit: W. M. Keck Observatory/Adam Makarenko

Together, these observations confirmed the existence of the planet, leaving us with the compelling question of how the planet actually survived, saidIfAastronomer Daniel Huber, second author of the study. The observations from multiple telescopes on Maunakea were critical in this process.

Escaping engulfment

At a distance of 0.46 astronomical units (AU, or the Earth-Sun distance) to its star, the planet Halla resembles warm or hot Jupiter-like planets that are thought to have started on larger orbits before migrating inward close to their stars. However, in the face of a rapidly evolving host star, such an origin becomes an extremely unlikely survival pathway for planet Halla.

Another theory for the planets survival is that it never faced the danger of engulfment. Similar to the famous planet Tatooine from Star Wars, which orbits two suns, the host star Baekdu may have originally been two stars, according to the team. A merger of these two stars may have prevented any one of them from expanding sufficiently large enough to engulf the planet.

A third possibility is that Halla is a relative newbornthat the violent collision between the two stars produced a gas cloud from which the planet formed. In other words, the planet Halla may be a recently born second generation planet.

Most stars are in binary systems, but we dont yet fully grasp how planets may form around them, said Hon. Therefore, its plausible that more planets may actually exist around highly evolved stars thanks to binary interactions.

Reference: A close-in giant planet escapes engulfment by its star by Marc Hon, Daniel Huber, Nicholas Z. Rui, Jim Fuller, Dimitri Veras, James S. Kuszlewicz, Oleg Kochukhov, Amalie Stokholm, Jakob Lysgaard Rrsted, Mutlu Yldz, Zeynep elik Orhan, Sibel rtel, Chen Jiang, Daniel R. Hey, Howard Isaacson, Jingwen Zhang, Mathieu Vrard, Keivan G. Stassun, Benjamin J. Shappee, Jamie Tayar, Zachary R. Claytor, Corey Beard, Timothy R. Bedding, Casey Brinkman, Tiago L. Campante, William J. Chaplin, Ashley Chontos, Steven Giacalone, Rae Holcomb, Andrew W. Howard, Jack Lubin, Mason MacDougall, Benjamin T. Montet, Joseph M. A. Murphy, Joel Ong, Daria Pidhorodetska, Alex S. Polanski, Malena Rice, Dennis Stello, Dakotah Tyler, Judah Van Zandt and Lauren M. Weiss, 28 June 2023, Nature.
DOI: 10.1038/s41586-023-06029-0


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