The Jupiter-like planet Hala survived the expansion of its sun Baekdu into a red giant, a process that should have engulfed it, according to astronomers at the University of Hawaii. The surprising survival has prompted theories about planetary evolution, including a potential binary star origin or that Halla is a newly formed “second-generation” planet. Courtesy: WM Keck Observatory/Adam Makarenko
Astronomers at the University of Hawaii’s Institute for Astronomy have discovered a planet that survived a catastrophic 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 similar doom as their host stars age. But not all hope is lost, as astronomers from the University of Hawaii Institute of Astronomy (If) made the remarkable discovery of the survival of a planet after what should have been certain destruction at the hands of its sun. The study was published June 28 in the journal Nature.
The Jupiter-as planet 8 UMi b, officially named Halla, orbits the red giant star Baekdu (8 UMi) only half the distance separating the Earth and the Sun. Using two observatories on Maunakea Hawaii Island—WM Keck Observatory and Canada-France-Hawaii telescope (CFHT)—a team of astronomers led by Mark Hohn, a NASA Hubble Fellow in If, discovered that Hala continues to exist despite Baekdu’s usually dangerous evolution. Using observations of Baekdu’s stellar oscillations by NASAOn the Transiting Exoplanet Survey Satellite, they found that the star was burning helium in its core, signaling that it had already massively expanded into a red giant star once before.
The planet Halla may have once orbited two stars that interacted with each other through mass transfer, as shown. The eventual merger between the stars allowed the Halla to avoid being swallowed up and continue to exist around a giant helium-burning star. Courtesy: WM Keck Observatory/Adam Makarenko
The star would have inflated to 1.5 times the planet’s orbital distance—swallowing the planet in the process—before shrinking to its current size just a tenth of that distance.
“A planetary eclipse has catastrophic consequences for either the planet or the star itself — or both,” said Hohn, the study’s lead author. “The fact that Halla managed to stay in close proximity to a giant star that would otherwise have engulfed it highlights the planet as an exceptional survivor.”
Maunakea observatories confirm survivors
The planet Halla was discovered in 2015 by a team of astronomers from Korea using the radial velocity method, which measures the periodic motion of a star due to the gravitational pull of the orbiting planet. After the discovery that the star must once have been larger than the planet’s orbit, If the team conducted additional observations from 2021 to 2022 using Keck Observatory’s high-resolution Echelle spectrometer and CFHTThe ESPaDOnS tool. This new data confirmed that the planet’s 93-day, nearly circular orbit has remained stable for more than a decade, and that the back-and-forth movement must be due to a planet.
Depicted is the violent merger between two stars that may have formed the giant helium-burning star Baekdu. The debris from the merger formed a disk from which the planet Hala formed, allowing for the unlikely survival of the planet around the star. Courtesy: WM Keck Observatory/Adam Makarenko
“Together, these observations confirmed the existence of the planet, leaving us with the pressing question of how the planet actually survived,” said If astronomer Daniel Huber, second author of the study. “Observations from Maunakea’s multiple telescopes were critical in this process.”
Escape from engulfment
At a distance of 0.46 AU (AU, or the Earth-Sun distance) to its star, the planet Hala resembles “warm” or “hot” Jupiter-like planets that are thought to have gone on larger orbits before migrating inward near their stars . However, in the face of a rapidly evolving host star, such an origin becomes an extremely unlikely path for the planet Hala to survive.
Another theory for the planet’s survival is that it never faced the danger of being engulfed. Like the famous Star Wars planet Tatooine, which orbits two suns, the lead star Baekdu may have originally been two stars, according to the team. The merger of these two stars may have prevented either of them from expanding enough to engulf the planet.
A third possibility is that Halla is relatively newborn—that a violent collision between the two stars produced a cloud of gas from which the planet formed. In other words, the planet Hala may be a recently born “second generation” planet.
“Most stars are in binary systems, but we still don’t fully understand how planets can form around them,” Hohn said. “Therefore, it is plausible that more planets exist around highly evolved stars due to binary interactions.”
Reference: A nearby giant planet avoids being swallowed by its star by Mark Hohn, Daniel Huber, Nicholas Z. Rui, Jim Fuller, Dimitri Veras, James S. Kushlevich, Oleg Kochukhov, Amalie Stockholm, Jakob Lisgaard Rørstedt, Mutlu Yildiz, Zeynep Çelik Orhan, Sibel Örtel, 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 , Thiago L. Campante, William J. Chaplin, Ashley Chontos, Stephen Giacalone, Ray Holcomb, Andrew W. Howard, Jack Lubin, Mason McDougall, Benjamin T. Monte, Joseph MA Murphy, Joel Ong, Daria Pidgorodetska, Alex S. Polanski, Malena Rice, Dennis Stello , Dakota Tyler, Judah Van Zandt, and Lauren M. Weiss, 28 Jun 2023, Nature.
DOI: 10.1038/s41586-023-06029-0