Astronomers find a planet that shouldn't exist

When the Sun reaches the end of its life in five billion years, it will expand 100 times its current size, likely enveloping the Earth in hot plasma and destroying it. Many planets in other solar systems face a similar doom once their host stars grow old — but an international team of astronomers has now discovered a planet that appears to have defied the odds and survived.

The Jupiter-like gas planet 8 UMi b, named Halla, closely orbits the giant star Baekdu in the constellation Ursa Minor. Using observations of Baekdu’s stellar oscillations from NASA’s Transiting Exoplanet Survey Satellite (TESS), the researchers found that the star is burning helium, rather than hydrogen, in its core. This signals that it had once expanded into a red giant; a period of violent stellar transition which should have destroyed Halla, the researchers explained in the journal Nature.

“Engulfment by a star normally has catastrophic consequences for close-orbiting planets,” said second author Dr Dan Huber, from The University of Sydney. “When we realised that Halla had managed to survive in the immediate vicinity of its giant star, it was a complete surprise.

“As it exhausted its core hydrogen fuel, the star would have inflated up to 1.5 times the planet’s current orbital distance — engulfing it completely in the process — before shrinking to its current size.”

Halla was discovered in 2015 by a team of Korean astronomers using the radial velocity method, which measures the periodic gravitational tug of the orbiting planet on its star. Following the discovery that the star must at one time have been larger than the planet’s current orbit, the team conducted additional observations from 2021 to 2022 using the W. M. Keck Observatory and the Canada-France-Hawaii Telescope. These new data confirmed that the planet’s 93-day, near-circular orbit had remained stable for more than a decade and that the radial velocity changes observed in the star must be due to this orbiting planet.

So how could Halla have escaped engulfment? At a distance of 0.46 astronomical units (equivalent to almost half the distance from Earth to the Sun) to its star, it resembles ‘warm’ or ‘hot’ Jupiter-like planets, so-called due to their proximity to stars. These gas giants are thought to have started their lives in orbits much further out from their stellar hosts before migrating inward. However, in the face of a rapidly evolving host star, such an origin story becomes an unlikely survival pathway for Halla.

“We just don’t think Halla could have survived being absorbed by an expanding red giant star,” Huber said. A more plausible theory for the planet’s survival is that it never faced the danger of engulfment.

“The system was more likely similar to the famous fictional planet Tatooine from Star Wars, which orbits two suns,” said co-author Professor Tim Bedding, also from The University of Sydney. Unlike our Solar System, most stars exist in binary pairs, but we don’t yet fully grasp how planets form around them.

“If the Baekdu system originally consisted of two stars, their merger could have prevented any one of them from expanding sufficiently to engulf the planet,” he said. This is because the two stars would have ‘fed’ off each other during the transition from hydrogen burning stars to what Baekdu is now, a helium burning red giant star.

A third possibility is that Halla is a newborn planet: that 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 ‘second-generation’ planet in the star system.

As the first known close-in planet around a core-helium burning star, Halla shows that exoplanet discoveries might still surprise us by appearing where they are least expected. Working together, stellar and planetary scientists will be able to use this new discovery to better understand whether death-by-star is a fate shared by all such close-in planets.

Image caption: A possible violent merger between two stars could have formed the helium-burning giant star Baekdu, enabling the planet’s unlikely survival around the star. Image credit: W. M. Keck Observatory/Adam Makarenko.