(Nanowerk News) When our Sun reaches the end of its life in five billion years, it will expand to 100 times its current size, most likely enveloping the Earth in hot plasma, destroying our rocky home. Many planets in other solar systems face a similar catastrophe as their host star ages.
But hope is not lost for all the planets. An international team including University of Sydney astronomers has made a remarkable discovery about the survival of a planet after what should have died at the hands of its sun, a star in the ‘Little Bear’ constellation Ursa Minor.
The Jupiter-like gas planet 8 UMi b, named Halla, orbits the giant star Baekdu. In a first for astronomy, a team – including Dr Dan Huber and Professor Tim Bedding from the University of Sydney – have discovered that Halla survived a violent stellar transition period that would normally destroy adjacent planets.
Their findings are published in Natural (“An approaching giant planet escapes engulfment by its star”).
Using observations of the Baekdu star’s oscillation from NASA’s Transiting Exoplanet Survey Satellite (TESS), the scientific team found that the star is burning helium, not hydrogen, in its core, indicating that it once expanded massively to become a red giant star.
“Sinking by a star usually has disastrous consequences for closely orbiting planets. When we realized that Halla had managed to survive around its giant star it was a real shock,” said Dr Huber, author of both studies and Australian Research Council Future Fellow in the School of Physics and Sydney Institute for Astronomy. .
“When its core hydrogen fuel is exhausted, the star will expand to 1.5 times the planet’s current orbit distance – swallowing it completely in the process – before shrinking to its current size.”
Baekdu is one of the stars in the constellation ‘Little Bear’, or Ursa Minor. It is almost 11 times the radius of our Sun, with 1.6 times its mass.
The Mauna Kea Telescope confirmed survivors
Planet Halla was discovered in 2015 by a team of Korean astronomers using the radial velocity method, which measures the periodic gravitational pull of the planet orbiting its star, which is about 520 light years from Earth.
Following the discovery that the star was once larger than the planet’s current orbit, the team made additional observations from 2021 to 2022 using the WM Keck Observatory and the Canada-France-Hawaii Telescope on Mauna Kea, Hawaii.
The new data confirms that the planet’s nearly circular 93-day orbit has remained stable for more than a decade and that the changes in the radial velocity observed in the star must be caused by the planet orbiting it.
“Together, these observations confirm Halla’s existence, making us wonder how this planet survived,” said lead author, Dr Marc Hon of the University of Hawaii. “Observations from several telescopes on Maunakea are very important in this process.”
Escape from engulfment
At a distance of 0.46 astronomical units (equivalent to nearly half the distance from Earth to the Sun) to its star, the planet Halla resembles a ‘warm’ or ‘hot’ Jupiter-like planet, so called because of its proximity to the star. These gas giants are thought to begin their lives in orbits much more distant from their stellar hosts before migrating inward.
However, in the face of a thriving host star, such an origin story made the planet Halla an extremely unlikely path of survival.
“We just didn’t think Halla could survive being absorbed by an expanding red giant star,” said Dr Huber.
A more plausible theory for the planet’s survival is that it never faced any danger of being engulfed.
“The system is more akin to the famous fictional planet Tatooine from Star Wars, which orbits two suns,” Professor Bedding said.
“If the Baekdu system originally consisted of two stars, their merging could prevent one of them from expanding enough to engulf the planet.”
This is because the two stars would ‘feed’ each other during the transition from a hydrogen-burning star to today’s Baekdu, a helium-burning red giant.
A third possibility is that Halla was a nascent planet: that a violent collision between the two stars produced a cloud of gas that formed the planet. In other words, the planet Halla may be a ‘second generation’ planet in the star system.
Unlike our Solar System, most stars are binary pairs, but we don’t yet fully understand how planets form around them. Therefore, it is plausible that more planets might exist around highly evolved stars thanks to such binary interactions.
As the first known close planet around a star burning a helium core, Halla points out that exoplanet discoveries may still surprise us by appearing around stars where we least expect them.
Many of the thousands of identified exoplanets are in orbits very close to their parent stars. Working together, star and planetary scientists will be able to use these new discoveries about Halla to better understand whether death for a star is the destiny shared by all adjacent planets.