The discovery of an enormous planet orbiting a dying star, known as 8 Ursae Minoris b, is shaking up our understanding of how planets form and the limits of stellar destruction. Astronomers have been surprised to find the planet in a stable, nearly circular orbit around the star, as it had been expected that the red giant would have expanded beyond the planet’s orbit before shrinking to its current size. This means that any planets orbiting closely would have been engulfed and ripped apart. This discovery relied on precise measurements from NASA’s Transiting Exoplanet Survey Satellite (TESS), and suggests that planet formation and destruction processes are more complex and unpredictable than previously thought.
As stars like our Sun approach the end of their lives, they exhaust their nuclear fuel and become red giants, expanding to their maximum size. In this case, the star would have grown to 0.7 AU – three-quarters the distance from Earth to the Sun. It would have destroyed any nearby planets, however planet b, a large gaseous world, sits at 0.5 AU, making it impossible for it to have survived engulfment. Lead author Marc Hon proposed two other possibilities for planet b: that it is the survivor of a star merger, or it is a newly formed planet out of the debris from the merger. This suggests that star mergers may be more common than previously thought, and opens up an interesting new avenue for research on planetary formation and evolution.
The first scenario begins with two stars that are similar to our Sun in close orbit around each other, with a planet orbiting them both. One of the stars evolves faster than the other, going through its red giant phase and turning into a white dwarf. The other star just reaches the red giant stage before they collide, resulting in the red giant we see today. This violent merger stops the red giant from expanding further, thus sparing the orbiting planet from destruction. In the second scenario, the merger ejects an abundance of dust and gas that forms a disk around the remaining red giant. This protoplanetary disk provides the material for a new planet to form. It is almost like having a second life for a planetary system, albeit one near its end.
Astronomers are able to infer chaotic series of events from present day observations by understanding the stellar physics. Thanks to the space telescope TESS, astronomers are able to monitor jitters and quakes on distant stars. This phenomenon is known as “asteroseismology” and by studying the oscillations on 8 Ursae Minoris, the discovery team found that it reflects a late helium-burning stage rather than a star that is still expanding and burning hydrogen. This suggests that the crisis of the star has already occurred, yet the planet still somehow continues to exist. In order to understand how this is possible, astronomers continue to study the stellar physics in order to gain insight into this extraordinary phenomenon.