A Planet Survives the Death of Its Star

In a groundbreaking discovery, astronomers have found a large planet, named Planet 8 Ursae Minoris b, orbiting a dying star against all odds. Situated approximately 530 light-years away, this star, in its red giant phase, should have expanded beyond the planet’s orbit and destroyed it. However, the planet remains in a stable and nearly circular orbit, defying expectations. This astonishing finding, made possible by NASA’s Transiting Exoplanet Survey Satellite (TESS), suggests that the process of planet formation and destruction is far more complex and unpredictable than previously believed.

As stars like our Sun near the end of their lives, they transform into red giants as they deplete their nuclear fuel. In this case, the star would have expanded to 0.7 astronomical units, about three-quarters of the distance from Earth to the Sun. Such expansion would have engulfed and torn apart any nearby planets. Surprisingly, Planet B, a gaseous world, orbits at around 0.5 astronomical units, indicating that it could not have survived engulfment. Marc Hon, the lead author of the study, proposes two alternative explanations for its existence: either the planet is the result of a merger between two stars or it formed from the debris left behind by that merger.

In the first scenario, two stars similar in size to our Sun orbit each other closely, with the planet orbiting both. One star evolves faster and becomes a white dwarf after going through its red giant phase. The other star reaches the red giant stage before the collision occurs, resulting in the red giant we observe today. This merger halts the red giant’s expansion, sparing the planet from destruction. In the second scenario, the violent merger of the two stars ejects dust and gas, forming a disk around the remaining red giant. This disk acts as a breeding ground for a new planet to form. It represents a late-stage second life for a planetary system, even as the star approaches its end.

Astronomers infer these complex events from present-day observations by leveraging their understanding of stellar physics. TESS, known for its planet-hunting capabilities, can also observe the oscillations and quakes in distant stars. These oscillations follow predictable patterns during the red giant phase, a field of study known as “asteroseismology.” The oscillation pattern observed in 8 Ursae Minoris matches that of red giants in the late helium-burning stage, indicating that the star has already gone through its expansion phase. The crisis has passed, yet the planet somehow persists.

The recent study, titled “A close-in giant planet escapes engulfment by its star,” was published in the prestigious journal Nature in June 2023. Led by astronomer Marc Hon from the University of Hawaii, an international team of scientists contributed to this groundbreaking research.

This remarkable discovery challenges our existing knowledge of planet formation and survival in the face of stellar evolution. It highlights the intricate and unpredictable nature of the cosmos, reminding us that there is still much to learn about the mysteries of our universe.