A large planet surprisingly orbits a star that was supposed to destroy it

The TESS space telescope discovered a planet that survived a merger between two stars as the merged star swelled to become a red giant

The planet 8 Ursae Minoris b orbits a star 530 light-years away, which is in the final stages of its life. The red giant star was supposed to expand beyond the planet's orbit before shrinking to its current (still giant) size. In other words, the star was supposed to swallow up and tear apart every planet orbiting it in conjunction. And yet the planet remains in a stable and almost circular orbit. This discovery, based on precise measurements by NASA's Transiting Planet Survey Satellite (TESS), shows that the creation and destruction of planets is likely much more complex and unpredictable than many scientists thought.

As stars like our Sun approach the end of their lives, they begin to run out of nuclear fuel. They become red giants, expanding to their maximum size. If this were to happen in this case, the star would grow outward from its center to 0.7 AU – that is, about three-quarters of the distance from Earth to the Sun. It would engulf and destroy any nearby planets in the process. But planet b, a large gas world, is about 0.5 AU away. Since the planet could not have survived being swallowed, Mark Hoon, the lead researcher on the latest paper on the discovery, offers two other possibilities: the planet is actually a survivor of a merger between two stars, or it is a new planet – formed from the remnants left behind by that merger.

The first scenario begins with two stars similar in size to our sun in close orbit, and the planet orbits both. One of the stars "evolves" a little faster than the other, passing through its red giant phase, shedding its outer layers and becoming a white dwarf - a tiny but massive remnant of a star. The other just reaches the red giant stage before the two collide; What remains is the red giant we see today. However, this merger stops the further expansion of the red giant, saving the planet surrounding it from destruction. In the second scenario, the violent merger of the two stars pushes out an abundance of dust and gas, which forms a disk around the remaining red giant. This "proto-planetary" disk provides raw material for a new planet to coalesce. This is a kind of late second life for a planetary system - even though the planet is still nearing its end.

How can astronomers infer a chaotic series of events from current observations? It all comes from a good understanding of stellar physics. Planetary probe TESS can also be used to observe tremors and noises on distant stars, and these follow known patterns during the red giant phase. (Tracking such fluctuations in the stars is called "astroseismology"). The discovery team found that the oscillating pattern of 8 Ursae Minoris matches that of a red giant in the late stages of burning helium – and not one that is still expanding while burning hydrogen. So it's not like the star is still growing and hasn't reached the planet yet. The crisis has come and gone, but the planet surprisingly continues to exist.

The paper describing the TESS findings, "A close-in giant planet escapes engulfment by its star", was published in the journal Nature in June 2023 by an international science team led by astronomer Mark Hoon of the University of Hawaii.

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