The inner planets are not expected to survive the expansions of dying stars. Somehow this did

Planet Haila accompanies a star that should, in the normal course of stellar evolution, have expanded well beyond Haila’s orbit before shrinking again. No planet could survive being engulfed by its star like that, leaving astronomers with a mystery they’re struggling to solve.

As stars age, they swell to become red giants, before shedding their outer layers and becoming white dwarfs. When the Sun does this, it will become large enough to engulf both Mercury and Venus, though a heat-stricken Earth might narrowly escape. After this there will be a period of helium fusion before its eventual collapse to become a white dwarf.

The 8 star Ursae Minoris, Baekdu to its friends, has completed this major expansion and now has a helium-fuse core. This makes Haila’s presence in an orbit just wider than Mercury’s a real puzzle. A new document confirms Haila’s existence and offers some possible explanations.

This is a planet that shouldn’t exist, Dr. Ben Montet of the University of New South Wales. It should have been ingested by its star.

The most obvious explanation for Haila is also the least interesting and least likely, the paper concludes. Haila may have once been on a more distant orbit and some disruption caused her to plummet. However, the stability and nearly circular shape of her orbit make this scenario highly unlikely, indicates the modeling, which requires a barely plausible series of events.

Instead, Montet and co-authors suspect that Baekdu was two stars locked in a tight orbit. One of these evolved during its life cycle and ended up as a small white dwarf that was eventually engulfed by its companion. This shorted out the supergiant stage, meaning the combined star never reached the size one would expect, allowing Haila to survive.

Haila would then have been a circumbinary planet or similar to Tatooine, but there would have been no one to appreciate Tatooine’s sunsets. Haila is a gas giant about 65% more massive than Jupiter. Even if it had moons, the searing heat of stars so close by would never allow them to be habitable.

When a red giant and a white dwarf merged to become Baekdu, a disk of debris would have been ejected.  It is possible that Haila formed from this record

When a red giant and a white dwarf merged to become Baekdu, a disk of debris would have been ejected. It is possible that Haila formed from this record.

Image credit: WM Keck Observatory/Adam Makarenko

There is also a third possibility, also much more likely than the orbital contraction idea, which is that Haila is a second-generation planet formed from a cloud of gas ejected during the merger of stars. Such second-generation planets have been theorized before, and the opportunity to study one is a tantalizing prospect for planetary scientists.

As if that weren’t complicated enough, the team studying Baekdu using Keck Observatory and the Transiting Exoplanet Survey Satellite (TESS) found traces of a more distant large object. They think this is more likely a small star than a large planet, although it hasn’t been seen directly, and it orbits at least as far from Baekdu as Jupiter is from the Sun.

Enough evidence emerged in 2015 for Haila that both planet and star were considered worthy of a name, but prior to this study there was still some uncertainty as to whether it was real. The latest observations confirmed this. More importantly, they determined that Baekdu is in the helium fusion or red lump stage, having used up all of its hydrogen, rather than still being rising, making Haila’s existence a pressing issue.

Evidence for the idea of ​​a recent stellar merger comes from Baekdus’s abundant lithium, something only one percent of the roughly 100,000 red giants we’ve studied have. Stars burn lithium in their atmospheres, so it’s usually only seen in very young stars or those that have recently consumed a planet or star.

Montet told IFLScience that no one expected to find planets in 90-day orbits around red giants, but a lot of searches have been done to look for them further out. Since such studies would likely notice a closer planet if it were there, it certainly seems like objects like Haila aren’t common. However, where there is one, there are likely others, and the authors are eager to seek them out to help solve the mystery of Haila’s unlikely existence.

The study is published in Nature.

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