The ‘forbidden planet’ that escaped a fiery doom

All across the Milky Way, dying stars are gobbling up their planets. Even Earth is likely to perish this way about 5 billion years from now, when the sun expands and devours its innermost worlds.

But the giant planet Halla, which closely orbits a star 520 light years from Earth, appears to have narrowly escaped such an apocalyptic fate. A new explanation for this planet’s survivor status hints that there may be a hidden population of death-defying worlds elsewhere in the galaxy, according to a study published Wednesday in the journal Nature.

Halla is “a forbidden planet of sorts,” said Marc Hon, a NASA Hubble fellow at the University of Hawaii at Manoa and an author of the study. “The star itself might have a very unusual history that somehow permitted this planet to survive at such a close distance around what is otherwise a rather inhospitable host star,” he added.

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As stars like the sun reach the end of their lives, they transition into red giants that exponentially puff up in size, incinerating any worlds that fall within their advancing boundaries. Scientists have spotted indirect signs of such planetary engulfments across the galaxy, and a team recently reported the first direct detection of a planet flaming out as a star consumed it. In some systems, planets may even cannibalize one another, according to another recent study that found evidence of a gas giant that ate a Mercury-size world.

Halla, first discovered in 2015 and resembling Jupiter, has added a new wrinkle to the evolving tale of planetary engulfment. Scientists realized Halla was in a precarious position only when they examined the star system a few years later with NASA’s Transiting Exoplanet Survey Satellite. Those observations revealed that Halla’s host star, Baekdu, has exhausted its hydrogen fuel and is now burning through helium.

By the time most red giants dip into their helium supply, they have already ballooned in size by orders of magnitude. In other words, Halla, which occupies a tight 93-day orbit, ought to be in Baekdu’s belly right now. But when Hon and his colleagues conducted follow-up observations with ground telescopes in Hawaii, they saw that Halla was still there, intact and flouting all expectations.

After ruling out other possible explanations, the team proposed that Baekdu, also known as 8 Ursae Minoris, could be the product of two stars that fused together in the past. That merger may have prevented either one from growing large enough to swallow surrounding planets. Halla could also be a newborn “second generation” planet that coalesced from the explosive detritus of the stellar combination, the researchers said.

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In either case, Halla is not safe forever. Baekdu, which is about 1.6 times as massive as the sun, is expected to swell up again in the near future.

“This planet might have evaded death once, but it seems unlikely that once the star starts expanding, it would actually continue to survive,” Hon said.

In addition to explaining Halla’s existence, the team’s merger hypothesis could account for Baekdu’s high concentrations of lithium, an element that is not normally found in red giants but that could be produced as two stars become one.

“The planet is really hard to explain, but their interpretation is the best idea that I’ve heard,” said Andrew Vanderburg, an assistant professor of physics at Massachusetts Institute of Technology who studies exoplanets and reviewed the study for Nature.

Melinda Soares-Furtado, a NASA Hubble fellow at the University of Wisconsin-Madison who studies planetary engulfment, called the study an “exciting” example of the “unexpected properties” revealed in star-planet interactions. She suggested that future research about the system involve experts on blue stragglers, a class of luminous stars that are thought to be formed by stellar mergers.

“I think new discoveries like this call for a cross-pollination of ideas,” said Soares-Furtado, who was not involved in the study.

To that end, Hon and his colleagues plan to continue unraveling the backstory of this curious system while also searching for similar worlds.

“Planets just end up in places that we least expect,” Hon said. “They are somewhat resilient to what we think would kill them.”