The ‘forbidden planet’ that has escaped fiery doom

All over the Milky Way, dying stars are devouring their planets. Even Earth will likely end this way in five billion years, when the sun expands and devours its inner worlds.

But the giant planet Halla, closely orbiting a star at a distance of 520 light-years from Earth, seems to have narrowly escaped such an apocalyptic fate. A new explanation for this planet’s survival status suggests there may be a hidden population of death-defying worlds elsewhere in the galaxy, according to a study published on Wednesday in the journal Nature.

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

As stars like the sun reach the end of their lives, they turn into red giants that expand exponentially and burn up all worlds that fall within their encroaching boundaries. Scientists have noticed indirect signs of such planetary engulfments in the galaxy, and a team recently reported the first direct detection of a planet flaring up as a star consumed it. In some systems, planets can even cannibalize each other, according to another recent study who found evidence of a gas giant eating a world the size of Mercury.

First discovered in 2015 and resembling Jupiter, Halla has added another wrinkle to the evolving story of planetary engulfment. Scientists didn’t realize Halla was in a precarious position until they surveyed the galaxy 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 dive into their helium supply, they’ve already exploded by orders of magnitude. In other words, Halla, who takes a tight 93-day job, should be in Baekdu’s belly right now. But when dr. Hon and his colleagues did follow-up observations with ground-based telescopes in Hawaii, they saw that Halla was still there, intact and beyond all expectations.

After ruling out other possible explanations, the team suggested that Baekdu, also known as 8 Ursa Minoris, could be the product of two stars that merged in the past. That fusion may have prevented either of them from growing large enough to gobble up surrounding planets. Halla could also be a “second-generation” newborn planet that coalesced from the explosive debris of the stellar combination, the researchers said.

In either case, Halla isn’t safe forever. Baekdu, which is about 1.6 times more massive than the sun, is expected to swell again in the near future.

“This planet may have escaped death once, but it seems unlikely that once the star begins to expand it will actually continue to survive,” said Dr. Hon.

In addition to explaining Halla’s existence, the team’s fusion hypothesis could explain the high concentrations of lithium in Baekdu, an element not normally found in red giants but which can be produced when two stars become one.

“The planet is very hard to explain, but their interpretation is the best idea I’ve heard,” said Andrew Vanderburg, an assistant professor of physics at MIT 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 on the system would involve experts on blue stragglers, a class of luminous stars believed to have been formed by stellar mergers.

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

To that end, Dr. 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 we least expect,” said Dr. Hon. “They’re somewhat resilient to what we think would kill them.”