Illustration by ESO/L. Calçada
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An artist's impression shows the triple star system HD 131399, which includes a newly discovered planet.

Illustration by ESO/L. Calçada

New Planet Is an Oddball With Three Suns

Captured in a rare direct image, the strange world may be doomed due to the dangerous dance of its orbital partners.

Imagine a world where seasons last approximately 140 years, where shadows occasionally come in triplets, where heat and pressure wring iron rain from the atmosphere, and where sunrises and sunsets are spectacularly variable: Sometimes there’s one sun in the sky, sometimes two, sometimes three.

This is planet HD 131399ab, a world four times as massive as Jupiter, and one of the few planets to have its picture taken directly.

As astronomers report today in Science, HD 131399ab lives 340 light-years away in the constellation Centaurus, and its heavens are a roiling canvas of triple-sunned activity.

That’s because the planet marches around one member of a three-star system, taking 550 Earth-years to complete one orbit. Two smaller stars also orbit the planet’s host, which is a bluish white giant almost twice the size of the sun. The smaller stars whirl around one another like a spinning dumbbell, creating a continually changing sunscape.

As far as planetary systems go, this one—while exotic and picturesque—is not entirely unique. Many planets orbit stars that come in pairs or even triples, sometimes tracing a single path around multiple stars at once. But this world is special because astronomers could see it directly, using the SPHERE instrument and the Very Large Telescope, and because of the somewhat important detail that it might not survive for long.

“We can’t say with certainty that the system is in fact stable and won’t eject the planet in the near future,” says University of Arizona graduate student Kevin Wagner, who discovered the system while surveying a hundred stars for giant planets.

Balancing Act

Our sun, a singleton, is unusual in its solitude. Most of the stars in the sun’s neighborhood have a stellar companion. Thus, astronomers expect many planets to orbit stars that grew up and stayed together.

“About 50 to 60 percent of the stars in our solar neighborhood are in binary systems, and we expect, based on the current statistics, that 10 percent of them host planets,” says the University of Hawaii’s Nader Haghighipour, who specializes in studying planet formation in complicated systems.

But such systems must strike a delicate balance between orbital dynamics and the availability of planet-building ingredients. If that balance is skewed, planets that manage to form in multistar systems could be doomed to any of several dismal fates, including annihilation as they hurtle inward toward a whirling stellar binary or ejection from their home systems, destined to wander for eternity in an endlessly dark galaxy.

For planets to survive in a multistar system—as HD 131399ab has done for at least 16 million years—certain requirements need to be met. And oddly, this planet’s system barely meets these needs.

Turns out, HD 131399ab’s current orbit puts it around 7.6 billion miles (12.2 billion kilometers) from its star, roughly twice as far as Pluto is from our sun, on average. That is just wide enough to cause trouble. The planet swings a bit too close to the spinning pair of stars that share its sun, putting it in an area where that stellar dumbbell is a problem.

“This planet is in the unstable region between star A and binary BC,” Haghighipour says. “That makes it really hard to explain.”

Because of that, scientists suspect the planet formed closer to its star, where the whirling binary couldn’t bother it, and somehow ended up farther out. It’s not yet clear whether that happened because a gravitational spat with a sibling planet punted it to the fringe, or because it formed around the whirling pair of stars and changed allegiance later in life, or because the stars themselves shifted.

And while the planet’s precise orbit hasn’t yet been determined, it’s possible the world will continue its journey outward, eventually ending up as one of the billions of free-floating planets wandering our galaxy.

“If the planet is on a very [elongated] orbit, it will likely be ejected over the next several thousand or 10,000 years. This is an exciting possibility, although unfortunate for the planet, because we know of many planet-mass objects floating freely in space unassociated with any star,” Wagner says.

Iron Rain

For now, astronomers are content to stare at this world and begin to find out more about what it’s like. Because it’s directly observable—as opposed to worlds detected through their gravitational tugs or silhouettes—scientists can learn more about its atmosphere.

HD 131399ab, like Jupiter and Saturn, has a predominantly hydrogen and helium atmosphere, Wagner says, with a bit of water and methane tucked in as well. But unlike the distinct clouds swirling on Jupiter, this world is remarkably cloud-free.

“This planet seems to be clear, or only partly cloudy at most,” Wagner says, describing how clouds of silicate rock particles could be forming deep within the planet’s atmosphere. “At even lower layers, the temperature will be yet hotter, and iron droplets may condense out of the hot atmosphere, forming iron rain.”

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