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Alien planet with 3 stars is actually a star itself, scientists determine

An artist's depiction of the hypothesized planet HD 131399 Ab at bottom left with its three stars to the right.
An artist's depiction of the hypothesized planet HD 131399 Ab at bottom left with its three stars to the right. (Image credit: European Southern Observatory/L. Calçada)

Finding alien worlds in distant solar systems is a delicate task. 

Astronomers' tools have come a long way since the first exoplanet discoveries in the 1990s, but the quest remains daunting: to find tiny objects hundreds of light-years away, far outshone by their host stars. It shouldn't be surprising that, sometimes, exoplanet discoveries turn out to be flukes.

Such is the fate of the purported planet HD 131399 Ab, which scientists once thought to have three suns. When astronomers tried to look at the planet again, they found that it was actually something entirely different and far in the background masquerading as an exoplanet. Now, the astronomers who originally found the planet are retracting their discovery.

Related: These 10 super extreme exoplanets are out of this world

The HD 131399 star system lies about 350 light-years away from Earth in the constellation Centaurus. The stellar system consists of three stars: an A-type white main star about twice our sun's mass, around which astronomers thought HD 131399 Ab orbited, and two smaller companions.

Scientists first announced so-called HD 131399 Ab's discovery around six years ago. The Jupiter-size newfound world garnered a fair amount of attention at the time, partly thanks to the novelty of a world with three suns. From an astronomer's point of view, the discovery was interesting because it had been directly imaged: rather than seeing the planet as a blip of darkness or a wobble of movement in its main star, scientists had spotted it directly with their telescopes.

But as years passed and astronomers tried to check in on the system, they found that something was not quite right. In particular, scientists analyzed the objects' parallax, how much they moved against the stellar background as seen at different points in Earth's orbit. That analysis by independent astronomers found that the supposed planet and the star system weren't at the same distance from Earth; later measurements by the planet's original discoverers confirmed this.

In other words, what scientists had thought was a planet was actually something moving very quickly in the background, many light-years farther away from Earth than the HD 131399 system. 

"It must be a very red star — probably a red giant, and maybe even one with a disk." Kevin Wagner, an astronomer at the University of Arizona and one of HD 131399 Ab's would-be discoverers, told Space.com. That red color would have made the star look deceptively like a planet, he noted. 

Additionally, this distant star apparently moves abnormally quickly, hence it appearing to match pace with a much nearer star system. This discrepancy suggests that some phenomenon gave the star a boost — perhaps it was ejected from its host system or experienced a close encounter with another system, scientists think.

"Each of these would be quite a rare occurrence on their own," Wagner told Space.com. "The combination of these two low-probability characteristics of the background object are what led us to misclassify it as a planet."

Wagner said he's not aware of any other instances in which a red star or another fast-moving object had been mistaken for a closer planet — but, as the HD 131399 Ab episode shows, the possibility certainly exists.

The research is described in a paper published Friday (April 15) in the journal Science.

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Rahul Rao
Contributing Writer

Rahul Rao is a graduate of New York University's SHERP and a freelance science writer, regularly covering physics, space, and infrastructure. His work has appeared in Gizmodo, Popular Science, Inverse, IEEE Spectrum, and Continuum. He enjoys riding trains for fun, and he has seen every surviving episode of Doctor Who. He holds a masters degree in science writing from New York University's Science, Health and Environmental Reporting Program (SHERP) and earned a bachelors degree from Vanderbilt University, where he studied English and physics.