We Found a Planet Twirling Between Three Suns

And scientists can't yet explain how it got that way.

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Astronomers have just detected a watery planet dancing between three separate suns. It's only 320 light years away.

A team of astronomers led by Kevin Wagner at the University of Arizona discovered the exoplanet. The scientists were conducting a survey of nearby worlds with incredibly vast orbits, scouring a small section of the sky with several ground-based telescopes based in the arid Atacama Desert of Chile. Through sheer serendipity, this odd triple-star planet popped up first.

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Its preliminary name is HD 131399Ab. For the sake of ease, let's call it Trisol (my choice, not the scientists'). The scientists describe the new planet today in the journal Science.

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Here's what the scientists know about Trisol, which, from the point of view of Earth, is near the the Scorpius constellation and the bright star Antares. The planet has an enormous orbit; each year there is around 550 Earth years. The world directly orbits a star roughly twice the size of our Sun. That star and the planet are also in a constant tango with a nearby pair of tightly twirling stars. One is about the size of our Sun and another half as large. For the entire system of both twirling pairs to make a single rotation requires 35,000 Earth years.

The exoplanet is about four times the size of Jupiter. Depending on the time of the year, from the perspective of somebody on Trisol all three stars could be tightly bunched together in the sky, or they could be coming from entirely opposite sides. That would provide planet-wide endless daytime.

This graphic shows the orbit of the planet in the HD 131399 system (red line) and the orbits of the stars (blue lines).

Wagner's team detected chemical fingerprints from the atmosphere of the planet using a process called near-infrared spectroscopy. They found it had significant quantities of water and methane. Although the planet is one of the coldest ever to be directly detected, at roughly 1,000 degrees Fahrenheit it's still too hot for that water to stay in liquid form.

The scientists are especially interested in learning how exactly this odd, three-star planetary system could have evolved. "We're guessing between three possible scenarios. We think it's unlikely that the planet formed where we see it today" he says.

One idea is that the planet formed much closer to the star it orbits but was flung into a long orbit by some unknown process, maybe involving unseen planets. The second idea is that the star Trisol orbits formed around the one of the nearby twin stars and was likewise flung out somehow and caught in a new orbit. Lastly, the scientists say the stars could have even formed while the stars were still getting into their current position, and was shoved out during the stellar shuffling.

Whatever the truth may be, this world has one amazing horizon to gaze at.

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