An artist's rendering of the triple-star system HD 131399 ESO / L. Calçada

Here we are, standing on a distant moon. And I mean really out there, 320 light-years from Earth, somewhere in the constellation Centaurus.

What are we doing here, you ask?

Why, we’re admiring HD 131399Ab, of course. Yes, it’s the unglamorous name for that ginormous planet over there, the one with three suns. You really can’t miss the thing; it is four times the size of Jupiter, after all. Though to be fair, most of the exoplanets that scientists have directly imaged are 10 times the size of ole Jupe—which has a circumference 11 times bigger than Earth’s. (If you need a moment to blink silently in awe of the vastness of space, I’ll wait.)

HD 131399Ab isn’t the first planet found to have three suns. But it is the first such planet to be found in a wide-orbit system—​by far the widest known within a multi-star system. This is weird because multi-star systems are usually so unstable that they ejects planets, ​which are subject to competing gravitation force from all those suns.​This particular planet’s unexpected survival is therefore peculiar, and perhaps revealing. It could mean that such systems are more commonly hosts to planets that scientists tend to think, according to a paper published in the journal Science on Thursday.

HD 131399Ab is still just a baby, really, one of the youngest known exoplanets at just 16 million years old. That makes it sound sort of sweet and non-threatening, but you really don’t want to go there. It’s good we’re standing on this moon, in other words. (A moon that’s really only theoretical—quite likely to exist, given what scientists know about moons and planets, but not confirmed.) From here, the planet is clearly the brightest object in the sky. Not only can we get a good look at HD 131399Ab from this vantage point, but we’re spared its intense 1,000-degree heat, and from the fact that it doesn’t actually have a surface to stand on. Where the ground should be there are just clouds made of “cigarette-sized particles of silicate rock,” says the astronomer Kevin Wagner, who discovered HD 131399Ab. Wagner’s here with us, by the way, because he knows the most about this whole triple-sun situation we’re facing.

He can tell you, for instance, how the biggest of the three suns up ahead is eight times more luminous than the sun we know on Earth—but that it doesn’t appear as bright since it’s much farther away from this planet than the Earth’s sun. The smaller two suns actually orbit the larger one, all while twirling around one another “like a spinning dumbbell, separated by a distance roughly equal to that between the Sun and Saturn,” as the ESO put it in a statement.

Those two smaller suns change in apparent brightness—and in distance from the gigantic sun—as the planet goes about its orbit. Which means that, from where we’re standing, we can admire three separate sunsets and sunrises. (Take that, Tatooine, with your measly twin suns.)


The red line in this graphic represents the orbit of the planet in the HD 131399 system, and the blue lines represent the orbits of the three suns. (The two smaller suns, at right, are very close together.) The planet orbits the brightest star in the system.

HD 131399Ab eventually gets to a point in its orbit—when it’s along a path between the three suns—in which it experiences near-constant daylight. That’s because the monster-sun is rising from one horizon as the smaller two suns are setting at the other.

Much of the time, though, HD 131399Ab’s orientation to its three suns is such that it gives the planet a sunny side and a dark side. This arrangement—daytime on one side, nighttime on the other—is the way things are on HD 131399Ab for most of the year. A year for the planet, however, lasts the equivalent of 550 Earth years.

“I’m personally most interested in learning more about the planet’s orbit,” Wagner says. “I think it’s unlikely, but there’s a chance that it might not be as stable as we think it is, and that the planet might even orbit all three stars. In the next few years we’ll be able to watch the planet move in its orbit and will be able to tell which is the case.”

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