An artist's concept of another triple-star system. (NASA/JPL-Caltech)

Scientists have found a rare three-star system 685 light years away. Instead of the more typical single star, or even a pair, it boasts a trio of suns that coexist in a complex dance. And the system is rare even among the triple-threat crowd: It hosts a stable planet, which is something scientists have seen only three times before.

Researchers used to think the KELT-4 system, home to a "hot Jupiter" planet called KELT-4Ab, was a binary system — which is much more common. But according to recent research published in the Astronomical Journal, one of those original stars is instead a binary pair.

The planet orbits only one of the one of the stars — KELT-4A — so technically that star is its only sun. KELT-4A is relatively close to us and bright, making it a better candidate for study than any of the stars in previously discovered triple systems. KELT-4Ab orbits it in just three days. To someone standing on the edge of the planet (though it would be impossible to stand on, as it's mostly gas), researchers estimate that the star would appear 40 times larger than our own sun.

The other two stars would appear much dimmer — about as bright as our planet's moon and not much larger than many of the stars we see in our own sky. KELT-4B and -4C, the pair of stars previously mistaken for a singleton, are caught in an orbit around KELT-4A, making them part of the system as well. But they're so distant from the larger star — eight times farther than Pluto is from our own sun — that it takes them 4,000 years to make the trip around KELT-4A; KELT-4B and -4C orbit each other every 30 years.

One thing that makes the system particularly interesting is the fact that its planet is what scientists call a hot Jupiter. These are Jupiter-like gas giants that sit close to their suns. Scientists believe, based on models of planetary evolution, that gas giants should form farther out — so the prevailing theory is that hot Jupiters are planets that have somehow been roped into a closer orbit than the one they were born in.

In this case, there's a tantalizingly obvious suspect for this orbital upheaval — or rather a pair of suspects.

"The binary system KELT-4BC may be what ultimately drove the planet KELT-4Ab so close to its star," lead author Jason Eastman, a research associate at the Harvard-Smithsonian Center for Astrophysics, told

Eastman and his colleagues plan on using the European Space Agency's Gaia satellite to study the orbital movements of these stars. They may be able to puzzle out how their gravity pushed KELT-4Ab into such a hot spot.

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