Having Two Or Three Suns Doesn't Seem to Hurt Alien Planets

The three-body problem, scientists' term for figuring out the orbital quirks of a planet circling not one star but two, is so tricky that its name has inspired an award-winning novel. But the three-body problem is a piece of cake compared to the next degree of complexity—how a planet interacts with three stars at once.

It turns out that situation isn't quite as chaotic as might have been expected, according to new research presented at the annual conference of the American Astronomical Society held this week in Denver, Colorado.

"Because of the complex dynamics between these stars and planets, it was previously thought improbable that many planets would have stable orbits in these regions," lead author Frank Busetti, a scientist at the University of the Witwatersrand in South Africa, said in a press release.

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We think of stars as individual things, but that's not really the case—only a slim majority of solar systems in the universe [possess just one star. Binary star systems are pretty common, accounting for about a third of systems. Although three-star systems are rarer, they still make up almost a tenth of the total.

And astronomers already knew that even those three-star systems could hold planets because they've spotted planets in 33 different such systems. None of those planets actually orbit all three stars—most orbit a single star that's at a bit of a distance from the other two. One orbits the two stars of its system that are closest together.

In all, there are five different possible orbits: around each of the three individual stars, around the close pair, and around all three. So the team set about modeling what would happen in thousands of given configurations of a planet at two or three stars.

An artist's impression of what an exoplanet orbiting binary stars might look like. Turns out, having more than one star isn't quite as chaotic as might have been expected. NASA/JPL-Caltech

Those models showed surprisingly large stable regions around the stars, where a planet would be able to survive and remain in orbit without anything too dramatic happening to it. That's good news for those who dream of finding more and ever-weirder exoplanets.

(Busetti and his colleagues were also intrigued to see that retrograde orbits, in which a planet moves backward, tended to be more stable than the traditional orbital direction.)

And the work isn't just theoretical—the team hopes that their results will help astronomers prioritize their search for exoplanets, as they can now reference the stable zones around these star systems.