If there’s one thing to be said for the Kepler mission, it’s certainly turning up some very strange new worlds.

Recently, the mission has spotted a Tatooine-like exoplanet; it’s found evidence for a “phantom” world; spied a tightly-packed six-pack and even found an exoplanet darker than coal.

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Now, a sun-like star — called Kepler-18 — has been spotted with a super-Earth and two Neptune-like worlds orbiting it. Although the discovery of a multi-world star system would have been considered “odd” only a few months ago, this isn’t why this particular star system is weird: two of Kepler-18′s worlds have rhythm.

At a joint meeting of the American Astronomical Society’s Division of Planetary Science and the European Planetary Science Conference being held in Nantes, France, a team from McDonald Observatory at The University of Texas at Austin announced their discovery: Kepler-18′s exoplanets orbit in resonance with one another.

Their research will be published in a special Kepler edition of The Astrophysical Journal Supplement Series in November.

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The three worlds all orbit their star well within the distance that Mercury orbits our sun — therefore, they have very fast orbital periods. Kepler-18b, c and d have periods of 3.5, 7.6 and 14.9 days respectively. Kepler-18b has a mass of around seven Earths (making it a super-Earth), whereas c and d are bigger; 17 and 16 Earth-masses (Neptune-like proportions).

Looking at those numbers, you may have noticed something: Kepler-18c orbits roughly twice as fast as Kepler-18d. And they appear to be interacting gravitationally.

When Kepler-18c and d pass in front of their star, they “are not staying exactly on that orbital period,” said lead researcher Bill Cochran. “One is slightly early when the other one is slightly late, (then) both are on time at the same time, and then vice-versa.”

This apparent cosmic dance can mean only one thing.

“It means they’re interacting with each other,” Cochran continued. “When they are close to each other … they exchange energy, pull and tug on each other.”

This condition is what astronomers refer to as an orbital “resonance” of 2:1.

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Having two interacting exoplanets in the same star system turns out to be pretty useful to astronomers. Confirming that the signal detected is indeed an exoplanet passing in front of its star (thus causing a small dip in starlight for “transit-method” missions like Kepler to spot), and not some other observational artifact, many transits need to be recorded. But in the case of Kepler-18c and d, this resonant dance highlighted that they were two worlds interacting with one another — therefore they are orbiting the same star.

But the existence of Kepler-18b (the super-Earth) is less certain, although the researchers are still confident it is there.

“There’s a small possibility that (Kepler-18b) is due to a background object, but we’re very confident that it’s probably a planet,” said Cochran. His team calculated that the likelihood the object is a planet is 700 times more likely than the likelihood that it’s a background object. I’d bet on those odds.

Image: Artist’s impression of a star system not too dissimilar to Kepler-18. Credit: ESO