Last September, NASA's planet-hunting Kepler mission announced the discovery of Kepler-16b, a "circumbinary" planet which, like Star Wars' Tatooine, orbits not one, but two stars.


Now, almost one year later, NASA has outdone itself: members of the Kepler mission announced today the discovery of Kepler 47, a binary star system, located about 5,000 light years from Earth, with two obiting planets. It's the first multi-planet circumbinary system we've ever observed, and unlike previously discovered circumbinary planets, one of Kepler 47's is actually within the stars' habitable zone, where liquid water — and life — could potentially exist.


Featured above: an artist's depiction of the Kepler-47 system, image courtesy of NASA/JPL-Caltech/T. Pyle

When a planet outside our solar system passes between its parent star (or stars) and the Kepler space telescope's line of sight, it becomes silhouetted in an event planet-hunters call a "transit". During a transit, the amount of starlight recorded by the telescope is temporarily reduced. By measuring these dips in brightness, members of the Kepler team can determine if there's an exoplanet orbiting a distant star — or, in the case of circumbinary planets, stars.

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Just as Kepler-16b was described as the first confirmed observation of a circumbinary planet, NASA is calling Kepler-47 (K-47) the first clear example of a circumbinary, multi-planet system. "Each planet transits over the primary star, giving unambiguous evidence that the planets are real," explains astronomer Jerome Orosz, lead author of the paper recounting the discovery in this week's issue of Science.

At first, that might not sound like a big deal — after all, scientists already showed that a planet can orbit a binary star system, how significant is it really that they've found two planets doing the same thing?


The answer: pretty significant. In an interview with io9, MIT astrophysicist Sara Seager explained that for planetary scientists and members of the Kepler team, "this is big."


It's a multi-planet system, Seager says — and yes, on some level you'd expect this to exist; "but you really need to see it to believe it," she explains, "and that's why people are so excited."

According to Seager, planet formation is currently one of the most challenging topics in all of astrophysics, because it involves non-linear physics:

"It's hard to describe every single possible interaction between planets and the stars, planets and each other, [or] planets in the disc of gas and dust" says Seager. She continues:

It just adds more complicating factors to include another star, another massive body into the equation, so every single time we make a discovery like this, it's a big deal. It's one of the things we've been waiting [to encounter], and it's another step along the path to understanding exoplanets and planetary systems.


But the presence of a second planet isn't the only thing that makes this binary star system noteworthy. According to Orosz and his colleagues, the outermost planet — dubbed Kepler-47c (K-47c) — is well within the stars' so-called "habitable zone" — the range of distances from the host stars, where liquid water could persist on the surface of an Earth-like planet:


At 4.6-times the diameter of Earth, K-47c is a big, and probably inhospitable, planet — but it's still an important find. "While Kepler-47c is probably a gas giant," write the researchers, "its location is notable as it demonstrates that circumbinary planets can exist in habitable zones."

What's more, even if K-47c is devoid of life, it's moons — if it has any — could be a very different story.


"Large moons, if present, would be interesting worlds to investigate," note the researchers. Seager, for one, agrees:

"Even though [K-47c] is a big planet, certainly too big for life," she explains, "it could have a moon that could support life."


The researchers' findings are published in the latest issue of Science.