A binary star system

It’s not enough anymore to simply announce the finding of a new exoplanet. We’ve found so many in the past few years (thousands to date) that they’re almost commonplace. Now, the new planets have to have something unusual about them to make them noteworthy. Case in point, a pair of planets orbiting the binary star system Kepler-47.

Circumbinary (orbiting two stars rather than just one) planets have been discovered before. However, this time, it’s a whole star system going around the pair of stars. Granted, that system only has two known stars, but more may yet be found.

The planets were discovered by Jerome Orosz of San Diego State University and more than three dozen collaborators who shared authorship. They had to observe the two stars eclipsing each other, as well as the planets transitting (passing in front of) the stars. Each of these events had to be observed multiple times in order to confirm the data. Luckily, the two stars orbit each other every seven and a half days and the inner planet orbits both stars in just under 50 days. Thus, it didn’t take that long to see several transits. At a little over 300 days, the outer planet has a year almost as long as ours. Cosmologists have only observed three transits of this planet.

An artist's depiction of the Kepler-47 system. Kepler-47c is the large planet on the left; Kepler

47-b appears as the small blue crescent to the right of the two stars.

Image courtesy of NASA/JPL-Caltech/T. Pyle.

What about habitability? Although one of the stars is similar in size to that of our sun (the other is about a third as big), neither of the planets is expected to be Earth-like. While the outer planet does technically reside in the habitable (liquid water zone), it’s about four and a half times as big as the Earth. The inner planet is also three times the size of the Earth and not even in the habitable zone.

So neither of these new planets is going to be a New Earth. We’ll have to keep looking for the place to evacuate our citizenry when our sun becomes a white dwarf. The good news is that we now know that planets can be found in the habitable zone of binary star systems as well as single star systems.


Orosz JA, Welsh WF, Carter JA, Fabrycky DC, Cochran WD, Endl M, Ford EB, Haghighipour N, Macqueen PJ, Mazeh T, Sanchis-Ojeda R, Short DR, Torres G, Agol E, Buchhave LA, Doyle LR, Isaacson H, Lissauer JJ, Marcy GW, Shporer A, Windmiller G, Barclay T, Boss AP, Clarke BD, Fortney J, Geary JC, Holman MJ, Huber D, Jenkins JM, Kinemuchi K, Kruse E, Ragozzine D, Sasselov D, Still M, Tenenbaum P, Uddin K, Winn JN, Koch DG, & Borucki WJ (2012). Kepler-47: A Transiting Circumbinary Multiplanet System. Science (New York, N.Y.) PMID: 22933522