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The First Double-Double: Astronomers Find Two Planets Orbiting a Two-Star System

Kepler-47

Artist’s rendition of the Kepler-47 system with two stars and two planets. In the movie “Star Wars Episode IV,” Luke Skywalker’s planet Tatooine has two suns. Science fiction has now become science fact. Courtesy NASA/JPL-Caltech/T. Pyle

A team of astronomers including IfA’s Nader Haghighipour has discovered the first two-planet system orbiting two stars. The system, called Kepler-47, is known as a circumbinary planetary system. It demonstrates that complete planetary systems can exist around a pair of stars.

Kepler is a NASA space observatory whose mission is to survey a portion of the Milky Way galaxy to discover Earth-like planets in or near the habitable zones of their host stars and to determine how many of the billions of stars in our galaxy have such planets.

The Kepler-47 system contains the smallest known planet orbiting a pair of binary stars. It is 4,900 light-years from Earth in the constellation Cygnus. The inner planet’s radius is only three times that of Earth, and it orbits the binary star every 49 days. The radius of the outer planet is 4.6 times that of Earth (about the size of Uranus), and it orbits the binary every 303 days, making it the longest orbital period transiting planet known to date. The planets were discovered using Kepler by monitoring the faint drop in brightness produced when both planets transit (eclipse) their host stars.

More important, the outer planet’s orbit and the spectral types of the stars (G and M) place the planet well within the “habitable zone,” the region where a terrestrial planet could have liquid water on its surface. While this planet is most likely a gas giant like Jupiter and not a terrestrial planet like Earth, and is thus not suitable for life, its discovery establishes that circumbinary planets can, and do, exist in habitable zones.

Kepler-47 system diagram

This diagram shows the size of the Kepler-47 system relative to our solar system.
Courtesy NASA/JPL-Caltech/T. Pyle

The stars orbit and eclipse each other every 7.5 days. Each planet transits over the primary star, giving unambiguous evidence that the planets are real. The Kepler-47 system contains at least two planets. There is evidence that at least one additional planet may be present, but the current data are not yet sufficient to establish its existence.

The planets are small, and thus unlike the previous Kepler circumbinary planets, they do not gravitationally disturb the stars or each other, so their masses cannot be directly measured. However, the data clearly show that these small objects are certainly planets and not brown dwarfs (objects larger than planets but smaller than stars). Based on their radii, they probably have masses of about 8 and 20 times that of Earth.

“In contrast to a single planet orbiting a single star, planets whirling around a binary system transit a moving target,” explains Jerome Orosz (San Diego State University), lead author of the study. “The time intervals between the transits and their duration can vary substantially, from days to hours, and therefore the extremely precise and almost continuous observations with Kepler space telescope were fundamental.”

A NASA video about Kepler-47: http://www.youtube.com/watch?v=Cl5EknhEsSQ