SDSU astronomers discover third planet in two-star solar system

SAN DIEGO - A group of researchers led by astronomers at San Diego State University announced the discovery of a third planet in the Kepler-47 solar system, making it the only known system with multiple planets that orbit two stars.

The researchers spotted the Neptune-to-Saturn-sized planet between the system's two previously observed planets while monitoring data from NASA's Kepler telescope. SDSU astronomers believe the planet has gone largely unnoticed until now because it may have passed in front of one of the system's two stars, weakening its transmission signal.

The planet has been dubbed Kepler-47d.

"We saw a hint of a third planet back in 2012, but with only one transit we needed more data to be sure,'' said SDSU astronomer Jerome Orosz, the lead author of the study published in the Astronomical Journal. "With an additional transit, the planet's orbital period could be determined, and we were then able to uncover more transits that were hidden in the noise in the earlier data.''

In addition to confirming the planet's existence, the research team discovered that it is the largest body on record in the Kepler-47 system. All three planets also have a lower density than Saturn, our solar system's least dense planet. Kepler 47 is roughly 3340 light-years from Earth toward the northern constellation Cygnus.

The system itself is fairly compact and could fit within Earth's orbit around our sun. One of the system's stars is roughly similar in size to the one in our solar system while the other is roughly one-third that size. All three planets are at least three times larger than Earth, with Kepler-47d registering at seven times larger.

"This work builds on one of the Kepler's most interesting discoveries: that systems of closely-packed, low-density planets are extremely common in our galaxy,'' said Jonathan Fortney, a UC Santa Cruz astronomer who was not involved in the study. "Kepler-47 shows that whatever process forms these planets -- an outcome that did not happen in our solar system -- is common to single-star and circumbinary planetary systems.''