In the Kepler-47 system, astronomers have discovered a third planet, and they secured the system's title as the most interesting of the binary-star worlds. Led by astronomers at San Diego State University, the team of researchers used data from NASA's Kepler space telescope to detect the new Neptune-to-Saturn-size planet orbiting between two previously known planets.
As the only known multi-planet circumbinary system, Kepler-47 has three planets orbiting two suns. Circumbinary planets are those planets that orbit two suns.
The researchers detected the planets in the Kepler-47 system through the "transit method." The planet can pass in front of the host stars if the orbital plane of the planet is aligned edge-on as seen from Earth, resulting in a measurable decrease in the observed brightness. The astronomers detected the new planet, named Kepler-47d earlier due to weak transit signals.
The usual aspect of circumbinary planets is the alignment of the orbital planes of the planets changes with time. In this case, the middle planet's orbit has become more aligned, leading to a stronger transit signal. The depth of the transit went from undetectable at the beginning of the Kepler Mission to the deepest of the three planets over four years.
The researchers from SDSU were surprised by both the size and location of the new planet. Kepler-47d appears to be the largest of the three planets in the Kepler-47 system.
Talking about the discovery, the paper's lead author and SDSU astronomer, Jerome Orosz said that they saw a hint of a third planet back in 2012, but with only one transit they needed more data to be sure. With an extra transit, it is possible to determine the planet's orbital period, and they were able to uncover more transit that was hidden in the noise in the earlier data.
A much better understanding of the system is possible with the discovery of the new planet. For instance, researchers now know the planets in this circumbinary system are very low density, less than that of Saturn, the Solar System planet with the lowest density.
Though a low density is not that unusual for the sizzling hot-Jupiter type of exoplanets, it is rare for mild-temperature planets. The equilibrium temperature of Kepler-47d is roughly 50 o F (10 o C), while Kepler-47c is 26 o F (32 o C). The innermost planet, which is the smallest circumbinary planet known is a much hotter 336 o F (169 o C).
Jonathan Fortney from the University of California, Santa Cruz astronomer and part of the team, said that this work builds on one of the Kepler's most exciting discoveries, that systems of closely-packed, low-density planets are prevalent in the galaxy. He further added that Kepler-47 revealed that whatever process forms these planets, an outcome that did not happen in the solar system, is common to single-star and circumbinary planetary systems.
