Astronomers have confirmed that a third planet is orbiting the two stars at the center of the Kepler-47 system. This discovery gives us more information about the dynamics of binary planetary systems. This is very important considering that about half of all Sun-like stars have a companion.
The first two planets, known as Kepler-47b and Kepler-47c, were discovered in 2012. A third candidate world was announced in 2013, but it took further analysis to confirm it. As reported in The Astronomical Journal, the third planet, Kepler-47d, sits in between the other two.
“We saw a hint of a third planet back in 2012, but with only one transit we needed more data to be sure,” lead author Jerome Orosz, from San Diego State University, said in a statement. “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.”
All three planets have low densities (they would “float” on water) and are smaller than Saturn. They orbit the two stars respectively in 49, 187, and 303 days, while the stars themselves orbit around each other every 7.45 days. The larger star has roughly the same mass as our Sun, while the smaller has about a third of its mass.
The two outermost planets are tightly packed together, so there is no chance that another world is orbiting between them. They are also quite temperate, and optimistic predictions put them in the system's habitable zone. The planets also orbit on the same plane and in nearly circular orbits, which suggests that if (or when) the planets moved about in the past, they did so gently.
"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,” added Jonathan Fortney, from the University of California, Santa Cruz, who was not part of 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."
The system is located 3,340 light-years from Earth in the constellation of Cygnus, the swan.