Artist’s rendering of binary system Kepler-47, with outer planet Kepler-47c
in the foreground and inner planet Kepler-47b in the bottom right corner
Growing up in a binary star system can be tough. As the system’s two suns orbit each other, they exert strong gravitational pressures capable of pushing away young planets or sending them careening into one another. It wasn’t until last year that astronomers found the first evidence of a world orbiting two stars. Now new data has revealed that the binary system Kepler-47 goes one step further: it contains not one but two planets.
The new discovery, published in the journal Science, proves that despite the challenging environment, a multi-planet system can form around two stars. The inner planet, Kepler-47b, orbits the central stars—a Sun-like one with a smaller companion—every 49.5 days. The outer planet, Kepler-47c, has a radius 4.6 times bigger than Earth’s, a year of about 303 Earth days, and an orbit located in the habitable zone where liquid water could exist on the planet’s surface.
What’s most interesting about this discovery is the challenge it poses to the current theory about how planets develop. According to the nebular hypothesis, disks of dust form around a star, and within that disk, a combination of small collisions and gravity eventually pulls the dust together into planets. But in a binary system, the pull from two stars and their tendency to smash planets into one another would make the process much more complicated. In light of these findings, astronomers may need to revise their understanding of how planets form, especially in systems where two stars are throwing their weight around.
Image courtesy of NASA/JPL-Caltech/T. Pyle