WASP-107b is a hugely bloated planet, with a mass of only two Neptunes, but a radius near that of Jupiter, making it one of the puffiest planets known. As such it has been heavily studied, and indeed was the exoplanet showing the first detection of helium.

Long-term monitoring of the WASP-107 system with the Keck telescope has now revealed a companion planet, WASP-107c, as announced by Caroline Piaulet et al.

The new planet is in a much wider orbit, with a period of 1088 days and a high eccentricity of e = 0.26. It likely does not transit, and has a mass of perhaps a third that of Jupiter.

The discovery of a second planet is important for understanding the nature of WASP-107b itself. The tight, 5.7-d orbit, and the fact that the orbit is mis-aligned with the star’s rotation, might be explained by gravitational interactions with the second planet. The bloated size could then result from tidal interactions with the host star, as the planet circularised in its orbit, after the interactions with WASP-107c.

The authors conclude that, “Looking ahead, WASP-107b will be a keystone planet to understand the physics of gas envelope accretion”, starting with a planned observation with the soon-to-be-launched JWST.