Planetrise: an odd pair of planets

Those who have witnessed the Moon rising above the horizon will know that it’s one of the most dramatic events in the night sky. A moonrise, however, would have nothing on a ‘planetrise’. If you lived on Kepler-36b, a planet in a newly-discovered solar system, you’d be able to watch a majestic gas giant slowly loom into view:

An artist's conception of a gas giant rising above Kepler-36b (credit: David A. Aguilar)

This two-planet system, recently discovered by an international team of astronomers using NASA’s Kepler spacecraft, make a very odd couple. One of the planets – Kepler-36b – is a rocky world, about 1.5 times the size of the Earth, while its companion – Kepler-36c – is a gaseous planet similar to Neptune.

The planets are in orbit around a star in the Cygnus constellation, and they're 20 times closer together than any pair of planets in our Solar System. Astronomers are now trying to work out how such vastly different planets could have ended up so close together.

In the most common model of planetary formation, rocky planets are created close to their host star, whilst the gas giants remain further away – the Solar System is a prime example of this. It’s therefore possible that the orbits of these planets have shifted, or even that their composition has changed over time.

Whatever the reason for this puzzling pair, us earthlings will have to make do with watching moonrises for now, at least until we've worked out how to colonise distant solar systems.