The planet WASP-15b is 1.43 times the radius of Jupiter, the largest planet in our Solar System. That’s about 14 times the radius of Earth.

WASP-15b is only half the mass of Jupiter so, being much bigger, it is much less dense than Jupiter. WASP-15b is a big ball of hydrogen gas that is likely hotter than Jupiter, and if you heat up a gas it expands.

The star WASP-15 is 1.48 times the radius of the Sun. Its spectral type is F7 which makes it hotter than our Sun. It has a brightness of visual magnitude 10.9.

WASP-15b orbits at a distance of 7.46 million km, taking 3.8 days to go round its orbit.

WASP-15 lies in the constellation of Centaurus. Its coordinates are right ascension 13:55:43.0 and declination −32:09:35. The system is at a distance of 280 parsecs from us.

The brightest stars in the plot have magnitude −1 and the faintest have magnitude 6, so, with a visual magnitude 10.9, WASP-15 is much fainter than these stars. You would need a telescope to see it.

WASP-15b is in a retrograde orbit! That means that the host star spins one way, but the planet orbits the other way! This is not seen in our Solar System, where all the planets orbit in the same direction that our Sun spins.

Why is WASP-15b in a retrograde orbit? We don’t fully know, but it is likely that gravitational interactions with other unseen planets in the same system pushed it into its current orbit.

WASP-15 has been observed by NASA’s Spitzer Space Telescope which observes infra-red radiation (heat). Spitzer detected a reduction in the total heat from the system when the planet passed behind the star.

WASP-15b was discovered by the WASP-South transit survey and was announced in a paper led by Dr Richard West of Warwick University. For more information visit http://exoplanet.eu/catalog/wasp-15_b/.