The planet WASP-17b is one the largest planets ever found, being nearly twice the radius of Jupiter, the largest planet in our Solar System. It is a vast, bloated gas-giant planet that is twenty times larger than Earth.
Despite being much bigger than Jupiter, WASP-17b has only half of Jupiter’s mass. It has been puffed up by extra internal energy (heat), likely caused by tides.
The host star, WASP-17, is 1.4 times the radius of our Sun and is slightly hotter, being a star of F4 spectral type.
WASP-17b orbits at a distance of 7.7 million km from its star, and takes only 3.74 days to orbit once.
WASP-17 lies in the constellation of Scorpius. Its coordinates are right ascension 15:59:51 and declination −28:03:42. The system is at a distance of 400 parsecs from us.
The brightest stars in the plot have magnitude 1 and the faintest have magnitude 6. WASP-17 has a visual magnitude of 11.5, and so is much fainter than these stars. You would need a telescope to see it.
WASP-17b was the first exoplanet found to be in a retrograde orbit! That means that, while the star spins on its axis one way, the planet orbits the other way.
WASP-17b has been observed with ESO’s Very Large Telescope, showing that its atmosphere is mostly clear, with signs of sodium and potassium.
For more information visit http://exoplanet.eu/catalog/wasp-17_b/.
References:
WASP Planets 