The planet WASP-21b is 1.21 times the radius of Jupiter, the largest planet in our Solar System. It is vastly bigger than Earth.

WASP-21b is about the same mass as Saturn. That’s about a third the mass of Jupiter, which means that WASP-21b is much less dense.

The star WASP-21 is 1.06 times the radius of the Sun. Its spectral type is G3 which makes it similar to our Sun, which is a G2 star.  WASP-21 has a brightness of visual magnitude 11.6.

WASP-21b orbits at a distance of 7.77 million km, taking 4.32 days to go round its orbit.

WASP-21 lies in the constellation of Pegasus. Its coordinates are right ascension 23:09:58.0 and declination +18:23:46. The system is a distance of 250 parsecs from us.

The brightest stars in the plot have magnitude 1 and the faintest have magnitude 6, so, with a visual magnitude 11.6, WASP-21 is much fainter than these stars. You would need a telescope to see it.

WASP-21b was the first exoplanet found to be transiting a star in the galaxy’s “thick disk”. Our galaxy has spiral arms surrounding a central bulge. It thus looks like this (plan view of the left, edge-on view on the right), with our Sun being out in one of the spiral arms:

But the flattened disk is itself composed of a thicker disk and a thinner disk.  Most stars we see are in the “thin” disk, which contains a lot of gas and dust and regions of star formation giving birth to new stars.   The “thick” disk contains little gas and dust and is composed of older stars born billions of years ago.

WASP-21b was announced in a paper led by Francois Bouchy, then of the Observatoire de Haute-Provence. Prof Bouchy used the SOPHIE spectrograph at OHP to make radial-velocity measurements of WASP-21, thus proving that the orbiting body, WASP-21b, has the right mass to be a planet.

For more information visit http://exoplanet.eu/catalog/wasp-21_b/.