How many generations of humans have looked up at the night sky and wondered how many of stars had planets around them? Perhaps it is only a few, since early humans would have considered our planet and our star to be unique, and would have looked at other stars without comprehending that they were the same as our sun, only at a vast distance. But ours is the first generation to know the answer, that most stars do indeed have planets around them.
My research involves looking for such extra-solar planets. We operate the WASP-South transit survey, which is an array of cameras out in the South African desert which photographs the night sky repeatedly, every clear night, building up light-curves of millions of stars, watching for small dips in their light caused by a planet passing in front of the star, once per orbit.
The depth of the transit dip tells you the fraction of the star that is occulted by the planet, and thus, if you know the size of the star, you obtain the size of the planet. You can then find the mass of the planet by the gravity it exerts on the host star, which you measure using the Doppler shift of the star’s light as it is rhythmically tugged to-and-fro as the planet orbits. The combination of size and mass tells you that planet’s density, and from that you have a fair idea of what it is made of. Continue reading