Detecting Exoplanets

An exoplanet is a planet orbiting a star other than the Sun. Thousands have now been found, but very few have been seen directly, because:
• a planet emits no light of its own and only reflects a tiny fraction of its star's light
• the planet is very close to its much brighter star as seen from Earth, so its light is swamped by the star's glareTwo indirect methods are used.Radial velocity method. The star and planet both orbit their common centre of mass, so the star wobbles slightly. As it moves towards and away from us, the lines in its spectrum are periodically blue shifted and red shifted (the Doppler effect). The period of the variation equals the planet's orbital period, and a larger shift indicates a more massive planet. This method works best for massive planets close to their stars.Transit method. If the planet's orbit is edge-on to us, the planet passes in front of the star once every orbit. The star's apparent brightness dips slightly and regularly. The time between dips gives the planet's orbital period, and the depth of the dip depends on the fraction of the star's disc blocked: dip ≈ (r_planet / r_star)².Worked example: a planet with radius one tenth of its star's radius blocks (0.1)² = 0.01 of the star's disc, so the brightness dips by about 1%. An Earth-sized planet around a Sun-like star gives a dip of less than 0.01%, which is why space telescopes are needed to find small planets.