Round and round the mulberry bush

A PLANET orbiting a star tugs it gently this way and that, so it oscillates between moving towards Earth and away from it. The velocities involved are tiny: for Proxima Centauri about two metres per second, a brisk walk. Nevertheless, the effect on the star’s spectrum can be measured from the ground. When a star is approaching Earth, its light is slightly bluer; when away, slightly redder. For this method, the plane of the planet’s orbit need not be aligned with Earth.

The transit technique, by contrast, requires that it is, so that the planet passes between Earth and its parent star every orbit. When that happens, the parent star’s light will dim accordingly. Transiting was used with great success by Kepler, an American space telescope which detected well over 1,000 distant planets earlier this decade. Statistical analysis of that sample suggests many—possibly all—red dwarfs have rocky planets, and that they are likely to crop up quite frequently in a red dwarf’s habitable zone. Since red dwarfs are the commonest stars, the most likely place to find an Earthlike planet is in orbit around one.

Source: Economist