Is the Keplar mission closing in on another Earth?
Until Kepler, the leading detection method used to discover exoplanets — planets outside the Solar System — was much more likely to find giant planets, resulting in a sampling bias. Known as radial velocity or Doppler spectroscopy, the method depends on identifying the shift in a star’s spectral lines as it wobbles around a mutual centre of gravity with a planet. The larger the planet and the closer to the star it lies, the faster the star’s movement towards and away from Earth, and the easier it is to detect the shift in the spectral lines. Almost all of the planets found by this technique have been larger than Jupiter and very close to their stars, sometimes completing an orbit in just a few days.
An alternative method was presented in 2000, when CFA astronomer David Charbonneau and his colleagues, working from a shed in a car park outside the National Center for Atmospheric Research in Boulder, Colorado, observed a planet passing across — or transiting — the face of its parent star. Within days, another group had made a similar observation. In this case, the researchers were confirming a transit predicted for a planet, HD 209458b, that had been spotted using the radial-velocity method.
Before long, planets were being detected by their transits alone. Those early detections also yielded large, close-in planets, which were easier to see because they obscured larger portions of their host stars than their Earth-size counterparts would do. But researchers were thrilled to realize that, in principle, a space telescope could be made sensitive enough to see the transits of Earth-sized planets in Earth-like orbits — and the idea of Kepler was born.