Since 1995, almost 200 extrasolar planets have made themselves known by spectral Doppler oscillation as they tug their host stars to and fro. In the last two years, a very different technique—gravitational microlensing—has revealed only three planets. But the most recent microlensing discovery makes clear the special advantages of that technique for finding Earth-like planets. In January, a worldwide collaboration of three microlensing groups—PLANET, OGLE, and MOA—reported 1 the discovery of a planet of roughly 5.5 Earth masses (M ⊕) circling a red-dwarf star with an orbital radius of about 2.6 astronomical units (1 AU is Earth’s orbital radius). It’s the lightest extrasolar planet ever discovered.
The great majority of the planets found by the Doppler method have been Jupiter-like gas giants, two or three orders of magnitude heavier than Earth. That’s because the amplitude of the telltale Doppler oscillation of the host star’s spectrum increases as the...
