Microlensing search for planets

Most exoplanets have been found by Doppler evidence of periodic stellar motion or by transits across a star’s face. Both methods are strongly biased in favor of planets with orbital radii R much smaller than 1 AU, Earth’s distance from the Sun. Microlensing, the transient brightening of a star through gravitational lensing by an intervening star or planet, is an alternative that’s most sensitive to planets several AU from their stars. As seen in the figure, the background star brightens as the lensing star crosses the line of sight. But a 6-M_⊕ (Earth-mass) planet trailing the lensing star by 3 AU produces a telltale blip in the light curve. Microlensing has as yet revealed fewer than two dozen planets. Now the PLANET network has reported a comprehensive analysis of its searches from 2002 through 2007. In those six years it discovered only three planets, with masses M ranging from 6 M_⊕ to 4 Jupiter masses. From that harvest and seven planets found by other networks, together with a careful determination of search efficiency as a function of M and R, the PLANET analysis reaches surprisingly broad conclusions: In the R range 0.5–10 AU, a region largely unexplored by other techniques, roughly 17% of all Milky Way stars have Jovian planets; about half have Neptunian middleweights; and more than 60% harbor “super-Earths” (5–10 M_⊕). On average, every star has 1.6 planets heavier than 5 M_⊕ in that R range. (A. Cassan et al., Nature 481, 167, 2012.)—Bertram Schwarzschild