Recent exoplanet detections like the ‘super Earth’ found orbiting a red dwarf 9000 light years away have put the spotlight on gravitational microlensing. The phenomenon occurs when light from a background star is deflected by the gravity of an intervening object; in other words, one star passing quite near or in front of a far more distant one (as seen from Earth) will cause a lensing effect that can be studied. We’ve seen that a distant quasar can be lensed by a foreground galaxy, producing eerie, multiple images of the same quasar.

But things get trickier when it comes to microlensing within our own galaxy using individual stars. We can’t resolve the images created by these events with current telescopes, but the lensing does produce a measurable amplification of the distant star’s light. And any planets in orbit around the intervening star can perturb that lensing effect enough to signal their presence. The beauty of this is that microlensing is sensitive to planets down to terrestrial size.

But for microlensing to work, the two stars involved must be precisely aligned. The method thus demands keeping a close watch on millions of stars to catch these rare events, which is why microlensing observations for exoplanets are focused on the galactic bulge and the Magellanic Clouds. And even there the wait can be long, for the chances of a background star being microlensed at any time are roughly 10 ^-6, according to a useful new study by Nicholas James Rattenbury (Jodrell Bank and University of Manchester).

“Microlensing is currently detecting planets in a previously unreachable region of the planetary mass-radius space,” Rattenbury writes. In fact, “Microlensing is returning detections of planets with masses approaching that of Earth.” But he goes on to note that most of the lens system planets are M-class dwarfs, and the planets thus discovered would be, because of the limits of the current method, well removed from their habitable zones. The clear implication is that we are still several technological steps away from being able to detect habitable Earth-class planets around such stars.

One way to improve our capabilities is through proposed space telescopes like the Microlensing Planet Finder, which Rattenbury believes would be capable of detecting Earth-mass planets in the habitable zones of G and K-type stars. In fact, MPF could, according to its proponents, detect planets down to 0.1 Earth masses, and at separations from 0.7 AU to infinity. For more on MPF, see Bennett, Bond, Cheng et al., “The Microlensing Planet Finder: Completing the Census of Extrasolar Planets in the Milky Way,” available here (PDF warning).

But back to Rattenbury, who comments here on microlensing’s bright future:

Improvements in survey and follow-up instrumentation and operation will increase the discovery rate of low-mass planets, leading to estimates of the Galactic planetary mass-function. Many more planets will be discovered via microlensing. New ground and space telescopes will have higher sensitivity to low-mass planets than current instrumentation. In particular, a space telescope such as the proposed MPF mission will be sensitive to habitable Earth-like planets around Sun-like stars. Within the next ten years we can expect that dozens of extra-solar planets will have been discovered via microlensing, possibly some very similar to Earth.

Centauri Dreams‘ take: As for habitable worlds around red dwarfs, an exciting and rapidly developing subspecialty of the exoplanet hunt, we seem unlikely to find them through microlensing even via missions like MPF, but promising projects like Transitsearch.org hold out the real possibility of detecting Earth-size worlds and smaller around relatively nearby red dwarfs. The exoplanet hunt thus continues as a cluster of techniques are fine-tuned, from ever more accurate radial velocity measurements to space-based microlensing proposals and transit searches that will one day snare us a terrestrial world.