While transit and radial velocity methods get most of the press when it comes to finding exoplanets, gravitational microlensing offers an independent alternative. Here a star passes in front of a far more distant object, causing the light from the source to be gravitationally ‘bent’ by the intervening star. The useful thing for exoplanet work is that if the ‘lensing’ star is orbited by one or more planets, they can leave their own signature in the microlensing event. And indeed, microlensing collaborations like MOA (Microlensing Observations in Astrophysics) and OGLE (Optical Gravitational Lensing Experiment) have made the method pay off in exoplanet discoveries.


Image: Gravitational microlensing relies on chance line-ups between an intervening star with planetary system and a more distant light source. Credit: California Institute of Technology.

Now researchers at the University of Auckland are proposing to measure low-mass planets, planets as small as the Earth, using these methods. Microlensing events vary in terms of the degree of magnification they provide, with both MOA and OGLE finding some events with extremely high magnifications. You would think the higher the magnification, the better, but this turns out to be not necessarily the case, as the university’s Phil Yock and colleagues explain in a new paper. Their simulations clarify the methods we can use to detect Earth-sized planets:

Our conclusion is that low-mass planets can be e?ciently searched for in events with moderately high magni?cations of order 50-200, and that the greater frequency of these events in comparison to those with higher magni?cations lends advantage to monitoring them. However, to take full advantage of their discovery potential, telescopes with apertures in the range 1-2m would be needed to photometer them with su?cient precision to detect low-mass planets.

A network like this is, in fact, being deployed by the Las Cumbres Observatory Global Telescope Network, working in collaboration with SUPA/St Andrews (Scottish Universities Physics Alliance). These are 1- and 2-meter instruments, with three telescopes each in Chile, Australia and South Africa, and one each in Hawaii and Texas. Working with a telescope in the Canary Islands and supplementing its data with several other southern hemisphere instruments, the network, according to Yock and colleagues, ought to be able to help us not only detect Earth-sized planets but measure them at substantial distances from their star. Says Yock:

“Our proposal is to measure the number of Earth-mass planets orbiting stars at distances typically twice the Sun-Earth distance. Our planets will therefore be cooler than the Earth. By interpolating between the Kepler and MOA results, we should get a good estimate of the number of Earth-like, habitable planets in the Galaxy. We anticipate a number in the order of 100 billion.”

It’s an intriguing idea. The Kepler effort is all about developing a statistical read on how often Earth-class planets occur in the galaxy by looking at a huge number of stars. Yock’s work demonstrates that low-mass planets can come out of microlensing observations through continuous network monitoring. The paper notes that in recent times, about 10 events with magnifications over 200 have been detected by MOA and OGLE every year. Dropping the magnification requirement to 50 should quadruple the detection rate, allowing us to develop an independent check on measurements made by radial velocity and transit studies.

Consider how useful microlensing can be. We are not burdened, as with radial velocity and transits, with a measurement more sensitive to larger worlds closer to their host stars. The planets thus far detected through microlensing have had masses ranging from a few Earth masses to a few Jupiter masses, while their separation from their hosts has been on the order of several AU. This provides, says the paper, ‘a representative sample of the distribution of planets between us and the centre of the Galaxy that orbit a relatively unbiased sample of host stars.’

Thus we can supplement other ways of estimating the abundance of cool planets in the Milky Way by developing a statistically significant sample of detections through the kind of microlensing events Yock describes. The paper is Abe et al., “Extending the Planetary Mass Function to Earth Mass by Microlensing at Moderately High Magni?cation,” accepted for publication at Monthly Notices of the Royal Astronomical Society (preprint).