Gravitational microlensing has been actively employed in the search for MACHOs (Massive Astrophysical Compact Halo Objects) in the galactic halo, although with ambiguous results. The idea here is to find large, dark objects by detecting the microlensing effects they produce on stars behind them. While these dark matter studies have looked toward the Large Magellanic Cloud, we are using the same technique elsewhere in the planet hunt, finding that exoplanets can magnify the light of stars behind them in the galactic bulge, producing a clear detection.

Remember, for this kind of work, you want a dense background field of stars because the alignment needed for microlensing is obviously rare. The Magellanics are ideal, as is the galactic bulge, and so, for that matter, is M31, the Andromeda galaxy.

And if our early exoplanet work, relying on radial velocity and transit methods, has naturally produced large planets in the Jupiter class, microlensing can be quite effective at smaller scales. Now a new paper examines yet another benefit of the technique, that it works better at large distances from the source star, giving us the chance to detect planetary systems not only here in the Milky Way but in other galaxies as well.

The paper, by Gabriele Ingrosso (INFN, Italy) and colleagues, notes that at these distances, only giant stars with large radii can produce detectable microlensing events. The work is tricky because the source stars cannot be resolved by ground-based telescopes. ‘Pixel lensing’ is the name used for gravitational microlensing in such situations, and applied successfully, it can tell us much about the distribution of matter in a galaxy like M31, showing us its own dark halo objects as they cause microlensing of starlight in the background disk.


Image: The M31 galaxy may be offering up evidence of planets in its halo, if pixel lensing data can be correctly interpreted. Credit: Space Telescope Science Institute.

In fact, there have been a small number of microlensing events already detected towards M31 by two different collaborations, and planets may well be in the mix. Note this from the paper, which explains how exoplanet discoveries tie in with ongoing work on compact dark matter objects in the galactic halo (I’ve deleted internal references for brevity):

…new observational campaigns towards M31 have been undertaken… and hopefully a few planets might be detected in the future, providing a better statistics on the masses and orbital radii of extrasolar planets. It is in fact expected, and supported by observations and numerical simulations, that almost any star has at least a planet orbiting around it… In other words… the rate of single lens events towards M31 may suffer of a strong contamination of binary lensing events, most of which are expected to be due to extrasolar planets.

Planets in Andromeda? The mind boggles at the thought of detecting such, not that we don’t assume they’re present, but who would have believed our technology capable of such a reach? One anomalous pixel-lensing event has already turned up, possibly indicating a planet some six times as massive as Jupiter. It’s too early to claim a planet in Andromeda’s halo, but the candidate event labeled PA-99-N2 looks suspiciously like one, and pixel lensing itself seems destined to flag more.

The paper is Ingrosso et al., “Pixel-lensing as a way to detect extrasolar planets in M31,” accepted for publication in Monthly Notices of the Royal Astronomical Society and available online.