The Galaxy is Not Enough

Megalomania can sometimes be good. Well, with a lot of emphasis on the 'sometimes'. Here we are as a species, scooping up detections of planets around other stars at a ferocious rate. Our galaxy is however a big place, and we're going to be sifting through its two hundred billion or so stars for a very, very long time. Eventually though, presuming that we remain aware of the deeper meanings of the word civilization, we might find ourselves not content to be just parochially curious. We talk about life in the universe but the Milkyway is merely one of a couple hundred billion galaxies in the observable cosmos. Can we seek out planets in other galaxies?

Even now the answer is a surprising 'almost'. A paper by Ingrosso et al. about a year ago in the Monthly Notices of the Royal Astronomical Society, discusses in detail the possibility of seeing evidence for planetary mass objects in gravitational microlensing events occurring in the Andromeda galaxy (M31) - a mere 2.5 million light years away across our backyard. As with lensing detection of planets in our own galaxy the mechanism is that a foreground star with planets (in this case within Andromeda) acts as a gravitational lens on the light from a background star (also in Andromeda), dramatically magnifying it as the two systems drift into close alignment from our point-of-view. At the distance of Andromeda the best we can do is see 'pixel-lensing', which is a variation in the number of photons coming from individual pixels in our digital cameras, each of which contains the light from many indistinguishable stars - too close together on the sky to be separated. During magnification the tiny lensing effect of planets can also be greatly enhanced, adding characteristic spikes and wiggles to the way the light varies during the alignment.

By carefully modeling what one might expect to see, Ingrosso et al. demonstrate that there is a pretty good chance that in a few percent of Andromeda lensing events you should find the signs of Jupiter-scale planets if they are there. They even go so far as to offer support for the detection of a 6 Jupiter mass planet in a rather funky lensing event seen in Andromeda by An et al. with the gripping name of PA-99-N2. As with all micro-lensing studies there are huge challenges owing to the amount of data needed to spot events and to fully characterize them as they play out over days to weeks. The pay-off is an incredible statistical cleanliness and sensitivity.

Probing for planets in Andromeda may therefore be fruitful. What of further afield? There's the rub. The same technique might be applied to more distant galaxies, but it will require bigger telescopes, capable of fitting more pixels across the faces of these stellar swarms. It could be worth it though. Suppose we were able to evaluate the planetary populations (albeit statistically) in a representative chunk of intergalactic space. That would be an incredibly interesting set of numbers. It would tell us what galaxy types - elliptical to spiral - are most fertile grounds for other worlds, and take us closer to answering the question of the true capacity of this universe for life.