‘Microlensing’ might reveal planets in other galaxies

We know that planets orbit stars in our own galaxy, but what about in other galaxies? It seems reasonable that other galaxies should be similar to our own Milky Way, but how can we be sure?

The mediocrity principle, from the field of philosophy, states that an object sampled at random from a larger set is most likely to come from the most common category. For example, in a set of 100 balls, with 98 of them green and two red, a ball chosen at random is likely to be green. Of course, it’s still possible you could draw a red one.

This principle has been used to argue that Earth, our sun and our solar system are not special. If life evolved here on Earth, then the mediocrity principle suggests there is life on planets orbiting other stars.

The problem with the principle of mediocrity is that it’s not always correct. For example, if there is intelligent life elsewhere in our galaxy, where is it? Some people believe that aliens have visited (or are visiting) Earth, but there is no scientific evidence of it. That suggests there might be something special about Earth or our solar system.

Two decades of evidence has shown that there are planets around many stars, called exoplanets. The most common observation is that a star will have a single, Jupiter-size planet that orbits close to the star, but in recent years, it was found that some stars have Earthlike exoplanets, along with other planets similar to our own solar system.

If planets are common in our own galaxy, and the laws of physics are the same everywhere in the universe, then it follows that there should be planets in other galaxies.

However, observations of other galaxies are difficult, because of the distance. To give a sense of scale, the nearest star is about 4 lightyears away, and the center of the Milky Way galaxy is about 25,000 light-years away. On the other hand, the nearest galaxy is about 2.5 million light-years away, and most galaxies are much farther.

Using a standard telescope, it’s impossible to see individual stars in another galaxy, unless it’s during an event like a supernova. Different techniques are needed.

A recent paper in The Astrophysical Journal Letters suggests that planets may be prevalent in other galaxies. Using a technique called microlensing, astronomers can use other massive objects such as black holes to magnify the light coming from faraway sources.

Albert Einstein first proposed that massive objects can bend light. But there are other ways to bend light, such as sending it through a material such as glass. A magnifying glass uses the principle of bending light, also called refraction, to focus light. If your eyes can’t distinguish the headlights of a car a mile away, then using a lens (such as in binoculars) can help. Similarly, if you want to distinguish details of stars in a faraway galaxy, bending the light can help. This is what a black hole can do, acting like a huge lens.

Of course, the problem is to get the black hole perfectly aligned between us and the galaxy in the background. That doesn’t happen often, but it does happen.

Using microlensing, the new paper reports the presence of iron in the spectrum of light from a background quasar. Based on frequent shifts of the iron emission light, this suggests that planets likely exist in the background galaxy.

Though some doubts remain about the interpretation of these data, it is a first step. Future observations using the microlensing technique are needed to be sure.