Until now, scientists haven't detected any planets outside of our own Milky Way Galaxy - it's simply been too difficult to discern such small things from so far away. That should make you wonder: are there any planets outside the Milky Way? A new paper might be able to answer that question.
Researchers at the University of Oklahoma looking at a galaxy 3.8 billion light years away spotted evidence of planets. More specifically, they think that there should be at least 2,000 objects, ranging from moon- to Jupiter-sized, per main-sequence star in the galaxy, based on how the galaxy's gravity warped the objects behind it. This is not direct evidence, mind you; no one has spotted any actual planets. But it's evidence nonetheless.
"It is natural to hypothesize that planets are common in external [non-Milky Way] galaxies as well," the authors write in the paper, published in Astrophysical Journal Letters. "However, we lack the observational techniques to test this hypothesis, because compared to their Galactic brethren, extragalactic planets are much farther away and much more difficult to separate from the host stars/galaxies." So they came up with a more roundabout method.
Normally, researchers spot exoplanets through the way they dim the stars they orbit when they pass between the star and our telescopes. Sometimes they even identify distant planets through direct imaging, actually resolving the planet from its host star.
That doesn't work if you're looking at distant galaxies where you can barely tell the stars from one another. So instead, the researchers used a technique called "quasar microlensing." What's that? Well, according to Einstein's theory of General Relativity, really massive things, like galaxies, stars, and even planets warp the shape of space itself. That means that they also bend the light travelling through the warped space. And if they bend a bunch of light rays the right way, they can actually act magnify the light behind it, acting like a huge magnifying glass.
In this case, a distant galaxy called RXJ1131−1231 has warped light in such a way that astronomers could see several magnified images of a galaxy behind it in the sky. As they looked closer and closer, the researchers noticed much tinier changes, like nicks or irregularities in a lens. In their new paper, the researchers demonstrated that a population of rogue planets (i.e. planets not orbiting stars) in RXJ1131−1231 could account for the lensing at the smallest scale.
Scientists have previously spotted this kind of microlensing event using the Andromeda galaxy as the magnifying glass, which some have proposed could be evidence for planets.
The paper doesn't name any planets or anything like that, as the study's author Eduardo Guerras says in a press release: "This galaxy is located 3.8 billion light years away, and there is not the slightest chance of observing these planets directly, not even with the best telescope one can imagine in a science fiction scenario." But they were still able to use their calculations to estimate the number of planets and masses.
We tend to assume that other galaxies have planets - after all, why should the Milky Way be unique? - but this could be the first, albeit indirect, evidence for these distant worlds.