We've Discovered Planets Outside Our Galaxy for the Very First Time

We "know" a lot of things exist without ever actually seeing them: the Earth's core, dark matter, a fitted sheet that's been properly folded. Of course, hard evidence is always best, so when we find it, it's extremely exciting. We've always "known" that there are planets outside of our own galaxy, but we've never actually discovered them. Until now: on Feb. 2, 2018, astrophysicists announced the first discovery of planets outside the Milky Way. This. Is. Big.

Distant Quasar RX J1131

NASA/CXC/Univ of Michigan/R.C.Reis et al

Come Out, Come Out, Wherever You Are

It's been hard enough to find exoplanets within the Milky Way. We didn't discover the first planet outside of our own solar system until 1992, and since then we've discovered just shy of 4,000. There are a few different ways scientists find these distant worlds, and each technique works best for planets at different distances. The radial velocity method, which detects the faint wobble of a star affected by the gravity of an orbiting planet, works best for close neighbors — planets up to 100 light-years from Earth. The transit method detects planets hundreds of light-years away by measuring the slight dimming of a star as a planet passes in front of it. But for really, really distant planets, you're gonna need gravitational lensing.

Gravitational lensing works thanks to a principle in Einstein's theory of relativity, which says that matter curves the fabric of spacetime. If you have enough matter in one place, light will actually curve around it. Just like light bends with the curve of a telescope's lens, light from distant objects bends around stars, galaxies, and black holes to expose and magnify it in a way we can detect.

How We Found Them

Gravitational lensing — or, more specifically, microlensing — was key to this big discovery, which was made by astrophysicists Xinyu Dai and Eduardo Guerras at the University of Oklahoma. They studied the quasar RX J1131-1231, which sits 6 billion light-years away, as it was gravitationally lensed by a galaxy passing in front of it at 3.8 billion light years away. Quasars are supermassive black holes that shoot jets of plasma that are rich in X-ray light. As the galaxy passed in front of the quasar, the gravitational lens magnified and multiplied it into four separate images, as you can see in the image below.

Image of the gravitational lens RX J1131-1231 galaxy with the lens galaxy at the center and four lensed background quasars. It is estimated that there are trillions of planets in the center elliptical galaxy in this image.

University of Oklahoma

Using data from NASA's Chandra X-ray observatory, the researchers found a a peculiar signature in the energy from the quasar that they concluded had only one explanation: planets in the lensing galaxy. It turned out to be around 2,000 planets ranging from the mass of the Moon to the mass of Jupiter, with trillions more waiting to be discovered in the galaxy.

Don't hold out for pictures, though. "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," said Guerras. But we know they're there, and that's reason enough to celebrate.

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