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Scientists may have found the light from two merging black holes, and a gas giant, without the gas.

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[SciShow Space intro]

It's been about five years since scientists first recorded evidence of a pair of black holes smashing and merging together. That evidence took the form of gravitational waves, the rippling of space-time itself.

Those waves have taught us a lot, but something has also been missing: unlike most things in astronomy, we haven't been able to see any of these mergers with light. That kind of makes sense, since black holes are, you know, completely black. But for a while, scientists have suspected that there is a way these collisions could light up. 

And last week in Physical Review Letters, astronomers reported that they may have finally seen it happen. By definition, black holes don't give off light of their own, but we can still see many of them because they tend to heat up nearby gas and dust. And as that material gets hotter, it glows. The problem is, this hasn't been relevant for the mergers we've detected among pairs of black holes because those mergers have generally happened way out in the middle of nowhere where there's not much dust or gas to be found.

But hypothetically, if a collision happened in a denser area of space, we might see something. Like, say, at the center of a galaxy. In the hearts of some galaxies, supermassive black holes are actively gobbling up the matter swirling around them in an accretion disk. Astronomers call these areas active galactic nuclei, or AGNs. 

But turns out AGNs can have more than just one black hole: they can also be home to a pair of much smaller black holes that might one day emerge together. Theoretically, when that happens, the new black hole from the merger gets a huge kick. Its path around the center of the galaxy shifts and it plows across the accretion disk, causing the gas to release an extra burst of light that fades over the following weeks. 

In last week's study, a team of astronomers was hunting for events like this. They were trying to match gravitational wave events with flares of light coming from the same point in the sky within a few weeks of one another. And in some data from last year, it seems like they found one.

In May 2019, two key observatories found a gravitational wave candidate likely coming from the AGN inside a distant galaxy. And based on the waves, it seems to have come from a pair of colliding black holes. Meanwhile, the ZTF sky survey recorded a flare of visible light coming from that same galaxy about five weeks later. 

If these events are related, this would confirm our hypothesis and give significant insight into the physics of black hole mergers. Like, it could teach us more about exactly where the black holes came from and how they got there.

But it's important to point out that the flare's origins still aren't 100% certain. The team has worked to rule out a supernova, a typical flare that happens when a black hole eats a star, and just normal activity from the supermassive black hole, but there's still some uncertainty.

And technically, the gravitational wave data has yet to be confirmed as an actual merger. Still, there's some good news here. If the models are correct, the new black hole will cause another flare in the accretion disk in a few years. So we just need to keep our eyes and our telescopes open.

 Gas Giant Core(3:04)

Meanwhile, in other news, astronomers seemed to have identified what's, honestly, a pretty weird planet. Their paper was published Wednesday in the journal, Nature, and it describes a planet that could be the core of a gas giant that has had most of its outer layers blown away.

The planet was spotted in 2018 by NASA's satellite Tess, and it's called TOI-849B. It's the first, and so far only planet found in its system and it orbits a Sun-like star so closely it takes less than a day to make one revolution. Thanks to a suite of other telescopes, we also think it's about 90% as wide as Neptune but more than twice as massive.

That means the planet has an extremely high density for its size. It's actually not that different from Earth's density which suggests it's mostly made of rock.  That might not seem like much of a big deal, but this is where most of the mystery comes in, because as far as we know, planets that massive should be gas giants with huge envelopes of Hydrogen and Helium.

But according to computer models, this planet's outer layer of gas makes up less than 4% of its mass. So either this planet never managed to build up the usual atmosphere most gas giants do when they're forming or this planet started as a much bigger planet and has lost most of its atmosphere.  And that could have happened in a number of ways.

The planet could have collided with another object or been heated up so much by its star that some of its atmosphere puffed up, up, and away. Or maybe, the gravitational tugging between the planet and its star ripped the layer off.  Whatever the mechanism ends up being, it'll be an important puzzle piece for understanding how planets form.

And TOI-849B could also help us solve the mysteries of gas giant interiors. The cores of Jupiter, Saturn, Uranus, and Neptune are very hard for us to study given that they're buried beneath kilometers of gas and liquid. So TOI-849B and other planets, even though they are lightyears away, could offer astronomers an easier view of what's inside gas giants.

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