[♪ INTRO] Most of what we know about the inside of our planet comes from looking down and studying Earth’s interior itself.

But there’s still a lot we don’t know about our planetary home, and scientists are always looking for better tools to help us learn more. And just last week, a group of physicists announced that we might be getting a new one.

In a paper published in the journal Physical Review D, they calculated that a detector that’s already being built will be able to study Earth’s interior using neutrinos, some of the most elusive particles in the universe, which are constantly raining down on us from space. We learn a lot about the inside of our planet from earthquakes. For example, based on the types of ripples we detect from an earthquake, we can tell if it traveled through a layer of liquid.

By combining what we know about how ripples travel through matter with measurements of things like gravity, scientists have formed pretty good models of what Earth and other planets look like beneath the surface. But today’s methods leave a lot of unanswered questions still, like how different layers of rock move and change to let heat flow through the inner Earth. The authors of this new paper argued that’s where neutrinos can help: The way they travel through the Earth can give us a brand-new way of understanding its internal structure.

Neutrinos are tiny particles that hardly ever hit anything else, and they come in three types, called flavors: electron neutrinos, muon neutrinos, and tau neutrinos. But a neutrino doesn’t always stay the same flavor. It can switch between flavors as it travels.

And even though they hardly ever directly interact with other matter, the density of matter around a neutrino helps determine how quickly it switches between flavors. Generally, they spend more time as electron neutrinos in denser matter. And the team pointed out that we can use this weird property to study Earth’s interior.

For example, if we see more electron neutrinos among the neutrinos that have passed through certain parts of the planet, we know those parts are denser. Versions of this idea have been around for a few years, but we haven’t had detectors sensitive enough to tell us how the number of extra neutrinos changes when they travel through different parts of the Earth. So these authors carefully calculated how sensitive a detector would need to be for this fine-grained measurement.

And they showed that a detector called DUNE, which should be operational in 2027, will finally be able to measure Earth’s internal structure using neutrinos. So within a decade, we might be using the universe’s ghostliest particles to study our own little planet. Meanwhile, other researchers are looking into some of the strangest galaxies we’ve ever seen.

In this week’s Nature, a group of astronomers published a study on jellyfish galaxies, and what they can teach us about why some supermassive black holes suddenly start feasting. And yeah, jellyfish galaxies are a real thing! Galaxies speed toward or away from the stuff around them at hundreds of thousands of kilometers per hour.

But as they travel, some galaxies ram into what’s called the intracluster medium, or ICM: Huge pockets of gas that aren’t connected to any particular galaxy. And when a galaxy passes through the ICM, its own gas can get dragged behind as it tries to push the ICM out of the way. This leaves long threads of gas and dust trailing behind the galaxy, making it look a lot like a jellyfish. When the team started studying a set of jellyfish galaxies, they noticed a connection to an unsolved problem in astronomy.

Almost all of the jellyfish galaxies had really active supermassive black holes at their centers that were eating lots of gas and stars. Just about all galaxies have supermassive black holes at their centers, but usually they’re not doing very much; gas and stars just orbit around them. Very occasionally, though, huge quantities of gas and stars are sucked toward the black hole, where they either fall in or get flung away with huge amounts of energy.

When that happens, astronomers will say that the galaxy or black hole is active. And they aren’t exactly sure what flips the switch from a black hole with gas and stars quietly orbiting it to one with stuff actively diving into it. The fact that all these jellyfish galaxies also had active black holes was an interesting connection, and the team wanted to know which came first: the active black holes or the jellyfish tentacles? If the black holes were active before the galaxies hit the ICM and became jellyfish, some of the material falling in toward the black holes might’ve missed and been flung out into the trails behind the galaxy, making even bigger jellyfish than would’ve otherwise been. But if the galaxies’ black holes were quiet before, the collisions with the ICM might’ve pushed a whole bunch of gas into the center to get eaten up, making the black holes active.

By measuring the galaxies’ speeds and where they were in their cluster, the authors found that for these jellyfish, collisions with the ICM almost certainly came first and activated the black holes. Almost all the jellyfish were moving quickly enough through the dense ICM that lots of gas would’ve been pushed toward their centers. And the one galaxy they observed that didn’t have an active black hole was moving really slowly compared to the others, meaning that it probably wasn’t moving fast enough to have its gas pushed into the center.

Astronomers already know that galaxy mergers and other large events can force gas toward a galaxy’s central black hole, activating it. But this new paper shows that something as minor as passing through a random pocket of gas can set galaxies off, which might explain a bunch of active, non-jellyfish galaxies, too. They might’ve slammed into something that pushed gas toward their centers, even if they didn’t come out looking like jellyfish afterward. So that’s one more reason to study some of the weirdest-looking galaxies out there: they can teach us a lot about all kinds of processes happening in the universe. If you’d like to learn about some more weird galaxies, you can check out our video about 3 galaxies that astronomers are pretty sure shouldn’t exist. [♪ OUTRO]