Could planet nine be a black hole?

We often think of blackholes like vacuums, but they are actually more like couch cushions...
03 February 2022

Interview with 

Becky Smethurst, University of Oxford


An artist's impression of a supermassive black hole


Becky Smethurst explains to Julia Ravey the possibility of a black hole affecting the Milkly Way as if it was a 9th planet...

Julia - So, the Hubble telescope recently detected a black hole which appeared to be giving birth to stars. Becky, is this what we expect black holes to do?

Becky  - So, this is something we call "outflows" or "jets" from supermassive black holes. It really confuses people because the definition of a black hole is something so dense that not even light itself can escape, so when people hear about something coming off a black hole, it's very confusing. It's not actually coming from the black hole itself when we talk about outflows or jets. Essentially, as you chuck material towards the regions of a black hole, obviously everything is fighting to get into that black hole as that gravity is pulling it inwards. But, the material also heats up: it gets accelerated, it goes up to huge speeds and it actually starts to glow. Radiation like light starts to come off this material and you can get pressure from light. We talk about this radiation pressure like equipping spacecraft with solar sails - instead of wind particles hitting a sail, you've got light particles hitting a sail on a spacecraft in space and that can power it. It exerts a pressure that pushes outwards if you have this material around a black hole that can heat up, and it's actually how we spot the majority of black holes as well. They're some of the brightest objects in the universe which, again, is a bit of a weird one, but it pushes outwards against gravity pulling inwards. And so, if you actually chuck too much material in there, there's too much material that starts glowing that it actually starts to push back again, and you end up with this outflow from the black hole instead. I like to call it a black hole burp: it's like you fed it too much and it's just goes, "Eurgh! No", and that's what's happening. What I study is how those black hole burps can stop a galaxy from forming stars, but what the Hubble space telescope has found is that that burp is that actually causing stars to form.

Julia - James asked if two black holes can merge.

Becky - Yes. Two black holes can merge and we have actually detected signals from two black holes merging. These are not light signals - which is usually how we get anything from the universe, any information, whether it's visible light that we can see, or if it's radio waves or gamma rays or x-rays, they're all form of light - but for merging black holes, we get gravitational waves, which are essentially like ripples through space itself. Einstein said that gravity is like something massive which curves space, so you can picture this if you think of a trampoline or a stretched bedsheet, right, and you put a football in the middle of it and it will stretch your sheet out and it will curve the trampoline or the bedsheet. If you think about removing that, you'll get a shock wave that will go through the trampoline. So, if you have two black holes merging that are spiralling around each other with this huge effect because they're so dense, it does create these ripples that we then detect as the squishing and stretching of space.

Julia - And the last question we have here is, there's been some debate around if a ninth planet exists in our solar system. Do you think there is another planet or could it be something else?

Becky - This has been raging for year, even back to the 1800s when Neptune was first discovered and Neptune's orbit was a little bit weird and they thought, "maybe there's something beyond Neptune." And then of course they discovered Pluto and then they thought they'd solve that one and then it turned out Pluto was about the size of our moon and they realised it wouldn't be enough to shift up Neptunes orbit. Then, they realised that Pluto's orbit was weird, and then they discovered a lot of things similar to Pluto that were near Pluto, which is one of the reasons why Pluto got demoted as well (I'm sorry to the internet who still haven't recovered from that news). All of those things have very strange orbits as well, so the idea is that there could be another very massive planet, 30 times the mass of the earth something like that, beyond the orbit of Neptune, that's shepherding all these things into these weird orbits. We haven't found anything, so it could be that it's so far away that it's so faint that we can't spot it. But, there was a paper that came out a few years ago that was suggested maybe we can't see it, maybe we can't find it because it's a black hole, and that's why. It would be a black hole that would be about the size of a tennis ball, but it would be 30 times as heavy as the earth, just to give you an idea of the density of black holes. The idea that the solar system could have this just like pet black hole that's just lurking around on the edges just makes me so happy. Dog might be a man's best friend, but a black hole in the solar system is a woman's best friend in my opinion.


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