The Supermassive Podcast - 49: BONUS - Double Yolker Galaxies

Episode Date: February 12, 2024

Do we see black hole mergers as a result of galaxy mergers? What did the night sky look like to the dinosaurs? Is a gas giant just a rocky planet with a huge and dense atmosphere? Do all orbits eventually end up in resonance?  The Supermassive Team is back with the first bonus episode for 2024! Send in your questions to [e-mail] or find us on Instagram, @SupermassivePod. The Supermassive Podcast is a Boffin Media production for the Royal Astronomical Society. The producers are Izzie Clarke and Richard Hollingham. 

Hello and welcome to another bonus episode of the Supermassive podcast from the Royal Astronomical Society with me, science journalist Izzy Clark, astrophysicist Dr Becky Smethurst and the society's deputy director Dr Robert Massey. Yeah, this is a place where Robert and I take on even more of your questions from the Supermassive mailbox because, let's face it, you all have such good questions, you keep sending keep sending them in there's just too many we can't fit them all in the normal episodes first though i just want to say a big thank you so much for the lovely messages about our multiverse episode from last year like they were just fab weren't they to read it was so nice to hear that you all enjoyed it um and you'll be pleased to hear that uh listener wendy entwistle almost crashed her car when she heard her name mentioned on that episode so i hope she doesn't

almost crash her car when she hears this one as well yeah that's something to bear in mind yes we do not take any responsibility no i'm joking uh yeah i'm not liable thank you okay robert let's go on to some questions can you help with this one from ewan from newcastle upon tyne uh izzy can we say it properly please what it's newcastle i think you'll find i was like his name's ewan what no newcastle is newcastle i i'm not going to offend the lovely people um so he says hi izzy becky and robert i'm a long-time listener first time emailer i love the podcast and haven't yet managed to stop myself listening the day it comes out and having to wait a whole month again for a new one yeah a lot of people complain about that

actually sorry two weeks two weeks now because we give you bonuses my question is why do some objects end up in orbital resonance such such as the Galilean moons, and others, such as the planets in the solar system and any of Jupiter's or Saturn's other moons, orbit at other more random seeming periods? Do all orbits eventually end up in resonance, given enough time, or is there something else to it that I'm missing? Thanks, Ewan. And thank you, Ewan, for listening as well. I listened with such enthusiasm. And it's really good to start with a question like this

that indeed makes me think. And there are a lot of resonances in the solar system, like the giant planet moons you mentioned around Jupiter and also, for example, Neptune and Pluto in a resonant interaction. And in practice, that means that the ratio of their orbits is a kind of neat multiple. So in the case of Neptune and Pluto, there's a two to three ratio in their orbital periods. But in some cases, you see things that look like they're close to resonance,

or actually coincidences like the Earth and Venus. And in some cases, rather than making things more stable, resonances actually remove objects. And the best example is where you get gaps in the asteroid belt. And that's a result of the interaction between mostly Jupiter and the asteroids clearing them out and then well thinking about our previous episode on how the world is actually sending some of those asteroids in towards the inner solar system and towards the earth but in a neatly formed planetary system the idea would be that you'd get more of those things start off a bit neater from a collapsing rotating disc and so you know those resonances are more likely however in astronomy everything rarely stays that neat and so any kind of

interaction sometimes say if you've got passing stars nearby all those kind of things tend to mess things about and simply the motion of larger planets with respect to each other and when we look outside the solar system so thinking about how often this happens from what i could tell about one percent of the solar systems that we found elsewhere show really obvious resonances so it's not quite as common as you think but it's undoubtedly a thing and it also just doesn't necessarily last over time okay i'm becky james evans on instagram asks hey guys love the podcast and as a non-scientist, I appreciate you making everything always very digestible. Good. I have a question though about the gassy planets. I struggle to understand why is it that a gas planet is not just a small rocky planet with a huge and

dense atmosphere? Some distinguishing here would help my brain compute this. Thanks. Okay, James. I mean, they sort of are, right? I mean, the cores of the gas giant planets are thought to be made of like the same composition as rock, at least rock with a very high concentration of metal in there, like nickel and iron. One that's obviously a few thousands, if not tens of thousands of degrees in temperature.

So they're sort of like lava surrounded by a thick dense atmosphere which i guess you could say is just a rocky planet with with an atmosphere i think james what you're getting at is asking almost why it's not like earth's atmosphere where we can like see down to the surface and see down to the rocky surface um like through the gas and that's just because like the atmosphere of gas giants um you know they are so much denser and they're made from different things to our atmosphere as well and i think people struggle with this because they think of like the air around us as being transparent but if you think about in terms of like gases when they get very dense and when they're very concentrated so think about steam from a kettle or when dry ice is sort of you know puffing off all of that gas as well like the gas right

sensor is opaque and then as it diffuses around the room it then it then turns transparent because the molecules get further apart and then they're not just transparent they're not opaque anymore so gas giant atmospheres because they're big, the atmosphere is very dense because there's so much of it as well. And it's held under these intense pressures. And so the molecules are very densely packed. And so the atmosphere is therefore opaque and we can't see through them down to that little larvary rock core right at the very center. It's the same as, you know, if Earth was just completely covered in clouds, if it was cloudy everywhere on Earth all at once, right? Because clouds is just very dense water vapors, very dense gas that would block the view down.

And that's pretty much what's happening on a gas giant. There we go. Robert, Emily has also slid into the DMs and says, I've been blissfully falling asleep while listening to you discussing dark matter, multiverse, fiery planets and all that jazz there's no better way to relax while keeping curiosity and hope alive in these dark times i wonder if she feels like that after our end of the world episode well i carry i'll carry on i'll carry on there's just one problem questions keep popping up but i fall asleep and forget them another one did pop up during the day, though, today. How did the night sky look when the first dinosaurs appeared on Earth?

Was it brighter since the universe was more contracted than it is now? Anyway, thank you for making my nights brighter throughout the year. You're welcome, Emily. And I was thinking about this all again. The first dinosaurs appeared about 250 million years ago. And coincidentally, that's about the time it takes for the sun and the planets the solar system to go around the galaxy to complete an orbit around the galaxy so you might think oh you come back to the same stars and they would look similar but of

course actually the difficulty with that is that all the stars are moving around the galaxy in their own right all at slightly different speeds in their own orbits and so the sky would look completely different we've absolutely no idea what it would look like actually there's probably really no way of telling because it is true also over that time period that some of the brightest stars would have would indeed have exploded as supernovae and are no longer there so really hard to say but what wouldn't have changed very much on that time is the size of the universe because if you remember that's 13.7 billion years old so the sun and the planets are really you know they're in recent history on cosmological time scales

so the universe would only is only a bit bigger than it was back then so the this you know and also within galaxies and so on it's not that galaxies are getting bigger as a result of the expansion of the universe at least not unless we go forward to things like a terrible big rip or something like that. So realistically, that wouldn't make a difference, but you would have a very, very different night sky. We just really have no idea what it would look like. Okay. And Becky, can you answer this question from Brian Russell?

He says, hello, Supermassive podcasters. Hello. Today's astronomy picture of the day, which is ARP 87 merging galaxies from Hubble inspired me to ask do we observe black hole mergers as a result of galaxy mergers cheers Brian Brian I love this question first of all because I got to look at a picture of ARP 87 again which is fantastic for anyone who's not seen it you should go look at it but you've also stumbled on a bit of a mystery in black hole astrophysics which you know is my favorite the smile on becky's face right now is like well well well

so after two galaxies have merged together we do see things that i like to call double yokers right because it's like when you crack an egg and there's two yolks in the egg and you're like yes never happened to me but i love today oh so many times it's the best thing um but essentially what it is when you have it in a galaxy essentially you've got two growing super massive black holes in the center of a galaxy and they're glowing because they're growing so that we can see them and it looks like almost like the galaxy's got like two little eyes in the middle right or two yolks of a fried egg and it's double yoker the thing is the time it takes for the super massive black holes to merge together is much longer than the time for the galaxies to merge you know for all the stars to slosh together and for it to stop looking

like a mess and start looking like a galaxy again that takes a few billion years supermassive black holes another couple billion years on top of that now the extra issue is something called the final parsec problem oh yes fantastic right we've got the final parsec problem which essentially says that when two black holes that are orbiting each other two super massive black holes when they get to about a parsec or so away from each other which is about three light years away you know as they're spiraling closer and closer and closer at that point there's no way for those black holes to lose enough energy for them to get off that orbit where they're about three light years apart from each other and if you can't lose energy you can't shrink the orbit

further so that they actually merge together that's what the maths tells us so technically supermassive black holes shouldn't merge but then we see like this really clear correlation between galaxies where the more massive the galaxy is the more massive the supermassive black hole is which we're like oh that's probably from mergers you know you merge a galaxy together you grow it you merge the two black holes in the same process so the mass is telling us there's something weird going on here that either we've we've overlooked in terms of our knowledge of black holes or maybe black holes don't actually merge when they're supermassive like we don't know because we never we never actually observed one because the time scale is so long that's the problem now the LIGO detectors that you know

detect mergers of star mass black holes right they're you know like 10 times the mass of the sun rather than 10 million times the mass of the sun rather than 10 million times the mass of the sun like a supermassive black hole they're only sensitive to the smaller mass black holes merging together right so we could never detect this with ligo if that's what you were thinking we need a much larger detector to do that and this is why i was asked for this got very excited last year do you remember izzy when there was that big announcement about the pulsar timing arrays that had detected the graph like the light year-long gravitational waves by using like pulsars scattered around the galaxy and that was like your gravitational wave detector and we figured those light year-long gravitational waves one of the most likely culprits for them

would be two super massive black holes spiraling you know around each other not just two but like many like a huge population of those kind of things happening not merging but spiraling yes so you know like it was sort of like a sort of i don't know like a pat on the back like okay this probably does happen these these pairs of supermassive black holes do exist but the hope that's riding on all of this is the lisa gravitational wave detector i think we've mentioned this on the podcast before because it's just so insane but essentially it's a gravitational wave detector in space that's the size of the earth sun orbit like you put like three detectors in a triangle just follow the earth around it's wild yeah it's crazy i love it so much and that's how big of a detector you would need to actually, you know, record like the gravitational waves from two supermassive black holes

merging if they do actually merge and they don't just stall because of the final parsec problem. So I did say, Brian, you've really asked a question there about a big mystery, but it's a really cool one. I mean, such a short question from Brian and it's blown my mind just a little bit. Isn't that the classic me though? Short question, horrendously long answer. I mean, it's wicked. It's wicked.

Okay, well, thank you everyone. Do keep the questions coming. We try and read all of them. You can email podcast at ras.ac.uk and we're also on Instagram at supermassivepod. We will be back next time with an episode all about quakes, whether that's here on earth with earthquakes or on other planets entirely. But until next time,

everybody happy stargazing.