Spacewalk: the Scale of our Solar System

Take a tour of our planets faster than the speed of light
22 August 2022
Presented by Julia Ravey, James Tytko
Production by Julia Ravey.


A giant model of the sun on Midsummer Common in Cambridge


If you were offered the chance to go up into space, would you want to go? A trip to Mars may sound like a big adventure but in reality, the journey to one of our nearest planets would take an estimated 7 months. And if you are wanting to move beyond Mars to Jupiter, you are talking years of commuting. The scale of space is difficult to wrap our heads around - there are zeros upon zeros when talking about distances between our neighbouring planets, never mind those in further reaches of the milky way. But a scaled-down sculpture trail of our solar system has allowed us to take a tour of our planets, whilst also learning about living on Mars, exploding stars and what those beautiful James Webb Space Telescope images are really telling us... 

In this episode

Purple arch with the word "Mercury" on top pointing to a tiny pea-sized planet

01:03 - How long does sunlight take to reach Earth?

And when compared to the size of the sun, just how small are the planets in our solar system?

How long does sunlight take to reach Earth?
Matt Bothwell, University of Cambridge

Julia Ravey takes a walk through space... Well, not really. But she went down to Midsummer Common in Cambridge where there is a space walk! Julia met up with Matt Bowell who gave her a guided tour of our planets...

Matt - It's the scale of 590 million to one. So the sun we are looking at here in front of us is about a bit less than two and a half meters in diameter; 590 million times smaller than the real thing. But the nice thing about this whole sculpture trail is that the entire solar system is to scale. Even when you see pictures of the solar system, it's never to scale. It's impossible to get the real scale of the solar system into one picture. The only way to do it is like this, to lay the solar system out over about 10 kilometres so you can see how much space there really is.

Julia - Shall we spacewalk?

Matt - Yes, let's space walk. We can start off if you want, by walking at the speed of light.

Julia - Relative to this?

Matt - Yes, relative to this, I'm not that fast a runner. So we are exploring a 590 million to one solar system. If we scale the speed of light down by 590 million, then you can walk at the speed of light on this scale. It's about this fast...

Julia - A bit of a shuffle...

Matt - It's a little bit less than two kilometres an hour. I think it's about a toddlers walking pace.

Julia - So if I ran along here and I mean, I am not fast, I'm slow - I would be running much faster than the speed of light?

Matt - You would, you would be super luminal.

Julia - Well, I'm going to take that and I'm going to do my run along here at some point and tell everyone I was running faster than the speed of light.

Matt - You could probably do 10 times faster than the speed of light easily.

Julia - Matt don't flatter me! But here we are, I'll take it.

Matt - It would take us eight minutes to do this short walk to the earth if we were going at the speed of light. So cosmically, the speed of light is sort of a crawl.

Julia - A cosmic crawl. I like that. I feel like that's what I should call this walk.

Matt - We're approaching Mercury now. Do you want to describe what you're seeing?

Julia - It's a nice big purple sculpture with "Mercury" on the top. And it's pointing down to what looks like a bead, the size of a generous garden pea.

Matt - It's sort of reddy-orange; a suspect garden pea, maybe. I think it's about five or six millimeters across. It's really extraordinary when you think of how enormous the sun is. And then you see this tiny thing that could fit on your fingertip. This is how big Mercury actually is.

Julia - It's an actual chickpea.

Matt - It is a chickpea.

Julia - What's your favourite fact about Mercury?

Matt - It has the highest temperature difference in the solar system. Because there's very little atmosphere, the temperature of the sun doesn't get conducted around the planet very well. And so even though the side of Mercury facing the sun gets cooked to like hundreds and hundreds of degrees, the dark side of mercury is freezing. It's like minus 200 C.

Julia - A tale of two planets.

Matt - Yeah, absolutely. Actually, can I give you two facts? So Mercury is very iron rich and dense. Mercury really resembles Earth's core quite a lot. So if you think about Earth, Earth has this iron rich core surrounded by the liquid rock mantle. And the crust of Mercury is really just a big ball of iron. So one theory is that mercury is the core of a planet that used to be much larger and maybe was involved in some sort of cosmic car crash that smashed off all the mantle and the crust and just left behind the planet's core. So it might be the leftover centre of a dead planet. One fact about this scale solar system, which I really enjoy, is how far it would take to reach Proxima Centauri, the nearest starter earth. Laid out on this same scale, you'd have to walk all the way around the Earth one and a half times, and then you'd be at the nearest star.

Julia - So if I was stood here in this scale, it wouldn't be like I could walk to London and find a star?

Matt - No, on this exact scale, eight kilometres up to Waterbeach, you would have to carry on walking until you go all the way around the earth and then carry on once you've returned back to the same point and then go all the way around the earth, down to Australia. And then at that point, that's the nearest star.

Julia - That is ridiculous.

Matt - It feels like a long walk Pluto, but then this is the dense parts of the universe where we live, right? This is our nice, hot, dense, bright cosmic backyard. Once you've left Pluto, you would have months and months and months of walking through nothingness to reach a star.

Julia - Wow. Space is big.

Matt - I've heard this feeling called cosmic vertigo. This idea of almost intense, overwhelming dizziness you get when comprehending the scale of the universe. I think astronomers like that feeling.

Julia - I think they like it - that's like adrenaline for them. For me, I feel a bit car sick.

Matt - Maybe we're cosmic rollercoaster junkies or something. We just like it.

Julia - Keep riding that rollercoaster!


07:02 - What will happen when the sun dies?

Studying exploding stars - or supernovae - requires observing millions of galaxies per night

What will happen when the sun dies?
Stephen Smartt, Queen's University Belfast

Our Place in Space is an incredible collaboration between art and science. James Tytko spoke to Stephen Smartt, astronomer at Queen’s University Belfast who studies supernovae and was key to making the scaled-down solar system project happen...

Stephen - It was interesting to get together originally with the artist Oliver Jeffers. It was his idea to create a scale model of the solar system. And so my job was to make it scientifically accurate and also to try and create a scale that we can relate to. So I think we have done it and you can walk it, you can walk to Pluto. It's a long way, but it puts the size of the planets in terms of their distances into perspective, and then puts into perspective, the size of our solar system within the galaxy and the broader universe. I think the interesting idea from Oliver was not just to make it a scientific story, but to put humanity at the centre of this. And that's the idea of the big earth at the start, and then to walk the solar system and to see in reality how small earth is.

James - We heard from Matt a bit earlier that mind-blowingly the nearest star on the scale of the Our Place In Space walk would be about a lap and a half of the earth away. This is your area of expertise, isn't it? Can you tell us a bit about your research?

Stephen - I study stars mostly in other galaxies and stars that explode in other galaxies. So the nearest star to us in our backyard is Proxima Centauri. And as Matt said, that's one and a half laps around the earth. So the size of our galaxy, which contains a hundred billion stars, on this scale would be about the true distance between the centre of the sun in Cambridge and Jupiter and Saturn in the real solar system. Now, what I study are supernovae, the deaths of stars. They're quite rare, so we look at millions of galaxies every night to find these supernovae. And the distances to those on this scale are at least a thousand times the distance between the sun in Cambridge and the distance to Jupiter or Saturn. So they are quite immense distances we deal with.

James - Absolutely. You mentioned the death there of supernovae. That's quite dramatic terminology. What actually happens when a star dies?

Stephen - All stars greater than about eight to 10 times the mass of the sun will end their lives when the fuel runs out in the core. They survive by creating thermal pressure in the core through nuclear reactions, and they're trying to collapse under gravity. For most of their life, those two forces are exactly balanced and that's why stars are circular. At the end of their lives, when they've burnt through the nuclear fuel in the core, they can't support themselves. The core collapses and that releases a huge amount of energy and destroys the star. And these supernovae can as bright as single galaxies and may last for about a few weeks to a few months. That's what we search for on a nightly basis.

James - What about on a day to day basis? What are you looking for?

Stephen - We partner with several telescope surveys around the world. The ones we work with at the minute are mostly in Hawaii, and the nice thing about that is the time difference. So when Hawaii's observing 10 to 11 hours behind us, then it's our daytime and we can sift through the data while the telescopes are surveying the sky. There's some specialized telescopes over there that survey the whole sky every night. There are two telescopes in Hawaii, one in Chile and one in South Africa, and they're the same type of design. Probably , for the first time in history, we've been able to get to the sensitivity that we can achieve with these survey telescopes. And we basically look for everything that changes or moves in the night sky.

James - So all stars eventually meet their demise. What about our star, the sun? What what's in store for that one in its lifetime?

Stephen - That one will not create a supernova. A supernova is produced by a star, which is at least eight times the mass of the sun. So what happens with a star like the sun, these low mass stars, is that again, the core will run out of fuel. So the hydrogen will burn to helium. The helium will burn to carbon and oxygen. And at that stage, it will not be hot enough for the carbon and oxygen to burn any further. What happens in these stars, and we see it across the galaxy, is that they become what we call 'red giants' and the atmosphere swells up. So what will happen is that if you think of the sun on Midsummer common, it will expand by about a factor of a hundred and left over in its core will actually be something about the size of the Earth made of carbon and oxygen. That's what we call a 'white dwarf' and the atmosphere, which is very extended, will then just float away and will be heated by the radiation from the white dwarf. It'll form a nebular planet. So effectively, the sun will just swell up and eventually the other layers will just puff away and we'll be left with a very hot white dwarf, the size of this earth at the position of the sun. The timescales that we infer are very long, so although it sounds worrying that the earth will end up within a very tenuous, low density atmosphere of the sun and potentially be vaporized, this won't happen for another four and a half billion years. So the sun is about halfway through its lifetime, and the earth is about four and a half billion years old. This will last for another 4 billion years or so, until the sun becomes a red giant. We have to infer the timescales through looking at stars at different stages in their lives, and then apply sophisticated computer models. And that's mostly what astronomy is about; applying the physics that we know and understand on earth through sophisticated models to the observations that we take.

Purple sign saying the distance to the Sun and Venus on the "Our Place in Space" trail

12:45 - Why is the moon moving away from Earth?

And why Earth has such a plain name compared to the other planets

Why is the moon moving away from Earth?
Matt Bothwell, University of Cambridge

We are on a space walk at the Our Place in Space trail in Cambridge this week. Julia Ravey was given a tour by Cambridge University's public astronomer Matt Bothwell... 

Julia - We're approaching Venus.

Matt - Yeah. So we've reached Venus. It's another trek away from Mercury. We've upgraded from chickpea to grape. This thing is maybe a centimetre across. So one interesting fact about these planets, because this entire solar system is to scale, when you stand at the distance of the planets and look back towards the sun, the sun is the same size, physical angular size, as it would actually look on that planet. When you look at the sun from Venus, that's how big the sun would be in the Venus sky. Wow. So it's about twice as big as it appears from earth.

Julia - I was going to say - it's really hot and sunny here today. So when we get to earth on our next little bit of the trek we'll have to have a look and give it a bit of a comparison.

Matt - Yeah, exactly. Well, without looking directly at the sun, of course.

Julia - Yeah. I have got my sunglasses on, but even I'm not that brave.

Julia - Well, let's head to Earth. Venus, it's been nice knowing you, we're on our way. We have arrived home. Well, nearly home. We've got a little pit stop first at the moon. The moon is pea sized as well.

Matt - Moon is pea sized. Yes. And earth is more cherry tomato size, maybe a bit more grape size. Earth and Venus are actually very similar sizes.

Julia - Yeah. Are they sister planets? Is that what they get called?

Matt - Yeah. You can think of the Earth and Venus as sort of sister planets, maybe Venus is Earth's evil twin. Because of its horrible climate and it's sulphuric acid rain - it's sort of like looking in a dark mirror or something.

Julia - So if we turn back now and look towards the sun.

Matt - Yeah. So if we look towards the sun, the sun is the same size that it appears in our sky. And there's actually a really nice trick you can do.

Julia - Oh, I love a trick.

Matt - So, because the entire thing is to scale, if you put your eye level with where the earth is, you can create an eclipse.

Julia - I can just see the spikes and it's a total eclipse.

Matt - It perfectly eclipses the disc of the sun.

Julia - Yeah. It's a spot on match. That is fab. So you've got your own little eclipse here as well.

Matt - Yeah, exactly. Eclipses won't be around forever, actually, because the moon is actually getting further away from the earth.

Julia - Why is that?

Matt - Because it's losing energy. So if you think about tidal energy, the moon raises and lowers the tides, we can get energy out of that to drive our renewable energy. But that energy has to come from somewhere. And so that energy in the tides, we are basically stealing it from the moon's orbits. And so the moon loses energy and moves away from the earth. It moves away at around the speed that your fingernails grow, about a couple of centimetres per year. But that means way back in the past, the moon would be much bigger in the sky and would've completely blocked the sun and you wouldn't get any night eclipses. 500 million years in the future, the moon would've moved so far away, it'll be smaller than the sun and you won't get eclipses anymore. So we are living at the right cosmic time to see these eclipses.

Julia - A very special time to be alive.

Matt - Yeah, exactly. We're very lucky. I find it interesting that Earth is the only planet that was named before we knew it was a planet. So all of the other planets in the sky, I guess we named them before we knew there were planets, but they were named after Gods. The ancient Greeks and the Romans looked in the sky and knew that they were special. And so they named them after their gods. Earth is named after just dirt. I sometimes feel we should name it something a bit more poetic than dirt. We should name it Gaia or something. That'd be quite nice.

Julia - Have you ever thought of that? Our planet is literally called mud.

Julia - So we've got Mars here. This looks like a, what are they called? Red currents - Christmas time. That's what I'm thinking of.

Matt - I feel like we're running out of small foods to name these planets.

Julia - In terms of science and research, what is something interesting that's going on about Mars at the minute?

Matt - There's a Mars rover called Perseverance, which is busy exploring the Jezero crater. It's the remains of a dried up lake on Mars and we're looking for any signs of ancient life. Because in terms of habitable planets, Mars, a few billion years ago was pretty spot on

Julia - Matt. I'm going to leave you on Mars or you might float back to the sun and I'm going to keep walk through space.

Matt - Amazing. Well, this has been really fun. I've enjoyed walking the solar system with you. It's been great.

Julia - Yeah. It's been brilliant. And now I must traverse the rest alone. Never know who I'll find on my way.

Matt - I believe in you.

Julia - Thank you.

Astronauts need faster spacecraft, better radiation protection and heat shields before they can enjoy the Martian landscape in person.

18:28 - Did Mars once have the conditions for life?

And what would it be like to be dropped on the planet today?

Did Mars once have the conditions for life?
Simon Morden

How would you fancy taking a trip to the red planet? Julia Ravey spoke to author of "The Red Planet" and planetary geologist, Simon Morden, who gave her an insight about what life might be like on our neighbouring planet, and told her how he once held a bit of Mars in his hands...

Simon - I was working on achondrite meteorites. These are igneous type rocks formed from molten rock, but out in space. I was looking for raw iron and nickel specs inside this meteorite, and I didn't find any. And I thought that was really disappointing. What I did find was I found a lot of iron oxides. So I just wrote a note saying, I think this sample is contaminated, it's been weathered. And it was six months later that I discovered that I was half right in that yes, that meteorite had indeed been weathered, but it hadn't been weathered on earth. It had been weathered on Mars.

Julia - If I were to land on Mars today, what would I be met with? What would I see? And how would I feel?

Simon - First of all, I definitely advise you to go in a spacesuit because the average pressure on Mars is six millibars. That's probably as good as vacuum as you'll get. It'll be cold. Summer temperatures on Mars get up to 20 degrees or so, but on that same day, just before dawn, you'll be looking at -80 C because of the cold. All of the water vapour in the atmosphere has basically frozen out. It's still there as ice under your feet, but there will be none in the air at all. It will be baron and you will have not seen anything like it at all. Not that you can see very far of course, because the curvature of the planet is such that if you're standing there on the surface of the planet, you will only able to see to the horizon, which is five miles away.

Julia - If we look back at Earth, it's had great freezes, it's had times when it's been extremely hot. Has Mars had similar changes to its terrain and to its climate over time as well?

Simon - Mars, because it formed further away from the sun, ended up with a big load of dust and gas and ice that went into the planet when it was formed, which meant that its atmosphere was ridiculously big to start off with. But the planet that small couldn't hang onto an atmosphere that big. So as that atmosphere was stripped away into space and the surface temperature cooled, the pressure lowered, and it was able to rain. So there was a time when Mars was warm and it was very wet. Literally half of Mars was covered with an ocean. But the loss of its atmosphere was only ever going to end up with Mars getting basically meaner and colder as the years progressed.

Julia - And Mars has a pretty big volcano on its surface. So how did volcanic activity affect the planet?

Simon - Mars is the site of the highest volcano in the solar system, Olympus Mons, which is 24 kilometers high. That's 15 miles, which is pretty beefy. It's not the only ludicrously massive volcano on Mars. Apart from a small handful that are elsewhere on the planet, they're all centered on this one place called Tharsis. What you've essentially got with Tharsis is this lump of rock that you've stuck onto the side of Mars. And if you imagine that someone has taken a small planet and stuck it to the side of your own planet, it's going to make the planet wobble a bit. And that's exactly what happened with Mars. Tharsis wasn't originally on the equator, but because Mars is rotating once every 24 and a bit hours, the whole of Mars has literally twisted in its orbit so that Tharsis is now exactly on the equator.

Julia - And the biggest question about Mars, ala David Bowie, is there - or was there - life on Mars do we think?

Simon - I am going to say yes. If we look at the Earth's oceans and where we think life originated on earth, it will be in the deep ocean where you have things called black smokers. That's where sea water has gone down through cracks in the crust and it's met hot rock. Now, hot water is great at dissolving soluble minerals, very primitive plants and bacteria can use that chemical soup that comes back out to power their own reactions. If we look at Mars, we have exactly the same conditions. We have a deep ocean. We have cracks in the crust. We have hot rock underneath the crust. If we get down there and trundle around on the Northern Plains, looking for the sites of these black smokers, wherever they might be, we may well find the same kind of primitive life there that we would've done 4 billion years ago on Earth.

An artist's impression of a black hole

24:05 - What can the James Webb Telescope see?

The James Webb Space Telescope uses infrared light to capture the universe (and black holes) like never before

What can the James Webb Telescope see?
Becky Smethurst, University of Oxford

We have an absolutely incredible tool, which is allowing us to see beyond our solar system. In fact, to galaxies so, so far away, they think they are the oldest and farthest we have ever seen. That is of course the James Webb Space Telescope. Julia Ravey spoke to Becky Smethurst from the University of Oxford and author of "A Brief History of Black Holes: And why nearly everything you know about them is wrong", who gave an insight into what we're actually looking at in those beautiful images...

Becky - It's almost easier to say 'what are we not looking at the minute?' Because it feels like JWST or 'James Webb Space Telescope' feels like it's just revealing so much at the minute. But what we're seeing is essentially infrared light from all these objects in space. So this is light that has a longer wavelength than visible light that we can see with our eyes. It's redder, so that means it can see through dust because this light has a longer wavelength that essentially just kind of goes around all these dust particles rather than getting scattered off it. So we can see into star forming regions, which is really cool. And also because the universe is expanding, and all the light from galaxies at great, great distances has been red shifted by that expansion, it's been stretched out to these longer wavelengths beyond what we can see. It means that we can actually see some of the most distant galaxies in the entire universe, the most distant of these islands of stars in our universe, which is amazing because if they're the most distant, the light that's going from them has traveled the most distance as well, which means it left those galaxies when the universe was much younger,

Julia - There's been all these claims around the James Webb images that we've seen the oldest stars in our entire universe, but what more needs to be done to confirm how old these stars actually are?

Becky - People are starting to pick out what we call candidates for the most distant galaxies. You know, the light from which is the oldest light we could detect. People are claiming, oh, you know, this light left these galaxies 13.6 billion years ago therefore it's the oldest light. The thing is, that we've only found evidence for that in images. Ideally, what you would want to do is take all the light from that galaxy, split it through a prism to confirm that they actually are at that distance and also declare which one actually is the furthest away. We're gonna need to, to get the spectrum of light as well, which James Webb will have already collected. People are working on it, but there's also cool things you can do with spectrum at those wavelengths. For example, you can take the light from an exoplanet, such as a planet in orbit around another star in our galaxy, our island of stars, the Milky way. And you can say, okay, take the little bit of star light that passed through that planet's atmosphere on its way to us and we'll see how it differs from the normal star light that we receive from that star. And any differences essentially tell you what's in the atmosphere of that planet, because say there is water or carbon dioxide, or maybe methane or something in that atmosphere, it will steal away a little bit of the light. And so we can actually use this to figure out what exoplanet atmospheres are made of and if they contain water.

Julia - That is really exciting. And then if we move away from planets, can you also see black holes in these images?

Becky - Amazingly enough, Yes. <laugh>. I mean, you could tell I'm excited about this, because this is my stuff. And people always get thrown by this, the idea that you can see a black hole, because the idea of a black hole is that it's so dense that light can't escape. But the thing is the material around a black hole, and especially a super massive black hole because of the black hole's extreme gravity, it gets accelerated to huge speeds, which means that it gets hot and it starts to glow, you know, in the same way that you shove a piece of metal in a fire, right? It all starts to glow like in a blacksmith forge or something. So it starts to glow incredibly brightly, so much so that super massive black holes are some of the brightest objects in the entire universe. They literally light up like Christmas trees <laugh>. But the thing is, the centres of galaxies where we find these super massive black holes, are really dusty. So we only ever see about half of them. And they're also not that bright in visible light. But they're very bright in infrared light. So one of the images that was actually released of what was called Stephan's Quintet, it was those five nearby galaxies that were all grouped together. One of the galaxies at the very, very top, if you look in the longer wavelengths of infrared light in the image that was released, there's just this giant bright, blazing thing in the center of it. And that is the gas that's swirling around the super massive black hole. And it's amazing that we're gonna be able to pick them out at that distance as well.

Julia - Now that we have these images and we're gonna be able to analyze them and we've seen the power of this telescope, how do you think this will change our thoughts on the universe?

Becky - I don't think that's gonna be an area of astronomy that isn't touched by what JWST is gonna find. And that's the thing is that we've designed it to do various different things, to see these exoplanet atmospheres, to peer through dust where stars are forming, and to see back the oldest light in the universe. Not only are we gonna get all these observations that we plan to take, but all these observations we didn't plan to take and who knows what we're going to find. And that I think is what I'm most excited for, what you call these sort of 'unknown unknowns', The things that we didn't even know to ask as we were designing this telescope. But think in 20 years time we're going to be like, 'wow, that thing that we never knew when JWST launched that now has changed everything'. We can't call what it's gonna be, but I think it's gonna be big, whatever it is.


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