Spacewalk: the Scale of our Solar System

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.

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.