A recipe for the solar system

Our solar system formed 4.6 billion years ago, but how did this happen?
25 August 2015

Interview with 

Dr Brendan Owens, Royal Observatory at Greenwich


Our solar system formed 4.6 billion years ago but how do we know that's the The Solar Systemcase? Well, new solar systems are forming all around us, all the time and with the help of a telescope, astronomers can photograph the drama as it unfolds. Graihagh Jackson spoke to Brendan Owens from the Royal Observatory to find out more...

Brendan - So, this is a selection from our Deep Space category from astronomy photographer of the year as a selection of star birth and star death.

Graihagh - There are some beautiful colours. I can blues and yellows, red, very, very rich.

Brendan - It's kind of amazing and maybe a little disappointing for some people sometimes is that the colour that you see in a lot of the images is what we call false colour. Those photos are corresponding to just particular energies from particular atoms - so iron, carbon, oxygen, and then you can choose to colour them in. Just to avoid confusion, we don't go all creative and colour them whatever we want. We try and stick to a palette called the Hubble palette. So, we all kind of recognise what the different colours are. So typically in images, the reds, and pinks, they can be hydrogen. Sometimes a different shade, it can be neon as well. If you have something like oxygen sometimes it's green or blue, so you can change them a little bit but they're just strictly so we can try to keep it to the same palette so we all understand.

Graihagh - How do we get from these sort of beautiful clouds of dust to what we know today as our Solar System - our sun, planets, rotating around the sun?

Brendan - Something has to trigger the collapse of the gas. One cause often is a supernova explosion from another star. Gravity pulls in clumps of gas and that starts to swirl, and it's a bit like when you take an ice skater, spinning, got their arms open, and then when they bring their arms in, they spin faster. When you get that, we're getting a squeezing and squeezing of gases and it gets hotter and hotter. So eventually, the star ignites so to speak, the stars getting to a temperature where it can fuse hydrogen into helium and start the process of a new star. But around it, you'll get a dusty donut.

Graihagh - Now that we have a dusty donut around our star, the rest of our Solar System can begin to take shape and weirdly enough, what happens next in the story of our Solar System is incredibly similar to when you combine water, washing up liquid, and pepper. Join Brendan and me in our experiment. I'll give you a couple of seconds to get everything you need... okay, ready?

Brendan - The first thing I'm going to have to do to recreate that idea of the gas collapsing and everything, swirling around, so we're going to have to swirl the water first of all. And then we're going to add in our planet pieces. So, our little dust pieces. We'll sprinkle that in around the area.

Graihagh - In case you hadn't realised, our planet pieces are the pepper. So, get sprinkling.

Brendan - The grains by themselves are quite small, but we can see that they're actually clumping together to form the early planets.

Graihagh - How would that happen in real life? How would they start to group together?

Brendan - Before gravity can take over, before they're big enough to have enough gravitational pull to pull in material, they actually do stick. They're smashing against each other. Everything is quite hot at this time as well. So, things are far more malleable than we think of them today as in rocks. So, more things are molten. So, they do stick together and you end up with a few - what we call planetesimals so really, the seeds of the planets that we have today. There is another step. We've took on the swirling around, we've ended up with quite a few planetesimals.

Graihagh - We have, I was going to say. There are at least 20 or 30 in there.

Brendan - And this is actually pretty good before everything settles down into the Solar System we're familiar with today, there's a bit of clear up job that's performed by our early star, the sun. It's what we call a T Tauri star and it's a young star that's actually got very powerful stellar winds. Every now and again, we'll get what we call solar storms, the gas from the sun material have been ripped off, these explosions that come towards the earth and across the Solar System. But it was much worse back in the early days. So what we're going to do is recreate those strong stellar winds by using washing up liquid and I'm going to put a little bit on my finger. The experiment is changing the surface tension of the water, but it's quite magical. I hope you're ready for this.

Graihagh - I'm very ready, okay. Pop a bit of washing up liquid on your finger and dip it gently into the middle of the bowl of the water and pepper. Did you see it? It's pretty quick, but all the clumps have shut out from their random clumps in the water right to the edges of the bowl. There wasn't one planetesmal left but then how did Earth, Mars and Venus form?

Brendan - It's a little bit exaggerated. It's known as catastrophic because in reality, you should be left behind with the biggest planetesmals. But what it's doing in this time and as the planets are forming, the stellar winds are clearing out excess gas dust. Basically, they're putting a stop to any further formation.

Graihagh - What sort of timescales are we talking about from that initial collapse of a supernova and other sort of star coming by and the star collapses, and you've got this swirling dust, all the way to what we have today? What sort of timescale are we looking at? I imagine it's in an order of millions.

Brendan - Yeah, actually relatively speaking. The early stages are in the order of up to 100 million years in that, getting to sort of the stage where you've got bigger pieces that can start to combine together into the Solar System formed today. But everything properly settled down, probably about 4.5 billion years ago or 4.4 billion years ago. So, it's still an incredibly long time ago that we've had that. So, things have settled down since then. So, altogether, we're talking about tens to hundreds of millions years' time to actually go through those stages. Now the wonderful thing is, that we've found recently that we can look at other planet systems in better detail than we have before. So, we found planets called exoplanets that orbit other stars. We can see those in their different stages of their lives. So, not just stars in different stages on their own, but also the planets as well.

Graihagh - So, we've talked a little bit about how the rocky planets formed, but what about the gas giants like Jupiter and Saturn?

Brendan - So, that is probably the most contentious area of planet formation theory, is trying to think whether or not which models suits best. So, there are two main models and one is called the core accretion model which is where the gas is falling on to the cores of the gas giants. So, a bit like the terrestrial planets but further out where gas can accrete onto these solid centres for the gas giants. A thing about that is, you need the time to allow that material to fall onto those cores before the young sun blasts away any of that material that would form the atmosphere around it. Alternatively, there's the disc instability model which is where if the disc around the sun can cool quick enough, it just clumps together to form the gas giants. But that relies on the disc cooling down fast enough again to beat the strong stellar winds clearing out the Solar System. So, we've got those two ideas for gas giants.

Graihagh - So, moving beyond the gas giants then and to these icy planets that we know as Neptune and Uranus, how did they form?

Brendan - So, they're further out. So, they are mostly caused of ice crystals in their atmosphere. They're even cooler so we think that they might have formed a little bit later even than the other gas giants like Jupiter and Saturn. We also think that there might have been an extra gas giant in there at some point. With planets moving around the young sun, if they're in sequence with each other, if you have say, one orbit of an outside planet, it's the same as two orbits of the next planet in. If they align with each other, they give a gravitational kick to the next planet out. So, we think we may actually have a lost brother in the Solar System flying off that we call a rouge planet.

Graihagh - Is that like a comet and will it come back eventually or is it just gone forever?

Brendan - It may be gone forever, depending on the kick it's got. We also have about - our closest star is 4 light years away, but that's just in one direction. So, if it went off in another direction, it's going to have a much longer journey to join up with a new planet system. There is that idea as well that planets possibly could be captured by another star system. So, this is really a speculation, a theoretical area of things, but there are theories out there, it still is possible.

Graihagh - So there's many chance there might be a planet in our Solar System that might be from another star.

Brendan - I think we've come to planets maybe that are further out. Not even planets or should I say, dwarf planets. We may have captured something else from another system so there may be one of those like Pluto that doesn't belong originally to us which would be quite amazing. I think it would be quite difficult to find out if that's the case. Getting, tracing again the nitty-gritty detail on how things formed is quite difficult when you start off like we did at the start of our journey with one big cosmic soup from an old star and who shared what, that's a tricky one to find out. But hopefully, technologies at least have been able to allow us to piece together the timeline better than we ever have done before.


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