How Planets are Made
Chris - Some of today's presentations at the AAAS science conference here in Boston were quite literally out-of-this-world. That's because scientists were explaining how they're looking for Earth-like planets. In other words, other worlds that could harbour life elsewhere in our Milky Way galaxy. How do planets form in the first place and why are they all different shapes and sizes? Earlier today I caught up with Michael Meyer who's been trying to solve this planetary puzzle at the University of Arizona.
Michael - I study star formation and planet formation. In particular, in the last few years we've been using NASA's Spitzer space telescope to understand the formation and evolution of planetary systems. We pick groups of stars that are very young, groups that are middle aged and groups of stars that are older and hope that we're watching a movie of what unfolds as one cohort evolves into the next.
Chris - This gives you some clues as to what's going on 4.5 billion years ago in our own solar system, in other words how we got here?
Michael - That's what we hope. Our programme is specifically focussed on studying sunlight stars in the disc of our Milky Way galaxy. We have a sample of 300 stars with ages only 3 million years after the stars first formed up to 30 million, 100 million, 300 million, a billion and then up to 3 billion years old. Not quite as old as our star, the sun.
Chris - How long does it take a system of planets to form in the first place?
Michael - It depends on which kind of planet were talking about. Our programme has been trying to study the evolution of dust and gas content. On the gas front we've determined that the amount of stuff needed to form Jupiter mass planets goes away on time scales of 3-to-10 million years. A planet like Jupiter has to form really fast. We think planets like Earth probably took 10-50 million years to form, at least in our own solar system. The new data we've received from the Spitzer space telescope seems to suggest that those processes might be very common about some of those stars.
Chris - That's not very long though, is it? That suggests that once planets are going to happen they happen quick.
Michael - That's right. If you sort of have a bunch of rocky things of order a kilometre in size and hit a go button gravity takes over and they will, on time scales of 10 to hundreds of millions of years, form most of the planets and the complement that we know of in our solar system.
Chris - Chat's the sort of step-by-step system of events that seems to happen to give rise to planets once'star begins to form?
Michael - Well, the disc of gas and dust that's left over from star formation is really a key part of that whole process or forming a young, sun-like star. Over time the gas and dust evolves in that the particles - the little dust grains that give a lot of the infra red emissions we see - can smash into each other and stick. It's kind of like the particles are in soot that you see rising from chimneys. Those tiny, tiny particles come together and form larger and larger things like pebbles and rocks and eventually the things that we might identify as boulders and then grow up to objects as large as the moon and Mars. That is the cascade of collisions that we think leads to planets like Earth.
Chris - Why are all the planets on the same planet though? Why do we find that they all go round lined up? Why does all that material settle into a disc, I suppose, rather than planets here and there all round the star?
Michael - If you imagine our star like the sun forming from a cloud of dust and gas in space that's rotating, albeit ever so slowly, as that cloud contracts a lot of the material if forced into a disc by a principle of physics known as the conservation of angular momentum. It's the same thing that whizzes you to the outer edge of your car as you're speeding round a curve in the motorway. That same physical principle operates on spinning clouds of gas that form sun-like stars. It almost forces you into a situation where you collect mass in the centre but you must be surrounded by a disc of material left out there.
Chris - What makes the difference between whether we end up with a planet a bit like the Earth or one that's made of gas, like Jupiter? How does that happen?
Michael - Well, even to form a planet like Jupiter you need to start with a core that probably looks similar to a few-to-ten-times the mass of the Earth. The processes that lead from those tiny dust grains up to pebbles and boulders and things like the Earth take a certain amount of time. If you can get ten Earth masses of rocky material together about the distance of Jupiter from its sun before the gas goes away you have the potential to serve as a focal point, a nucleation site if you will of swarming and gathering the gas together. If that process takes too long and the gas is blown away before it can happen you might not have a Jupiter. Earth-like planets, we think, form anyway. Whether there is or isn't gas there you can still over a more leisurely period of time - 10, 30, 40, 50 million years - form rocky planets. We think that's one reason why we're seeing evidence of planet formation like rocky planets like Earth much more common[ly] than the evidence so far that we see for gas giant planets.
Chris - Is there any kind of magic formula where you always end up with planets that are certain sizes or are you now finding that in fact it's just a mixed bag?
Michael - Much more so a mixed bag. Our idea of planet formation suggests that it's messy. It's a very complex process. We think that's what leads to the large diversity of planetary systems that we see around stars like the sun.
Chris - How does that effect our prospects of finding another Earth-like planet out there somewhere?
Michael - Well, as I said, we think our research suggests that planets like Earth could be very common. Like Earth simply means that they're rocky and that they're relatively close to their host star. Exact analogues are probably rare but what is it about the Earth that gives the possibility for the emergence of life? That's what all of us would really like to know. It would help us to better assess whether the sixty per cent of sun-like stars that we think might be forming terrestrial planets, whether those rocky planets could be places where life could develop.
Chris - Do you know what those magic ingredients are?
Michael - The elemental compositions that lead to life? Well, have a favourite word that we use in the emerging interdisciplinary field of astrobiology that's CHONSP: carbon, hydrogen, oxygen, sulphur, nitrogen and phosphorous. Those are the elements, the stuff of life that leads to the biochemistry of life.
Chris - Is it, based on your measurements, likely that most of these rocky worlds that form around planets will have a fairly even smattering of those or are there special accumulations of certain things in certain places that happen?
Michael - The chemistry of forming planetary systems is very complicated. We believe in a general way that the farther out you go the more likely you are to retain the carbon, nitrogen and oxygen in icy materials that could be very important to the emergence of life. In the inner part it's a bit of a mixed bag. The chemistry and the dynamics are so complex I can imagine a wide range of compositions for terrestrial planets.
Chris - Is it just stars a bit like our sun that we think give the best prospects for the evolution of life? Could we just be looking at any old star and hope to find some life there?
Michael - I think in our ignorance we focus on things we know. We only know of one planet that has given rise to life so we focus on terrestrial planets and we focus on stars like the sun. Again, in our ignorance that's what we know. I think over the years we're getting a sense that our perspective is a bit myopic. We need to step back and look at the formation and evolution of planetary systems around a whole host of stars. We shouldn't forget that the very first planets were discovered around the dead remnant of a very massive star, a pulsar. The first rocky terrestrial planets were in fact found in such an extremely exotic environment. As we learn more about how planets could form around stars much less massive than the sun, the cold, red end dwarfs: those are the most common stars in the Milky Way galaxy and if they are suitable places for life then life could be more common than we even could imagine.
Chris - I have to ask you to speculate with that point. What do you think the chances are during your professional life that we're going to find it?
Michael - I don't know. What I do know is that in the coming decades we will learn a tremendous amount about how common Earth-like planets are around stars like the sun. Whether or not we detect the biochemical signatures of disequilibrium which is really the stuff of life in the atmospheres of planets around other stars I can't venture a guess. I do believe that the biochemical origins of life will remain one of the key problems in science for the next century.