Science Interviews

Understanding our stars and planets

Diana -   Where do stars and planets come from?  Here to answer that question is Professor Mark McCaughrean from the European Space Agency, where he’s the head of research and scientific support.  Hello, Mark.

Mark -   Hello, Diana.  How are you?

Diana -   I’m very good.  Thanks.  Well, let’s start off now with some of the earliest models of planet formation.  So how did humans really start to think about where the universe came from?

Mark -   Well, I think there’s lots of models which we know from all of the religious textbooks and so on, so if we step ahead from those and look where we were in the 1700’s, 1600’s, or so, and the important thing there is that this is after we realized and convinced ourselves that the sun was the middle of the solar system rather than the Earth.  So once we got away from a Geocentric model into heliocentric, things start to fall in place.  The few important pieces of evidence that come into that, firstly, all the planets go around the Sun in the same direction in the sky.  They all,are more or less in circular orbits, and they are more or less all in one plane.  And so immediately you might think that the solar system formed out of some kind of flattened disc around the Sun.  So we started with Descartes, Kant, and Laplace, well-known philosophers of the day who figured out that if the Sun and the planets around it formed in a flattened disc, how would you actually get a disc to form in the first place?  So as Chris just pointed out a moment ago, we now know that planets and the stars that they go around form from giant clouds of gas and dust.  And those clouds have to collapse.  And as they collapse down, they’re going to flatten because they’re rotating very slightly at the beginning, these giants clouds, and through conservation of angular momentum are going to spin up on one axis more than another, and end up being great, big, flattened rotating discs.  And the philosophers knew that pretty well.  So that’s how we started off in the 1700’s.

Diana -   How did they get to that understanding?  What kind of technology did they have at their disposal?

Mark -   All they were really doing was applying, looking at observations of the sky with the naked eye, and with the first telescopes.  So Kant for example, was well aware that the telescopes in the mid-1700’s were able to start seeing faint, fuzzy, blobs out in space.  And he thought those were perhaps dusty clouds which were beginning to form stars, and the planets that go around them.  So there was technology in the use of telescopes, so mostly in terms of calculus and arithmetic.  It was mostly theory work at that point.

Diana -   And coming to our understanding in the 20^th century, what do you think were the key markers of our new, modern understanding?

Mark -   Well, what happened was very interesting.  And so Kant and Laplace and later on, Swedenborg and others put this model together.  There became a major problem with it.  And that is if you look at the solar system which we live today, almost all of the mass of the solar system is in the sun.  The planets have very little, less than a percent.  But almost all of the angular momentum, the rotational angular momentum is actually in the planets.  And that was a real problem because the model they had been developing couldn’t account for that.  How did that evolve?  So, most of the mass was in the centre, but most of the angular momentum outside.  So then it was for 60 or 70 years actually until the early 1970’s a major diversion into what we would call “catastrophe theories” of the formation of planets.  Catastrophe, in the sense that, nothing happened on its own.  It happened in an event, a particular event, such as the passage of a massive star, a pre-existing star, passed a nebula, which then pulled-out through tidal forces, pulled-out a lump of material from this forming star.  And that was then able to collapse the planet.  There were lots of models like that.  They were very, very reputable for 60 or 70 years.  But then it was observations that brought us back to where we are today.  And that is in the mid-70’s, 80’s, and then the 90’s, we actually began to see that stars elsewhere in our universe were surrounded by discs of gas and dust.  And that has brought us all the way back to the Laplace model, which is today called the “solar nebula model” in today’s parlance.

Diana -   So it’s kind of come full circle in a, sort of, century-like way.  Can we actually see the stars and the planets forming?  What does it look like in the telescopic images?

Mark -   Well, when we look at the stars today even the birth of stars, is an interestingly modern phenomenon.  It wasn’t until in the 1940’s or so that we began to realise that stars had to be forming in our universe.  And then we began to get evidence of that happening.  There was an assumption before that many stars had just existed forever.  They may have been born at one time, but the idea of star formation going on today is quite new.  And so when we look at these places where stars are forming, and a very typical example is the Orion nebula, which many people will be familiar with in the night sky as a fuzzy patch in the sword of Orion just below its belt.  What we see there are giant clouds of gas and dust glowing, being heated up by the hot stars, which have just been born in the middle.  But we actually don’t see much else with visible wavelengths.  And that’s because stars and planets are made out of gas and dust.  And in particular the dust obscures our vision of the young stars.  They’re surrounded by this dust still, and you just can’t see into those star-forming regions at visible wavelengths.  However, if you look in the infrared or at much longer wavelengths, radio wavelengths, millimetre wavelengths, you can see a clear picture, and you can see these young stars slowly condensing, rotating at fairly high speed with discs of gas and dust around them, making planets.

Diana -   But stars are quite bright and exciting, but what about the slightly duller things like moons and captured asteroids, that sort of thing?

Mark -   Yeah, well that’s an interesting question.  I mean, how, for example, how the satellites in our own solar system, how the things going around the planets occurred.  In fact, there’s a whole list of things.  It may well be, for example, in the Jovian system around Jupiter and Saturn that most of the big moons there formed the same way that planets formed around the sun, through angular momentum being conserved, and condensation in the disc to form those planets.  But then you have our own Moon, for example.  How did that form?  That’s much bigger than any of the others, as a ratio to the size of the planet that is going around, and that may well have been a catastrophic event, where a rather large object, pre-existing in the solar nebular in the early stages in the first tens hundreds of millions of years.  Our solar nebular was probably full of planets which aren't there today, big lumps of material.  Some of which have fallen into the sun.  Some of which have collided with other planets.  And that’s what we think happened to the moon, is that something hit the Earth, the young Earth.  There was no life, of course there, and spun off a huge amount of material from the mantle, from the outer part of the Earth, and that then condensed into the Moon.  Other moons going around other planets, for example, Phobos and Demos, the two main moons of Mars, are captured asteroids.  They’re very, very odd objects.  They’re not circular at all, and it seems as they got closer to the asteroid belt they just captured gravitation of it.

November 2009