Another golden oldie from my archive, but topical still.
Douglas Lin, of the University of California, Santa Cruz, has looked in detail at what happens to the solid chunks of material that build up in the disc around a young star like the Sun. One crucial feature of his calculations is the way the solid objects interact with the gas that is still present in the disc during the early stages of planet formation. Because of the interaction between pressure, gravity and rotation, the gas at any chosen distance from the central star moves around the star slightly more slowly than the speed with which the particles and lumps of material are moving in their orbits. This means that the particles are overtaking the gas; in effect, as Lin puts it, ‘running into a headwind that slows them down and causes them to spiral inward, toward the star’. A piece of material a metre across can halve its distance from the star in this way in just a thousand years, and the bigger the pieces grow the faster they move inwards – up to a point.
That point is picturesquely dubbed the ‘snowline’. It is the distance from the star where frozen water, ammonia and other volatile substances evaporate; in the case of the Sun at a distance of between 2 AU and 4 AU, between the orbits of Mars and Jupiter. This is why the boundary between the rocky planets and the icy objects in our Solar System lies where it does.
At the snowline, water vapour released by the icy grains as they evaporate changes the properties of the gas in such a way that it now rotates faster than the solid grains, giving them a boost which tends to make them move outwards in their orbits. So material piles up at the snowline, where grains are packed closer together and can quickly grow into larger lumps. Within a million years of the formation of the Sun, many of these lumps are a kilometre or so across and very little dust remains. As they grow, and as gas is being dissipated from the inner part of the disc by the heat of the Sun, the planetisimals, as they now are, are less influenced by interactions with the gas, and many of them migrate inwards towards the Sun, into the region where rocky planets are found today. The exact positions that the planets end up in when this migration stops depends on many factors, including the temperature in different regions of the disc and the size of the planet, but the overall picture is clear from many computer simulations.
Planetisimals gather up the remaining dust gravitationally and collide and merge with one another, with the survivors settling into roughly circular orbits which have been swept clean of debris. Chunks of material left over from these collisions may still be with us, in the form of some of the asteroids. Because there is more dust to feed on farther out from the Sun, embryonic planets grow bigger farther out. According to Lin’s calculations, at a distance of 1 AU from the Sun a planetary embryo can grow to one tenth of the mass of the Earth within 100,000 years, but then all the available dust is gone; at a distance of 5 AU, there is more dust and an embryo can continue to grow for a few million years, reaching a size of about four Earth masses. But this isn’t the end of the story. Interestingly, Lin points out that there is no room for any more planets in our Solar System today – the planets we have are as close together as the complex interaction of gravitational forces between them will allow. It is very likely that more planets formed when the Solar System was young, but that the surplus were ejected from unstable orbits before the present stable pattern was established.
It cannot be a coincidence that Jupiter, the largest planet in our Solar System, lies just beyond the snowline; but astronomers are still not able to explain just how a Jupiter-sized planet ended up in a stable orbit there. Interactions between an embryonic planet in the outer part of the Solar System and the gas in the disc, still significant that far from the Sun, explain why the embryonic Jupiter ended up close to the snowline, and accumulated a great deal of gas from the material available there. But what stopped it spiralling inwards into an orbit like those of the many ‘hot Jupiters’ that have now been discovered? If it had done so, it would have pushed any rocky planets in the inner Solar System into the Sun ahead of it.
Once Jupiter had formed, it helped the other giant planets to form by stopping the inward flow of material in the disc and by disturbing the orbit of planetisimals so that many of them migrated to the outer part of the Solar System. The first effect aided the formation of the second gas giant, Saturn; the second effect provided enough frozen chunks to make the massive cores of the ice giants, Uranus and Neptune. All of this, prior to the processes which sent the giant planets into their present orbits and disturbed the Kuiper Belt, only took about 10 million years after the formation of the Sun. But the formation of the Earth took a lot longer.