The Beginning of Planet Formation

The four innermost planets of our solar system-Mercury, Venus, Earth, and Mars-are terrestrial, or rocky planets. The beginning of their creation process occurred when the cloud of leftover material from the formation of the Sun settled into disc around the young star. Most of the material in the cloud, like the material of the Sun itself, was in the form of hydrogen and helium. But there was a trace of dust, no more than 2 percent of the original material, in the form of particles as fine as the particles in smoke. Heat from the young Sun blew much of the gas away, but the rotation of the original cloud ensured that the dust settled into a disc around the young Sun-a protoplanetary disc like the ones seen around young stars today.

Within the disc, all the particles were moving in the same direction around the Sun, like runners going round a track. This meant that when they bumped into one another, they did so relatively gently, not in head-on collisions, giving the particles a chance to stick to one another. The tendency to stick may have been helped by electric forces produced by particles rubbing against one another, in the same way that you can make a child's balloon stick to the ceiling after rubbing it on a woolen sweater. Another important factor was turbulence in the gas, creating swirling structures like whirlwinds which gathered pieces of material together and gave them a chance to interact. Computer simulations show how objects as big as Ceres can form in this way-provided the particles can stick together.

Something else may also have helped the particles to stick together something else that is special about the solar system. Studies of pieces of rock from meteorites show that the dusty disc around the young Sun contained tiny globules of material,known as chondrules, formed by melting at temperatures between 1,200 degrees Centigrade and 1,600 degrees centigrade.Molten, or partly molten, blobs would be more sticky and encourage the buildup of larger lumps of stuff in the disc. But how did they get so hot? The most likely explanation is that the heat was released by radioactive elements that had been sprayed by a nearby star in the process of dying into the gas cloud from which the planets formed. One possibility is that a supernova occurred close to the cloud that became the Sun just before the Sun formed; it is even possible that the blast wave from this explosion triggered the collapse of the gas cloud that became the Sun and solar system. Supporting evidence for this idea comes from measurements of the proportions of various isotopes (different forms of an element found in meteorites）. Radioactive aluminium-26 seems to have been present in the proto-solar system from the beginning, but a pulse of iron-60 arrived about million years later.This matches what we know about the fate of a very large star, with more than 30 times as much mass as the Sun. In the late stages of its life, the star first blows away much of the outer layers of material,which by then is relatively rich in aluminium-26，in a wind easily strong enough to cause any nearby gas cloud to collapse.The star only explodes at the very end of its life, showering the neighborhood with elements including iron-60.

There is a rival idea, developed in Barcelona by Josep Trigo-Rodriguez and colleagues, which suggests that the radioactive material was fed into the solar system as it was forming from much less massive star which came much closer to the Sun. The right proportion of isotopes could have come in the wind of material being blown away from a star with only six times as much mass as our Sun in the last stage s of its life. But the star would have to be very close to the Sun for this to happen-closer than 10 light-years-which makes such an event unlikely, statistically speaking.