The current hypothesis of Solar System formation is the nebular hypothesis, first proposed in 1755 by Immanuel Kant and independently formulated by Pierre-Simon Laplace.[8] The nebular theory holds that 4.6 billion years ago (a date determined via radiometric dating of meteorites),[9] the Solar System formed from the gravitational collapse of a gaseous cloud called the solar nebula. It had a diameter of 100 AU and was 2–3 times the mass of the Sun. [citation needed] Over time, a disturbance (possibly a nearby supernova) [10] squeezed the nebula, pushing matter inward until gravitational forces overcame the internal gas pressure and it began to collapse. As the nebula collapsed, conservation of angular momentum meant that it spun faster, and became warmer. As the competing forces associated with gravity, gas pressure, magnetic fields, and rotation acted on it, the contracting nebula began to flatten into a spinning protoplanetary disk with a gradually contracting protostar at the center.[11] Studies of young, pre-fusing solar mass stars, called T Tauri stars, show that these discs extend to several hundred AU and are rather cool, reaching only a thousand kelvins at their hottest.[12] From this cloud and its gas and dust, the various planets formed. The currently accepted method by which the planets formed is known as accretion, in which the planets began as dust grains in orbit around the central protostar, which initially formed by direct contact into clumps between one and ten kilometres in diameter, which in turn collided to form larger bodies (planetesimals), of roughly 5 km in size gradually increasing by further collisions by roughly 15 cm per year over the course of the next few million years.[13] The inner solar system was too warm for volatile molecules like water and methane to condense, and so the planetesimals which formed there were relatively small (comprising only 0.6% the mass of the disc) [citation needed] and composed largely of compounds with high melting points, such as silicates and metals. These rocky bodies eventually became the terrestrial planets. Farther out, the gravitational effects of Jupiter made it impossible for the protoplanetary objects present to come together, leaving behind the asteroid belt. [14] Farther out still, beyond the frost line, where more volatile icy compounds could remain solid, Jupiter and Saturn were able to gather more material than the terrestrial planets, as those compounds were more common. They became the gas giants, while Uranus and Neptune captured much less material and are known as ice giants because their cores are believed to be made mostly of ices (hydrogen compounds).[15] [16] After 100 million years, the pressure and density of hydrogen in the centre of the collapsing nebula became great enough for the protosun to begin thermonuclear fusion, which increased until hydrostatic equilibrium was achieved.[17] The young Sun's solar wind then cleared away all the gas and dust in the protoplanetary disk, blowing it into interstellar space, thus ending the growth of the planets.[18] [