solar nebula with proto star: a young solar system

Current theory holds that our own solar system formed from the collapse of a gaseous cloud called the "solar nebula". Originally 2–3 times the mass of the Sun and 100 AU in diameter (1 AU is the Earth–Sun distance; Pluto’s orbital diameter is 80 AU), some disturbance like a nearby supernova pressed our nebula inward. At that point gravitational forces overcame the internal gas’ outward pressure, and the nebula began to collapse. If a skater pulls in her arms while spinning, her rate of spin increases; this is called conservation of angular momentum. So as our spinning solar nebula contracted due to gravitational collapse, it spun faster. Now gravity, angular momentum, internal gas pressure and magnetic fields acted on our solar nebula; and in concert caused it to flatten into a spinning, planet-forming disk of dust and clumps of matter, with the largest clump in the center.

Above is an artist's concept of a young solar system. In the center is a protostar, a stage in the development of a star after it has started to contract within a solar nebula (a "solar nebula" is a primordial cloud of gas and dust that under internal gravitational attraction begins to coalesce into planetary bodies orbiting a central protostar). In the picture below the protostar is drawing material from a surrounding disk of dust and gas roughly the size of Jupiter’s orbit. The developing gas giants (rings indicate debris remaining from their formation) sweep up infalling dust from the outer disk creating the open area; the gap is about the distance between Jupiter’s and Saturn’s orbits. This is how solar system's like our own evolve, or so we hypothesize.

Although space in general is just a few degrees above absolute zero, because of our Sun’s radiation the young inner solar system was too warm for volatile molecules like methane and water to condense. Thus planetesimals accreting there were largely constituted of rocky or metallic compounds with high melting points. And because such compounds formed less than 1% of the solar nebula, such bodies were small. They eventually coalesced into the so-called terrestrial planets: Mercury, Venus, Earth and Mars.

Between Mars and Jupiter, large coalescing bodies were easily torn apart or destabilized by Jupiter’s gravitational stresses. The result is the asteroid belt: millions of small bodies orbiting primarily between Mars and Jupiter. Their total mass is about 4% of our Moon’s; 26 have diameters greater than 200 km. Ceres, the largest, is 952 km. across and ac-counts for ¼ the mass of all asteroids.

Moving outwards from the protostar we reach a distance where the solar nebula is cool enough for volatile hydrogen compounds like water, ammonia and methane to condense to ice. This is a very significant boundary, for such compounds comprise most of the solar nebula, and beyond this "frost line" solid ice grains are more available to accrete into planets. Here the giants Jupiter, Saturn, Uranus and Neptune formed; within this frost line the terrestrial planets reside.

The gas giants were large enough to retain their original hydrogen-helium atmospheres from the solar nebula, one reason they are giants. The terrestrial planets were too small to retain such light gases, generating their atmospheres by volcanism, comet falls, or through life. After 100 million years the protostar began thermonuclear fusion in its core; its resultant solar wind blew all the gas and dust in the protoplanetary disk into interstellar space, ending planetary growth.

This Solar Nebula with Protostar: a Young Solar System page and much of this 550-page website are excerpted from You and the Universe.