[58] A secondary depletion period that brought the asteroid belt down close to its present mass is thought to have followed when Jupiter and Saturn entered a temporary 2:1 orbital resonance (see below). The process was most efficient for Jupiter and Saturn; hence, their compositions are most nearly cosmic. Much less gas was captured by Uranus and Neptune, which is why these two planets have compositions dominated by the icy and rocky building blocks that made up their large cores rather than by hydrogen and helium. Some of them grew big enough for their gravity to shape them into spheres, becoming planets, dwarf planets, and large moons. [11] The terrestrial embryos grew to about 0.05 Earth masses (MEarth) and ceased accumulating matter about 100,000years after the formation of the Sun; subsequent collisions and mergers between these planet-sized bodies allowed terrestrial planets to grow to their present sizes. However, in 2006, the International Astronomical Union declared that Pluto does not count as a planet. For part of its red-giant life, the Sun will have a strong stellar wind that will carry away around 33% of its mass. The cloud collapsed, possibly due to the shockwave of a nearby exploding star, called a supernova. OpenStax is part of Rice University, which is a 501(c)(3) nonprofit. Proplyds in the Orion Nebula. Still others, such as Earth's Moon, may be the result of giant collisions. It seems that we live in a universe packed with planets a web of countless stars accompanied by families of objects, perhaps some with life of their own. Planets emerge from the dense disk of gas and dust encircling young stars. They did this by determining the age of stony materials from the asteroid belt. Most of the hundreds of billions of stars in our galaxy are thought to have planets of their own, and the Milky Way is but one of perhaps 100 billion galaxies in the universe. [31], The various planets are thought to have formed from the solar nebula, the disc-shaped cloud of gas and dust left over from the Sun's formation. The solar nebula is a cloud of interstellar gas and dust that condensed to form the entire solar system, including the sun and planets. [81] Objects with large mass have enough gravity to retain any material ejected by a violent collision. [123], Gradually, the hydrogen burning in the shell around the solar core will increase the mass of the core until it reaches about 45% of the present solar mass. Good question. No planet formed in this area. [120][121], As the Sun expands, it will swallow the planets Mercury and Venus. Our solar system began forming about 4.6 billion years ago within a concentration of interstellar dust and hydrogen gas called a molecular cloud. Moons have come to exist around most planets and many other Solar System bodies. Many moons have formed from circling discs of gas and dust around their parent planets, while other moons are thought to have formed independently and later to have been captured by their planets. [2][61] The panspermia hypothesis holds that life itself may have been deposited on Earth in this way, although this idea is not widely accepted. [34] Today, the four giant planets comprise just under 99% of all the mass orbiting the Sun.
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