The planets of our solar system.
Scientists aren't completely sure how the solar system formed, but most agree the best explanation is that a cloud of molecules collapsed inward on itself, forming our solar system about 4.6 billion years ago.
In this description, called the nebular model, our sun coalesced first, surrounded by a spinning disk of gas and dust.
How the sun formed
Some evidence, such as a 2010 study from scientists at the Carnegie institution, suggests this contraction could have been spurred by a burst from nearby supernovas . Other forces like differences in density could also have caused the cloud begin collapsing according to "From Suns to Life: A Chronological Approach to the History of Life on Earth" (Springer, 2004), an astronomy review text.
Initially, gas collected in the dense center of this spinning disk, creating a protosun. Collisions between molecules heated things up, eventually raising temperatures to about ten million degrees Celsius. These increasingly hot and violent crashes sparked nuclear reactions, which turned the protosun into a star. This process took about 100,000 years, according to "From Suns to Life."
How the planets formed
Meanwhile, in the disk of material around the young sun, a process called accretion formed the planets, moons, comets and asteroids. Small particles crashed together to form larger and larger bodies, eventually reaching the size of planetesimals up to a few kilometers across. Massive enough to create their own gravity, these bodies drew even more collisions, with only the largest surviving the destructive crashes, according to the Lunar and Planetary Institute.
In the hot area near the evolving Sun , water on the planetesimals tended to evaporate away, gasses were swept outward and only heavier material, like silicon and metals could condense into solids. Young planets in the inner solar system (like Earth) formed from this rocky, dense material.
Farther away from the new star, cooler temperatures and abundant ice allowed for much larger bodies to form, creating the cores of planets such as Jupiter and Saturn. These cores were large enough for their gravity to pull in gas from the surrounding nebula, creating the gas giants of the outer solar system, according to the third edition of "The Solar System," an astronomical reference book.
The other objects in the solar system
Beyond Neptune, in the even colder reaches of the disk, there was not enough material available for planetesimals to grow to gas giants. These stunted chunks make up the Kuiper belt.
Between Mars and Jupiter, another field of planetesimals became the asteroid belt, which is thought to have been kept from clumping into a planet by the gravity of Jupiter, according to "The Solar System."
The nebular model explains why all of the planets orbit in the plane of the Sunâ??s rotation that plane contains the remnants of that early gaseous disk.
Telescopes have captured images of early moments in the formation of other solar systems throughout the universe, showing in live action what we believe took place around our own Sun.
- Gallery: The New Solar System
- What's at the Center of the Milky Way?
- How Do Scientists Know the Universe is Expanding?
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