SUMMARY OF KEY IDEAS
Formation of the Solar System
Hydrogen, helium, and traces of lithium, the three lightest elements, were formed shortly after the formation of the universe. The heavier elements were produced much later by stars and are cast into space when stars die. By mass, 98% of the observed matter in the universe is hydrogen and helium.
The solar system formed 4.6 billion years ago from a swirling, disk-
shaped cloud of gas, ice, and dust, called the solar nebula. The planets and other debris in the solar system today formed from gas, ice, and dust in the solar nebula orbiting the protosun.
The outer solar system, beyond the snow line, had both dust and ice (including hydrogen and helium), while inside the snow line, such ices were vaporized by the protosun.
Jupiter and Saturn were initially worlds of rock and metal that pulled onto themselves large amounts of hydrogen and helium, along with some water.
Uranus and Neptune were also initially worlds of rock and metal, but they attracted more water and less hydrogen and helium than the other giant planets.
The Nice model of solar system formation proposes that in the outer solar system Jupiter formed first, followed by Saturn, and then by Neptune and Uranus, which were then flung out to their present orbits by gravitational forces from Jupiter and Saturn.
The four inner planets formed through the collisions of Moon-
sized bodies, probably after the outer four planets were formed. The Sun formed at the center of the solar nebula. After about 100 million years, the temperature at the protosun’s center was high enough to ignite thermonuclear fusion reactions.
For 800 million years after the Sun formed, impacts of asteroidlike objects on the young planets dominated the history of the solar system.
Categories of Solar System Objects
Astronomical objects smaller than the eight planets are classified as dwarf planets or small solar system bodies (SSSBs).
A variety of other names, including asteroids, comets, meteoroids, trans-
Neptunian objects, plutinos, plutoids, Kuiper belt objects (KBOs), and Oort cloud objects, overlap with the designations “dwarf planet” and “SSSB.” KBOs and Oort cloud objects are trans-
Neptunian objects— they orbit farther from the Sun than the outermost planet. To date, five objects—
Pluto, Ceres, Eris, Haumea, and Makemake— have been classified as dwarf planets. Other objects orbit the Sun beyond Neptune. At least 1500 KBOs have been observed. A few potential Oort cloud objects have also been identified.
WHAT DID YOU THINK?
How many stars are there in the solar system? The Sun is the only star in our solar system.
Were the sun and planets among the first generation of objects created in the universe? No. All matter and energy were created by the Big Bang. However, much of the material that exists in our solar system was processed inside stars that evolved before the solar system existed. The solar system formed billions of years after the Big Bang occurred.
How long has Earth existed, and how do we know this? Earth formed along with the rest of the solar system about 4.6 billion years ago. The age is determined from the amount of radioactive decay that has occurred on Earth.