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.

Comparative Planetology

• The four inner planets of the solar system share many characteristics and are distinctly different from the four giant outer planets.
• The four inner, terrestrial planets are relatively small, have high average densities, and are composed primarily of rock and metal.
• Jupiter and Saturn have large diameters and low densities and are composed primarily of hydrogen and helium. Uranus and Neptune have large quantities of water as well as much hydrogen and helium. All four giant planets have terrestrial cores.
• Pluto, once considered the smallest planet, has a size, density, and composition consistent with other large Kuiper belt objects.
• Asteroids are rocky and metallic debris in the solar system, are larger than about 10 m in diameter, and are found primarily between the orbits of Mars and Jupiter. Meteoroids are smaller pieces of such debris. Comets are debris that contain both ice and rock.

Planets Outside Our Solar System

• Astronomers have observed disks of gas and dust orbiting young stars.
• At least 1000 exoplanets have been discovered orbiting other stars.
• Exoplanets ranging in mass from less than the mass of the Earth to many times the mass of Jupiter have been detected.
• Most of the exoplanets that have been discovered have masses roughly equal to the mass of Jupiter.
• Some exoplanets are observed directly, while most are detected indirectly as a result of their effects on the stars they orbit.
• Exoplanets orbiting virtually all types of stars have been observed.
• Some planets that are not orbiting stars have been observed.