Key Ideas and Terms

4-1 The solar system has two broad categories of planets orbiting the Sun: terrestrial (Earthlike) and Jovian (Jupiterlike)

• Two defining characteristics of a planet are its orbit around a star and its diameter being large enough to form a spherical shape.
• A planet’s average density is its mass divided by volume, measured in kilograms per cubic meter (kg/m³), which characterizes its interior.
• All four inner, terrestrial planets have dense iron cores and hard, rocky surfaces with mountains, craters, valleys, and volcanoes.
• All four outer, Jovian planets are relatively large with low densities and are Jupiterlike, having thick atmospheres.

4-2 Seven large moons are almost as big as the inner, terrestrial planets

• Naturally forming, rocky moons orbit the Jovian planets much as Earth’s Moon orbits Earth.
• The Jovian planets have many natural moons that are almost as large as the terrestrial planets.

4-3 Spectroscopy reveals the chemical composition of the planets

• Spectroscopy reveals the chemical composition of planets both with and without a surrounding envelope of gas, known as its atmosphere.
• The inner planets have solid surfaces composed primarily of silicates.
• The outer planets have thick gaseous atmospheres rich in hydrogen, helium, and methane.

4-4 Small chunks of rock and ice also orbit the Sun: asteroids, trans-Neptunian objects, and comets

• Most asteroids are rocky objects orbiting our Sun in an asteroid belt between the orbits of Mars and Jupiter, the largest of which are known as minor planets.
• Dwarf planets are the largest of the trans-Neptunian objects (TNOs) orbiting our Sun at and beyond Neptune’s orbit.
• Icy balls known as comets that orbit our Sun just beyond the orbits of the trans-Neptunian objects are found in the Kuiper belt, whereas the most distant comets held by our Sun’s gravitational attraction are part of the Oort cloud.

4-5 The Sun and planets formed from a rotating solar nebula

• A collection of dust and gas illuminated by the star it surrounds is known as a nebula.
• The nebular hypothesis is that our solar system formed by a flattening and spinning collapse of an initial solar nebula, which explains why all planets orbit in the same direction in the same plane.
• This constant relationship between the size of an object and its rotation speed is an example of a general principle called the conservation of angular momentum.
• The flattening disk eventually forming the planets of our solar system surrounding our early protosun is known as a protoplanetary disk or proplyd.

4-6 The planets formed by countless collisions of dust, rocks, and gas in the region surrounding our young Sun

• In the early solar system, whether a planet was made of solid or a gas depended on a material’s condensation temperature.
• Planets formed first as kilometer-sized planetesimals, which gravitationally combined through accretion to become protoplanets.
• In a molten protoplanet, material is free to move, and through chemical differentiation, denser, iron-rich minerals sank to the centers of the planets while the less dense, silicon-rich minerals floated to their surfaces.
• Young stars emit a T Tauri wind of mass similar to how the present-day Sun loses mass from its outer layers in the form of high-speed electrons and protons known as the solar wind.

4-7 Understanding how our planets formed around the Sun suggests planets around other stars are common

• Planets orbiting stars other than our Sun are called extrasolar planets.
• Indirect detection using the transit method, measuring a star’s dimming, and the radial velocity method, measuring tiny shifts in a star’s position, find more exoplanets than direct imaging.