Planetary science

Solar system formation, planetary interiors, and exoplanet detection

What is planetary science?

Planetary science studies planets, moons, asteroids, comets, and other objects that orbit stars — including our own solar system's eight planets and the countless smaller bodies orbiting alongside them.

Our solar system

The Sun sits at the center of our solar system, and everything else orbits it because of its enormous gravitational pull (from the Mechanics article). The four inner planets — Mercury, Venus, Earth, and Mars — are relatively small and rocky. The four outer planets — Jupiter, Saturn, Uranus, and Neptune — are much larger and made mostly of gas and ice. Between Mars and Jupiter lies the asteroid belt, a region containing countless smaller rocky bodies that never came together to form a full planet.

How planets form

Planets form from the same swirling cloud of gas and dust that gives birth to a star. As the young star ignites at the center, the remaining material flattens into a spinning disk, and within that disk, dust particles gradually stick together, forming larger and larger clumps that eventually grow into full-sized planets — a process that has been directly observed happening around other young stars.

Moons and rings

Many planets have moons — smaller bodies orbiting them the way planets orbit the Sun. Earth has one; Jupiter and Saturn each have dozens of confirmed moons of widely varying size. Saturn and, to a lesser extent, the other outer planets, also have rings: countless small particles of ice and rock orbiting in a flattened disk around the planet.

Why planetary science matters

Planetary science helps us understand Earth itself by comparison with other worlds, guides the search for potentially habitable environments elsewhere in the solar system, and has directly enabled space missions that have landed on Mars, orbited every planet in the solar system, and sent spacecraft beyond it entirely.