The Sun-centered model of the solar system is called the Copernican or heliocentric model. Galileo's telescope observations of the phases of Venus helped prove that the planets circle the Sun.

Galileo Galilei also discovered Jupiter's four largest moons: Io, Europa, Ganymede and Callisto. Ganymede, one of those four, is the largest moon in the solar system.

In the 1600s Johannes Kepler showed that planets move in ellipses, not perfect circles. Kepler's three laws of planetary motion also say that a planet sweeps out equal areas in equal times, and that a planet's orbital period grows with the size of its orbit.

William Herschel discovered Uranus in 1781 with a telescope. Because Uranus did not follow the orbit people expected, Urbain Le Verrier concluded that an unknown planet must be affecting it.

Neptune was found with the help of calculations: Johann Galle located it through a telescope after scientists predicted where it should be. Galileo had seen Neptune in 1612 and 1613 but recorded it as a star.

The first asteroid found, Ceres, was discovered on New Year's Day 1801. There is a belt of asteroids, which are rocky space rocks, between Mars and Jupiter, and more than a million asteroids orbit the Sun in this main belt. Clyde Tombaugh discovered Pluto in 1930.

Pluto is a dwarf planet because it is nearly round but has not cleared its orbit of debris. Pluto was once counted as a planet. As of November 2026, it is called a dwarf planet. Also as of November 2026, astronomers have recognized five dwarf planets.

As of November 2026, our solar system has eight planets that circle the Sun. Also as of November 2026, the International Astronomical Union says a planet must orbit its star, be mostly round, and have cleared other objects of similar size out of its orbit.

Words to know

• heliocentric: Sun-centered
• ellipses: oval paths
• asteroid: rocky space rock
• calculations: worked-out math
• discovered: first found