Overview

As a result of discoveries made by the New Horizons mission and other observations, the new solar system may have many more planets than the accepted eight or nine. The new solar system may have hundreds of planets, including asteroids, moons, and dwarf planets. Rather than memorize hundreds of names, it’s more important to know where the planet is located.

Definition

The accepted definition of “planet” according to the 2006 International Astronomical Union had three parts: that the body was round, held together by its own gravity; that it did not generate its own nuclear fusion, unlike stars; and that it and its satellites move alone through their orbit. Pluto was demoted to “dwarf planet” as it did not meet all the criteria. However, recent scientists have proposed that the only important part of the planetary definition is that the body is round, held together by its own gravity. Once again, Pluto is a planet. In fact, it and its largest moon Charon are part of a binary system.

Inner Planets and Asteroids

The closest planets to the Sun are relatively small, made of rocks and metals, and include Mercury, Venus, Earth and its Moon, and Mars and its satellites Phobos and Deimos. Most asteroids are also made of rocks and metals, and the largest asteroid Ceres meets the definition of a dwarf planet.

Outer Planets

The outer planets are giants, including Jupiter, Saturn, Uranus, and Neptune. All have rings, although Saturn’s are the most visible from Earth. Sometimes Jupiter and Saturn are known as gas giants because they consist mostly of hydrogen and helium. Uranus and Neptune are smaller and consist of various ices, and are sometimes called ice giants.

Beyond Neptune

The mysteries beyond Neptune have become better known because of the New Horizons space mission. Pluto is the best known object in the Kuiper Belt, and it and Charon form a binary planet. Some other named planets include Makemake, Haumea, and Eris. As the New Horizons mission and others travel further, more objects will be discovered, and what is known about the Solar System expanded.

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