A Guide to Things Orbiting Our Sun
Discovery
For millennia, astronomers have followed points of light that seemed to move among the stars. The ancient Greeks named them planets, meaning "wanderers." Mercury, Venus, Mars, Jupiter and Saturn were known in antiquity, and the invention of the telescope added the Asteroid Belt, Uranus, Neptune, Pluto and many of these worlds' moons. The dawn of the space age saw dozens of probes launched to explore our system, an adventure that continues today. Only one spacecraft so far, Voyager 1, has crossed the threshold into interstellar space.
Inner Planets
The four inner four planets Mercury, Venus, Earth and Mars are made up mostly of iron and rock. They are known as terrestrial or earthlike planets because of their similar size and composition. Earth has one natural satellite; the moon, and Mars has two moons; Deimos and Phobos.
Between Mars and Jupiter lies the Asteroid Belt. Asteroids are minor planets, and scientists estimate there are more than 750,000 of them with diameters larger than three-fifths of a mile (1 km) and millions of smaller asteroids. The dwarf planet Ceres, about 590 miles (950 km) in diameter, resides here. A number of asteroids have orbits that take them closer into the solar system that sometimes lead them to collide with Earth or the other inner planets.
Outer Planets
The outer planets Jupiter, Saturn, Uranus and Neptune are giant worlds with thick outer layers of gas. Between these planets, they have dozens of moons with a variety of compositions, ranging from rocky to icy to even volcanic. Nearly all the planets' mass is made up of hydrogen and helium, giving them compositions like that of the sun. Beneath these outer layers, they have no solid surfaces — the pressure from their thick atmospheres liquefy their insides, although it is believed that they could have rocky cores. Rings of dust, rock, and ice encircle all these giants, with Saturn's being the most famous.
Pluto and Planet 9
Pluto, now considered a dwarf planet, dwells in the Kuiper Belt. It is not alone: recent additions include Makemake, Haumea and Eris. Sedna, which is about three-fourths the size of Pluto, is the first dwarf planet discovered in the Oort Cloud.If Planet Nine exists, it orbits the sun at a distance that is 20 times farther out than the orbit of Neptune. (The orbit of Neptune is 2.7 billion miles from the sun at its closest point.) The strange world's orbit is about 600 times farther from the sun than the Earth's orbit is from the star. Scientists have not actually seen Planet Nine directly, and some astronomers debate its existence.