O Inventário do Sistema Solar

Planetary scientists often refer to the inner worlds of

the solar system as the “terrestrial” planets, from the word

terra, which is Latin for “earth.” It indicates worlds that

have a similar rocky composition to Earth. Earth, Venus,

and Mars have substantial atmospheres, while Mercury

appears to have a very thin one. The big outer worlds are

called the “gas giants.” These planets consist mostly of very

small rocky cores buried deep within massive spheres

made of liquid metallic hydrogen, and some helium,

covered by cloudy atmospheres. The two outermost planets

—Uranus and Neptune—are sometimes described as “ice

giants” because they also contain significant amounts of

supercold forms of oxygen, carbon, nitrogen, sulfur, and

possibly even some water.

The Oort Cloud

The entire solar system is surrounded by a shell of

frozen bits of ice and rock called the Oort Cloud. It

stretches out to about a quarter of the way to the nearest

star. Both the Kuiper Belt and the Oort Cloud are the origin

of most of the comets we see.

Moons and Rings

Nearly all the major planets, some of the dwarf planets, and some asteroids

have natural satellites called moons. The one we’re most familiar with is Earth’s

Moon. The lunar (from the name Luna) surface is the only other world on which

humans have set foot. Mars has two moons, called Phobos and Deimos, and

Mercury and Venus have none.

The gas giants swarm with moons. Jupiter’s four largest are Io, Europa,

Ganymede, and Callisto, and they’re often referred to as the Galileans, honoring

their discoverer, astronomer Galileo Galilei. Over the past few decades, at least

sixty more have been discovered orbiting Jupiter. Saturn, Uranus, and Neptune

also sport dozens of smaller icy worlds. Out in the Kuiper Belt, dwarf planet Pluto

has at least five satellites, while Eris has at least one.

Each gas giant planet has a set of rings. Saturn’s is the most extensive and

beautiful. It’s possible Earth had a ring in its early history, and planetary

scientists now look at rings as somewhat ephemeral (short-lived) objects.

Orbiting the Sun

All the planets of the solar system travel around the