The Solar System Explained
The Solar System Explained
The Solar System is the Sun and everything held by its gravity. It includes eight planets, many moons, dwarf planets, asteroids, comets and huge amounts of smaller material. The Sun contains most of the mass in the Solar System. Everything else formed from material around the young Sun about 4.6 billion years ago. The planets do not all look or behave the same because they formed under different conditions. Studying those differences helps scientists understand how planetary systems develop.
The four inner planets are rocky
Mercury, Venus, Earth and Mars are called terrestrial planets. They are made mostly from rock and metal and have solid surfaces. Mercury is the smallest planet and the closest to the Sun. Venus has a thick carbon-dioxide atmosphere and an extremely hot surface. Earth has liquid surface oceans and a biosphere full of life. Mars is cold today, but its surface shows evidence that liquid water existed there in the distant past.
The outer planets are much larger
Jupiter and Saturn are called gas giants. Uranus and Neptune are usually called ice giants. These four worlds are much larger than Earth and do not have a simple solid surface you could stand on. Jupiter is the largest planet in the Solar System. All four outer planets have ring systems, although Saturn's are by far the easiest to see. They also have many moons with very different sizes, surfaces and histories.
Moons can be complex worlds
A moon is a natural object that orbits a planet or another small world. Some moons are little more than irregular rocky bodies. Others are large enough to have complex geology. Jupiter's moon Io is extremely volcanic. Europa and Saturn's moon Enceladus are strong examples of worlds thought to contain oceans beneath icy surfaces. Moons are therefore major targets in the search for environments that could possibly support life.
Asteroids, comets and dwarf planets
Not every important object in the Solar System is a planet or moon. Asteroids are mostly rocky or metallic bodies, with many found between Mars and Jupiter. Comets contain large amounts of ice and can develop bright tails when they approach the Sun. Dwarf planets are round worlds that orbit the Sun but have not cleared other objects from their orbital neighborhood. Pluto and Ceres are two well-known examples. These smaller bodies preserve useful clues about the early history of the Solar System.
Why the Solar System keeps changing
The Solar System may look stable, but its objects are always moving and changing. Asteroids collide, moons crack, planets lose atmosphere and comets slowly change after repeated trips near the Sun. Spacecraft continue to reveal details that cannot be seen clearly from Earth. New small bodies and moons are also still being discovered and confirmed. That means some counts, such as the number of known moons, can change over time. Paleon focuses on the larger structure and clearly dates numbers that are likely to change.