At the center, the sun accounts for over 99% of the mass in our solar system. The intense surface heat is fueled by the fusion binding of hydrogen.
The eight planets in orbit which most are familiar with are (in order of proximity to the sun): Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus, and Neptune. “Dwarf planets" such as Pluto also exist.
The Inner Planets
Mercury is the smallest of the eight planets. It rotates around the sun every 88 (Earth) days and travels at an average of 108,000 MPH.[1]
Venus, named after the goddess of love and beauty, has nearly the same diameter and density as Earth. There is a thick, cloudy, atmosphere covering this planet's surface.[2]
Earth is man's planet, she harbors life, and she orbits at one Astronomical Unit (AU) around the sun. Of the four inner planets, Earth is the most dense, and she is the only planet with observed plate tectonics.[3]
Luna, is Earth's only natural satellite. It is in synchronous rotation, meaning that it keeps nearly the same face turned towards the Earth at all times.
Mars is named after the Roman god of war. At one time Mars probably had seas and lakes, now found in the form of ice on its surface. It is likely that water exists under the surface.[4]
There are two moons, Phobos and Deimos, in the gravitational pull of Mars. Phobos is larger than Deimos, and in millions of years will likely disperse into a planetary ring.
Beyond Mars is our solar system's main asteroid belt. Asteroids are mostly non-volatile rock and metal, and they have a vast range in size.
Ceres is the only dwarf planet located in our asteroid belt. It contains about a third of the belt's total mass, and it may harbour an ocean of liquid water.
The Outer Solar System
The outer four planets of our solar system are enormous, and they all have rings gracing their orbits. Jupiter itself is 318 times as massive as Earth.[5]
Jupiter, composed mostly of hydrogen and helium, is the largest planet in our solar system. If it were about 60 times more massive it would begin collapsing under its own gravity and likely ignite into a star. There are 63 satellites known to be in Jupiter's orbit.
Europa, a major moon of Jupiter, is likely to be covered with ice. There may exist liquid oceans which lend to the suggestion about possible (albeit primitive) life on this satellite.
Saturn is similar in atmospheric composition to Jupiter. It also has the most extensive ring system in our solar system, and it has at least 60 moons orbiting it.[6]
Uranus is the lightest of the outer planets. Its core is much colder than the other outer gas giants, and it has a tilted orbit with 27 moons.[7]
Neptune's temperatures vary from about 7,000 degrees Celsius at its center to 218 degrees at its cloud tops. It has 13 moons in orbit.[8]
The Edgeworth-Kuiper Belt is similar to, but much larger than, our asteroid belt. It consists primarily of small (and often frozen) bodies of methane, ammonia, and water.
Pluto is the largest known object in the Kuiper belt. Pluto has three moons, of which the most notable is Charon. It and Pluto orbit each other in a binary system.[9]
Hypothetical Objects
Many believe there is a vast cloud of comets encircling our planetary system. These comets are collectively labeled the Oort Cloud.
Our sun may have a companion star named Nemesis beyond the Oort Cloud. This star may explain the periodic mass extinctions on Earth.
1. "Mercury: Facts & Figures". Solar System Exploration. NASA.
2. "Venus: Facts & Figures". Solar System Exploration. NASA.
3. "Earth: Facts & Figures". Solar System Exploration. NASA.
4. "Mars: Facts & Figures". Solar System Exploration. NASA.
5. "Jupiter: Facts & Figures". Solar System Exploration. NASA.
6. "Saturn: Facts & Figures". Solar System Exploration. NASA.
7. "Uranus: Facts & Figures". Solar System Exploration. NASA.
8. "Neptune: Facts & Figures". Solar System Exploration. NASA.
9. "Pluto: Facts & Figures". Solar System Exploration. NASA.
