||The solar system consists of the Sun and its orbiting planets. The Sun, vastly larger than any of the planets, is a small star, similar to many of the stars in the night sky. Its light is generated by hydrogen fusion. This process occurs because the gases in the star (mostly hydrogen) are subjected to incredibly high pressures and temperatures, due to the gravitational field of the star. These conditions force the hydrogen atoms into close proximity, whereupon two hydrogen atoms fuse to form one deuterium atom. Energy is released by this process, and much of it is emitted as visible light.
The rest of the bodies which make up the solar system, the planets, are indistinguishable from stars to the naked eye. However, with a telescope it may be seen that they have surface features and sometimes moons, whereas stars remain point-like, due to their distance. Patient observations reveal that the planets move against the background of stars as they orbit the sun.
The closest planet to the Sun is Mercury, an airless ball of rock about the size of our Moon. The temperatures on its night side and day side differ by 670 degrees centigradeâ€”the largest such contrast in the solar system.
Next is Venus, similar to Earth in size and distance from the Sun. Venus has a daytime temperature of 450 degrees centigrade, and an atmosphere of 90% carbon dioxide. It is covered with a thick layer of sulphur clouds, which have recently been penetrated by spacecraft which have sent back information about its surface.
The next planet out from the Sun is Earth, whose oxygen-rich atmosphere and small temperature variations combine to support life. Earth has one moon.
Mars is the next planet out; slightly smaller than Earth and with similar daytime temperatures, it offers the best hope of supporting human life. However, it has a carbon dioxide and carbon monoxide atmosphere, and the night time temperatures are aboutâ€”100 degrees centigrade. Mars is orbited by two moons, Phobos and Deimos.
After Mars we find the asteroid belt, a huge ring of orbiting rocks. Many of the meteorites that we see burning up in our upper atmosphere as shooting stars come from here.
Jupiter, the largest planet, lies beyond the asteroid belt. This gas giant could fit 1,000 Earths within it, but is still small compared to the sun, which has 1,000 times the volume of Jupiter. Jupiter has a tiny solid core surrounded by a dense atmosphere of hydrogen, helium, methane, ammonia and a little water. Its swirling cloud systems produce distinctive patterns including the Great Red Spot. Jupiter has 12 large moons and many smaller ones, which lie in the same band as Jupiter\'s faint ring system.
Further out is Saturn, a gas giant similar to Jupiter but smaller, with 10 large moons and extensive ring systems. Beyond Saturn we find Uranus and Neptune, both gas giants with five and two large moons respectively. Uranus has a highly unusual mode of rotation in that its axis points toward the sun, unlike all the other planets whose axes are at 90 degrees to the plane of the solar system.
Last of all is tiny Pluto, a system of two frozen rocks orbiting each other. Due to its eccentric orbit, Pluto is sometimes closer to the Sun than Neptune.
All of the planets and the asteroid belt lie in a plane, so that the solar system has a disc-like appearance. This indicates that the cloud of gases from which it formed had an overall rotation. JJ
See also astronomy; nuclear fission/fusion.