What is a Planet?

Photo courtesy NASA Our solar system

There are eight other planets in our solar system besides Earth. Many of these planets have moons orbiting them. But what exactly is a planet? By definition, a planet is a large body that orbits a star and shines by reflecting starlight from its surface. Planets vary in their mass, composition and distance from the star. In our solar system, the planets are divided into three major categories:

• Inner-terrestrial planets - Mercury, Venus, Earth and Mars. These planets are made of rock and orbit close to the Sun.
• Outer gas giants (Jovian planets) - Jupiter, Saturn, Uranus and Neptune. These planets are massive (with hundreds of times the mass of Earth). They have dense, gaseous, hydrogen-rich atmospheres that also contain helium, ammonia and methane. These atmospheres probably surround inner cores made of rock.
• Other bodies - Pluto, comets, asteroids and Kuiper Belt objects. These bodies are made of rock and ice mixtures. Despite controversy, Pluto is still categorized as a planet, though its composition is more similar to asteroids and comets than to other planets.

Photo courtesy NASA Formation of a solar system

The planets in our solar system were made from the disc of swirling gas and dust that formed our sun. As the hydrogen gas and dust of the early solar system fell into the center of disc, forming the protosun, the gas and dust heated up to a temperature that could sustain nuclear fusion. At the same time, smaller clumps of dust and gas, called planetismals, formed in the outer parts of the disc. When the protosun "ignited," it blew the dust and gas away from its immediate vicinity. The planetismals coalesced to form the planets (see How Stars Work: The Life of a Star for details). Scientists believe that other solar systems would be formed in the same way.

Habitable Zone Stars are much too hot to support life, so planets or moons are the most likely places for life to develop. Light from a star warms the orbiting planet and supplies the energy necessary for life. In addition to energy, life seems to need a liquid, chemical solvent of some type in which to develop. On Earth, this solvent is water, but it is conceivable that other solvents (such as ammonia, methane or hydrogen fluoride) might also work. With this in mind, it seems that the planet must lie within a certain range of distances from the star so that the solvent can remain in liquid form -- if the planet is too close to the star, the solvent will boil away; if it is too far from the star, the solvent will freeze. For our sun, the habitable zone appears to be between the orbits of Venus and Mars.