Planets and stars differ in their mass, composition and life cycle. Stars are usually structurally simple bodies of high mass that produce energy by way of nuclear fusion of hydrogen into helium. Planets are much smaller, do not generate light and usually orbit stars.
Solar systems form when clouds of gas and dust coalesce into protostellar accretion disks. Most of the mass in such a disk falls toward the center, which provides the mass and energy required to drive the nuclear fusion engine that powers a star throughout its life. Outside the star, small eddies in the accretion disk collapse locally to form small bodies. These small objects grow in size as they collide with one another over millions of years. The largest of these bodies become planets.
Unlike stars, planets can be gaseous or rocky. Some worlds, such as the dwarf planet Pluto, incorporate ice as a major component of their makeup. At the lower size range, dwarf planets can resemble comets, which are formed in ways similar to planets. The largest possible planet would have a mass 75 times that of Jupiter. Above that threshold, the world's mass is sufficient to sustain fusion, and it is, by definition, considered a brown dwarf star.Learn More
Mercury is the planet with the shortest period of revolution of approximately 88 Earth days. However, Mercury’s period of rotation is about 59 Earth days.Full Answer >
The planet Venus has a thick, dense atmosphere made up almost entirely of carbon dioxide. Though Venus is farther from the sun than Mercury, its surface is actually hotter because the atmosphere traps the heat like a giant, planetary greenhouse.Full Answer >
The planet Mercury completes a single rotation in just under 176 days. Mercury was once thought to be tidally locked, and its rotation was not discovered until 1965.Full Answer >
While no specific date can be identified for the discovery of Mars, knowledge of the planet dates back to ancient civilizations. Ancient Romans, Egyptians and Chinese astronomers independently discovered and named the planet.Full Answer >