SPACE.com -- Space.com's Guide to the Solar System


	Space.com's Guide to the Solar System posted: 01:30 pm ET 11 November 1999

Mars reference page

Planets are just part of our solar system. Comets, asteroids and the planets' moons are part of the solar system, too, all orbiting around a star we call the Sun. And it's all held together by gravity. The planets are often divided into two groups: the four inner planets (Mercury, Venus, Earth and Mars) and the five outer planets (Jupiter, Saturn, Uranus, Neptune and Pluto). Click for more detail:

X PLANET	DISTANCE FROM SUN IN MILES	DIAMETER IN MILES	ROTATION IN EARTH TIME	ORBIT IN EARTH TIME
Mercury	36 million	3,031	58.6 days	88 days
Venus	67 million	7,521	243 days	225 days
Earth	93 million	7,926	23 hrs, 56 min	365.24 days
Mars	142 million	4,217	24.6 hrs	687 days
Jupiter	483 million	88,730	9.9 hrs	11.9 years
Saturn	886 million	74,900	10.2 hrs	29.5 years
Uranus	1.78 billion	31,763	17.2 hrs	84 years
Neptune	2.79 billion	30,755	16.1 hrs	165 years
Pluto	3.67 billion	1,430	6.4 days	248 years

How the solar system formed

Scientists believe the solar system began forming 10 to 12 billion years ago as swirling gas and dust formed a dense core. The core, with most of the mass, collapsed around 5 or 6 billion years ago and later became the Sun.

The small amount of remaining material swirled into a disk. Some of it crashed together and formed planets. That's the main theory, at least, and though most scientists think that's how it happens, there are a handful of other explanations.

Final fact

Most scientists suspect there are oodles of other solar systems like ours. And as of late, they had found nearly two dozen other planets orbiting faraway stars. None of them appear to have the right conditions to support life, though.