Solar Nebula. Our solar system began forming within a concentration of interstellar dust and hydrogen gas called a molecular cloud. The cloud contracted under its own gravity and our proto-Sun formed in the hot dense center. The remainder of the cloud formed a swirling disk called of the solar nebula.

Planets are generally divided into two groups: the terrestrial and the giant planets. The terrestrial planets are the four inner planets: Mercury, Venus, Earth, and Mars. They are generally small in size (about the size of the Earth) and are predominantly rocky in composition.

Four Major Characteristics of the Solar System

• smaller size and mass.
• higher density (rocks and metal)
• solid surface.
• few moons and rings.
• closer to the Sun.

The orbits of the planets are ellipses with the Sun at one focus, though all except Mercury are very nearly circular. The orbits of the planets are all more or less in the same plane (called the ecliptic and defined by the plane of the Earth’s orbit).

Venus

The Days (And Years) Of Our Lives

On February 7, 1984, Bruce McCandless became the first human to float free from any earthly anchor when he stepped out of the space shuttle Challenger and flew away from the ship. He later helped deploy the Hubble Space Telescope into orbit from the space shuttle Discovery in 1990.