Solar system; stability of orbital motions; satellites (physics only)

Our Solar System is heliocentric — the Sun (a star) sits at its centre, and everything else orbits it: the eight planets, several dwarf planets (including Pluto and Ceres), asteroids, and comets. Moons are natural satellites that orbit planets rather than the Sun directly. The Solar System itself is just a tiny part of a much larger galaxy, the Milky Way.

In order of increasing distance from the Sun, the eight planets are: Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus, Neptune.

• •The smaller, inner planets (Mercury, Venus, Earth, Mars) are made primarily of rock; the larger, outer planets (Jupiter, Saturn, Uranus, Neptune) are made primarily of gas.
• •All the planets orbit the Sun roughly in the same plane.
• •Every planet rotates on its own axis, but at different speeds; a few rotate in the opposite direction to the others, or on a noticeably tilted axis — likely the result of collisions earlier in the Solar System's history that knocked their axis off balance.
• •The largest planets have prominent rings, because their strong gravitational fields are able to capture and hold onto orbiting debris.

• •Mars appears to briefly reverse its direction across the sky (retrograde motion) as seen from Earth — this is naturally explained by a faster-orbiting Earth overtaking the more slowly-orbiting Mars, but is very difficult to explain if Earth is stationary at the centre.
• •Galileo observed moons orbiting Jupiter, proving that not everything in the sky orbits the Earth.
• •Kepler showed that planetary orbits are better described as ellipses, not perfect circles.

A star begins life as a large cloud of dust and gas within a galaxy, called a nebula. Gravitational attraction between the particles in the cloud pulls them closer together; as the cloud contracts, it becomes denser, and its temperature and pressure rise.

1. 1Once the core is hot and dense enough, hydrogen nuclei begin to fuse together to form helium nuclei — nuclear fusion — releasing a huge amount of energy.
2. 2This release of energy creates outward pressure that opposes the inward pull of gravity trying to collapse the cloud further.
3. 3An equilibrium forms once the outward pressure from fusion exactly balances the inward pull of gravity — the star has now fully formed, as a stable main sequence star, and will stay in this state for billions of years.
4. 4Eventually, the star runs out of hydrogen to fuse in its core. Fusion can no longer balance gravity, so the equilibrium breaks and the star collapses.

What happens next depends on the star's mass:

A planet orbiting the Sun (or a satellite orbiting a planet) is kept on a roughly circular path by the gravitational force pulling it toward the object it orbits. This force continuously changes the orbiting object's direction, without necessarily changing its speed.

A satellite is any object that orbits a larger body. Natural satellites, such as the Moon, occur without human involvement; artificial satellites, such as communication or weather satellites, are built and launched by humans and placed into orbit around the Earth.