The Solar System

The Solar System formed approximately 4.6 billion years ago from a giant molecular 
cloud. It consists of the Sun and all celestial objects gravitationally bound to it.

The Sun contains 99.86% of the Solar System's mass. It is a G-type main-sequence 
star and is composed primarily of hydrogen (73%) and helium (25%).

The eight planets of the Solar System in order from the Sun:
1. Mercury - Smallest planet, closest to Sun, no atmosphere
2. Venus - Hottest planet, thick atmosphere, rotates backwards
3. Earth - Only known planet with life, has one moon
4. Mars - Red planet, has two moons (Phobos and Deimos)
5. Jupiter - Largest planet, has Great Red Spot, 95 known moons
6. Saturn - Known for prominent rings, 146 known moons
7. Uranus - Ice giant, rotates on its side, 28 known moons
8. Neptune - Windiest planet, has Great Dark Spot, 16 known moons

Pluto was reclassified as a dwarf planet by the International Astronomical Union 
(IAU) in 2006. Other dwarf planets include Eris, Haumea, Makemake, and Ceres.

The asteroid belt is located between Mars and Jupiter. It contains millions of 
rocky objects, with Ceres being the largest at about 940 km in diameter.

The Kuiper Belt extends beyond Neptune's orbit and contains icy bodies including 
Pluto. The Oort Cloud is a theoretical spherical shell of icy objects at the 
outer edge of the Solar System.

Notable spacecraft missions:
- Voyager 1 launched in 1977, entered interstellar space in 2012
- Mars rovers: Spirit (2004), Opportunity (2004), Curiosity (2012), Perseverance (2021)
- New Horizons flew by Pluto in 2015
- Cassini orbited Saturn from 2004 to 2017

The habitable zone, also called the Goldilocks zone, is the region around a star 
where liquid water could exist on a planet's surface. Earth is within the Sun's 
habitable zone.