Sun

The Sun is a yellow dwarf star about 4.6 billion years old. It has a system of eight planets in concentric orbits, including an inner system with four rocky planets and an outer system with four gas giants, with an asteroid belt separating the two. Earth is the third-closest planet to the Sun and the largest rocky body in the system, with Venus a close second and Mars third. Interestingly, there are two moons of gas giants intermediate in size between the planets Mars and Mercury, one of which, Titan, has an atmosphere thicker than Earth's. In addition to planets and moons, the solar system also includes countless small Sun-orbiting bodies, rocky asteroids (some of which range well outside the asteroid belt and have been known to collide with Earth) and icy comets which spend most of their existence very far from the Sun.

The Sun generates massive amounts of energy by fusing hydrogen into helium, a process that will continue for several billion years into the future. As it burns up more of its fuel, the Sun becomes gradually hotter, a key discovery that led to the question of how Earth's climate has been maintained within the relatively narrow range of temperatures conducive to life. This led James Lovelock to formulate the hypothesis that climate homeostasis on Earth has been maintained by Gaia for billions of years. The Daisyworld model is a simplification of this concept.

Incoming sunlight is the energy source for almost all life on Earth, whether directly through photosynthesis, or indirectly through consuming or parasitizing plants. One major exception is the ecosystem of species that cluster around hydrothermal vents on the ocean floor, which are powered by Earth's internal heat, the result of nuclear decay of heavy elements in the planet's interior.