Structure and the order of the planets
The Sun holds about 99.8% of the Solar System's mass; planets and smaller bodies share the rest. Moving outward, Mercury, Venus, Earth and Mars are rocky terrestrial planets; Jupiter and Saturn are gas giants made mostly of hydrogen and helium; Uranus and Neptune are ice giants rich in water, ammonia and methane ices. The asteroid belt lies between Mars and Jupiter, the Kuiper belt of icy bodies such as Pluto lies beyond Neptune, and much farther out the hypothetical Oort cloud is thought to supply long-period comets. Distances are usually given in astronomical units (AU), the Earth–Sun distance of about 150 million km.
| Planet | Distance from Sun (AU) | Orbital period |
|---|---|---|
| Mercury | 0.39 | 88 days |
| Venus | 0.72 | 225 days |
| Earth | 1.00 | 365 days |
| Mars | 1.52 | 687 days |
| Jupiter | 5.20 | 11.9 years |
| Saturn | 9.58 | 29.5 years |
| Uranus | 19.2 | 84 years |
| Neptune | 30.1 | 165 years |
Formation and changing ideas
The accepted nebular hypothesis says that about 4.6 billion years ago part of a giant molecular cloud collapsed, forming the Sun at its centre while dust and gas in the surrounding disc clumped into planets. First proposed by Kant and Laplace in the 18th century, the idea is now strongly supported by telescopes that see protoplanetary discs around young stars. For most of history the Earth-centred model dominated, until Copernicus proposed a Sun-centred system in 1543; Galileo's telescope, Kepler's laws of planetary motion and Newton's law of gravitation completed the picture. The "Nice model", in which Jupiter and Saturn migrated and scattered asteroids and comets, is an active area of research.
Pluto and exploration
Discovered by Clyde Tombaugh in 1930, Pluto was the ninth planet for 76 years. After the similar-sized Eris was found in the Kuiper belt, the International Astronomical Union adopted a formal definition in 2006: a planet must have cleared the neighbourhood around its orbit. Pluto became a dwarf planet. Voyager 1 and 2, launched in 1977, flew past the giant planets from Jupiter to Neptune and have since left the heliosphere, and in 2015 New Horizons revealed Pluto's heart-shaped ice plain. With thousands of exoplanets now known around other stars, how typical our own system is has become a research question in its own right.
FAQ
- What is the largest planet in the Solar System?
- Jupiter. Its diameter is about 11 times Earth's and its mass is roughly 2.5 times that of all the other planets combined.
- Could Pluto become a planet again?
- Some planetary scientists reject the IAU definition, but Pluto is officially a dwarf planet. Reinstating it would logically mean adding Eris and other similar bodies as well.
Sources reviewed: 2026-10-26
References
- Woolfson, M. (2000). The origin and evolution of the solar system. Astronomy & Geophysics, 41(1), 1.12–1.19.
- International Astronomical Union. (2006). Resolution B5: Definition of a Planet in the Solar System. IAU General Assembly, Prague.
- Gomes, R., Levison, H. F., Tsiganis, K., & Morbidelli, A. (2005). Origin of the cataclysmic Late Heavy Bombardment period of the terrestrial planets. Nature, 435, 466–469.
- NASA Solar System Exploration. (2024). Planets: Overview.
- Copernicus, N. (1543). De revolutionibus orbium coelestium. Nuremberg.
- Kepler, J. (1609). Astronomia nova. Prague.
- Newton, I. (1687). Philosophiæ Naturalis Principia Mathematica. London.
- Laplace, P.-S. (1796). Exposition du système du monde. Paris.
- Safronov, V. S. (1969/1972). Evolution of the Protoplanetary Cloud and Formation of the Earth and the Planets. NASA TT F-677.
- Stern, S. A., et al. (2015). The Pluto system: Initial results from its exploration by New Horizons. Science, 350(6258), aad1815.