UNDERSTAND · 12 ESSENTIAL ANSWERS
The big questions about the Solar System
Start with short answers, then follow links to scientific fact sheets and the 3D experience to explore each idea further.
How many planets are there in the Solar System?
There are eight: Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus and Neptune. They all orbit the Sun.
What is the order of the planets from the Sun?
Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus, then Neptune. The first four are rocky; the next four are giant planets.
Is the Sun a planet?
No. The Sun is a star, an enormous sphere of plasma that produces energy through nuclear fusion. It contains about 99.8% of the Solar System's mass.
Why is Pluto no longer a planet?
Since 2006, the International Astronomical Union has classified it as a dwarf planet. Pluto orbits the Sun and is nearly round, but has not cleared its orbital neighbourhood.
Where is the asteroid belt?
The main belt lies between Mars and Jupiter. It contains countless rocky bodies, including the dwarf planet Ceres, but its total mass is far smaller than the Moon's.
Which is the largest planet?
Jupiter. Its diameter is close to 140,000 kilometres, about eleven times Earth's. Roughly 1,300 Earths could fit inside its volume.
Which is the hottest planet?
Venus, at about 465 °C at the surface. It is hotter than Mercury because its dense, carbon-dioxide-rich atmosphere creates a powerful greenhouse effect.
What is the difference between rotation and orbital revolution?
Rotation is a body's spin around its own axis, linked to its day length. Orbital revolution is its movement around another body; for a planet orbiting the Sun, this defines its year.
What is an astronomical unit?
An astronomical unit, or AU, is approximately the average Earth–Sun distance: about 149.6 million kilometres. Neptune is about 30 AU from the Sun.
Why do the planets not fall into the Sun?
Gravity pulls them towards the Sun while their velocity carries them forwards. Together, these produce an orbit.
Where is the Solar System in the Milky Way?
It lies in the Orion Spur, about 26,000 light-years from the galactic centre. It completes one orbit around the Milky Way in roughly 230 million years.
Is there life elsewhere in the Solar System?
No extraterrestrial life has been confirmed. Mars and the oceans of Europa and Enceladus are among the places most studied for conditions that could support microbial life.