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For seventy-six years, ever since Pluto was discovered in 1930, we had 9 planets in our Solar System. This all changed in 2006 when Pluto was demoted to the category of dwarf planets.
Mercury was only the second smallest planet back when there were 9. Mercury is closer to Earth than a number of other planets, but we cannot get a very good look at it because of its proximity to the Sun. Astronomers cannot use the Hubble Space Telescope to look at the planet because the Sun’s light would permanently damage the piece of equipment.
Venus is the brightest of all 9 planets. The only objects brighter in the Solar System are the Sun and the Moon. Venus is so bright that it can actually cast shadows. If the Moon is not out one night, you may be able to find some shadows thrown by the planet.
Earth is the densest of all planets in our Solar System. Our planet is composed mostly of iron, silicon, magnesium, and oxygen. Almost one-third of the planet (32.1%) is iron. There is nearly as much oxygen in the planet – 30.1%. There are lesser amounts of silicon (15.1%) and magnesium (13.9%). The materials are not spread equally throughout the planet. For example, most of the iron is in the core of the planet.
Ever since astronomers spotted what looked like canals on Mars, they have been searching for water and signs of life. While life has not been discovered yet, scientists have found deposits of water underneath the surface of the planet.
Not only is Jupiter the largest and most massive planet in our Solar System, but it is also the fastest spinning planet. Jupiter completes a full rotation in about 10 hours. The planet has actually flattened slightly at both ends due to the speed at which it spins.
Saturn does not have the most moons of any planet in our Solar System – that distinction goes to Jupiter with 63 moons – but Saturn comes in a close second with 60 moons. When Galileo first saw Saturn with a telescope, he thought that the planet’s rings were moons. Astronomers were not able to determine what the rings were until they developed better telescopes.
Uranus is the only planet to rotate on its side. This planet has the greatest axial tilt of any planet in our Solar System – 98°. As a result of this extreme tilt, the north pole is in darkness for 42 years then it gets 42 years of light before repeating the cycle.
Neptune is quite a bit larger than Earth, but its gravity is very similar. If you could stand on Neptune – you cannot because it does not actually have a surface – then you would only experience approximately 17% more gravity than you would standing on Earth.
Pluto, which was discovered in 1930, was the ninth planet. Pluto was by far the smallest planet. In fact, Pluto is even smaller than the Earth’s moon. This tiny planet was also the coldest one. Although its temperatures can drop to -240°C, the average temperatures on Pluto are -219°C.
Universe Today has articles on all 9 planets of the solar system including list of the planets and planets in the Solar System.
If you’d like more info on the nine planets, check out NASA’s Solar System exploration page, and here’s a link to NASA’s Solar System Simulator.
Astronomy Cast has episodes on all the 9 planets of the Solar System including Mercury.