February 19: Solar System Day

It's Solar System Day, so here are ten things you didn't know about our solar system:

1. The address of the Solar System is Local Interstellar Cloud, Local Bubble, Orion–Cygnus Arm, Milky Way. It is located 26,000 light-years from the middle of the Milky Way. The nearest star is Proxima Centauri, 4.22 light years away.
2. It consists of: one star (the Sun), 8 planets (Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus and Neptune), probably several hundred dwarf planets, of which we have recognised five - Pluto, Ceres, Haumea, Makemake and Eris, 468 natural satellites (moons), 702,663 minor planets (eg Asteroids) and 3,388 comets.
3. The four smaller inner planets, known as the “terrestrial planets” (Mercury, Venus, Earth and Mars), are primarily composed of rock and metal. The outer four are “gas giants”. Jupiter and Saturn, the larger of the four, are composed mainly of hydrogen and Helium. Uranus and Neptune are composed largely of ices (Water, ammonia and methane).
4. There are theories which have suggested the Solar System once had more planets than it does now. There was allegedly a planet called Vulcan, which orbited closer to the sun than Mercury. Modern science has proved it never existed (it would have been too hot for Mr Spock to live on anyway). Recent studies have suggested that there was once a fifth giant planet — similar to Jupiter — but it was ejected from the Solar System through gravitational interactions with the other planets.
5. 99.86% of the Solar System's mass is accounted for by the sun. Most of the remainder is Jupiter.
6. The Solar System doesn't stop at Pluto. It's a common misconception that Pluto is the outer limit. In fact, the Solar System may extend as much as a thousand times further than that. The point at which the Solar System ends and interstellar space begins is not precisely defined, but would be where solar winds can reach, or the heliopause.
7. There are two moons within the system which are larger than Mercury, the smallest planet. They are Ganymede, a moon of Jupiter and Titan, a moon of Saturn.
8. Saturn isn't the only planet to have rings. It is merely the only one whose rings can be seen from a small telescope. Jupiter, Uranus and Neptune also all have rings, but they are dark and difficult to observe.
9. The asteroid belt between Mars and Jupiter contains thousands of object, but they are much more widely spaced out than the iconic asteroid belts which cause havoc to space craft in science fiction films. They are so far apart that rather than risking being hit by one, a spacecraft would have to work very hard to get close enough to an asteroid to even take a photo of it. Scientists believe that Hollywood style asteroid belts probably don't exist (but who knows what's out there - nobody's ever actually been!)
10. Although Mercury is closest to the Sun, Venus is hotter. That is because Venus has a very thick atmosphere made up of greenhouse gas carbon dioxide which stops the heat escaping into space as it does on Mercury.