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1. The Solar System only has 8 planets
That’s right, only 8 planets. Not the 9 planets you grew up with. That’s because the International Astronomical Union reclassified Pluto as a dwarf planet in 2006. Why is Pluto not a planet? To qualify as a planet, an object needs to orbit the Sun, have enough mass to pull itself into a spherical shape, and have cleared out its orbit of other material. It’s this third requirement that Pluto hasn’t fulfilled; it’s just a fraction of the mass in its orbit, while the other planets are millions of times more massive than everything else in their orbits. You may want to read more about the 10 facts about Pluto.
2. But the Solar System has 4 dwarf planets
Pluto has been downgraded, but there are now 4 dwarf planets in the Solar System: Ceres, Pluto, Eris and Makemake. Dwarf planets are objects that orbit the Sun and have enough mass to form a sphere, but they share their orbit with other objects. And as telescopes improve, more dwarf planets will be discovered. There might eventually be more dwarf planets than planets. See how much you already know about our Solar System?
3. The influence of the Solar System extends out for almost 2 light-years
The theoretical size of the Solar System goes out as far as the the Sun’s gravity overpowers anything else in the region; and this is almost 2 light-years away, nearly halfway to the nearest star. It’s thought that the Oort Cloud – a region where the long-period comets come from – extends out to 100,000 astronomical units from the Sun (1 astronomical unit, or AU, is the average distance from the Earth to the Sun).
4. Most of the mass of the Solar System is the Sun
In fact, the Sun contains 99.86% of the mass in the Solar System. And the Sun is 73% hydrogen, so most of the matter in the Solar System is hydrogen, with the remaining amount being mostly helium, oxygen and carbon. Everything else, like the metals and rocks is just a tiny fraction of a fraction of the mass in the Solar System.
5. The Solar System is 4.6 billion years old
Since the Earth is constantly resurfacing itself, we can’t find out how old it is, but there’s another way to find out. Meteorites, which date back to the formation of the Solar System, have been raining down on Earth for millions of years. Scientists have sampled meteorites and learned that they’re all 4.6 billion years old. That means that everything in the Solar System formed around the same time – give or take a few million years.
6. Everything orbits in the same direction
All the objects in the Solar System orbit the Sun in a counter-clockwise direction. This matches the theory that the Solar System formed all at once from a cool cloud of hydrogen. As the gas came together, it began to spin, so that the Sun collected in the middle, surrounded by an accretion disk of gas and dust. All the planets and other material in the Solar System formed within this rotating disk. There are a few exceptions, however, like Halley’s Comet.
7. The Sun is just one star in 200 billion in the Milky Way
Doesn’t that give you perspective? Our whole world is just one planet orbiting one star in a galaxy of 200 billion stars.
8. But they’re really far apart
There are only a few stars within 10 light-years of the Sun. The closest system is the Alpha Centauri system, 4.4 light-years away. Barnard’s Star is 5.9 light-years away, and then Wolf 359 (7.8 light-years), Lalande 21185 (8.3 light-years), Sirius (8.6 light-years), Luyten 726-8 (8.7 light-years), and finally Ross 154 at 9.7 light-years away. Interstellar travel will be difficult when you’re dealing with these distances.
9. Astronomers used to think the Earth was the center
You take it for granted that the Sun is the center of the Solar System, but astronomers used to think that the Earth was the center of the entire Universe. It wasn’t until the 17th century, when Nicolaus Copernicus first presented the concept, and showed how it accurately predicts the positions of the planets.
10. Humans have sent spacecraft almost everywhere
Spacecraft from Earth have visited or orbited every planet in the Solar System, and more are on their way to visit some of the dwarf planets. We’ve explored the Sun, the Moon, and many asteroids. And now some of the oldest spacecraft still active – NASA’s Voyager spacecraft – have almost reached the Sun’s heliosphere; the point where the solar wind slows down as it bumps against the interstellar wind.
We have written articles about the solar system for Universe Today. Here are facts about the planets in the Solar System.
We have recorded a whole series of podcasts about the Solar System at Astronomy Cast. Check them out here.