What do you think about when you stare out into the night sky? Do you WONDER how far away those shiny objects really are? Or do you instead imagine what types of alien life might be out there staring back at you?
If you've spent much time here on Earth, you know it's a pretty amazing place. Compared to a single individual, Earth is humongous. It would take most, if not all, of a lifetime to see even a fraction of its treasures.
That's probably why it blows your mind when you realize how unbelievably tiny Earth is compared to the rest of the solar system and the larger universe. In fact, it can be nearly impossible to comprehend truly the size of the universe.
Even if you concentrate on just Earth's neighborhood — our solar system — its size can boggle the mind. Scientists had to come up with a new unit of measurement just to get a graspon the immense distances within the solar system. The astronomical unit (AU) is based upon the average distance from Earth to the Sun, or approximately 93 million miles.
For example, Mercury, the closest planet to the Sun, is 0.39 astronomical units from the Sun. But what about the planet farthest out there? Today, scientists consider Neptune to be the farthest official planet, but dwarf planet Pluto is still out there in our solar system. Pluto sits 39.2 astronomical units from the Sun, or about 3.67 billion miles.
So is that the size of our solar system? Around 3.67 billion miles? Nope! Our solar system extends well beyond Pluto. Pluto sits within the Kuiper Belt with other dwarf planets that extend from 30 AU out to 50 AU.
But even the Kuiper Belt isn't at the end of the solar system. Astronomers know that the Sun's solar wind travels farther out into space where it eventually meets the interstellar medium, which is the cold, dark background material of the galaxy that exists between stars. This point is known as the heliopause or the termination shock, and astronomers believe it's approximately 122 AU away from the Sun.
While some astronomers are content to claim that the size of the solar system is around 122 AU, others point out that the solar system should really be defined by the reach of its gravity. In other words, if an object can be said to orbit the Sun, then it should be considered part of the solar system.
Using this expanded definition, astronomers point to the theoretical Oort Cloud as the approximate boundary of the solar system. A cloud of icy objects that could be the source of comets that enter the inner solar system from time to time, the Oort Cloud sits more than 100,000 AU away from the Sun.
Using the Oort Cloud as an approximate boundary would mean that the size of our solar system approaches nearly 2 light years! That's equivalent to almost 12 trillion miles. Try to wrap your mind around that. And once you have wrapped your mind around that, remember that's just the size of our tiny solar system, which is just a speck in terms of the whole universe!
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