Our Place in Space

“A person’s a person, no matter how small”
- Horton the Elephant, Horton Hears a Who, Dr Seuss

I used to judge elementary and middle school science fairs on occasion. Aside from exhibits showing how carbonated soda would corrode steel nails, the most ubiquitous project theme seemed to be the tried and true model of our Solar System. Oh, what wondrous concoctions and contraptions they were, fashioned almost universally from colorfully painted and carefully decorated plastic foam balls of various sizes and a rainbow array of pipe cleaners and wire coat hangers. The more imaginative ones would incorporate movement, the planets able to revolve around the Sun. Without exception, they all fit within the allotted space for a project—certainly within a square yard or so. I always thought the youngsters, at least the geeky ones, would get a kick out of knowing how big their projects would be if they were modeled to scale…

Let’s say the Earth, which is about 8000 miles in diameter, were approximately the size of a large pea—a hair bigger than a quarter inch in diameter. Our Moon, about one quarter the diameter of earth, would then be slightly less than the size of the tiniest of baby peas. Put in proper reference to one another, baby pea Moon would be placed in orbit around big pea Earth at a distance of about 9 inches.

Using the same scale, our Sun, which is about 870,000 miles in diameter, would be a very large beach ball, about three feet in diameter. Earth orbits the Sun at a distance of 93 million miles. Using our big pea Earth and our big beach ball Sun, we would need to place them at the opposite goal lines of a football field to appropriately model the distance—about a hundred yards apart.

How big is our Solar System? Heck, what comprises our Solar System? Let’s go with the latest thinking, which excludes Pluto as an actual planet. Yes, if you have not heard, Pluto was demoted to dwarf planet status by the International Astronomical Union in 2006. So, all that stuff you learned about nine planets? Throw it away. (This was really bad news for plastic foam ball manufacturers, who saw an immediate ten percent slump in annual sales and were forced to compensate by launching an all-out marketing blitz promoting miniature hand-made snowmen during the winter holiday season, but I digress.)

We now consider Neptune, the eighth and final planet, to be the farthest planet from our Sun. Neptune is about 30,000 miles in diameter, just under four times the diameter of Earth. Compared with our large pea planet, Neptune would be a small cherry tomato (only blue). Neptune orbits the sun at an average distance of about 2.8 billion—yes that’s billion—miles. In our model, our cherry tomato Neptune would need to be placed about 1.73 miles away from our beach ball Sun.

So, if our science fair project model is going to be accurate, we’re going to need a pretty big room to put it in—a room three and a half miles across to accommodate the diameter of our system!

There are regions of the Solar System well beyond Neptune, such as the Kuiper Belt. This cloud of orbiting objects, much like the Asteroid Belt between Mars and Jupiter, only much larger and more massive, is the place to which poor Pluto was demoted. Including this outer region in our model would roughly double its size to seven miles in diameter.

Let’s not stop our journey just yet, shall we? Let’s take big step back—one giant leap for humankind, if you will—and imagine our Sun, our star, as a single grain of sand, say about one thirty-second of an inch in diameter. In that scale, you’d have to have some pretty amazing vision (probably a microscope) to see the tiny speck that would be Neptune, in its orbit a little over nine feet away from our grain of sand Sun. So, with our Sun as a tiny grain of sand at the center, our Solar System would be a circle a little over 18 feet across. If we included the Kuiper Belt region, our circle would be about 30 to 35 feet across.

Other than our Sun, the nearest star to us is Alpha Centauri, the third brightest star in the night sky (behind Sirius and Canopus) and visible in the Southern Hemisphere as the brightest star in the constellation Centaurus. Alpha Centauri is actually a system of three stars in close proximity to each other, all sharing a complex gravitational and orbital arrangement. This star system is approximately 4.37 light years away from our Sun. That’s how far light, travelling at 186,000 miles per second (about 5.88 trillion miles per year), will travel in about four years and four-and-a-half months, which is about 25.8 trillion miles.

So, in our new model, Alpha Centauri would be represented by three tiny grains of sand. And how far would this small collection of sand grains be from the sand grain that is our own Sun, the 18-foot circle that is our Solar System? 16 miles away! We, indeed, live in a sparsely populated neighborhood.

Our galaxy, the Milky Way, is a spiral disc, the arms and center of which contain approximately 200 billion stars (and perhaps as many as 400 billion). This disc is about 100,000 light years across (that’s 100,000 multiplied by 5.88 trillion miles, folks). Still using our grain of sand—a single tiny grain of sand—as our Sun, our galaxy would be 366,000 miles across!

Perhaps our scale is too big to fathom this. Let’s take another step back. Let’s say our entire solar system, about 5.6 billion miles in diameter (using Neptune’s orbit as the outer limit while fully acknowledging the true Solar System extends far beyond that), is the size of a dime, about five eighths of an inch in diameter. The Alpha Centauri system is now about 240 feet away. In this new scale, our galaxy would still be about 1040 miles across! Our Solar System, our little dime, would be about 27 miles from the center of our galaxy.

Head hurt? Let’s take one more step back, shall we? Let’s say our entire home galaxy, our Milky Way, all 100,000 light years across (100,000 times 5.88 trillion miles), is now just a dime. Scientists agree there are billions of galaxies in the observable universe—billions of them. Excluding the Large and Small Magellanic Clouds (small galaxies associated with the Milky Way and visible in the Southern Hemisphere), our nearest galactic neighbor is Andromeda. Although less massive than our own galaxy, scientists have estimated Andromeda contains as many as a trillion stars and is slightly larger than twice the diameter of the Milky Way, which would make it about the size of a fifty-cent piece in our final model. This fifty-cent piece Andromeda would be positioned about fifteen-and-a-half inches from our dime Milky Way.

And, so, we come to the size of our universe (not to mention well beyond my outer limit to discuss this topic with any confidence—well past time to scare up an astrophysicist). Here, there is broader disagreement as to size; however, the mainstream scientific community seems to accept that the visible universe is a sphere approximately 92 billion light years in diameter. I’m not sure exactly where our Milky Way dime fits into this universe. Our study—our understanding—of the limits of our universe is necessarily Milky Way centric. So, if our dime were at the center of the visible (to us, humankind) universe, our universe would be a sphere about nine miles in diameter.

Feel small? I do. Maybe it’s time we stick together and love each other more.