What is a planet? Indeed, for that matter what is space?
The definition of “planets” has been changing since the days of ancient Greece. The Greeks coined the word planets meaning “wanderers.”
The seven wanderers visible to the naked-eye back then were the sun, Moon, Mercury, Venus, Mars, Jupiter and Saturn.
Uranus was too faint to be seen and wasn’t discovered until 1871 by astronomer William Herschel. Although it was observed many times previously, Uranus was always mistaken for a star.
Neptune, the eighth planet, and the outermost gas planet, was discovered by Johann Galle, a German astronomer.
Finally, the ninth planet, Pluto – now considered a dwarf planet – was discovered in 1930 by astronomer Clyde William Tombaugh.
So, over the centuries we have gone from officially recognizing seven planets, then redefining what a planet is and eliminating the sun and Moon. Then there were five . . . and then six, seven, eight and nine. Now in 2009 we are back to eight again.
So, how many planets are there in space? We’ll have to find the answer outside our solar system.
Once outside our system we enter the cold, dark regions of interstellar space within our galaxy, the Milky Way.
Our galaxy is a spiral galaxy. It looks like a giant glowing pinwheel containing about 300 billion stars.
The galaxy takes up a great deal of space. It’s estimated to have a diameter of 100,000 light years. Since a light year is six trillion miles, 100,000 light years.
NASA estimates our galaxy has billions of planets. That’s not a very exact number. Within the past year, however, two astronomers from the University of South Wales, Australia, developed new calculations. Drs. Charley Lineweaver and Daniel Grether determined that “at least a billion, but probably more like 30 billion [planets are in our galaxy].” But that figure only accounts for the gas giants – Jupiter type worlds. What about the smaller ones like Earth, Venus or Mars? Is it possible to arrive at a rough calculation of those types?
Lineweaver thinks so. “A reasonable guess is the same number of Earths as Jupiters.”
That means another 30 billion planets. So we have an approximate total of 60 billion planets in our galaxy.
But our galaxy is just one of many, and all of them have planets orbiting some of their stars as well. Currently, it’s thought that approximately eighty billion galaxies populate the known universe.
If we project the number of planets calculated to be in our galaxy across all the other galaxies we arrive at roughly two hundred and ten sextillion planets.
But even that figure is not entirely correct as it’s based on the extrapolation of planets in the visible – or known – universe. There’s a lot of universe out there we can’t see because the universe is expanding at such a fast rate that light from the farthest galaxies hasn’t reached our planet.
So there are roughly 210 sextillion planets in space, give or take a few trillion. When you consider that our universe may be only one of many, the number could well reach into infinity.
The next time you happen to stare up at the stars, consider all the trillions of planets out there. It makes one wonder sometimes how many billions of eyes may be staring back towards us!