An evening under the Milky Way in Marfa, Texas (C) Steph Goralnick 2011 (used by permission)

As far as we know, it was Galileo who first aimed a telescope at the night sky. This is easy to believe, since we can imagine that most other men of Galileo’s time were interested more in enemy encampments, brothel windows, and then, maybe, and a very distant third, the night sky.* Galileo was a nerd even by Renaissance standards.

Anyway, it was Galileo who first resolved the Milky Way into its constituent stars, in 1610, thus confirming what many philosophers, from 5th century BC Greece to Galileo’s 17th-century Italy, had surmised: that the Milky Way was a collection of stars, too distant and dim to be resolved by the human eye, and that our sun was but one of many stars in an island universe, a galaxy.

And this has remained the predominant understanding of our Milky Way home, until today. Astronomers at the 221st meeting of the American Astronomy Society have reported that their best estimate, according to the data presently on hand, is that the Milky Way galaxy may contain up to 17 billion earth-like planets. “Earth like” doesn’t mean you should book your vacation to one of these worlds, but it means terrestrial (rocky) planets of a size comparable to the earth, orbiting their parent stars at a distance that might be favorable to the conditions of life. In other words, 17 billion (somewhat) earth-like planets that might support life.

Growing up, I would wonder, sometimes aloud with parents or friends, if there were planets orbiting around the stars we see in the night sky. And, of course, if those planets had people on them. And if those people had telescopes, looking at us. (Okay, my thoughts were more along the lines of: if those people slimy creatures were donning latex human masks, marching onto troop ships with laser guns charged, and setting forth in our direction at ludicrous speeds for some high-tech interstellar pillage. But anyway.)

The first hard evidence for extrasolar planets, or exoplanets, came in 1992. Our view of the universe once again shifted, and one of our fundamental questions were answered. Since then scientists–and even some amateur astronomers with pretty nice telescopes–have discovered 874 exoplanets. Nasa’s modest but amazing Kepler orbiting space telescope, which has found over 100 confirmed worlds, is providing some of the richest data we have yet had access to. And it’s got another 2000 possible planets, called candidates, that it’s keeping it’s one-meter eye on. The 17 billion estimate comes from Kepler findings.

To quote Malcolm Reynolds, “It’s getting awful crowded in my sky.”

I can now open an Ipad app called Exoplanet, and pull up a list of all 874 planets. I can tell you when they were discovered, by whom and what method was used to detect them. Orbital period and eccentricity, which is not how weird they are but rather how ovular their orbits are. Mass and even basic type: terrestrial, gas giant, hot jupiter. I can look at a simulation of the solar system the planet belongs to, and even go to a wide angle model of the Milky Way and then zoom in to that particular star. I can find a list of links to publications about that system. Every couple of days, I get an alert that a new exoplanet is discovered, and I have to update the app’s database. It’s almost the Hitchhiker’s Guide to the Galaxy.

Exoplanet will even tell me where in the sky I can find the parent star. And I can go outside, find that star, and know there’s a planet there. I can find the band of the Milky Way, and know that I’m seeing not just a river of billions of stars, but planets.

It’s an incredible time to be alive.

I will never look up in the same way again. I will never again have to say to a gathering of the interested public, “Well, we believe there are lots of planets orbiting around those stars, but we don’t really know…”

Now I can talk about evidence. I can tell people that, based on the publically-funded Kepler mission, astronomers now have evidence that there are as many planets as there are stars in the Milky Way, and that 17 billion of those are similar to earth: rocky, about earth-sized, and orbiting in a potentially habitable zone. And then I can tell them that this number is based on the idea that there are 100 billion stars in the Milky Way. There could be as many as 4 times as many stars, so…do the math.

Our sky is full of worlds.

* If the women of Galileo’s day were allowed to be educated, you can bet we wouldn’t be flabbergasted today at the idea that, of all the Europeans with telescopes in the 17th century, only one of them though to look at the moon with it. For more on this, see Kim Stanley Robinson’s excellent historical sci-fi novel, Galileo’s Dream.