It has been a week dedicated to space exploration. Elon Musk outlined SpaceX‘s plan at the 67th annual International Astronautical Congress in Guadalajara, Mexico to colonize Mars and pretty much save humanity from extinction. NASA's Hubble has imaged what may be water vapor plumes erupting off the surface of Jupiter's moon Europa, increasing the possibility that missions to Europa may be able to sample Europa's ocean, one of the most promising places that could potentially harbor life in the solar system, without having to drill through miles of ice.
All humans (I hope) dream of expanding to other planets (at some point). Maybe not a myriad of us share the exact vision of wanting to send a million people to Mars and start a colony, but sometimes a faction of dreamers is all it takes.
Mars isn't the "promised land", but it has a reasonable amount of gravity, sunlight, and an atmosphere of sorts. More importantly, it is the only planet in our solar system, aside from Earth, that we'd have a chance to live on, though our own moon and some of Jupiter's moons (Europa among them) might be candidates for colonization with serious effort.
But we don't have to stop there. Currently, scientists know of over 3,300 planets orbiting other stars. Most of them would present serious barriers to human habitation. Many of those planets are either too hot or too cold to survive, even in a controlled environment. There are a few gas giants like Jupiter, without even a solid surface to stand on. Some have so little gravity that our bodies would deteriorate over time, or so much that our own weight would crush us.
A few exoplanets, though, are similar enough to Earth that we could imagine establishing a protected environment where humans could live. Later on, a massive terraforming project might make one of these planets suitable for surviving out in the open. Once we did that, it could become a base for more colony ships.
There's a planet orbiting Proxima Centauri that humans could live on in a controlled environment. If we had a spaceship that could sustain the speed of light, we could get there in 4.2 years. How many other planets in our galaxy have a consistency, size, and temperature that humans might live on? We have no idea.
Before 1995, when astronomers at the University of Geneva detected a planet orbiting 51 Pegasi, we had no solid evidence of any planets outside the Solar System. In 2015 alone, astronomers discovered 45 new planets. The number of planets that we still can't detect surely is far greater than the number astronomers have detected. Some of them must be habitable, or even inhabited.
How long would it take to planet-hop across the galaxy? In short - very very (very) long. We can put a lower bound on it by imagining spaceships jumping from one planet to the next at the speed of light, colonizing it, and then taking off for the next planet without a pause. The galaxy has a radius of 50,000 light years, perhaps 60,000 to the outermost suburbs. We're about 25,000 light years from the center of the galaxy. So in the best-case scenario, it would take 75,000 years to reach the other end of the galaxy.
This number is unrealistically low, of course. It's impossible for a ship to fly at the speed of light, and we have not yet figured out how to construct one that can even come close. We'd have to stop to colonize each planet and then build new spaceships. A realistic time period, even assuming an all-out effort that never stopped, would be many orders of magnitude greater. But let's be really optimistic and just round the number up to 100,000 years. If you think in terms of the Milky Way age since its inception 13.4 billion years ago, humans will reach for the stars (literally) in approximately 0.000007 of that time, or just in a half of humankind existence time (approximately 200,000 years).
But we don't even have to stop there. There are lots of other galaxies past the Milky Way. We don't really know how many, but estimates range around 100 billion. That's 100 billion galaxies. Our galaxy has something like 100 billion stars. That's a lot of zeros. Other galaxies could be bigger or smaller by a few orders of magnitude. Each one probably has millions or billions of planets. What it comes down to is that Carl Sagan's "billions and billions" is a serious understatement for the number of potential planets in the universe.
Don't get sidetracked by those numbers just yet and let's get those lightspeed colony ships going. It's a long trip. To get to the Andromeda Galaxy, our closest full-sized galactic neighbor, would take over two million years even at that speed. The most distant galaxies are much farther. Scientists currently estimate the radius of the observable universe as 46.5 billion light years. We don't know how close we are to the center of the universe, but let's just assume (typically human) and say we're at the exact center. Then our colony ships would reach the farthest galaxies in 46.5 billion years, plus a little to speed up and slow down.
That's a little bit (actually it is a lot) longer than it took life on Earth to evolve from its earliest beginnings, so I can assume that whatever arrives there will no longer be human. Still, they'll be our descendants.
There's another problem, though. While we're trying to reach those distant galaxies, they're running away from us. The whole universe is expanding, and the farther an object is from us, the faster it's fleeing. (Or we're fleeing from it; it's the same thing.) In 46.5 billion years, those galaxies will be still farther from us. A lot farther. It's like Zeno's parable of Achilles and the tortoise. Each time we get to where the galaxy was when we started, it won't be there any longer.
Will there even be anything to colonize when we get there? That's questionable. The lifespan of a Sun-like star is believed to be about 10 billion years. Of course, there may be new stars and new planets, which won't even come into existence until after our Sun is gone.
Or maybe they won't. At some point, according to most cosmologists, the universe will be spread too thin to form new stars. The last ones will eventually die out. By the time we reach the most distant galaxies, they might not be there any more.
At that point, we can (easily) declare our colonization victory. We'll have reached all the habitable worlds still remaining in the universe.