We don't know, but there could be thousands, and possibly millions, of Earth-like planets studding the dark latitudes of the Milky Way. Our Galaxy could be thick with worlds that host not just life, but intelligence. In this putative club of sentients, is it possible that we are the newest arrivals?
This question can be trivially answered.
Although Homo sapiens has been plodding the planet for a few hundred thousand years, our technical competence to build rockets and radios is only a century old. Anyone who's mastered seventh grade science knows that's not a heck of a lot of time compared to the age of the Earth.
As a kid, you were probably encouraged to make a paper strip chart of the history of our planet from its formation 4.6 billion years ago, through bacteria, trilobites, dinosaurs and humans. If the chart ran the walls from the back of the classroom to the front blackboard, the representation of time since the invention of radio was only a hundredth of a hair's-width wide. If your chart began with the formation of the Galaxy, 13 billion years ago, the era of technological competence would be even thinner.
So we're surely among the newest pledges in the frat house, if club membership requires radio technology or better. That means that if we pick up a signal from extraterrestrials, you can be smugly confident that those broadcasters are far beyond our own level.
But there are other matters of relevance: how many club members exist and how much more advanced would they be?
If we want to estimate how many contemporary worlds have technically sophisticated inhabitants, we can begin with the Drake Equation. This fabled formulation estimates the number of hi-tech galactic societies as the product of the rate at which they arise times their average lifetime. This is just like computing how many students are on campus at the local university by multiplying the number of new admissions by the average length of a college stay (close to four years).
We don't know much about the average lifetime of technological societies, other than the fact that ours has, so far, managed to survive for a century. We also don't know at what rate sentient societies spring up in the Galaxy. But we do know that this rate is surely tied to the frequency with which stars are born. Clearly, a greater flux of new stars will ultimately produce a larger number of planets with thinking beings.
What is the star formation rate? Well, there are roughly 200 billion stars in the Milky Way, and that means that the average rate over the last 13 billion years has been about 15 new stars per year. In fact, however, this average rate is two tads misleading. Anyone who's used a radio telescope to study galaxies knows that when you examine a big spiral like the Milky Way, you find that the total amount of interstellar gas is typically a few percent of the mass of all the stars. Since interstellar gas is the stuff from which stars are built, it's obvious that there's little material around today for constructing new ones.
Sure, stars explode as they die, spewing some of their contents back into space. Even the Sun will blow off some steam as it heads for the stellar bone yard. But the great majority of what's inside the Sun will stay there forever, locked in by gravitation. The ingredients for new stars are sparse, and most of the stars that our Galaxy will ever make... have been made.
Mary Barsony, a Research Scientist with the Space Science Institute, notes "these days, the stellar birth rate in the Milky Way is only about one solar mass per year. The Galaxy's not nearly as fertile as it once was. It seems that there was a real burst of star formation more than 10 billion years ago, though. Those early years were when the stellar population boomed."
In other words, our Sol is a real sunny-come-lately.
Clearly, this must affect the roster for our club of intelligent beings. But how? There are two obvious possibilities. One is that intelligence is such a useful attribute that technological societies last a really long time - billions of years. Heck, trilobites lasted a half-billion years, and they weren't even smart (by any reasonable standard). So maybe the thinking-beings club is home to really, really old societies, and we're like preschoolers surrounded by grad students.
The other possibility is that, no, technology doesn't survive for such long time spans. And while the Galaxy may have spawned great civilizations in the deep and distant past, they are mostly gone now. In this scenario, other club members are not quite so ancient, but they're in short supply.
Which, if either, of these possibilities is true will only become clear when we've decoded a signal from elsewhere. But the fact with which we began our discussion remains just that: an indisputable fact. We are the new arrivals on the technological scene. Our strut and fret on the galactic stage has just begun.