By Adam Frank
It is not every day that a question haunting humanity for 2,500 years is answered once and for all. It is not every day that the gates of awe, possibility and potential are thrown open. It is not every day that for a brief moment we get to know -- truly know -- something wonderful for the first time.
That day came and went 15 years ago even if most of the planet wasn't paying much attention. It did not matter. A step had been taken and a line had been crossed. Now, deep into new terrain, it's getting more wonderful everyday.
A little less than 15 years ago, the first extra-solar planet was discovered. For astronomers "extra-solar" means a planet orbiting another star. The question of other worlds orbiting other stars is very, very old. For millennia humans have looked into the dark night sky and asked if those mysterious pinpricks of light might be other Suns and home to their own Earths. The "plurality of worlds" debate stretches at least as far back to the classical world of Hellenistic Greeks and their intellectual brethren. In the first century BC the Roman poet Titus Lucretius Carus, a dedicated atomist, surmised:
You must necessarily suppose that there are other orbs of earth in other regions of space and various races of men and generations of beasts.
Later, when the dark ages descended across Europe, classical inquiry and inquisitiveness fell victim to the Church's strangle hold on power. Discussion of the plurality of worlds became dangerous. Even as late as the 16th century, Giordano Bruno's advocacy of a sky rich with suns and planets and Life brought him the Inquisition's ire and was (at least) one factor that led to his burning at the stake.
Still the debate continued. In 1686 Bernard de Fontenelle would argue from the principle of plenitude that other worlds must exist. Our ignorance is nothing but lack of appropriate instruments for seeing these other planets:
"All philosophy...is based in two things only: curiosity and poor eyesight"
From Newton to Einstein, from Hubble to Sagan, our instruments (and eyesight) got better but were still too poor to (literally) resolve the arguments. Countless false claims of planet detection failed to clear the muddy waters. The ancient but simple question remained: "Do other planets exist outside our solar system"?
Then on October 6, 1995, Michel Mayor and Didier Queloz walked us over the threshold announcing the first definitive detection of an exoplanet orbiting an ordinary solar-type star (51 Pegasi). In the 15 years that followed a true census of exoplanets has begun. We now hold a list topping 400 other worlds including true exo-solar systems -- complete families of planets.
As often is the case in science, once there was actually something to study we found our ideas about solar systems were hopefully egotistical. Many of the newly discovered exo-systems look nothing like our own. Some stars have Jupiter-sized behemoths dancing around them in tighter orbits than even our scorched Mercury. Others stars have their giant planets locked in wild orbits that look more like a comet's path than the stately circles we expect from our own home system.
Most importantly we now know the magic number on which the cosmic plentitude life might depend. From our census of the new worlds it appears that somewhere between 10% and 15% of the Milky Way galaxy's solar-type stars harbor planets. When the dust settles and the math is done and that leaves (I hate to say this) at least billions and billions of worlds with orbiting planets.
How many of these (almost) countless worlds harbor life of any kind? We have no idea. How many of these worlds have watched life evolve to sentience and technological prowess? Anything we say would be pure speculation. The question of a plurality of life remains unanswered. Perhaps that question will fall next. Perhaps it will remain unanswered forever.
But for right now, let that larger question go. For right now, recognize what has happened. Stop what you are doing at some point today and look up. Recognize that after 2,500 years of questions, we now have answers. After 2,500 years of ignorance we are now enlightened. After 2,500 years of debate and discussion, hypothesis and heresy, we have crossed the Rubicon and no longer live in the bardo of the unknown. This is where the path and practice of science has taken us. We have taught ourselves to learn and learned our way into a vision as deep as space itself. We have answered at least one of our own oldest questions.
It does not happen every day and yet every day we need to remember that this is the miracle of being human in so rich a cosmos. Like our best attempts in art and poetry, like our most heartfelt longing of spirit to connect with each other and the wider world, this one vast question answered is an emblem of what is best in us and what we can achieve.