Life in the Universe: What are the Odds? - NASA Science
We don't know when, or even if, we'll find life beyond Earth, but NASA scientists continue the hunt among the thousands of exoplanets confirmed in the galaxy so far.
exoplanets.nasa.gov
As humanity casts an ever-wider net across the cosmos, capturing evidence of thousands of worlds, an ancient question haunts us: Is anybody out there?
The good news: We know vastly more than any previous generation. Our galaxy is crowded with exoplanets – planets around other stars. A healthy percentage of them are small, rocky worlds, of a similar size and likely similar composition to our home planet.
Now the bad news. We have yet to find another "Earth" with life, intelligent or not. Observing signs of possible microbial life in exoplanet atmospheres is currently just out of reach. No convincing evidence of advanced technology – artificial signals by radio or other means, or the telltale sign of, say, massive extraterrestrial engineering projects – has yet crossed our formidable arrays of telescopes in space or on the ground.
Drake's list can be a good conversation starter, and a useful way to frame the complex questions around the possibility of other life. But these days, scientists don't spend a great deal of time discussing it, Domagal-Goldman said.
Instead, they use a narrower yardstick: the habitable zone.
Every star, like every campfire, has a definable zone of radiated warmth. Too close, and your marshmallow – or your planet – might end up as nothing more than a charred cinder. Too far away, and its surface remains cold and unappetizing.
The habitable zone concept is not yet definitive. Small, rocky worlds like ours that orbit other stars are too far away to determine whether they have atmospheres, at least using present-day technology.
That's where teams like the one co-led by Kopparapu and Domagal-Goldman come in. The space telescopes and instruments now on their drawing boards are meant to be powerful enough to peer into these atmospheres and identify the molecules present. That will tell us which gases dominate.
She studies how to use the data gathered so far on exoplanets to refine designs for future space telescopes.
Over the past quarter century, thousands of exoplanets have been confirmed in a Milky Way galaxy that likely holds trillions. Thousands more will come to light in the years ahead. Tools like the habitable zone will help planet hunters sort through these growing ranks to pick the most likely candidates for supporting life.
"This is one of the questions we get from the public often: If there are aliens, how are we going to recognize them if they're really weird?" Domagal-Goldman said. "How do we find what we would consider to be weird life? And how do we make sure not to be tricked by strange, dead planets that look alive – mirages in the desert?"
Life on planets around other stars also might be hidden in a subsurface ocean encased in ice, invisible even to our most powerful space telescopes. Moons of Jupiter and Saturn are known to harbor such oceans, some revealing through remote sensing at least a few of the characteristics we expect for habitable worlds.
Where is everybody?
The question has fueled more than 70 years of debate, but boils down to a simple observation. Our Milky Way galaxy has plenty of stars, plenty of planets, and plenty of time to develop intelligent lifeforms – some of whom might well have had billions of years to develop interstellar travel.
Yet so far, we've seen no sign of such technology, nor heard a peep of conversation. Why is the cosmos so profoundly silent?
"If life had so much time to evolve, why haven't we found it?" Batalha asks, to summarize the question. "Why isn't life just crawling everywhere in the galaxy, or the universe? It could be a combination of a lot of things. Space travel is very difficult for us."
Experts offer many reasons why somebody, or something, might be out there, yet beyond our detection. On the other hand, the ultra-cautious might remind us that, while a lifeless cosmos seems unlikely, we have exactly zero information one way or the other.
Still, scientists like Kopparapu say they like our chances of finding some form of life, and are hard at work on the telescopes and instruments that could make that future, party-starting epiphany a reality.
"It's not a question of 'if,' it's a question of 'when' we find life on other planets," he said. "I'm sure in my lifetime, in our lifetime, we will know if there is life on other worlds."
