The question as to whether or not we're alone in the Universe is one of the most exciting mysteries of our time.
Since the 1990s, astronomers have discovered and studied thousands of planets beyond our Solar System, known as exoplanets.
The more we learn about these alien worlds, the more we learn about the likelihood that life exists somewhere else out there in the cosmos.
More on the search for life
And through our studies we may even find alien life – or signs of it – on some of these exoplanets.
Lisa Kaltenegger is the director of the Carl Sagan Institute at Cornell University and a professor of astrophysics, who investigates exoplanet atmospheres.
We asked her about the chances of finding alien life.
How are you helping in the search for alien life?
I’m trying to figure out, remotely through a telescope, how we can find life on planets orbiting other stars.
I look at the starlight filtering through the air of planets to tell me what’s in the atmosphere of these other worlds.
Earth’s life has significantly changed our atmosphere, so I’m looking for where life has altered either the atmosphere or the surface of the planet so much that I can tell the difference.
What are you looking for in exoplanet atmospheres?
The combination of chemicals that we cannot explain without invoking life, known as 'biosignatures'. The best combination we have right now is oxygen with a gas like methane.
Those two gases react with each other, so if they’re in the air right now it tells you that both are being produced in huge amounts.
For that combination, we have no other explanation than there being life. What that doesn’t do is tell us what kind of life – it could mean slime mould or dinosaurs, or us.
Do we have the technology we need to find life?
What I find so fascinating is that we went from 'it’s impossible' to 'it’s possible'.
That’s the James Webb Space Telescope. But we’re not anywhere close to 'it’s easy'!
The more light we can get, the more time we get to look at these planets and the more we can tell you.
There is hope that we will get a space telescope just for the search for life.
Is alien life out there waiting to be found?
What we know right now is that one out of five stars has a planet in the habitable zone that’s small enough to be rocky.
We have 200 billion stars in our Galaxy alone, and there are billions of galaxies.
The biggest surprise would be if we find nothing. However, the key point is ‘find’, because it’s so hard to actually find it.
I think that’s a numbers game. Right now, we know of 48 worlds that could be like ours.
If we’re really lucky and a planet close to us shows signs of life that we can spot, then we can do it with JWST.
What impact would it have if we found alien life?
People of my generation think it will have this huge effect.
But 1995 was when the announcement was made of the first planet around another star; my students have never lived in a world where they didn’t know whether there were planets around other stars
It’s not going to be shocking if we find something. For them the question is, when are you going to figure out if there’s life out there?
However, if there were a UFO landing on the White House lawn saying “Take me to your leader”, I don’t know what effect that would have!
This interview appeared in the May 2025 issue of BBC Sky at Night Magazine
