We are not alone: Evidence builds for alien life outside our solar system
In the wake of Kupe, Zheng He and Cook, a new breed of mostly deskbound adventurers are making incredible breakthroughs in the search for life-supporting planets.
We live in the middle of a golden age of space exploration – most of it by people sitting over computers in their offices, analysing data downloaded from telescopes on Earth or in the far reaches of the galaxy.
Technology has enabled our knowledge of the universe to expand at a rate undreamt of even 50 years ago. There’s every chance that many people who witnessed the first moon landing in 1969 will live to see the discovery of life on another planet. As computing power and the number of techniques for finding planets continue to grow, so does the number of exoplanets – planets outside our solar system – that have been identified.
First, a little arithmetic to give some perspective. It is estimated there are between 200 billion trillion stars, in between 200 billion and 2 trillion galaxies in the observable universe. That’s 200,000,000,000,000,000,000,000 stars. Most of these have planets. In the years since the first exoplanet was confirmed in 1992, we have discovered 5500 planets at last count. If, therefore, there are 200 billion trillion stars, then 5500 is at best 0.00000000000000000275% of the planets that probably exist. So, even though we haven’t yet discovered one capable of supporting life, there are still plenty left to check out.
Out of the past
To understand planets beyond the solar system, we first need to know a lot about this one. Lisa Kaltenegger is an astrophysicist, astrobiologist, and co-founder of the Carl Sagan Institute at Cornell University in Ithaca, New York. Her bio describes her as a pioneer and world-leading expert in modelling habitable worlds and their light fingerprints. She is also the author of more than 100 peer-reviewed research papers, and her recently published book, Alien Earths, is an approachable and inspiring introduction to the search for alien life.
One of Kaltenegger’s main focuses is astrobiology and the history of our own planet. When she began considering the problem of identifying alien Earths, she discovered it was necessary to learn a lot about how this one developed, because that tells us what signs to look for out there.
“When I was working on the Darwin mission at the European Space Agency to find life in the universe,” says Kaltenegger, “it was clear that if we looked for modern Earths, we would miss signs of life. When you look at the Earth through time, life has changed our planet. And that’s the only way I can find life on another planet – if the biosphere didn’t change it, I wouldn’t have anything to look for.”
Which, she explains, is why there is so much about life on Earth in her book. “If you are looking for a carbon copy of modern Earth, even if you have billions of options, you’re probably going to miss it because you are looking for something specifically that went through all the same evolutionary steps.”
If we restricted our search for evidence of life to identifying radio signals from other planets – as was once the case –that could exclude other technologies. In the case of our own planet, for instance, radio signals reflect less than 200 years of our existence.
But if we allow ourselves to look for other signs, such as those in the biosphere, we widen our search to billions of years in a planet’s history. Kaltenegger points out that single-celled organisms have been on this planet for two billion years, “and we have no idea what the evolutionary scale is on other planets”.