How scientists are exploring what extraterrestrial life could look like Premium

Exploring the search for alien life beyond Earth, from alternative biochemistry to detecting technosignatures in exoplanet atmospheres.

We have only one example of biology forming in the universe: life on earth. But what if life can form in other ways? How do you look for alien life when you don’t know what alien life might look like?

These questions are preoccupying astrobiologists, who are scientists who look for life beyond earth. Astrobiologists have attempted to come up with universal rules that govern the emergence of complex physical and biological systems both on earth and beyond.

I’m an astronomer who has written extensively about astrobiology. Through my research, I’ve learned that the most abundant form of extraterrestrial life is likely to be microbial, since single cells can form more readily than large organisms. But just in case there’s advanced alien life out there, I’m on the international advisory council for the group designing messages to send to those civilisations.

Since the first discovery of an exoplanet in 1995, over 5,000 exoplanets, or planets orbiting other stars, have been found. Many of these exoplanets are small and rocky, like earth, and in the habitable zones of their stars. The habitable zone is the range of distances between the surface of a planet and the star it orbits that would allow the planet to have liquid water, and thus support life as we on earth know it.

The sample of exoplanets detected so far projects 300 million potential biological experiments in our galaxy – or 300 million places, including exoplanets and other bodies such as moons, with suitable conditions for biology to arise. The uncertainty for researchers starts with the definition of life. It feels like defining life should be easy, since we know life when we see it, whether it’s a flying bird or a microbe moving in a drop of water. But scientists don’t agree on a definition, and some think a comprehensive definition might not be possible.

NASA defines life as a “self-sustaining chemical reaction capable of Darwinian evolution.” That means organisms with a complex chemical system that evolve by adapting to their environment. Darwinian evolution says that the survival of an organism depends on its fitness in its environment. The evolution of life on earth has progressed over billions of years from single-celled organisms to large animals and other species, including humans. Evolution is the process of change in systems. It can describe how a group of something becomes more complex – or even just different – over time.

Exoplanets are remote and hundreds of millions of times fainter than their parent stars, so studying them is challenging. Astronomers can inspect the atmospheres and surfaces of earth-like exoplanets using a method called spectroscopy to look for chemical signatures of life. Spectroscopy might detect signatures of oxygen in a planet’s atmosphere, which microbes called blue-green algae created by photosynthesis on earth several billion years ago, or chlorophyll signatures, which indicate plant life. NASA’s definition of life leads to some important but unanswered questions. Is Darwinian evolution universal? What chemical reactions can lead to biology off earth?