By Emily Levesque, University of Washington
Looking for life on other planets could mean searching for signs of any living thing, from microbes to megaflora and fauna. But what are the telltale signs that life might be present on a planet? How can we know, from so many light years away, whether some alien form of bacteria, or shellfish, or dinosaurs, might be thriving on the surfaces of planets we can barely even spot? The answer lies in looking for biosignatures. But, what are they?
The signs of life on a far off planet can be in understanding how creatures impact the planets they’re living on: What they leave behind, how they affect their environments, and how all of that might translate into something we can observe. These effects are known as , the observable effects of organisms breathing or eating or excreting, changing the land masses or oceans or atmosphere of planets in ways that could alert us to their presence.
Vikki Meadows, an astronomer at the University of Washington, leads a group that specializes in habitability and biosignatures by asking an interesting question: What would our own Earth look like as an exoplanet? If an alien astrobiologist trained a powerful telescope on us, what would they see? We can imagine the answer today. With a teeming mass of humans on the planet, they might pick up electromagnetic signals thanks to radio and television, or unfortunately detect the chemical signatures of the pollution that we pour into our atmosphere. However, humans have only been on Earth for a few hundred thousand years, and technologically savvy humans, for only a few hundred.
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Alien Astrobiologists Observing the Earth
What if the alien astrobiologist had observed the Earth a 100 million years ago, during the time of the dinosaurs? Or even earlier, when life on Earth was dominated by early bacteria? They might have recognized the presence of dinosaurs by detecting signs of something like dinosaur digestion, releasing methane that would show up in Earth’s atmosphere.
As for the bacteria, we know that about 2.5 billion years ago early life forms, known as cyanobacteria, were the dominant source of life on Earth.
Those first bacteria survived, thanks to photosynthesis, using energy from the Sun to produce and release oxygen. They did this so efficiently that we think they fundamentally altered Earth’s atmosphere, changing it from oxygen-poor to oxygen-rich. An enterprising astrobiologist in another planetary system might have been able to spot this effect.
Studying Exoplanet’s Atmospheres
Studying exoplanet atmospheres is immensely challenging. It is comparable with the difficulties of simply taking a spectrum of a planet and studying its atmosphere in the same way that we study stars. However, applying the same creative methods that astronomers use to discover exoplanets can give us at least a glimpse of what these planetary atmospheres might look like.
Just a few years after we began discovering exoplanets, Sara Seager, an astronomer at MIT, began imagining how we might study these planets’ atmospheres.
It is worth remembering how Pluto’s atmosphere was discovered. Astronomers observed a background star, as Pluto passed in front of it, and noted the initial gradual drop in the star’s brightness as Pluto’s atmosphere partially blocked it. Sara Seager first imagined doing something similar with an exoplanet, discovered thanks to the transit method, passing in front of its host star. If that planet has an atmosphere, there should be a brief but precious window of time when the light from the star is passing through that foreground planetary atmosphere.
Capturing the Transmission Spectrum
Thus, observations taken at exactly the right point could capture what’s called a transmission spectrum. This involves watching the starlight pass through the planet’s atmosphere and measuring the effects of the atmosphere on that light to suss out some atmospheric chemistry. Early observations of transmission spectra have shown that it’s possible to identify molecules like methane, carbon dioxide, and water—all exciting prospects if we’re searching for signs of life.
Finally, efforts to spot signs of life, or at least former life, have also been happening much closer to home. Right now, it looks like our nearest planetary neighbor, Mars, is only home to robots, with rovers roaming its surface to study the Red Planet in incredible detail. Heroic teams of scientists and NASA engineers have worked for years to keep these rovers running, giving us by far our best and closest look at what life would be like on another planet.
Missions to Mars
Today’s missions to Mars are still studying its atmosphere, polar ice caps, and geology in the hopes of spotting any signs that life used to exist on the planet. Even evidence of long-dead microbes would be an incredible discovery, giving us our first look at what alien life might be like.
Research by Seager and Meadows is still ongoing, driving astrobiologists’ efforts to predict and observe the elusive signs that life might be present on another world. Our continued study of Mars also offers us a potentially compelling close look at alien life that may have existed in past eras.
To conclude, when most of us imagine aliens, we don’t picture microbes, as exotic as they might be. Anyone who’s ever pondered about life on other worlds has imagined what it might mean if the life was like us; if it was intelligent, if it could communicate, and if we could talk back.
The search for life on other planets is, indeed, without a doubt, one of the most compelling and potentially ground breaking pursuits in all of astronomy. The idea gets at the very heart of our fundamental human curiosity about the universe and our questions about how we got here, and whether or not we’re alone.
Common Questions about Biosignatures
Biosignatures are the observable effects of organisms breathing or eating or excreting, changing the land masses or oceans or atmosphere of planets in ways that could alert us to their presence.
Vikki Meadows is an astronomer at the University of Washington, who leads a group that specializes in habitability and biosignatures.
Early observations of transmission spectra have shown that it’s possible to identify molecules like methane, carbon dioxide, and water.