Have you ever thought about how we would know if other planets have life on them? How could we tell that life existed on a planet from afar, without actually landing on it? Well, 30 years ago, an ambitious experiment attempted to give us some answers by searching for life on the only planet we can be certain is home to life: Earth.
The experiment was conducted by the great astronomer and science popularizer Carl Sagan, who persuaded NASA to have the Galileo spacecraft take Earth in view during its flyby in 1990. Galileo was headed to explore Jupiter and its moons, but Sagan believed he could use it as a testing ground for the search for extraterrestrial life.
We were looking for a way to find life on worlds where we can’t visit the surface. We realized that we had a spacecraft that was going to do just that, explained David Grinspoon, a collaborator of Sagan’s and planetary scientist at the Planetary Science Institute.
Galileo’s probes examined Earth’s atmosphere and surface, searching for physical and chemical signs that would show the presence of life. They found oxygen, ozone, methane, and nitrous oxide in the atmosphere, all of which are generated by living things. They observed oceans, clouds, vegetation, ice caps, and polarized light from the land, all of which are signs that point to a living planet.
Sagan and his colleagues reported their research in the journal Nature 30 years ago this week. Their article demonstrated how to search for biosignatures – evidence of life – on other planets, and galvanized the development of astrobiology as a science. Astrobiology is the science of life in the universe, and how it begins, develops, and responds to its surroundings.
“The Galileo experiment was audacious and visionary. It demonstrated how to search for life on planets around other stars before we had any evidence that such planets existed,” said Victoria Meadows, an astrobiologist at the University of Washington and principal investigator of the NASA Astrobiology Institute’s Virtual Planetary Laboratory.
Astronomers have since found thousands of exoplanets – planets revolving around other stars – and a few of them could be habitable. The new generation of telescopes, including the James Webb Space Telescope and the Extremely Large Telescope, will have the capability to study these planets in finer detail and search for biosignatures in their atmospheres.
But the discovery of biosignatures is not sufficient to establish life. There could be other ways in which certain chemicals or characteristics could be found on a planet. Oxygen, for instance, can be generated by non-biological means like photodissociation or splitting of water by sunlight.
“We have found a kind of fingerprint of life on Earth. But we must be careful not to assume that if we find this same fingerprint elsewhere, it has the same cause,” said Christopher Chyba, another colleague of Sagan’s and a professor of astrophysical sciences and international affairs at Princeton University.
To. exclude false positives or false negatives, astrobiologists have to take into account the history and context of a planet, and seek for several lines of evidence that can confirm or deny the presence of life. They also have to contrast observations with models and simulations that can forecast what a living or non-living planet would be like.
“The search for life beyond Earth is one of the most profound questions in science. It is also one of the most challenging,” declared Science in the News, a Harvard University graduate student organization that seeks to fill the communication gap between scientists and non-scientists.
Sagan’s experiment not only showed us how to search for life elsewhere, but also how to value life here. By viewing Earth as an outsider would view it, he exposed the fragility and singularity of our home planet, and the urgency of preserving it from human-made changes.
