In early 1993, a manuscript landed in the Nature offices announcing the results of an unusual—even audacious—experiment. The researchers, led by planetary scientist and broadcaster Carl Sagan, were looking for evidence of life on Earth that could be detected from space. The results, published 30 years ago this week, were “strongly suggestive” that there is indeed life on the planet. “These observations represent a control experiment for the search for extraterrestrial life from modern interplanetary spacecraft,” the team wrote (C. Sagan and others. Nature 365, 715–721; 1993).
The experiment was a master stroke. In 1989, NASA’s Galileo spacecraft launched on a mission to orbit Jupiter, where it was scheduled to arrive in 1995. Sagan and his colleagues wondered whether Galileo would find definitive evidence of life at home if its instruments could be trained to The Earth. They convinced NASA to do just that when the craft flew past the home planet in 1990.
How do we know if there is life on Earth? This daring experiment revealed
As we describe in an essay, a major concern for the journal’s editors was that the paper did not report a new discovery. Nature published it because it was a convincing control experiment to test the accuracy and appropriateness of the methods used to detect extraterrestrial life. If the study had found less evidence for life than it did, it would have been even more significant – it would have called into question the relevance of the parameters that scientists have proposed as evidence of life on other worlds.
The experiment was possible because Galileo had to circle Earth and Venus on his way to Jupiter to get a boost from the gravity of the two planets. It passed 960 kilometers from Earth at its closest point, over the Caribbean Sea.
From the spacecraft’s spectrometers, the researchers found evidence of oxygen, water vapor, ice and snow, along with carbon dioxide, methane and other greenhouse gases. Its imaging system spots clouds, oceans, coastlines and rocky surfaces. Although the technology did not have sufficient resolution to be able to detect actual life, it was able to detect electromagnetic signals whose amplitude varied in pulses. These amplitude modulated (AM) waves were widely used at the time to transmit radio and television broadcasts and were of a type not known to occur naturally. “Of all Galileo’s scientific measurements, these signals provide the only indication of intelligent, technological life on Earth,” the team wrote. This was a great twist because Sagan was constantly on radio and TV, one of the most famous science TV shows of his generation.
The research now being taught has stood the test of time and contributed to further thinking about frameworks for reporting evidence of life on other planets. Since the early 1990s, astronomers have discovered more than 5,500 planets orbiting stars outside the solar system. Additionally, a cascade of discoveries is expected in data from NASA’s powerful James Webb Space Telescope (JWST), which is uniquely well-equipped to study the atmospheres of exoplanets.
Reproducing scientific results is difficult but essential
Three decades later, Sagan’s classic experiment has three important lessons for researchers and scholarly publishers. The first is that it is important to test not only what we don’t know, but also what we think we know. The second is a reminder to those of us who publish science that control experiments—such as replication studies—are as important as studies that describe new results.
Last but not least, the lesson is implicit in the great care the team took in reporting the findings, including the detection of chemical signatures such as the presence of water or greenhouse gases. It would be easy, given what is known about life on Earth, to assume that the first piece of evidence settles the matter. Instead, the researchers built a nuanced conclusion, bringing together all the available evidence. Their approach shows why the search for extraterrestrial life will always be one of the most difficult problems in science.