Carl Sagan died on December 20, 1996, at the age of 62. He spent his career doing something that was not fashionable in the professional scientific community for much of that time: taking the question of life beyond Earth seriously as science, not speculation.
His books, his television work, his scientific papers, and his testimony before Congress shaped how a generation understood the cosmos. He was also wrong about some things, and being honest about that is the most Sagan-like thing we can do.
What he got right
The cosmos is full of the ingredients for life. Sagan argued throughout his career that the chemistry of life, carbon chemistry, amino acids, nucleotides, is not unusual. It is cosmically common. The universe makes these molecules everywhere. The synthesis of complex organics in interstellar clouds, on comets, in the atmospheres of giant planets, has been confirmed repeatedly since his death. Amino acids have been found in meteorites, in interstellar gas clouds, on asteroids. The premise of Sagan’s argument, that life’s chemistry does not require special conditions for its raw ingredients, is strongly supported.
Mars probably had a wet past. Sagan was an early advocate for the view that Mars had significant liquid water on its surface in the distant past. This was contested when he made it. It is now one of the best-supported conclusions in planetary science. Mars had rivers, lakes, and possibly an ocean. The evidence is in the geology, the mineralogy, and the atmospheric models.
The search for extraterrestrial intelligence deserves scientific resources. The SETI program, which Sagan championed, was dismissed by many mainstream scientists as a waste of telescope time. Sagan’s argument was probabilistic: with hundreds of billions of stars in our galaxy, even if intelligent life is rare, the numbers suggest we should look rather than assume we are alone. SETI remains underfunded relative to what a serious search would require, but it is no longer scientifically disreputable. The question of life beyond Earth is now mainstream science.
Skepticism and wonder are not in conflict. This is perhaps Sagan’s most important intellectual contribution, and it holds up perfectly. The instinct to be moved by the scale and age of the universe, and the instinct to demand evidence before accepting any specific claim about it, are not in tension. They reinforce each other. Wonder without skepticism becomes credulity. Skepticism without wonder becomes sterility. Sagan modeled both simultaneously, and the field of astrobiology, which essentially did not exist as a discipline when he died, embodies that combination.
What he got wrong
The pace of exoplanet discovery. Sagan expected that identifying Earth-like planets around other stars would take much longer than it did. The Kepler mission launched in 2009 and within a few years had confirmed thousands of exoplanets. We now know that rocky planets in the habitable zones of their stars are not rare. We have found them around the nearest stars to our own. Sagan was right that they would exist; he significantly underestimated how soon we would find them and how many there would be.
The near-term prospects for Mars. Sagan was optimistic about Mars exploration in the 1970s and 1980s and believed human missions would happen within decades. The political and institutional realities of spaceflight funding meant that hope was deferred by several generations. He died before the Mars Pathfinder mission proved the viability of rover exploration. He never saw Mars from the perspective of the Curiosity rover’s cameras. His timeline was wrong; his science was right.
The Fermi paradox resolution he preferred. Sagan leaned toward solutions to the Fermi paradox, the question of why we have not detected evidence of other civilizations despite the apparent abundance of planets, that emphasized the self-destructive tendencies of technological civilizations. His reasoning was shaped in part by Cold War nuclear anxiety. The silence of the cosmos, he suggested, might reflect the tendency of civilizations to destroy themselves shortly after developing the ability to communicate across interstellar distances.
This is not wrong. It is one of many possible explanations. But subsequent work has added considerable depth to the question, from the “rare Earth” hypothesis to the “dark forest” theory to arguments about the resolution of the observable universe. The landscape of possible answers has expanded significantly since Sagan’s death, and his preferred answer, while not refuted, looks less like the obvious solution than it once did.
The lasting contribution
The thing about Sagan that holds up most robustly is not his specific predictions but his framework. He insisted that the universe is vast enough to contain surprises, that life is probably not unique to Earth, and that the only way to know is to look carefully and honestly. He also insisted that the method, not just the conclusions, matters. Extraordinary claims require extraordinary evidence.
Thirty years later, that framework is more relevant than ever. We are building the telescopes to look for biosignatures in exoplanet atmospheres. We are sending spacecraft to Europa and Titan. We are analyzing Martian rocks in laboratories. We have organizations systematically collecting UAP data. All of this is Sagan’s program, expanded and professionalized.
He would, it seems fair to say, have been pleased by the pace of discovery. He might have been impatient with how long the human Mars mission is taking. And he would almost certainly have something specific, rigorous, and memorable to say about fast radio bursts.