Are we alone in the universe? This depends on what “we” means – whether one considers the simplest forms of microbial life, or solely planets populated with conscious, technologically advanced creatures. Geoscience is becoming increasingly essential to addressing these questions in astrobiology and space probing projects.
The James Webb Space Telescope placed at an orbit 1.5 million kilometers from Earth in 2022 has provided new data about the universe and motivated a huge number of new publications, including popular books such as “Is Earth Exceptional? The Quest for Cosmic Life,” by astrophysicist Mario Livio and Nobel-laureate biochemist Jack Szostak.
Where is Everybody?
The universe is 13.8 billion years old, and the solar system is 4.6 billion years old. Scientists estimate that there are at least two trillion galaxies in the observable universe, each with at least 100 billion stars, taking the Milky Way as a yardstick. If there are millions of Earth-like planets and human-like species out there, why are there no signals from extraterrestrial beings? This great cosmic silence, known as the Fermi Paradox, was articulated by Enrico Fermi in 1950: “Where is everybody?” Perhaps the signals are not detectable by our technology, or they have passed us, or they will come one day or will never come at all because of distances too great to bridge.
A Matter of Habitability
Since the 1960s, programs like the SETI (Search for Extraterrestrial Intelligence) Institute have been proactive efforts to solve the Fermi Paradox. Astrophysicist and astrobiologist Frank Drake first suggested a probabilistic seven-factor equation for SETI that considers fractions of stars with planets and fractions of planets that develop and support life forms, especially intelligent beings capable of cosmic communication. Using this approach, Carl Sagan estimated that a million civilizations might exist in our own galaxy alone.
While a good start, the Drake equation might be too simple and require additional parameters, especially with regard to emergence and evolution of life on a planet. This consideration has led scientists to examine the factors that might create a “habitable or Goldilocks zone” in a solar system, including proper distance from the sun, sufficient planetary size and gravity and the presence of rocks, water and atmosphere.
In their book “Rare Earth: Why Complex Life is Uncommon in the Universe” Peter Ward and Donald Brownlee argue that the “habitable zone” is not sufficient and that life on Earth evolved toward humans under a complex sequence and unique set of conditions including plate tectonics, glacial ages, mass extinctions, atmospheric changes, the moon’s influences and protection from giant planets against cosmic hazards.
Physical Determinism versus Historical Contingency
Similarly, Australian theoretical physicist Brandon Carter in 1983 suggested that it took 4.5 billion years (almost half the sun’s lifespan) for humans to emerge on Earth after successful passage of life through “hard steps,” each with its own improbability. That is why, Carter suggested, Earth is “exceedingly rare” in the universe.
In a recent study in Science Advances, researcher Daniel Mills and colleagues critically refuted Carter’s “hard-step hypothesis.” They instead suggest that intelligent life on a planet does not require lucky breaks. Humans appeared on Earth as the right conditions developed, not through improbable hard steps, but via opening of the “windows of habitability” in Earth’s history. In other words, organic evolution by natural selection succeeds and “biospheric evolution generally proceeds in a coarsely deterministic or predictable fashion.”
This deterministic hypothesis implies that if we replay “life’s tape” on Earth we will get almost the same results. Paleontologist Stephen Jay Gould, who coined the phrase “replaying life’s tape” in his book “Wonderful Life,” believed that evolution is highly contingent on history and that each time we replay life’s tape, we may get strikingly different results. Why? Think of the butterfly effect: tiny events may result in significant and unpredictable changes.
Perhaps, life trajectories on exoplanets develop somewhere between hard determinism and chaotic randomness, with each planet having its own historical conditions. The spectrum of life probably tapers from microbial toward intelligent organisms, and the latter, like mammalian species, are very diverse. Planets like Earth with species resembling us may be very rare.
Cosmic Suprises
Our concepts of life and planetary habitability are limited to Earth. If microbial life is discovered in ponds of liquid methane (not water) on Jupiter’s or Saturn’s moons far from the sun, we will have to redefine the habitable zone. Our first encounters will probably be with inorganic artificial intelligence created by long-gone or inaccessible creatures.
The possibility of finding Earth-like planets may be slim or strong, but as physicist Philip Morrison once said, “If we never search, the chance of success is zero.”