We have yet to discover any signs of aliens, a troubling observation that has led to much speculation. One possible solution to the Great Silence is that nobody's out there. It's a conclusion that sounds impossible to believe, but there may be something to it. Here's why we may be alone in the universe.
Photo: "Lost Souls" by Julie Fletcher.
Ever since physicist Enrico Fermi posed the question — where is everybody? — people have been wondering why we haven't seen any signs of extraterrestrial civilizations. As Fermi pointed out, the math just doesn't add up. Our galaxy, at 13 billion years old, has been around long enough for aliens to explore and colonize it many times over by now (recent work shows it should take less than a billion years, perhaps even as little as a few tens of millions of years). Clearly, we should have seen somebody by now.
This surprising observation led astronomer Michael Hart to conclude that spacefaring life in the Milky Way should be either galaxy-spanning or non-existent. But the exclusive presence of "non-existent spacefaring" aliens could be attributable to any number of things, including a reluctance to explore space, or owing to technological intractability. But it could also imply that aliens simply don't exist. Indeed, despite all the recent discoveries of potentially habitable exoplanets, along with the general feeling that our universe is primed for life, there are many reasons to suspect we're truly unique in the large scheme of things.
The Right Place at the Right Time
As astronomer Paul Davies has said, "If a planet is to be inhabited rather than merely habitable, two basic requirements must be met: the planet must first be suitable and then life must emerge on it at some stage."
Indeed, life is dependent on the presence of five critical elements, or metals in the parlance of astronomers: sulfur, phosphorus, oxygen, nitrogen, carbon (or SPONC for short). These heavier elements were cooked in nuclear reactions inside stars and became part of the interstellar medium only when stars reached the end of their energy-producing life. So, as time went by, the concentration of metals in the universe gradually increased.
But here's the thing — these heavier elements only recently became sufficiently concentrated in the interstellar medium to allow life to form. Planets around older stars, therefore, are likely to be low in SPONC. Only around relatively young stars, like ours, can life emerge. So humanity would thus be among the first civilizations — perhaps the first — to arise.
But as Stephen Webb points out in his book, Where is Everybody?, the suggestion that chemical enrichment explains our solitude is, by itself, way too overstated — it's insufficient to completely explain the Great Silence. For example, we don't know the degree of metallicity required of a star for it to possess viable planets, and we know that the metallicity of stars vary considerable between different classes of stellar populations. Simply put, we don't know enough about this variable to make a definitive conclusion about it.
GRBs: The Evolutionary Reset Button
Another intriguing possibility is that our galaxy is subject to frequent gamma-ray bursts (GRBs). And by frequent we're talking about one every few billion years or so. A GRB is one of the most energetic phenomenon yet discovered in the universe. These blasts are probably caused by a hypernova — the sudden collapse of massive star to form a black hole — or the product of the collision between two neutron stars, those ultra-dense remnants of supernovas. Across the observable universe, GRBs happen at a rate of about one per day.
The ensuing blast of radiation from a hypernova has the capacity to destroy the biosphere of an Earth-like planet, instantly killing most living organisms on or near the surface (underwater or lithoautotrophic ecosystems would survive). The gamma-rays would also instigate chemical reactions that create ozone-killing molecules powerful enough to destroy more than 90% of a planet's ozone layer, allowing fatal ultraviolet light from the parent star to cook any complex biological molecules it strikes.
Back in 1999, James Annis of Fermilab in Illinois proposed that GRBs could cause mass extinction events on any habitable planet within a distance of 10,000 light-years from the source. To put that into perspective, the Milky Way is 100,000 light-year across and about 1,000 light-years thick. Thus, a single GRB would extinguish life across a sizeable portion of the galaxy.
According to new work conducted by astronomers Tsvi Piran and Raul Jimenez, the odds that a planet could be hit by a GRB depends on its place in space and time. The closer that a planet is to the galactic core, where the density of stars is much greater, the odds increase. Their models show that a planet near to the core has a 95% chance of being hit by a catastrophic GRB at least once every billion years. Pulling back a bit, about half of the solar systems in the Milky Way are close enough such that there's an 80% chance of a GRB per billion years.
But here's where it gets interesting: The frequency of GRBs were greater in the past owing to lower levels of metallicity in the galaxy. Metal-rich galaxies (i.e. those with significant accumulations of elements other than hydrogen and helium) feature less gamma-ray bursts. Thus, as our galaxy becomes richer in metals, the frequency of GRBs decreases. What this means is that prior to recent times (and by recent we're talking the past 5 billion years or so), GRB extinction events were quite common. And in fact, some scientists suspect that the Earth was struck by a GRB many billions of years ago. Piran and Jimenez figure that these events were frequent and disbursed enough across the Milky Way to serve as constant evolutionary reset buttons, sending habitable planets back to the microbial dark ages before complex life and intelligence had a chance to develop further. Fascinatingly, before about 5 billion years ago, GRBs were so common that life would have struggled to maintain a presence anywhere in the cosmos (yes, the entire cosmos).
This would suggest, in the words of Annis, "the Galaxy is currently undergoing a phase transition between an equilibrium state devoid of intelligent life to a different equilibrium state where it is full of intelligent life."
Humanity, therefore, may not be alone, but one of many intelligent civilizations emerging at roughly the same time.
It's a compelling theory, but one that's still unconvincing. Astronomer and astrobiologist Milan M. Ćirković was at one time a big proponent of this theory, but has since lost confidence.
"For the moment, I have somewhat retreated from the idea that GRBs are the main astrobiological regulatory mechanism in the Galaxy," he explained to io9. "The required sharpness of the phase-transition is too large — or too fine-tuned, if you wish — to account for the lack of manifestations of advanced technological civilizations (ATCs)."
He says the frequency of galactic GRBs is decreasing, but it's not enough to explain the Great Silence. The difference between the typical timescale for the sterilizations of habitable planets and the time it should take for an ATC to colonize the galaxy is still way too large. Moreover, Piran and Jimenez may be overstating the extent of these mass extinctions; it's an open question as to how much damage these bursts unleash, and how quickly life can snap back.
Ćirković says this leaves us with two options to explain why we may be alone in the galaxy: One is that GRBs, together with some other catastrophic process or processes (be it natural or artificial), are together stifling the emergence of ATCs. He says that together they may occur with sufficient frequency to do this, since the risks are cumulative. Alternatively, we have to seek some completely different resolution to explain the Great Silence.
Our Rare Earth
One of these resolutions is the so-called Rare Earth Hypothesis — the suggestion that the parameters required to spawn a space-faring species is excruciatingly narrow. It's an idea that was put forth in 1999 by paleontologist Peter Ward and astronomer Donald Brownlee. By synthesizing the latest findings in astronomy, biology, and paleontology, the two put together a list of variables that, in their opinion, make our planet exceedingly rare in the cosmos. So rare, in fact, that it may explain why we may be the only ones out there.
According to Ward and Brownlee, the prerequisite conditions for complex life include:
- The right location in the right kind of galaxy: Galaxies have dead zones owing to varying levels of star metallicity, X-ray and gamma-ray radiation, and gravitational perturbations of planets and planetesimals by nearby stars (which can facilitate impacts of bolides).
- Orbiting at the right distance from the right type of star: Our planet is in the so-called Goldilocks Zone of our solar system where the conditions are just right for complex life to emerge.
- A solar system with the right arrangement of planets: Without the presence of outer gas giants, like Jupiter and Saturn, complex life may not have arisen. Interestingly, star-hugging hot Jupiters are very common.
- A continuously stable orbit: Planets in binary systems have wacky orbits that potentially take them in and out of habitable zones. That's a problem. And binary systems are exceptionally common in the Milky Way, accounting for at least half of all systems.
- A terrestrial planet of the right size: There needs to be enough surface area, a stable atmosphere, and level of gravity that's not too heavy or light for evolutionary processes to occur.
- A planet with plate tectonics: This process has a moderating influence on temperature excursions in the Earth's climate. A planet without plate tectonics would lack a temperature regulation mechanism — and without a stable temperature, the evolution of complex life becomes increasingly unlikely.
- A large stabilizing moon: Our moon puts the Earth on a stabilizing axis, allowing for seasonality, which some astrobiologists say is vital for the emergence and development of complex life.
- An evolutionary trigger for complex life: The transition from simple cells (prokaryotes) to complex ones (eukaryotes) may in fact be the most difficult step for evolution to take.
- The right time in evolution: Life has it difficult in the early stages of galactic and planetary evolution, including such things as periodic bombardments of celestial objects, extreme volcanism, atmospheric factors, and as already mentioned, the increased chance of GRB extinction events.
Admittedly, this looks like a daunting list. And it just might be enough to account for the Fermi Paradox. But as Ćirković told me, it's an "ugly choice" given its known deficiencies. We're learning, for example, that Earth-like planets abound in the Milky Way (there could be as many as 40 billion habitable planets in our galaxy), that complex life is capable of emerging in extreme environments, and that the various parameters presented by Ward and Brownlee (such as the role of Jupiter and plate tectonics), may be overstated as potential filters. In fact, a recent study pointed out that our galaxy may host planets considerably more suitable for life that Earth.