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Look up at the night sky, look through a telescope, look at the thousands of images brought back by the Hubble Space Telescope and similar huge “light buckets.” What do you see? What do you see in your mind’s eye when you think of space? Vastness, emptiness, vacuum. Coldly shining stars which, up close, are actually maelstroms of searing fire, writhing gases, fractured plasma, and immense gravity. Stellar neighborhoods—our own included—which are fields of deadly radiation at all frequencies and with particles flying through at nearly the speed of light. Interstellar neighborhoods filled with dust, ice chips, and gases that are probably poisonous and definitely unbreathable.
The universe upon which we gaze is a place of chaos, silence, and death. … Or so one would imagine.
Despite this surface appearance, I believe the universe teems with life. Life is out there, waiting to meet us, maybe to greet us, maybe to eat us. But life exists. Life, this wondrous “temporary reversal of entropy”—and ultimately the consciousness that life has led to, at least here on Earth—is the whole point of having a universe.1 Otherwise it is just empty space and explosively fusing hydrogen.
Or consider the contrary proposition: that it only happened once, and it happened here. Of all the hundreds of billions of galaxies, each containing hundreds of billions of stars, only Sol, a minor sun a third of the way out from the center of the Milky Way, and only our own Earth, a medium-sized rocky planet in Sol’s inner orbit, were visited by this phenomenal accident of chemistry. Everywhere else, just fire, gas, and inert dust.
That’s like thinking your family’s house out in the suburbs was the actual birthplace of the arts of cooking, reading and writing, or television simply because you first encountered them under that one roof. A toddler thinks this for about five minutes between the ages of two and four. Then he or she discovers neighboring children and the household next door.
Why do I think life is common? First, because we find its building blocks elsewhere in the solar system. Amino acids, the precursors to proteins, have been found on comets,2 which means that they were scattered—perhaps intentionally seeded—among the dusts and gases out of which our star and its planets formed. And carbon-based, organic chemicals are found in quantity beyond Earth’s sky. For example, the atmosphere of Saturn’s moon Titan is rich in methane. The possibility also exists that at least some of the fossil fuels we drill from the Earth’s crust were not the products of decayed life on earth—old swamp forests becoming seams of coal and methane, and tiny diatoms becoming pools of oil and domes of natural gas—but instead some of these hydrocarbons existed in the coalescing planetary matrix and were squeezed together during the Earth’s formation, rising toward the surface like the veins of gold and uranium.
Second, if you look at the Earth today, you see a planet covered with life, teeming with life, and transformed by life. But it was not always so. The atmosphere contains breathable oxygen and the soil contains nutrients only because the first microbes and then the larger organisms that evolved from them have been softening up the rock and freeing up gases for more than two billion years.3 If you had come to Earth back then, before the first life got to work on the planet, you would have found a stony surface, sterile seas, and an unbreathable atmosphere composed mostly of nitrogen with admixtures of carbon dioxide, methane, ammonia, and water vapor.4 This is a planet shaped by life, made habitable for life by its own processes. Life is the ultimate terraforming service and, through the process of evolution, it always gets the details exactly right for the type of life that will eventually live there.
If humans were to discover this solar system from afar during an interstellar scouting expedition, we would see two obvious places to look for our kind of life. First would be the Earth itself, which we would deem a remarkable and wholly unaccountable paradise. Second would be the watery world under Europa’s icy crust. We can suppose that heat from an active inner core or gravitational kneading from nearby Jupiter keeps the water under Europa’s ice liquid and perhaps warm enough for life’s processes. Europa may even offer volcanic vents rich in minerals, like those under Earth’s oceans, capable of supporting its own active biological ecology.
Mars may once have held life, although it now seems pretty dead. Perhaps some planetary disaster killed Mars. But perhaps any early atmosphere that Mars possessed leaked away because the planet was too small, its gravity too weak, to hold onto gas molecules lighter than carbon dioxide. Perhaps Mars once had an active iron core that was able to generate a magnetosphere to deflect the solar wind, as Earth’s core does. But now Mars has a thin atmosphere and high surface radiation, unsuitable for any kind of advanced life.
Venus may once have held life, because its starting conditions were very much like those on Earth, and its orbit, while nearer to the Sun, is not so near as to account for the planet’s high ambient temperature—about 800 degrees Fahrenheit—all by itself. Venus apparently suffered some kind of runaway greenhouse accident that increased both the atmospheric pressure and the cloud base. Its atmosphere is also not very nice, being mostly carbon dioxide like Mars’s but vastly denser. The sky rains sulfuric acid because of the high concentration of sulfur dioxide. In addition, radar studies of the planet’s surface indicate suspiciously few “astroblemes,” or visible impact structures. This apparent lack of asteroid bombardment, which is found everywhere else in the solar system, suggests that either the weather wears old craters away at a phenomenal rate, or the lithosphere lacks a tectonic plate structure like that of Earth. Without shifting plates and occasional volcanic eruptions along the plate boundaries, the heat from Venus’s core might build up until the entire surface simply melts and subsides, as if it was being groomed by a some kind of planetary Zamboni machine.
As for the probability of finding life, the rest of our solar system would appear to that scouting mission as either gas giants with no discernible surface or barren rocks and balls of ice without atmospheres—Titan excepted. And both of our immediate neighbors, Mars and Venus, have interesting and perhaps tragic histories that render them inimical to life. But tragic for whom? If Earth was reshaped by life as it grew here, what influences might a different kind of life have brought to these planets?
We define life fairly closely.5 We recognize our kind of life by carbon-based chemical processes, a fragile cellular structure, and some degree of mobility and interaction with the environment. That may be an overly narrow definition. Who is to say that some kind of piezoelectric circuitry flowing through a crystal, or a chemical reaction among various metals, could not create a fully satisfied form of life, perhaps one with an active and questing intelligence?
In our journeys out among the stars, we will have to step carefully. Otherwise, we might mistake the local inhabitants for paperweights or some aggressive form of corrosion. But my bet is we’ll meet a fair number of carbon-based life forms out there, with some analogue of proteins shaping cellular structures that are active, mobile, and interacting with their environment. My guess is they’ll come right up and tap us on the shoulder—either to greet us or proceed to eat us.
1. “Or what’s a heaven for?”—in the words of Robert Browning.
2. See Found: First Amino Acid on a Comet, from New Scientist, August 17, 2009. Similar articles trace further discoveries. At the time of this posting, I expect the European Space Agency’s Philae probe to find similar chemicals on comet 67P/Churyumov-Gerasimenko.
3. See my blog post DNA is Everywhere from September 5, 2010.
4. But scientific views of the early Earth’s atmosphere differ. See, for example, Earth’s Early Atmosphere in Astrobiology Magazine from December 2, 2011.
5. See Between Life and Not-Life from November 9, 2014.