We still don’t know what critical factors determine a planet’s destiny as one of these very different types of worlds. The fact that we don’t really understand the Earth-Venus difference in tectonic evolution shows that we can’t yet predict whether a given planet will have plate tectonics. This is currently one of the greatest weaknesses in our ability to predict which planets elsewhere in the universe are good candidates for life. We are understandably obsessed with finding other Earth-size worlds, but we still don’t really know the history of our nearby twin. Future exploration of Venus will help us here. We know the secrets are sitting there, on and below the surface of Venus, in that crushing atmosphere, beneath that shroud of acid cloud. We can go and find them. It won’t be easy, but we’ll do it. The answers may clue us in to some of the deep connections among climate, geology, and biology on worlds like Earth, and prepare us to make sense of the new harvest of rocky planets we are just starting to discover around other stars.
Billions and Billions of Worlds
For at least as long as we’ve been human, we’ve looked at the night sky and wondered what the stars were and how far away they were. For hundreds of years we’ve known they are distant suns, and wondered if they shine on worlds of their own. Now, finally, we know. When you look at a night sky studded with stars, most of them are orbited by unseen planets.
Planetary scientists long assumed that our sun was not unusual in having a family of orbiting planets, and that many of the hundreds of billions of other stars in the Milky Way, and the trillions and trillions in other galaxies, were likely centers of their own extrasolar systems. Indeed, this is what I was taught in grad school, in the late 1980s, long before we knew it was true. Our professors told us that the formation of the planets was a predictable by-product of the messy birth of the Sun, an inevitable fusing of orbiting bodies from the leftover debris, and therefore something that should have happened around most stars. Yet, at that time, this belief was a kind of informed faith. We believed other planets were there, but we had no direct evidence.
Science marches onward. Better instruments were built, and some clever, dogged astronomers refused to take “We don’t know” for an answer. The breakthrough came in October 1995, when astronomers in Switzerland observed that 51 Pegasi, a star similar to the Sun and fifty light-years away, was wobbling, responding to the gravitational tug of a small, unseen companion. A week later this observation was confirmed by American astronomers who caught the same wobble from a telescope in California. The gates were cracked open, and a flood of discovery began. More and more extrasolar planets (exoplanets, for short) were found by this and other techniques. Now we’ve acquired the solid data to transform our longtime belief in exoplanets from a well-justified hunch to a known fact.
The watershed came from NASA’s Kepler spacecraft, the brainchild of Bill Borucki, another longtime denizen of Ames Research Center. Bill had been advocating for this planet-hunting mission for decades, seemingly forever. I remember him doggedly pushing the concept when I was a postdoc at Ames, twenty years before Kepler became a reality. His concept was to launch a small telescope into orbit just to obsessively stare at one little area of sky. He proposed that if we could precisely monitor the brightness of a large number of stars in one random area of the galaxy, watching for any flickering, we could tell if planets ever passed in front of any of them. A lot of people thought it was clever but kind of “out there.” Things changed when observers started to find planets from ground-based telescopes. Bill Borucki finally got his mission. Kepler was launched in March 2009 and began revealing to us the approximate number and demographics of planets in one tiny (and presumed typical) patch of the Milky Way, covering one wingtip of Cygnus the Swan. Now we know planets are ubiquitous. They’re a normal thing for a star to have, and they exist in a stunning diversity of size, density, and orbital arrangement.
The question of what these newly discovered planets are really like as worlds, as places, is both overwhelmingly interesting and incredibly difficult to answer. When we started exploring the solar system, we knew about only nine planets. Now our effort to put Earth in context takes place against a backdrop of “billions and billions” of them. Planets are diverse and quirky, so the handful of local examples may not be sufficient for us to learn universal patterns of planetary evolution. We need many more, and fortunately we are about to get them as we slowly learn more details about exoplanets. Only extrasolar planets are all so ridiculously far away, and each is orbiting close to a star that is billions of times brighter, making it a ridiculously daunting technical task to observe the planets themselves directly. Detecting them and determining size, density, and orbit has become relatively routine, but finding out much more about them is going to be really, really hard. How many of them are similar to Earth in the ways we most care about? Do they have tectonic cycles, stable climates, watery surfaces, and conditions otherwise copacetic for our kind of life?
The only way we have right now of gaining any hints is through painstaking analysis of data sets at the hairy edge between noise and meaning. Yet a fantastic amount of cleverness is being displayed by the community of scientists, many of them young, who are now bravely trying new ways to use the tools we do have to glean information about these distant new worlds.
For a very long time, perhaps forever, we will know much, much more about the handful of planets within our solar system. To understand exoplanets, we’ll always rely heavily on our detailed knowledge of the local bunch—because planets are incredibly complex. Climate and atmospheric evolution are intricately bound up with interior, and tectonic (and biological? and technological?) evolution. Our progress will be limited until we explore our own solar system much more deeply. If we want to know what makes planets tick, we have to do a much more thorough job with the ones we can get to.
Exoplanets and solar system planets—we won’t be able to understand one without the benefit of exploring the other. Fortunately, we live at a time when we can proceed with discovery in both realms, and the net effect will take us a great way toward learning how planets like ours function. The modelers are starting to map out the possible climate states of exoplanets. I’ve participated in some workshops about this and have found it fascinating watching astronomers and terrestrial climate modelers try to talk to one another. There is a huge gulf in scale and perspective. Our knowledge of exoplanets is so sparse. Each of these worlds is known to us as, at best, a few numbers: mass, distance from a star, and in some cases vague inferences about temperature or atmospheric composition. In contrast, our Earth climate models are supplied with dense grids of data: millions of points of temperature, humidity, and wind velocity. The mismatch in perspectives and techniques makes communication challenging. Yet the questions raised are vital: increasingly, we are banking on our climate models for the future well-being of human civilization on this planet. If our models really are any good, shouldn’t they also be able to predict the climate on any other world as well? At these workshops, I’ve felt as if I were seeing the future, a new era of climate understanding that will come when we can study our own planet’s qualities in the context of thousands of its peers.
The discovery of exoplanets has been a transformative watershed for planetary science. These planets will test our theories and deepen our insights in so many ways that were impossible during the phase of human history (which ends right now) when we knew nothing about virtually all planets in the universe. Earth climate modelers, so mired in the details of their difficult and urgent work, may be slow to realize it, but they need exoplanets, too.
Another Earth
A half century into the space age, we’ve learned that we really can’t discover all that we need to know about our planet without looking beyond it. Planetary exploration has become crucial for understanding and protecting Earth. Astrobiology is closely tied to the topic of climate change on Earth because the starting point for thinking about planetary habitability is to look at the qualities that make our own planet h
abitable.
We’re still just beginning to take the census of planets in our galaxy. A statistical study of Kepler’s harvest suggests that one in five Sun-like stars has Earth-size planets in its habitable zone.6 This is a pretty loose estimate, with some arbitrary definitions. “Earth-size” here simply means having a diameter between one and two times Earth’s diameter, and “habitable zone” is defined very crudely. Yet this study succeeded in showing that, however one chooses to define these things, the news is good: there are planets in abundance, including plenty of the types of worlds where we can most easily imagine life evolving. In fact, in August 2016, we learned that Proxima Centauri, the nearest star to the sun, apparently has a planet that is slightly larger than Earth orbiting in its habitable zone!
Still, what does an Earth-size world really imply? Just because a planet is Earth-size, does that mean it is Earth-like? This might seem a silly semantic question, on a par with “is a dwarf planet a planet?” but it has generated some heated and interesting debate. What do we really mean by Earth-like? Perhaps a good criterion would be the continuous presence of both stable surface water and vigorous geological activity over billions of years, because the most meaningful way for a planet to be Earth-like, to have the quality we really care about, would be to favor our kind of carbon-based, water-immersed life.
Finding “another Earth” has long been an obsession of the exoplanet science community, often referred to as the Holy Grail. We are primed for such an announcement. We can expect many exciting discoveries of worlds that are tantalizingly similar in enticing ways. We may even soon find one with an atmosphere strange enough to hint at the presence of life. Yet, despite the sex appeal of “another Earth,” I don’t believe we will find one. Why? Because planets are more like people than protons. Particles such as protons are interchangeable. See one, you’ve seen ’em all, and it is likely that we can learn all the types of elementary particles. We may never learn all the types of planets. Each planet is the result of a complex and contingent history. Like people, they are individuals, and no two will be exactly alike.
What is it really that makes Earth Earth? The surface, where we live, exists at the interface between two giant heat engines: below us the churning mantle, above us the restless, windy troposphere. All rocky worlds large enough to hold an atmosphere will have some combination of these, and the dynamic possibilities of each realm have been substantially illuminated by the variations we’ve found on other worlds. We live on the convoluted, shifting shoreline between cycles of earth and sky that are incessantly driven by the Sun above and the heat below. So much about our world can be understood as the interplay between these inner and outer cycles. Life thrives at the boundary, enabled and sustained by the great cyclic flows of carbon, nitrogen, oxygen, sulfur, and phosphorous.
Still, something else is going on here. It’s a two-way exchange. These cycles feed, but also feed off, Earth’s biosphere. Gradually, we’ve come to realize that there is another great force at work on Earth: life itself. Life is so enmeshed in all the cyclical workings of Earth, and the recycling of the elements, that these geochemical cycles are now more commonly referred to as biogeochemical cycles, a word whose multiplicity of prefixes reveals the causal complexity of our world.
During the formative eons when Venus and Mars were undergoing their climate catastrophes, Earth was also in the earliest phases of its own radical transformation. Had anyone been there to observe its humble beginning, it would have seemed innocuous at first: a strange kind of chemical scum that started forming somewhere around a hot vent on the ocean floor or in some warm tidal pond. This tiny disturbance had a peculiar property. It was self-perpetuating, and thus marked a propitious branching point, the lowly start of a major transition that forever changed our planet’s fate. Not only was this stuff self-propagating, but its imperfections made it adaptable. It changed with the planet, and then it changed the planet. The phenomenon persisted and spread and eventually globalized, taking over Earth.
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CAN A PLANET BE ALIVE?
Wake up to find out that you are the eyes of the world.
—Robert Hunter
Life Disturbs
The search for life elsewhere presents some juicy puzzles. Do we know what we’re looking for? What should we assume that we and aliens have in common? Despite our cultural saturation in Spielbergian imagery, we don’t really expect little green men with big heads and teardrop eyes. Yet must life everywhere be built of the same molecules? Are DNA and proteins universals or just locally frozen-in accidents? Must other life, at least, be built of carbon molecules in liquid water, or might there be some entirely different basis for complex biochemistry that would operate better in an alien environment? What is life, anyway? Would we know it if we found it?
We know of only one biosphere, and we’re in it, so we have no perspective. On Earth we’re all related, stemming from the same beginning, so we have only one example of life, the opposite of what you need for scientific perspective on a complex system. How can we try to define it before we’ve found multiple cases? Yet how well can we search for something we haven’t defined? As space exploration began in the early 1960s, scientists in the nascent field of exobiology were aware of this philosophical conundrum, but it became a practical problem in the mid-’60s, as NASA began to plan the search for life with the first Martian landers, a process that would culminate in 1976 with the landings of Viking 1 and 2 on Mars. Out of this effort came an idea that changed the way we think about life on Earth.
NASA hired James Lovelock, an iconoclastic, independent British scientist/inventor,1 to help devise instruments for finding life on Mars. Lovelock realized that a quality of life on Earth, one that should indeed be truly universal, is that it radically changes the global environment. He proposed that searching for such anomalies on another planet was a better idea than trying to find actual organisms. It should be easier to spot these large-scale deviations than to observe living creatures, which might well be microscopic and/or unrecognizable. Such a search strategy doesn’t assume anything specific about alien life, only that life interacts chemically with its surroundings, extracting energy and leaving its mark. He concluded that life would always knock an atmosphere out of chemical equilibrium, leaving a strange brew of gases that would differ recognizably, detectably, from the air of a lifeless world. He began to suspect that to find life on Mars, we merely had to make very precise observations of the atmosphere. This could be done from Earth with sophisticated telescopes and spectrometers. This advice didn’t really help NASA with their question of what instruments to put on a lander to find life. Jim Lovelock is definitely not the type to give you the answer he thinks you want to hear.
He worked on this problem at the Jet Propulsion Laboratory in Pasadena, where he shared an office with Carl Sagan. In 1965, he learned of new infrared observations of Mars, from the Pic du Midi telescope in France, showing that its atmosphere, like that of Venus, was made almost entirely of CO2. This is fundamentally different from Earth, and what you would expect if the atmosphere were in perfect equilibrium, completely undisturbed by any biological activity. As far as he was concerned, these measurements showed that there was no life on Mars. Lovelock concluded that a lander mission was no longer needed to determine if anything was alive and breathing on the Red Planet. Of course his officemate vehemently disagreed. Sagan’s mentor, astronomer Gerard Kuiper, had earlier argued that the “regenerative dark areas” seen on the surface of Mars were caused by vegetation, and Sagan had developed his own speculative hypotheses about the kind of life we might discover there with surface landers.
I would like to have been a fly on the wall for that exchange. Lovelock is an erudite provocateur who seems to love tweaking the establishment. To the degree that there was an establishment line of thought in exobiology, still a small and barely reputable effort at that time, it was Sagan’s. Carl relished a good intellectual argument and loved kicking around new ideas, but he was also heavily in
vested in sending a lander to Mars to look for the microbes or spores he suspected just might be crawling or hibernating in the dirt there. NASA ignored Lovelock’s advice and proceeded to design, build, and fly the Viking orbiters and landers—two of each.
The Viking landers were magnificent, among the most complex and elegant machines ever built. They both arrived on Mars in 1976 and gave us our first vivid portraits of the Martian surface. As a teenager, I was enchanted by these missions, the drama of their landings, and the first pictures of that rock-strewn, alien desert where Viking would search for life. When I went to college, Viking provided my first taste of real scientific research, in the form of a summer job working for Tim Mutch, my freshman planetary geology professor at Brown University, who led the camera team for the Viking landers. My first task was recording the size and position of every rock at both Viking landing sites, which would allow us to investigate the history of the site by doing statistical comparisons with different kinds of rock fields on Earth. (Yes, my job was counting rocks, which sounds like a parody of a boring occupation. But these rocks were on Mars!) I spent months immersed in the dusty plains of the Red Planet. By the end of that summer, I felt I knew what it was like to tromp around on Mars with the crimson dust blowing around my ankles in thin, frozen breezes.
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