Exoplanets: Diamond Worlds, Super Earths, Pulsar Planets, and the New Search for Life beyond Our Solar System

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NOT YOUR GRANDFATHER’S GALAXY

There are more things in heaven and earth, Horatio, than are dreamt of in your philosophy.

Shakespeare, Hamlet, Act 1, Scene 5

The universe used to be a simple place. We lived in a sedate solar system with nine planets circling an ordinary star in an unremarkable part of the galaxy. We assumed there were other solar systems out there—systems pretty much like ours. We had genteel arguments about whether some of those systems might support life, and we enjoyed science fiction adventure stories such as Star Trek and Star Wars that populated the galaxy with interesting (and often combative) beings who spoke English. But the central fact was that we knew about only one planetary system, so we labored under what we can call the curse of the single example.

If you have only one example of something—be it a planetary system or a butterfly—the natural assumption is that every other thing you find will be like the one you know about. Take butterflies as an example. If the only kind of butterfly you had ever seen was a monarch, it would be reasonable to assume that all butterflies have to be big and orange and migrate to a particular spot in California every year. Confronted with a cabbage butterfly—small, white, and nonmigratory—you might understandably be confused. Some of your scientific colleagues might even argue that what you were seeing wasn’t a butterfly at all, but a kind of beetle. Eventually, though, you would begin to explore a little more and find that the discovery of the cabbage butterfly was just the beginning of a journey into a world of amazing complexity and diversity, and that there were thousands of different kinds of butterflies in nature. You would realize that your original paradigm—the notion that there was only one kind of butterfly—was simply wrong and that it had blinded you to the true complexity of the living world.

We argue in this book that the butterfly analogy is a perfect description of humanity’s recent discovery of the universe of exoplanets: planets outside our solar system. Only 30 years ago, most scientists would have asserted that we had a perfectly good explanation of the origins of our own solar system, an explanation based on the solid bedrock of the laws of physics and chemistry. These laws, they would have said, dictate that any other solar systems out there would have an inner contingent of small, rocky planets and an outer set of gas giants. These other solar systems, in other words, would be just like ours. And like the hypothetical butterfly collector in our analogy, we would begin our exploration of the worlds beyond our solar system with the wrong paradigm in mind, and, again like that hypothetical collector, we would be overcome by the incredible complexity we found when we actually looked at what is out there.

Planetary surprises were not slow in coming. Before we even got out of our own backyard, the way we looked at our solar system underwent a revolution. We began to see that, instead of a handful of planets in sedate orbit around the Sun, the moons of the outer planets constitute a group of diverse worlds in their own right. One of them, Jupiter’s moon Europa, turned out to have a vast ocean of liquid water under its icy exterior, a fact that instantly made it a target for scientists interested in finding life away from Earth. Since that early discovery, such interior oceans have been found on other Jovian moons; on Enceladus, a moon of Saturn; and perhaps even under the frozen surface of Pluto. Instead of being a rarity found only on Earth, liquid water appears to exist in many other places even within our own solar system. The paradigm that told us that water has to be in surface oceans, as on Earth, was just wrong.

Things got more curious as we started exploring the outer reaches of our system. We’ll touch briefly on the silliness involved in the demotion of Pluto in chapter 4, but the fact of the matter is that Pluto is actually the gateway to a whole new part of the solar system. Called the Kuiper belt after the Dutch astronomer Gerard Kuiper (1905–73), who suggested its existence in 1951, this is a flat disk of material that extends out beyond Pluto. We have known about the belt for a long time, but it was usually considered a kind of afterthought to the inner planets. Indeed, one of the authors of the book you are holding (James Trefil, hereafter JT) once compared it to a scrap pile left at a construction site after the important building was done.

This attitude changed quickly when astronomers discovered that, far from being an inconsequential pile of rubble, the Kuiper belt is actually home to an incredible variety of planets. Some of these planets are the size of Pluto, and some even have moons. Today, some astronomers estimate that dozens of planets may be lurking out there, a number that completely dwarfs the familiar inner group that includes Earth. Even before we left the solar system, in other words, the simple paradigm of nine planets orbiting the Sun was breaking down. Instead of being a lonely, demoted outsider, Pluto became the beginning of a previously unknown collection of worlds.

The Search for Exoplanets

Our search for planetary systems circling other stars has a long history. We’ll discuss the daunting problems involved in this search in chapter 3. Even so, as you might guess, when we finally nailed down the existence of such a system in 1992, the discovery came as a complete surprise. The new planets, which are indubitably there, turned out to be circling the wrong kind of star, a kind of star called a pulsar. Pulsars are small, unbelievably dense, rapidly rotating masses of matter left when a large star explodes in a supernova. These supernova events mark the end of the line in the evolution of some types of stars. The titanic explosion blows huge amounts of material out into space, and you would expect that any planet unfortunate enough to be in orbit around such a star would be completely destroyed. Yet here these planets are, where no planet ought to be.

If the pulsar planets were the first surprise, the detection of planets circling normal stars was the next. The technique originally available for exoplanet detection, described in detail in chapter 3, involved measuring the small motion of the star ascribable to the gravitational pull of its planet. Such a technique is best at detecting large planets—those capable of exerting strong gravitational pulls on their star. Someone observing our own solar system with this technique, for example, would see the effects of Jupiter before he or she (or it) saw the effects of Earth.

In any case, when this technique was used to search for exoplanets, the first positive results were the discovery of what came to be called hot Jupiters. These are massive planets—typically several times larger than Jupiter—orbiting close to their stars, often closer to their stars than Mercury is to ours. But according to the paradigm that other solar systems should be like ours, this was impossible. Gas giants such as Jupiter were supposed to form only far away from their star, not close in. Another surprise; another failure of the paradigm. As the collection of hot Jupiters grew, astronomers began to wonder if any system out there is like ours.

As it turned out, they need not have worried. The fact that we were finding hot Jupiters first was simply a result of the detection system available. The situation changed with the launch of the Kepler satellite in 2009. We’ll describe this incredible instrument in more detail in chapter 5, but basically it searches for the small dimming of a star’s light due to the passage of a planet across the star’s face.

It’s important to realize that this type of search will be successful only if the orbit of the planet is oriented so that the planet passes between its star and Earth. A planet whose orbital plane is perpendicular to that line of sight is invisible. Also, the satellite searched only a small segment of the sky—think of it as searching an area a couple of times bigger than a full moon. Despite the limited nature of the search, however, Kepler found over 4,000 exoplanet systems in its four years of operation.

Talk about surprises! The first surprise that came from the Kepler satellite was the sheer number of exoplanets out there. Extrapolating from the small volume that Kepler searched to the entire galaxy, astronomers quickly realized that the Milky Way must contain more planets than stars. Far from being a rare event, in other words, the formation of planetary systems seems to be pretty much the norm. Like the butterfly collector in our example, we are having to adjust to the notion that the universe is a lot more complex and diverse than we imagined.

After that initial shock, surprises continued to emerge. As we refined our detection techniques, all sorts of new and strange worlds began to show up. Hot Jupiters faded into the background and a complex array of planets came into sight. These are discussed in detail in later chapters, but the new assortment of planets includes:

• Super Earths—rocky planets several times the size of the Earth. There seem to be a lot of these out there.

• Styrofoam worlds—planets so light that we cannot figure out why they don’t collapse under their own gravity.

• Diamond planets—planets made of pure carbon, with diamond mantles and cores of liquid diamond, a material unknown on Earth.

• Multistar worlds—planets that circle up to four stars, systems that were supposed to be dynamically impossible.

• Hot Earths—worlds so close to their stars that their surface rocks are vaporized. When such a planet rotates, snowflakes made of solid rock fall from the sky.

• Rogue planets—planets wandering around unattached to stars. It is possible that the majority of planets in the galaxy are of this type.

Faced with this incredible (and growing) diversity, we have to give up our old ideas about how planetary systems form and recognize that our own system is only one of many types that can exist. We must, in other words, develop new paradigms to deal with what we are learning about exoplanets.

As this list of strange worlds grows, we have begun to realize that the intense concentration on what has come to be called the Goldilocks planet was simply misplaced. The Goldilocks planet is a hypothetical body that, like Earth, is situated in a position near its star that makes it not too hot, not too cold, but just right. By just right, we mean that it can have oceans of liquid water on its surface. The reason for this concentration, of course, is that we might expect such a world to be the home of life like ours.

What about Life?

And this brings us to the issue that generates the most interest in exoplanets: the question of whether any of these new worlds is a home to life. Once we turn our attention to the existence of life, however, we have to realize that we are once again confronting the curse of the single example. We know of only one type of life, the result of only one experiment. At the most basic molecular level, every living thing on Earth is descended from a single first cell and operates through the use of the same genetic code, the same basic DNA structure. At the molecular level, you have a lot more in common with the grass on the lawn than you might think. As we did when we first began exploring the realm of exoplanets, we approach the question of life with the assumption that whatever we find out there (if anything) will be like us to some degree.

We can think of the origination of life on Earth as occurring in two stages, rather like gears shifting in a car. The first stage was the development of the first living cell from inorganic materials, and the second was the process by which that first cell produced the diversity of living forms we see around us today.

We actually have a pretty good notion of how life evolved on Earth once the first cell showed up—it’s contained in the theory of evolution. Some of the pieces of the puzzle involved in how that first cell developed are in place, and intense research efforts are being carried out to fill in the gaps. We know that life established itself on our planet 3.5 billion years ago, and that for the next 3 billion years Earth was a pretty dull place. An extraterrestrial visiting Earth then would have found a planet whose oceans were full of green pond scum. It is only in the past half billion years that complex multicelled life showed up, with intelligence and technology appearing much later than that. We can expect, then, that even if we do find life on an exoplanet, the discovery will most likely be that of a pond scum planet.

The prevailing paradigm is that any life we find out there will be carbon based and will operate in a way similar to that of life on Earth, although not necessarily with the same molecules. If life is based on molecular chemistry, as it is on Earth, there will have to be some molecular mechanism that plays the same role as DNA in passing genetic information from one generation to the next. Such a molecule will have to be large and complex, and, so the argument goes, it will have to involve carbon chains. Carbon chemistry proceeds most quickly in liquid water, and this explains why we are searching for the Goldilocks planet.

Yet even if we confine our attention to molecular-based life, the sheer number and variety of exoplanets suggest that we should be prepared for surprises, for patterns that we don’t see on Earth. To mention just one example, Earth’s pattern of natural selection and evolution is driven in part by the fact that plate tectonics is constantly shifting the geography of the planet, constantly changing ecosystems. This means that organisms are constantly playing catch-up, constantly trying to adapt to new realities. It has been suggested, for example, that the development of upright posture and intelligence in early humans was driven by the drying up of rain forests in north-central Africa millions of years ago. We can ask, however, what evolution would look like on a world without a constantly changing surface. Would it come to a stop? Would the progression in complexity we see in Earth’s fossil record show up on such a world? Would intelligence and technology evolve? Somewhere out in the array of exoplanets are the answers to questions such as these.

There are deeper questions we can ask, too: Does life really have to be based on molecular chemistry? Does it have to evolve according to the dictates of natural selection, as it does on Earth? It has become a standard quip among scientists that life is like pornography—we can’t define it, but we know it when we see it. We argue that this may not be true and try to stretch our imaginations by suggesting the possibility of entities that are (arguably) alive but are not like us. In chapter 12, we suggest that, just as we needed a new paradigm to deal with exoplanets, we will need a new paradigm to deal with life—a paradigm that inevitably takes us away from the Goldilocks planet and toward something much richer and more exciting.

The marvelous variety of planets actually raises an old question known as the Fermi paradox. Named after the Italian American physicist Enrico Fermi (1901–54), it involves an incident in which, after hearing an argument that the galaxy should be full of advanced technological civilizations, he asked a simple question: Where is everybody? Given the rich variety of worlds we know to be out there, why do we seem to be alone?

In the end, this might be the most important question raised by our new