Summers Michael E - Exoplanets: diamond worlds, super earths, pulsar planets, and the new search for life beyond our solar system

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

Shakespeare, Hamlet, Act 1, Scene 5

T he 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 theresystems 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 somethingbe it a planetary system or a butterflythe 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 butterflysmall, white, and nonmigratoryyou might understandably be confused. Some of your scientific colleagues might even argue that what you were seeing wasnt 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 paradigmthe notion that there was only one kind of butterflywas 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 humanitys 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, Jupiters 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. Well touch briefly on the silliness involved in the demotion of Pluto in , 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 (190573), 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. Well discuss the daunting problems involved in this search in . 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.