BENN6189_05_C13_PR5_V3_TT.QXD 10/31/07 8:03 AM Page 394 13 Other Planetary Systems The New Science of Distant Worlds LEARNING GOALS 13.1 Detecting Extrasolar Planets 13.3 The Formation of Other Solar Systems ◗ Why is it so difficult to detect planets around other stars? ◗ Can we explain the surprising orbits of many extrasolar ◗ How do we detect planets around other stars? planets? ◗ Do we need to modify our theory of solar system 13.2 The Nature of Extrasolar Planets formation? ◗ What have we learned about extrasolar planets? ◗ How do extrasolar planets compare with planets in our 13.4 Finding More New Worlds solar system? ◗ How will we search for Earth-like planets? 394 BENN6189_05_C13_PR5_V3_TT.QXD 10/31/07 8:03 AM Page 395 How vast those Orbs must be, and how inconsiderable Before we begin, it’s worth noting that these discoveries this Earth, the Theatre upon which all our mighty have further complicated the question of precisely how we Designs, all our Navigations, and all our Wars are define a planet. Recall that the 2005 discovery of the Pluto- transacted, is when compared to them. A very fit like world Eris [Section 12.3] forced astronomers to recon- consideration, and matter of Reflection, for those sider the minimum size of a planet, and the International Kings and Princes who sacrifice the Lives of so many Astronomical Union (IAU) now defines Pluto and Eris as People, only to flatter their Ambition in being Masters dwarf planets. In much the same way that Pluto and Eris of some pitiful corner of this small Spot. raise the question of a minimum planetary size, extrasolar —Christiaan Huygens, c. 1690 planets raise the question of a maximum size. As we will see shortly, many of the known extrasolar planets are con- siderably more massive than Jupiter. But how massive can a planet-like object be before it starts behaving less like a little more than a decade ago, all of planetary sci- planet and more like a star? In Chapter 16 we will see that ence was based solely on the study of our own objects known as brown dwarves, with masses greater than Asolar system. Then, beginning in 1995, a dramatic 13 times Jupiter’s mass but less than 0.08 times the Sun’s change occurred as scientists began to detect planets mass, are in some ways like large jovian planets and in other around other stars. More than 250 such planets were already ways like tiny stars. As a result, the International Astronom- known by 2007, and new discoveries are coming rapidly. ical Union defines 13 Jupiter masses as the upper limit for We are even beginning to learn about the characteristics a planet. of these distant worlds. The discovery of planets around other stars represents a triumph of modern technology. It also has profound ◗ Why is it so difficult to detect philosophical implications. Knowing that planets are com- mon makes it seem more likely that we might someday planets around other stars? find life elsewhere, perhaps even intelligent life. Moreover, We’ve known for centuries that other stars are distant having many more worlds to compare to our own vastly suns (see Special Topic, p. 386), making it natural to sus- enhances our ability to learn how planets work and may pect that they would have their own planetary systems. The help us better understand our home planet, Earth. nebular theory of solar system formation, well established The study of other planetary systems also allows us by the middle of the 20th century, made extrasolar planets to test in new settings our nebular theory of solar system seem even more likely. As we discussed in Chapter 8, the formation. If this theory is correct, it should be able to ex- nebular theory explains our planetary system as a natural plain the observed properties of other planetary systems consequence of processes that accompanied the birth of as well as it explains our own solar system. In this chapter, our Sun. If the theory is correct, planets should be com- we’ll focus our attention on the exciting new science of mon throughout the universe. But are they? Prior to 1995, other planetary systems. we lacked conclusive evidence. Why is it so difficult to detect extrasolar planets? You already know part of the answer, if you think back to the Detecting Extrasolar Planets Tutorial, Lessons 1–3 scale model solar system discussed in Chapter 1. Recall that on a 1-to-10-billion scale, the Sun is the size of a 13.1 Detecting grapefruit, Earth is a pinhead orbiting 15 meters away, and Jupiter is a marble orbiting 80 meters away. On the Extrasolar Planets same scale, the distance to the nearest stars is equivalent The very idea of planets around other stars, or extrasolar to the distance across the United States. In other words, planets for short, would have shattered the worldviews of seeing an Earth-like planet orbiting the nearest star besides many people throughout history. After all, cultures of the the Sun would be like looking from San Francisco for a western world long regarded Earth as the center of the uni- pinhead orbiting just 15 meters from a grapefruit in Wash- verse, and nearly all ancient cultures imagined the heavens ington, D.C. Seeing a Jupiter-like planet would be only a to be a realm distinct from Earth. little easier. The Copernican revolution, which taught us that Earth The scale alone would make the task quite challeng- is a planet orbiting the Sun, opened up the possibility that ing, but it is further complicated by the fact that a Sun- planets might also orbit other stars. Still, until quite re- like star would be a billion times as bright as the light cently, no extrasolar planets were known. In this first sec- reflected from any planets. Because even the best tele- tion we’ll discuss why the detection of extrasolar planets scopes blur the light from stars at least a little, the glare presents such an extraordinary technological challenge of scattered starlight would overwhelm the small blips and how astronomers have begun to meet that challenge. of planetary light. chapter 13 • Other Planetary Systems 395 BENN6189_05_C13_PR5_V3_TT.QXD 10/31/07 8:03 AM Page 396 SPECIAL TOPIC How Did We Learn That Other Stars Are Suns? Today we know that stars are other suns—meaning objects that produce enough energy through nuclear fusion to supply light and heat to orbiting planets—but this fact is not obvious from looking at the night sky. After all, the feeble light of stars hardly seems comparable to the majestic light of the Sun. Most ancient observers guessed that stars were much more mundane; typical guesses suggested that they were holes in the celestial sphere or flaming rocks in the sky. The only way to realize that stars are suns is to know that they are incredibly far away; then, a simple calculation will show that they are actually as bright as or brighter than the Sun [Section 15.1]. The first person to make reasonably accurate estimates of the distances to stars was Christiaan Huygens (1629–1695). By assuming that other stars are indeed suns, as some earlier astronomers had guessed, Huygens successfully esti- mated stellar distances. The late Carl Sagan eloquently described Christiaan Huygens (1629–1695) the technique: Huygens drilled small holes in a brass plate, held the plate up to the Sun and asked himself which hole seemed as bright as he remem- his results explained a fact known since ancient times: Stellar par- bered the bright star Sirius to have been the night before. The hole allax is undetectable to the naked eye [Section 2.4]. Recall that was effectively 1 the apparent size of the Sun. So Sirius, he rea- 28,000 the lack of detectable parallax led many Greeks to conclude that soned, must be 28,000 times farther from us than the Sun, or about Earth must be stationary at the center of the universe, but this half a light-year away. It is hard to remember just how bright a star lack also has an alternate explanation: Stars are incredibly far is many hours after you look at it, but Huygens remembered very away. Even with his original estimate that Sirius was only half a well. If he had known that Sirius was intrinsically brighter than the light-year away, Huygens knew that its parallax would have been Sun, he would have come up with [a much better estimate of] the far too small to observe by naked eye or with the telescopes avail- right answer: Sirius is [8.6] light-years away.* able at the time. Huygens thereby “closed the loop” on the an- Huygens could not actually prove that stars are suns, since cient mystery of the nature of stars, showing that their appear- his method was based on the assumption that they are. However, ance and lack of parallax made perfect sense if they were very distant suns. This new knowledge apparently made a great im- *From Cosmos, by Carl Sagan (Random House, 1980). Sagan pression on Huygens, as you can see from his quotation on the demonstrates the technique in the Cosmos video series, Episode 7. top of p. 385. As recently as the early 1990s, these challenges made Butler of San Francisco State University.1 More than 250 even some astronomers think that we were still decades other extrasolar planets have been discovered since that away from finding extrasolar planets. But human ingenuity time, many of them by these same teams of astronomers.