Strange New Worlds: The Search for Alien Planets and Life Beyond Our Solar System - Ray Jayawardhana (2011)
Chapter 4. Planet Bounty
Hot Jupiters and other Surprises
It had been a long time coming. There were the many decades of failed attempts and refuted claims, of denied research grants and scarce telescope time, and of painstaking refnements to the instruments and software. Finally, in the fall of 1995, astronomers announced de-fnitive evidence of the frst planet orbiting a normal star other than the Sun. It was an unusual beast in an unexpected location—a gas giant a hundred times closer to its star than Jupiter is to the Sun—raising serious doubts about its nature and fueling a sharp debate about its very existence. Within a year, there were six more, all found with the Doppler technique, by measuring subtle velocity shifts in the parent star’s spectrum of light. The new discoveries marked the culmination of an age-old quest and turned planet hunters into media stars practically overnight. Yet they also raised new questions about the planetary birth process, revealed odd behaviors not seen among the Earth’s brethren, and challenged our preconception of the solar system as the norm. Fifteen years later, the pace of discovery continues unabated, as astronomers scour the skies targeting a wide variety of stars with ever-more sophisticated instruments mounted on telescopes around the globe, unraveling a hitherto-unimagined diversity of worlds.
The star-hugging orbits of the frst extrasolar giant planets came as a surprise to most scientists. But one person had anticipated them decades earlier. You could say that astronomy was in Otto Struve’s blood. His prolifc great-grandfather Wilhelm published 272 works, had eighteen children, and founded the Pulkovo Observatory near St. Petersburg in 1839. His grandfather and uncle were prominent astronomers as well. Struve was born in 1897 in Ukraine, where his father served as the director of a university observatory. He enlisted in the Russian Imperial Army during World War I and later fought with the White Army against the Bolsheviks during the Russian civil war. Wounded and on the losing side, he went into exile in Turkey, where he ate at soup kitchens and found work as a lumberjack for a while. Later, through family connections in Germany, he landed a job offer at Yerkes Observatory near Chicago as an assistant for stellar spectroscopy. The Yerkes director was “perfectly willing to take him on his lineage,” even though Struve confessed that “I am only marginally familiar with the area of astronomical spectral analysis and that I practically have never worked in that area.” He did not disappoint: after obtaining a PhD at Chicago, Struve went on to become one of the world’s foremost experts in stellar spectroscopy. He also played a key role in the growth of American astrophysics, becoming director of Yerkes himself, helping establish McDonald Observatory in Texas, and serving as editor of the Astrophysical Journal.
Struve believed that planets and life were common in the universe. He suggested that most normal stars rotate slowly because much of their initial spin (or angular momentum) had been transferred to the orbital motion of planets. In a remarkable two-page paper published in the Observatory in 1952, he wrote, “there seems to be no compelling reason why the hypothetical stellar planets should not, in some instances, be much closer to their parent stars than is the case in the solar system.” He added, “We know that stellar companions can exist at very small distances. It is not unreasonable that a planet might exist at a distance of 1/50 astronomical unit. . . . Its period around a star of solar mass would then be about 1 day.” He concluded that such planets could be detected from radial velocity measurements or from observing “eclipses” of the star by the planet (see chapter 5), correctly prognosticating two of the most successful exoplanet detection methods decades before anyone else.
Success . . .
By the 1990s, scientists had pretty much forgotten Struve’s suggestion about close-in giant planets. Or perhaps they treated it as pure speculation, without a physical basis. A few theorists had considered the possibility of planets migrating from their birthplaces, but planet hunters were searching for Jupiter-like planets in Jupiter-like orbits, taking years if not decades to circle their stars. So when Michel Mayor and Didier Queloz of Geneva Observatory announced the discovery of a Jovian planet in a four-day orbit at a workshop in Florence, Italy on October 5, 1995, the reaction was mostly astonishment.
Mayor took on Queloz as a PhD student fve years earlier to help build a new spectrograph for the 1.93-meter telescope at the Observatoire de Haute Provence in France. His team had been using an older instrument on a 1-meter telescope at the same observatory to survey binary stars. Now they wanted to achieve higher velocity precision to look for punier companions. “It was the natural prolongation of our study of binary stars, but with the new instrument we would have the possibility to access giant planets.” Mayor told me. “We were pragmatic. . . . We wanted to look for whatever low-mass companions were out there,” said Queloz. In their design, unlike in the setups of the Canadian and American teams, starlight did not pass through a gas cell (see chapter 3). Instead, one optical fber fed starlight into the spectrograph while another fber brought in light from a thorium-argon lamp. The spectra of the star and the lamp were recorded simultaneously, allowing the researchers to measure subtle shifts in the stellar lines against the “fxed” lines due to thorium and argon. The Swiss team tested the new instrument in 1993. “It worked even better than we had hoped,” Queloz explained. “We realized that our precision was good enough to look for giant planets.” The following year, they started a program to monitor the radial velocities of 140 nearby Sun-like stars. Most of the stars were stable, but a handful showed changes in velocity well above the measurement error of 15 meters per second. The variations of one star, 51 Pegasi, implied a Jupiterlike planet in a 4.2-day orbit, Queloz realized in the fall of 1994. “At frst I didn’t believe it was real. I was convinced there was a bug somewhere,” he recalled. “The instrument was brand new, and we didn’t know if it was misbehaving in some way.” But the stability of the other stars gave him confdence. In February 1995 he faxed Mayor, who was on sabbatical in Hawaii, a note about this strange planet in a frenzied orbit along with a plot of the measured velocities. “Michel’s response was ‘Why not?’ ” Queloz told me.
Figure 4.1. The velocity curve of the star 51 Pegasi as it wobbles toward and away from us every 4.2 days in response to its planet. Credit: M. Mayor and D. Queloz (Geneva Observatory)
When Mayor returned to Geneva in the spring, the two did more checks of the data. The detection held up. But, to be safe, they decided to wait until they could observe the star again that summer. In July, they monitored 51 Pegasi almost continuously for eight nights. The new data confrmed that a planet at least half the mass of Jupiter circled its star at a distance much closer in than Mercury is to the Sun. “At that point we opened a bottle of champagne and had a little private celebration with our families,” Queloz recalled. The planet’s extreme proximity to its star would make for a world of scorching temperatures—over 1000 degrees Celsius— quite unlike anything seen or imagined until then. Just a few months earlier, the prominent planet-formation theorist Alan Boss of the Carnegie Institution of Washington had published a paper in Science predicting that extrasolar giant planets would be found around low-mass stars in orbits more or less similar to Jupiter’s. Here Mayor and Queloz were about to challenge that dogma in the most dramatic way.
First they wanted to be doubly sure, especially given the unexpected nature of their planet. So they spent the rest of July and August trying to rule out other possibilities. If the star pulsates, alternately expanding and contracting like a human heart, that could cause Doppler shifts too. As a star expands, its surface moves toward us, producing a slight shift to the blue in its spectral lines; when it contracts, the surface moves away, resulting in a shift to the red. If 51 Pegasi is indeed pulsating, its brightness should also vary, rising as it expands and fading as it contracts. Mayor and Queloz asked other astronomers to monitor the star to test this scenario. Since the star stayed steady, they ruled out pulsations as the cause of the observed Doppler shifts. Could the culprit be starspots instead? Dark areas on the star that rotate in and out of view as the star spins on its axis also could produce small velocity shifts. The presence of large starspots, as in the case of sunspots, would imply strong magnetic activity, especially on a fast-spinning star as 51 Pegasi would have to be to account for the four-day period. Such activity gives rise to certain characteristic lines in the spectrum, which were not seen in this case. So the starspot scenario was also ruled out.
There remained one last worry: the companion is indeed real, but it is much bigger than half the mass of Jupiter, placing it well beyond the planetary regime. After all, Doppler measurements only give us the minimum mass of a companion. Depending on the tilt of its orbit relative to the sky, the actual mass could be a lot higher than the minimum. Even a faint stellar companion would produce only small radial velocity changes if its orbit is oriented nearly pole-on (because the wobble would be on the plane of the sky, rather than toward and away from us). Mayor and Queloz calculated the odds: there was a 1 percent chance that the companion is more massive than four times Jupiter, and a one-in-40,000 chance it is massive enough to be a red dwarf star. Still, with 140 stars in their sample, there was some chance that one of them would happen to have a companion in a pole-on orbit. Luckily, the spectra of 51 Pegasi could be used to check. The rotation of a star smears out spectral lines, because half the star is moving toward us, producing a tiny blue shift, while the other half moves away, causing a tiny red shift. That is unless the star is seen pole-on: in that case, no part of it is moving toward or away from us. Mayor and Queloz found that the spectral lines of 51 Pegasi were somewhat smeared out, thus confrming the star was not being viewed pole-on.
Convinced that 51 Pegasi’s companion is indeed a planet, they rushed a paper to Nature at the end of August and monitored the star again for eight nights in September. By early October, they received reports from three anonymous referees that the journal had consulted: two were positive while the third was negative. Still, there were no showstoppers among the objections raised by the referees. So they decided to present their discovery at the Ninth Cambridge Workshop on Cool Stars, Stellar Systems and the Sun in Florence. The news of the announcement spread quickly around the globe. “Most people were skeptical. . . . They thought it’s crazy to have a Jupiter so close to its star,” Que-loz told me. “The expectation was to fnd giant planets in long period orbits, like in our solar system—we had challenged that paradigm.”
Within a week, Geoffrey Marcy and Paul Butler started monitoring 51 Pegasi at the Lick Observatory in northern California. So did a team led by Robert Noyes of Harvard and Timothy Brown, then at the High Altitude Observatory in Colorado, with a telescope in Arizona. Both teams quickly confrmed the Swiss pair’s fnding. When the discovery paper came out in Nature on November 23, it included a note thanking the other two teams for the independent confrmation.
But how did such a planet come to be? Its mass was comparable to that of Jupiter, and its orbit was circular like the orbits of giant planets in the solar system. But “this certainly does not imply that the formation mechanism of this planet was the same as for Jupiter,” Mayor and Queloz wrote in their paper. Planet formation models did not allow for gas giants to form so close to their stars: the temperatures are too high for the material needed to form the planet cores to remain solid. On the other hand, it was not clear that the orbit of a planet formed at 5 astronomical units could shrink enough to carry it a hundred times closer to the host. The researchers wondered whether it could be a brown dwarf, sort of a failed star (see chapter 6), which formed as a close companion to 51 Pegasi—since tight stellar binaries are common—and somehow lost its envelope through evaporation.
Figure 4.2. The star-hugging orbit of the 51 Pegasi b planet in comparison to planets in the inner solar system.
Prompted by the exciting discovery, theorists went back to the drawing boards. Douglas Lin and Peter Bodenheimer of the University of California at Santa Cruz along with Derek Richardson (then) of the Canadian Institute for Theoretical Astrophysics in Toronto suggested that the planet had spiraled inward from its birthplace farther out through interactions with the remnants of a protoplanetary disk. It is an idea Lin and a few others had frst considered in the 1980s. The surfing planet may have come to a halt at the disk’s inner edge, or the star’s tidal forces may have put the brakes on it. Other theorists, like Frederic Rasio and Eric Ford, both then at the Massachusetts Institute of Technology,
proposed that interactions between two or more planets could propel one inward while boosting another’s orbit or even kicking the latter out of the system altogether. Model calculations by Tristan Guillot and his colleagues (then) at the University of Arizona showed that a fully formed gas giant planet could survive extremely close to a Sun-like star without losing much of its envelope.
Meanwhile, Marcy and Butler mined their observations of 120 stars from the Lick radial velocity survey. In part due to limited computing power, they had not searched thoroughly for periodic signals in the entire dataset until now. In January and February 1996, they announced two planets of their own discovery. The frst, around the star 70 Virginis, weighs over six times more than Jupiter and takes almost four months to complete its oval-shaped orbit. The other, a 3-Jupiter-mass object in a circular orbit around 47 Ursae Majoris, seemed more akin to solar system planets. That summer they announced three more planets—orbiting 55 Can-cri, tau Bootis, and upsilon Andromedae—all of them “hot Jupiters,” as the new class of close-in giant planets came to be called. One more announcement came in the fall. The University of Texas astronomers William Cochran and Artie Hatzes, observing at McDonald Observatory, and the California team independently found a 1.5-Jupiter-mass planet in a highly elongated orbit around 16 Cygni B, a member of a triple star system.
Suddenly, the foodgates were open. Some astronomers sounded positively giddy as they talked about the dawn of a new golden era. The breathtaking pace of discovery—seven planets in one year—fed the public imagination. “Despite years of searching with the most powerful telescopes, despite decades of listening for the faint crackle of radio signals from distant civilizations, despite endless theorizing about how life might or might not arise, nobody had ever found concrete evidence to suggest that our planet, our civilization, our life-forms were anything but unique in the cosmos. Now, suddenly, everything has changed,” wrote veteran science journalist Michael Lemonick in Time magazine. “Even if the new planets are sterile, though, their very existence is a powerful piece of astronomical news,” he added. “Other worlds are no longer the stuff of dreams and philosophic musings. They are out there, beckoning, with the potential to change forever humanity’s perspective on its place in the universe,” declared John Noble Wilford, writer renowned for his coverage of the historic Apollo missions, in the New York Times.
Table 4.1 First Planets around Normal Stars
. . . and Controversy
Not everyone was convinced, though. In fact, there were a number of outspoken skeptics in the scientifc community. Some argued the newly found objects must have formed in a very different way from solar system planets. The elongated orbits of two of the companions raised questions: planets born in a disk should end up in circular orbits, due to friction with the surrounding material. In the 16 Cygni B system, the gravitational tug of the companion stars could be responsible for pulling the planet into an elongated orbit, but the same explanation does not apply to the single star 70 Virginis.
Besides, the radial velocity method by itself can only measure the minimum mass of a companion. That is because the tilt of the orbit relative to the sky is unknown.
Even a fairly massive companion would cause only small shifts in the line-of-sight velocity of a star if its orbit happens to be oriented face-on from our vantage point. Thus brown dwarfs or even stellar companions could masquerade as planets, critics such as David Black of the Lunar and Planetary Institute in Houston pointed out. “We could conceivably be viewing [51 Pegasi] system’s orbital plane at a wide angle. If so, 51 Peg B’s mass could well be 10 times greater than the 0.6-Jupiter lower limit. The jury is still out, but at this time I would bet on 51 Pegasi B being a brown dwarf, not a member of a planetary system,” he wrote in the August 1996 issue of the Sky & Telescope magazine.
The planet hunters countered that there was no observational bias toward face-on orbits and that there were too few brown dwarfs around stars to account for the new discoveries. Besides, if the companions were unseen cool stars, they should contribute to the infrared light of the system. Still, the debate raged on for several years. “Science is a pretty conservative system. So I can understand the skepticism,” Queloz told me.
Perhaps the strongest challenge to the planet claims came in February 1997, from David Gray of the University of Western Ontario in Canada. The author of a widely used text on stellar spectroscopy, he had taken some thirty-nine spectra of 51 Pegasi over eight years, during the course of monitoring bright solar-type stars. With all the fuss about a hot Jupiter in its midst, Gray decided the data in his hands deserved close scrutiny. “I was aware for some years that [51 Pegasi] was variable,” he told Sky & Telescope. “This didn’t concern me particularly. But about six months ago, I became aware that the whole planet thing was almost hysterical. [So]
I thought it was incumbent on me to look at my data.” What he found was a spectral line due to iron that tilted alternately toward the blue and the red. The wiggles in the line were suffcient to mimic the radial velocity signal of the 51 Pegasi’s planet, Gray reported in a Nature paper. Orbital motion would not change a line’s shape, only its position. Therefore, he concluded that the star’s own pulsations, of a kind not seen before, must be responsible. “The planet hypothesis is no longer an adequate interpretation of the data,” he wrote, unleashing a frestorm of debate. “Dr Gray’s work leaves a question mark over the presence of other extrasolar planets,” observed Leslie Sage, the astronomy editor at Nature, adding that “No criticism of Drs. Mayor and Queloz is intended or justifed—they did the best they could with their equipment.” His prognostication: “While almost all astronomers agree that there probably are planets around most stars, it will be some time before their presence is demonstrated conclusively.”
Even before Gray’s paper appeared in print, the planet hunters posted a rebuttal on Marcy’s web site at San Francisco State University, raising several objections. First, they referred to a forthcoming paper by Artie Hatzes and colleagues, whose high-resolution data— albeit covering less than the four-day period—failed to confrm the wiggles that Gray’s intermittent observations had revealed. Second, they pointed out that 51 Pegasi’s brightness is extremely constant, whereas oscillating stars should exhibit periodic changes. Third, they noted that the Doppler measurements showed only a single period and amplitude, while pulsating stars, like ringing bells, have a variety of overtones or harmonics.
“The ensuing debate was at times less than civil,” observed journalist James Glanz in Science. “The dispute has thrown light on the underside of a high-stakes feld where new claims are followed like sports scores by the wider public. Astronomers grumbled privately about the attacks on Gray’s work that appeared on an elaborate web site—complete with links to corporate sponsors— maintained by the planet searcher Geoff Marcy of San Francisco State University. Gray responded in kind on his own web site, calling some objections ‘arguments of ignorance.’ ”
The planet sleuths were vindicated a year later. Gray retracted his objection in the January 8, 1997, issue of Nature. The same issue included a paper by Hatzes and colleagues: their extensive new observations, using an instrument with more than twice the resolution of Gray’s, did not fnd changes in 51 Pegasi’s spectral line shapes. Independently, Timothy Brown and colleagues came to the same conclusion in a paper published in the Astrophysical Journal the following year. “The weakness of my 1989-96 observations was that they were spread widely in time. To properly study a period as short as 4.23 days, one should have observations much more closely spaced in time,” Gray conceded on his web site. “None of us sees the profle variations shown by my older data. . . . Therefore, the best conclusion is that the profles do not vary, and presumably the signal seen in my earlier observations was indeed noise no matter how small the calculated probability of occurrence,” he added. His admission perhaps betrayed a slight tinge of bitterness: “This means two things. First, the interesting non-radial oscillations of 51 Pegasi are no longer available with all their potential for revealing the physics of the star. A pity. And certainly a disappointment for many of us. Second, the planet hypothesis is now the frontrunner. People interested in extra-solar-system planets will be pleased.” The quote attributed to Gray in Glanz’s article is even more blunt: “It looks like we’re back to an ugly old planet.”
Wedged uncomfortably in a middle seat at the back of the plane, Debra Fischer had no reason to think it would be a life-changing fight. She was returning home to San Francisco from the Tenth Cambridge Workshop on Cool Stars, Stellar Systems and the Sun held in Boston in July 1997. Growing up in Des Moines, Iowa, she always had an interest in science. As an undergraduate at San Diego State University, she took pre-med classes and “imagined I would become a surgeon someday.” Along the way, however, she fell in love with physics. It was after moving to San Francisco in the mid-1980s with her husband, a cardiologist, that she decided to pursue a master’s degree. Geoff Marcy was a newly minted faculty member at San Francisco State University. After taking a course of stellar astrophysics from him, Fischer signed up to do research on binary stars under his supervision. “When I went to Lick, I was totally hooked. I loved being at the observatory,” she recalled. “Observatories are like monuments to humankind’s curiosity about the universe. Everything else seemed inconsequential in comparison.” She also met Paul Butler, a chemistry student taking physics courses. Butler left for Maryland to earn a PhD and later returned to SFSU as a post-doc with Marcy. By the time all three attended the 1997 Cool Stars meeting, Marcy and Butler were already basking in the limelight as successful planet hunters, while Fischer was wrapping up her PhD thesis at the University of California, Santa Cruz, about how often red dwarf stars come in pairs.
On that propitious fight, Marcy and Butler were seated toward the front. At one point, Marcy came to the back of the plane, to where Fischer was seated, and asked her: “Paul and I wondered if you would like to join the team?” Together with Steve Vogt from UC Santa Cruz, they had begun a radial velocity survey at one of the 10-meter Keck telescopes in Hawaii, and needed someone to take over the Lick effort. Fischer had taken spectra for them occasionally during her Lick runs, so she was familiar with the project. “I literally jumped at the chance,” Fischer told me. “I promised to pour my heart and soul into it.” The next month, she took over the Lick planet search, initially targeting 100 nearby Sun-like stars. “It looked like it was going to be a numbers game, so we increased the sample to 450 stars pretty soon,” said Fischer, who moved back to SFSU as a postdoc after completing her thesis. By now a mother of three children, she kept up a grueling schedule. “I observed at Lick every clear night and worked at SFSU during the day,” she explained.
One of the stars on Fischer’s list was upsilon Androme-dae, which was already known to harbor a Jupiter-mass planet in a 4.6-day orbit. Marcy and Butler had noticed hints of a second planet in the data early on, so she kept close watch to see the putative outer planet complete a full orbit. Meanwhile, a team led by Timothy Brown (then) of Colorado’s High Altitude Observatory and Robert Noyes of Harvard was also monitoring the same star since 1994. The latter team used a 1.5-meter telescope at the Whipple Observatory in southern Arizona.
“One day Geoff called to tell me that Bob Noyes’s team had found a second planet around this star and asked whether we wanted to collaborate on a paper,” Fischer recalled. “Initially, I was a bit disappointed.” She had hoped to confrm the outer planet herself with the Lick dataset spanning eleven years. The two teams agreed to collaborate, and Fischer was tasked with deriving the best-ft parameters of the system. But she failed to get a good match with two planets. “When I subtracted the 4.6-day inner planet and the newly found outer planet in a three-and-half-year orbit, I expected to see just noise. Instead I saw a curve with a period of about 240 days, as if there was an interloper between them,” she said. “I was afraid to tell Geoff. He might think I’m nuts,” she added. She wondered if the system of three planets would be stable. So she called her graduate school buddy Gregory Laughlin, a theorist with a knack for dynamical calculations, for a quick check. “He ran the analysis overnight, and confrmed the three pla nets were in a stable confguration,” she continued. The next morning, she took a printout of her triple-planet-ft to show Marcy. “Walking across campus to meet with Geoff, I felt deeply moved looking at this piece of paper. I thought that if a star could make such a well-packed planetary system, planets must form naturally, robustly. And here I had in my hands the data to prove it.” The two teams jointly announced their discovery of the frst multiple planet system in April 1999.
The “frsts” have continued, with the Geneva and California teams dominating the headlines, even though there are several other notable research groups in the running. The target lists run into the thousands. Michel Mayor’s team extended its hunt to the southern hemisphere, using telescopes at La Silla, Chile, and later installing perhaps the world’s most stable spectrograph at one of them (see chapter 8). The Americans also went south, using the 4-meter Anglo-Australian Telescope, together with British and Australian partners. Scientists raced to fnd lower-mass planets, by improving the precision of their measurements, and planets in wider orbits, by monitoring stars for longer periods. Meanwhile, confrmation that at least one of the companions—a hot Jupiter orbiting HD 209458—is a planet came from observing its “transit” (see chapter 5). Once its size and true mass were measured, any lingering doubts about stars in face-on orbits masquerading as planets all but disappeared. In March 2000, the Keck survey yielded the frst two planets with masses below that of Saturn. Orbiting the stars HD 16141 and HD 46375, they were still hefty beasts at about 70 Earth masses. In 2006, the Geneva group announced a system of three Neptune-size planets around HD 69830, accompanied by a remnant dusty disk.
Figure 4.3. Radial velocity curve of upsilon Andromedae: when multiple planets are present, multiple periods are visible. Here the innermost planet has been removed to clearly show the periods due to the other two planets. Credit: exoplanets.org
Planet discoveries have become routine: announcements often arrive in bunches of half-dozen or more. At a conference in Portugal in October 2009, the Geneva team announced thirty-two new planets at once. The total planet count reached two hundred in 2006 and doubled again by 2010. The vast majority still come from Doppler surveys, though that is likely to change in the near future (see chapter 8). The number of known multiple-planet systems has also grown signifcantly, as have planets in orbits similar to Jupiter’s. One multi-planet system, 55 Cancri, is now known to harbor at least fve members. “The competition used to be quite extreme,” Debra Fischer told me recently. “Now with the enormous number of planets found, that has softened in some ways.” Stephane Udry from the Geneva team concurred: “The race between radial velocity teams has calmed down a bit, because now there are so many planets and so many different aspects to pursue.”
In 2005, Michel Mayor and Geoff Marcy shared the million-dollar Shaw Prize. The citation read: “For fnding and characterizing the orbits and masses of the frst planets around other stars, thereby revolutionizing our understanding of the processes that form planets and planetary systems.” In recent years, there has been feverish speculation about planet hunters being contenders for the Nobel Prize in Physics. Most astronomers agree that the discovery of extrasolar planets ranks among the most important advances of the last century, but some feel it is may not be appropriate for the Physics Nobel because it does not involve “new physics.”
As the planet census grew, interesting trends started to show. Some confrmed our expectations. For example, despite the greater challenge of detecting them via the Doppler technique, lower-mass planets have turned out to be common. At the high end of the scale, however, brown dwarfs—with masses between ten and eighty times that of Jupiter—in tight orbits around stars proved to be rarer than expected, though they are common as free-foating objects (see chapter 6). That led astronomers to suggest the existence of a “brown dwarf desert” in the vicinity of solar-type stars. The implication is that brown dwarfs and planets form through different processes and that smaller worlds are easier to make than bigger ones.
The origin of the elongated orbits, occupied by the majority of known exoplanets, has remained a mystery. In comparison, the Earth’s siblings appear to be the exception rather than the norm in having almost perfectly circular orbits. Friction between newborn planets and the surrounding gaseous disk should circularize planet orbits. How did worlds like the 70 Virginis planet elude this outcome? In some cases, such as 16 Cygni B, the gravitational tug of a companion star may distort the orbit. In other instances, slingshot games between planets may be responsible. After all, comets in our solar system are hurled into elliptical orbits as a result of close encounters with planets. If so, the early days of many planetary systems may be more chaotic than we had imagined, based on the solar system’s well-behaved gas giants. Planets locked into resonance—for example, with the outer planet completing two orbits in exactly the time it takes the inner planet to go around once— also hint at wanderings after birth. Such pairs would have almost certainly captured each other into a stable resonance during migration, rather than being born with simple orbital period ratios. The large population of hot Jupiters also points to a dynamic if not violent youth. As a newborn giant planet spirals in toward a star, many things can go wrong. Its inward migration can push terrestrial planets into the stellar incinerator. Or the giant planet itself could fall in. UC Santa Cruz theorist Douglas Lin has suggested that “infant mortality among planets” may be rampant and that the hot Jupiters we observe today could just be “the last of the Mohicans.”
Moreover, several research groups recently have found hot Jupiters that orbit in the “wrong” direction— opposite to the rotation of their host stars. That’s not the case in our solar system, and is not what we would expect for planets forming in a disk that spins in the same direction as its star. In a number of other cases, a planet’s orbit appears to be tilted relative to the star’s equator. Perhaps most surprisingly, Barbara McArthur of the University of Texas at Austin and her colleagues reported that the orbits of two massive planets around the star upsilon Andromedae are inclined sharply relative to each other, by nearly 30 degrees. In contrast, the eight major planets in our solar system orbit in nearly the same plane. All of this evidence, taken together, implies that planets move around often from their birthplaces. The misaligned orbits point to migration due to gravitational tugs-of-war among multiple planets or between a planet and a distant stellar companion of the host star, rather than due to interactions with the planet-forming disks. In the upsilon Andromedae case, the interactions may have been strong enough to throw out one or more of the newborn planets completely out of the system.
One trend that has intrigued scientists for over a decade is the tendency of giant planet host stars to be richer in “metals”—a term astronomers use to lump together all elements heavier than hydrogen and helium—than the average in the solar neighborhood. The correlation is so strong, especially for hot Jupiters, that planet hunters can increase their chances at least threefold by targeting the most metal-rich stars. So much so that Greg Laugh-lin of UC Santa Cruz drew up a list of twenty bright stars for a quick pilot survey with Fischer at the 1.5-meter telescope at Lick. They nicknamed the stars after heavy-metal bands, for ease of keeping track: stars with the highest metal abundances got names of speed-metal and death-metal outfts like Slayer while less “metallic” ones were named after glam-metal bands like Skid Row. Later, Fischer and Laughlin founded the so-called N2K Consortium to target two thousand metal-rich stars in quick-look mode, monitoring each star for just three nights to pick out those that likely harbor hot Jupiters.
When the trend with metal abundance emerged initially, some theorists, including Lin, suggested that in-falling planets or planetesimals might have enriched the stars’ atmospheres. If that were the case, stars with deeper convection zones should show less of an effect, because the dumped metals would be distributed through a large fraction of the star’s interior, thus diluting their signature. Studies, based on both the Geneva and California samples, rule out the scenario. The best bet now is that the heavy elements are the cause rather than the effect: in other words, the heavy-element content in a star’s protoplanetary disk helps spawn massive planets. The higher the heavy-element abundance, the easier it would be for massive solid cores to build up in time to accrete a gas envelope before the disk disperses (see chapter 2). On the other hand, if a star and its disk are metal-poor, giant planets may not form at all. We do not know yet whether the same requirement applies to forming terrestrial planets like the Earth.
Figure 4.4. Stars with higher abundances of heavy elements seem more likely to harbor close-in giant planets. Credit: D. Fischer (Yale University) and J. Valenti (Space Telescope Science Institute)
At frst, planet hunters focused on stars similar in mass and age to the Sun, avoiding all but the widest binaries. The primary reason was a propensity to search for other planetary systems that resemble ours, thus perhaps harboring conditions suitable for life (see chapter 9). But there were some practical reasons too. Lower mass stars are cooler and dimmer, making it harder to take good-quality spectra of them. One solution is to use bigger telescopes to gather more of a faint star’s light. Indeed, astronomers now use 6-to 10-meter telescopes such as the Keck in Hawaii, the Magellan and the Very Large Telescope in Chile, and the Hobby-Eberly Telescope in Texas to survey red dwarf stars (also known as M dwarfs). The results to date suggest that giant planets are less common around lower-mass stars. That is not surprising, since we know that protoplanetary disks around them contain less raw material (see chapter 2). On the other hand, terrestrial planets may be both common and easier to detect (see chapter 8).
Searching for planets around high-mass stars presents a different set of problems. Hotter stars contain fewer spectral lines. They spin faster too, smearing out those lines. Without a large number of sharp spectral features, the Doppler technique’s precision is compromised. Stellar evolution offers astronomers a way to get around this problem to some extent: they can target stars more massive than the Sun that have already evolved into sub-giants, after exhausting hydrogen in their cores. In the bloated old age, these stars have cool atmospheres with lots of spectral lines, and they rotate slower. As it turned out, one of the frst subgiants found to harbor a planet is gamma Cephei, a 1.6-solar-mass star that caught the attention of Bruce Campbell and Gordon Walker back in the 1980s. The Canadian team announced a 1.7-Jupiter-mass planet in a 2.7-year orbit around it in 1987, and retracted the claim fve years later, fearing confusion due to stellar pulsations (see chapter 3). In 2003, with new data from the McDonald Observatory, a team led by Artie Hatzes and including the Canadians was able to confrm its existence.
John Johnson of the California Institute of Technology is working with Marcy and Fischer to target a few hundred subgiants with the Lick and Keck telescopes. The fndings so far, by his team and others, support the trend of higher-mass stars harboring more giant planets. The likelihood of fnding a Jovian planet within 2 AU is about 1 percent for red dwarfs, 4 percent for Sun-like stars, and 9 percent for stars between 1.3 and 2 solar masses. Interestingly, none of the planets orbiting subgiants is within 0.8 AU of the star, whereas over 40 percent of planets around Sun-like stars are inside that distance. The gap is striking, according to Johnson. It could be that closer-in planets were destroyed as the host star expanded in size. Or, it could be telling us something about how a star’s mass or energy output affects the planet migration process.
Finding planets around young stars could provide valuable insight on the timescales related to the birth and migration of planets. For example, if a hot Jupiter were to be found orbiting a 10-million-year-old star, we will know that it had to fully form and migrate inward from its birthplace within that timeframe. But young stars pose special challenges for Doppler searches. In many cases, their lines are smeared out by fast rotation. Line shapes also vary, due to enhanced activity like st-arspots and magnetic fares. Between 2004 and 2007, using the Magellan telescope in Chile, my colleagues Alexis Brandeker, Duy Nguyen, Marten van Kerkwijk, and I carried out a pilot study of over 400 nearby young stars with ages of 2–40 million years. We observed each target a mere four to six times, not enough to trace companion orbits but suffcient to get a sense of the typical radial velocity scatter due to various sources of noise. We found that for a subset of young stars, those that rotate slower, it may be possible to detect massive close-in planets with the Doppler technique. In fact, in January 2008, a team led by Johnny Setiawan of the Max Planck Institute for Astronomy in Heidelberg, Germany, reported a 10-Jupiter-mass planet orbiting the nearby young star TW Hydrae every 3.6 days. Another European team who did not see the same velocity shifts in infrared spectra of TW Hydrae has called the claim into question, however. The stellar activity tends to affect infrared lines less, so it makes sense to conduct Dop-pler surveys of young stars in the infrared rather than the optical. But the number of suitable instruments is severely limited at present.
The diversity of worlds that has emerged already is absolutely remarkable. Still, with its strong bias toward fnding massive planets in short-period orbits around quiescent middle-aged Sun-like stars, the Doppler technique has revealed only the tip of the proverbial iceberg. If we are to have any hope of getting a sense of the true diversity of worlds out there, we need to marshal all the complementary planet-search techniques at our disposal.