The Life of Super-Earths
Page 4
As soon as the first list of OGLE candidates appeared on the Internet, my younger colleague Kris Stanek walked into my office and challenged me to find planets orbiting the stars. He felt that the Harvard-Smithsonian Center for Astrophysics might have the resources and—most importantly—the expertise to pull this off. I agreed with him about the expertise; after all, the first planet showing transits, HD 209458b, had been our local success just two years earlier. But finding planets on the OGLE list needed a new approach. In fact, the entire transiting method of finding planets needed to be sorted out. The simple recipe of the 1990s—using photometry to look for blinking stars—had not produced any results.
The first hint of what needed to be done came from the OGLE list itself. A few of the transit candidates on it showed changes in their light between consecutive blinks. I had seen this many times before, but it had nothing to do with planets. When two stars, known as binary stars, orbit each other very closely—in orbits similar in size and period to those of the hot-Jupiter planets—the stars literally pull each other into pear-shaped forms. Their asymmetric forms mean that the shape of the surface we distant observers can see differs over the course of the orbit, and, as a result, the amount of light we see varies too. In addition, stars also illuminate each other, and that adds to the light variation. Now, if the two orbiting stars happen to be aligned just right, we also see the stars eclipse each other. There was only one problem—stars are large and their eclipses are very deep (so deep that the first such binary star was discovered by the unaided eye of John Goodricke in 1782).10 The OGLE eclipses were ten times smaller; how could that be?
At this point my background in stellar physics came in handy. These OGLE candidates were certainly eclipsing stars, but with one of two possible differences from the typical binary pair. Either a third star was also in the picture or a very small star was in an orbit with a very big star. In the first case, the third star washes out the depth of the eclipse, making it appear shallow; in the second case the eclipse is shallow to begin with. One or two of the stars in the OGLE list even showed the telltale sign of a mismatched pair of stars, as there were barely visible eclipses due to the smaller star. The problem was figuring out whether the rest of the OGLE list was composed of similar eclipsing binaries as well.
The resolution to our primary problem—distinguishing false positives from real transiting planets—lay in fifty years of understanding of how stars work, known as the theory of stellar evolution. Stars of different masses have highly predictable temperatures and luminosities at any given age, and binary stars are of the same age by definition. Convinced that this basic stellar knowledge could be used to solve the problem, I began to lay out the steps needed to confirm that a blinking star was in fact being dimmed by a transiting planet.
I am a theorist—about stars—and although I love using telescopes and their instruments, I wasn’t able to manage the OGLE challenge by myself. The problem needed a team. Guillermo Torres (a.k.a. Willie), an expert on binary stars and spectroscopic observation, was ready for the challenge, and we agreed that we should talk to one of our graduate students who knew how to do spectroscopy of faint stars, or their explosions, as it happens. That student was Saurabh Jha, who then was spying on very distant supernovae to understand dark energy. Saurabh was already excited about extrasolar planets; he had collaborated with our senior graduate student, David Charbonneau, in observing the transits of HD 209458b. In the meantime, Dave had already been building his own telescope for seeking out transiting planets, while working at Caltech.
Willie, Saurabh, and I got to work right away. Our new method involved multiple steps. After using photometry to identify the potential transits, we obtained a single low-resolution or medium-resolution spectrum of the star, in order to identify whether it was a massive star. If it was, we excluded it. If it was not, we obtained more spectra, to look for a large Doppler-shift wobble. A large wobble would indicate that the “transits” are due to a star, not a planet. Next, we used more sensitive instruments to look for a wobble due to a planet. That step required the largest telescopes on Earth, with the best spectroscopy possible. If we detected the small wobble, the next step was to analyze the spectra for distortions in the absorption spectral lines. If such distortions were not present, and no second set of spectral lines was visible either, we put all the accumulated observations together and compared them to the predictions of the range of stellar models with different possible configurations of foreground and background intervening stars. At the end of the day, if all this cohered consistently, the planet was confirmed, and its size and mass were precisely determined.11 Willie and I also had to prepare the stellar and binary star models that we would need to analyze the systems and determine if they were stars or planets.
But at Harvard we had no access to a large telescope with a precise spectrograph for Doppler shift measurements of the very faint OGLE stars. Only the largest—the Keck telescope—would do, and the lion’s share of observing time belonged to the partner institutions, the University of California and Caltech, that had built it. Fortunately, I had an ongoing collaboration on both Keck telescopes with colleagues at Caltech, the wizards of optical astronomy. At Caltech, Maciej Konacki—a young researcher working with my colleague Shri Kulkarni, and also a Pole (like the rest of the OGLE team)—was more than excited and ready to bring in the Keck at the last step of our transiting method. The Keck instrument—the old reliable HIRES spectrograph designed by Steve Vogt and used by Geoff Marcy to discover many extrasolar planets with the Doppler shift method—would be made available to us, and Maciej made sure we completed the crucial last step for planet discovery.
We had a busy summer, first doing the observations in Chile and Hawaii, where the Keck observatory sits atop Mauna Kea on the Big Island, and then analyzing the data as fast as possible in between. We were in a hurry not only because we knew that there was a race and the competition was fierce, but also because the core of our idea was to complete our steps in the right order, eliminating false positives along the way, and to make the best use of the precious time on Keck at the end. The results from the first few steps, done in Chile, were stunning: most of the OGLE transiting candidates were not transits and not planets, eliminating more than 90 percent of the “planets” on the OGLE list.
Fortunately we still had five candidates left for the observations at Keck. We were optimistic, but it was already clear that the initial high expectations, that more than half of the transiting candidates could be planets, were severely dashed; what’s more, at least two more tests remained. It wasn’t impossible that all the candidates would have to be struck from the list. In retrospect, we were very glad to see that our transiting method was working so far—for the first time I felt that we had a clear edge and were ahead of the competition, who were mired in a heap of false positives. If false positives had not been such a major problem, it was conceivable that any of the other competing teams would stumble on a planet from the list by chance and beat us by a month or less in announcing the first one!
The Keck observations went fine, and we seemed to have a clear winner among the four candidate stars we had observed: OGLE-TR-33. It was given this less than poetic name because none of the stars on the list had been observed before, so it was named for its place in the catalog of the team that first observed it. Star 33 had a clear wobble, with an amplitude that corresponded to a very large planet or, more likely, a brown dwarf, the name for a small failed star. Finding a transiting brown dwarf was almost as exciting back then as finding a large planet, so we rushed testing OGLE-TR-33, only to find that it failed the last test. We could not believe it—we had even started writing a paper to the journal Nature, while doing our test and models for a second time. Now we had to abandon it. In the meantime, another one of our top candidates had passed all its tests with flying colors. Initially we had neglected it because OGLE-TR-33 had a clear large wobble and had seemed an easier nut to crack. This star was OGLE-TR-56, and it looked l
ike a Jupiter-mass planet.
As November was ending, we had finally discovered the nature of OGLE-TR-33: it was a system of three stars, two of which orbit each other very closely and eclipse each other, while a large star nearby, the big and brightest in the system, washes out the deep eclipse of the other two. The third star does not have a wobble of its own, but a large wobble of another star in the system (its spectral lines have a large Doppler shift) causes a small distortion in the spectral lines of the third star. Because the third star rotates fast and its spectral lines are broad, that small distortion was just enough to give us the impression of a small wobble, as if due to a planet that matches the shallow washed-out eclipse. OGLE-TR-33 was the ultimate tricky false positive!12
Now we focused our full attention on OGLE-TR-56. It had passed all our tests, including the spectral lines distortion test that had uncovered OGLE-TR-33 as a false positive. We felt very confident that OGLE-TR-56b was a planet precisely because of our experience with OGLE-TR-33 and the other false positives our new transiting method had helped uncover. The method was working, and we quickly got a paper accepted for publication in Nature.13 In the first week of January 2003 I flew to Seattle to present our discovery to the meeting of the American Astronomical Society. Just like Captain James Cook 235 years earlier, we had crisscrossed the Pacific Ocean to catch a glimpse of a transit, and we had succeeded.
To top it off, OGLE-TR-56b was an exotic planet—a record holder in several ways: the shortest known orbital period (only twenty-nine hours), hence the hottest known planet (close to 2000 K), as well as the most distant extrasolar planet (at about 5,000 light-years from Earth). The exotic properties caught the attention of the media, while for us and the planet hunters the biggest excitement was that the transiting method for planet discovery was finally figured out. In short, the unexpected large fraction of false positives had been the major obstacle, and our set of tests and use of stellar models solved the problem. Within the next three years we and several other teams would use our approach successfully to confirm more than a dozen new transiting planets. The path to discovering a true Earth was now open. A new age of exploration was upon us.
The first age of exploration began in the fifteenth century. In 1484 one of the men whose efforts would define the era, Christopher Columbus of Genoa, was trying to convince King João II of Portugal to finance his expedition to cross the Atlantic. The king was reluctant, not because he thought Earth was flat, but because Columbus insisted that it was only 10,000 miles around the equator, and that the westward route to India and the Spice Islands would be short. Portuguese sailors (who, thanks to the support of Prince Henry the Navigator earlier in the century, had sailed up and down the Atlantic by the African coast) had estimated a much larger size for the planet, pole to pole, and had gotten a number much closer to the actual value of about 25,000 miles. (Back in the third century BC, Eratosthenes, a Greek mathematician, had already estimated the same size.) As a result, King João II did not fund Columbus, who then left for Spain. He had the wrong number, but luck was on his side and he stumbled upon the New World.14 The Portuguese, in the meantime, went to India sailing around Africa.
The search for new Earths is no different. Since the late 1990s we have had the knowledge and the technology to do it. We have debated numbers and methods. Now we are sailing and waiting for the day when one of us will shout “Terra!”
Just like the Portuguese sailors, who started exploring nearby areas, planet hunters must do the same. The sailors would venture into the Atlantic and find islands like the Azores or farther down the coast of Africa, or perhaps even get an early glimpse of South America off the coast of modern Brazil. Our team also began small and cheap during the initial “gold rush” on discovering transiting planets. Most notable of our early stakes is the Hungarian-made Automated Telescope, known as the HAT project or network (HATNet, for short). HATNet is led and literally put together by a young colleague of mine, Gaspar Bakos. Gaspar came to the Harvard-Smithsonian Center for Astrophysics as a graduate student at the enthusiastic recommendation of one of my mentors (and Gaspar’s mentor too), Bohdan Paczynski (1940–2006) of Princeton. The rationale for the recommendation was that Gaspar was the person to take advantage of a revolution in digital imaging (cheap CCDs, such as you can find in any digital camera) and precise image processing.15 Together, the two technologies meant we could look at “all the sky, all the time.” Or at least lots of the sky, very often. That, Bohdan believed, could bring a revolution in astronomy. As so often before, Bohdan turned out to be right.
There are many applications for the “all the sky, all the time” approach, but discovering planets by transits is an obviously good one. My colleague Robert Noyes and I thought so and convinced Gaspar as well. Thus HATNet was born on two continents and on a shoestring budget—with photography equipment for telescopes and amateur-grade CCDs, but with professionally designed and machined hardware (in Hungary, Gaspar’s home country) and software.
HATNet comprises six small telescopes (not much different from the large cameras with zoom lenses used by professional photographers), four on Mount Hopkins in southern Arizona and two on Mauna Kea.16 They are automated, following a cleverly written computer program that receives inputs (e.g., priorities for what should be observed) from the astronomers during the day; then they work all night like robots. HATNet shuts down in case it detects too many clouds or inclement weather, and Arizona communicates important updates to Hawaii. Remember that Hawaii experiences sunset and sunrise later than Arizona. Therefore, the two HATNet telescopes in Hawaii begin their work night later and take over fields that the Arizona telescopes can no longer see. By having “eyes” in Arizona and Hawaii, HATNet effectively extends its work night from twelve to fifteen hours, and can catch and see more transits.17 HATNet was designed to discover transiting planets like Jupiter and Saturn at nearby stars. It has found thirty so far, with some of them (e.g., HAT-P-11b) the size of Neptune; what’s more, HATNet and other projects like it have set off a transiting “gold rush.”
Just as anyone could set off for California with a sluicing pan and some grub, the would-be astronomer of today can set herself up to find a new planet simply by maxing out a credit card. (I hope nobody does that literally.) It hasn’t all been shoestring budgets, though, as the big guns, such as NASA and the European Space Agency, did not wait long to join the rush. Although a transit-hunting operation can be set up on a limited budget, the big agencies have a real advantage—they can get things lifted into space. Stars twinkle when seen from Earth because the air moves constantly, and not just in one direction; there are different currents at different altitudes. The air currents act as sheets with multitudes of lenses that shift the point-like images of stars, just like the sunlight playing on the bottom of a swimming pool. Much of this twinkling happens at high frequencies—about once every hundredth of a second. And, as it turns out, all this makes detecting a transit of an Earth in front of a Sun-like star practically impossible even with the largest telescopes. A telescope in space has its own challenges (although as the Hubble space telescope taught us, they are not insurmountable), but to find small planets, it’s the only way to go.
The Europeans already had a mission in the pipeline that could easily accommodate the transit hunting. Hunting the convection and rotation of stars (known as COROT, and referring to the famous pointillist impressionist painter), the telescope was to obtain thousands of images of stars, resembling pointillist paintings, in order to study their subtle light variations and help understand basic things about stars, like their rotation and internal convection. Of course, COROT could do an excellent job detecting planet transits in the process, with its name now standing for convection, rotation, and transits (CoRoT).
In the meantime, NASA had missed an early opportunity to fund a space telescope dedicated to discovering transiting planets, one with a stated goal to discover planets as small as Earth and determine how common planets like ours actually are. William Borucki
of the NASA Ames Research Center in California had been trying to convince the agency that his experiment would succeed. Even a null result, meaning that no transiting Earths were discovered, would be meaningful, implying that planets like ours are very rare. NASA panels had turned him down before, but in 1999 Bill Borucki was assembling a crew to propose again; he asked me and a dozen more colleagues to join. The successful discovery of more and more exoplanets with the Doppler shift method was a powerful new motivation.
Even though I hadn’t done much work on the problem at the time, I had first thought of discovering such planets in 1999, when a group of us, mostly at the Harvard-Smithsonian Center for Astrophysics, and mostly observers and engineers, came together to propose to NASA an innovative space telescope design for planet detection—with a square mirror, as opposed to a round one. My colleagues Costas Papaliolios and Peter Nisenson had invented this unusual design in order to minimize stellar glare and allow glimpses of planets huddled close to their stars. With a team of about twenty and led by our experienced space mission scientist Gary Melnick, we prepared a detailed scientific and engineering proposal.
My job on the team was to work out what kind of planets our telescope might be able to discover. It seemed then that super-Earths were in reach. (I liked to call them super-Earths and super-Venuses for short, as it had been common in astronomy to use the adjective “super” for newly discovered or hypothesized objects that are larger in size or energy than known ones. For example, stars that are larger than giant stars are called supergiants, explosions that are stronger than novae are called supernovae, and so on.) The shorthand stuck, as you’ve probably already surmised.18