Dr Sara Seager and the Search for Extrasolar Planets

For scientists studying the formation of planetary systems and the presence of life, there is one big problem: Humankind has only a single example, this solar system and life on Earth. To learn the most about a subject, it is best to study a number of cases. Where can other examples to compare to Earth be found? Other planets must be found orbiting other stars. Finding these extrasolar planets, and trying to understand what their surface environment is like and what they are made of, is the challenge and pursuit taken on by Dr. Sara Seager.

Extrasolar planets are necessarily far away and thus difficult to detect: The planets do not produce light, like stars do, and must be detected by their influences on the star they orbit. Before recent advances in telescopes, digital photography, and computer analysis, these planets could not be detected from Earth. In 1991 Alexander Wolszczan and Dale Frail at the National Radio Astronomy Observatory discovered the first real extrasolar planet. This planet is closely orbiting a pulsar star 978 light-years from Earth. A pulsar star constantly emits high levels of energetic radiation, so this new planet, which orbits at only 0.19 AU (closer to its star than Mercury is to ours), is extremely inhospitable to life. This planet is not an analog of Earth or of any of the other planets in this solar system.

In the mid-1990s Seager was a graduate student at Harvard University, ready to choose her thesis topic, when the first several extrasolar planets orbit Sun-like stars were discovered and confirmed. These planets are all far larger than the Earth and orbit much closer to their stars. Seager was fascinated by the discovery and made a risky choice, deciding to carry out her doctoral thesis research in the area of extrasolar planets. The content of a scientist's doctoral thesis is a dominant influence on their career; they become experts in the field of their original doctoral research, and will be hired for their first jobs on the basis of this work. In the case of extrasolar planets, the question remained whether the field would develop and become a legitimate area of study at all. Very few planets had been found, none Earth-like or even resembling the giant planets in this solar system. The four planets orbiting Sun-like stars are all of the type referred to as "hot Jupiters": giant planets close to their stars, and not in the least resembling the Earth. If the field did not prove to be a topic large and tractable enough for many scientists to pursue, with interesting and relevant results, Seager risked largely wasting her years of work toward a doctoral thesis. She completed her thesis in 1999, and her gamble paid off. The field is fascinating, fruitful, and relevant, and she is one of its first and most prominent researchers.

Dr. Seager is now a research scientist at the Carnegie Institute of Washington. To search for extrasolar planets Dr. Seager and her research team collect digital photos of a wide field of stars, taking snapshots every few minutes through a telescope. The digital photos are downloaded into computer clusters, where programs compare the amount of light emitted over time (the flux) by each star in each snapshot. The programs are written to look for stars that show short-term decreases in the flux of a star. A planet moving in front of the star (known as transiting) and blocking some of its light from coming to Earth may create decreases in flux. Once the computer programs locate stars with short-term decreases in flux, the scientists study the candidates in much more detail to make sure the cause of the dip is a transiting planet and not a dimmer companion star or some other anomaly.

More than 20 groups around the world are searching for extrasolar planets with this method. Some researchers are using telescopes as small as one-inch backyard scopes to search for extrasolar planets, teaming with researchers in different parts of the world so they can take photos of the same part of the sky at nighttime around the clock as the Earth turns. Small telescopes look at nearby, bright stars. Seager's team uses more powerful telescopes in a complementary approach: Looking at fainter, more distant stars. Scheduling time on the few large optical telescopes around the world appropriate for this kind of research is a constant challenge. Seager's team uses the Carnegie one-meter telescope in Chile and has used the National Optical Astronomical Observatory's four-meter telescopes at Kitt Peak, Arizona, and the Cerro Tololo Inter-American Observatory near La Serena, Chile. Seager's team and other groups such as the Optical Gravitational Lensing Experiment (OGLE) at Princeton University have found three planets with the transit search technique. They monitor up to tens of thousands of stars at one time. The team is perfecting its techniques and has been studying stars in large groups called open star clusters to maximize the chances of seeing a star with a transiting planet.

One of Dr. Seager's long-term research goals is to learn how to detect signs of life elsewhere in the universe. To recognize planets with oceans like Earth, more needs to be known about what the Earth and the rest of this solar system look like from space. There is a lot of dust in and around this solar system from asteroids and comets. What does this look like from far away? If scientists saw an extrasolar planetary system likely to hold planets like Earth, how would they recognize it? Perhaps the small planets like Earth, so hard to detect, leave wakes in the dust as they orbit, so observers could look


Dr. Sara Seager and the Search for Extrasolar Planets


for wakes. After finding a small, dark planet, perhaps its atmospheric characteristics will indicate the presence of liquid water.

Seager's main line of research is about extrasolar planetary atmospheres. Her work was used to help detect an extrasolar planet atmosphere for the first time. She studies the theory of atmospheres and planetary systems to determine what scientists on Earth can detect about extrasolar atmospheres, and she is continuing to work on characterizing the atmospheres of planets that transit their parent stars. The formation and persistence of atmospheres are little understood, even in this solar system. Can a planet orbiting at 0.05 AU (seven times closer to its star than Mercury is to the Sun) have an atmosphere? How can an observer tell if an extrasolar planet has clouds? Most importantly, how can an observer tell from these great distances whether an extrasolar planet has life?

Dr. Seager is involved with NASA's Terrestrial Planet Finder mission to locate extrasolar planets. Looking at distant stars is much easier outside the blurring influence of the Earth's atmosphere, and there are plans to launch an instrument into orbit to look for planets. She is on the mission's scientific working group, which decides which stars and how many stars the instrument should look at, and what kinds of measurements it needs to be able to make to detect what the science team wants it to. They think about how to detect small, dark planets like Earth, and collaborate with the mission's engineers to come up with an instrument that can do the necessary work. When Terrestrial Planet Finder is launched in the next decade, Dr. Seager hopes to be able to find planets like Earth, small and dark when seen from a great distance, and thus locate the best chances for finding other life in the universe.

star from the human point of view, never passing in front of it. Requiring a transit narrows the candidate extrasolar systems consid-erably.This kind of detection is far more desirable, though, because as the planet passes in front of the star the star's light is also changed by the planet's atmosphere, if it has one, and much more can be learned about the planet. The mass, size, and atmospheric compositions of transiting planets can be determined. For more, see the sidebar "Dr. Sara Seager and the Search for Extrasolar Planets" on page 110.

An example of a giant planet orbiting close to its star is found at HD209458, a faint star in the constellation Pegasus.The giant planet orbiting this star makes a transit of the star from the viewpoint of Earth. During transit of the planet, the star's light is dimmed by 2 percent (an Earth-size planet would only dim the starlight by about 0.02 percent).This regular dip in light is the clue that allowed scientists to identify the planet, since it is far too small and dim to be seen with telescopes from Earth. The size and mass of the planet were then determined by studying the pattern of light from the star and the wobble of the star as the orbiting planet circles it.This planet appears to be 35 percent larger than Jupiter, but it has 30 percent less mass. This strange, low-density giant planet is nothing like any planet in this solar system. As it cools over the age of the solar system, Jupiter is contracting. Its current rate of contraction is three millimeters per year. How, then, can this distant system maintain a larger, lower-density planet? Perhaps the heat from the star may be keeping the planet hot and inflated. This system raises the question of Jupiter's future: Will it eventually migrate inward in the solar system, disrupting the inner planets, to take up an orbit closer to the Sun?

Of the many extrasolar planets now known, only two Earth-size and one Moon-size planets have been detected. More than 5 percent of nearby stars show evidence of giant orbiting planets, but these are actually the least likely to have Earth-size planets. Giant planets close to their stars are likely to have thrown smaller planets out of their solar systems through gravitational interference. To find smaller, Earth-like planet around other stars, some scientists are more closely watching the inner planets Mercury and Venus. Mercury transits the Sun from the Earth's point of view several times per century, and Venus transits the Sun once or twice per century. By watching these transits, scientists hope to learn how to detect Earth-like planets transiting other stars.

Finding Earth-like planets that may harbor life is an even larger challenge. Sara Seager from the Carnegie Institute would like to identify habitability from spectral analyses of the atmospheres of extrasolar planets. Oxygen is the big signal on Earth: There are no known nonbiological sources that can create large amounts of O2 in an atmosphere. Nitrogen is another important atmospheric tracer of life: Only microbes on Earth manufacture nitrogen oxide (NO). Water clouds might signal oceans, and high albedo contrasts on the surface may indicate oceans and continents.To learn to identify these signals from other planets, scientists would like to see the spectra of Earth from a distance.There is a way to do this from Earth: Scientists measure the light that is reflected off the Earth, shines onto the Moon, and is bounced back to Earth. This light, called Earthshine, gives the spectral signature of Earth from space. Earthshine needs to be distinguished from sunshine, though; during a full Moon, the light shining back to Earth comes from the Sun. Earthshine can only be measured from the relatively dark portions of the Moon when the Moon is not full.

Scientists and nations around the world are becoming involved in the search for extrasolar planets. In 2007 NASA is launching a mission called Kepler to examine stars from outside the Earth's atmosphere. The European Space Agency will launch a similar mission, Eddington, in 2008, and in 2006 the French space agency (CNES) will launch a mission called Corot. Together these missions will be able to examine 100,000 stars over three years, searching for signs of life outside the solar system.

Part Two: The Moon

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