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Planet Hunter's Paradise
Perpetually frozen, relentlessly barren, and shrouded in darkness for months on end, the remote terrain of the Antarctic is a hunter's paradise for astronomers whose quarry is a certain type of extrasolar planet. The icy South Pole offers SETI Institute scientist, Dr. Douglas Caldwell and his team the best vantage point for observing "hot Jupiters," a class of gas giants (like Jupiter) in tight orbits around their suns.
Caldwell's Antarctic planet hunt uses the "transit method," which measures the dimming that occurs as a planet passes in front of its star. Caldwell notes the simplicity of the science behind transit searches. "It's simple physics. Newton's laws." The planet-star relationships reveal valuable information about both celestial objects.
"The time between dimming (transits) tells you the period of the orbit. From Kepler's law, you then know the distance between the planet and its star. From the amount of dimming, geometry tells you a lot about the planet. For example, if you know the size of the star, you know the size of the planet."
"Simple physics," Caldwell notes with a smile.
Physics Background
Caldwell attended Carnegie Mellon University after a childhood spent in urban Pennsylvania, where pallid night skies lacked the rich star fields that so often engage young imaginations. Carl Sagan's wildly popular television show, Cosmos aired when Caldwell was in high school, and was to be a major influence on the future astrophysicist. The series, and reading "a lot of science fiction," cultivated a life-long interest in astronomy and an avid desire to understand what was happening in the depths of space.
While thinking "astronomy was neat," Caldwell had little exposure to the discipline in academia. "I was always interested in physics," he explains, and physics allowed him to "grasp the details of what's going on, to understand what's happening in very simple ways."
Never losing his interest in astronomy, Caldwell worked briefly in southern California as a civilian engineer for the Navy upon graduation. Before long, he found himself back in school, this time focusing on his interest in the cosmos. In the late 80s, Caldwell undertook a PhD. program at New York's Rensselaer Polytechnic Institute, in astrophysics.
A research fellowship at NASA Ames in the late 90s led to Caldwell's current occupation as a professional planet hunter. Caldwell describes the Antarctic search as a "side growth" of another mission, NASA's Vulcan project, itself a "practice project" for the space-based Kepler. While the transit method itself is simple, it requires continuous observation of the star, a process which, when Earth-based, is complicated by clouds and the rising and setting of our Sun.
Advantages of the South Pole
Caldwell recalls a talk by Kepler's Principal Investigator, William Borucki, on the observing advantages of the South Pole with its long nights, "Earth-surface projects present problems, because we never know when transits will occur. At our latitude, a meager one-third of our 24-hour day is dark."
At the Lick Observatory, where the Vulcan photometric equipment looks for transits, a six-to-eight-hour observation per day is the most Project Vulcan astronomers can hope for. "You have to look longer to get a good detection," says Caldwell. "At the South Pole, you can look continuously and see transits in short order."
With seed money from the NASA Ames Research Center to investigate the potential for an Antarctic search, Caldwell and his team made a reconnaissance trip in the summer of 2001. "We took the Vulcan equipment, did some observing and tested the infrastructure. There is a lot of good astronomy support and infrastructure at the South Pole."
Caldwell grasps for a good description of the weather at the research station, which experiences few storms and surprisingly little snow. He uses the term "mild," with some hesitation. The biggest issue is dealing with the constant cold. While the buildings are very well insulated, "going in and out is a pain."
Asked about adapting to life at the research station, he responds, "It's hard to say what a day is when the sun never sets. In summer, the Sun is up all day and goes around in a circle." The research team does not stay over during the observations. Caldwell has actually never been on the site during an Antarctic night, when, from mid-February to November, there is very limited access to or from the station.
The project will take images of the night sky with the equipment that remains on site. With a wide, seven-degree field of view (which is about the size of the Big Dipper's "scoop") the camera focuses upon areas that are dense with stars. "We'll take images of the same part of the sky, the same set of stars, for as long as we can - for a month or so."
During the remote observations, the photometer, the device that actually measures the star's brightness, will take images every three minutes or so. A computer controls the entire operation, and stores the data until the researchers return to collect the results. "There just isn't a good data connection at the South Pole," explains Caldwell. "The Internet is only available about six hours a day, so we can't send images back to Mountain View." What they do send back to Mountain View is the hard disk from the computer.
Candidates Checked with the Doppler Method
Should a planet candidate emerge when the hard drive is checked, nothing "will be certain right away," says Caldwell. "Lots of things mimic planets--binary star systems, for example," His team will check their results against those obtained from the radial velocity method, which measures the Doppler shifts in the light from a target star to see whether a large planet is tugging on the star and influencing its motion. Because their search is looking for large, close-in planets that take short times to circle the star, Caldwell's team can use the Doppler method to quickly check their findings. Says Caldwell, "Observing for over a week or so, we can see enough of the motions to confirm a detection."
For Caldwell, the most exciting part of searching for planets is the unpredictability of the findings. "The first planets we detected didn't look at all like ours. This made us realize that there are many possible planetary configurations, and we don't really know that much about how they form, move around and evolve."
Caldwell's team will not be expecting to find a world similar to ours. Two things, he explains, are currently keeping us from looking for Earth-like planets from Earth. Because earths are so much smaller than their sun, the amount of dimming is about one part in ten thousand of brightness--a very small amount. Right now, no one on the ground can do photometry that can measure such dimming. Also, when looking for an object like the Earth, with an orbit of approximately one year, astronomers must observe without pause throughout the entire orbital cycle, a relatively long period of continuous observing interrupted by day and night.
Thus, Earth-based planet searches find worlds that are very different than our own, helping astronomers who are compiling a growing and increasingly diverse catalogue of extrasolar planets. For astrobiologists finding such planets, where life is unlikely to develop, leads to a better understanding of processes that drive planet formation. Finding these bodies will also impose better constraints on the search for Earth-like bodies.
Caldwell explains. "Current theories are that if you do see a close-in planet, it probably formed far away from its star, and migrated in. If this planet is a big Jupiter, you can imagine that it plowed in, and that couldn't have been good for any Earth-like planets with orbits in the way. So you can look at these and check them off as systems that probably won't have Earth-like planets."
Narrowing the search space will be a boon to Caldwell and his colleagues when they zero in on Earth-like planets during the Kepler mission, scheduled to launch in 2006. Several Institute scientists eagerly await the mission, which could yield results directly relevant to astronomers using the Allen Telescope Array for SETI observations. Caldwell is enthusiastic about the work of his colleagues at the Institute. "SETI is a great project," he says. "It's something that seems so obvious to do. If we think there is other life out there, one way to find it is to see if it wants to be found."
Caldwell believes that SETI astronomers exemplify the sort of bold thinking that "should be a characteristic of all scientists." He urges young scientists to be open-minded, not to look at things the way others do. "Young people come in with fresh perspectives," he notes, and are eager and willing to try new ideas. Extrasolar planet hunting was a new idea not so long ago, and a universe of other new ideas seems just on the horizon for the young astronomers who wish to explore our galactic home.
October 30, 2003
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