Sometimes being left out in the cold is a good thing. Or so thinks SETI Institute astrophysicist, Dr. Douglas Caldwell, whose planet hunting team has set up shop in one of the most cold, remote areas of the planet, the South Pole.
At this location, the sun sets in April and rises in September. During the four-month night, the stars shine constantly against crisp black skies, never rising nor setting, but wheeling about in circles above the automated photometric search equipment. Weather, despite the cold (temperatures average a bone-chilling negative 49 degrees Centigrade!) is relatively mild with few storms and moderate winds.
These are perfect conditions for a "transit search," the planet hunting technique that monitors the brightness of a star over time for regular and periodic "dimming" in that stars brightness. Such "winks" could indicate that an orbiting body like a planet has passed between the star and the observer. Transit searches use sensitive "photometers," basically telescopes coupled with digital camera-like recording devices that capture images of the stars under observation. Signal processing experts like Caldwell analyze the stored data for dips in brightness.
Caldwell cites the advantages of observing in the South Pole, no doubt the best place on Earth for such a search. Observations uninterrupted by cycles of night and day are more efficient and accurate, he explains. They can be conducted three to four times more quickly (at higher latitudes, observers may have only a six hour period to monitor a star), and subsequent data analysis will not need to consider "diurnal extinction," the dimming of a stars brightness that is induced as the viewing angle changes in tracking the star, which must be observed through an ever thicker layer of atmosphere once it descends from its zenith.
Caldwell describes what happens at the South Pole during the long winter night. "Well take images of same part of skyareas where there are the most starsfor as long as we can. Probably 4 months or so." The equipment takes images every three minutes. While the project is highly automated, Caldwell says the team "makes spot checks of a couple of images a day just to make sure that every thing is working right." All of the science, however, must be done in California once the data is shipped back for computer analysis at the end of the season. It is not possible to do the data analysis in real time, as the data connection at the South Pole is not adequate. The Internet is available to researchers about 6 hours a day, so they cannot send images back. "We send back the hard disk from a computer," explains Caldwell.
With funding from the National Science Foundation, Caldwell and his team members from the SETI Institute, NASA Ames Research Center, the Rochester Institute of Technology and the University of New South Wales, are looking for "hot Jupiters," large gas giants that tightly orbit their stars
Once the search identifies a candidate planet, more work begins. "Lots of things mimic a planet," says Caldwell, "binary systems for example, so well run through a series of steps to eliminate other possibilities." They will then check their results with the radial velocity Doppler method, the technique used by most planet hunters including Dr. Geoffrey Marcy and his team. Logistics of this verification step are still being worked out. According to Caldwell, there are "fewer radial velocity Doppler projects in the southern hemisphere" and his group has been talking to facilities in Australia.
Because this search looks for close-in large planets with short orbital periods, the Doppler method can perform its check quickly. In just over week or so, observers can see enough of the motions to verify the results of the transit search.
Large planets with short orbital periods are the most likely type to be found with an Earth-based transit search. Such planets, however, are clearly not cozy abodes for life and one may wonder what this search contributes to astrobiology. Plenty, according to Caldwell. "The first planets detected didnt look at all like ours. They made us realize that there are a lot of ways there can be planets out there and we dont really know that much about how they form, move around and evolved." The more we learn about the individual "species" of planets in this cosmic bestiary, the more we can infer about the formation of planets like our own.
Caldwell also notes that according to current theories, large hot Jupiters form far from their stars, migrating in and wreaking havoc on smaller rocky bodies with orbits crossing its path. Says Caldwell, "If a big Jupiter did migrate in, you can imagine that it plowed in and, that couldnt have been good for Earth like planets." He notes that we can "check off" these systems as unlikely to harbor Earth-like planets.
While astrobiologists may need to wait for projects such as Kepler, set to launch in 2007, to search for planets like our own, much remains to be discovered in the meantime. Says Caldwell, "the exciting part is seeing what cannot be predicted."
advertisement
