Starz in the Hood
There are more stars in our celestial backyard than we once thought.
- By Michael Milstein
- Air & Space magazine, May 2000
(Page 2 of 5)
Cataloging stars is not a new idea, but the NStars project will compile computerized dossiers packed with information never before included in conventional star catalogs—details such as a star’s age and the amount of dust swirling around it—that bear on the ability to support Earth-like planets. If such details are lacking, the project will sponsor research to nail them down. “This isn’t a matter of just punching numbers into a computer,” says Backman. “It’s a matter of aggressively identifying what we need to know but don’t know, and going after it.”
Researchers eager to test the limits of bigger and better telescopes have routinely skipped past the ho-hum objects close to home and focused instead on the biggest, hottest, and most distant attractions in the universe. Neutron stars, supernovae, and other stellar exotica make up only a small fraction of the population of stars, but for several decades they’ve been the most attractive objects to study, according to Backman, who today is a professor of astronomy at Franklin and Marshall College in Lancaster, Pennsylvania, with a dual appointment at NASA’s Ames Research Center in California.
“Astronomers are just big kids—they like things that blow up, that shoot off sparklers or collide,” says Thronson. “But the majority of everything falls into the range of what we call normal.” That neglected category actually includes stars with different sizes, shapes, and histories, he adds. “Really the greatest variety lies in normalcy, because ‘normal’ stars are by far the most abundant, and probably the most likely to have planets like ours. ‘Are we alone?’—a fundamental question—will probably be answered in these stars.”
Indeed, many of the 30 or so planets discovered beyond our solar system orbit commonplace stars. Encouraged by the growing list of extrasolar planets detected from ground-based telescopes, NASA plans to launch a series of space-based observatories, beginning with the Space Interferometry Mission in 2005, with the ultimate goal of finding Earth-like planets that have the potential to harbor life. The best chance for success lies in nearby stars, because the resolution of the images and spectral data will be higher. But which nearby stars?
“There’s a list of maybe 10 things, a kind of rap sheet for these stars that you need to fill out,” says Chas Beichman, chief scientist for NASA’s Origins program at the Jet Propulsion Laboratory in Pasadena, California. For example, thick dust in other solar systems could obscure the vision of the Terrestrial Planet Finder, a more powerful telescope that’s slated to follow the interferometry mission into space around 2011. So it wouldn’t make sense to waste valuable observing time on dusty stars, even though these may be the incubators of future solar systems. It won’t do much good in general to look at very young stars, since it takes a long time for swirling dust to turn into planets that are even remotely Earth-like. It would probably be useful to target stars with high concentrations of heavy metals, since those elements seem to contribute to planet formation. But they have to be close—within the 50-light-year range that the Terrestrial Planet Finder can examine in detail. All in all, the 200 to 300 star systems to be investigated by that advanced instrument will have to meet a stiff list of criteria.
So far, though, the rap sheets are distressingly blank. When Beichman, Thronson, Backman, and their colleagues convened a meeting in 1997 to discuss dust in other solar systems, it quickly became obvious that there was hardly any information on the subject. In fact, there was hardly any information on neighboring stars, period—and nowhere near enough to start picking suitable targets for the Terrestrial Planet Finder.
Today’s ground-based planet hunters know the problem all too well. “Right now we sift through an incredible amount of information looking for candidate stars, because we know so little about these stars,” says Debra Fischer, an astronomer at the University of California at Berkeley and member of the prolific planet-finding team that includes Geoffrey Marcy of San Francisco State University and Paul Butler of the Carnegie Institution of Washington. The list of candidates includes a lot of bright stars, because stars that appear bright in the sky are more likely to be close. But that general rule doesn’t always hold. For example, the brightest and third brightest stars in the sky—Sirius and Alpha Centauri—are both less than 10 light-years away, just around the corner in interstellar terms. But the second brightest star, Canopus, is 74 light-years distant. And giant Betelgeuse, number 10 on the list, is a whopping 500 or so light-years away.
Although most easily visible stars have been named or numbered and we know their coordinates in the sky, in many cases we don’t know how far away they are. That third dimension, distance, is critical for planet-hunting. It’s also by far the most difficult variable to nail down.