“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.
The most common means of finding the distance to a star is to determine its parallax, which is an angular measure of its apparent movement against other, more distant objects in the background. The closer a star is to Earth, the greater its parallax. The same principle holds when driving on a multi-lane expressway: Cars in the next lane appear to be moving faster against the surrounding landscape than do cars in the far lanes. Astronomers repeatedly photograph a target star, and by measuring its gradual movement against the background (it usually takes at least two years to get a good parallax), they can use trigonometry to calculate its distance from Earth. It isn’t the most glamorous work in science, which probably explains why parallax data is simply lacking for most stars, particularly in the southern hemisphere, where historically there haven’t been as many astronomers.
The lack of good distance estimates for most stars has long nagged Todd Henry, an astronomer at Johns Hopkins University in Baltimore and the deputy project scientist for Backman’s small NStars research effort at Ames, which, confusingly, has the same name as the larger NASA-NSF program just getting under way. Henry’s first job, funded by the Search for Extraterrestrial Intelligence back in 1991 when NASA was still running that program, was to identify stars that might warrant further scrutiny in a search for radio signals from alien civilizations. Henry worked off a list of the 100 stars known to be closest to Earth, but soon found himself puzzling over four objects that weren’t on the list, even though they were quite bright.
That curiosity led Henry and six other researchers to create an informal group they called the Research Consortium on Nearby Stars, or RECONS, to investigate unidentified shining objects. In 1995 the group got time at a complex of mountaintop observatories in Chile, where they measured the brightness of Henry’s four objects at different wavelengths. The photometry data revealed that three of the objects were in fact stellar giants, very bright but also very distant. The fourth star, which had seemed the most promising from the beginning, turned out to be a more common star known as a red dwarf, but its distance from Earth was not recorded in the catalogs.
As it happened, Henry lucked out. Astronomer Philip Ianna of the University of Virginia had already collected images of this particular star from 1976 through 1990, but hadn’t yet found time to extract parallax data and crunch the numbers to come up with its distance from Earth.