Starz in the Hood
There are more stars in our celestial backyard than we once thought.
- By Michael Milstein
- Air & Space magazine, May 2000
One day while in junior high school in the late 1960s, Dana Backman strayed into the adult section of his local library, wondering what sort of books adults read that weren’t available on the other side of the building. One of them was Habitable Planets for Man, written by astronomer Stephen Dole. A pioneering report commissioned by the RAND Corporation years before the first moonshot, Dole’s book assessed the possibility that planets suitable for human colonization were circling stars other than our sun.
“I was really just a science fiction fan at that point, but this was real stuff,” Backman recalls of his teenage discovery. “I was fascinated. I ate it up. I memorized it.”
Based on the ages and masses of stars in the galaxy and the many criteria necessary for habitable planets—water, an atmosphere with oxygen, light, certain chemicals, some gravity but not too much—the book estimated that up to 10 stars within 20 light-years of Earth may have human-friendly planets orbiting them. Especially striking to Backman was the suggestion that planet-bearing stars may be relatively unimpressive to look at. Indeed, an intelligent being on a planet circling the nearest star, about four light-years away from us, would regard our sun as pretty run-of-the-mill.
Later, while studying astronomy in college and graduate school, Backman was puzzled by the fact that most other researchers showed little interest in our stellar neighbors, preferring instead to look toward the far edges of the universe. When he talked excitedly about the possibility of planets beyond our solar system, other scientists looked at him as if he were reporting a fleet of UFOs. Still, he wondered to himself what astronomers must be missing.
Now he thinks he knows: Stars. Lots of ’em.
Today, Backman is one of the principal scientists behind a new NASA and National Science Foundation research initiative called the Nearby Stars Project, which seeks to fill a gaping hole in our knowledge of our own celestial neighborhood by cataloging stars within 25 parsecs, or about 80 light-years, of the sun. Rough estimates suggest that astronomers have measured the distances to only about half the stars within that range. The rest have yet to be recognized as nearby objects. A good number of them are perfectly visible, may even have a name, yet they remain anonymous, hidden in the vast crowd of more distant stars like an astronomer’s version of “Where’s Waldo?”
Ironically, nearby stars are the best places to look for planets beyond our solar system—the kind of planets that once earned Backman funny looks from his colleagues but which now make headlines around the world. The stars we should know best, it turns out, we hardly know at all.
“To be told that half the stars in our neighborhood are basically missing is something of a shock,” says Harley Thronson, a senior program scientist at NASA headquarters in Washington, D.C., who initially fostered the Nearby Stars Project, also called NStars. It’s also “a big handicap when you’re planning multimillion-dollar missions to look for planets around such stars. It means you don’t know where to look.”
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.
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.
“I said, ‘Phil, do the numbers now!’ ” Henry recalls. It took only a few weeks to determine that the star, which goes by the unassuming tag GJ1061, was just 12 light-years away, making it the 20th nearest star—scarcely three times farther than our closest neighbor, the Alpha Centauri system, which includes the stars Alpha Centauri A, Alpha Centauri B, and, closest of all, Proxima Centauri.
“I’ll let you in on a secret,” Henry says. “We have better numbers now, and they show that it’s actually even closer. A few years ago, no one knew it was there.” The team gave the star another, newer name: RECONS 1.
The discovery led Henry to some simple calculations. Within five parsecs, or about 100 trillion miles, we know of 60 stars and one planet beside our own solar system. Let’s say this represents most, if not all, of the objects actually out there. Simply assuming that the density of stars remains the same out to 10 parsecs, there should be roughly 500 stars within that volume of space. Yet astronomers have counted only about 315.
Within 20 parsecs, there should be about 4,000 stars. “We know of about half that,” Henry says. “I like to compare it to a baseball field, where you have quite a few infielders but far fewer in the outfield. In fact, the density of players in the outfield of space should be about the same as the infield. We just haven’t located them yet. People ask me, ‘Why don’t we know where the rest of them are?’ The simple answer is ‘Because there are a lot of dots in the sky.’ It takes time to map everything out, and we just haven’t been at it that long.”
Astronomers also don’t always share information as effectively as they could, so it may be that parallax data for other nearby stars actually will be found in Ianna’s or some other researcher’s files. Or it could be that different astronomers have pieces of the puzzle that, once put together, could reveal stars much closer than anyone thought. So Backman’s team will build an unprecedented database from as much information as they can round up from the existing scientific literature, and make it available to astronomers and the general public over the Internet. The team’s search for relevant information has already turned up other candidate nearby stars. An expected influx of additional data from new sky mapping projects such as the Sloan Digital Sky Survey—which aims to chart more than a hundred million celestial objects, including stars, galaxies, and quasars—should yield even more discoveries.
The second element of NStars calls for additional research—using telescopes around the world to identify “new” nearby stars based on their parallaxes and to learn more about those that are already known. University of Texas astronomer Fritz Benedict, for instance, plans to seek funding to examine about 500 stars for signs that they are binary, or double stars; he expects that around 20 percent will be. Researchers could then exclude those stars from the list of candidates for planet hunting, because stable planets are unlikely to form in binary systems.
The research will have a wider purpose than just adding to the roster of nearby stars, says Benedict. “It’s not only filling in the blanks on our star catalogs, but also filling in our understanding of the way solar systems might work.”
The idea of NStars had so much appeal that when NASA launched the project last year, the National Science Foundation quickly offered to contribute half the initial funding of $1.2 million. By October, the funding agencies had received around 80 proposals, twice as many as they had expected, which led them to try to scrounge up more money.
Backman and Henry sound like kids on Christmas Eve when they talk about the wonders NStars should soon reveal. “I’ve been waiting for this for 10 years,” beams Henry. “I’m sure we will find new things, all kinds of crazy things. The nearest star, Proxima Centauri, may not be the nearest star. It’s really hard to guess what we’re going to find.”
But he hazards a guess anyway. Henry figures that most of the 2,000 or so uncounted stars within 25 parsecs of us will turn out to be faint red dwarfs about the size of Jupiter. Stellar censuses suggest that these dim objects, which are far fainter than the sun, dominate the star population, and our own neighborhood should be no exception. Although such stars shine too feebly to offer much hope of finding planets suitable for life, Berkeley’s Debra Fischer and her planet-hunting colleagues have already found one planet revolving about a red dwarf. They believe that most red dwarfs—in fact most stars—sport planets of some type.
“I think they all have planets,” Fischer says. “The mounting evidence we have is basically that if we can find planets around a star—if our technology is good enough to see them—then we do. It really looks like planets are manufactured very efficiently around stars.”
There’s one last reason, of course, to tote up a list of nearby stars, and to find out if they have habitable planets. If we ever learn how to travel the immense distances of interstellar space, we’ll want to visit the closest solar systems first.
Sadly, though, Fritz Benedict already appears to have ruled out the first stop in this cosmic voyage. After years of observing, Benedict found no evidence of planets around Proxima Centauri, the closest member of the Alpha Centauri triad. It was perhaps no surprise, given that the star doesn’t seem to be rich in the heavy metals that help planets glom together. Still, it’s disappointing. But as Todd Henry will happily tell you, there are plenty of other stars in the sky. And they may be closer than you think.