Captured on Camera: Are They Planets?

Hunting planets in some of the nearest--and least-explored--star-forming regions

Astronomers have found a new frontier right in Earth's cosmic backyard. Instead of peering at faint galaxies or imaging the most distant reaches of the universe, some researchers are looking to make groundbreaking discoveries by examining a few nearby groups of newborn stars.

These youthful denizens, the closest known arrays of very young stars, lie less than 200 light-years from Earth. Yet until a few years ago, scientists didn't even know most of them existed.

It isn't just the nearness of these youthful stars that has piqued the curiosity of astronomers. It's also their age. Ranging from a few million to about 30 million years old, the stars are mature enough to have fully emerged from their gaseous birthing clouds. But they're still young enough that any planets they possess would have only recently finished forming and would still be aglow. That makes these planets a compelling target for an infrared telescope outfitted with high-resolution optics, notes Glenn H. Schneider of the University of Arizona in Tucson.

Indeed, astronomers exploring the closest of these star groups, dubbed the TW Hydrae association, unveiled an image 2 years ago that may be a baby planet (SN: 1/9/99, p. 20). Ray Jayawardhana of the University of California, Berkeley told SCIENCE NEWS that he and other astronomers have a handful of images that may also show planets.

The attempt to image extrasolar planets has a checkered history, riddled with hyped announcements and embarrassing retractions. The scientists studying the new candidates caution that they, too, could be fooled. Some of the faint objects could turn out to be distant background stars or even galaxies that happen to lie at the same position in the sky as one of the nearby, newborn stars.

The researchers have a strategy, however, for distinguishing a planet from a more-distant object. A remote star or galaxy has no discernible motion across the sky, but a nearby star moves ever so slightly, and its orbiting planet tags along.

For instance, if a faint dot of light in the TW Hydrae association really is a planet, it should move in 1 year a distance roughly equal to one-thirty-thousandth the width of the full moon on the sky, says Schneider. That tiny motion is just large enough for scientists to discern by using an infrared telescope with high-resolution optics, he notes.

Tracking the motion of the candidate orbs, astronomers will probably know by the end of the year if they have a winner, says Schneider. Scientists may be just months away from unveiling the first bona fide portrait of a planet orbiting a star outside the solar system.

Other astronomers have recently obtained images of objects that, based solely on estimates of their mass, may qualify as planets. However, these free-floating bodies don't seem to orbit any star and may in fact have formed as stars do (SN: 5/19/01, p. 312).

In late March, at NASA's Ames Research Center in Mountain View, Calif., researchers described their progress.

"These newly discovered groups of stars are prime targets for studying anything about planet formation--for directly imaging them, for understanding details of their evolution," says Jayawardhana. Because each of the young stellar groups is a different age, astronomers hope that by examining several, they can pin down the time line for the series of events necessary for planet formation.

The details include when stars shed the gaseous cocoon in which they were born and at what age they form disks of gas and dust--the raw material for planets. The researchers also want to learn how much time it takes for a planet to coalesce from such a disk.

"We don't really know exactly when planets form," notes Jayawardhana. It could be anywhere between 1 million and 10 million years after a star is born, he says. If astronomers only find planets among stars older than a certain age, that would provide a huge clue, he adds.

It may seem strange that until just a few years ago, astronomers were unaware of most of the groups of newborn stars closest to Earth. But their discovery relied on an X-ray survey of the entire sky and information about stellar motions, neither of which was available a decade earlier.

Although the individual stars had been recognized before, it wasn't obvious that any of them had been born together. Familiar star-forming regions, such as the Orion nebula, are compact. They span just a few degrees of sky, although they contain thousands of stars, making them easy to identify. In contrast, the nearby TW Hydrae association includes just a few dozen newborn stars but covers a section of sky 15 to 20 degrees wide.

"That's such a big chunk of sky that you wouldn't even notice they were physically associated," says Schneider.

In part, these groups of newly formed stars appear more spread out simply because they lie closer to Earth than the Orion nebula does. The young stars are also about 10 times as old as those in the Orion nebula and so have had more time to separate since birth. With their birth cloud long gone, "there was no easy way to tell that these stars were born together," Jayawardhana says.

In the 1970s, George H. Herbig, now at the University of Hawaii in Honolulu, examined a new map of the southern sky and noted that a star in the constellation Hydra has an unusual spectrum. The star emits high-intensity red light produced by hydrogen atoms.

That radiation, the researchers knew, is characteristic of a very young star. According to theory, when the disk of gas and dust that surrounds some newborn stars begins to disintegrate, hydrogen gas in the disk heats up and generates the light as it spirals onto the star's surface.

The newborn in Hydra, however, seemed to lack youthful partners or even a trace of its birth cloud. Dubbed TW Hydrae, the star seemed odd in another respect. Located in a sparsely populated region above the plane of the Milky Way, it lies far from our galaxy's highest concentrations of newborn stars.

In 1983, two other astronomers, Slavek M. Rucinski of the University of Toronto and J. Krautter of Landessternwarte in Heidelberg-Koenigstuhl, Germany, measured the abundance of lithium in the star and confirmed its youthfulness. Because nuclear reactions in a blossoming star quickly destroy lithium, significant amounts of the element indicate a star is still in its first blush of youth.

In 1989, Brazilian astronomers reported that TW Hydrae had company after all. Using the Infrared Astronomical Satellite as their guide, the researchers identified two other neighboring stars that looked to be equally young.

The unusually high intensity of infrared light that the satellite detected comes from the dusty disks that surround young stars. Each disk absorbs the visible light from the star it encircles and reradiates it in the infrared, notes Eric L.N. Jensen of Swarthmore (Pa.) College.

Examining a large swath of the southern sky in 1992, the Brazilian team found that TW Hydrae had several more companions. By then, another telescope, the German-British X-ray satellite ROSAT, had began surveying the entire sky. Because young stars emit intense X rays, they stand out prominently in the ROSAT survey. The X-ray data also indicated other groups of newborn stars near Earth.

"The ROSAT all-sky survey in X rays really made the biggest difference" in finding these groups, says Jayawardhana.

The Hipparcos satellite, launched in 1989, has also aided the search for young stellar groups close to Earth. During its 4-year mission, Hipparcos measured parallax--the apparent motion of stars caused by Earth's changing vantage point as it orbits the sun. This satellite gauged the distances and measured the true motions of bright, nearby stars. The data indicated which ones move together across the sky. Those that travel in concert are considered to have a common origin.

Since 1997, astronomers have identified nine or so new groups of young, nearby stars, estimates Jayawardhana.

Next month, Inseok Song and Ben M. Zuckerman of the University of California, Los Angeles will provide new information about what may turn out to be the closest grouping of newborn stars. When European astronomers discovered this tiny stellar group about a year ago, they reported two low-mass stars that move in sync with Beta Pictoris, the first star for which astronomers had imaged a circumstellar disk. Song and Zuckerman have since found two additional members of the association, known as the Beta Pictoris moving group.

If the five stars constitute a genuine group, they would range in age from 15 to 20 million years and lie only about 65 light-years from Earth. Song and Zuckerman will present their findings at a June meeting of the American Astronomical Society in Pasadena, Calif.

Among the other recently discovered groups is MBM12, the second-nearest menagerie of young stars. It resides about 200 light-years from Earth. A more youthful version of the TW Hydrae association, MBM12 still has traces of the gas cloud from which its stars emerged.

In contrast, Eta Chamaeleontis, a cluster of about a dozen stars, lies far from any birth cloud, but its members are much less widely dispersed than those in TW Hydrae. It may represent a group intermediate in age between TW Hydrae and MBM12.

"This is an emerging field--it's only in the last 4 or 5 years that people took notice," says Jayawardhana. "When we just had the TW Hydrae group, people kind of ignored it because it was just one example."

Just as astronomers began discovering these nearby groups of young stars, planet hunters made a major breakthrough. In 1995, two teams succeeded in detecting massive planets by an indirect method. They measured the wobble that these orbiting bodies induce in the motion of their parent stars (SN: 10/21/95, p. 260).

Once those teams demonstrated a successful method for finding planets, it "provided a lot of impetus to look for other ways for looking," including direct imaging, notes Jensen.

The wobble and imaging methods are complementary, says Jayawardhana. The wobble technique works best for deducing massive planets that closely orbit their parents because these orbs exert the largest tug on their stars. Imaging requires that a planet lie a fair distance from its star, so that the faint infrared glow from the orbiting body can be distinguished from the blindingly bright light of its parent.

Soon after the planet hunters reported their first successes, observational astronomy got a technological boost from a method known as adaptive optics. In such a system, a computer-controlled device flexes a telescope mirror hundreds of times a second to continuously correct for the blurring caused by Earth's turbulent atmosphere.

For ground-based telescopes, adaptive optics are essential for capturing faint images of planets and concentrating their light precisely enough to track their motion, says Schneider. These optics have been installed on several large, ground-based telescopes, including the Keck II atop Hawaii's Mauna Kea. One of the quartet of instruments known as the Very Large Telescope in Paranal, Chile, will soon be outfitted with a similar system.

Flying above Earth's turbulent atmosphere, the Hubble Space Telescope doesn't require adaptive optics. Another technology, however, has bolstered Hubble's ability to image planets. The telescope's cryogenically cooled, near-infrared camera, NICMOS, relies on a precisely fashioned mask to block the light from a bright star. This coronagraph enabled astronomers to search the region around a star for faint companions.

In 1997, Schneider, Zuckerman, Eric E. Becklin of the University of California, Los Angeles and their colleagues embarked on a search using NICMOS to find planets or disks around young stars. As the importance of the TW Hydrae association became generally known, the team juggled its observing plan to include several stars in the association.

The study yielded a bonanza. Among the seven TW Hydrae stars that they examined, they found a disk around two, a dusty ring around another, and, for a fourth, an orbiting companion whose large mass qualifies it as a failed star. Moreover, in proximity to the star called TWA 6, the researchers found a faint object that could be a planet.

Schneider's team estimates that this object's brightness indicates a mass about twice that of Jupiter. However, the body appears to lie more than three times as far from its star as Pluto, our solar system's outermost planet, lies from the sun.

The object's sizable distance from its presumed parent star could indicate that it's not a planet after all but some dim background star or galaxy that happens to appear near TWA 6 in the sky. On the other hand, notes Jayawardhana, "we shouldn't assume that planets don't exist so far from their parents."

He speculates that even if a planet weren't born far from its parent star, it might have been kicked out to that distance by the gravity of an unseen planet that lies much closer in.

In late 1998, NICMOS ran out of coolant before Schneider's team could track the motion of the object near TWA 6. A final observation with NICMOS did show that the object was relatively red, an indication that it wasn't some spurious foreground object or an energetic quasar.

"There's no proof that it's a planet, but the fact that it's very red ... makes it a very interesting candidate," says Schneider. "I don't know of another candidate in the same ballpark."

NASA plans to revive NICMOS early next year. But Schneider and his colleagues are hoping they won't have to wait that long to find out if the fuzzy point of light they've detected is a planet.

With the adaptive optics system on the Keck II Telescope, Schneider and his coworkers, including Bruce Macintosh of the Lawrence Livermore (Calif.) National Laboratory, have continued to track the object, dubbed TWA 6B. Observations taken in February could suffice to pin down the motion of the faint body, says MacIntosh. If not, the team plans to observe TWA 6B again in December.

Macintosh is still analyzing the February data, but he and Zuckerman say the object may turn out to be a mere background star. "I'm a pessimist," says Zuckerman.

Skeptics may still doubt the object is a planet even if it does move in concert with TWA 6, says Jayawardhana. It could be a heavier partner, such as a brown dwarf.

To assess his own planetary candidates, Jayawardhana says he would like to obtain their spectra as well as track their motion. Recording the spectra of such a faint body isn't easy, he notes. The resulting data, however, can reveal the composition of the object and shed further light on whether it's a planet or a failed star.

The region around nearby newborn stars isn't the only place that astronomers are hoping to capture an image of a planet, notes Jayawardhana. His team is also surveying the environs of several mature stars just a few light-years from Earth whose wobbles indicate they harbor closely orbiting planets as massive as Jupiter. Although those planets would lie much too close to their parent star to be imaged, they could be part of a larger system of planets, one of which resides far enough from the star to be seen.

With adaptive optics systems and coronagraphs under development for several new telescopes, researchers have the capability to image planets, says Schneider.

"Some of the candidates may pan out, some may not," he concludes. "If it's not TWA 6B, then unquestionably someone in the next few years is going to have the first real image of what people will think of as a bona fide planet."