http://www.sciencenews.org/20010526/bob14.asp

Captured on Camera: Are They Planets?

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

Ron Cowen

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.

A checkered history

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.

Star-forming regions

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.

Newborn stars

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."

Planet hunters

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.

A fuzzy point

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