Download Planet Finding

As astronomers find more
and more planets orbiting distant stars,
an NCAR scientist and his colleagues have for the
first time detected the atmosphere of an extrasolar
planet. Now they plan to analyze the atmosphere in
more depth, which may offer clues about the creation
of solar systems—and possibly provide a method
for finding life on other worlds.
Planet finding
Detecting the atmosphere of a faraway world
I
It was more than 200 years ago—on June 6, 1761, to be precise—that a Russian
scientist named Mikhail Lomonosov observed what looked like a black dot
crossing the surface of the Sun. The dot was Venus, making a rare direct passage between the Sun and Earth, and Lomonosov noticed something odd about
the planet’s edge: it appeared to be enveloped in a thin haze. This marked the
first time a scientist detected an atmosphere on another planet.
Last year, NCAR’s Timothy Brown and a team of fellow astronomers also
detected an atmosphere by observing light passing by the edge of a planet. Their
discovery made scientific history because the planet isn’t in our solar system.
Instead, the massive sphere—known prosaically as HD 209458 b—orbits a star
some 150 light years from Earth (one light year is equivalent to about 6 trillion
miles, or more than 10 trillion kilometers).
Building on this breakthrough, scientists believe they can analyze distant
atmospheres to learn more about the formation of other solar systems—and
potentially even gather clues about life elsewhere in the galaxy. Brown and his
collaborators are turning to the next step in their research: determining the
molecular composition of the atmosphere of HD 209458 b.
Brown’s colleagues include Robert Noyes (Harvard-Smithsonian Center for
Astrophysics), Ronald Gilliland (Space Telescope Science Institute), David
Charbonneau (California Institute of Technology), Edward Dunham (Lowell
Observatory), and Juan-Antonio Belmonte (IAC, Astrophysics Institute of the
Canary Islands). Despite the enormous difficulties in making observations
from a distance of 150 light years, Brown has little doubt of success. “We should
be able to detect common molecules in the atmosphere,” he says.
A lifelong interest
Brown has been interested in the stars ever since
he was a child, reading about the launch of
Sputnik and other satellites in the 1950s. “I
can’t remember wanting to be anything
but an astronomer,” he says. With his
father, James Brown (former chancellor of Southern Illinois University), he
built a telescope when he was 14 years
old—then built a small observatory for the
telescope so he and his friends could be out of
the wind as they watched the stars.
Timothy Brown used this
telescope to spot planets
beyond our solar system.
After earning a doctorate in astrophysics from the University of Colorado,
Brown joined NCAR’s High Altitude Observatory to pursue solar
research. His work took a dramatic
turn in 1992, when he and a team of
scientists at NCAR and the Smithsonian Astrophysical Observatory
UCAR Highlights 11
mounted a spectrograph they’d designed and
built on a 60-inch (1.5-meter) telescope at
the Fred Lawrence Whipple Observatory on
Mt. Hopkins in Arizona. They’d created the
spectrograph to study subtle oscillations in the
light coming from the Sun and other stars. But
Brown soon realized it could be used for planet
hunting as well, and he began his search, just
as others were launching similar quests. At
that time, no planets in
other solar systems had
ever been detected—but
scientists such as Brown
were certain of their
existence.
While stars exert gravitational pull on planets, the
reverse is also true. The detected radial velocity is a
faint wobble in a star created by the gravitational pull
of its orbiting planet. This method yields three kinds
of data: how close the planet is to its star, how massive
the planet is, and the shape of its orbit.
It wasn’t until 1995 that Michel Mayor and Didier Queloz of the Geneva Observatory found the first extrasolar
planet, circling the star 51
Pegasi. Brown’s team, which
had also been investigating
the star, was one of the first to
confirm the groundbreaking
observations.
Four years later, teams
Such planets were too
at Harvard-Smithsonian
far away and too small to
(which included Brown as a
be observed directly by
member) and San Francisco
even the most powerState University revealed
ful telescopes. Instead,
that not one, but three
Brown and other scien- Timothy Brown and David Charbonneau detected
planets were orbiting the
tists turned to a techa planet crossing in front of its star, HD 209458, as
star Upsilon Andromedae.
nique involving radial
shown in this artist’s rendition.
This was the first time an
velocity. This method
entire solar system had been
uses Doppler spectroscopy to look for tiny
detected—and it provided strong evidence that the
shifts in spectral lines that indicate motion
galaxy may well be teeming with planets. “When I
toward (blue) or away (red) from the observer.
look up at the stars now at night, I can imagine easily
that every one of them has planets around [it],” says
Debra Fischer, a physicist and astronomer who was a
member of the San Francisco State team.
A distant STARE
In transit
Under the remote and starry skies of the Canary Islands, a
In the summer of 1999, Brown and Charbongroup of NCAR researchers is probing the galaxy for hints of distant
neau
(who was then an HAO Newkirk Fellow and
planets. The project, known appropriately as STARE (Stellar Astrophysics
a
graduate
student at Harvard) set up a small
and Research on Exoplanets), consists of viewing thousands of stars for weeks
telescope
in
the parking lot of NCAR’s Foothills
or months at a time to detect the faint dips of light that could indicate an orbiting
Lab
and
pointed
it skyward. The telescope (with
planet. Using a telescope that NCAR scientist Timothy Brown built, the researchers
lenses
that
Brown
ground himself) used precise
(including Donald Kolinski of NCAR and Roi Alonso Sobrino of IAC) take timed
time-series
photometry
to detect the dimming
exposures of the same field of view, then analyze the resulting data for light curves.
caused
when
a
planet
transits
(crosses in front
The STARE telescope was installed in a newly constructed dome on the island of
of)
its
parent
star.
When
the
two
men trained the
Tenerife by a team from NCAR's High Altitude Obsevatory. There’s no guarantee
instrument
on
the
star
known
as
HD
209458 in Sepof success—Brown and Ronald Gilliland of the Space Telescope Science Institute
tember
1999,
they
found
the
classic
dimming
pattern
in Baltimore failed to detect planets after a survey of 35,000 stars in 2000.
that
indicated
the
existence
of
a
planet.
But STARE will provide other scientific payoffs, such as cataloging stars
that have significant variations.
Brown turned to the transit method to augment the
data gleaned from radial velocity. The method relies
on the alignment of extrasolar systems. If a system
is oriented so that Earth lies near the plane of the
distant planet‘s orbit, then once per orbit the planet
passes between its star and Earth, making the star
appear dimmer. A Jupiter-sized planet transiting a
Sun-sized star, for example, would cause the star’s
light to dim about 1% for several hours.
By combining the data on diameter with data on
a planet’s mass from radial velocity measurements,
“you can determine its density,” Brown explains.
“And if you know its density, then—eureka!—you
know what it’s made of.”
The transit method also can be used for determining whether a distant planet has an atmosphere. That’s because an atmosphere, if it existed,
would distort or leave a chemical signature on
the small amount of the star’s light that passed
through it (as Lomonosov saw when he detected
the atmosphere of Venus by noticing a slight haze.)
Different molecules absorb starlight in different ways, and Brown’s team decided to look for
sodium, which leaves a distinctive signature that
shows up in the yellow-green part of the spectrum.
“Sodium has a strong spectroscopic signature,”
Brown explains. “The analogy I like to use is that
it’s like skunk. You don’t have to have a lot of it in
the air to know that it’s there.”
Last year, Brown and his team observed several
transits using the imaging spectrograph on
NASA’s Hubble Space Telescope. They compared
these data with spectra taken at other times.
The result: traces of sodium in the spectra of
the transits that could only have come from an
atmosphere around HD 209458 b.
The discovery itself didn’t come as a major
surprise. Scientific models predict that gas-giant
planets such as HD 209458 b are surrounded by
atmospheres that contain sodium. But the finding
is an astronomical landmark because it points
the way to observing atmospheres of other distant
planets. “This opens up an exciting new phase of
extrasolar planet exploration, where we can begin
to compare and contrast the atmospheres of planets around other stars,” says Charbonneau.
Within the next few years, scientists will have
powerful new space- and ground-based tools to
hunt for extrasolar planets. Astronomers believe
they can use spectrographs to look for signs of life
elsewhere in the galaxy. If they detected oxygen in
an atmosphere, for example, that would be an indication of plant life. That’s because oxygen, which
easily combines with other elements, is rarely
present in pure form unless it is being regularly
replenished.
Brown doubts that HD 209458 b harbors life as
we know it. The planet is so hot that any clouds
in the atmosphere would probably be composed
of liquid minerals, such as molten drops of iron.
Given its rapid rotation and proximity to its sun,
the atmosphere is probably extremely turbulent.
“It’s probably crazy by Earth standards,” he says.
“You can bet that there are winds on that planet,
and they may even be supersonic.”
Shown in an artist’s depiction are three extrasolar
planets revolving around the
star Upsilon Andromedae.
Brown believes the data gathered by his team and
other astronomers will yield valuable insights into
the formation of planets and enable scientists to
determine whether the composition of our own solar system is the exception or the rule in the galaxy.
“For a long time we’re going to be quite limited in
what we can detect,” he warns, because the planets
are so far away. But, he adds, “the proposition is so
exciting we ought to see what we can deduce.”
On the Web
NASA/JPL PlanetQuest
http://planetquest.jpl.nasa.gov
STARE
http://www.hao.ucar.edu/public/research/stare/stare.html
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