Next Step: What Color Is It?

A recent student visitor to the National Center for
Atmospheric Research and an NCAR senior scientist have made the first
direct detection of the atmosphere of a planet orbiting a star
outside our solar system and have obtained the first information
about its chemical composition. Their unique observations from NASA’s
space-based Hubble telescope demonstrate that it is possible to
measure the chemical makeup of extrasolar planetary atmospheres and
potentially to search for chemical markers of life beyond Earth.

Lead investigator David Charbonneau, a former graduate student fellow
at NCAR, and NCAR’s Timothy Brown, used the Hubble telescope’s
spectrometer to detect the presence of sodium in the extrasolar
planet’s upper atmosphere. Charbonneau is now at the California
Institute of Technology. The National Science Foundation, NCAR’s
primary sponsor, and NASA supported the research.

“Only a decade ago, planets outside the solar system were still in
the realm of science fiction,” says Charbonneau. “Searching for a
star’s unseen planetary companion was crazy. Hoping to see its
atmosphere was even crazier.” Now planets are discovered monthly, and
even their atmospheres are in reach, he says. “Suddenly, discussing
searches for Earth-like planets seems quite reasonable.”

The graduate student and the senior scientist worked together at
NCAR’s High Altitude Observatory to develop their computer model and
refine their observational approach. The model predicted which
chemicals and how much of each to look for. To take advantage of the
particular strengths of the Hubble spectrograph, they narrowed their
search to sodium. Last month, two years after their initial proposal,
they were rewarded when the anticipated thick black lines showed up
at the predicted position in the yellow-green region of the
starlight’s visible spectrum —
a sure sign of sodium atoms absorbing
and scattering starlight. The Hubble’s highly precise measurements
confirmed that the sodium exists not only in the star’s atmosphere
but in the planet’s as well.

“It’s hugely exciting to nail down anything at all about something as
mysterious as planets outside our solar system,” says Brown. “Is this
planet’s chemical personality unique or is it typical of a certain
class of extrasolar planets? We have no clue. We hope to find out.”
Extrasolar planetary atmospheres are tough to crack scientifically
because the observable quantities are tiny and the measurements must
be extremely precise, he says.

First color in a planet’s portrait

The planet orbits a yellow, sun-like star called HD 209458, a
seventh-magnitude star (visible with an amateur telescope), which lies
150 light-years away in the autumn constellation Pegasus. Its
atmospheric composition was probed when the planet passed in front of
its parent star, allowing the scientists for the first time ever to see
light from the star filtered through the planet’s atmosphere. Before
their observation, there was no direct proof of the existence of an
atmosphere on any of the 76 extrasolar planets discovered to date, let
alone a measurement of specific chemical composition.

Charbonneau and Brown are currently using the Hubble’s spectrograph
to scrutinize faint starlight reflected off the planet toward Earth
just as the planet passes behind the edge of its star. The results,
based on a barely measurable .01% whisper of starlight, should reveal
the color and reflectivity of the planet and indicate whether clouds
of dust or metallic elements might be present.

“In some computer models, this planet is blacker than coal,” says
Brown. “In others, it’s bright white, like Venus. Only observations
can tell us what’s real.” As soon as next spring, artists may be
using the scientists’ results to paint the planet’s portrait in true-
to-life color.

From low-tech breakthrough to the Hubble

Charbonneau already earned a place in astronomy textbooks during his
visit to NCAR. While working on his Ph.D. at the Harvard-Smithsonian
Center for Astrophysics, he was selected as a Gordon Newkirk Graduate
Student Fellow in NCAR’s High Altitude Observatory from August 1999
to August 2000. In the fall of that year, he and Brown experimented
with a small telescope, designed and fabricated at NCAR using Brown’s
homemade optics. Brown built it to test what was then an untried
technique, called the transit method, to confirm the existence of the
HD 209458 planet by observing the dimming of starlight as the planet
passed between its star and the Earth.

The same planet had been discovered a few months earlier using a more
common detection procedure based on its slight gravitational tug on
the star. From that observation, the planet was estimated to be 70
percent the mass of the giant planet Jupiter (or 220 times more
massive than Earth).

The low-tech telescope first searched for transits from a chicken
coop outside Boulder, then from a makeshift shed next to an NCAR
parking lot. (It now scans the skies for planet-bearing stars from
its perch on Tenerife in the Canary Islands.)

By November 1999, Charbonneau and Brown had results. To date the
planet circling HD 209458 remains the only extrasolar planet whose
physical existence has been confirmed using the transit method of
detection. Within a month of that success, the two scientists
proposed using the Hubble spectrograph to identify elements in an
extrasolar planetary atmosphere.

In the current Hubble experiment, the astronomers actually saw less
sodium than predicted for the Jupiter-class planet. One
interpretation suggests that high-altitude clouds in the alien
atmosphere may have blocked some of the light.

The Hubble observation was not tuned to look for gases expected in a
life-sustaining atmosphere (which is improbable for a planet as hot
as the one observed). Nevertheless, this unique observing technique
opens a new phase in the exploration of extrasolar planets, say
astronomers. Such observations could potentially provide the first
direct evidence for life beyond Earth by measuring unusual abundances
of atmospheric gases caused by the presence of living organisms.

A gaseous giant

The planet is an ideal target for repeat observations because it
transits its star every 3.5 days —
the extremely short time it takes
the planet to whirl around the star at a distance of merely 4 million
miles from the star’s searing surface. This precariously close
proximity to the star heats the planet’s atmosphere to a torrid 2,000
degrees Fahrenheit (1,100 degrees Celsius).

The team —
which also included Robert Noyes of the Harvard-Smithsonian
Center for Astrophysics and Ronald Gilliland of the Space Telescope
Science Institute in Baltimore, Maryland —
had previously used transit
observations by Hubble and ground-based telescopes to confirm that
the planet is primarily gaseous, rather than liquid or solid, because
it has a density less than that of water. (Earth, a rocky rather than
a gaseous planet, has an average density five times that of water.)
These earlier observations thus established that the planet is a gas
giant, like Jupiter and Saturn.

The planet’s swift orbit allowed for observations of four separate
transits to be made by Hubble in search of direct evidence of an
atmosphere. Though the star also has sodium in its outer layers, the
spectrograph precisely measured a very slight additional filtration
of sodium (an enhancement of less than one percent) as the starlight
passed through the planet’s atmosphere.

The team next plans to look at HD 209458 again with Hubble in other
colors of the star’s spectrum to see which are filtered by the
planet’s atmosphere. They hope eventually to detect methane, water
vapor, potassium, and other chemicals in the planet’s atmosphere.

Hot Jupiters and the search for Earth-like planets

Once other transiting giants are found in the next few years, the
team expects to characterize chemical differences among the
atmospheres of these planets. These anticipated findings would
ultimately help astronomers better understand a bizarre class of
extrasolar planets discovered in recent years that are dubbed “hot
Jupiters.” They are the size of Jupiter but orbit closer to their
stars than the tiny innermost planet Mercury in our solar system.
While Mercury is a scorched, airless rock, these planets have enough
gravity to hold onto their atmospheres, though some are hot enough to
melt copper.

Conventional theory is that these giant planets could not have been
born so close to their stars. Gravitational interactions with other
planetary bodies or gravitational forces in a circumstellar disk must
have carried these giants via spiraling orbits precariously close to
their stars from their birthplace in cooler regions farther out,
where they bulked up on gas and dust as they formed.

Proposed moderate-sized U.S. and European space telescopes could
allow for the detection of many much smaller Earth-like planets by
transit techniques within the next decade. The chances for detection
will be more challenging, since detecting a planet orbiting at an
Earth-like distance will mean a much tighter orbital alignment is
needed for a transit. And the transits would be much less frequent
for planets with an orbital period of a year, rather than days.
Eventually, study of the atmosphere of these Earth-like planets will
require meticulous measurements by future larger space telescopes.


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Ray Villard
Space Telescope Science

Cheryl Dybas