Astronomers using NASA’s Hubble Space Telescope have
made the first direct detection and chemical analysis of the
atmosphere of a planet outside our solar system. Their unique
observations demonstrate it is possible with Hubble and other
telescopes to measure the chemical makeup of extrasolar
planets’ atmospheres and potentially to search for chemical
markers of life beyond Earth.

The planet orbits a yellow, Sun-like star called HD 209458, a
seventh-magnitude star (visible in an amateur telescope) that
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
astronomers for the first time ever to see light from the
star filtered through the planet’s atmosphere.

Lead investigator David Charbonneau of the California
Institute of Technology, Pasadena, and the Harvard-
Smithsonian Center for Astrophysics, Cambridge, Mass.;
Timothy Brown of the National Center for Atmospheric
Research, Boulder, Colo.; and colleagues used Hubble’s
spectrometer (the Space Telescope Imaging Spectrograph, or
STIS) to detect the presence of sodium in the planet’s

“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.
The astronomers actually saw less sodium than predicted for
the Jupiter-class planet, leading to one interpretation that
high-altitude clouds in the alien atmosphere may have blocked
some of the light. The team’s findings are to be published in
the Astrophysical Journal.

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 exoplanets, or 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.

The planet was discovered in 1999 through its slight
gravitational tug on the star. The planet was estimated to be
70 percent the mass of the giant planet Jupiter, or 220 times
more massive than Earth. Subsequently, astronomers discovered
that the tilt of the planet’s orbit makes it pass in front of
the star — relative to our line-of-sight from Earth —
making it unique among all the approximately 80 extrasolar
planets discovered to date. As the planet passes in front of
the star, it causes the star to dim very slightly for the
duration of the transit. Transit observations by Hubble and
ground-based telescopes confirmed that the planet is
primarily gaseous, rather than liquid or solid, meaning that
the planet is a gas giant, like Jupiter and Saturn.

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

Observations of four separate transits were made by Hubble in
search of direct evidence of an atmosphere. During each
transit a small fraction of the star’s light on its way to
Earth passed though the planet’s atmosphere. When the color
of the light was analyzed by STIS, the telltale “fingerprint”
of sodium was detected. Though the star also has sodium in
its outer layers, STIS precisely measured the added influence
of sodium in the planet’s atmosphere.

The team, including Robert Noyes of the Harvard-Smithsonian
Center for Astrophysics and Ronald Gilliland of the Space
Telescope Science Institute in Baltimore, 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. Once other transiting giants are found in the
next few years, the team expects to characterize chemical
differences among the atmospheres of these planets.

The Space Telescope Science Institute (STScI) is operated by
the Association of Universities for Research in Astronomy,
Inc. (AURA), for NASA, under contract with the Goddard Space
Flight Center, Greenbelt, Md. The Hubble Space Telescope is a
project of international co-operation between NASA and the
European Space Agency (ESA). The National Center for
Atmospheric Research is sponsored primarily by the National
Science Foundation.


Astrophysics, abstract

From: David Charbonneau <>
Date: Wed, 28 Nov 2001 21:24:02 GMT (146kb)

Detection of an Extrasolar Planet Atmosphere

David Charbonneau,
Timothy M. Brown,
Robert W. Noyes,
Ronald L. Gilliland
Comments: 26 pages, 8 figures, accepted by ApJ 2001 November 19

We report high precision spectrophotometric observations of four planetary
transits of HD 209458, in the region of the sodium resonance doublet at 589.3
nm. We find that the photometric dimming during transit in a bandpass centered
on the sodium feature is deeper by (2.32 +/- 0.57) x 10^{-4} relative to
simultaneous observations of the transit in adjacent bands. We interpret this
additional dimming as absorption from sodium in the planetary atmosphere, as
recently predicted from several theoretical modeling efforts. Our model for a
cloudless planetary atmosphere with a solar abundance of sodium in atomic form
predicts more sodium absorption than we observe. There are several
possibilities that may account for this reduced amplitude, including reaction
of atomic sodium into molecular gases and/or condensates, photoionization of
sodium by the stellar flux, a low primordial abundance of sodium, or the
presence of clouds high in the atmosphere.

Paper: Source (146kb), PostScript, or Other formats
(N.B.: delivery types and potential problems)refers to ,
cited by

NOTE TO EDITORS: Electronic images, illustrations, animations
and additional information are available at:

and via links in

Artist's Concept: Light curve changes during transit of HD 209458
Artist’s Concept: Light curve changes during transit of HD 209458

This animation illustrates the dimming effect of a planet on its host star, as represented by the graphic light curve.

View this image: 


Date Published: Tuesday, November 27, 2001

Artist's Concept: Planet transits HD 209458
Artist’s Concept: Planet transits HD 209458

A planet passes in front of its host star – a celestial event called a transit.

View this image: 


Date Published: Tuesday, November 27, 2001

Artist's concept of the planet orbiting HD 209458
Artist’s concept of the planet orbiting HD 209458

Though the planet’s detailed appearance is an artist’s concept, it is known to be a gas giant similar to Jupiter, and is locked very close to its host star.

View this image: 


Date Published: Tuesday, November 27, 2001