BOULDER — Scientists have made their first direct discovery of a planet
orbiting a bright star using a network of small telescopes and the
“transit method” of detection. A periodic dimming of light from a bright
star 500 light years away revealed the planet’s presence. The star’s
intense light will allow scientists to explore the chemical makeup of the
planet’s atmosphere in future observations. A paper on the recent
discovery will appear in The Astrophysical Journal Letters.

This is the first extrasolar planet discovery made by a dedicated survey
of many thousands of relatively bright stars in large regions of the sky.
It is also the first using the Trans-Atlantic Exoplanet Survey (TrES,
pronounced “trace”), a network of small, relatively inexpensive
telescopes designed to look specifically for planets orbiting bright
stars. The telescopes make use of the transit technique, in which
scientists analyze the shadow cast by a planet as it passes between its
star and Earth.

The discovery team includes scientists from the Astrophysical Institute
of the Canaries (IAC), National Center for Atmospheric Research (NCAR),
Harvard-Smithsonian Center for Astrophysics (CfA), Lowell Observatory,
and California Institute of Technology.

A team of scientists led by Timothy Brown (NCAR), David Charbonneau
(CfA), and Edward Dunham (Lowell Observatory) developed the TrES network.
Brown built the optical system of the telescope used in the discovery and
located on Tenerife in the Canary Islands. A graduate student of Brown’s,
Roi Alonso Sobrino, of the IAC, discovered the planet, called TrES-1,
after three years of persistent planet hunting.

“The fact that we can learn anything at all about a planet 500 light
years away is astonishing, ” says Brown.

The network’s other two telescopes are located at the Lowell Observatory
in Arizona and at Mt. Palomar, California.

“It’s almost paradoxical that, with the transit method, small telescopes
are more efficient than the largest ones, in a time when astronomers are
planning 100-meter telescopes,” says Alonso.

Of the approximately 12,000 stars examined by the TrES survey, Alonso
identified 16 possible candidates for planet transits.

“The TrES survey gave us our initial lineup of suspects. Then, we made
follow-up observations to eliminate the imposters,” says co-author
Alessandro Sozzetti (CfA/University of Pittsburgh).

After compiling the list of candidates in late April, the researchers
used telescopes at CfA’s Whipple Observatory in Arizona and Oak Ridge
Observatory in Massachusetts to obtain additional photometric
(brightness) observations, as well as spectroscopic observations that
eliminated eclipsing binary stars.

In a matter of two month’s time, the team had zeroed in on the most
promising candidate. High-resolution spectroscopic observations by
Guillermo Torres (CfA) and Sozzetti using the 10-meter-diameter Keck I
telescope in Hawaii clinched the case.

“Without this follow-up work the photometric [brightness] surveys can’t
tell which of their candidates are actually planets. The proof of the
pudding is a spectroscopic orbit [using the Doppler method] for the
parent star. That’s why the Keck observations of this star were so
important in proving that we had found a true planetary system,” says co-
author David Latham (CfA).

More than 120 planets have been found by the Doppler method, which
detects the gravitational pull of the planet on its star, but only
gigantic planets can be “seen” this way. Moreover, the Doppler method
gives indirect information about a planet. In 1999, the transit method
was first used successfully to confirm the existence of a planet that had
been discovered through its gravitational effect.

Only now has the transit method resulted in a discovery involving a
Jupiter-size planet circling a bright star. The success of the transit
method opens the possibility of directly determining key information
about the planet, such as its mass and radius (size), and its atmospheric
components.

Next Step: Exploring the TrES-1 Atmosphere

Scientists study an extrasolar planet’s atmosphere by using a technique
called spectroscopy. As starlight passes through the planetary
atmosphere, light at some wavelengths disappears. This occurs as
elements and compounds in the atmosphere, such as methane and carbon
monoxide, absorb light at specific wavelengths.

By observing which wavelengths are absorbed, Brown and colleagues will
learn which elements are present in TrES-1’s atmosphere. The scientists
plan to search for water vapor first, since it can give clues about other
chemical components.

“All that we have to work with is the light that comes from the star,”
says Brown. “It’s much harder to learn anything when the stars are
faint.” Three planets have been found with the transit method using large
telescopes aimed at faint stars. However, the starlight is too dim to
examine the planetary atmospheres.

Brown’s research is funded by the National Science Foundation, NCAR’s
primary sponsor, and by NASA.

Opinions, findings, conclusions, or recommendations expressed in this
publication are those of the author(s) and do not necessarily reflect the
views of the National Science Foundation.

Note to Editors: “TrES-1: The Transiting Planet of a Bright KOV Star” is
available on the Web at http://xxx.lanl.gov/archive/astro-ph (number
astro-ph/0408421).

The paper’s authors are Roi Alonso Sobrino, IAC; Timothy Brown, NCAR;
Guillermo Torres, David W. Latham, and Alessandro Sozzetti, CfA; Juan
Belmonte, IAC; David Charbonneau, CFA; Hans J. Deeg, IAC; Edward W.
Dunham and Georgi Mandushev, Lowell Observatory; Francis T. O’Donovan,
California Institute of Technology; and Robert Stefanik, CFA.

On the Web:

Find this press release and images on the Web at
http://www.ucar.edu/news/releases

More about the transit method (with graphic)
http://www.hao.ucar.edu/public/research/stare/overview.html

and Doppler spectroscopy
http://www.hao.ucar.edu/public/research/stare/search.html#methods