Periodic Dimming of Bright Starlight Reveals Distant Planet
August 24, 2004
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.
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