WASHINGTON, D.C. — Using NASA’s Spitzer Space Telescope,
researchers have for the first time detected light from confirmed
planets orbiting stars outside our solar system.

Two teams of researchers have observed two planets in our galaxy, the
Milky Way: One, cataloged as HD 209458b, is orbiting a star 153 light
years distant — about 29 quadrillion miles from Earth — in the
constellation Pegasus, the winged horse. The second planet, TrES-1,
is even more distant — 489 light years away in the constellation

At least 130 stars outside our solar system have been shown to have
orbiting planets, based on a slight wobble in the stars’ positions or
a variation in intensity as planets pass in front of them. Both HD
209458b and TrES-1 were initially detected by these indirect
methods. However, “Our Spitzer observations are the first direct
measurements of light from confirmed extrasolar planets,” says Joseph
Harrington, a senior research associate in the Center for
Radiophysics and Space Research at Cornell University, Ithaca, N.Y.

Harrington is a member of a team led by Drake Deming at NASA’s
Goddard Space Flight Center, which measured infrared radiation from
HD 209458b. Almost simultaneously, a team led by David Charbonneau at
the Harvard-Smithsonian Center for Astrophysics measured infrared
emissions from TrES-1. Both planets are gas giants similar to the
planet Jupiter in our solar system. But they are “hot Jupiters”
because they orbit very close to their stars and absorb so much heat
that they radiate strongly in the infrared. Both teams used
instruments on the Spitzer Space Telescope, which was launched in
August 2003. Deming’s team used the Multiband Imaging Photometer for
Spitzer and Charbonneau’s team used the Infrared Array Camera.

The two teams announced their discoveries today (March 23, 2005) at a
NASA press conference in Washington, D.C. The journal Nature has
posted a paper from the Deming group in its online edition and will
publish the paper in its April 7, 2005, issue. A paper from the
Charbonneau group has been posted on the arXiv preprint server at
Cornell and will appear in the June 20, 2005, issue of Astrophysical

Both teams used a similar technique: observing the change in
intensity of light as the planet passed behind the star. Subtracting
this from the intensity of star and planet system, measured prior to
the transit of the planet behind the star, leaves a direct detection
of the radiation from the hot planet.

Even the nearest stars to Earth are so far away that most telescopes
see them only as points of light, although future specialized NASA
missions hope to obtain images showing a planet separated from a
sunlike star. Meanwhile, astronomers can deduce a great deal by
observing the variations in a star’s light and by observing its
spectrum. Based on spectral type, HD 209458b is orbiting a star very
much like our sun. TrES-1’s star is smaller and cooler.

Both planets are closer to their stars than is Mercury to our sun,
and both complete one orbit about every three days. Tidal forces
cause their rotation to match that time, so that they always turn the
same face to their stars, just as the moon always has the same face
to Earth. Based on HD 209458b’s light output, Deming’s team
determined the planet’s temperature at 1,130 Kelvin (857 degrees
Celsius or 1,574 degrees Fahrenheit), which is about double the
temperature of a pizza oven. Charbonneau’s team estimates TrES-1’s
temperature at 1,060 Kelvin (787 degrees Celsius or 1,448 degrees
Fahrenheit). The temperature measurement of HD 209458b has a possible
error of plus or minus about 150 Kelvin, while for TrES-1 it is plus
or minus 50 Kelvin.

Gas giants like Jupiter become very hot when they are formed, as the
gas that makes them up is compressed by gravity. Jupiter cooled over
time, and now has a temperature at its cloud tops of 123 Kelvin, or
150 degrees Celsius below zero. “Hot Jupiters,” by contrast, receive
heat from their stars and remain hot. It is this heat that makes
these planets a choice for infrared detection.

A significant finding by the Deming group is that HD 209458b’s
eclipse behind its star occurs exactly halfway around its orbit from
its transit in front, meaning that its orbit is likely circular
rather than elliptical. (It could be elliptical only if the long axis
of the ellipse points directly toward Earth, which would be a
remarkable coincidence, the researchers say.) This finding refutes a
hypothesis that the planet has an elliptical orbit due to the
influence of another planet in the system. Such an orbit would have
heated the planet by means of a varying tide, but this heating
mechanism is now ruled out. The finding doesn’t rule out other
planets, only those in certain positions.

Deming’s and Harrington’s co-authors on the Nature paper are Sara
Seager of the Carnegie Institution of Washington, D.C., and Jeremy
Richardson of NASA Goddard. Co-authors of the Astrophysical Journal
paper are Lori E. Allen, S. Thomas Megeath, Alessandro Sozzetti,
David W. Latham and Guillermo Torres of the Harvard-Smithsonian; Roi
Alonso of the Instituto de Astrofffisica de Canarias, Spain; Timothy
M. Brown of the High Altitude Observatory, NationalCenter for
Atmospheric Research, Boulder, Colo.; Ronald L. Gilliland of the
Space Telescope Science Institute, Baltimore, Md.; Georgi Mandushev
of Lowell Observatory, Flagstaff, Ariz.; and Francis T. O’Donovan
of the University of Pittsburgh.

The Spitzer telescope is a NASA mission managed by the Jet Propulsion
Laboratory in Pasadena, Calif. The third science instrument for
Spitzer, the Infrared Spectrometer, was developed by a Cornell team
led by astronomy Professor James Houck.

Related World Wide Web sites: The following sites provide
additional information on this news release. Some might not be part
of the Cornell University community, and Cornell has no control over
their content or availability.

Spitzer Telescope home page: http://www.spitzer.caltech.edu/

NASA Spitzer site: http://www.spitzer.caltech.edu/spitzer/index.shtml

Cornell’s Spitzer telescope site: http://www.news.cornell.edu/releases/SIRTF/

The arXiv preprint server: http://arxiv.org/archive/astro-ph