Earth's strongest winds wouldn't even be a breeze on these planets(Forwarded)

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Jan. 9, 2007

Earth's strongest winds wouldn't even be a breeze on these planets

Earth's inhabitants are used to temperatures that vary, sometimes greatly,
between day and night. New measurements for three planets outside our
solar system indicate their temperatures remain fairly constant -- and
blazing hot -- from day to night, even though it is likely one side of
each planet always faces its sun and the other is in permanent darkness.

The reason apparently is supersonic winds, perhaps as strong as 9,000
miles an hour, that constantly churn the planets' atmospheres and keep
temperatures on the dark side from plunging.

The planets, gas giants similar in size to Jupiter, were discovered in the
last decade orbiting stars about the same size as our sun and less than
150 light years from Earth. All of them orbit within about 5 million miles
of their stars, far less than Mercury's distance from our sun.

Astronomers have wondered whether planets orbiting so close to their stars
but with one side in constant daylight and the other permanently dark
would exhibit sharp temperature differences between the day side and the
night side. For the three planets in this study, the temperatures appear
to be constant, likely because of the strong winds that mix the atmosphere
planetwide, said Eric Agol, a University of Washington assistant professor
of astronomy and co-author of a poster presenting the findings today at
the American Astronomical Society national meeting in Seattle.

"We can't say for sure that we've ruled out significant day-night
temperature differences, but it seems unlikely there is a very big
contrast based on our measurements and what we know about these systems,"
said Agol, who is lead scientist for a project using the Spitzer Space
Telescope to measure the temperature properties of extrasolar planets.

Agol and colleagues Nicolas Cowan, a UW astronomy doctoral student and
lead author of the poster, and David Charbonneau of the
Harvard-Smithsonian Center for Astrophysics measured infrared light from
each of the planetary systems at eight different positions in their orbits
in late 2005. They measured the thermal brightness of the systems when the
planets' day sides faced the Earth, when the night sides faced the Earth
and at various phases in between. They detected no infrared brightness
variations in any of the systems, suggesting there are not big differences
in temperatures on the day and night sides.

Instead the planets appear to have a fairly uniform temperature of about
925 degrees Celsius, or about 1700 degrees Fahrenheit.

"If heat from the parent star is carried to the dark side, then the
overall temperature would be lowered somewhat because the heat is being
distributed across the planet," Agol said. "Some theorists believe that
supersonic winds are responsible for recirculating the heat."

Measuring the planets' temperatures is a painstaking process because a
planet's radiation is drowned out by the light from its host star. Even
when a planet goes behind the parent star and disappears completely from
view, the decline in light from the entire system is almost imperceptible,
on the order of 0.25 percent, Agol said. Making the observations requires
precise calibration and light measurements.

The three planets are 51 Pegasi, about 50 light years from our sun,
HD179949b about 100 light years distant, and HD209458b about 147 light
years away. A light year is about 5.88 trillion miles. In 1995, 51 Pegasi
became the first planet orbiting another star to be discovered. Since then
numerous planets -- gas giants the mass of Jupiter or larger -- have been
observed from Earth. Most orbit very close to their stars. A common theory
is that they formed far away from their stars, perhaps in about the same
position as Jupiter is to our sun, and then migrated close to their stars.
Their distance makes it difficult to gather much direct data about the
planets.

To date no Earth-sized planets have been reported orbiting other stars
like our sun.

Agol noted that the planets probably have the same side always facing the
star because they are so close to their parent stars. The effect is the
same as the Earth has on the moon, which has had its rotation slowed so
much by Earth's gravity that the same side always faces Earth.

"These planets are so close to their host stars that the tidal forces are
enormous, a few thousand times as strong as on Earth," he said. "The tides
are so strong and form a bulge that distorts the planet so much that the
orbit is slowed by the star's tug on the tidal bulge."

The Spitzer telescope is managed by NASA's Jet Propulsion Laboratory in
Pasadena, Calif., and its science operations are conducted at the
California Institute of Technology.

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For more information, contact Agol at (206) 543-7106; or Cowan at (206)
251-0093.

See an animation at

http://sscws1.ipac.caltech.edu/Image..._name=svg07-01

IMAGE CAPTION:
[http://uwnews.washington.edu/ni/phot...97&spid=29398]
An artist's conception shows a gas-giant planet orbiting very close to its
parent star, creating searingly hot conditions on the planet's surface.
New research suggests that for three such planets lying from 50 to 150
light-years from Earth, strong winds thousands of miles per hour mix the
atmosphere so that the temperature is relatively uniform from the
permanently light side to the permanently dark side. This illustration
represents an infrared view of a planetary system, in which brightness
indicates warmer temperatures. For example, the bright band around the
equator of the planet denotes warmer temperatures on both the dark and
sunlit sides. The planet's poles, shown in darker colors, would be cooler.
NASA/JPL-Caltech/R. Hurt