SEATTLE -
Heated winds on at least three planets
outside our solar system blow so
strongly that the worlds are kept at a constant toasty temperature, even the night sides that never face their parent stars.
The planets
are called "hot
Jupiters" because they are large and gaseous like Jupiter, but orbit much closer to
their stars. All three orbit within about five million miles (eight million
kilometers) of their stars, much closer than Mercury is to our Sun. Scientists
think winds on the planets blow at supersonic speeds, as strong as 9,000 miles
(14,500 kilometers) per hour, churning their home worlds' atmospheres and keeping
temperatures on their dark sides from plunging.
For more
than a decade scientists have wondered whether, like Jupiter, the temperature
on hot Jupiters are constant, or if there are sharp temperature differences
between their day and night sides.
The
findings
Last
October, a team of astronomers led by Brad Hansen of the University of
California, Los Angeles discovered evidence of the latter on a nearby hot
Jupiter called Upsilon Andromeda b [image].
Hansen's
team found that one half of Upsilon Andromeda b was as hot as molten lava,
while the other half was chilled possibly below freezing. The researchers
speculated that the sunlit side of the fire
and ice planet was perhaps tidally
locked to its stellar parent, like the Moon
is with Earth, so that only one side
of the planet faced the star. Another possibility was that Upsilon Andromeda b
was somehow shedding heat from its star rapidly into space, before its winds
could circulate it to the night side.
The new
finding, presented here at the 209th meeting of the American
Astronomical Society, suggests the alternative heating scenario predicted by
scientists can occur as well.
In late 2005,
the researchers used NASA's
Spitzer Space Telescope to gather infrared readings from each of the
planets at eight different positions in their orbits. They measured the thermal
brightness of the planets when their day sides faced Earth, when their night sides faced Earth
and at various phases in between. They found no variation in the infrared
brightness, suggesting no widely varying temperature differences between the
planets' day and night sides.
Instead, the
planets all appeared to have a uniform temperature of about 1,700 degrees
Fahrenheit (925 degrees Celsius). The planets are all slightly cooler than they
would be if they did not have winds to circulate heat.
"The
cooling occurs via thermal infrared radiation, which is why the planet has a
more uniform brightness," said study team member Eric Agol of the
University of Washington. "The heat absorbed on the dayside is carried by
fast winds, partly releasing the heat on the day side and partly on the night
side."
The three planets
are 51 Pegasi, HD179949b and HD209458b. They are located about 50, 100 and 147
light-years away from Earth, respectively. Because they each orbit so close to
their stars, the researchers think the planets are probably tidally locked to
their stellar parents.
51 Pegasi was
the first planet discovered orbiting another star in 1995. Since then, the
number of known extrasolar planets has swollen to more than 200 and the
majority of them are hot Jupiters. Astronomers expect to eventually find large
numbers of smaller planets, when technology allows.
More to
learn
Whether the
majority of hot Jupiters are evenly heated or experience sharp temperature
contrasts remains unclear.
"Variations
in their atmospheric chemical composition or dust content could change how fast
they absorb and reradiate heat, and thus whether the winds can carry heat to
the night sides," Agol told SPACE.com.
Agol cautions
that both his findings and those of Hansen's team require further follow-up. In
both cases the statistical significance of the findings is low, he said in an
email interview. "With better data we will learn whether there really is a
puzzle."
Knowing how
hot Jupiters are heated could have implications for the search
for extraterrestrial life.
"There
has been some speculation that life might be able to exist on the cooler
night-side of hot extrasolar rocky planets, but if those planets had similar
weather patterns in their atmospheres as these hot Jupiters, that may not be
the case," Agol said.
Editor's
Note: All
week, SPACE.com is providing complete
coverage of the 209th meeting of the American Astronomical Society.
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