One of the biggest risk-takers in
the planet world orbits so close to
its parent star that its upper atmosphere bleeds into space like high-speed steam
rising from a boiler. Now astronomers have measured the "layer-cake" structure
of the extrasolar planet's heated
atmosphere.
Like a freshly-made cotton candy, HD
209458b is so hot that its atmosphere appears to be puffed up. The Jupiter-size
planet is just 4.4 million miles (7 million kilometers) from its star, or 20
times closer than the Earth is to our
Sun, and about 100 times closer than Jupiter
is to our Sun, hence the nickname of "hot Jupiter" for extrasolar planets this big and close to their parent
stars.
The new study, detailed in the Feb.
1 issue of the journal Nature, could shed light on other hot Jupiters.
"This planet's extreme
atmosphere could yield insights into the atmospheres of other hot Jupiters," said lead scientist Gilda Ballester of the University
of Arizona in Tucson.
Light filter
With the Hubble
Space Telescope, astronomers measured starlight that filtered
through the planet's puffy
outer atmosphere. Astronomers
can calculate an atmosphere's
structure and chemical make-up by studying how starlight filters through it.
"The layer we studied is
actually a transition zone where the temperature skyrockets from about 1,340
degrees Fahrenheit (1,000 Kelvin) to about 25,540 degrees (15,000 Kelvin),
which is hotter than the Sun," Ballester said.
"With this detection we see the details of how a planet loses its
atmosphere."
They observed that ultraviolet radiation
from the host
star bakes the hydrogen gas in the upper atmosphere [image],
causing it to balloon. With such an intense heat source, the gas streams from
the planet's gravitational pull at a rate of 10,000
tons a second, or three times the rate of water flowing over Niagara Falls.
Even still, astronomers don't expect
the planet to fade away any time soon. It has a prognosis of 5 billion
years or more.
Hot insights
Discovered in 1999, HD
209458b is one of a few known alien worlds that can be
observed as it transits
its parent star, called HD 209458. With a 3.5-day orbit, HD 209458b is 150
light-years from Earth in the constellation Pegasus.
Astronomers estimate about 10 to 15
percent of the more than 200 known extrasolar planets are hot Jupiters
like HD
209458b. A recent Hubble survey revealed 16 hot Jupiter candidates in
our Milky Way Galaxy, which suggests there might be billions of these gassy
star-huggers in our galaxy.
Previous Hubble observations
revealed oxygen, carbon, and sodium in HD
209458b's atmosphere, as well as in its fan-like tail. Those findings combined with the
new ones
provide the first detection of the chemical makeup of an extrasolar
planet's atmosphere.
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