Unlikely To Harbor Life, But Still A Find
A scorching-hot gas planet beyond our solar system is steaming
up with water vapor, according to new observations from NASA's
Spitzer Space Telescope.
The planet, called HD 189733b, swelters as it zips closely
around its star every two days or so. Astronomers had predicted
that planets of this class, termed "hot Jupiters," would contain
water vapor in their atmospheres. Yet finding solid evidence for
this has been slippery. These latest data are the most convincing
yet that hot Jupiters are "wet."
"We're thrilled to have identified clear signs of water on a
planet that is trillions of miles away," said Giovanna Tinetti, a
European Space Agency fellow at the Institute d’Astrophysique
de Paris in France." Tinetti is lead author of a paper on HD
189733b appearing today in Nature.
Although water is an essential ingredient to life as we know it,
wet hot Jupiters are not likely to harbor any creatures. Previous
measurements from Spitzer indicate that HD 189733b is a fiery 1,000
Kelvin (1,340 degrees Fahrenheit) on average.
Ultimately, astronomers hope to use instruments like those on
Spitzer to find water on rocky, habitable planets like Earth.
"Finding water on this planet implies that other planets in the
universe, possibly even rocky ones, could also have water," said
co-author Sean Carey of NASA's Spitzer Science Center at the
California Institute of Technology in Pasadena. "I'm excited to
tell my nephews and niece about the discovery."
The new findings are part of a brand new field of science
investigating the climate on exoplanets, or planets outside our
solar system. Such faraway planets cannot be seen directly;
however, in the past few years, astronomers have begun to glean
information about their atmospheres by observing a subset of hot
Jupiters that transit, or pass in front of, their stars as seen
from Earth.
Earlier this year, Spitzer became the first telescope to
analyze, or break apart, the light from two transiting hot
Jupiters, HD 189733b and HD 209458b. One of its instruments, called
a spectrometer, observed the planets as they dipped behind their
stars in what is called the secondary eclipse. This led to the
first-ever "fingerprint," or spectrum, of an exoplanet's light.
Yet, the results came up "dry," probably because the structure of
these planets' atmospheres makes finding water with this method
difficult.
Later, a team of astronomers found hints of water in HD 209458b
by analyzing visible-light data taken by NASA's Hubble Space
Telescope. The Hubble data were captured as the planet crossed in
front of the star, an event called the primary eclipse.
Now,
Tinetti and her team have captured the best evidence yet for wet,
hot Jupiters by watching HD 189733b's primary eclipse in infrared
light with Spitzer. In this method, changes in infrared light from
the star are measured as the planet slips by, filtering starlight
through its outer atmosphere. The astronomers observed the eclipse
with Spitzer's infrared array camera at three different infrared
wavelengths and noticed that for each wavelength a different amount
of light was absorbed by the planet. The pattern by which this
absorption varies with wavelength matches that created by
water.
"Water is the only molecule that can explain that behavior,"
said Tinetti. "Observing primary eclipses in infrared light is the
best way to search for this molecule in exoplanets."
The water on HD 189733b is too hot to condense into clouds;
however, previous observations of the planet from Spitzer and other
ground and space-based telescopes suggest that it might have dry
clouds, along with high winds and a hot, sun-facing side that is
warmer than its dark side.
HD 189733b is located 63 light-years away in the constellation
Vulpecula.
(Artist's rendering of HD 189733b by C. Carreau,
ESA)