http://www.facebook.com/ScienceReason ... Hubblecast 12: Hubble sees an Extrasolar Planet's Atmosphere (Murk on a monster planet).
Using
the NASA/ESA Hubble Space Telescope, astronomers have found the first
clear evidence of high altitude haze or clouds in the atmosphere of an
extrasolar planet (exoplanet). This discovery reveals a deeper
understanding of the class of giant planets that astronomers call 'hot
Jupiters'.
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Over
the last 15 years astronomers have discovered more than 270 planets
around other stars. A burning question is naturally: What are these so
called extrasolar planets like?
Most of them are actually giant
gaseous worlds several times the size of Jupiter which is our own Solar
Systems largest planet. Others are smaller rocky or icy worlds that are
still several times larger than our own home planet Earth. We have yet
to discover a world that is similar to our own planet.
Using the
Advanced Camera for Surveys (or ACS), the Hubble Space Telescope has
recently observed a fascinating large gaseous planet around the star HD
189733. Careful analysis of these very high precision observations by a
team lead by Frédéric Pont from the Geneva University Observatory showed
that this planet, designated HD 189733b, possesses a murky haze layer
extending over an altitude range of about 1000 km in the planets upper
atmosphere. The atmosphere of this gaseous planet is at around 800
degrees Centigrade.
This is due to its proximity to its parent
star and is the reason that astronomers call this type of planet a hot
Jupiter. These hazes are probably made of tiny condensed particles less
that 1/1000th of a millimetre across, similar to those already known on
Venus and Saturns moon, Titan. Their presence means that the sky over HD
189733b would look very much like a red hazy sunset viewed from an
industrially polluted city on Earth.
Whats so special about this
particular star and its planet? Bob Fosbury: "This is a rather special
planetary system because the star itself is quite small, its only about
75% the size of our Sun and yet the planet itself is larger than
Jupiter. So when the planet transits in front of the star it obscures
quite a large fraction of light from the star which enables a very
accurate measurement."
What exactly made these observations so
precise? Why was this only possible now? Bob Fosbury: "We have to do
this from space because when we try and do this from the ground the
atmosphere makes it very difficult to make precise measurements of
brightness, so we do it from space. And the special thing about this
particular observation is that the scientists spread the light out over
many pixels in the detector. So rather than just having a little point
of light representing the star on the detector the starlight was spread
out into a spectrum using this so called grism mode of the Advanced
Camera for Surveys."
"Now, that enables a very precise
measurement because you can measure over many many pixels (a large area
of detector)but it also, by spreading out the colours, enables you to
measure the brightness (or the reduction in brightness) of the starlight
over many colours. So having these different measurements in different
colours enables you to characterize the nature of the atmosphere."
The
reason why this work was possible at all was because from our vantage
point the orbit of HD 189733b is seen almost exactly edge on. Now what
that means is that every two days or so the planet actually moves across
the face of its parent star as seen from here. Now when that happens
some small fraction of the light from the star has to pass through the
atmosphere of the planet in front of it in order to reach Earth. Because
of this process the composition of the atmosphere of the planet is
stamped onto the light like a unique fingerprint. Astronomers can then
see this fingerprint in the spectrum of the stars light.
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Hubblecast features news and Images from the Hubble Space Telescope (HST).
The
space-based observatory is a collaboration between NASA and ESA. The
observations are carried out in visible, infrared and ultraviolet light.
In many ways Hubble has revolutionised modern astronomy.
The
Hubble Space Telescope has made some of the most dramatic discoveries in
the history of astronomy. From its vantage point 600 km above the
Earth, Hubble can detect light with "eyes" five times sharper than the
best ground-based telescopes and looks deep into space where some of the
most profound mysteries are still buried in the mists of time.
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http://hubblesite.org.
