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HD-189733b : A planet in orbit around a star in the constellation of Vulpecula

12 July 2007:  A huge hot gas planet similar to Jupiter has been discovered in orbit around a distant star about 64 light years away.

The exciting news is that water vapour, presumably in the form of hot steam has been detected.  The detection was achieved by waiting till the planet crossed in front of its local "sun" and watching for changes in the spectrum of the light passing through the planet's atmosphere.  A dark absorption line, corresponding to water vapour, was detected. The telescope used to do the detection was on board the Spitzer space telescope satellite, so it has a clear view of the planet and its nearby star, without having to look through the earth's atmosphere.

The power level of the infra red light spectrum was recorded at three wavelengths:  3.6 microns, 5.8 microns, and 8 microns.  The results were slightly different amounts of absorption, between 2.36 % and 2.44%.   This must have been very difficult to detect as the power level vary by less than 0.01 dB.    You often have to take many measurements and average the results to get high accuracy.  Anyway the results corresponded closely with what would be expected if there were water vapour or steam in the transmission path - so that is how the conclusion was reached.

The high  temperature of this particular planet makes it highly unlikely that there is extraterrestrial intelligent life there but the discover does give hope that there might be other cooler planets nearby, like the earth, where life might thrive.

At a distance of 64 light years radio communication would be barely feasible.  Apart from the long delay getting a signal each way, very powerful transmitters would be required and large receive dishes at the other end.  You can try your own experimental transmission calculations here : SETI communications: radio range calculator and link budget

When I do on line satellite spectrum measurements I am lucky to get 0.1 dB accuracy.  My on line spectrum analyser is connected to a satellite dish and occasionally the sun passes through the beam and the noise level increases all across the band.  My receiver works in the microwave spectrum 10.7 - 11.7 GHz, which is far lower frequency then infra red.  However next time it happens I will plot the full spectrum and see if I can detect any water vapour on our own earth's atmosphere by observing the spectrum of the sun and looking for any dips.

Spitzer Space Telescope
The Spitzer space telescope used to detect the infra-red spectrum from HD189733b. The telescope uses a 85 cm diameter reflector and the low noise infra-red detector is cryogenically cooled to reduce its background noise level.
Courtesy NASA/JPL-Caltech.

Infra-red absorption spectrum
The three measured infra-red power levels and the full spectrum of water vapour for
comparison. The yellow bars up and down and sideways represent possible errors in the measurements.
Credit: ESA, NASA/ JPL-Caltech/G. Tinetti (Institute d'Astrophysique de Paris, University College London)
Courtesy NASA/JPL-Caltech.

Amended 7 Feb 2010, amended 10 June 2021