Undreamed Shores - Ocean Worlds: The Story of Seas on Earth and Other Planets - page 256

black holes. Water is everywhere. Astronomers, therefore, have been
looking for water—and oceans—on the exoplanets that lie beyond
our solar system.
The electromagnetic radiation that travels at the speed of light
through space gives us information on the size, density, and orbital
characters of planets very far from our solar system. It also gives us
clues to the chemistry of those distant worlds, as we can interrogate
the faint radiation patterns by spectroscopy. For instance, as radia-
tion passes through the Earth's atmosphere, atoms and molecules
absorb that radiation in different wavelengths of ultraviolet, infrared,
and visible light. This produces an absorption spectrum which can
be detected by a spectroscope. The patterns of absorption lines indi-
cate the presence and abundance of oxygen, water vapour, or carbon
dioxide, all gases that have clear absorption bands in the thermal
infrared. To any alien spaceships that may one day view our own
solar system, the absorption spectra of the atmospheres of Earth,
Venus, and Mars will appear quite different. Venus and Mars show
the clear signal of carbon dioxide, but neither show strong signals of
oxygen or water vapour. The Earth, with all three gases, would stand
out as a fundamentally different planet, one where life might have
evolved. Viewed from outer space, even Earthshine—sunlight that
has bounced back to the Earth off the Moon—has been found to
reveal a clear spectral 'biosignature'. 168 It follows that if we can collect
enough light from distant planets, then it might be possible for us to
decipher the atmospheres of those planets too. The greatest promise
for this work lies with telescopes (or rather spectroscopes) in space.
The hot Jupiter planet HD 189733 b was discovered in 2005, orbit-
ing a star 63 light years from Earth. This planet is more massive than
Jupiter and is in a very close orbit around its star. A year takes only
two Earth days, and its gaseous atmosphere is heated to 1,000 degrees
Celsius. As the planet transits across the face of its star, light passing