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Paris (AFP) July 24, 2014
A study of unprecedented detail has revealed astonishingly little water on three distant planets, hinting at potential obstacles in the search for Earth-like worlds rich in H2O, astronomers said Thursday.
Using the Hubble Space Telescope, a team conducted detailed measurements of the atmospheres of three gas giants orbiting stars similar to our Sun, expecting to find "lots of water".
Between 60 and 900 light years from Earth, these worlds are so hot, with temperatures between 900 and 2,200 degrees Celsius (1,650-4,000 Fahrenheit), that they were thought to be ideal candidates for detecting water vapour.
The researchers, however, "have come up nearly dry," said a statement from Cambridge University, which took part in the research.
"The low water abundance we are finding is quite astonishing," said research leader Nikku Madhusudhan of the university's Institute of Astronomy.
"These results show just how challenging it could be to detect water on Earth-like exoplanets in our search for potential life elsewhere."
One of the planets, dubbed HD 209458b, was subject to the highest-precision measurement of any chemical compound ever carried out on a planet outside our solar system, the team said.
The other two were HD 189733b and WASP-12b.
The three planets had between one-tenth and one-thousandth the amount of water predicted by standard planet-formation theories, said the statement.
"As we prepare to search for biosignatures on rocky (Earth-size) exoplanets in the future we should be prepared to find planets with significantly less water than our expectations based on Earth," Madhusudhan told AFP by email.
Future telescopes searching for signs of water may now need to be designed with higher sensitivity to account for the possibility of planets being significantly drier than predicted.
The finding also "opens a whole can of worms" in the existing theory of planet formation, Madhusudhan added.
Under the accepted theory, giant planets form around young stars on a cosmic "disc" composed of hydrogen, helium and ice and dust particles.
The dust particles stick together, forming larger and larger grains drawn together by the gravitational forces of the disk.
The forming planet core may continue attracting solid matter and gas until it becomes a gas giant, whose atmospheric oxygen had been thought to be largely in the form of water.
"The very low levels of water vapour found by this research raise a number of questions about the chemical ingredients that lead to planet formation," said the statement.
One possibility is that the primordial disks the planets came from may have contained less water than previously thought.
The team used Hubble to study the planets as they passed in front of their parent stars, looking for signature water vapour influence on the starlight shining through the gas giants' atmosphere, as seen from Earth.
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