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Dry exoplanets surprise astronomers

Jenny Winder, News Writer
Jul 27, 2014, 13:53 UTC

Sen—Astronomers looking for water vapour in the atmospheres of three planets orbiting stars similar to the Sun have been surprised by how little they found.

Measurements revealed only one-tenth to one one-thousandth the amount of water predicted by standard planet-formation theories.

The three planets, HD 189733b, HD 209458b, and WASP-12b, lie between 60 and 900 light-years away from Earth and were thought to be ideal candidates for detecting water vapour in their atmospheres because of their high temperatures. These “hot Jupiters” are so close to their star that they have temperatures between 800° and 2200° C (1,500° and 4,000° F). 

"Our water measurement in one of the planets, HD 209458b, is the highest-precision measurement of any chemical compound in a planet outside our Solar System, and we can now say with much greater certainty than ever before that we've found water in an exoplanet," said Nikku Madhusudhan of the Institute of Astronomy at the University of Cambridge, in the UK. "However, the low water abundance we have found so far is quite astonishing."

This finding presents a major challenge to exoplanet theory. "It basically opens a whole can of worms in planet formation. We expected all these planets to have lots of water in them. We have to revisit planet formation and migration models of giant planets, especially “hot Jupiters,” and investigate how they're formed."

These results may have major implications in the search for water in potentially habitable Earth-sized exoplanets. Instruments on future space telescopes may need to be designed with a higher sensitivity if target planets are drier than predicted.

"We should be prepared for much lower water abundances than predicted when looking at super-Earths (rocky planets that are several times the mass of Earth)," Madhusudhan added in a statement.

The planets were selected because they orbit relatively bright stars that provide enough radiation for an infrared-light spectrum to be taken using the Hubble Space Telescope.

The currently accepted theory on how giant planets form, known as core accretion, says that a planet forms around the young star in a protoplanetary disk made primarily of hydrogen, helium, and particles of ices and dust composed of other chemical elements.

The dust particles stick to each other, forming larger and larger grains. The gravitational forces of the disk draw in these grains until a solid core forms. This leads to runaway accretion of solids and gas to eventually form a giant planet.

This theory predicts that the proportions of the different elements in the planet are enhanced relative to those in its star, especially oxygen. Once the giant planet forms, its atmospheric oxygen is expected to be largely encompassed within water molecules. The low levels of water vapour found by this research raise a number of questions about the chemical ingredients that lead to planet formation.

Drake Deming of the University of Maryland, who led one of the earlier studies said, "The problem is that we are assuming the water to be as abundant as in our own Solar System. What our study has shown is that water features could be a lot weaker than our expectations."