It looks like finding a habitable alien world could be much more difficult than the human race was hoping after three hot-Jupiter exoplanets turned out to have far less water than astronomers predicted.
The chosen worlds were reckoned to be ideal candidates for the detection of water vapour in their atmospheres because they orbit so close to their suns, getting high temperatures of between 1,500 and 4,000 degrees Fahrenheit that turn water into a measurable fog. But the researchers were astonished when the planets came up nearly dry, despite the predictions of planet formation theory.
An artistic illustration of the gas giant planet HD 209458b in the constellation Pegasus. To the surprise of astronomers, they have found much less water vapour in the hot world's atmosphere than standard planet-formation models predict. (Credit: NASA, ESA, G. Bacon (STScI) and N. Madhusudhan (UC))
"Our water measurement in one of the planets 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, who led the study.
"However, the low water abundance we have found so far is quite astonishing .
"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."
As well as upsetting planet theories, the research also has major implications for the search for water in potentially habitable Earth-sized exoplanets. If all alien worlds are going to be drier than scientists previously thought, instruments on future space telescopes will need to have a much greater sensitivity to find what little water is there.
"These very hot planets with large atmospheres, orbiting nearby stars, are the best possible candidates for measuring water levels. And yet the levels we found were much lower than expected. This shows just how challenging it could be to detect water on Earth-like exoplanets in our search for potential life elsewhere ," Madhusudhan said.
Only space telescopes can detect water on distant worlds. Any instrument on Earth has its view interrupted by our planet's own atmosphere, which is laden with water and contaminates the observation.
These three exoplanets were picked for the study because they orbit relatively bright stars, providing enough radiation for an infrared-light spectrum observation. Absorption features from the water vapour in each planet's atmosphere can then be seen because they are superimposed on the amount of starlight filtering through.
The worlds, known as HD 189733b, HD 209458b, and WASP-12b are between 60 and 900 light-years away from Earth and turned out to have only a tenth to a thousandth the amount of water predicted by the currently accepted theory of how giant planets in our Solar System form.
According to core accretion theory, planets form around the young star first in what's called a "protoplanetary disk" – a cloud made primarily of hydrogen, helium and particles of ice and dust made of other chemical elements. These dust particles clump together making larger and larger pieces, which are drawn in by the gravitational forces of the disk to form a solid core. Once the core is in place, it steadily accretes solids and gas to eventually make a giant world.
The theory predicts that the proportion of different elements in the planet are enhanced relative to those in its star, particularly oxygen. Once the planet has formed, that atmospheric oxygen is then expected to be largely encompassed in water molecules. But this research raises questions about the chemical ingredients in world-building.
"There are so many things we still don't know about exoplanets, so this opens up a new chapter in understanding how planets and solar systems form," said Drake Deming of the University of Maryland, who led one of the precursor studies.
"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."
The findings were published in the July edition of The Astrophysical Journal Letters.
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