Planetary water? No says astronomical data

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  • Published: Aug 1, 2014
  • Author: David Bradley
  • Channels: Infrared Spectroscopy
thumbnail image: Planetary water? No says astronomical data

Following the vapour trail

Artist's impression of the gas giant planet HD 209458b (unofficially named Osiris) Credit: NASA, ESA, and G. Bacon (STScI)

Infrared spectroscopy has revealed that three planets orbiting Sun-like stars at distances of 60 to 900 light years from Earth are a lot drier than astronomers hoped. HD 189733b, HD 209458b and WASP-12b were thought to be ideal candidates for detecting extraterrestrial water vapour, but the data has come up dry.

The three "hot Jupiters" are gas giants resembling the planet Jupiter but are not the chilly worlds of our outer system, instead they have temperatures of between 800 and 2200 degrees Celsius. Astronomers thought these places would be ideal candidates for detecting water vapour in their atmospheres. However, the near-infrared spectroscopic data from the Hubble Space Telescope reveal, rather surprisingly, that the planets have as little as a tenth of the water and sometimes as low as one thousandth the water predicted by our conventional theories of planet formation. This may well suggest that astronomers need to rethink models of distant solar systems and perhaps even the evolution of our own.

"Our water measurement in one of the planets, HD 209458b, is the highest-precision measurement of any chemical compound in a planet outside the solar system, and we can now say with much greater certainty than ever before that we have found water in an exoplanet," explains research leader Nikku Madhusudhan of the Institute of Astronomy at the University of Cambridge, UK. "However, the low water abundance we are finding is quite astonishing," he adds.

Planetary revelations

Madhusudhan adds that the surprising revelations about planetary water vapour, or lack thereof, presents astronomers with a significant problem in the fledgling science of exoplanet theory. "It basically opens a whole can of worms in planet formation," he suggests. "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 are formed."

The hot Jupiters have orbits very close to their parent stars but the findings could have implications for finding water on "Goldilocks" exoplanets, planets that are like Earth - not too hot, not too cold. We might need to design instruments for future space telescopes that are even more sensitive to water, if we hope to investigate distant planets that much drier than theory predicts. "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 adds.

Madhusudhan worked with colleagues from Johns Hopkins University and the Space Telescope Science Institute in Baltimore, the University of Maryland in College Park and the Dunlap Institute at the University of Toronto, Ontario, Canada, to estimate the amount of water vapour in the planetary atmospheres based on sophisticated computer models and used statistical techniques to explain the near infrared data they obtained from Hubble observations. The data represent the infrared in the starlight passing through the planet's atmosphere and are compared with direct observation of that starlight to reveal the chemistry of the atmosphere itself. Such observations from the ground would be almost impossible because the Earth's atmosphere contains such a large amount of water vapour itself. "We really need the Hubble Space Telescope to make such observations," explains Dunlap's Nicolas Crouzet one of Madhusudhan's co-authors.

Theory in its element

Current theories of planet formation suggest that solid planetary cores form by accretion as hydrogen, helium, and particles of ices and dust containing other chemical elements circulate around a new star with gravity pulling together larger and larger grains that coalesce over millions of years. Once a large enough solid core forms gas is accreted massively forming an atmosphere. If a big enough core forms, then the planet can accumulate huge quantities of gas to become a giant. The theory also suggests that oxygen in the form of water ice accreted from  the protoplanetary disk must form a large component of such a gas giant  and should exist today in the form of water vapour. The lack of water vapour in the atmospheres of these exoplanets raises a number of questions about the chemical ingredients that lead to planet formation, the researchers say.

"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," adds team member Drake Deming of the University of Maryland. "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 next step is first to build detailed theoretical models to  investigate what could explain the low water levels we are observing,  and secondly to conduct similar observations of other exoplanets to see  how common or unique our present finding is," Madhusudhan told SpectroscopyNOW. "Ultimately, we would like to be able to understand the formation of  planetary systems by observing their chemical abundances."

Related Links

Astrophys J Lett 2014, online: "H2O Abundances in the Atmospheres of Three Hot Jupiters"

Article by David Bradley

The views represented in this article are solely those of the author and do not necessarily represent those of John Wiley and Sons, Ltd.

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