A team of astronomers headed by Dr Nikku Madhusudhan of the Cambridge University’s Institute of Astronomy has found that the abundance of water vapor in the atmospheres of the three newly discovered hot Jupiters – HD 189733b, HD 209458b, and WASP-12b – is between ten and a thousand times less than what standard planet formation theories predict.
This is an illustration of a hot Jupiter orbiting a Sun-like star.
According to the core accretion theory of planet formation, planetary systems form from a huge disk of hydrogen gas and dust around a star.
Over a period of a million years or more, the dust particles stick together, forming larger and larger grains.
Eventually, the grains collect to form planetesimals which eventually coalesce into a planet.
At the same time, the gravity of the planet attracts an atmosphere of gas from the disk.
The theory predicts that a planet’s most abundant element will be oxygen, which would take the form of water vapor in the atmosphere.
The very low levels of water vapor discovered in the atmospheres of HD 189733b, HD 209458b and WASP-12b raise questions about our understanding of the chemical processes involved in planet formation.
Located between 60 and 900 light-years away from Earth, these exoplanets belong to a class of planets known as hot Jupiters. They have average temperatures between 900 and 2,200 degrees Celsius and were thought to be ideal candidates for detecting water vapor in their atmospheres.
“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,” explained Dr Madhusudhan, who is the first author of a paper published in the Astrophysical Journal (arXiv.org pre-print).
Dr Madhusudhan and his colleagues analyzed near-infrared spectra, obtained using NASA’s Hubble Space Telescope, of HD 189733b, HD 209458b, and WASP-12b.
The Hubble did not directly resolve the exoplanets. Instead, it collected light from both the parent star as well as light that passed through the planet’s atmosphere as the planet passed in front of the star.
Subtracting the light of the star from the total light collected left the light that passed through the planet’s atmosphere. Spectroscopic analysis of this light allowed the astronomers to measure the presence of water vapor.
“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,” Dr Madhusudhan said.
“We can now say with much greater certainty than ever before that we’ve found water in an exoplanet. However, the low water abundance we are finding is quite astonishing.”
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Nikku Madhusudhan et al. 2014. H2O Abundances in the Atmospheres of Three Hot Jupiters. ApJ 791, L9; doi: 10.1088/2041-8205/791/1/L9
