Three Jupiter sized planets with no water vapor baffle scientists

A team of astronomers has made the most precise measurements yet of water vapor in the atmospheres of three Jupiter-like exoplanets and found them to be between ten and one-thousand times drier than models predicted.

“The low water vapor levels are surprising,” said Dr. Nicolas Crouzet of the Dunlap Institute for Astronomy & Astrophysics, University of Toronto in a statement. “Our models predict a much higher abundance of water vapor, and so these results challenge our current understanding of planet formation. And they raise questions about our ability to identify water in an Earth-like exoplanet.”
The team analyzed near-infrared spectra, obtained using the Hubble Space Telescope, of three exoplanets between 60 and 900 light years away, identified as HD 189733b, HD 209458b, and WASP-12b. The exoplanets belong to a class of planets known as “hot Jupiters” and have average temperatures between 900 and 2200° C.

The Hubble Space Telescope collected light from the parent star as well as light that passed through the planet’s atmosphere as the planet passed in front of the star. The scientists then accounted for the recorded light differences in order to come up with an estimate of the amount of light that passed through the atmosphere of the planet.

A subsequent spectroscopic analysis of this light allowed the astronomers to measure the presence of water vapor. The very low levels of water vapor discovered by the researchers raise questions about the current understanding of the chemical processes involved in planet formation.

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.

Although this prediction was wholly unverified by this analysis, it is possible that there were other factors that hampered the analysis. There could have been clouds at high altitudes that obscured the Hubble’s view of water vapor in lower levels of the atmosphere, which could have “completely undermined [their results],” said Adam Burrows, an astronomer from Princeton University, in a Nature news blog. Burrows worked on a study of HD 209458b whose data was analyzed in the current study, and was not involved in the current study.

The results also give astronomers an idea of what they face in searching for an exoplanet capable of supporting life.

“These very hot planets with large atmospheres, orbiting nearby stars, are the best possible candidates for measuring water levels,” says Dr. Nikku Madhusudhan of the Institute of Astronomy at the University of Cambridge and lead author of the paper. “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.”