Wind And Clouds Play An Important Role In Hot Jupiter Atmospheres
July 5, 2013

Eight ‘Hot-Jupiter’ Exoplanets Analyzed By Astronomers

Lee Rannals for - Your Universe Online

Astronomers studying eight exoplanets falling into the "hot-Jupiter" class suggest winds and clouds play an important role in the atmospheric make up of these exotic planets.

Hot-Jupiters are giant planets similar in size to Jupiter, but orbiting very close to their stars. Astronomers are able to detect which gases are present in their atmosphere by analyzing the spectrum of starlight filtered through the planet's atmosphere when it transits between Earth and its star.

A team led by the University of Exeter used the NASA and European Space Agency's Hubble Space Telescope to examine eight hot-jupiters. Catherine Huitson of the University of Exeter presented the team's findings this week at the Royal Astronomical Society's National Astronomy Meeting in St Andrews.

"These hot Jupiter planets are expected to have a vastly different composition from planets in our own Solar System like Jupiter, where temperatures at the cloud tops are around -150 degrees Celsius," said Huitson. "The first planet we measured is one of the hottest to be observed, with a temperature of over 2000 degrees. The early results of the survey are now in, and they present a diverse range of puzzling properties."

The first planet observed showed an unexpected absence of titanium oxide. Computer models of hot-Jupiter atmospheres suggest grains of this heavy molecule should be circulated by fast winds, allowing gaseous titanium oxide to reach the observable upper atmosphere. The non-detection of the gas would suggest either the winds are not as strong as models show or the molecule is forming much larger grains that are too heavy to be lifted.

"Titanium oxide is a solid on Earth, but we expect it to be present in the atmosphere of the hottest hot Jupiters because of the extreme temperatures. This molecule is important because it could trap atmospheric heat high up forming a stratosphere - the same role ozone plays on Earth," Huitson said. "However, our results show that this molecule is not present in the upper atmosphere, meaning that we need to revise our understanding of how wind processes distribute materials."

The astronomers were able to confirm the detection of water vapor in the atmospheres of two planets. The water was found in the quantities predicted by theory.

"While our models tell us that water (as steam) should be present in hot Jupiter atmospheres, until now the molecule has only been seen in limited quantities and in fewer planets than expected," said Huitson. "Seeing steam in two exoplanets is a great confirmation of current theory. Our new findings suggest that previous non-detections were caused by opaque, high-up clouds obscuring the parts of the atmosphere where steam is present."

She said a surprising diversity is emerging from continuing observations among planets with similar temperatures. The team is still analyzing all the data from the eight-planet Hubble survey. Huitson said, "the remaining results are sure to present even more surprises as we try to understand such extreme and unknown objects."