Alien atmospherics

Sodium in the atmosphere of the planet filters out light from its parent star. Image Credit: A. Feild, Space Telescope Science Institute (STScI). Click image to magnify.

A planet orbiting a distant star has an atmosphere, although it is nothing like that found on earth.

Instruments on the Hubble telescope have made the first measurements of the chemistry of an alien atmosphere outside the solar system. NASA and other researchers are now keenly scouring the data for the chemical markers that could betray life on this distant world.

	Artist's impression of the distant star and planet. Credit G. Bacon at STScI. Click here to find animation movie.

A Jupiter-sized planet circling the sunny star HD 209458 in the constellation of Pegasus, some 150 light-years from earth has revealed its atmospheric chemistry as it passed in front of the star. The alien sunlight is filtered by the planet's atmosphere as it passes in front of the star - every three and a half days - and has given astronomers a unique first-time view of starlight filtered in this way. "This opens up an exciting new phase of extra-solar planet exploration," explains Caltech's David Charbonneau, "[Now] we can begin to compare and contrast the atmospheres of planets around other stars."

HST detects additional sodium absorption. Image Credit: A Feild. Click image to magnify.

Charbonneau and colleague Timothy Brown of the National Center for Atmospheric Research were using Hubble's Space Telescope Imaging Spectrograph (STIS) to look for the presence of sodium in the planet's atmosphere. But, instead of seeing high levels of sodium the levels were much lower than they expected. This suggested that high-altitude clouds in the alien atmosphere might have blocked some of the starlight.

	Star map. Credit: Z. Levay, STScI.

The present observations were not fine-tuned to detect the gases that would likely appear in a life-sustaining atmosphere; the planet is too hot anyway at 1100 Celsius. The approach hints at the possibility of remote exploration of other extra-solar planets, which may one day reveal the telltale chemical fingerprint of a living planet. The team, which includes Robert Noyes of the Harvard-Smithsonian Center for Astrophysics and Ronald Gilliland of the Space Telescope Science Institute, are planning to look again at HD 209458 in the hope of detecting methane, water vapor, potassium and other chemicals.

The planet revealed itself to the earthbound in 1999 when astronomers recognised perturbations in the movements of the star that could only be caused by the gravitational tug of a planet close to the mass of Jupiter, more than 220 times more massive than the Earth.