Full steam ahead on planet GJ1214b

An exoplanet discovered in 2009 is a 'super-earth' a bit like our own, but scientists say it's unlikely to be a good place to live given its sizzling temperatures and atmosphere of clouds or steam, writes DICK AHLSTROM,Science Editor

SCIENTISTS HAVE for the first time gathered direct information about the atmosphere surrounding a distant, earth-like planet. It wouldn’t offer the best option for a home away from home however given astronomers believe it could have an atmosphere made up almost entirely of steam.

This is one of the key theories to emerge following an analysis of the exoplanet GJ 1214b. While astronomers can readily calculate the size and mass of the more than 500 exoplanets so far discovered in orbit around distant stars, they have struggled to get hard information about their atmospheres.

This information gap has now narrowed in a study of the atmosphere surrounding GJ 1214b, published this morning in the journal Nature. The planet is classed as a “super-earth”, a rocky planet like our own but measuring between twice and 10 times the Earth’s mass. It was discovered in 2009 and is one of a special category of exoplanets because it “transits” – that means the planet orbits directly in front of its star as we view it.

This immediately opens up the possibility of trying to peer into its atmosphere, not directly but by analysing the starlight passing through it from behind.

That is exactly what a research team involving US and German scientists, led by Prof Jacob Bean of the Harvard-Smithsonian Centre for Astrophysics, did. They used the European Southern Observatory’s Very Large Telescope to capture spectrographic information that offers intriguing but complex data about what surrounds GJ 1214b.

The planet is our near neighbour in a galactic sense but still lies 40 light-years away in the constellation Ophiuchus, the Serpent Bearer. Its radius is 2.6 times bigger than Earth’s, it is about 6.5 times as massive and it orbits its sun once every 38 hours.

Whatever the atmosphere it must certainly be a sizzling place given that the planet orbits just two million kilometres away from its sun. Yet it is the spectrographic data rather than the orbiting diameter that makes the scientists speculate about a steamy atmosphere.

They knew the planet had a substantial atmosphere because its apparent mass – calculated by how strongly its gravity tugs on its star – was much bigger than that of the rocky planet alone.

Three theories had evolved on what the atmosphere contained. One held that it was mostly hydrogen gas, but the spectrographic data did not show hydrogen. Another suggested that it was mostly hydrogen but that this was masked by a cloudy haze of some kind floating high above the planet. While the scientists acknowledged this as a possibility, they could identify “no candidate” for what the clouds might be. The third theory held that the atmosphere was made up almost entirely of steam. “Of the models proposed for the planet . . . only the predicted spectra from cloud-free atmospheres composed predominantly of water vapour (steam) agree with our measured transmission spectrum,” the authors write in Nature.

“We have reached a real milestone on the road toward characterising these worlds,” Prof Bean said. “Although we can’t yet say exactly what that atmosphere is made of, it is an exciting step forward to be able to narrow down the options for such a distant world to either steamy or hazy.” They want to have a return look at GJ 1214b, but this time looking for spectrographic data for a different set of light wavelengths. These will give scientists a clearer view of whether it is clouds or steam that surround the planet.

Looking for planets in the Goldilocks zone

An exoplanet or extrasolar planet is a planet outside of our own solar system. Astronomers always believed that exoplanets must exist around other stars but it was not until 1992 that the first such planet was found in orbit around a pulsar. Many more followed and as of 23rd November the tally stands at 504, with numbers increasing almost daily.

The number rises rapidly today because astronomers have much better tools for finding and confirming exoplanets including ground and space-based telescopes and specialised orbiting satellites. One such satellite is Nasa’s Kepler orbiting observatory. Launched in March last year, Kepler will stare for the next three years at a single slice of sky, allowing it to monitor 145,000 stars at one time.

It is watching for the minute dip in a star’s light intensity caused when an orbiting planet transits or passes in front of the star as observed by Kepler. The current assumptions hold that at least 10 per cent of sun-like stars will have orbiting planets of their own and probably many more than that.

The planets come in all shapes and sizes and the most commonly and easily detected planets are similar to our own gas giants Jupiter and Saturn. Kepler is designed however to spot Earth-sized planets, ones that might orbit their sun-like stars in the “Goldilocks” zone, not so close as to be overly hot and not so far away as to leave the planet too cold for habitation.

Being able to detect an exoplanet’s atmosphere is the next step, with astronomers looking for signs of biological activity.