The winds of change blow for exoplanetology

Thinking back to the time at which I got my first telescope, way back in the mists of time when the Berlin Wall still stood and Mrs Thatcher was in power, I could be confident of pointing it up into the sky and seeing evidence of about as many planets outside of our solar system as the professionals knew of – none.

Fast forward more than two decades later and the extrasolar planets catalogue lists 463 confirmed planets, with a similar amount expected to drop out of the recently collected list of 706 candidate stars glanced by the Kepler mission. Observations of starlight passing through the atmospheres of some of these planets, or reflected off them, have provided absorption spectra (when compared to the unaltered light of the star), giving a glimpse of the sorts of stuff in those atmospheres. More careful spectral measurements then provided estimates of temperatures, providing maps of temperatures on the planets that could in theory then be used to estimate wind speeds.

But not all that glistens is gold and not every temperature refers to a weather bearing one. Just because a layer of stuff is absorbing and emitting light, doesn’t mean it is sharing the energy of that light with all around it in the manner suggested by a couple of points on a spectrum, so comment has often been reserved on temperature maps gained so far. However, new observations have come in that have taken this process one step further along.

Ignas Snellen, of Leiden Observatory, The Netherlands, and colleagues have been making observations of the planet HD 209458b, lying 150 light years away in the constellation of Pegasus. The planet was first seen in 1999, its presence inferred by the dip in the light of the star as the planet passed between it and us. It is a hot Jupiter style exoplanet, nice and close, making it a good target for observations of the atmosphere. Those observations have revealed a number of constituents over time including hydrogen, oxygen, carbon, methane, carbon monoxide, carbon dioxide and water vapour.

Improved spectroscopic techniques and instruments zoomed in on fluctuations in the light, providing changes in the signal of as little as one in one hundred thousand. This enabled not just the presence of spectral lines, but also the shape of those lines and how they moved due to Doppler shifts – velocity induced changes in frequency. From the shifts, they were able to measure atmospheric wind speeds of 6,200 miles per hour, essentially using the same basic physics as the Doppler radar used to measure windspeeds on Earth.

The planet, sometimes known as Osiris, was already known to be a turbulent world. It lies one eighth the Sun-Mercury distance and has inflated to two and a half times the volume of Jupiter with only 0.69 times the mass. Modelling of the atmospheric response has suggested that the atmosphere is expanding beyond the point at which the planet can hold onto it and it is this huge extension as well as the closeness of the planet that makes the atmosphere an easier target than might otherwise be the case.

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