HD 209458b: High Wind Rising

by Paul Gilster on June 23, 2010

HD 209458b is perhaps the most persistently studied exoplanet we have, a transiting ‘hot Jupiter’ that has already revealed a slew of its secrets, including the detection of carbon dioxide, water vapor and methane. I confess that it sometimes seems like black magic to me that we are able to ferret out the signature of organic compounds on worlds we cannot even see. But the transit method is fruitful, and when scientists examine the light of the star during a planetary transit, the tiny portion of that light filtering through the planet’s atmosphere can be analyzed.

In the case of HD 209458b, we’re talking about a three hour transit, one that occurs every 3.5 days as this ‘hot Jupiter’ makes its rounds. Now we learn that the carbon dioxide detected here can also be studied in terms of its velocity. The result: We have indications of a vast storm, a wind flow that’s moving at speeds that defy the imagination. Ignas Snellen (Leiden Observatory, The Netherlands) led the team that performed this work:

“HD 209458b is definitely not a place for the faint-hearted. By studying the poisonous carbon monoxide gas with great accuracy we found evidence for a super wind, blowing at a speed of 5,000 to 10,000 km per hour.”

The storm itself is not surprising, but the fact that we are able to detect it is something of a coup. The planet in question is about 60 percent as massive as Jupiter, orbiting a Sun-like star 150 light years from Earth in the direction of Pegasus. Orbiting at 0.047 AU, the world is tidally locked, with surface temperatures thought to reach about 1000 degrees Celsius on the star-side, while the other remains much cooler. The temperature differential is what kicks up the enormous winds, now measured using the ESO Very Large Telescope and the CRIRES spectrograph, which produced spectra sharp enough “…to determine the position of the carbon monoxide lines at a precision of 1 part in 100,000,” according to team member Remco de Kok. From the paper on this work:

Since with transmission spectroscopy we probe the atmospheric region near the planet’s terminator, the blue-shift indicates a velocity-flow from the day side to the night side at pressures in the range 0.01-0.1 mbar as probed by these observations. Such winds may be driven by the large incident heat flux from the star on the dayside. Indeed three-dimensional circulation models indicate that at low pressure (<10 mbar) air should flow from the substellar point towards the antistellar point both along the equator and the poles.

But HD 209458b has yielded even more, as the astronomers were able to measure the velocity of the exoplanet as it orbits its star, using the information to refine estimates of the mass of both star (1.00 plus or minus 0.22 Sun masses) and planet (0.64 plus or minus 0.09 Jupiter masses). Further measurements showed how much carbon dioxide is present in the planet’s atmosphere. HD 209458b turns out to be about as carbon-rich as Jupiter and Saturn, leading to speculation that it was formed in the same way.

The paper is Snellen et al., “The orbital motion, absolute mass, and high-altitude winds of exoplanet HD 209458b,” Nature 465 (24 June 2010), pp. 1049-1051.

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Vagueofgodalming June 23, 2010 at 14:39
James Jason Wentworth June 23, 2010 at 14:49

This is *very* exciting! It leads me to wonder–if there just happens to be a transiting “exo-Earth” closer than the 150 light years in the case of this “hot Jupiter,” could instruments existing today glean similar atmospheric spectral data on it (in particular, detect oxygen co-existing with methane)? Being able to directly measure such a planet’s wind speeds and orbital velocity would be “icing on the cake.”

Le Moustier June 23, 2010 at 15:00

Somehow, the knowledge that these winds are very probably raging at this second, makes this a real place for me, in a way that other exoplanets are not (yet!). I look forward to further ingenious research from other teams in enriching our knowledge of the diversity of the exoplanet population.
Thanks for the website Paul.

andy June 23, 2010 at 16:04

While the weather conditions are a nice result, one thing that comes out of this is that for the first time we have a transiting exoplanetary system where orbital velocities are available for both the star and the planet, making it essentially a double-lined eclipsing spectroscopic binary. That means we can get a direct determination of the mass of the host star, which turns out to agree with previous determinations based on more indirect means (modelling from the spectrum).

Carl Keller June 23, 2010 at 18:53

“… surface temperatures thought to reach about 1000 degrees Celsius on the star-side, while the other remains much cooler.”

Venus is a very slow retrograde, and its temperatures do not vary widely due to it’s atmosphere’s density, at least at its surface. I wonder what dynamics the supersonic winds do to the distribution of temperature.

The analysis is from the ‘cloud tops’, and what the world does deeper down is also driven by tidal energies.

Eric June 23, 2010 at 22:38

Wow! This really is an amazing time to be alive, despite the horrible mess we’re seeing in the Gulf of Mexico.

5-10 thousand KPH….. Is there any analog for this anywhere in our experience? A scram jet? Reentry of the shuttle?

david lewis June 24, 2010 at 10:35

5-10 thousand kph. The power in such a wind would be astounding!!!

Katrina had a maximum wind speed of 280 kph. So the wind speeds there are around 17.8 to 35.7 times that of katrina. Or 5600 to 45000 times as much force.

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