Impressive results released today show just how much we’re learning about the ‘hot Jupiters’ that comprise about a quarter of known exoplanets. The first concern HD 149026b, a distant world which the infrared Spitzer instrument has shown to be the hottest planet ever studied. It’s somewhat smaller than Saturn but more massive, and is thought to contain more heavy elements than could be found in our entire Solar System outside the Sun itself, with a core as much as 90 times the mass of the Earth.
And the odd thing about HD 149026b is that for it to reach the measured temperature — a smoking 3,700 degrees Fahrenheit, or 2,300 degrees Kelvin — it would have to be absorbing just about all the starlight reaching it. The upshot is a planet whose surface is blacker than charcoal, re-radiating incoming energy in the infrared. What a view for the nearby traveler: “The high heat would make the planet glow slightly, so it would look like an ember in space, absorbing all incoming light but glowing a dull red,” said Joseph Harrington (University of Central Florida).
You study things like this by measuring the drop in infrared light that occurs when the transiting planet moves behind its star. This is an unusual planet indeed — Drake Deming (NASA GSFC), a co-author of the paper on this work, says it’s “…off the temperature scale that we expect for planets.” HD 149026b is 279 light years away in the direction of the constellation Hercules, and it is the smallest and densest known transiting exoplanet. Its conditions sunside make the second exoplanet we have to discuss seem almost pleasant by comparison.
For HD 189733b, some 60 light years away and the closest known transiting exoplanet, shows temperatures (via Spitzer again) of 1,200 Fahrenheit on the day side (922 K) and 1,700 F at night (1,200 K), a relatively even range. We can thus deduce something more about its weather: powerful winds must be spreading the heat from dayside to nightside (assuming that this world is tidally locked to its star). These winds may be moving at more than 9,600 kilometers per hour, and the Spitzer studies show a warm spot — about twice the size of Jupiter’s Great Red Spot — 30 degrees east of the point directly below the star. From this, scientists extrapolate easterly moving winds that redistribute the intense heat.
Image: Four views of HD 189733b’s cloudtops in infrared light, each centered at a point of longitude 90 degrees from the last. A grid of longitude lines is superimposed on the map. These views clearly show a hot spot that is offset from the substellar point (high noon) by about 30 degrees. The offset may indicate fast “jet stream” winds of up to 6,000 mph (9650 km/h). Credit: NASA/JPL-Caltech/Heather Knutson (CfA)
HD 189733b is a huge planet in an extremely close orbit, circling its star every 2.2 days. With more than a quarter million data points to play with, the team was able to assemble a simple map of this distant world. “We can see the changes in brightness as features in the planet’s atmosphere rotate into and out of view,” said Heather Knutson, a graduate student at Harvard and lead author of the paper on this work.
Will the James Webb Space Telescope permit even more finely detailed work, perhaps mapping weather patterns on planets as small as Earth? It’s certainly a possibility, and one to be mindful of as we look forward to getting the Webb instrument into space around 2013. The paper on HD 189733b is Knutson et al., “A map of the day–night contrast of the extrasolar planet HD 189733b,” Nature 447, (10 May 2007), pp. 183-186 (abstract here). The HD 149026b paper is Harrington et al., “The Hottest Planet,” Nature online publication 9 May 2007.