For some ‘hot Jupiters,’ at least, changes in the weather aren’t much of an issue. In a new study presented at the American Astronomical Society meeting in Seattle, three Jupiter-class planets orbiting within five million miles of their host stars were found to have remarkably similar temperatures globally, even though they’re tidally locked. You would expect that a planet with one side turned perpetually toward its star would show considerable temperature variation between the day and night sides, but that does not appear to be the case.
“We can’t say for sure that we’ve ruled out significant day-night temperature differences, but it seems unlikely there is a very big contrast based on our measurements and what we know about these systems,” said Eric Agol (University of Washington). Agol is lead scientist for the project, which used the Spitzer Space Telescope to measure infrared light from the three systems at eight different positions in their orbits. The study showed no infrared brightness variations in any of them.
Image: This artist’s animation (click to proceed to animation page) shows a gas-giant planet orbiting very close to its parent star, creating searingly hot conditions on the planet’s surface. New research suggests that for three such planets lying from 50 to 150 light-years from Earth, strong winds thousands of miles per hour mix the atmosphere so that the temperature is relatively uniform from the permanently light side to the permanently dark side. Credit: NASA/JPL-Caltech/R. Hurt (SSC).
Of course, uniform temperatures don’t mean agreeable ones. These planets weigh in at about 925 degrees Celsius (1700 degrees Fahrenheit). The probable mechanism that keeps their temperatures constant is supersonic wind that recirculates heat, mixing the atmosphere planetwide. The stars that host these furnace-like worlds are 51 Pegasi, about 50 light years from our sun, HD179949 (100 light years distant), and HD209458 (147 light years away). And yes, 51 Pegasi should ring a bell. It’s the home of the first exoplanet ever discovered orbiting a main sequence star. That discovery is only a little more than a decade old, amazing when you consider how much has happened since.
I’m not surprised, the idea of tidally locking a gas-giant planet is a bit tenuous. Taking Jupiter as an example, the atmospheric rotational period varies by slightly less than 1% from the equator to the poles, not to mention that it contains weather bands which run in opposite directions. Similarly, the Solar rotation rate varies over a range of about 5% of the average rate. Larger and hotter gas-giants are likely to have similar phenomena.
What I find really fascinating about these sorts of studies is the degree to which we have been able to glean data on objects which have been so recently discovered. I don’t think many people appreciate the many implications of some of the research that’s been done. Studies of planetary density and temperature not only give us information about composition, they rule out other possibilities such as large rocky bodies. This may seem mundane because we have always suspected bodies in this size range to be gas-giant planets, but it is no less valuable regardless. And it represents the rapid transformation and maturation of this field. 15 years ago even professional “experts” could have a serious debate about the very existence of extra-terrestrial planets, whether they were common or bizarre, ultra-rare flukes. Today we have a lot more answers and we’re asking very different questions, in a mere 5 more years we’ll have yet more answers and yet different questions (likely about Earth-sized planets, with data from Corot, Kepler, etc. in hand). It’s certainly a good time to have an interest in exoplanets.
I couldn’t agree with you more, Robin. In fact, I often marvel at my luck at being alive at just this time, when the next ten to fifteen years should bring us thousands of new exoplanet discoveries, including the kind of terrestrial worlds that so many of us are hoping for. Now if we can just find some NASA funding for New Worlds Imager!
First Observation of Planet-Induced X-ray Emission: The System HD 179949
Authors: S. H. Saar, M. Cuntz, V. L. Kashyap, J. C. Hall
(Submitted on 19 Dec 2007)
Abstract: We present the first observation of planet-induced stellar X-ray activity, identified for the HD 179949 system, using Chandra / ACIS-S. The HD 179949 system consists of a close-in giant planet orbiting an F9V star. Previous ground-based observations already showed enhancements in Ca II K in phase with the planetary orbit. We find an ~30% increase in the X-ray flux over quiescent levels coincident with the phase of the Ca II enhancements. There is also a trend for the emission to be hotter at increased fluxes, confirmed by modeling, showing the enhancement at ~1 keV compared to ~0.4 keV for the background star.
Comments: 3 pages, 1 figure; Exoplanets: Detection, Formation and Dynamics, IAU Symposium 249, eds. Y.S. Sun and S. Ferraz-Mello (San Francisco: Astr. Soc. Pac.)
Subjects: Astrophysics (astro-ph)
Cite as: arXiv:0712.3270v1 [astro-ph]
Submission history
From: Manfred Cuntz [view email]
[v1] Wed, 19 Dec 2007 20:45:31 GMT (40kb)
http://arxiv.org/abs/0712.3270
Rayleigh scattering by H2 in the extrasolar planet HD209458b
Authors: A. Lecavelier des Etangs, A. Vidal-Madjar, J.-M. Desert, D. Sing
(Submitted on 5 May 2008)
Abstract: Transiting planets, such as HD209458b, offer a unique opportunity to scrutinize the planetary atmospheric content. Although molecular hydrogen is expected to be the main atmospheric constituent, H2 remains uncovered because of the lack of strong transition from near-ultraviolet to near-infrared. Here we analyse the absorption spectrum of HD209458b obtained by Sing et al. (2008a) which provides a measurement of the absorption depth in the 3000-6200 AA wavelength range.
We show that the rise in absorption depth at short wavelengths can be interpreted as Rayleigh scattering within the atmosphere of HD209458b.
Since Rayleigh scattering traces the entire atmosphere, this detection enables a direct determination of the pressure-altitude relationship, which is required to determine the absolute fraction of other elements such as sodium. At the zero altitude defined by the absorption depth of 1.453%, which corresponds to a planetary radius of 0.1205 times the stellar radius, we find a pressure of 33+/-5 mbar. Using the variation of the Rayleigh scattering cross-section as a function of wavelength, we determine the temperature to be 2200+/-260 K at 33 mbar pressure.
Comments: To be published in A&A
Subjects: Astrophysics (astro-ph)
Cite as: arXiv:0805.0595v1 [astro-ph]
Submission history
From: Alain Lecavelier des Etangs [view email]
[v1] Mon, 5 May 2008 20:03:20 GMT (38kb)
http://arxiv.org/abs/0805.0595