* Astronomy

A scorching-hot gas planet beyond our solar system is steaming up with water vapour, according to new observations from NASA's Spitzer Space Telescope.
The planet, called HD 189733b, swelters as it zips closely around its star every two days or so. Astronomers had predicted that planets of this class, termed "hot Jupiters," would contain water vapour in their atmospheres. Yet finding solid evidence for this has been slippery. These latest data are the most convincing yet that hot Jupiters are "wet."

"We're thrilled to have identified clear signs of water on a planet that is trillions of miles away"  -  Giovanna Tinetti, a European Space Agency fellow at the Institute d'Astrophysique de Paris in France.  

Tinetti is lead author of a paper on HD 189733b appearing today in Nature.

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Title: A map of the day-night contrast of the extrasolar planet HD 189733b
Authors: Heather A. Knutson, David Charbonneau, Lori E. Allen, Jonathan J. Fortney, Eric Agol, Nicolas B. Cowan, Adam P. Showman, Curtis S. Cooper, S. Thomas Megeath

"Hot Jupiter" extrasolar planets are expected to be tidally locked because they are close (<0.05 astronomical units, where 1 AU is the average Sun-Earth distance) to their parent stars, resulting in permanent daysides and nightsides. By observing systems where the planet and star periodically eclipse each other, several groups have been able to estimate the temperatures of the daysides of these planets. A key question is whether the atmosphere is able to transport the energy incident upon the dayside to the nightside, which will determine the temperature at different points on the planet's surface. Here we report observations of HD 189733, the closest of these eclipsing planetary systems, over half an orbital period, from which we can construct a 'map' of the distribution of temperatures. We detected the increase in brightness as the dayside of the planet rotated into view. We estimate a minimum brightness temperature of 973 ± 33 K and a maximum brightness temperature of 1212 ± 11 K at a wavelength of 8 microns, indicating that energy from the irradiated dayside is efficiently redistributed throughout the atmosphere, in contrast to a recent claim for another hot Jupiter. Our data indicate that the peak hemisphere-integrated brightness occurs 16 ±6 degrees before opposition, corresponding to a hot spot shifted east of the substellar point. The secondary eclipse (when the planet moves behind the star) occurs 120 ± 24 s later than predicted, which may indicate a slightly eccentric orbit.




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-- Edited by Blobrana at 11:52, 2007-05-09

Astronomers using the Spitzer Space Telescope have detected a strong flow of heat radiation from a planet orbiting the nearby star HD 189733b. The findings allowed the team to "take the temperature" of the planet.

"This is the closest extrasolar planet to Earth that has ever been detected directly, and it presents the strongest heat emission ever seen from an exoplanet" - Drake Deming, NASA's Goddard Space Flight Centre, Greenbelt, US.

Deming is the lead author of a paper on this observation to be published in the Astrophysical Journal on June 10.

The planet "HD 189733b" orbits a star that is a near cosmic neighbour to our sun, at a distance of 63 light years in the direction of the Dumbbell Nebula. It orbits the star very closely, just slightly more than three percent of the distance between Earth and the sun. Such close proximity keeps the planet roasting at about 844 Celsius, according to the team's measurement.

The planet was discovered last year by François Bouchy of the Marseille Astrophysics Laboratory, France, and his team. The discovery observations allowed Bouchy's team to determine the planet's size (about 1.26 times Jupiter's diameter), mass (1.15 times Jupiter), and density (about 0.75 grams per cubic centimetre). The low density indicates the planet is a gas giant like Jupiter.
The observations also revealed the orbital period (2.219 days) and the distance from the parent star. From this distance and the temperature of the parent star, Bouchy's team estimated the planet's temperature was at least several hundred degrees Celsius, but they were not able to measure heat or light emitted directly from the planet.

"Our direct measurement confirms this estimate" - Drake Deming .

This temperature is too high for liquid water to exist on the planet or any moons it might have. Since known forms of life require liquid water, it is unlikely to have emerged there.
Last year, Deming's team and another group based at the Harvard-Smithsonian Centre for Astrophysics used Spitzer to make the first direct detection of light from alien worlds, by observing the warm infrared glows of two other previously detected "Hot Jupiter" planets, designated HD 209458b and TrES-1.

Infrared light is invisible to the human eye, but detectable by special instruments. Some infrared light is perceived as heat. Hot Jupiter planets are alien gas giants that zip closely around their parent stars, like HD 189733b. From their close orbits, they soak up ample starlight and shine brightly in infrared wavelengths.
Deming's team used the same method to observe HD 189733b. To distinguish the planet's glow from its hot parent star, the astronomers used an elegant method. First, they used Spitzer to collect the total infrared light from both the star and its planet. Then, when the planet dipped behind the star as part of its regular orbit, the astronomers measured the infrared light coming from just the star. This pinpointed exactly how much infrared light belonged to the planet. Under optimal circumstances this same method can be used to make a crude temperature map of the planet itself.

"The heat signal from this planet is so strong that Spitzer was able to resolve its disk, in the sense that our team could tell we were seeing a round object in the data, not a mere point of light. The current Spitzer observations cannot yet make a temperature map of this world, but more observations by Spitzer or future infrared telescopes in space may be able to do that"- Drake Deming.

Deming's team includes Joseph Harrington, Cornell University, Ithaca, N.Y.; Sara Seager, Carnegie Institution of Washington; and Jeremy Richardson, NASA Postdoctoral Fellow at Goddard, in the Exoplanets and Stellar Astrophysics Laboratory.

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Title: Strong Infrared Emission from the Extrasolar Planet HD189733b
Authors: Drake Deming, Joseph Harrington, Sara Seager, L. Jeremy Richardson

We report detection of strong infrared thermal emission from the nearby (d=19 pc) transiting extrasolar planet HD189733b, by measuring the flux decrement during its prominent secondary eclipse. A 6-hour photometric sequence using Spitzer's infrared spectrograph in peak-up imaging mode at 16-microns shows the secondary eclipse depth to be 0.551 ±0.030%, with accuracy limited by instrumental baseline uncertainties, but with 32-sigma precision (0.017%) on the detection. The 16-micron brightness temperature of this planet (1117±42K) is very similar to the Spitzer detections of TrES-1 and HD209458b, but the observed planetary flux (660 micro-Jy) is an order of magnitude greater. This large signal will allow a detailed characterisation of this planet in the infrared. Our photometry has sufficient signal-to-noise (~400 per point) to motivate a search for structure in the ingress/egress portions of the eclipse curve, caused by putative thermal structure on the disk of the planet. We show that by binning our 6-second sampling down to 6-minute resolution, we detect the modulation in the intensity derivative during ingress/egress due to the overall shape of the planet, but our sensitivity is not yet sufficient to distinguish between realistic models of the temperature distribution across the planet's disk. We point out the potential for extending Spitzer secondary eclipse detections down to the regime of transiting hot Neptunes, if such systems are discovered among nearby lower main sequence stars.


Upper Panel: Baseline-removed aperture photometry of the HD189733 secondary eclipse. Points are individual 6-second measurements, with error bars suppressed for clarity, but showing the eclipse curve having the best-fit amplitude (0.551 ± 0.03%) and central phase. Lower Panel: Data from the upper panel averaged in bins of width 0.001 in phase (~ 3 minutes), with error bars and the best-fit eclipse curve.


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