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Exoplanetary Weather: From a Single Pixel

How much information can you extract from a single pixel? That’s a key question for exoplanet studies as we look to the day when advanced telescopes can actually see a planet orbiting another star. But a single point of light seems to offer scant value, which is where Enric Pallé (Instituto de Astrofísica de Canarias) and colleagues go to work. They’ve been looking at how that single pixel changes over time, and what we might glean from it in terms of planetary details.

A key factor is cloud cover. Using data from Earth’s weather satellites, the scientists have been able to discover consistent patterns associating clouds with arid or rainy landmasses. Pallé explains:

“The trick lies in interpreting the movement of the Earth’s surface and the clouds as periodical signals, just as if we were to observe the spots on a spinning ball appearing and disappearing…[O]n a global scale clouds aren’t as random and chaotic as is generally believed, but instead follow a pattern marked by continental orography and oceanic currents.”

Cloud cover analyzed

The team studied two decades’ worth of data to produce its computerized model of Earth’s brightness. Over time, cloud patterns would help astronomers figure out the rotation period as particular brightening showed up with each rotation. And with the length of the day established, variations during that period would provide some indication of weather changing on the surface.

Image: Clouds are indicative of atmospheric pressure and existing temperature. Terrestrial cloud cover can be seen this three-dimensional representation, obtained from combined measurments from various meteorological satellites. The cloud distribution is represented during the phenomenon known as “El Niño” (1997-98) and shows anomalies in sea surface temperatures. Credit: NASA.

So an extraterrestrial civilization looking at Earth just might be able to say it had detected a living planet, one probably containing clouds and liquid water. Pallé and team had a poster at the Extreme Solar Systems conference last summer at Santorini that explains the latter:

Such variability is likely to be related to the atmospheric temperature and pressure being near a phase transition. Thus, such observations would support the possibility of liquid water on an extrasolar planet.

The assumption is that the repeated appearance and disappearance of clouds indicates active weather, and the contrast to worlds like Venus, whose brightness doesn’t change, is obvious. So are the limitations of the method, but add it to spectroscopic observations identifying elements in the planet’s atmosphere and you begin to piece together a picture of a living world.

How best to put such data to work? From the paper:

…we could learn if dynamic weather is present on an Earth-like exoplanet, from deviations from a fixed phase curve. In contrast, a cloud-free planet with continents and oceans would not show such light curve deviations. With phased light curves we could study local surface or atmospheric properties with follow-up photometry, spectroscopy, and polarimetry, to detect surface and atmospheric inhomogeneities and to improve the sensitivity to localized bio-markers. Finally, we have also provided guidance for the necessary specifications for future space missions.

The paper is Pallé, et al., “Identifying the rotation rate and the presence of dynamic weather on extrasolar Earth-like planets from photometric observations,” slated to appear in The Astrophysical Journal; full reference when available.

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  • ljk January 24, 2008, 0:12

    The Influence of Dust Formation Modelling on Na I and K I Line Profiles in Substellar Atmospheres

    Authors: C.M.S.Johnas, Ch.Helling, M.Dehn, P.Woitke, P.H.Hauschildt

    (Submitted on 23 Jan 2008)

    Abstract: We aim to understand the correlation between cloud formation and alkali line formation in substellar atmospheres. We perform line profile calculations for Na I and K I based on the coupling of our kinetic model for the formation and composition of dust grains with 1D radiative transfer calculations in atmosphere models for brown dwarfs and giant gas planets.

    The Na I and K I line profiles sensibly depend on the way clouds are treated in substellar atmosphere simulations. The kinetic dust formation model results in the highest pseudo-continuum compared to the limiting cases.

    Comments: 5 pages, Accepted for publication in MNRAS

    Subjects: Astrophysics (astro-ph)

    Cite as: arXiv:0801.3544v1 [astro-ph]

    Submission history

    From: Christiane Helling [view email]

    [v1] Wed, 23 Jan 2008 10:26:34 GMT (655kb)


  • ljk February 5, 2008, 10:25

    Atmospheric Circulation of Hot Jupiters: Three-dimensional circulation models of HD 209458b and HD 189733b with Simplified Forcing

    Authors: Adam P. Showman, Curtis S. Cooper, Jonathan J. Fortney, Mark S. Marley

    (Submitted on 4 Feb 2008)

    Abstract: We present global, three-dimensional numerical simulations of the atmospheric circulation on HD 209458b and HD 189733b and calculate the infrared spectra and light curves predicted by these simulations, which we compare with available observations. Radiative heating/cooling is parameterized with a simplified Newtonian relaxation scheme.

    Our simulations develop day-night temperature contrasts that vary strongly with pressure. At low pressure (less than 10 mbar), air flows from the substellar point toward the antistellar point, both along the equator and over the poles. At deeper levels, the flow develops an eastward equatorial jet with speeds of 3-4 km/sec, with weaker westward flows at high latitudes. This basic flow pattern is robust to variations in model resolution, gravity, radiative time constant, and initial temperature structure.

    Nightside spectra show deep absorption bands of H2O, CO, and/or CH4, whereas on the dayside these absorption bands flatten out or even flip into emission. This results from the strong effect of dynamics on the vertical temperature-pressure structure; the temperature decreases strongly with altitude on the nightside but becomes almost isothermal on the dayside.

    In Spitzer bandpasses, our predicted planet-to-star flux ratios vary by a factor of 2-10 with orbital phase, depending on the wavelength and chemistry. For HD 189733b, where a detailed 8-micron light curve has been obtained, we correctly produce the observed phase offset of the flux maximum, but we do not explain the flux minimum and we overpredict the total flux variation. This discrepancy likely results from the simplifications inherent in the Newtonian relaxation scheme and provides motivation for incorporating realistic radiative transfer in future studies.

    Comments: 17 pages, 14 figures, submitted for publication in ApJ

    Subjects: Astrophysics (astro-ph)

    Cite as: arXiv:0802.0327v1 [astro-ph]

    Submission history

    From: Adam Showman [view email]

    [v1] Mon, 4 Feb 2008 02:28:00 GMT (1579kb)


  • ljk June 13, 2008, 15:32

    Differential rotation in giant planets maintained by density-stratified turbulent convection

    Authors: Gary A. Glatzmaier, Martha Evonuk, Tamara M. Rogers

    (Submitted on 12 Jun 2008)

    Abstract: The zonal winds on the surfaces of giant planets vary with latitude. Jupiter and Saturn, for example, have several bands of alternating eastward (prograde) and westward (retrograde) jets relative to the angular velocity of their global magnetic fields. These surface wind profiles are likely manifestations of the variations in depth and latitude of angular velocity deep within the liquid interiors of these planets.

    Two decades ago it was proposed that this differential rotation could be maintained by vortex stretching of convective fluid columns that span the interiors of these planets from the northern hemisphere surface to the southern hemisphere surface. This now classic mechanism explains the differential rotation seen in laboratory experiments and in computer simulations of, at best, weakly turbulent convection in rotating constant-density fluid spheres. However, these experiments and simulations are poor approximations for the density-stratified strongly-turbulent interiors of giant planets. The long thin global convective columns predicted by the classic geostrophic theory for these planets would likely not develop.

    Here we propose a much more robust mechanism for maintaining differential rotation in radius based on the local generation of vorticity as rising plumes expand and sinking plumes contract. Our high-resolution two-dimensional computer simulations demonstrate how this mechanism could maintain either prograde or retrograde surface winds in the equatorial region of a giant planet depending on how the density scale height varies with depth.

    Comments: Geophysical and Astrophysical Fluid Dynamics, in press

    Subjects: Astrophysics (astro-ph)

    Cite as: arXiv:0806.2002v1 [astro-ph]

    Submission history

    From: Gary Glatzmaier [view email]

    [v1] Thu, 12 Jun 2008 06:07:23 GMT (520kb)