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Light Off an Alien Ocean

If you want to put the hunt for planets around other stars in perspective, consider this. For almost all of our species’ time on this planet, we have looked at the planets in our own Solar System as unresolved points of light that seemed to move upon a celestial sphere. The brief time that we have been able to see more is measured since the invention of the telescope, a tiny window compared to the millennia that went before.

We are now working hard to see extrasolar planets as unresolved, moving points of light. In doing so, we’re looking at ways to image these planets that would yield the greatest scientific return. Recall former NASA administrator Dan Goldin’s wish to actually see the surfaces of distant exoplanets — he talked to putting such images on the walls of our schools. One day, starshade technologies coupled with space-borne telescopes may make that possible. For now, though, there is the real potential of something closer: identifying exoplanets with oceans.

The beauty of such an identification, writes Peter McCullough (Space Telescope Science Institute) is that we don’t actually need to resolve the planet to find out whether it has an atmosphere and an ocean. Here the scientist writes about what we can do with near-term technologies:

…we propose to exploit the linear polarization generated by Rayleigh scattering in the planet’s atmosphere and specular reflection (glint) from its ocean to study Earth-like extrasolar planets. In principle we can map the extrasolar planet’s continental boundaries by observing the glint from its oceans periodically varying as the rotation of the planet alternately places continents or water at the location on the sphere at which light from the star can be reflected specularly to Earth.

Got that? We’re talking about mapping continents on planets around other stars, using equipment that could be within our capabilities soon. The ‘Rayleigh scattering’ McCullough talks about is what happens when light is scattered off molecules in the air. It’s more effective at short wavelengths and is in fact responsible for the blue color of the sky. A surprising amount of work has gone into the study of Rayleigh scattering and specular reflection — glint — on extrasolar planets already.

Both oceans and atmospheres polarize reflected light. The important point here is that Rayleigh scattering and glint off an ocean can be differentiated, allowing us to mine data from their interplay. Mccullough uses a parallel with lighting techniques in our own oceans. A laser can be used to light up the sea floor, with ocean water scattering the light to create a haze visible to a camera. The laser light that does hit the sea floor creates a well-defined spot as well. Let me quote McCullough again:

By scanning the laser across the sea floor and simultaneously recording the location and brightness of the peak of the image, the light scattered by the turbid water is suppressed and detection of objects on the sea floor is enhanced. In the proposed technique for imaging extrasolar planets, the glint acts like the localized spot of the laser beam, and the rotation of the planet under the glint serves much the same purpose as the scanning of the laser beam.

If the method works, we should be able to tell the difference between terrestrial-size planets and terrestrial worlds with oceans. That’s a big step forward for exoplanet studies and it’s one available in the near-term. And if we can extend that model to learning about the shapes of continents on such worlds, we’ve moved a bit closer to making Goldin’s vision a reality. Space telescopes or the Moon itself could provide an ideal base for pursuing such studies, and tomorrow I want to turn to McCullough’s ideas on lunar exploration and its implications for this work.

The paper is McCullough, “Observations of Extrasolar Planets Enabled by a Return to the Moon,” to be published in Astrophysics Enabled by the Return to the Moon, Ed. M. Livio (Cambridge: Cambridge University Press), 2007 (abstract here). For greater detail, see the same author’s “Models of Polarized Light from Oceans and Atmospheres of Earth-like Extrasolar Planets,” submitted to The Astrophysical Journal and available here.

Comments on this entry are closed.

  • Darnell Clayton March 15, 2007, 13:35

    This is great news to hear of indeed!

    My question is this: How will we be able to tell the chemical difference between oceans, whether it is water, organic, methane, etc.?

    I would hate to have a future exploration ship travel all that way and end up finding a world full of hydorgen peroxide oceans (which wouldn’t taste all that great).

  • Administrator March 15, 2007, 13:39

    Good question, Darnell. The answer is that we’ll be working with a pretty good knowledge of where a given planet fits within a star’s habitable zone. If we can calculate approximate temperatures given the separation of the planet from its star, we can rule out certain kinds of substances from the start. No liquid methane on an Earth-type ocean world, for example. Finer points may be learned from spectroscopic studies of the planet’s atmosphere, the kind of thing we’re just starting to do with transiting exoplanets. Starshade technologies like New Worlds Imager will make measuring an exoplanet atmosphere much easier and far more precise.

  • ljk April 24, 2007, 13:43

    Are extrasolar oceans common throughout the Galaxy?

    Authors: David Ehrenreich (IAP), Arnaud Cassan (ARI)

    (Submitted on 23 Apr 2007)

    Abstract: Light and cold extrasolar planets such as OGLE 2005-BLG-390Lb, a 5.5 Earth-mass planet detected via microlensing, could be frequent in the Galaxy according to some preliminary results from microlensing experiments. These planets can be frozen rocky- or ocean-planets, situated beyond the snow line and, therefore, beyond the habitable zone of their system. They can nonetheless host a layer of liquid water, heated by radiogenic energy, underneath an ice shell surface for billions of years, before freezing completely. These results suggest that oceans under ice, like those suspected to be present on icy moons in the Solar system, could be a common feature of cold low-mass extrasolar planets.


    Accepted in Astronomische Nachrichten (Astronomical Notes)


    Astrophysics (astro-ph)

    Cite as:

    arXiv:0704.3024v1 [astro-ph]

    Submission history

    From: David Ehrenreich [view email]

    [v1] Mon, 23 Apr 2007 19:19:39 GMT (114kb)


  • ljk October 28, 2007, 0:13

    Ocean Planet or Thick Atmosphere: On the Mass-Radius Relationship for Solid Exoplanets with Massive Atmospheres

    Authors: E. R. Adams, S. Seager, L. Elkins-Tanton

    (Submitted on 25 Oct 2007)

    Abstract: The bulk composition of an exoplanet is commonly inferred from its average density. For small planets, however, the average density is not unique within the range of compositions. Variations of a number of important planetary parameters–which are difficult or impossible to constrain from measurements alone–produce planets with the same average densities but widely varying bulk compositions. We find that adding a gas envelope equivalent to 0.1%-10% of the mass of a solid planet causes the radius to increase 5-60% above its gas-free value. A planet with a given mass and radius might have substantial water ice content (a so-called ocean planet) or alternatively a large rocky-iron core and some H and/or He.

    For example, a wide variety of compositions can explain the observed radius of GJ 436b, although all models require some H/He. We conclude that the identification of water worlds based on the mass-radius relationship alone is impossible unless a significant gas layer can be ruled out by other means.

    Comments: 5 pages, 3 figures, accepted to ApJ

    Subjects: Astrophysics (astro-ph)

    Cite as: arXiv:0710.4941v1 [astro-ph]

    Submission history

    From: Elisabeth Adams [view email]

    [v1] Thu, 25 Oct 2007 19:51:39 GMT (275kb)


  • ljk January 15, 2008, 0:56

    Detecting the Glint of Starlight on the Oceans of Distant Planets

    Authors: D.M. Williams, E. Gaidos

    (Submitted on 11 Jan 2008)

    Abstract: We propose that astronomers will be eventually be able to discriminate between extrasolar Earth-like planets with surface oceans and those without using the shape of phase light curves in the visible and near-IR spectrum. We model the visible light curves of planets having Earth-like surfaces, seasons, and optically-thin atmospheres with idealized diffuse-scattering clouds.

    We show that planets partially covered by water will appear measurably brighter near crescent phase (relative to Lambertian planets) because of the efficient specular reflection (i.e., glint) of starlight incident on their surfaces at a highly oblique angle. Planets on orbits within 30 degrees of edge-on orientation (half of all planets) will show pronounced glint over a sizeable range of orbital longitudes, from quadrature to crescent, all outside the glare of their parent stars. Also, water-covered planets will appear darker than a Lambertian disk near full illumination.

    Finally, we show that planets with a mixed land/water surface will polarize the reflected signal by as much as 30-70 percent. These results suggest several new ways of directly identifying water on distant planets.

    Comments: 41 pages, 7 figures. Icarus in press

    Subjects: Astrophysics (astro-ph)

    Cite as: arXiv:0801.1852v1 [astro-ph]

    Submission history

    From: Darren Williams [view email]

    [v1] Fri, 11 Jan 2008 21:10:05 GMT (2043kb)