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.