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Starlight on a Distant Sea

Planets around other stars are too faint to be imaged directly, and although claims have been made for such detections (2M1207b is a case in point), it’s safe to say that our current techniques need significant upgrading to achieve reliable images of such distant worlds. But studying terrestrial planets is a long-term objective and numerous studies have gone into concepts like Terrestrial Planet Finder and Darwin. One day and with some instrument we will indeed be looking at an exoplanet as small as the Earth, working with estimates of surface temperatures and checking its atmosphere for biomarkers that flag the presence of life.

So let’s suppose that in fifteen years or so we’re looking at actual reflected light from a terrestrial world. What else can we learn about the place? The brightness of a planet like this can be affected by many things, including the presence of deserts on the surface or bright clouds above it. An active weather pattern would indicate the presence of a hydrological cycle like the one we see here on Earth. Such changes in brightness would be difficult to detect on planets that rotate in a few days or less, but the presence of oceans on these worlds may well be apparent. Imagine being able to look at starlight reflected off an alien sea.

Darren Williams (Penn State Erie) and Eric Gaidos (University of Hawaii), who address the question in a new paper, say it’s possible. And as we improve our instruments, potentially measurable variations might even include the seasonal blooming of land plants or oceanic algae or the coming and going of snow. All such fluctuations will vary depending on the planet’s obliquity and orbital inclination with respect to the observer. The staggering thing is the amount of potentially recoverable information from a source so dim in relation to its parent star that we cannot see it today.

In terms of liquid surfaces, the chances of detection look quite interesting. From the paper:

A reflected light curve also contains information about the scattering properties of the surface, independent of any seasonal changes. Planets with water will reflect light toward the observer more efficiently in crescent phase than in gibbous phase because of the higher reflectance at low incident angles. This glint from water will make a planet appear anomalously bright in crescent phase compared to diffuse-scattering surfaces observed in the same geometry. Light reflected from water will also impart some polarization to the disk-averaged signal, which might be measurable under idealized (i.e., optically-thin, cloud-free) atmospheric conditions.

The paper goes on to study how starlight reflected off water might be detected, and examines the light curves from a variety of surfaces in the visible and near-infrared spectrum. Half of all extrasolar planets should have the kind of orbital inclinations that would make the glint of existing oceans apparent. But the reflection of starlight from an ocean surface begins to dominate only under certain circumstances:

Specular reflection of starlight from an ocean surface occurs at all phase angles, but only begins to dominate the wholedisk signal when a planet is nearest its star as a thin crescent. Observations at such phase angles can be obtained of planets around G and F stars where they have adequate angular separation and orbit within the habitable zone.

The authors’ figures show that detections within 0.66 AU of a parent star are unlikely. To use these methods, we’ll need to stick with G and F stars, where the angular separation should make such observations possible. Indeed, planets with a surface of continents and oceans, like ours, should polarize the reflected signal by as much as 30-70 percent. Of all the tricky measurements a terrestrial planet finder instrument of this class might make, the identification of reflected water may well be the easiest.

The paper is Williams and Gaidos, “Detecting the Glint of Starlight on the Oceans of Distant Planets,” in press at Icarus and available online.

Comments on this entry are closed.

  • James M. Essig January 18, 2008, 22:09

    Hi Folks;

    Perhaps very long baseline optical telescopes will one day give us the ability to view even any existent tree lines on mountains of terrestrial like planets around stars as far as 100s of light years away. It has been suggested that building large VLBI optical telescopes using the craters of the moon to shield numerous reflectors each of which could be 1000s of meters wide and distributed over a width or area of 100s of kilometers wide would enable us to do such. Obviously, optical signal timing would have to be coordinated and catalogued with the utmost precision of the best atomic clocks and the various inputs would have to be collated computationally with vast supercomputing power, but I feel it should eventually be doable.

    The use a VLBI optical scopes distributed throughout Earth or near Earth solar orbit should allow us in theory to see the tree lines on any line of sight accessable planets within the Milky Way Galaxy.

    The use of VLBI reflectors with optical collection area of trillions of square kilometers distributed over distances of billions of kilometers as in beyond the orbit of Pluto in order to avoid interference by solar power fluctuations, planetary gravity based perturbations, etc, might will enable us to image the tree lines of line of sight accessable planets near the edge of the observable universe thus enabling us to look back in time 13 billion + years. This astounding technology would potentially enable us to read any news paper headlines on planets within the Andomedea Galaxy and other nearby Galaxies.

    Such huge optical telescopes might be composed of thin film inflatable reflectors wherein the this films have nanotech actuators and differential-area-element, relative position recorders to faciliate mass savings and real time microscopic-time-interval adjustment of each differential area of the reflectors for enhanced imaging capabilities.

    I think the optical frequency observational astronomers are going to have a field day as optical telescopy continues to improved. Optical engineers, software engineers, and computer scientists will no doubt enjoy the challenges of developing space based VLBI optical systems in the future.


    Your Friend Jim

  • george scaglione January 19, 2008, 13:26

    jim i sure would like to get a good view of those earthlike worlds myself.preferably as i put my boot down in the soil of one.lol though i don’t see that happening real soon ! a real shame. but,thanks for the good comments, george ps ONE MORE THING!!!!!!! you know if we could indeed get a real good look at an earthlike world – and i’ve said this before – it would be the best thing imaginable for the space program! people would get all excited and want to go.forget that the place might be 34 light years away! lol buddy,FINE BY ME!!! thanks again g

  • Hans Bausewein January 19, 2008, 15:48

    Interesting idea. Maybe *they* are already reading *our* newspapers and watching our destruction of a nice planet, provided that *they* exist of course.

    And considering the limit of the speed of light, those observers cannot be too far away or they would see our prehistoric planet.

    What would one see with such a large telescope looking into another large telescope? A big gap? Or the electronic ‘eye’ of the alien?

  • James M. Essig January 19, 2008, 22:15

    Hi George and Hans Bausewein;


    If they ever find an Earth like planet during the next couple/few decades. I will definately have to set up the old turkey smoker and have one hell of a party. I would love to work on the design of a manned vessel that would take us there if only as an assembly technician, code writter, or spokesperson. Better yet would be to take part on the mission as a crew member although I would probably be deemed to old to participate. Too bad we do not already have 2X or 3X fold life extension technology, I love to go.

    Hans Bausewein;

    It would be interesting if any such distant ETI life forms wouild intercept and recieve signals from our telecom activities from our online Tau Zero dialogues by the time we would launch a manned mission to investigate.


    Your Friend Jim

  • george scaglione January 20, 2008, 13:26

    jim and hans…very interesting and thought provoking concepts. also,if they can read what we are saying here by some stretch of the imagination(!) : hello i hope you will see fit to in some way communicate with us soon! what do you know about spacecraft propulsion? would be a real honor to have you join in the conversation. thank you one and all “hypotetical aliens” included! your friend george

  • forrest noble January 20, 2008, 20:25

    Congrats Jim, first in on this one.

    Yeah, the idea seems good. Reflections off water. Everything would have to be just right though. Just the right distance from the star, and just the right position in its orbital path for us to see the crescent of its reflection. It seems that reflections off snow or maybe ice might also be possible or other frozen liquids more likely to be at a planets poles.

    George, maybe I’m a hypothetical alien. My sisters seem to think so.
    your friend forrest

  • Hans Bausewein January 22, 2008, 13:53

    A bit more serious now (weekend is over)

    I think there’s still a lot more possible with current telescopes. We’ve recently seen one using polarization already. In the end it’s all about signal-to-noise ratio. Modelling the target system to see what to look for will certainly help. Taking snapshots at the expected most favourable time in the best frequency range and polarization will probably do a lot better than just random observations.

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