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Pale blue dot or not? What the colour of alien worlds can tell us

The views expressed are those of the author and are not necessarily those of Scientific American.


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The pale blue dot (click to get a bigger version). Earth, as seen by Voyager 1 in 1990 when it was around six billion kilometres away from us. Credit: NASA/JPL

Most people are familiar with the pale blue dot image of Earth taken by Voyager in 1990. Its blueness is significant, of course, because it is Earth’s abundant liquid water that makes it look that way.

But if you looked at the light that is reflected from Earth carefully, you would see several interesting features. One, caused by vegetation, is called “red edge”. Green plants absorbs a lot of red light creating a big, sudden jump in reflectivity in the red bit of the visible light spectrum. An alien, if it could get a good look, would be able to tell than Earth had plenty of vegetation because of this red edge.

A paper recently uploaded to arXiv (accepted for publication in the journal Astrobiology) looks at what features, like vegetation, we might be able to discern on a planet outside of our solar system.

Siddharth Hegde and Lisa Kaltenegger at the Max Planck Institute for Astronomy in Heidelberg, Germany, then looked at how extremophiles — creatures that live and even thrive in extreme conditions here on Earth — might fare in the different kinds of habitat. “Extremophiles provide us with the minimum known envelope of environmental limits for life on our planet,” the researchers say.

View of the Earth as seen by the Apollo 17 crew traveling toward the moon. Credit: NASA

I wrote about a similar topic at New Scientist in August when Hajime Kawahara at Tokyo Metropolitan University and Yuka Fujii at the University of Tokyo in Japan, published a paper that describes how they created 2D maps of what the light from an Earth-like planet would look like with various features on its surface (see ‘Finding a Blue Marble’ here). By watching a planet over time their technique is able to build up a more detailed image – a blue marble, rather than a pale blue dot. Maps like these may one day provide us with an indication of what the environment is like on a faraway exoplanet.

Hegde and Kaltenegger’s paper covers a more rough and ready technique that could be a first step in deciding which exoplanets are the ones we should be studying in more detail – the planets where organisms, albeit extreme ones, can survive here on Earth.

They note, however, that the characteristics of vegetation (or any organism with chlorophyll) could vary depending on their planet’s host star. The signature of chlorophyll near a hot star could have a blue, rather than red, edge to protect a plant’s leaves from overheating. Or on a planet that orbits a cool, dim star chlorophyll may appear black as it tries to absorb as much light as possible across the whole range of the visible spectrum.

A day where we have to use these techniques to decide which of an abundance of potential Earth 2.0s to travel to seems a long, long way away. But that doesn’t mean we can’t start daydreaming about which we will aim for first.

(Hat tip to Technology Review for flagging up the paper.)

Kelly Oakes About the Author: Kelly Oakes has a master's in science communication and a physics degree, both from Imperial College London. Now she spends her days writing about science. Follow on Twitter @kahoakes.

The views expressed are those of the author and are not necessarily those of Scientific American.





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  1. 1. Suttkus 2:56 pm 09/28/2012

    Doesn’t this all seem a bit earth-centric? Look for a “red edge” because vegetation is green? Why presume alien vegetation is green? Not even every organism that uses chlorophyll on Earth uses green chlorophylls. Terrestrial plant life could just as easily have ended up descending from the red algae or the brown algae.

    Why presume alien vegetation uses chlorophyll at all? Certainly there must be billions of non-chlorophyll molecules that could have provided the same functionality. The opsins in our eyes turn light into moving electrons, too and would work as a substitute.

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  2. 2. Russell Seitz 2:44 am 09/29/2012

    Kelly Oakes perpetuates a serious misconception about Earth’s albedo. While the iconic ” image of Earth taken by Voyager in 1990″ is disticntly blue, it is not pale ” because it is Earth’s abundant liquid water that makes it look that way.”

    In any substantial thickness, including the deep blue sea that covers most of the Earth , water is very dark indeed, rivaling black asphalt in its capacity to absorb sunlight- at ordinary sun angles oceans and parking lots have reflectivities of under 10% .

    But though bulk water is grossly absorbing, the refractive index contrast at air-water interfaces can, witness cloud droplets , snow fields and air bubble filled glacial ice , backscatter light with considerable efficiency- fresh snow can have an albedo of up to .95.

    because fresh , salt and frozen water cover ~ 80% of the planet , what Voyager recorded was in large part the average of bright white clouds circulating above Earths deep blue oceans.

    Seen from deep space on a largely cloud-free day , Earth seems a dark blue ark indeed.

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  3. 3. Kelly Oakes in reply to Kelly Oakes 3:21 pm 09/29/2012

    Suttkus: You’re right, and the authors of the arXiv paper I talk about do mention some of your points too. But if we are looking for an Earth-like planet (and I’m not suggesting that’s the *only* type of exoplanet we should be interested in), this is a good way to do it.

    Russell Seitz: As I understand it, the “pale” in the phrase pale blue dot refers specifically to that picture taken by Voyager 1. I’m sure you’re right about the actual albedo of the sea. I didn’t say Earth’s liquid water that makes it look pale, just blue. I wonder how many largely cloud-free days there are, because we certainly don’t have many where I am ;)

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  4. 4. luhng 4:29 am 09/30/2012

    ☻Geometaphysicalhyperastophysics☺ Good paper Kelly. made me think. props.

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  5. 5. DrFunkySpoon 9:42 pm 10/2/2012

    You don’t really mean chlorophyll here. You mean another photosynthetic pigment functioning in the way that chlorophyll does here on Earth. Chlorophyll is a specific molecule with certain spectral properties that will not be radically different elsewhere. But other pigments may well (probably do) evolve on other worlds that will be spectrally matched to take advantage of the local radiative and thermal environments.

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  6. 6. Kelly Oakes in reply to Kelly Oakes 3:30 am 10/3/2012

    DrFunkySpoon: I followed the authors’ lead in using the word chlorophyll, but I see your point.

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  7. 7. eeddee 11:37 am 10/21/2012

    i find anything to do with space all very interesting and scientific american is one more source ti feed my appetite

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