How can we find tiny particles in exoplanet atmospheres?

August 29, 2014 by Seth Shostak, SETI Institute

It may seem like magic, but astronomers have worked out a scheme that will allow them to detect and measure particles ten times smaller than the width of a human hair, even at many light-years distance.  They can do this by observing a blue tint in the light from far-off objects caused by the way in which small particles, no more than a micron in size (one-thousandth of a millimeter) scatter light.

In a recent study conducted by Adrian Brown of the SETI Institute, the broad outlines of this process have been worked out.  "The effect is related to a familiar phenomenon known as Rayleigh scattering," says Brown.  "And that's something everyone has seen: it makes the sky blue."

By analyzing spectroscopic data from the Cassini orbiter, the Mars Reconnaissance Orbiter, and ground-based telescopes, Brown has managed to document this blue enhancement in many nearby objects, including the rings of Saturn, its moons Dione and Epimetheus, Mars, the moon, and the tail of Comet 17P/Holmes.

Brown's theoretical study of the phenomenon showed that the spectral bluing occurs any time sufficiently small objects are in our field of view.  In his studies, he considered particles between 0.1 and 1.0 microns in size.  A is roughly 17 microns in diameter. 

So why isn't the ground beneath our feet blue?  Brown's research suggests that the effect is quickly damped by other objects that, despite being of the same type, have different size distributions.   The effect depends on having many particles within a narrow range of size.  In addition, too many might turn objects white.  As an example of the latter, a glass of milk appears white because of multiple scattering from fat globules, and clouds appear white due to multiple scattering from water aerosols (droplets).

How can we find tiny particles in exoplanet atmospheres?

Consequently, the bluing effect requires some process that forms lots of particles of almost identical size.  Simply establishing that such a process is present can give researchers clues to the history and conditions on extraterrestrial bodies.

"This technique would, in principle, allow us to find extremely tiny particles in the atmospheres or on the surfaces of exoplanets that are tens or thousands of light-years away," Brown says.

The research was published in the September 1 issue of Icarus.

Explore further: Astronomers find evidence of water clouds in brown dwarf atmosphere

More information: Adrian J. Brown, "Spectral bluing induced by small particles under the Mie and Rayleigh regimes," Icarus, Volume 239, 1 September 2014, Pages 85-95, ISSN 0019-1035, . On Arxiv:

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5 / 5 (2) Aug 29, 2014
Cool. Find particles of industrial activity that can't be naturally occurring, and spark a direct search for intelligent life.
5 / 5 (1) Aug 29, 2014
If this is bears out it's by far the best chance we have to find life, and even intelligent life. This is pretty exciting and the ramifications are earth shattering.

THIS is what I've been waiting for if it actually works....
5 / 5 (1) Aug 30, 2014
Awesome! A new technique to learn about conditions in atmospheres down to comet tails and even the Moon tenuous atmosphere and its migrating dust. The more we learn...

@Mm: I'm not sure how helpful learning about dust conditions is in assessing habitability or habitation. The best technique presented so far is the oxygen biomarkers, using the quadruple O2, O3, H2O and CO2 to distinguish between biotic and abiotic sources of a thermodynamically unstable oxygen atmosphere.

That is hard, so it will take a few decades of developing sufficiently large telescopes with coronagraphs/starshades and spectrometers to find a large enough statistical sample to declare "within this group of systems we are less than 1 % uncertain there are some life bearing planets". We may never be able to point to a specific planet apart from our own and say "here is life" with 1 % remaining uncertainty. [Source: Sara Seager, especially her recent participation in the NASA press conference on space 'scopes.]

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