A New Collaboration to Aid the Search for Life on Distant Worlds

MOUNTAIN VIEW – A new NASA initiative is embracing a team approach to the problem of finding life on planets around other stars. 

Termed NExSS (Nexus for Exoplanet System Science), this virtual institute will benefit from the expertise of several dozen scientists in the effort to find clues to life on far-away worlds.  

Hiroshi ImanakaHiroshi Imanaka, a research scientist at the SETI Institute and a specialist in the chemistry of planetary atmospheres, is part of a team recently selected to be part of NExSS.  He notes how using a “crowd” of experts can help the search:

“One major thrust of the exoplanet community has been to find worlds orbiting in the so-called habitable zone,” notes Imanaka.  “That’s the range of distances from a star where a planet could have temperatures permitting liquid oceans. 

However, liquid oceans are not the only condition under which life can exist.  Some of the moons of Jupiter and Saturn are examples of places that are not in the conventional habitable zone, but might be nonetheless habitable.  We want to take further steps to characterize habitable environments that lie beyond the solar system.”

The study of planets around other stars – so-called exoplanets – is a relatively new field.  Since the launch of NASA’s Kepler space telescope six years ago, thousands of exoplanet candidates have been found, and it’s this sudden storm of new worlds that has prompted efforts to learn if any exhibit clues to the presence of biology, such as oxygen or methane in their atmospheres.  Discovering exoplanets has largely been the work of astronomers, but it’s planetary scientists and astrobiologists who have the expertise to characterize planetary environments and examine them for biology.  The intention of NExSS is to bring practitioners of these multiple disciplines together so they can collaborate on efforts to not simply find exoplanets, but see if any are home to life.

Titan in front of Saturn A giant of a moon appears before a giant of a planet undergoing seasonal changes in this natural color view of Titan and Saturn from NASA's Cassini spacecraft. Image Credit: NASA/JPL-Caltech/SSI

Imanaka and his colleagues have investigated one world that might give useful clues in this endeavor: Titan, Saturn’s largest moon.

“Our group is like an all-star Titan team,” says Imanaka.  “We’ve long studied the organic chemistry of this intriguing moon, one that’s swathed in a thick smoggy atmosphere and has lakes of liquid methane and ethane.  We’ve investigated the organic haze in Titan’s atmosphere.”

But is Titan really a good analog for a life-bearing world?

“It’s possible that Titan could have life,” says Imanaka. “I’m not going to say ‘no’.  But what’s certain is that Titan can teach us about a pre-biotic world, as it’s producing the most complex organic compounds known beyond Earth.  And thanks to its exceedingly low temperatures, all the chemical reactions on Titan are sluggish.  It’s a slow-motion world, and that’s why it can tell us about the conditions on the early Earth – and maybe on some exoplanets, too.  

Being part of the NExSS network will allow us to apply this extensive knowledge of Titan to the examination of smoggy exoplanet atmospheres that might be similar.” 

The NExSS initiative is all about collaboration, and brings together sixteen teams in an effort to better sift distant planets for indicators of life.  In this, the space agency is borrowing a leaf from celebrated basketball player Michael Jordan:

“Talent wins games, but teamwork and intelligence win championships.”

How Can We Find Tiny Particles In Exoplanet Atmospheres?

MOUNTAIN VIEW – 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 human hair 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 tiny particles 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).

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.

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