Life Light

Life Light

The way light scatters off distant planets may reveal whether life exists on them.


A new way to search for alien life. I'm Bob Hirshon and this is Science Update.

Finding life on distant planets may someday become easier by analyzing the light that reflects off of them. That's an idea being developed by Bill Sparks of the Space Telescope Science Institute in Baltimore, and colleagues like physicist Thomas Germer of the National Institute of Standards and Technology. Germer explains that the molecules in living things tend to scatter light in a particular direction—a phenomenon called polarization.

If we see a net circular polarization coming from the planet, it's hard to imagine that coming from anything other than life.

Their team has developed an instrument that can detect that kind of polarization. So far, it's all in the lab, but the next step is to try it from a distance outdoors, and ultimately, from across space. I'm Bob Hirshon for AAAS, the Science Society.

Making Sense of the Research

Our universe is so vast that many scientists think that extraterrestrial life must exist somewhere out there. The problem is finding it. Our spaceships can barely reach the far corners of our solar system: practically next door, in the context of the whole universe. Telescopes can see stars that are farther away, but can detect planets beyond our solar system only indirectly (for example, by observing their gravitational pull on the stars).

As for visiting or receiving guests from other worlds, that's a long shot, to say the least. The nearest star beyond our sun is over 25 trillion miles away. Traveling that distance within a human lifetime would require a spacecraft that could move at or near the speed of light—which theoretically may be impossible. And even if some super-intelligent (or very long-lived) form of extraterrestrial life has solved this problem, the chances of them happening upon our little corner of the universe during their interstellar travels are tiny.

So, scientists are left with the option of detecting the presence of extraterrestrial life from a great distance. For a long time, one strategy has been carried out by a group called SETI, which stands for the Search for Extraterrestrial Intelligence. SETI uses the world's largest radiotelescope, in Arecibo, Puerto Rico, to scan the skies for patterns in radio waves that could not have occurred by random, natural means (and therefore, were generated by intelligent beings). The idea is that if intelligent civilizations are intentionally broadcasting radio waves like we do, some of those waves would leak into space, and eventually drift into our neighborhood. In fact, our own early radio and TV broadcasts have already passed by hundreds of stars.

However, SETI's approach is limited: intelligent life that uses radio-wave technology represents only a tiny sub-set of all the possible forms of life that may be out there. Germer's technique, on the other hand, could potentially detect any kind of life at all, as long as that life covers a substantial portion of a planet's surface. It's based on the fact that many molecules in nature come in two shapes, which are mirror images of one another. This property is called chirality; chiral molecules are said to come in “right-handed” and “left-handed” forms. Non-living minerals, like quartz, generally contain a random mix of right- and left-handed molecules. However, handed molecules in living tissues tend to be almost all right-handed or all left-handed. That's because it's easier for life to reproduce itself if the molecules in its components all orient the same way.

As it turns out, right- or left-handed substances scatter light in a particular way. Therefore, when light that reflects off a planet's surface polarizes in a right- or left-handed direction, that's a strong indication that there's some kind of life there. Germer's team has shown that living cells in a petri dish generate handed light, which can be detected by scientific instruments. Next, they plan to see if these instruments can also detect handed light from a distance—for example, reflected off a tree outside. If that works, then the ultimate goal would be to analyze light coming in from outer space, look for patterns of handedness, and then trace that light back to its planetary source.

Now try and answer these questions:

  1. How can light from outer space indicate the presence of life?
  2. What are right- and left-handed molecules? How do they fit into this technique?
  3. How does Germer's technique differ from SETI's?
  4. What are the limitations of looking for extraterrestrial life?

You may want to check out the May 8, 2009 Science Update Podcast to hear further information about this Science Update and the other programs for that week. This podcast's topics include: life on the ancient earth and beyond, the comet theory of the origins of life, how oxygen became abundant enough to support an explosion of life, a new method for locating life on other planets, and refuting the asteroid theory of dinosaur extinction.

For Educators

In the Access Excellence activity Project SETI, students learn to look for alien life in outer space, the SETI way.

In the National Geographic News article Alien Contact More Likely by "Mail" Than Radio, Study Says, read about a study that argues that intelligent alien life, should it exist, is more likely to communicate with Earth by sending artifacts, rather than radio waves. Links to related stories and websites are also found on this page.

The website Chiral Molecules, Structures, and Materials, created by University of Pennsylvania physicist Randall Kamien, gives on overview of the role of chirality in our universe.

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