Artist's depiction of an extrasolar planet encircled by small asteroids as it orbits a bright star. Telescopes could analyze the sunlight reflected of a distant planet to learn whether it is inhabited.
Credit: David A. Aguilar, Harvard-Smithsonian Center for Astrophysics.
Although Captain Kirk and crew could zip over to a planet at warp speed and teleport down to the surface to check if it was inhabited, current-day scientists will generally have to search for life from a distance. New research gives some hope that we could detect a "handedness" beacon from a planet full of microbes.
This handedness, or homochirality, is characteristic of life on Earth. The molecules that make proteins and DNA all have either a left-handed or right-handed orientation. Both orientations are made in equal quantities by non-biological processes, but life prefers to have just one type of hand over the other.
"Homochirality is a fundamental aspect of self-replication," says William Sparks of the Space Telescope Science Institute. "It is a reasonable proposition that life on other planets will exhibit a particular handedness."
Many of the molecules in organic chemistry come in left and right-handed varieties. When life starts building up larger compounds from these organic ingredients, scientists suspect that having only one hand to deal with is an advantage, if not a necessity.
Detecting homochirality in purified samples is easy in the lab, but could we see the signal if we stepped back 40 light-years and looked at an entire planet?
"We are testing whether we can remotely sense something that we think is a generic aspect of life," says Sparks.
As described in a recent paper in the Proceedings of National Academy of Sciences, Sparks and his colleagues found that the light coming off tiny microbes is partially polarized. If a similar amount of polarization reflects off alien microbes from a distant planet, it might be measurable with future telescopes.
Deciding what might constitute a smoking gun for life on faraway planets is an active field of research.
The classic biosignature is an atmosphere far from equilibrium. Certain molecules can only survive for a short time, so if astronomers detect them in the spectrum of light coming from a distant planet, we might assume that living things are replenishing the supply.
Another calling card of life might be the vegetation red edge, which is due to plants absorbing strongly in the visible wavelengths but reflecting infrared light. Alien plants may capture the light from their host star in a similar fashion.
However, there are geological phenomena that can mimic these supposed biological indicators. Volcanoes can replenish the atmosphere with short-lifetime gases, and some minerals have a red-edge-like reflection spectrum.
This ambiguity led Sparks and his colleagues to consider an alternative biosignature.
The homochirality of organic material was discovered in 1848 by Louis Pasteur, who explained why some biologically-derived molecules rotated the direction of polarized light.
Even now, scientists use polarization of light as a way to study biological samples. But typically these experiments work with purified extractions of single molecules. It is rare to study the polarized light signal coming from a full living organism.
However, a number of earlier experiments studied plant leaves and found that about 1 percent of the reflected light was circularly polarized (meaning the electric field rotated in a clockwise or counterclockwise direction).
Sparks and his fellow researchers decided to look further down the evolutionary tree, since simple microbial organisms might be a more common form of life on other planets than leafy plants.
The researchers chose marine cyanobacteria, which are photosynthesizing bacteria that appeared on Earth between 2 and 3 billion years ago. Organisms that photosynthesize light are probably the best bet for being an observable part of the reflected light from a planet. They live out in the open (as opposed to deep underground or underwater) and absorb light at the dominant frequencies of the host star.
The team put the microbes in a Petri dish and shone unpolarized light on them. A special polarimeter measured the circular polarization of the reflected and transmitted light.
The team found a polarization signal between 0.1 and 0.01 percent (the value of the signal depended on the wavelength of light). Minerals and other non-living objects typically do not polarize light to this extent.
Showing their hand
State-of-the-art polarimeters can detect signals of 1 part in a million. "It's not easy, but it can be done," Sparks says.
That might seem good enough to spot the microbes that Sparks and colleagues tested, but the light coming from a planet will be a mixture of light scattering off clouds, rocks and other sources. To be detectable, the authors contend that microbial colonies will need to provide 1 to 10 percent of the light from a viewing scene.
Sparks and others have actually looked for this signal from a not-so-far-away planet. Using ground telescopes they scanned the surface of Mars in 2005 looking for polarization at the level of 1 part in 1000. They didn't see anything, which is somewhat reassuring, Sparks says, since it implies that the polarization signal from terrestrial planets will not be overwhelmed by some non-biological source.
Observing polarization from planets outside our solar system will be harder. Sparks says getting enough photons will be "a stretch" for the Terrestrial Planet Finder, which is a proposed space mission to directly image exoplanets. The chances might be better with huge telescopes being planned for the ground, such as the European Extremely Large Telescope. This 42-meter-wide "light bucket" could potentially collect enough photons to see a small planetary polarization effect.
An interesting question is whether our own blue planet emits a polarization signal. "We don't know since no one has looked," Sparks says. His team calculated that regions of the ocean with lots of cyanobacteria should have a circularly polarized signal of 1 part in 10,000. The team is currently looking into possible field measurements over the ocean, as well as over forests and other vegetation, to precisely measure the polarization signal of Earth?s own life from a distance.
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