Image: Strain of the arsenic-eating bacterium called GFAJ-1
This scanning electron micrograph shows a strain of the arsenic-eating bacterium called GFAJ-1.
updated 7/10/2012 7:59:19 PM ET 2012-07-10T23:59:19

The ongoing brouhaha over arsenic-munching microbes on Earth shows just how tough it can be to search for "life as we don't know it" on our home world — and the challenges would be even greater on other planets.

On July 8, two new studies threw further doubt on a bacterium's supposed ability to swap out phosphorus for arsenic in its basic molecular machinery. The microbe known as GFAJ-1 apparently does need phosphorus to survive, according to the new research, meaning it likely follows the same basic rules as all the other lifeforms we know about on our planet.

The uncertainty and controversy surrounding GFAJ-1 — whose discovery was announced in December 2010 — suggest that it would be tough for a robotic rover or lander, with its stripped-down instrument suite, to confirm the presence of life as we don't know it on another planet or moon.

Tough — but not impossible, if you cast a wide enough net, scientists say.

"You never know what you're looking for until you find it," said Seth Shostak, a senior astronomer at the SETI (Search for Extraterrestrial Intelligence) Institute in Mountain View, Calif. "About all you can say is, make as wide a variety of tests as you can afford to cram onto the spacecraft." [ Extreme Life on Earth: 8 Bizarre Creatures ]

Looking for life
Searching for Earth-like alien life is a tough enough task, as the ambiguous results from NASA's Viking mission to Mars in the 1970s demonstrate.

But there's no guarantee that microbial life elsewhere in the solar system — if it exists — is Earth-like. Alien creatures may encode their genetic blueprints in a molecule other than DNA or RNA, for example. They may not even be carbon-based.

Standard biochemical techniques would have a hard time identifying such lifeforms in a scoop of Mars dirt or a thimbleful of ice from Jupiter's moon Europa. But other methods might have better luck.

For instance, microscope observations could discover extraterrestrial organisms, regardless of their biochemical particulars.

A morphology-based identification would not necessarily be definitive; after all, scientists are still arguing over the possible "microfossils" spotted in the Mars meteorite ALH 84001 in the mid-1990s. But the potential is there.

"That falls under the rubric of what Justice Potter Stewart said about pornography — you'll know it when you see it," Shostak told, referring to a famous 1964 Supreme Court case that considered whether obscenity is protected under the First Amendment.

Left-handed life?
Another possible tactic, Shostak said, is to zero in on molecules' chirality, or handedness.

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Complex molecules often come in two different mirror-image forms, a left-handed version and a right-handed version. Here on Earth, biomolecules tend to be one of these versions, but not the other. For example, life utilizes only left-handed amino acids for protein synthesis.

So finding a trove of complex molecules on another world that are exclusively right-handed or left-handed — or "homochiral" — could be a strong indicator of life, the thinking goes.

Confirming the discovery of alien microbes that are fundamentally different than Earth organisms would probably require a multitude of tests and a variety of evidence, Shostak said. And in the end, it may come down to Justice Stewart's "know it when you see it" test.

"You think of all the properties you think that biology would exhibit — it grows, and it needs an energy source, and it moves around a little bit and it's maybe got a cell wall," Shostak said. "You do all these tests, and what does the evidence say to you — guilty or not guilty? Most science isn't done by that sort of vote. But that's probably what it's going to come down to inevitably, unless it's very, very obvious."

Discovering intelligent extraterrestrial life, on the other hand, would probably be a bit more clear-cut.

"When you're looking for advanced lifeforms, I think you've got a much easier job," said Shostak, who is doing just that at the SETI Institute, searching for signals from alien civilizations. "I mean, if you see an interstate highway system, it's not as ambiguous as finding things that sort of look like a microbe but might not be."

Follow senior writer Mike Wall on Twitter @michaeldwall or @Spacedotcom. We're also on Facebookand Google+.

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Photos: Month in Space: January 2014

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  1. Southern stargazing

    Stars, galaxies and nebulas dot the skies over the European Southern Observatory's La Silla Paranal Observatory in Chile, in a picture released on Jan. 7. This image also shows three of the four movable units that feed light into the Very Large Telescope Interferometer, the world's most advanced optical instrument. Combining to form one larger telescope, they are greater than the sum of their parts: They reveal details that would otherwise be visible only through a telescope as large as the distance between them. (Y. Beletsky / ESO) Back to slideshow navigation
  2. A balloon's view

    Cameras captured the Grandville High School RoboDawgs' balloon floating through Earth's upper atmosphere during its ascent on Dec. 28, 2013. The Grandville RoboDawgs’ first winter balloon launch reached an estimated altitude of 130,000 feet, or about 25 miles, according to coaches Mike Evele and Doug Hepfer. It skyrocketed past the team’s previous 100,000-feet record set in June. The RoboDawgs started with just one robotics team in 1998, but they've grown to support more than 30 teams at public schools in Grandville, Mich. (Kyle Moroney / AP) Back to slideshow navigation
  3. Spacemen at work

    Russian cosmonauts Oleg Kotov, right, and Sergey Ryazanskiy perform maintenance on the International Space Station on Jan. 27. During the six-hour, eight-minute spacewalk, Kotov and Ryazanskiy completed the installation of a pair of high-fidelity cameras that experienced connectivity issues during a Dec. 27 spacewalk. The cosmonauts also retrieved scientific gear outside the station's Russian segment. (NASA) Back to slideshow navigation
  4. Special delivery

    The International Space Station's Canadian-built robotic arm moves toward Orbital Sciences Corp.'s Cygnus autonomous cargo craft as it approaches the station for a Jan. 12 delivery. The mountains below are the southwestern Alps. (NASA) Back to slideshow navigation
  5. Accidental art

    A piece of art? A time-lapse photo? A flickering light show? At first glance, this image looks nothing like the images we're used to seeing from the Hubble Space Telescope. But it's a genuine Hubble frame that was released on Jan. 27. Hubble's team suspects that the telescope's Fine Guidance System locked onto a bad guide star, potentially a double star or binary. This caused an error in the tracking system, resulting in a remarkable picture of brightly colored stellar streaks. The prominent red streaks are from stars in the globular cluster NGC 288. (NASA / ESA) Back to slideshow navigation
  6. Supersonic test flight

    A camera looking back over Virgin Galactic's SpaceShipTwo's fuselage shows the rocket burn with a Mojave Desert vista in the background during a test flight of the rocket plane on Jan. 10. Cameras were mounted on the exterior of SpaceShipTwo as well as its carrier airplane, WhiteKnightTwo, to monitor the rocket engine's performance. The test was aimed at setting the stage for honest-to-goodness flights into outer space later this year, and eventual commercial space tours.

    More about SpaceShipTwo on PhotoBlog (Virgin Galactic) Back to slideshow navigation
  7. Red lagoon

    The VLT Survey Telescope at the European Southern Observatory's Paranal Observatory in Chile captured this richly detailed new image of the Lagoon Nebula, released on Jan. 22. This giant cloud of gas and dust is creating intensely bright young stars, and is home to young stellar clusters. This image is a tiny part of just one of 11 public surveys of the sky now in progress using ESO telescopes. (ESO/VPHAS team) Back to slideshow navigation
  8. Fire on the mountain

    This image provided by NASA shows a satellite view of smoke from the Colby Fire, taken by the Multi-angle Imaging SpectroRadiometer aboard NASA's Terra spacecraft as it passed over Southern California on Jan. 16. The fire burned more than 1,863 acres and forced the evacuation of 3,700 people. (NASA via AP) Back to slideshow navigation
  9. Where stars are born

    An image captured by NASA's Spitzer Space Telescope shows the Orion Nebula, an immense stellar nursery some 1,500 light-years away. This false-color infrared view, released on Jan. 15, spans about 40 light-years across the region. The brightest portion of the nebula is centered on Orion's young, massive, hot stars, known as the Trapezium Cluster. But Spitzer also can detect stars still in the process of formation, seen here in red hues. (NASA / JPL-Caltech) Back to slideshow navigation
  10. Cygnus takes flight

    Orbital Sciences Corp.'s Antares rocket rises from NASA's Wallops Flight Facility on Wallops Island, Va, on Jan. 9. The rocket sent Orbital's Cygnus cargo capsule on its first official resupply mission to the International Space Station. (Chris Perry / NASA) Back to slideshow navigation
  11. A long, long time ago...

    This long-exposure picture from the Hubble Space Telescope, released Jan. 8, is the deepest image ever made of any cluster of galaxies. The cluster known as Abell 2744 appears in the foreground. It contains several hundred galaxies as they looked 3.5 billion years ago. Abell 2744 acts as a gravitational lens to warp space, brightening and magnifying images of nearly 3,000 distant background galaxies. The more distant galaxies appear as they did more than 12 billion years ago, not long after the Big Bang. (NASA / NASA via AFP - Getty Images) Back to slideshow navigation
  12. Frosty halo

    Sun dogs are bright spots that appear in the sky around the sun when light is refracted through ice crystals in the atmosphere. These sun dogs appeared on Jan. 5 amid brutally cold temperatures along Highway 83, north of Bismarck, N.D. The temperature was about 22 degrees below zero Fahrenheit, with a 50-below-zero wind chill.

    Slideshow: The Year in Space (Brian Peterson / The Bismarck Tribune via AP) Back to slideshow navigation
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