Could One of These Worlds Be E.T.’s Home?

by Gregory Mone

Of the more than 700 planets discovered outside our solar system, none yet fit the description alien hunters dream about: an Earth-like planet in an Earth-like orbit around a sunlike star. But some scientists want to broaden the parameters of their search. In November a team led by Washington State University astrobiologist Dirk Schulze-Makuch devised the Planetary Habitability Index, or PHI, a scoring system for distant worlds that measures their suitability for any kind of life, not merely life as we know it. “We can’t go after only the Earth model of life,” he says. “You really want to be open-minded.”

Courtesy Habitability Laboratory at UPR Arecibo; Courtesy NASA (3)

Under Schulze-Makuch’s criteria, a faraway world racks up points if it has a solid surface and an atmosphere, which act together to support chemical reactions and deflect damaging radiation. Liquid water is not a prerequisite for a high score: A planet with liquids on the surface receives more points than a dry world, but the presence of water confers no additional advantage. “If you didn’t know that water worked on Earth,” Schulze-Makuch says, “you might think methanol would work much better for life.”

The PHI scores of bodies within the solar system reflect Schulze-Makuch’s hypothesis that the most Earth-like places are not necessarily the friendliest for life. Earth gets a near-perfect score of 0.96 on the 0 to 1 scale (it just has less available energy now than it did when life originated 4 billion years ago). But second place goes to Saturn’s moon Titan (0.64), which hosts vast lakes of liquid hydrocarbons but has surface temperatures of –300 degrees Fahrenheit. Mars, the target of more than a dozen robotic missions to hunt for signs of microbial life, comes in third at 0.59.

None of the planets yet found outside our solar system score particularly well. Gliese 581d, a rocky world nestling a cool, dim star, nets a rating of 0.43. Kepler-22b, the most Earth-like planet NASA’s Kepler space telescope has found so far, gets a similar score. However, Schulze-Makuch emphasizes that the numbers are subject to change. Astronomers have been able to determine the surface and atmospheric composition of only a few exoplanets, so for most planets the data are incomplete. Future telescopes that are powerful enough to probe these worlds, such as NASA’s proposed Terrestrial Planet Finder, should make the PHI much more useful.

Alien Planets With No Spin May Be Too Harsh for Life

By Nola Taylor Redd, Contributor | – Fri, Dec 16, 2011

Tidally-locked planets — planets with one side perpetually facing their star while the other remains shrouded in darkness — tend to be warmer on one side than the other. The presence of an atmosphere can help distribute the heat across the planet, equalizing the temperatures. But tidal locking could result in wide climate variations, a result that could threaten the evolution of life on the surface of these planets.

Tidal locking depends on the planet’s mass and its distance from its star. For planets orbiting M-type stars, which are slightly smaller than our sun, the region where planets become tidally locked overlaps with the so-called habitable zone, where water can remain as a liquid on a planet’s surface.

In the solar system, the moon is tidally locked in orbit around Earth.

According to new research published in the December edition of the Astrophysical Journal,strong heating of a planet at a single point can change or even control how much weathering occurs on the planet, which can lead to significant and even unstable climate changes. These dramatic climate effects could make planets that otherwise have the potential for life to instead be uninhabitable.

Whatever the weather
When rocks and minerals are exposed to the air, they react to the gases within it. As the rocks erode, a fresh face comes into contact with the air, allowing even more gas to be converted. If the erosion process keeps pace with the output of fresh gas into the atmosphere — say, from volcanic eruptions — the climate remains stable.

On tidally-locked planets, a single region is consistently close to the star. Known as the substellar point, this region receives more direct sunlight, and thus more heat. The recent paper proposes that such constant attention could affect weathering, and thus could influence the climate of the atmosphere. [Photos: The Strangest Alien Planets]

The process, referred to as enhanced substellar weathering instability (ESWI), is based on the fact that the influx of heat would cause an increase of weathering at the substellar point. The higher temperatures can also result in stronger rainfall, which go on to affect weathering.

“The harder it rains, the more it erodes,” said principal investigator Edwin Kite, of the University of California at Berkeley.

More rain means an abundance of fresh rock to react with the atmosphere, removing more of its components.

Similarly, if the substellar point cools for any reason, the weathering process slows. Less rock is available to chemically react, and the atmospheric gas builds up. Volcanism could put more material into the atmosphere than the rocks can absorb — and since volcanoes on Earth release greenhouse gases like sulfur dioxide and carbon dioxide, presumably a runaway greenhouse effect could take flight, leading to additional heating.

All of this happens because the heat is focused on a single region that is constantly closest to its star.

“What controls the weathering rate of the planet is just that patch,” Kite said.

On Earth, carbon dioxide from the air reacts with calcium silicate, creating calcium carbonate and silicone dioxide. The process removes carbon dioxide from the air and controls the greenhouse effect.

“Weathering regulates the climate on Earth on long time scales, and makes sure it doesn’t get too hot or too cold,” explained Dorian Abbot, of the University of Chicago. Abbot studies climate dynamics on Earth and on extrasolar planets.

The same thing could happen on other planets, but if the conditions are right (or wrong) the results could be more detrimental.

“We sometimes see catastrophic exits from the habitable zone,” Kite said.

For instance, a habitable planet could find itself moving to a Venus-like situation, with clouds of gas significantly increasing the surface temperature to points where water would boil off.

Or it could simply boast wide swings over its lifetime, significant shifts from cold to hot and back again. Such fluctuations could mean trouble for life trying to evolve on a planet.

“It has taken a very long time for life to develop complexity on Earth,” Kite said.

Kite explained that a number of key steps were required to get life to the point as we know it today.

“It would require a long period of habitability on a planet to allow these different steps to take place,” he said. “It’s not enough just to rain on a planet for ten thousand years and expect interesting things to happen.”

Abbot agreed.

“Climate instabilities are not good for the life we usually think about.”

Catastrophic changes: Rare or common?
How many planets could find their atmosphere destabilized by tidal locking?

To narrow that down, one must first look at how frequently tidally-locked planets might exist.

For M-type stars, “We would expect that a lot of the planets in the habitable zone would be tidally locked,” Abbot said.

For ESWI to occur, certain conditions must be met on these planets.

The substellar point, closest to the star, cannot be underwater. Land is required for the strongly temperature-dependant weathering.

Similarly, the gas that is absorbed by weathering must be the prevalent gas in the atmosphere.

Kite notes that, even if Earth were moved to another star and became tidally locked, it would not be in danger. Though weathering on Earth consumes carbon dioxide, nitrogen makes up most of the atmosphere.

Such results are not just limited to planets that can only reveal a single face to their star.

According to Kite, “All that’s really important to get this process going is a large day/night temperature contrast.”

As an example, the team worked with Itay Halevy, of the Weizmann Institute of Science, to consider a Martian mystery. Mars is not tidally locked but has wide temperature variations across a Martian day.

The Red Planet lost its atmosphere long ago, and scientists are still trying to determine exactly how that happened. Kite thinks ESWI could be a potential contributor.

“It’s an open question whether enough weathering occurred over its geological history to draw down a significant amount of carbon dioxide,” he said.

But Kite was clear that such conditions shouldn’t stop astronomers from studying planets that fall in the danger zone. These bodies could still have the potential to be habitable.

“Ultimately, only observing can tell.”

This story was provided by Astrobiology Magazine, a web-based publication sponsored by the NASA astrobiology program.

NASA Unveils Arsenic Life Form

By Rachel Ehrenberg, Science News

When cooking up the stuff of life, you can’t just substitute margarine for butter. Or so scientists thought.

But now researchers have coaxed a microbe to build itself with arsenic in the place of phosphorus, an unprecedented substitution of one of the six essential ingredients of life. The bacterium appears to have incorporated a form of arsenic into its cellular machinery, and even its DNA, scientists report online Dec. 2 in Science.

Arsenic is toxic and is thought to be too chemically unstable to do the work of phosphorus, which includes tasks such as holding DNA in a tidy double helix, activating proteins and getting passed around to provide energy in cells. If the new results are validated, they have huge implications for basic biochemistry and the origin and evolution of life, both on Earth and elsewhere in the universe.

“This is an amazing result, a striking, very important and astonishing result — if true,” says molecular chemist Alan Schwartz of Radboud University Nijmegen in the Netherlands. “I’m even more skeptical than usual, because of the implications. But it is fascinating work. It is original, and it is possibly very important.”

The experiments began with sediment from eastern California’s Mono Lake, which teems with shrimp, flies and algae that can survive the lake’s strange chemistry. Mono Lake formed in a closed basin — any water that leaves does so by evaporation — making the lake almost three times as salty as the ocean. It is highly alkaline and rich in carbonates, phosphorus, arsenic and sulfur.

Led by Felisa Wolfe-Simon of NASA’s Astrobiology Institute and the U.S. Geological Survey in Menlo Park, California, the researchers cultured microbes from the Lake Mono sediment. The microbes got a typical diet of sugar, vitamins and some trace metals, but no phosphate, biology’s favorite form of phosphorus. Then the team started force-feeding the critters arsenate, an analogous form of arsenic, in greater and greater quantities.

One microbe in particular — now identified as strain GFAJ-1 of the salt-loving, mostly marine family Halomonadaceae — was plucked out and cultured in test tubes. Some were fed loads of arsenate; others got phosphate. While the microbes subsisting on arsenate didn’t grow as much as those getting phosphate, they still grew steadily, doubling their ranks every two days, says Wolfe-Simon. And while the research team couldn’t eliminate every trace of the phosphate from the original culture, detection and analytical techniques suggests that GFAJ-1 started using arsenate as a building block in phosphate’s place.

“These data show that we are getting substitution across the board,” Wolfe-Simon says. “This microbe, if we are correct, has solved the challenge of being alive in a different way.”

Arsenic sits right below phosphorus in the periodic table and so, chemically speaking, isn’t that different, Wolfe-Simon notes. And of the six essential elements of life — carbon, hydrogen, nitrogen, oxygen, phosphorus and sulfur (aka CHNOPS) — phosphorus has a relatively spotty distribution on the Earth’s surface. If a microbe in a test tube can be coerced to live on arsenic, perhaps life’s primordial home was also arsenic-rich and life that used phosphorus came later. A “shadow biosphere” of arsenic-based life may even exist unseen on Earth, or on some lonely rock in space.

“It isn’t about arsenic, and it isn’t about Mono Lake,” says Wolfe-Simon. “There’s something fundamental about understanding the flexibility of life. Any life, a microbe, a tree, you grind it up and it’s going to be CHNOPS. But we have a single sample of life. You can’t look for what you don’t know.”

Similarities between arsenic and phosphorus are also what make the element so poisonous. Life often can’t distinguish between the two, and arsenic can insinuate itself into cells. There, it competes with phosphorus, grabs onto sulfur groups, or otherwise gums up the works, causing cell death. Some microbes “breathe” by passing electrons to arsenic, but even in those cases the toxic element stays outside the cell.

Researchers are having a hard time wrapping their minds around arsenate doing the job of phosphate in cells. The ‘P’ in ATP, the energy currency for all of life, stands for phosphate. And the backbone of the DNA double helix, the molecule containing the genetic instructions for life, is made of phosphate. Basic biochemistry says that these molecules would be so unstable that they would fall apart if they were built with arsenate instead of phosphate.

“Every organism that we know of uses ATP and phosphorylated DNA,” says biogeochemist Matthew Pasek of the University of South Florida in Tampa. He says the new research is both fascinating and fantastic. So fantastic, that a lot of work is needed to conclusively show exactly how the microbe is using arsenate.

Both phosphate and arsenate can clump up into groups, and with their slightly negative electric charge, slightly positive DNA would be attracted to such clumps, says Pasek. Perhaps the arsenic detected in the DNA fraction was actually a nearby clump that the DNA wrapped itself around, he speculates.

The microbe may be substituting for phosphate with discretion, says geochemist Everett Shock of Arizona State University in Tempe, using arsenic in some places but not others. But Shock says the real value of the work isn’t in the specifics. “This introduces the possibility that there can be a substitution for one of the major elements of life,” he says. Such research “stretches the perspective. Now we’ll have to see how far this can go.”

For an audio report go here or download the mp3.