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Life where you least expect it.

IN 1977, SCIENTIFIC researchers aboard a submarine dubbed Alvin descended down to the seafloor, where they observed a hydrothermal vent located off the Galapagos Islands. What they saw shocked them. Hydrothermal vents are fissures situated at the ocean bottom, which spit scalding, acidic water out into the surrounding sea. Looking out through the sub's tiny windows, the amazed alien visitors saw thickets of tube worms--some as tall as four feet high--waving in the ocean currents like crimson tulips in the winds of March.

Hydrothermal vents form where Earth's crustal plates gradually are spreading apart Hot magma is seeping up from below to create mountain ranges termed midocean ridges. As cracks develop where this spreading occurs, the seawater sinks a mile or two down into the hot rock below. Now, enriched with precious minerals obtained from the hot rock, the scalding hot water rises to the ocean floor to create a vent.

In addition to the tube worms, which so far have been seen only in the Pacific, there are other denizens of the vents. For instance, there are small benthic worms that frolic through the mud, as well as pencil-thin Jericho worms sporting accordion-like tubes. There also are finger-length crimson palm-worms that stand upright, displaying red leg-like structures on their heads. A unique class of tiny worms termed Alvinellids--named in honor of the sub that discovered them--dwell on the walls of mineral deposits that form near the vents. Crabs, mussels, clams, and shrimp also inhabit the vents in huge numbers--but they are not the same species that we are familiar with. For example, the tiny shrimp that reside around the vents in the mid-Atlantic have no eyes.

Scientists are uncertain about how such shrimp and other creatures are able to thrive in the chemical-rich seawater that would kill ordinary shellfish. Indeed, biologists have observed a rich variety of crustaceans hanging around vents, including amphipods resembling sand fleas, and tiny lobsters called galatheids.

Bacteria also inhabit this environment. They are the first living tidbits to colonize newly formed vents, arriving in a snowy blast, and then floating down to create white sheets glued to the seafloor. Bacteria have been discovered underneath the ocean's floor as well. It seems likely that they emerge from beneath when the opportunity presents itself. Vent bacteria can endure much higher temperatures than any other known organism. Creatures such as the tube worms and other species that live near the hydrothermal vents are termed extremophiles, which are life forms that inhabit environments human beings consider "extreme"--habitats that seem to be unsuitable for life. Extreme life forms have been found in venues that scientists once would have considered uninhabitable--very hot, cold, dry, or acidic environments, as well as those doused with high doses of radiation. For instance, nematode worms of the Antarctic spend most of their lives in a state of complete dehydration, during which lime they blow around helplessly in the fierce winds of dry desert valleys. The worms shut down completely, and dry out their cellular contents. Eventually, they revive when their environments become less hostile.

The existence of such extremophiles on Earth strongly suggests that life may exist on other worlds--in habitats that our species would find "extreme." When we observe populations of tube worms and other species near deep sea hydrothermal vents, we are amazed by the obvious fact that they are living very well without any energy derived from the sun. Therefore, it is possible that, beneath the jumbled, cracked ice of Jupiter's small moon, Europa, there are similar communities living around hydrothermal vents. Astronomers strongly suspect that there is a dark, frigid global ocean flowing beneath the cracked icy surface of Europa. Scientists now are better prepared to search for such communities on other worlds thanks to their observations of extremophiles on our own planet.

Jill Tarter, director of research at the SETI (Search for Extraterrestrial Intelligence) Institute, Mountain View, Calif., explains: "At the same time that we have been developing the capabilities to detect distant Earths, we have also been finding that life on Earth occurs in places that earlier scientists would have considered too hostile to support life. Scientists were wrong. We now know that extremophiles can exist (and sometimes thrive) in the most astounding places: at the bottom of the ocean around hydrothermal vents; in ice, in pure salt, in boiling acid, and irradiated by massive doses of UV and X-rays. There do appear to be places on Earth that are too dry for even these [mostly microbial] extremophiles, or perhaps our sensors aren't yet sensitive enough to find them."

Since 1995, astronomers have spotted a veritable treasure trove of exoplanets; bodies in space that run the gamut from frozen, icy spheres to searing-hot worlds sporting molten surfaces. Most planet hunters consider the discovery of another "tiny blue dot" resembling our own watery world to be the "holy grail" of their dedicated quest to find life elsewhere in the universe. However, this endeavor may be too narrow in scope. While the crown jewel of exoplanetary explorations indeed would be a world with most of the same attributes as our Earth, new research suggests that life might be able to survive on some of the more exotic oddballs that are twirling around with us in our Milky Way galaxy.

Stephen Kane, an astronomer with the NASA Exoplanet Science Institute at the California Institute of Technology, notes in a NASA Jet Propulsion Laboratory press release that "when we're talking about a habitable planet, we're talking about a world where liquid water can exist. A planet needs to be the right distance from its star--not too hot and not too cold."

This temperature range is determined by the heat and size of the star, which is termed its habitable zone. Indeed, the most apparent physical necessity for the existence of life-as-we-know-it is the presence of liquid water. Life, as we experience it on Earth, is limited by the freezing and boiling points of water. All earthly chemistry depends on liquid water as a solvent.

However, some organisms have managed to develop the means to triumph over such limits. For example, by adding antifreeze compounds such as highly concentrated sugars, amino acids, and salts to their cells, a minute crustacean known as a "water bear," along with numerous other species of microorganisms that dwell in subfreezing temperatures, can survive in frozen regions of our planet. Species such as Antarctic icefish actually need the cold temperatures (to which they have become adapted) in order to survive.

Kane and colleague Dawn Gelino have developed a valuable resource that they call the Habitable Zone Gallery. It determines the distance and size of the habitable zone for every exoplanetary system discovered so far. It also shows which exoplanets circle within their star's habitable zone.

Out-of-round orbits

Most importantly, not all exoplanets sport Earthlike orbits that keep a fairly constant distance from their star's fiery furnace. One of the greatest surprises for planet hunters was the revelation that a large number of exoplanets travel in very eccentric (out of round) orbits that are elliptical (football shaped). In their journeys around their parent stars, they vary greatly in their distance from their incandescent parents.

Kane explains that "planets like these may spend some--but not all--of their time, in the habitable zone. You might have a world that heats up for brief periods in between long, cold winters, or you might have brief spikes of very hot conditions."

Kane and Gelino's study suggests that the habitable zone surrounding stars could be considerably larger than previously suspected. Exoplanets that may not be hospitable to our kind of life might be to some other forms--forms that resemble the extremophiles dwelling on Earth.

Therefore, even though planets circling their stars in eccentric orbits would be very different from our planet, this may not prevent them from being able to support an alien form of life. "Some organisms can basically drop their metabolism to zero to survive very long-lasting, cold conditions," Kane continues. "We know that others can withstand very extreme heat conditions if they have a protective layer of rock or water. There have even been studies performed on Earth-based spores, bacteria, and lichens, which show they can survive in both harsh environments on Earth and in extreme conditions of space."

This research indicates that life potentially can exist in many forms in our galaxy--not just on small, beautiful watery blue dots like our own world. For example, moons circling gas giant planets potentially can host life. Such moons exist in our own solar system--including Europa and Ganymede of Jupiter, and Titan and Enceladus of Saturn. "There are lots of giant planets out there, and all of them may have moons, if they are like the giant planets in the Solar System," indicates Kane. "A moon of a planet that is in or spends time in a habitable zone can be habitable itself." He furthers notes, "Life evolved on Earth at a very early stage in the planet's development, under conditions much harsher than they are today."

There probably are a vast multitude of eccentric and gas-giant planets dwelling in our galaxy, and some astronomers have suggested that there may be more planets than there are stars.

Scientists today are not in agreement about what constitutes an extreme environment. On our own planet, we have a variety of habitats running the gamut from superheated waters near subsurface volcanic vents to the extremely dry, frozen regions that compose the Antarctic valleys. There are living creatures living in caves that are being pelted constantly with droplets of sulfuric acid, as well as organisms surviving in extremely alkaline solutions. Likewise, there are creatures making homes in saturated salt solutions, as well as organisms that are able to survive in environments where they are subjected to high doses of ionizing radiation---and there are tidbits of life that can derive their energy and food from such unlikely inorganic elements as manganese, sulfur, and iron compounds.

Kane contends that, in addition to the habitable zone, other characteristics of exoplanets should be taken into consideration, such as the planet's mass, size, and atmosphere. "It's difficult to really know about a planet when you don't have any knowledge about its atmosphere." For example, both Earth and its "evil twin" Venus experience an atmospheric "greenhouse effect"--but the runaway greenhouse effect on Venus, which sports a surface hot enough to melt lead, renders it the hottest world in our solar system. "Without analogues in our own solar system, it's difficult to know precisely what a habitable moon or an eccentric planet in orbit would look like," Kane adds.

Nevertheless, Kane and Gelino's research suggests that habitability might exist in numerous and diverse ways in our Milky Way--not limited only to planets like our own. They currently are investigating which already-known exoplanets potentially might host extremophile life or habitable moons. "There are lots of eccentric and gas giant planet discoveries," Kane says. "We may find some surprises out there as we start to determine exactly what we consider habitable."

Judith Braffman-Miller is a freelance journalist.
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Title Annotation:Science & Technology; habitability in exoplanets
Author:Braffman-Miller, Judith
Publication:USA Today (Magazine)
Geographic Code:1USA
Date:Jan 1, 2013
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