In astronomy and astrobiology, a habitable zone is the scientific term for the region around a star within which it is theoretically possible for a planet with sufficient atmospheric pressure to maintain liquid water on its surface. Astronomers have discovered a veritable rogues' gallery of odd exoplanets (from scorching hot worlds with molten surfaces to frigid ice balls. And while the hunt continues for the elusive blue dot; a planet with roughly the same characteristics as Earth) new research reveals that life might actually be able to survive on some of the many exotic exoplanetary oddballs that exist.
In astronomy and astrobiology, a habitable zone is the scientific term for the region around a star within which it is theoretically possible for a planet with sufficient atmospheric pressure to maintain liquid water on its surface. Astronomers have discovered a veritable rogues' gallery of odd exoplanets (from scorching hot worlds with molten surfaces to frigid ice balls. And while the hunt continues for the elusive blue dot; a planet with roughly the same characteristics as Earth) new research reveals that life might actually be able to survive on some of the many exotic exoplanetary oddballs that exist.
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Stephen Kane, a scientist with the NASA Exoplanet Science Institute at the California Institute of Technology in Pasadena said, "A planet needs to be the right distance from its star -- not too hot and not too cold." Determined by the size and heat of the star, this temperature range is commonly referred to as the habitable zone around a star.
Kane and fellow Exoplanet Science Institute scientist Dawn Gelino have created a resource called the Habitable Zone Gallery. It calculates the size and distance of the habitable zone for each exoplanetary system that has been discovered and shows which exoplanets orbit in this so-called "goldilocks" zone. The Habitable Zone Gallery can be found at www.hzgallery.org . The study describing the research appears in the Astrobiology journal and is available at http://arxiv.org/abs/1205.2429 .
But not all exoplanets have Earth-like orbits that remain at a fairly constant distance from their stars. One of the unexpected revelations of planet hunting has been that many planets travel in very oblong, eccentric orbits that vary greatly in distance from their stars.
"Planets like these may spend some, but not all of their time in the habitable zone," Kane said. "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."
Though planets like these would be very different from Earth, this might not preclude them from being able to support alien life. "Scientists have found microscopic life forms on Earth that can survive all kinds of extreme conditions," Kane said. "Some organisms can basically drop their metabolism to zero to survive very long-lasting, cold conditions. 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 the extreme conditions of space."
Kane and Gelino's research suggests that habitable zones around stars might be larger than once thought, and that planets that might be hostile to human life might be the perfect place for extremophiles, like lichens and bacteria, to survive. "Life evolved on Earth at a very early stage in the planet's development, under conditions much harsher than they are today," Kane said.
Kane explained that many life-harboring worlds might not be planets at all, but rather moons of larger, gas-giant planets like Jupiter in our own solar system. "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," Kane says.
As an example, Kane mentioned Titan, the largest moon of Saturn, which, despite its thick atmosphere, is far too distant from the sun and too cold for life as we know it to exist on its surface. "If you moved Titan closer in to the sun, it would have lots of water vapor and very favorable conditions for life."
Still, the research suggests that habitability might exist in many forms in the galaxy -- not just on planets that look like our own. Kane and Gelino are hard at work determining which already-discovered exoplanets might be candidates for extremophile life or habitable moons. "There are lots of eccentric and gas giant planet discoveries," Kane says.
Habitable zones, however, are not stable. Over the life of a star, the nature of the zone moves and changes. Astronomical objects located in the zone are typically close in proximity to their parent star and as such are more exposed to adverse effects such as damaging tidal forces and solar flares. Combined with galactic habitability, these and many other exclusionary factors reinforce a contrasting theory of interstellar dead zones where life cannot exist.
Some planetary scientists have suggested habitable zone theory may prove limiting in scope and overly simplistic. There is growing support for equivalent zones around stars where other solvent compounds (such as ammonia and methane) could exist in stable liquid forms. Astrobiologists theorise these environments could be conducive to alternative biochemistry. Additionally there is probably an abundance of potential habitats outside of the habitable zone within subsurface oceans of extraterrestrial liquid water. It may follow for oceans consisting of ammonia or methane.
For further information see Weird Planets.
Habitable Zone image via NASA.
