The Growing Habitable Zone: Locations for Life Abound

In a galaxy filled with billions of stars, scientists searching for alien life need some way to pick out those which are most likely to harbor habitable planets and moons. For more than 150 years, an important tool in this screening process has been the concept of a "circumstellar habitable zone."

Traditionally, this zone has been defined as a narrow disk around a star where temperatures are moderate enough that water on the surface of a planet can exist in a liquid form. The idea is that where liquid water exists, life might arise.

Beginning in the latter half of the 20^th century, new information began to emerge that challenged the traditional view. Scientists on Earth began finding rugged organisms thriving in harsh conditions that were off-limits to most other creatures. Meanwhile, images beamed back by robotic probes in space revealed that other moons within our solar system were much more interesting geologically--and perhaps biologically--than our own.

However, beginning a decade ago, planets discovered around other stars began to reveal a diversity of planetary systems that was beyond expectations.

More recently, scientists have gone back and reexamined their ideas about the possibility of habitable planet forming around red dwarf stars. Despite being the most abundant stars in the galaxy, red dwarfs have traditionally been shunned by scientists as being too small and too dim to support life. Those prejudices are beginning to fade and the recent discovery of a small, rocky world in orbit around a red dwarf 28,000 light-years from our corner of the solar system has refueled speculations that these stars might harbor planets with life.

Extremophiles

Extremophiles are a diverse group of organisms that thrive in harsh environments intolerable to virtually all other creatures. Since the late 1960s, scientists have discovered hundreds of different extremophile species, most of them bacteria.

This hardy group includes members that can survive scalding waters, subzero temperatures, bone-crushing pressures, corrosive acid, extreme salt and arid conditions. Extremophiles have been found that can withstand massive doses of radiation, breath rust, eat sulfur, belch methane and live without oxygen or sunlight.

"Finding extremophiles on Earth has just been mind-blowing," said Carol Tang, a researcher from the California Academy of Sciences who studies extremophiles. "If you think about how there's very few places on Earth where there isn't life, you can't think about the solar system and the universe in a very limited way anymore."

Habitable moons

In 1979, NASA's two Voyager spacecrafts shocked scientists with images they beamed back of Jupiter's moon Europa. The images showed a shiny world covered in water ice, but what was really remarkable was how smooth its surface was.

Unlike our own moon, Europa has relatively few impact craters. Because it doesn't have an atmosphere to burn up incoming objects like asteroids, scientists concluded that Europa had an internal heat source that kept its waters fluid, allowing the moon to periodically repave its icy shell and erase away the craters that must routinely be carved.

"Before the Voyager missions, scientists used to think that the moons of the other planets were old, rocky, battered bodies like our moons," said Cynthia Phillips, a SETI planetary scientist.

Scientists think Europa stays warm by a process called tidal heating. All moons, including our own, are stretched and pulled by the planet they orbit. Jupiter is so massive and its gravity so strong that it actually causes Europa's surface to bulge and shrink as it circles around in its orbit. This constant motion generates friction and heat.

Saturn's cloud-covered moon, Titan, is thought to be warmed by the same process. Other moons generate heat through different means. Scientists recently discovered that Saturn's moon Enceladus, for example, contains a mysterious hot spot in its southern hemisphere that might be caused by radioactive material left over from the moon's formation billions of years ago.

Widening zone

This revelation, that not all the moons in our solar system are as dead and barren as our own, meant that places outside the traditional habitable zone might sustain liquid water and support life.

"If you have a fairly sizable planet with plenty of internal energy to keep warm it might not need to be close to the Sun," said biologist Ken Nealson from NASA's Jet Propulsion Laboratory in California."It might have plenty of energy to support a perfectly good biomass without having a lighted surface."

Scientists believe that beneath Europa's icy shell lies an ocean vaster than Earth's. For this reason, many scientists figure the Jovian moon may be a better bet for finding alien life than Mars.

"There might have been liquid water on Mars in the past and there could have been life then, but it's pretty unlikely that we'll find life living there today," Phillips said. "But on places like Europa, there could be and probably is water there today. Instead of looking at an extinct biosphere, we could be looking at a currently active one."

Red dwarfs

Last fall, a group of about 30 scientists from different fields got together in Mountain View, California for a workshop sponsored by the SETI Institute. The workshop was convened to answer a single question: are the planets orbiting red dwarfs habitable?

SETI scientists will soon begin looking for radio signals from intelligent extraterrestrial life using the Allen Telescope Array and they wanted to know whether red dwarfs should be included in the list of stars to search.

Red dwarfs are believed to make up about 85 percent of the stars in the universe, but they are so small and so dim that scientists have traditionally ignored them as possible havens for habitable planets.

One of the main objections was that the habitable zones of red dwarfs would be very narrow and very close to the stars. For a planet orbiting a red dwarf to be warm enough to have liquid water, it would need to be located closer to the star than Mercury is to our own Sun. At such a close distance, the planet would become tidally locked to the red dwarf the way our Moon is to Earth. Any water existing on such a planet would be boiled away on the side facing the star and frozen solid on the other.

In recent years, however, new computer models have suggested that the situation isn't as impossible as it might seem. The models predict that if an orbiting planet had a thick enough atmosphere, heat could be redistributed from the lit side of the planet to the side that was dark.

As for the criticism that a red dwarf's habitable zone is very narrow, Todd Henry, an astronomer at Georgia State University, has an interesting view. Because there are so many more red dwarfs than stars like our Sun, Henry has performed calculations suggesting that if the narrow habitable zones of all the red dwarfs in our galaxy were combined, they would equal the habitable zone of the all the Milky Way's Sun-like stars.

"You open up a lot more territories if you put [red dwarfs] back on the table," Henry said.

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