Is Earth Unique or Is Life Common Across the Universe?
This artist's conception shows the inner four planets of the Gliese 581 system and their host star, a red dwarf only 20 light-years from Earth. The large planet in the foreground is Gliese 581g, whose discovery was announced in September 2010. The planet is in the middle of the star's habitable zone and is only three to four times as massive as Earth.
Credit: Lynette Cook

NEW YORK ? The discovery of Gliese 581g, an alien planet orbiting in the habitable zone of its parent star, has added new fuel to the debate over the uniqueness of Earth and whether life exists elsewhere in our universe.

"Any planet is unique in its details, but what we're really asking is: are the general properties of Earth something we can expect to be common in the galaxy, or rare?" said astrophysicist Neil deGrasse Tyson, director of the Hayden Planetarium at the American Museum of Natural History, this week.

Tyson spoke during a Sunday (Oct. 10) panel discussion here to commemorate the 10th anniversary of the museum's Rose Center for Earth and Space. The panel's chief topic: the possibility of extraterrestrial life, and whether or not Earth is truly one-of-a-kind.?

The Gliese 581g finding ignited the scientific community because the exoplanet was found in the habitable zone of its star ? a region where a planet's range of temperatures could sustain liquid water on its surface.

This area is also known as the Goldilocks zone, because surface temperatures are neither too hot nor too cold, meaning liquid water and other potentially life-supporting conditions could be found on the exoplanet.

"On Earth, where there is liquid water, there is life," Tyson said. "So, water is a tantalizing tag for us as we conduct our search for life in the universe."

The expert panel discussion occurred a day before some scientists cast doubt on the existence of Gliese 581g. The planet's co-discoverer Steven Vogt said he stands behind the planetary find.

Earth as a model

In order to determine the conditions required to nurture and sustain life on other planets, understanding our home planet is a good place to start.

"The Earth is undoubtedly rare in the solar system," said Don Brownlee, a professor of Astronomy at the University of Washington in Seattle, during the panel. "If you think of the universe as a whole, it's almost a totally hostile place."

Even on Earth, the existence of animals and humans make up only a small sliver of the planet's history.

"Earth will last about 10 billion years, yet it took 4 billion years of geological and biological evolution on our planet to get animals on it," Brownlee pointed out. "Earth has changed a lot. Even if there was life on Earth for most of its lifetime, it was microbial life, not animal life."

Still, there are unique mechanisms at work on our planet that help maintain its habitability, such as the complex movement of its tectonic plates, which drives earthquakes and volcanic activity, and contributes to our planet's atmosphere and climate stability.

"Tectonics stopped on Mars and it lost its atmosphere," said panel member Paul Falkowski, a professor of Geological and Marine Sciences at Rutgers University in Newark, N.J. "On Venus, it's much too hot for life to exist as we know it. Earth is the only one in the so-called habitable zone."

If life was found on another planet in our solar system ? for instance, on Mars ? the discovery would be very telling of our chances of finding life elsewhere in the universe, said another member of the panel, Chris McKay, a research scientist at NASA's Ames Research Science Center in Moffett Field, Calif.

"Right here in our solar system, if life started twice, that tells us some amazing things about our universe," he said. "It means the universe is full of life. Life becomes a natural feature of the universe, not just a quirk of this odd little planet around this star."

A matter of time and patience

So, is life unique to Earth, or could it be common in our universe? And if it is common, how close are we to finding extraterrestrial life?

"I'm much more optimistic that, in my lifetime, we're going to know the answer to, 'Are we alone?' [before we know] the origin of life," Falkowski said.

And while some panelists disagreed on whether Earth itself is unique, they were more readily able to agree that finding some form of alien life ? be it microbial or otherwise ? is only a matter of time.

"I would say that Earth-like planets are common," said Fred Adams, a professor of Physics at the University of Michigan in Ann Arbor. "The planetary properties and the beds for forming life, we're there already. Life is just a physical process, and physical processes happen everywhere. It would be remarkable if life in some form isn't common."