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Editor's Note: We asked several scientists from various fields what they thought were the greatest mysteries today, and then we added a few that were on our minds, too. This article is one of 15 in LiveScience's "Greatest Mysteries" series running each weekday.
Life can be found in almost every nook and cranny of our planet Earth. Leaping, swimming, flying, sprinting, slithering, crawling or rooted firmly in place, organisms appear, die, and are replaced by new generations and new species.
Whether a similar bounty of life exists elsewhere in the universe is one of the oldest and most tantalizing questions of science. Considering the wide breadth of the universe and the countless stars it contains, the odds would seem in favor of the answer being "yes."
"We are here, made of stardust. Therefore, it is at least possible that there are others," said Jill Tarter, director of the Center for SETI Research in California.
Hardy critters
But today's scientists hope to get beyond mere statistics to find something more substantial, and more edifying. Perhaps more than at any other time in history, scientists are optimistic that extraterrestrial life does exist, and that a firm confirmation can be had.
Their hope is buoyed by recent discoveries of worlds beyond our solar system and new revelations recently learned about the hardiness of life here on our own planet.
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"As we learn more about the diversity of life, particularly microbial life, we expand our definition of what life is and how life can exist in some very hostile (to humans) environments," said biologist Diana Northup of the University of New Mexico.
Scientists have discovered microbes that are resilient to levels of heat, cold, salt, acidity, and radiation that would kill humans. Some of these so-called "extremophiles" have been found thriving in complete darkness, in parched deserts and even miles below ground.
All of this is good news for astrobiologists who dream of finding life beyond Earth's confines, as many of the extreme environments on our planet are thought to be the norm for other worlds. Earth's deserts, for example, have analogues on dry, dusty Mars. Saturn's moon Titan is a world of meandering rivers and lakes, and beneath the icy crust of another Saturn moon, Enceladus, might lie environments resembling the frigid ocean depths of Earth.
Brave new worlds
Astrobiologists are also heartened by the recent explosion of new planets discovered outside our solar system. Since 1995, when astronomers spotted the first planet in orbit around another normal star, the number of extrasolar planets, or "exoplanets," has swelled to over 200. Scientists now know of more than 20 times more planets outside our solar system than in it.
The majority of exoplanets discovered so far are bloated, fast-spinning gas giants, known as "hot Jupiters," that orbit extremely close to their stars and are thus probably unsuitable for life.
But some exoplanets are wondrously Earth-like. Scientists recently spotted one world only 20.5 light-years away that lies within the habitable zone of its star—the region around a star where liquid water, and thus life, might exist. (It was later discovered the planet might be too hot for life, but another potentially habitable world in the same system was quickly found to take its place.)
With the ongoing refinement of current planet-finding techniques and the launch of new satellites, scientists expect not only to find a truly Earth-like world, but to also be able to probe it for life's spectral fingerprints carried by a planet's reflected light.
"Depending on what level of seeking and finding we are prepared to do, we could make discoveries in the next two decades that entirely change the way we understand the universe and life," said Margaret Turnbull, an astrobiologist at the Space Telescope Science Institute in Baltimore, Maryland.
SETI
Of course, there is always the chance that extraterrestrial life will find us first. Perhaps not in the form of a visiting UFO, but a radio transmission from an advanced alien civilization is still considered within the realm of possibility.
"Mankind has achieved scientific-technological civilization only in the last 200 years or so, out of about 4.5 billion years of life on Earth," said Frank Wilczek, a Nobel-Prize winning physicist at MIT. "So it seems we ought to expect there to be many scientific-technological civilizations that have had many millions, or even billions, of years to develop."
But even the discovery of one single-celled microbe on a distant world would be enough—enough to finally answer that age old question of "Are we alone in the universe?" and enough to change how humanity views itself.
"The discovery of life forms inhabiting the unexplored extremities of our own planet, and eventually, the discovery of life on other planets, will bring into greater awareness the magnificence of a living universe," Turnball told LiveScience, "and, hopefully, a better understanding of ourselves."