A little over a year and some three hundred exo-planet discoveries ago, astronomers at the European Southern Observatory in Chile announced that they had found what might be the first habitable planet outside the solar system. Known as Gliese 581c, the new planet is only five times as massive as the Earth and inhabits a rare sweet zone around a dim red star in the constellation Libra where it is neither too hot nor too cold for liquid water.
Gliese is but a cosmic hop, skip, and jump from Earth -only some 20 light years away (or 120 trillion miles!). Voyager 1, now leaving the solar system at a speed of about 39,000 miles per hour, would need more than 300,000 years to travel that far. Or, maybe someday we'll actually invent a Star Trek-type transporter that reassembles our atoms and transports us to the farthest reaches of the Cosmos.
For decades, scientists have been debating the conditions that are needed to replicate an Earth-like probablility of complex beyond the microbial level. There's not much doubt in the minds of most astrobiologist that based on extremophile life we've discovered recently on Earth (see prior posts below), that life on the microbial level will be discovered sometime in the next twent years on Mars or on one of Jupiter or Saturn's moons.
The three recent key findings for astrobiology are extremophiles, extrasolar planets, and a sense that water may be more ubiquitous even in our own solar neighborhood (in meteors like the Mars' Lafayette, Europa, and the ice frost on polar Mars). This picture has evolved quite suddenly with 100-plus extrasolar planets found in just the last decade (and none known before around 1995). We now know that the number of planets in our own galaxy could easily tally in the hundreds of billions. The discovery of Gliese is a visible clue that a great number of these could be carpeted in the dirty chemistry we call life. Life on Earth may be unique, but it might not be miraculous.
Even in the oldest globular cluster star systems in our Milky Way galaxy -- choked with stars that were born more than 10 billion years ago -- there's enough "metals" to make earth-like worlds. According to models of planet formation developed by Dr. Sasselov and his colleagues of the Geneva discovery team, such a planet should be about half again as large as the Earth and composed of rock and water, what the astronomers now call a “super Earth.”
The most exciting part of the find, Dr. Sasselov said, is that it “basically tells you these kinds of planets are very common.” Because they could stay geologically active for billions of years, he said he suspected that such planets could be even more congenial for life than Earth. Although the new planet is much closer to its star than Earth is to the Sun, the red dwarf Gliese 581 is only about a hundredth as luminous as the Sun. "So seven million miles is a comfortable huddling distance." But for evolved animal life to be present we need to find that sweet "Goldilocks" planet with an exceedingly complex host of conditions present that have given rise the "Rare Earth" hypothesis.
In their book of that title, Rare Earth authors Peter Ward and Donald Brownlee, both of the University of Washington have outlined a short list of conditions needed: Right distance from a star; habitat for complex life; liquid water near surafce; far enough to avoid tidal lock; right mass of star with long enough lifetime and not too much ultraviolet; stable planetary orbits; right planet mass to maintain atmosphere and ocean with a solid molten core and enough heat for plate tectonics; a Jupiter-like neighbor to clear out comets and asteroids; plate tectonics to build up land mass, enhance bio-diversity, and enable a magnetic field; not too much, nor too little ocean; a large moon at the right distance to stabilize tilt; a small Mars-like neighbor as possible source to seed Earth-like planet; maintenance of adequate temperature, composition and pressure for plants and animals; a aglaxy with enough heavy elements, not too small, ellipitcal or irregular; right position the galaxy; few giant impacts like had 65 million years ago; enough carbon for life, but not enough for runaway greehouse effect; evolution of oxygen and photosythesis; and, of course, biological evolution.
Dr. Sasselov noted that aliens could have been pointing their antennas at Earth for 4.6 billion years, without picking up a signal. "Maybe the inhabitants of Gliese 581c are at the level of the classical Romans . . . or maybe trilobites." We need to check out hundreds of thousands of Earthlike worlds.