exoplanetology: astronomers have announced that they have found the first rocky exoplanet that orbits its star in the Habitable Zone, the narrow band of space where it's possible for liquid water to exist on the surface of a planet. This zone is affectionately known as the Goldilocks Zone, because it is neither too hot nor too cold... it's just right.
Exoplanetology is such a young field that we get to experience new "firsts" all the time. It certainly is an exciting time in astronomy. There was the first exoplanet discovered, then the first rocky planet discovered, then the first planet to be photographed, then the first planet to have been discovered with photography. Now we have a new one: the first planet that could be habitable.
("Habitable" in this case just refers to the planet's rocky composition and its potential for liquid water. On Earth, of course, it takes more than just water to keep us alive. Extraterrestrial life, if it is very different from life on Earth, could have radically different requirements for survival. Planets where liquid water is possible are good places to start looking, but we should be careful not to conflate this potential for liquid water with being Earth-like in the general sense, i.e. a place where humans could live comfortably).
Gliese 581g's existence has yet to be confirmed by other astronomers, but it is thought to be about 3 to 4 times the mass of Earth, and probably about 1.3 to 2 times as large. Its surface gravity would be somewhere between 1.1 and 1.7 times that of the Earth, which would be enough to sustain some kind of atmosphere. It orbits a dim red dwarf star, which puts out much less energy than our own Sun. But it orbits much closer, with an orbital period of only 37 days, so the energy output of the star is sufficient to allow for liquid water on the surface. Average temperatures have been estimated to be in the range of -84 to -49 degrees Fahrenheit, but since the planet is tidally locked, with one side permanently facing the star and the other side in permanent shadow, temperatures may range from extremely cold to scorching hot. Much will depend on the nature of its atmosphere... if it has an atmosphere. Astronomers have suggested that life could find a happy medium temperature somewhere near the terminator, where light meets darkness. But an atmosphere of sufficient thickness and appropriate composition could be capable of distributing the heat more evenly about the planet, warming the dark regions.
Amusingly, the media always manages to distort discoveries like these, and yesterday was no exception.
The most egregious of these distortions is that "potentially habitable planet" becomes simply "habitable planet"... a big leap. A decidedly Earth-like rendering published in the New York Times and elsewhere (pictured above) certainly fed the notion that this is Earth 2.0 . And it probably didn't help that one of the planet's discoverers, Steven Vogt of the University of California, Santa Cruz, told the media he thought the chances of life existing on Gliese 581g was "100 percent." He's not talking about intelligent life, of course; simple organisms like bacteria would count. Even so, it's quite a statement... but I won't begrudge him his excitement.
The reality is, we still know very little about this planet. We don't know anything about its atmosphere, so even though it orbits in the Goldilocks Zone, it could have an atmosphere something like that of Venus, with crushing pressures and extremely high temperatures. Or, it could be devoid of any substantial atmosphere, like our Moon (let us not forget, the Moon also lies in the Sun's habitable zone). If the planet has no magnetosphere (like Mars), life as we know it could be impossible, as the solar wind could strip away the atmosphere and make liquid water impossible. The good news is, Gliese 581g is massive enough that a magnetosphere could be present, despite its slow rotation. And a runaway greenhouse effect, like the one that may have brought Venus to its present state, is less likely in the habitable zone. But there are still lots of unknowns.
What is exciting about this discovery is not that Gliese 581g is the second Earth we've been looking for, but that its discovery suggests there are many more like it to be discovered! Red dwarf stars are the most common in the galaxy, and are extremely long-lived (some red dwarfs may have the potential to burn for trillions of years... longer, by far, than the present age of the Universe). It was once thought that life was unlikely around red dwarfs, but as the science is now suggesting otherwise, we have lots of new places to look. Gliese 581 is also pretty close by, only about 20 light years away. That means that if there were an advanced civilization there, with radio astronomy, we could send them a message and receive an answer in less than one human lifetime (unfortunately, visiting Gliese 581 is out of the question for the moment... traveling at the speed of our fastest spacecraft it would take us more than 1.5 million years to get there). The chances of having a civilization there with which we could communicate is remote, but it's nice to know the system is relatively nearby... in a galaxy 100,000 light years across, 20 light years feels like just a stone's throw away.
It's unfortunate that the media has to sensationalize the story to get the attention of the general public. It may or may not be intentional, but we know the discovery is already exciting enough without having to oversell it. For what it's worth, though, any press for exoplanetology is good press. And anyway, even if we temper our expectations of this particular world, we can still relish its implications. With more than 450 exoplanets discovered in the last 15 years -- and that number is sure to skyrocket as more sensitive technology comes online -- we are coming closer to finding worlds that are indeed Earth-like, which in turn brings us closer to finding life beyond our solar system. The possibilities are tantalizing.