Oct
3
Written by:
admin
10/3/2010 9:32 AM
Last week's announcement of an Earth-like planet in the habitable zone around its star, brings back to mind a recent discussion in our forums about exoplanet gravity. The new planet, Gliese 581g, is believed to be between 3.1 and 4.3 times the mass of the Earth, which begs the question: even if we could get there (Gliese 581 is approximately 20.5 light years away), could this be a usable planet for humans?
Since the radial velocity method was used to determine the planet's position and mass, we don't know much about its physical size in terms of radius. It is believed to be between 1.3 and 2 Earth radii in size, depending upon its composition (if it is mostly water, then it could be between 1.7 and 2 Earth radii; if rocky, then 1.3 to 1.5 Earth radii). As a result, surface gravity is expected to be in the range of 1.1 to 1.7 Earth gravities.
Those ranges mean that people could probably live on this planet, if the other conditions are found to be right. At 1.1 g, things wouldn't be terribly different than here on Earth, gravity-wise. Colonists and their belongings would weigh about 10% more than here on Earth, however, and it would be correspondingly more difficult for spacecraft to land and take off again. But that's the lowest end of the estimation. At 1.7 g, would we even find it feasible with current technology to take off from this planet once we've landed on it?
The higher gravity means that escape velocity would be higher, and slower rotation would also mean that you would gain less starting velocity from the planet's rotation; thus, rockets would have to be correspondingly more powerful and use correspondingly more fuel in order to launch. Given the difficulty we have in reaching orbit around Earth, I don't think our current technology would allow us to take off from Gliese 581g (or "Zarmina's World" in honor of the wife of one of the co-discoverers) once we arrive.
But what about future technologies? Given that the planet is believed to be tidally-locked to its parent star, a space elevator is probably completely out of the question: the reduced rotational speed and increased gravity would require a space elevator considerably longer than here on Earth, and we don't have even a theoretical material strong enough to build an elevator to orbit above Zarmina's World. A rail gun launch system might work, but would have to be very powerful (and thus very long) in order to launch cargo or passengers to orbital velocity. If we eventually develop some means of controlling gravity that would, of course, make the whole process much easier.
In the absence of those technologies, though, I think we can just assume that if we could get to Gliese 581 right now, a trip to Zarmina's World to settle there would be a one-way trip.
Of course, we don't know if Zarmina's World is even remotely habitable yet, notwithstanding the irrational exuberance of one of its discoverers. Red dwarf stars such as Gliese 581 are notorious for their variability in output, which means that the side of Zarmina' World facing the star would be constantly bombarded by flare-ups or intense stellar minimums and maximums. We also know nothing of the atmosphere: for example, does the planet have an ultra-thin atmosphere like Mars, or possibly an ultra-dense atmosphere like Venus? Either of those cases would make the planet virtually uninhabitable. Even if the atmosphere is at one Earth atmosphere of pressure, would it contain the right mix of gases to sustain life as we know it?
Over time, further observations of Gliese 581 will tell us more about this and the other worlds orbiting this star. Meanwhile, our best guess is that sometime within the next 18 months, the Kepler team will announce the discovery of dozens or possibly hundreds of new candidate planets within the habitable zone around their stars (albeit much farther away from us than Gliese 581), and the COROT team has a reasonable chance of finding one or more such planets as well. As we find more and learn more, we'll have a better chance of understanding whether or not life—and possibly even human life—can possibly ever exist around planets other than Earth.
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