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Updated Model Puts Earth On the Edge of the Habitable Zone
cylonlover writes with news of an update to the model used for calculating the habitable zone around stars shifting things out a bit. From the article: "Researchers at Penn state have developed a new method for calculating the habitable zone (original paper, PDF) around stars. The computer model based on new greenhouse gas databases provides a tool to better estimate which extrasolar planets with sufficient atmospheric pressure might be able to maintain liquid water on their surface. The new model indicates that some of the nearly 300 possible Earth-like planets previously identified might be too close to their stars to to be habitable. It also places the Solar System's habitable zone between 0.99 AU (92 million mi, 148 million km) and 1.70 AU (158 million mi, 254 million km) from the Sun. Since the Earth orbits the Sun at an average distance of one AU, this puts us at the very edge of the habitable zone."
1. I'm pretty sure that Venus doesn't have an appreciable magnetic field.
2. Even if it did, its day is about the same length in its year (e.g. about 250 earth days) so nobody could live in any fixed place on the planet without freezing or melting, even if we got rid of the thick atmosphere. You'd have to live in trucks rolling slowly around the planet in the ... pardon the pun ... twilight zone.
Mars on the other hand has normal days and could be warmed up with a greenhouse effect. Also, the thicker atmosphere would provide additional sheilding at the surface level. One could imagine the last few percentage points of shielding being made up with local magnetic field "bubbles" around settled areas, powered by fusion reactors, assuming we have that technology in the next century or so.
Surface? No, but Europa is believed to have an icy surface hiding a massive liquid water ocean. Although it is far from the habitable zone, gravitational interactions with Jupiter generates heat which keeps the oceans liquid. Add in some organic materials (which asteroids might supply) and life could have developed deep under the surface of Europa. Perhaps right now, as I type this, some big Europan fish-like creature is swimming through the cold oceans on the moon. (Or perhaps there are just Europan bacteria... even single celled alien life would be a major find.)
My sci-fi novel, Ghost Thief, is now available from Amazon.com.
Mars had liquid water at some point and is outside the habitable zone, for some definitions of habitable zone. So it is entirely possible that planets with liquid water can exist outside the habitable zone. The real issue is with stability. An interesting take on this is to consider the flux of radiation from the Sun hitting the Earth. For a disk the size of the Earth, one can calculate the distance where water freezes and where water boils as a rough estimate of a "zone" of sorts. When looked at in this way, the Earth is at a point just barely above freezing. That we have the climate that we do beyond that near freezing point is due entirely to greenhouse effects.