Venus is basically the same size as the Earth.
Earth's mean radius is 6,371 km. Venus' mean radius is 6052 km.
The masses are also similar, as are their compositions.

A more likely control on whether plate tectonics may be initiated is the existence of liquid water at the surface and within the lithosphere of the planet in question. Water greatly reduces the yield strength of plates (by as much as 62% when going from low to moderate temperatures compared with a drop of only 39% for dry olivine). So, while plate tectonics seems to be necessary for life, water (necessary for life) may be necessary for plate tectonics. Venus is just at the range from the Sun where it could have lost all of its water too quickly for plate tectonics to initiate (especially if it lost the water long before the planet was mostly still molten).

So, someone represented a company that has different ideas than you do...and that's a problem because?
Do /.ers really believe that their employer is their sole identity defining characteristic?
Are all of you who work for asshole-bosses also assholes?
It sure seems that that's what you're all saying when you go on these witch-hunts.

I'm not sure what you mean when you talk about 10% of a planetary surface and AUs.

Earth's surface area is 5.1x10^8 km^2. 10% of that is, obviously, 5.1x10^7 km^2. The land area of the US is about 9.8x10^6 km^2, so we're talking about 5-times the land area of the US. None of this has anything to do with distance from the star, just to do with the radius of the planet.

But, as you say, the point of 10% isn't that it's a special number; it's a starting point. Notice that this definition explicitly excludes any gaseous planets from the get-go. That's not necessarily fair, of course, but we've got to start somewhere, and rocky planets are a LOT simpler to understand w.r.t. possibilities of life.

Earth's average albedo isn't really all that controversial or problematic. For example, we can say that the poles probably had such and such an albedo at such and such a time (based on climate models based on core samples), the clouds are difficult for a specific times (decades or so), but again the climate will dictate some average cloud cover that is relatively accurate over long periods over the entire Earth.

Going into more detail would require an actual climate model (such as the Hadley model), which doesn't make much sense for extrasolar planets since we know next to nothing about atmospheres (especially their composition) on most extrasolar planets. Of course, we can speculate and use places like Titan, Mars, Venus, Earth, and Triton as jumping-off points for planets that are certain distances from their parent star.

Milankovitch cycles are certainly included in long-term habitability or continuous habitability zone research, but we're really limited by not knowing anything about the obliquity/precession cycles of extra-solar planets, which are quite dependent on specific circumstances of those planets.

The T-tauri phase of the pre-main sequence stars would strip almost any magnetic field-protected atmosphere from most planets, so we're fairly confident that any planets found orbiting such stars are uninhabitable. In fact, there are a lot of stars that can be ruled out (of having any sort of habitable zone) just by looking at them.

Habitability zones are for "well-behaved" stars with well-behaved planets. Some researchers are looking at double or triple star systems, so we'll have a better understanding of the possibility of life in such systems as time goes on.

I had expected this kind of troll from Timothy, but CmdrTaco?

WTF?

Yes, the moon is within the habitable zone, but it's not habitable. If we discover a rocky planet in the habitable zone of another star, the first thing we'll be looking for is an atmosphere (which is quite a bit more difficult than finding the planet, but techniques are being developed and tested). If we discover evidence for an atmosphere, the habitability of that planet jumps into a realm that is much more interesting. Then we start looking for evidence of certain gases in the atmosphere (water vapor, CO2, Nitrogen, etc.).

Well, first, let's go into some history.

A habitable zone around a main sequence star was originally (1959) defined as a (virtual) ring around that star in which at least 10% of the surface of a planet, with an Earth-like atmosphere, in that zone had a mean temperature of between 0 and 30 C with extremes not exceeding -10 and 40 C. This is appropriate for humans to survive.

The zone was quickly expanded to mean wherever liquid water was stable. The term "biostable" was employed to mean where liquid water was stable and the term "habitable" was restricted to mean a place suitable for humans. Soon, though, "habitable" was expanded to replace "biostable" and to include anywhere that liquid water is stable.

All (peer-reviewed) models since the original definition have used one type of atmosphere or another, usually an atmosphere chemically similar to Earth's. Most have also considered planetary albedo (surface brightness), solar evolution (as a star moves along the main sequence, the habitable zone changes or disappears, depending on the details), etc.

Several models have pessimistic estimates to the width and/or lifetime of a habitable zone, most often because an atmosphere like the Earth's is only metastable and it could collapse with only a few % change in solar energy input (distance from or luminosity of the sun, for example can greatly affect the stability of an atmosphere). Other models have included climate stabilization by linking CO2 and surface processes such as the creation/weathering of certain types of rock that remove/add CO2 from/to the atmosphere. There are a lot of these kinds of details that are included in most models of the habitable zone. A lot of the work is in determining which details are more important than others.

For my graduate work, we had to define the habitable zone around the sun, at the beginning of the solar system (4.556 Ga), and now. To do so, we had to start from the proplyd, condense all of the elements at the right distances from the sun, build the planets (we were allowed to assume that they formed in their current positions unless we wanted to make our work more difficult), allow atmospheres to condense or form, depending on where the planets were, etc., and finally determine which planets were possibly in the habitable zone as the sun evolved (Venus, Earth, and Mars, depending on the details and assumptions), and then determine whether the planets that are here now are in the habitable zone, and why or why not.

We, of course, used some pretty simple 1-D models for atmosphere, or used published models and argued why they were valid. We used simple models for planetary albedo, didn't evolve the albedo unless the atmosphere collapsed or changed dramatically in some other way (ignored Earth-like clouds, for example), etc. We used simple estimates for the concentrations of radioactive elements that could contribute to the surface temperature, used a simple model for luminosity evolution, etc., etc., etc.

For these kinds of simple models, the inner edge of the habitable zone is defined by when water will be lost from the atmosphere (through photolysis of the water vapor and escape of the hydrogen) and the outer edge is defined by when CO2 condenses and causes runaway glaciation.

Technically, the Moon is within the habitable zone, but it's obviously not habitable. Neither are Venus or Mars. This is because they don't have the right atmosphere, and may never have had the right conditions.

I make no guarantees to that assertion's applicability other than in the context in which it was originally intended.

The "holy grail" (so to speak) right now is finding evidence for ANY life outside of our planet. Doing so would change our relationship with the universe in many ways (even though most relevant scientists are much less agnostic than they should be when it comes to the question of whether life exists elsewhere in the universe). Once we find life in one place not on Earth, we'll be much more open to the idea of looking all over for it and for other forms of life than what we're familiar with. So, until that first goal is achieved, we'll look where we think we're most likely to find life.

In my graduate studies, we defined and derived it with and without an atmosphere.

Oh, you're not "very wrong."

We can only recognize life as we already understand it. A common medical exam question is to define life. A common graduate school exam question is to define life. How do we do that? Based on what we know.

We know that life (as-we-know-it) requires a few conditions, so we look for planets that could support those conditions.

Nobody thinks that's the only place to find life, but it's probably the easiest place to find life that we would understand...

They mean its proximity to the sun, the so-called "habitable zone" that everybody wants to talk about, regardless of the type of planet.

I presume you're complaining about the article linked to by Slashdot. That's not a scientific article; that's a popular science article.

Here's NASA's report, which isn't much better. I can't find the actual journal article yet.

www.nasa.gov/topics/universe/features/rocky_planet.html