New research shows that if Earth were slightly smaller and less massive it would not have plate tectonics. This may be the main reason that Venus doesn't have it.

http://www.astronomy.com/asy/default.aspx?c=a&id=6462

From that article:Hmmmm. There seem to be some unexplored assumptions in there.

Here is a one-page pdf abstract from these authors (although it doesn't look like it relates to the same meeting mention on the Astronomy page), which contains a handy number for world-builders: the continent cycle time is predicted to vary as the -0.3 power of planet mass.
And here is the arXiv page for Inevitability of Plate Tectonics on Super-Earths, by the same authors.

Grant Hutchison

If Earth is on the borderline, I'd hate to see what planets in the heart of the "sweet spot" look like!

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LOL. I suspect that barely reaching the borderline of a wide variety of factors related to Earth’s development is as sweet as it gets in this universe.

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I was just thinking of all the science fiction I've read where every life form on a planet considered every other life form (including humans) to be prime candidates for lunch!

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T. Anderson

If you think about it from the standpoint of the anthropic principle, it's quite natural that the Earth should be on the size borderline for life. That's because since presumably there are more smaller terrestrial planets than larger ones in the normal distribution of sizes, there should be more life found at the small end of the viable population of planets.

We have an example of one of typical solar systems with terrestrial planets. If any of the extra terrestial planets that we have found so far can be reasonably assumed terrestrial, they are two or more times more massive than Earth. That if, suggests Earth is a midsize terrestrial planet, or even of smallish mass. Perhaps we will know this year if we start finding Mercury, Venus and Mars mass extra terrestrial planets. Neil

So far we can't say anything, because all we're seeing are our own detection limits. This is called an "observational selection effect", and makes it impossible to say anything at all about the size distribution of terrestrial planets. However, most formation mechanisms of the type we generally imagine for terrestrial planets yield a distribution of more smaller examples than larger examples, with a lower cutoff at some "minimum size" for the described process to be in effect. Unless the Earth is already at or near that minimum size cutoff, I find it very unlikely that there are not vastly more smaller terrestrial planets than larger (notwithstanding the fact that the definition of "planet" may be pretty useless in this context).

Ken, it's great to see that you're finally coming around my way of thinking!
However, it may not be the case that small terrestrial planets are typical. I would say the size of a solar system's terrestrial planets largely depends on the metallicity of the parent star. The higher the metallicity, the easier it is for large rocky cores to form--up to a point. Too much metallicity, and the planets get too big and turn into gas giants instead. Too little metallicity, and you might not get planets at all. So the size distribution of terrestrial planets will depend on the distribution of metalicity among stars. (I have no idea if our sun is typical in that regard, though.)

(Granted, there are thousands of little asteroids compared to the handful of terrestrial planets. I take it we're limiting the discussion to "major" planets functionally defined by their ability to sweep out and dominate a particular orbital realm.)

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As I recall, our debate about the anthropic principle was around whether or not it qualified as a physical explanation of anything. I say that physical explanations are held to a different standard. The idea that life is out there, occuring most places it has a good chance to do so, and that we are just a part of that larger statistical situation, does not seem scientifically controversial to me. Hence it is normal to expect that the conditions under which intelligent life appeared on Earth should be characteristic of lots of other places where intelligent life appeared. That is not the same as saying it explains any physics. Perhaps what I'm using here is what is known as the weak anthropic principle.That is all certainly possible. All I'm saying is that if you look at all the stars in our galaxy, with all their metallicities and every other relevant factor, you should find a distribution of terrestrial planet sizes that I expect to peak at smaller rather than larger sizes. That just comes from the expectation that there are generally more ways to make small things than big things, and the more variables you include as relevant factors, the truer that becomes. Possibly not in all cases, one would need more details-- I'm not suggesting a theory of planet sizes here.
That's what I meant by the "useless" aspects of the definition of "planet". All we care about to invoke the anthropic principle are rocks that are capable of sustaining intelligent life. If it was easier to get such life on the moon of gas giant, then that is where we might have expected humanity to develop. Of course, it's always possible that there are several different planet types that are roughly equally prominent in the life in our galaxy, and then Earth could be just one general possibility.

Given earth like consistency, as a planet gets more volume, it is going to have more water content per surface area. As the planet ages this water migrates out to the surface. Therefore larger worlds are likely to have more oceanic coverage as a percentage of surface area. If the tectonics of a planet are completely submerged will they be suppressed? In the case of a large water world is the surface gravity likely to higher than earth, or lower? The atmosphere much deeper?

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With earthlike composition but larger mass a planet would have higher surface gravity. The atmosphere would be more compressed, so shallower in a sense, but likely with a higher surface pressure. The atmosphere could depend drastially on irradiance - think of Venus with an earthlike mass and (probably) composition, but a very different atmosphere.

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Water is seen as promoting tectonics on Earth, and almost all of Earth's spreading and subduction zones are submerged already, so I doubt there would be a problem for tectonics. Surface gravity is going to be higher, atmospheric scale height smaller, as AndreasJ says.

Grant Hutchison

Maybe not easy worlds to develop technology on..

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That may be a good point-- if Earth is the largest you can get and still have continents, and yet if smaller sizes are not geologically active enough, then the size range for intelligent life might be pretty narrow. That would also tend to force Earth to be near to the edge-- maybe it is also near to the upper size limit. If so, that's all a new term in the Drake equation-- if a Venuslike planet could never sustain intelligent life even if it were at Earth's location, that's a potentially severe blow to intelligent life-- especially if larger planets are awash in water (in the best case scenario).

An earth without plate tentonics would still cycle carbon in and out of the crust. Volcanic activity might be fairly constant with volcanos forming over hot spots. These would release some carbon, similar to shield volcanoes on earth. The material they eject would push the crust they lay on down into the mantle, causing it to mix with the mantle. Another option is that the earth might cycle through occasional periods of extreme volcanism, but this would still cycle the crust. But either way, the average amount of CO2 in the atmosphere might be very low.

It be mentioned here that in the Januar 4 issue of Science, there was an article called "Intermittent Plate Tectonics" by Paul G. Silver and Mark D. Behn, arguing that plate tectonics has effectively shut down during some periods of the Earth's history. If true, plate tectonics presumably isn't all that necessary for life ...

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The whole point of subduction is that it recycles the carbonate sediments that get subducted. If there were only hot-spot volcanoes, carbonate sediments would just sit on the ocean floor and wouldn't get recycled.

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