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Comment by cstross | Hacker News Reader

I want answers to three questions before I get excited:

1. Does it have active plate tectonics?

2. Does it have a working deep carbon cycle?

3. Has its atmosphere hung on to its hydrogen (that is, has it managed not to lose it all to high-altitude UV splitting and solar wind)?

If the answer to all three of those is "yes" then this suddenly gets a lot more interesting, whether or not we can see any free oxygen in the atmosphere. And while we won't be able to get answers to them using current technology, next-generation direct-observation planet seeking telescopes might be able to deliver within the 15-30 year time scale (if the astrophysicists I know are correct).

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How would the physics of those three observations work, and what angular resolution would be required? I can see spectroscopy working for #3 and #2, #1 stumps me.

In theory #1 could be answered by looking for an atmosphere temperature that are similar to Earth’s despite being in an orbit that should make the planet cooler. If there is no plate tectonics then all the CO2 gets locked up in carbonates and the temperature falls (e.g. Mars) or you get a run away greenhouse like Venus. It is plate tectonics that have kept earth in the habitable zone. In theory we should be able to detect such a temp delta.

Yeah, good questions. The thing that indicates to me is that, if we can find this one (and we exist)...there are probably lots of planets like this. Probably billions.

And if there are billions of them, then what are the odds that some of them would have plate tectonics, a deep carbon cycle and hydrogen?

Seem pretty high to me.

It's amazing that Kepler continues to discover new planets and (I hope) it will get to the point where we just expect it to be the norm. Oh, another planet in the Goldilocks zone? It's just a shame it'll be a long time before we can actually step foot on any of those planets.

I'm curious why active plate tectonics makes the planet more interesting. Do those contribute to the "liveability" of a planet in some way?

Google on "great oxygen catastrophe" and prepare for an eye-opening experience.

(Note that neither Mars nor Venus have active plate tectonics. The surface of Venus melts every couple of hundred million years due to trapped heat, while Mars has no magnetosphere to speak of ...)

You omit a key point. The oxygen event (initial uplift from near zero at 2.3Ga) is not directly related to plate tectonics. Instead, plate tectonics appears to have helped the oxygen boost at 1Ga: increased volcanic activity released large stores of phosphorus, which is required for photosynthesis, which produces oxygen.

You're attributing a statement of causality to me that I never made; I merely said that plate tectonics would make an earth-like exoplanet more interesting -- for exactly the reason you note: it appears to be a prerequisite for an oxygenation event.

We may never know the answers to these questions. Though it is currently possible to say something about the atmospheric composition of some close exoplanets, and that ability will be improved with better telescopes, the planets that Kepler observed during its mission are too far away to learn much about for the foreseeable future.

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