Post by petrifiedwalnut on Jun 22, 2017 5:18:29 GMT
I've been skeptical about the ability to detect Earthlike planets for a while - we have two basically Earth-sized planets in our solar system, and they are really different. One of them is substantially closer to the sun and has weird geology that may either result in or be a consequence of its super-greenhouse atmosphere. What other possibilities are there?
Right now, the thing that a telescope would notice first that is odd about Earth is its oxygen atmosphere - you can't get that without something continually producing oxygen, because the stuff is so reactive. On Earth, this is photosynthesis. But are there any non-biological processes that produce oxygen gas that we don't know about?
The sample size is one. That's enough to know that the Earth is possible, since it exists, and we also know that the Earth is resilient to perturbations, because it's survived a number of quite severe perturbations that we can detect in the geologic record. What I don't think we know is how fine-tuned the celestial environment has to be in order to get an Earthlike planet in the first place - I can see arguments, for example, that the Earth would not be possible were it not for the gravitational influence of the moon, and moons are going to be much more difficult for Kuiper to detect than planets.
We are currently coming to the conclusion that, from all the exosolar systems spotted, our particular configuration is really rare. A part of it can be owed to the methods being used, (particularly the transit and the barycenter methods) but the outstanding majority of all systems had planets only 1.5 times larger than Earth and more, and most of those seemed to be outer planets at that. So far, we've mostly found hot jupiters and candidates for gas giants, but it's hard to find anything about the Earth sized planets other than their density, size and orbital period with our current methodology. We can barely even sample atmospheric light from any exoplanets, and even then only under very specific conditions. "Earth-like" basically just means that it's a planet (very) roughly in the size and density range of Earth orbiting in or slightly outside the zone where you could find liquid water somewhere on the surface. Just in our solar system, both Venus and Mars would also fit into that equation, as well as some bodies in the asteroid belt (were they large enough) - and those aren't very Earth-like under general terms.
But we've filled at least one field in the Drake equation: basically every star is very likely to have another body in its orbit, be it a telluric planet, a gas giant, a star or something in between. Exceptions would be rogue stars and white dwarfs, but even those have been observed to occasionally have bodies.