Re: In as many cases
Is this just a claim, or do you have any reasoning or research behind this?
I can see that it may be at least as many ways to push it out, but I can not see that it has to be that way. You bring up Venus which is a planet radical different from Earth. First of all it has extreme length days, second the solar radiation is double of that of Earth, third it has no plate tectonic and fourth it has no internally generated magnetic field. Yes, we call it our sister planet, mainly because we are about the same size, we have an atmosphere and neither is orbiting Jupiter, but that is about it.
On the other hand we can easily argue that a planet too far on the inside of the goldilocks zone will have their atmosphere blown away regardless. Then we have a narrow band where the heat from the star is sufficient on it's own (possibly Venus as you point out), after that you have an extremely broad band of planets that would be too cold without a greenhouse effect (Earth and beyond). So you then end up a very small percentage where greenhouse gases could make an otherwise habitable planet inhabitable, then we have a much larger percentage of inhabitable planets that could be knocked into habitable range, and we have the planets that could not be habitable no matter what.
It is like this. You have a lottery, with a 1:100 chance of winning. Then you hand out a second ticket with a 1:10 chance of winning. The only problem is if you win on both your winning is canceled. Still, the second ticket would improve your chance of winning. The only way the second ticket would lower your winning chances is if the first lottery is more than 1:2. And clearly, not half the planets are habitable.
It is a little more complicated than this of course. For instance, the further away from being habitable the less chance there is of course to make them so with greenhouse gases. But unless you can put some actual reasoning or research behind your claim, I fail to see how you can be so sure.
Then we have that you failed to see the main point. That regardless of all of this, the conclusion still stands. That not finding a right sized planet in the goldilocks zone is not enough to conclude there can be no life in the system.
At last I would like to point out that we havn't actually looked at that many places yet. We have basically a good idea that quite a few of the objects orbiting the same star as us are so inhospitable that they are have no chance of having life. The ones that could maybe support life we have not been able to examine closely enough to actually confirm that they don't. Outside the solar system we have no way to check anything.
My hat and coat, please, I'll go and check.