And what percentage of the planets in the universe have we explored?
Just over zero percent - the same as the percentage of cod fish in the ocean that we have tagged. Yet we know to within plus or minus 1% how many cod fish there are in the ocean. How? We catch a few hundred, tag them and release them. Then we come back later and fish until we've caught a few hundred more. If most of the second batch are tagged, then we know that there are few fish out there and we are repeatedly catching the same ones. If few of the fish in the second batch are tagged, then we know that there is a large population that the tagged ones disappear into.
We've found a few hundred planets now and the fact that none of them meet any of the conditions for life, while not conclusive, is not particularly hopeful either.
It's oversimplifying things to state that the size of the planet controls the thickness of the atmosphere, but even if we used that theory, what's to stop us from finding a planet smaller than Venus but larger than Mars that fits your criteria for a "correct size"?
It is not really an oversimplification. Barring catastrophic events like collisions, gravity is really the only thing that determines how much atmosphere a planet has.
When I say that there is no correct size, I mean that the atmosphere of a young planet with lots of volcanoes is either increasing or decreasing, depending on how much gravity they have. In the former case, they will eventually have a runaway greenhouse effect and, in the latter case, they will eventually have a trace atmosphere.
But "eventually" needs clarification. Tiny objects like Titan cannot hold gas molecules for more than a few days; they just drift off into space and are replaced by new eruptions. Large objects like Venus can hold gas molecules forever; they just keep accumulating. But middle-sized objects like Mars can hold gas molecules for a while - possibly a billion years in Mar's case - so the question that divides us is: Can advanced life evolve in only a billion years? It took a lot longer than that for it to evolve on Earth.
There is no correct size that retains an atmosphere, but not a crushing one, for the three billions years that it took advanced life to evolve on Earth. If you agree that it takes that long for advanced life to evolve, then you
must invoke catastrophy theory to explain the existence of an atmosphere, but not a crushing one.
This is highly speculative. First of all, it's not entirely proven that a collision with another planetoid formed the moon at all (although I would say that that's the most likely theory). The rest of it is ENTIRELY unproven...there's certainly no consensus that we were hit by a planetoid, which then circled the sun and hit the earth again. It's EXTREMELY unlikely that any planetoid-sized object could survive a collision with the earth...even a glancing one.
Maybe twenty years ago, but today there
is a consensus. And, incidentally, the planetoid didn't survive. Its mantle, if it had one, was stripped off like the jacket of a hollow-point bullet and it's core plunged through the Earth and stuck to our core. The Moon was formed almost entirely from Earth debris.
There are hundreds of billions of galaxies. Even if 99.99% of them have X-rays, high radiation, or the improper mixture of elements, there would still be billions of galaxies...each with hundreds of billions of stars.
"The stellar disk of the Milky Way galaxy is approximately 100,000 light-years (9.5×1017 km) in diameter, and is believed to be, on average, about 1,000 ly (9.5×1015 km) thick. It is estimated to contain at least 200 billion stars and possibly up to 400 billion stars, the exact figure depending on the number of very low-mass stars, which is highly uncertain.
"The Andromeda Galaxy is a spiral galaxy approximately 2.5 million light-years away." - Wikipedia
We only have to consider the Milky Way. Any discussion of what exists in other galaxies is entirely academic. The nearest one, Andromeda, is 2.5M ly away. Even if a spaceship achieves 10% the speed of light, it would take 25 million years to get there and it would be a one-way trip. Humans certainly will not be around 25 million years from now and, even if they are, how will they know that their ancestor's spaceship had arrived?
Of course, there are many people who assume - and find it so obvious that it hardly needs proof - that every race of little green men is in possession of an amazing, super-duper, zip-around-the-universe technology that requires nothing more complicated than hitting the "warp drive" button on their ship's control panel. But that is pure speculation. The reality is that it takes an infinite amount of energy to reach light speed and a darned lot to reach 10% of light speed. The U.S. Government would have to sell 25% of itself to finance the construction of even
one 1000-pound ship that could go that fast. It's not like you're Hans Solo and you're just going to pick up a used ship on e-bay, hit the warp-drive button and - Presto! - you are on the other side of our 100K ly wide galaxy.
Within this galaxy, the very low-mass stars won't work, so we only have to consider the 200B stars that we can see. I don't have statistics - I'll try to find some - but, of those 200B stars, there are probably no more than a few hundred thousand with a habitable zone where liquid water can exist and which have heavy elements like iron to form rocky planets.
Of those, it appears that the great majority either have gas giant planets in very close circular orbits or in wide elliptical orbits, either of which would preclude having small rocky planets in circular orbits.
Intelligence is one of many traits that makes survival more likely, just like sharper teeth or faster feet. Over time, it is likely that a planet with diverse life forms would harbor at least one species that acquires a high level of intelligence. Even on Earth, the intelligence of the dominant life forms has steadily been increasing over time (even before the existence of mammals).
Actually, no, it is not. Sharp teeth are always an advantage in a fight, but intelligence is not. Intelligence makes humans slow. Have you ever fought a dog? You may think, from past boxing experience, that you are fast, but the dog will make you look like you are stuck in mollasses.
Also, a big brain is
very expensive. Compared to a similar-sized carnivour, like a wolf, we have to eat a
lot of meat just to keep our brains functioning. A malnourished person's biggest problem is not the weakness in his limbs but the loss of his intelligence - without enough food, it is
very hard to concentrate.
You are wrong that "the intelligence of the dominant life forms has steadily been increasing over time." The dinosaurs in the Cretaceous were no smarter than those in the Triassic after 200M years of evolution. If "intelligence is one of many traits that makes survival more likely," then why aren't there other intelligent animals today? Dog lovers like to think that their sheep dog is "as smart as a person," but that is just not true. Our species produced physicists like Einstein and chess masters like Kasparov. To compare a sheep dog's intelligence to a human's is nonsense; it would make more sense to compare his strength to a tiger's. Anyway, sheep dogs were
bred to be smart; that did not occur naturally.
The fact is, nobody knows why humans became intelligent. The existence of intelligence flies in the face of everything we know about evolution.
The Phanerozoic eon lasted for 500,000,000 years and, until 100,000 years ago, there was no intelligence. Then there was exactly
one intelligent species. If the Phanerozoic eon where a day, then humans accomplished in 0.72 seconds what the dinosaurs did not accomplish in 23 hours and 59 minutes.
Why did this one intelligent species arise? Nobody knows. There is no reason to think that it has happened elsewhere.
doh:lol
