If you think the rare earth hypothesis was religiously motivated than you don’t know a thing about it. Peter Ward is a serious scientist, a Paleontologist at the University of Washington specializing in the earth’s extinction events. The book “Rare Earth, Why Complex Life is Uncommon in the Universe” and co-authored by astronomer Donald Brownlee received excellent reviews from The New York Times, Science, and Discover Magazine. You can go to Blogging Heads and watch a one hour discussion between Carl Zimmer and Peter Ward regarding the Rare Earth Hypothesis. It is a laundry list of ifs, no doubt about it. What we have is a sample size of one when it comes to earth like planets so you are correct to call it highly speculative. But the book isn’t one of those sloppy sales jobs where the reader is deceived by bad science, the author admits what he doesn’t know. Perhaps I poorly represented The Rare Earth Hypothesis, but at least I read the book, which I am quite certain you have not.

We don’t know.

It is a fun thought exercise, that is all. It is fun to lob an idea out there devil’s advocate style and let the good folks here shred it. It isn’t known what kick started the Cambrian Explosion, probably a number of factors. As a gambler I would put my money on intelligent life being very very rare. But then again, I’m as bad at gambling as I am at convincing folks of oddball science theories.

“…our part of the universe doesn’t have nearly enough dark matter to bring a planet to habitability” How about NONE??? No Dark Matter, and the theory of it’s existence is also WRONG!!! Oh but, that can’t be because people in our past tell us it must exist or else fundamental physics and astronomical mathematics wouldn’t be able to make any sense. Hogwash. As a child I learned that if the hypothesis had no proof to support it then the hypothesis was itself wrong and you came up with a new hypothesis, not new fangled ways to explain why the original premise was correct.

Dark Matter and Dark Energy, the science communitys greatest witch hunt. By the way, I’m not a crazy theologin raging against Science. On the contrary, I would like to see science get back to it’s origins and PROVE theories to be correct before it becomes a religion unto itself, where we are told by our science gods (Einstein, Keppler, Newton etc) how the Universe and life itself came to be, where their theories are taken as truths even when they are unproven, where current scientists preach the gospil and try to explain that something is a truth when there is no proof, and where these same people call on our FAITH to believe in these truths even though there is little evidence to support them as being real.

@ dc:

The definition of a habitable planet for complex life has to be very strict

For biologists there is no such list, and specifically much of Rare Earth hypothesis has been rejected. Essentially it is a laundry lists of what ifs, and not good science. (To poison the well to boot: IIRC it was religiously motivated!? That would explain the essentially non-testable idea.)

[I don't know if your claim on oxygen atmosphere places on it, it is a consequence of life and we now know that photosynthesizers were early @ ~ 3.5 Ga.]

What is required biologically is a habitable planet, apparently a common occurrence, enough time and a great deal of contingency. Multicellularity in itself is common, today we know of bacteria that diversify as they build spore complexes AFAIU.

But in the energy hypothesis of eukaryotes, it is the mitochondria that provides 4-6 order of magnitude more energy for the genes and gene products. Endosymbiosis is now known to happen between bacteria, but have only once resulted in energy providers (what we know of). So it may be a rare occurence at an average delay of ~ 2-3 Gy.

With given data, it may be that not every habitable planet is inhabited @ ~ 3 Gy (but almost certainly @ 4-5 Gy, using a Poisson model for prediction). Of those fewer still will have energy non-constrained multicellulars. If a simplest possible stochastic model of Poisson process is used for prediction, the relative likelihood for the later would be on the order of tens of percent.

Given the number of habitable planets, multicellularity is likely a common enough occurence. It is, again, the contingency of intelligence that is the question. Noting other intelligent animals and so permitted to go through the same machinery of modeling stochastic contingent attempts, and assuming worst case of independent contingency, we would land in the order of percents or less of 5 Gy old or older suns having planets in the ETI set.

If intelligent was out there and they found another planet they could inhabit it makes sense they would have.

That doesn’t follow. If for example “they” expand naturally to inhabit the Oort cloud, they could colonize the Milky Way in some Gy but would have no sensible reason to go down the gravity well to visit planets, if they even had the technology to find them. Redeveloping landing and launching technology would be difficult, the return small and the venture risky.

The Fermi Paradox is the apparent contradiction between the high estimates of the probability of extraterrestrial civilizations and the lack of evidence or contact with them. The earth has had primitive life for at least 3.5 billion years and an atmosphere rich enough in oxygen for 550 million years. Sorry UFO fans, but we are very alone. If intelligent was out there and they found another planet they could inhabit it makes sense they would have.

Simple life will probably be very common and it wouldn’t surprise me at all if it is found in our own solar system maybe below ground on Mars or in the liquid water expected below the frozen surface on some moons in our own solar system.

http://blogs.discovermagazine.com/badastronomy/2010/10/18/gallery-of-exoplanets-real-pictures-of-alien-worlds/

White dwarfs are the final stadium of main sequence stars, aren’t they?
This means they have been red giants before and some very violent
things happened between the red giant and white dwarf situaton.
Any chance for a planet to survive in the habitable zone of the white dwarf?