Mars doesn't have internal dynamo because it it tiny, it cooled off already. Earth sized rocky planets with similar composition take billions of years to cool, and so will have magnetic field.
Why does Venus then, which is about the size of Earth and fairly similar in composition so lack a magnetic field?
I currently subscribe to a variant of this climate change theory. (Natural, not anthropogenic.)
My variant is that all, or almost all the civilizations the aliens know about formed around red dwarf stars. It's nice and stable there for very long periods of time. We're only stable here by luck - and our big moon helps some.
It shows how little you know about red dwarfs. There are some big problems with life on a red dwarf. 1. The damm things are rather cold as stars go. So to get the kind of heat that's needed for liquid water, you've got to be pretty close to the parent star... and that has two major consequences. The first is tidal locking which means the same face is facing the start constantly. The more serious problem is proximity.... At that distance the solar wind is so dense it would overwhelm what would be a nearly non existent magnetic field. (because of the slow rotation from part 1). The planet's atmosphere would literally be blown away by the highly ionized solar wind.
We are kinda in the middle of the sticks in our galaxy. We are a good bit out in one of the arms.
Actually as life goes in the Galaxy, we're in prime real estate. We're far enough in for the metal density to be reasonably high enough to form nice rocky worlds. We are however not so close to be irradiated by the higher density of active stars in the Galactic Core. Gamma Ray Bursters and other nasty effects are far more common there. No to mention the occasional jet that would be emitted by the central black hole. I expect the Core to be pretty much a sterile place.
There are 88 objects (known) in our solar system larger than 200 miles in diameter. We know one has life, we believe 3 others have a promising chance to have life (Enceladus, Titan. Europa), as well as the possibility of subterranean life on Mars (methane venting).
There's a world of difference between the promise of microbial life, (which seems the best that they're hoping for) and the star-spanning kind of life this thread is interested in. For a long time the dominant species on this planet, were trilobites whose nearest present descendant are sand crabs.
...or how about that space faring races would tend to travel towards the center of the galaxy, instead of way out here in a spiral arm?
Because they enjoy higher levels of X Rays and radioactivity that much? The Center is a hostile place for living things.
- Finally, IIRC there's the matter of the Oort cloud to consider - I believe I heard somewhere that it is expected to cause attenuation and scattering of low-intensity radio signals. I suppose we'll know more about that if Voyager is still transmitting when it reaches it in a few (hundred?) centuries.
Voyager's plutonium battery is good for at most, another couple of decades.
Or maybe the universe is so competitive that anyone who announces their presence eats the bad end of a relativistic weapon...
But why? That's the question you need to answer. Why would any civilization advanced enough for true interstellar travel even be slightly interested in smashing the Earth with a relativistic weapon, or any other kind of weapon?
Because the only motives we can ascribe to alien species are our own.
R is the one I understand the least so you could be right. In my view star formation on its own is not enough as it doesn't take the star lifecycle into account. In all there are 10 types of star, of which it might be reasonable to assume F/G/K can support planets with life, in which case it would be (R * 0.227 or R=1.589). (22.7% of main-sequence stars are F/G/K type)
F stars are comparatively short lived on the order of 100 million years or so... K stars are so dim that the theoretical "life belt" is so close that the solar winds would overwhelm the magnetosphere and blow away the planet's atmosphere.
You are ignoring the fact that it seems like one highly intelligent and technology-developing species could probably not evolve in coexistence with another one on the same planet, at some point one would win and kill off the other one.
I'm sure it's been proposed/discussed many times before, but I don't know if this concept has an "official" name or not.
it does... It's called natural selection. A niche can only be occupied by one species at a time. If there is competition, the species that outbreeds the other will drive the loser to extinction.
Also, while you're taking the past into account you're not taking the future. In 4.6 billion years whether you want to say 1 to 12 species evolved depending on how you want to frame it. The Earth has an estimated 5 billion years remaining... so lets say in the next 100 years, even a million years, there's an extinction event and primates all die. That's 4.9 billion years for another intelligent species to develop.
Actually the window for Earth closes a lot earlier than that. In about one billion years, the Sun's steadily increasing luminosity will grow to the point that no amount of Gaian adjustment can compensate for, and Earth will join Venus in the Runaway Greenhouse club.
Lastly, and more of a concern than the two previous is that a majority of police training today is geared toward attacking the public. There have been ample leaks from DHS training materials showing this to be true. Military and Law Enforcement agencies are using material claiming that "Patriots" and "Tea Party" type groups are potential terrorists.
This is not an unfounded concern. America has had periods where every now and then it became fashionable for whackos to gather in para-military groups put together frequently in reaction to progressive strides the country had made. In the post Civil War period it was the Klu Klux Klan drawn originally from Ex-Confederate troops. In more modern times there were Fascist and Nazi-Sympathizer BUNDS that would form for pretty much the same motivation only with anti-Semitism and anti-Catholicism spiced with a good deal of anti-immigrant hatred. When you put this together that the largest recent surge in gun ownership was not driven by a reasonable fear of crime, but the unreasoned fear by the election of a Black President, lots of things tend to add up. These studies aren't targeting the Tea Party, they are a recognition that the Tea Party DOES draw in a lot of the extreme whacko type among it's members. Gun ownership and crime are harder things to track, but what we are seeing in a new wave of shootings is a rise of impulse shootings, which have no real clear end to them... not even the survival of the shooters. So when it comes to trying to correlate trends in gun ownership, the real question to be asked is who's now buying guns in greater quantities than before. If the rise is that of the impulse, especially fear or angst-driven buyer than the decrease in crime is DESPITE the increase in gun ownership, not because of it.
"The one charm of marriage is that it makes a life of deception a neccessity." - Oscar Wilde
