Aliens - Are they out there? Have they visited us? And related questions

[Welsh cake]

It appears Jupiter like planets are very common in planetary systems. Very large Jovian planets are to be found in majority of the planetary system we know of. So I think the odds of a planetary system having large planets to sweep up debris from primodia disk and deflect later comets is high.

That much is apparent from looking at systems such as 55 Cancri, but that's not the point I was making. The point was, where there are also terrestrial planets in addition to gas giant whose mass acts like a gravity well that offers some means of protection from the onslaught of celestial bodies with rogue orbits, now currently only four planetary systems including Cancri and our Solar System are known to consist of at least five planets.

Also, I think you unnecessarily narrow the range of planets that might harbor life. There is no reason to suppose, for example, that large Jovian planets with long lasting internal heat sources can not support life in it's own atmosphere without being in the goldilock zone.

That's because we don't know if such life-forms can actually exist, asserting anything else at this point is pure speculation. I've heard all sorts of pseudo-science-fiction hypotheses that life may thrive inside stars! I can't really dismiss it as 'utter nonsense', as tempting as that is, since its plausible the cosmos may harbour extraterrestrial life in ways and forms which we've not yet observed or anticipated, but until we do discover them, we've no reason to believe otherwise. What we do know to a fairly high degree of certainty is that life can come from non-life on terrestrial planets such as Earth. The whole goal of proposed projects such as the Terrestrial Planet Finder is to try and find extrasolar planets like Earth that are more likely to harbour alien life-forms, than say a black hole, or a hypergiant star without a planetary system even, or a runaway planet and so on and so on.

As to the chances of intelligent life evolving and not destroying itself. At this point we have a sample of only 1.

I think you'll agree we've very nearly destroyed ourselves quite a few times (and still might do so in the near-future when overpopulation takes hold and our resources start to run out); bearing in mind internal threats such as war and conflict were being used as just an example.

We don't know how long it takes to detect earth from the average distance any civilizations are separated from each other in the universe. We don't know how long it takes for a typical civilization to get around to visiting enough planets to have a high chance of visiting earth should that civilization evolve at a specific distance from earth.

Given the current age of the universe, our existence as a species and current progress at space exploration, the answer is obvious, they would have to evolve and develop the interstellar travel technology at an exponential rate than our own.

It seems unwarranted to extrapolate from such slim collection of data a position so far from neutrality as "infinitesimal". Also, exactly how much is infinitesimal? Is it larger than 1 when multiplied by 6e18?

It was used as an expression, you'll appreciate that I did not state it was "impossible", nevertheless the odds, like you and me being born from an unbroken chain of life going back millions of years and having this discussion over a telecommunications network developed in the space of a century, are also incalculably small.