Extraterrestrial life on Europa or Enceladus could be 'indigenous,' study says

One of our other planets could be almost identical to earth, yet have zero life on it, because science has no theory on how that could come about.

Exactly. We know what conditions are necessary for existing Earth life to survive. We also know that hydrocarbons and complex organic molecules exist elsewhere in our solar system and in other solar systems, and how those are created in the absence of life.

All you need to do to create hydrocarbons and complex organic compounds is put hydrogen, carbon dioxide and some other basic elements under extreme pressure in a contained system with an energy source, stir it for a few hundred million or billion years, and eventually you get hydrocarbons and complex organic molecules. The lower atmosphere of a gas giant (such as Jupiter and Saturn), inside a newly formed and still molten but slowly cooling rocky planet (live Venus or Mars a couple of billion years ago), or deep in the crust or in the upper mantle of a rocky planet that still has a molten core (like Earth) are all perfect environments for creating hydrocarbons and complex organic molecules from scratch over millions of years.

We have no idea, however, how you make the leap from the most complex organic molecule to even the simplest living organism. If you reject the possibility that life was created by a higher power, then the only scientific explanation is to speculate that if you keep stirring complex organic molecules (under pressure and with an energy source), eventually some of them will combine into a simple self-replicating microbe, then somehow evolve into a single celled organism, then somehow evolve into a multi-celled organism.

Unless and until we actually find life somewhere else we have no idea if there is life anywhere else and can only speculate based on the vastness of space.

So, with very basic laws of probability, there are huge number of planets out there which have the right temperatures, water & air to sustain evolving life.

We can at least speculate that there is a huge number of planets in the universe with conditions necessary to sustain life based on available data, including the fact that we have found several exoplanets in neighboring solar systems that are similar in size and mass to Earth and located close enough to their respective stars for liquid water to exist.

So far, the most similar planet to Earth that we have ever found has almost exactly the same size and mass as Earth and orbits a yellow dwarf star at relatively the same distance as Earth orbits the sun. It is called Venus. If we observed Venus from several light years away we would be certain that we had found Earth's identical twin. There are a lot of factors, however, that we cannot yet detect in exoplanets that make the difference between a Venus-like planet and an Earth-like planet, including the existence of a molten iron core generating an electromagnetic field and a large moon generating tides and weather.

Also, many of the exoplanets that we have found orbit red dwarf stars and must orbit much closer to their suns in order to sustain liquid water. Those planets are probably bombarded with so much solar radiation due to their proximity to their stars that life cannot survive. There are countless other factors that also have to be considered such as the relative distance of a solar system from supernovas in other systems, which would have bombarded the planet with sufficient gamma and other radiation to sterilize any budding or existing life, and the relative distance that a solar system is from the center of the galaxy with much higher levels of cosmic radiation. We could owe the existence of life on Earth to the fact that our solar system is located "far out in the uncharted backwaters of the unfashionable end of the western spiral arm of the Galaxy."*

There is zero chance intelligent life has not evolved somewhere in zillions of planets.

We do not have enough data to even speculate whether there is life, let alone intelligent life, anywhere else in the universe. To begin to do that, we have to first find life somewhere else in the universe. If we are able to find independently evolved life anywhere else in our solar system, then some form of life is probably common in countless solar systems in our galaxy. That is why it is not pointless to keep looking. So far, we have not excluded the possibility that simple microbes once lived or still live on Mars, let alone on any of the moons of Jupiter or Saturn, which appear to have liquid oceans buried underneath miles of ice. If the rest of our solar system is completely sterile, then life could still exist in other solar systems, but it requires a lot more factors (other than just the presence of complex hydrocarbons and an atmosphere capable of supporting liquid water), that are currently unknown.

It is possible that the initial leap from complex organic molecules to the most simple self replicating microbe took so many countless hundreds of trillions of environmental conditions, freak occurrences, and other factors, all occurring on the same planet in the correct order, that so far our planet is the only one in the universe to ever develop any form of life. It is like the old trope about how if you have an infinite number of monkeys hitting keys on an infinite number of typewriters eventually one of them will reproduce all of the works of Shakespeare. Perhaps you must have an infinite number of planets with an infinite range of conditions and coincidental occurrences in order for one of them to eventually develop life.

Even if simple life is common in the rest of the universe, it would still require countless unknown conditions and occurrences for that life to eventually evolve into some form of intelligent life. On Earth, there were at least five mass extinctions, in which 99% of species that existed on Earth abruptly died off, along the way to the development of intelligent life. Without the most recent of those mass extinctions over 65 million years ago (most likely caused by an asteroid strike) dinosaurs would still be the dominant life form on Earth. From an evolutionary standpoint, intelligence provides no survivorship advantage over simply being the biggest, baddest carnivore capable of eating anything else around.

* Douglas Adams, The Hitchhiker's Guide to the Galaxy

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