Now that water has been found on Mars, the question of life outside of our own planet is likely to be a major discussion topic for a while. Since liquid water is a necessary (but insufficient) requirement for life, journalists, bloggers, and everyone else will be even more certain that life, including intelligent aliens, must exist throughout the galaxy.

When people claim that there must be millions or billions of advanced civilizations in the universe, I often hear a subtext:  “After all, why should anyone think humans are unique or special?” Sometimes there is even a sub-subtext that goes “Finding other life in the universe will put a nail in the coffin of theism”.

This is an odd idea. I admit that many creationists and fundamentalists think that only the Earth holds life, and that humanity is specially graced as the image bearers of God, ruling out aliens. But that is the view of a small number of Christians, who also reject evolution, the Big Bang, and most of science. We can forget about them. Most Christians, including some very prominent scientists in the search of ET life (I think of Jennifer Wiseman and Stephen Freeland, whom I have met) have no religious qualms about the existence of life on other worlds. I don’t either.

My skepticism is based on science, and I am a bit baffled why other scientists have not publicly mentioned why the science doesn’t support the existence of numerous alien civilizations, ready to be contacted and…well…whatever comes after that.

Here are some of the facts about what is needed for a planet to sustain life: (We know nothing about what is needed for the genesis of life, even on Earth.)

1. A rocky crust (as opposed to a gas planet) with liquid water available year round, or at least on a regular and frequent basis.
2. A circular or almost circular orbit. Elliptical orbits cause extreme temperature fluctuations that won’t allow for life to persist.
3. A single sun that allows for a regular orbit. At least half of all star systems are dual or multiple suns, and planets rotating around them will have unpredictable and irregular orbits that would not allow for life to exist.
4. A roughly Earth size mass. Gravity is proportional to mass. Planets that are too small would not have enough gravity to hold their atmospheres. Planets much larger than Earth have other issues (see below)
5. A molten core to allow for a magnetic field that can shield the planet from solar flares.
6. A very large nearby planet with a gravitational field strong enough to attract most meteors and asteroids, thus protecting the planet from bombardment that would create adverse conditions for life. Jupiter and Saturn fill this role for us.
7. A large moon to stabilize the planetary rotation. Unstable rotation can lead to large shifts in temperature and to other conditions not conducive to life.
8. A plate-tectonic or other variable crustal environment that would cause slow yet continuous variations in environmental conditions to allow for a constant evolution of life forms. A very stable planet might harbor life, but life would not evolve on it. An overly unstable planet would not allow enough time for evolution to occur. (Evolution must occur at a specific rate due to constraints on mutational rates).

The above are the constraints on planets required to host any kind of life. Planets that meet all of the above criteria are probably not terribly rare, and given the huge number of stars and planets in our galaxy, there must be some large number (Hundreds? Thousands?) of candidates that pass all of the above tests. But that means they would be amenable to single cell life. There are another set of conditions required for large multi-celled animals with complex nervous systems.

1. A planet much larger than Earth would have too strong a gravitational field to allow for larger animals.
2. An atmosphere containing oxygen similar to that of Earth. Oxygen-based metabolism is far more efficient than any form of non-oxygen-based metabolism and is required for complex multicellular life forms; this is especially true for any organism with a brain. Too much oxygen in the atmosphere is toxic to life. The right amount of oxygen is produced as a by-product of the metabolism of some rare species, such as blue-green algae in Earth.
3. Even with planets where all this were true, the odds of self-aware, intelligent creatures evolving appear to be very small. Dinosaurs had 250 million years to evolve and it does not appear that intelligence ever showed up in any dinosaurs sufficient to allow for civilizations. Even those terrestrial species that exhibit some degree of intelligence (canines, elephants, dolphins, corvids, other primates) failed to form technological civilizations, despite millions of years of existence.

The Drake equation, used for discussions about the chances of finding intelligent life, has two parameters that need rethinking. The first is the fraction of formed stars, f_p, that have planets. The value of this parameter, which was unknown at the time the equation was proposed, now appears to be quite high. The second is the average number of planets per star, n_e that can potentially support life. That number could be quite low for life in general, and much lower (possibly near 0) for life that is capable of technological advancement.

It should be noted that I am only talking about our own galaxy here, since life in other galaxies would have no way of contacting us, or vice versa.  Even highly advanced civilizations cannot exceed the universal speed limit for light, and if we ever did get a message from them, it would be over 2 million years old. Our answer would take just as long; not a great way to have a meaningful conversation.