The search for planets outside our solar system has been much in the news for the past couple of years, with most of the attention, and questions, focused on "habitable" planets -- do they exist, and if so are at least some in fact harboring life?

And the biggest question of all: What might that life look like?

While scientists might argue over what would make a distant exoplanet habitable -- and they do -- one inviolate requirement on which most have agreed is the necessity of liquid water.

That figures strongly in what most people would envision as a habitable planet, based on the only model we have -- Earth.

That brings up for most people a vision of a habitable planet with streams and rivers and oceans, and water vapor forming clouds in an atmosphere at least somewhat Earth-like.

Which understandably leads to some preconceived notions of what alien life might look like, again based on our only available model, so that we might expect insect-like life, or fish-like life, or mammal-like life.

Or at least something Earth-like.

And that may be expecting too much.

If our own planet has shown us anything, it's that life can exist under the most extreme conditions, and those conditions can create life forms we might consider "alien" even here on Earth.

First of all, "liquid" water covers a wide range, from near-boiling -- or even above -- to near-freezing, with both extremes seemingly less than hospitable to life.

Yet in the deepest parts of the Earth's ocean, there are what are known as hydrothermal vents, spewing out super-heated water and other chemicals at temperatures of more than 850 degrees Fahrenheit.

Yet despite the extreme conditions, these vents and the areas surrounding them are habitable, supporting diverse lifeforms such as "alien-looking" giant tube worms and other creatures in a food chain based on the chemicals dissolved in the vent fluids.

At the other end of the spectrum, there are cold-adapted single-cell microorganisms that can live happily in waters where the temperature hovers around freezing.

Such lifeforms existing at both extremes are collectively known as extremophiles, and as the name suggests can come in configurations that can seem alien in a world populated largely by more familiar shapes -- the fish and birds and mammals with which we are familiar and comfortable.

The lesson to take away from this is that if and when we get to the point of being able to examine distant planets closely enough to see if life exists, we should put away any preconceived notions of what form that life might take.

That would go for the most primitive up to the most complex -- and yes, even to intelligent life, if it is ever discovered.

Any expectation of bi-pedal, "humanoid" inhabitants of an alien planet -- an expectation nurtured by decades of science fiction books, movies and television shows -- must be put aside.

Does that mean that such creatures, looking at least a bit like us, don't or can't exist?

Of course not. But it is more likely that life off Earth, if and when we find it, will come in shapes and forms that will surprise us in ways we can't begin to anticipate.

It suggests our most effective research tool is an open mind.