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We've all seen movies

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about terrible insects from outer space

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or stories of abduction by little green men,

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but the study of life in the universe,

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including the possibility of extraterrestrial life,

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is also a serious, scientific pursuit.

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Astrobiology draws on diverse fields,

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such as physics,

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biology,

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astronomy,

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and geology,

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to study how life was formed on Earth,

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how it could form elsewhere,

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and how we might detect it.

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Many ancient religions described

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other worlds inhabited by known human beings,

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but these are more like mythical realms

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or parallel universes

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than other planets existing

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in the same physical world.

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It is only within the last century

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that scientists have been able

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to seriously undertake the search

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for extraterrestrial life.

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We know that at the most basic level

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organisms on Earth need three things:

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liquid water,

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a source of energy,

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and organic, carbon-based material.

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We also know that the Earth

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is just the right distance from the Sun,

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so as not to be either frozen or molten.

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So, planets within such a habitable range

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from their own stars

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may be able to support life.

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But while we used to think

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that life could only exist

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in such Earth-like environments,

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one of the most amazing discoveries of astrobiology

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has been just how versatile life is.

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We now know that life can thrive

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in some of the most extreme environments

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that'd be fatal for most known organisms.

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Life is found everywhere,

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from black smoke of hydrothermal vents

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in the dark depths of Earth's oceans,

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to bubbling, hot, acidic springs

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on the flanks of volcanoes,

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to high up in the atmosphere.

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Organisms that live in these challenging environments

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are called extremophiles,

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and they can survive at extremes

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of temperature,

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pressure,

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and radiation,

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as well as salinity,

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acidity,

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and limited availability of sunlight,

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water,

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or oxygen.

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What is most remarkable about these extremophiles

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is that they are found thriving in environments

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that mimic those on alien worlds.

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One of the most important of these worlds

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is our red and dusty neighbor, Mars.

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Today, astrobiologists are exploring places

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where life might once have existed on Mars

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using NASA's Curiosity rover.

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One of these is Gale Crater,

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an impact crater created

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when a meteor hit the surface of Mars

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nearly 3.8 billions years ago.

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Evidence from orbit suggest past traces of water,

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which means the crater

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might once have supported life.

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Planets are not the only places

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astrobiologists are looking at.

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For example, Europa, one of the moons of Jupiter,

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and Enceladus and Titan,

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two of Saturn's moons,

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are all exciting possibilities.

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Although these moons are extremely cold

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and two are covered in thick ice,

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there is evidence of liquid oceans beneath the shell.

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Could life be floating around in these oceans,

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or could it be living around black smoker

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vents at the bottom?

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Titan is particularly promising

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as it has an atmosphere

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and Earth-like lakes, seas, and rivers

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flowing across the surface.

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It is very cold, however,

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too cold for liquid water,

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so these rivers may instead be flowing

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with liquid hydrocarbons

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such as methane and ethane.

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These are composed of hydrogen,

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and, more importantly, carbon,

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which is the basic building block

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of all life as we know it.

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So, could life be found in these lakes?

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Although instruments are being designed

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to study these distant worlds,

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it takes many years to build them

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and even longer to get them

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where they need to be.

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In the meantime, astrobiologists work

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in our own natural laboratory, the Earth,

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to learn about all the weird

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and wonderful forms of life that can exist

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and to help us one day answer

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one of humanity's oldest questions:

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Are we alone?