We've all seen movies

about terrible insects from outer space

or stories of abduction by little green men,

but the study of life in the universe,

including the possibility of extraterrestrial life,

is also a serious, scientific pursuit.

Astrobiology draws on diverse fields,

such as physics,

biology,

astronomy,

and geology,

to study how life was formed on Earth,

how it could form elsewhere,

and how we might detect it.

Many ancient religions described

other worlds inhabited by known human beings,

but these are more like mythical realms

or parallel universes

than other planets existing

in the same physical world.

It is only within the last century

that scientists have been able

to seriously undertake the search

for extraterrestrial life.

We know that at the most basic level

organisms on Earth need three things:

liquid water,

a source of energy,

and organic, carbon-based material.

We also know that the Earth

is just the right distance from the Sun,

so as not to be either frozen or molten.

So, planets within such a habitable range

from their own stars

may be able to support life.

But while we used to think

that life could only exist

in such Earth-like environments,

one of the most amazing discoveries of astrobiology

has been just how versatile life is.

We now know that life can thrive

in some of the most extreme environments

that'd be fatal for most known organisms.

Life is found everywhere,

from black smoke of hydrothermal vents

in the dark depths of Earth's oceans,

to bubbling, hot, acidic springs

on the flanks of volcanoes,

to high up in the atmosphere.

Organisms that live in these challenging environments

are called extremophiles,

and they can survive at extremes

of temperature,

pressure,

and radiation,

as well as salinity,

acidity,

and limited availability of sunlight,

water,

or oxygen.

What is most remarkable about these extremophiles

is that they are found thriving in environments

that mimic those on alien worlds.

One of the most important of these worlds

is our red and dusty neighbor, Mars.

Today, astrobiologists are exploring places

where life might once have existed on Mars

using NASA's Curiosity rover.

One of these is Gale Crater,

an impact crater created

when a meteor hit the surface of Mars

nearly 3.8 billions years ago.

Evidence from orbit suggest past traces of water,

which means the crater

might once have supported life.

Planets are not the only places

astrobiologists are looking at.

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

and Enceladus and Titan,

two of Saturn's moons,

are all exciting possibilities.

Although these moons are extremely cold

and two are covered in thick ice,

there is evidence of liquid oceans beneath the shell.

Could life be floating around in these oceans,

or could it be living around black smoker

vents at the bottom?

Titan is particularly promising

as it has an atmosphere

and Earth-like lakes, seas, and rivers

flowing across the surface.

It is very cold, however,

too cold for liquid water,

so these rivers may instead be flowing

with liquid hydrocarbons

such as methane and ethane.

These are composed of hydrogen,

and, more importantly, carbon,

which is the basic building block

of all life as we know it.

So, could life be found in these lakes?

Although instruments are being designed

to study these distant worlds,

it takes many years to build them

and even longer to get them

where they need to be.

In the meantime, astrobiologists work

in our own natural laboratory, the Earth,

to learn about all the weird

and wonderful forms of life that can exist

and to help us one day answer

one of humanity's oldest questions:

Are we alone?