NARRATOR So what is it about Earth, the third planet out from the sun, that makes life possible?

The answer lies in three key ingredients. First, all life is made up of organic molecules consisting of carbon, in compounds that include nitrogen, hydrogen and oxygen, among others. Although organic molecules aren't alive themselves, they are the basic building blocks of every living organism.

Life also needs a liquid, like water. In water, the basic organic molecules can mix, interact and become more complex.

The last ingredient is an energy source, like the sun, to power the chemical reactions that drive all life, from the smallest microbe to us.

NARRATOR Earth has two of the three ingredients needed for life: water and energy from the sun. But what about organic molecules, the chemical building blocks of life; how did they, get to Earth?

Some scientists believe the answer may lie in the furthest reaches of the solar system, beyond Jupiter, Saturn, Uranus and even Neptune. Here, three billion miles from the sun, is a vast ring of comets and other debris, called the Kuiper Belt.

Like asteroids, comets are remnants from the dawn of the solar system, but, as well as rock, they are also made of ices that only freeze this far from the sun.

Astrobiologist Danny Glavin and his team think comets are the key to understanding how the final ingredients necessary for life arrived on Earth.

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