Anatomy of a Living World
BBC Focus Magazine, December 2010
What conditions would be required for a planet to host life? So we now know just how tough life can be. But what are the most fundamental requirements for a planet to host hardy bacteria? And what else might be necessary for fussier, more complex lifeforrns to exist? Understanding these requirements — the anatomy of a living world — will help us In the search for life elsewhere in the galaxy. The Earth is a living world, and has been thriving continuously for over three and a half billion years. So It provides a lot of clues about the conditions required for life to begin — and survive.
Large Moon
Having a large moon may be important forevolving complex animal life on an Earth-like planet. The gravity of the moon acts like a stabiliser to hold the planet upright, stopping the spin axis swaying too far back and forth over millions of years arid playing havoc with the global climate.
Volcanism
Plate tectonics and erupting volcanoes would play a key role in keeping a habitable planet’s climate stable and cosy for life. Together, they act to regulate the amount of atmospheric carbon dioxide (C02) and thus the greenhouse effect, keeping a linger on the planet’s thermostat.
Greenhouse Gases
Certain molecules in the atmosphere, such as C02, methane, and water vapour, act as greenhouse gases to help trap the star’s warmth. Without this planetary blanket, thewhole planet would be a frozen wasteland. And for complex life-forms like humans, oxygen-rich air is crucial.
Ozone Layer
An ozone layer high in the atmosphere is crucial for protecting land-based plants and animals from harmful ultraviolet radiation from the sun.
Water
Water is an absolute requirement of life as we know it, and oceans provide the perfect wet environment for organisms to develop in, but perhaps extremophiles could emerge in much drier environments. Water would soak deep into the rocks of the crust and help to lubricate the movements of plate tectonics.
Gas Giant
Some scientists think that having a massive planet like Jupiter in the outersolar system would help nurture life on inner Earth-like worlds. The gas giant’s powerful gravitational field would pull in stray asteroids and comets that might otherwise have hit the habitable planet. Microbes living deep in a planet’s crust would be better protected, however.
In the Habitable Zone
One of the most important factors that determines whether life could ever develop on a world is how closely it orbits its star. Wandering too close would roast the planet’s surface and boil its oceans dry. On the other hand, shying too far away would leave the planet a frozen wasteland. The warm sweet-spot in-between is called the habitable zone. Extremophiles living deep underground could possibly survive over a wider range of orbits than we once thought. It is also important, especially for complex life, for the planet’s orbit to be stable and circular.
Magnetic Field
Our habitable planet would need a magnetic field to act like a vast deflector shield, stretching far out into space. It would deflect the solar wind (a fast stream of particles from the Sun) around the planet. This wind would otherwise blow the planet’s atmosphere away. The magnetic field also protects the surface from cosmic radiation, although bacteria such as D.radiodurans could survive elevated radiation levels.
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