Life on Earth: Is our planet special?

Life on Earth: Is our planet special?

For Greek philosophers like Aristotle, Earth lay at the centre of a small universe and the idea of alien life was unthinkable.

Since then the tide of opinion has turned. Astronomers have shown that Earth may be just one of myriad habitable worlds.

Meanwhile biologists have shed light on how life might have originated here, and therefore on other planets too.

Far from being unique, many now regard Earth as an ordinary lump of space rock and believe that life "out there" is almost inevitable. But could the truth be somewhat more complex?

On Friday, top scientists are meeting at the Geological Society in London to debate this very issue, posing the question: "Is the Earth special?". What emerges is that aspects of our planet and its evolution are remarkably strange.

Prof Monica Grady is a meteorite expert at the Open University. She explained in what sense the Earth could be considered special.

"Well, there are several unusual aspects of our planet," she said. "First is our strong magnetic field. No one is exactly sure how it works, but it's something to do with the turbulent motion that occurs in the Earth's liquid outer core. Without it, we would be bombarded by harmful radiation from the Sun."

"The next thing is our big Moon," continued Prof Grady. "As the Earth rotates, it wobbles on its axis like a child's spinning top. What the Moon does is dampen down that wobble… and that helps to prevent extreme climate fluctuations" - which would be detrimental to life.

"Finally, there's plate tectonics," she added. "We live on a planet that is constantly recycling its crust. That's another way that the Earth stabilises its climate." This works because plate tectonics limits the amount of carbon dioxide escaping into the atmosphere - a natural way of controlling the greenhouse effect.

Earth that never was

If these factors were important for life flourishing on Earth, an obvious question is what went wrong for our moribund neighbours, Venus and Mars?

One popular explanation is the Goldilocks Effect. This states that Venus was simply too close to the Sun and overheated while Mars was too far away and froze. Between these extremes - like the baby bear's porridge - Earth was "just right" for life.

Indeed, just this week astronomers confirmed the discovery of an Earth-like planet in this "habitable zone" around a star not unlike our own.

One effect of Venus's lower density is that its interior melts more easily. So, whereas the Earth has a swirling core that is part solid and part liquid, the core of Venus is entirely liquid - and strangely calm.

In Dr Ghail's opinion, this has led to a spiral of doom for Venus. Without a turbulent core, no magnetic field was generated. And no magnetic field meant that Venus was mercilessly battered by solar radiation, causing it to lose all its water.

 Life on Earth: Is our planet special?