Sunday, April 08, 2012

Weekend Post. We're not alone

We’re not alone

No, I don’t mean that aliens have been found, either here on Earth or anywhere else. Despite what many of the lunatics on the internet will tell you, there’s no evidence that aliens are here on Earth. Much as I like science fiction movies about alien invasions and extra-terrestrial conspiracies I recognize that they’re just fiction. Honest, they are. Also, there’s precisely no credible evidence that they’ve EVER been here. Despite what some will say, the stories about us being created by visitors from other parts of the universe are just hogwash.

However, our planet is not alone. As well as our neighbor planets in our solar system, the other planets we see in the sky, from Mercury out as far as Neptune, it seems that other stars also have planets, perhaps even some like ours.

For several years astronomers have been able to detect signs of planets orbiting distant stars. These stars, although they are close by galactic standards, are nevertheless staggeringly far away. I suspect that it’s impossible to truly comprehend how far away they are but let’s try.

A beam of light travels at an astonishing 300,000 km/s. It could travel round the Earth seven times in a second. The light you see reflected by the moon took just over a second to reach your eyes. The light from the Sun took 8 minutes to reach you. At the moment the light from Venus takes just over 5 minutes. Light from Jupiter, the biggest planet in our system, and the one you can currently see to the left of Venus just after dark, took about 45 minutes to get here. [See here for fantastically nerdy live distances to our neighbours in our solar system.]

So far everything is measured in seconds and minutes. When you consider the distances to other stars the numbers become even more extraordinary. The light from the nearest star to the Sun took more than 4 years to get here.

These other planets that have been discovered are even further away. The nearest that’s been discovered is more than 20 light-years away, meaning that the light from it takes 20 years to get here. It’s more than a million times further away from us than the Sun.

Despite them being so far away, astronomers do have firm evidence that they’re there. A small proportion has actually been seen directly but the majority have only given us indirect evidence of their existence. Some can be identified because as they rotate around their distant star they regularly pass in front of it and the amount of light we see dips slightly as a result. Others can be inferred because as they rotate around their star the star wobbles very slightly. Our most sensitive instruments can detect that miniscule wobble.

So far, using a variety of techniques, astronomers have detected over 750 of these “extrasolar planets” orbiting over 700 stars. Many of them are so massive that they’re probably gas giants like Jupiter and Saturn and are probably uninhabitable by any form of life we’d be familiar with. Others appear to be so close to their star that they’d too hot to support life. Others are probably too cold.

Although it might sound egocentric (or perhaps geocentric?), most scientists thinking about these things expect that life can only exist in conditions a bit like those on Earth, most importantly on planets where liquid water can exist. They’re looking for extrasolar planets that are just the right distance from their star, in the so-called "habitable zone". So far they’ve found just a handful.

However a recent survey of the galaxy suggests that we should expect to find more. A lot more.

The majority of stars in our galaxy are “red dwarves”, stars are generally smaller than our sun and that burn more slowly. A survey of just over 100 of these stars showed that nine of them had rocky planets like ours circulating them. Given that our galaxy contains something like 160 billion red dwarf stars and that our methods of detecting planets remain primitive, the number of planets is probably going to be measured in billions. Some have estimated that it might be as much as 40% of them. They reckon that perhaps a hundred are within 30 light years of us.

Even if a small proportion are in the realm where liquid water is possible that still leaves a very large number where life as we know it is possible. Given that life seems so abundant on Earth it would be surprising if it didn’t emerge elsewhere. We might never meet them but that doesn’t mean there aren’t aliens reading a newspaper on a distant planet right now, just like you.

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