ALLIEN WORLDS IN FARAWAY GALAXIES

The search for alien worlds continues, with surprising consequences

Reported by Colin Stuart

© Tobias Roetsch

We should count ourselves lucky. For thousands of years humans have gazed up at the night sky, wondering whether or not we are alone. We are among the first generations to know for sure that our Solar System is just one of many strung out across the vast cosmos. Our discovery of around 4,000 alien worlds dancing around distant stars – known as exoplanets – over the last three decades has quite rightly been lauded. It’s a wonderful achievement that helps put us in our astronomical place. The haul has thrown up a zoo of exciting and enchanting worlds, from those that dance around two suns to planets with diamond rain, bubbling metal lava and possibly even liquid water.

Despite the accolades, these discoveries are rarely put into their true context. It is easy to forget just how much of the universe’s real estate remains unexplored. All of the confirmed planets found to date reside in our own Milky Way galaxy, and in our tiny corner of it to boot. There are around 2 trillion other galaxies in the observable universe. To see how much we’ve poked around in so far, imagine that this hoard of galaxies is represented by the total land area of the Earth. On that scale, our Milky Way accounts for the space taken up by the average UK bungalow. The area that includes the thousands of exoplanets we’ve found so far is smaller than a sock drawer in the spare bedroom.

It’s no surprise that astronomers would love to know more about planets beyond our current horizons, and those outside the confines of our own galaxy in particular. “They are so unimaginably far away that we’re unlikely to ever be able to reach them, but knowing about extragalactic planets is still really useful,” says Lewis Dartnell, an astrobiologist at the University of Westminster. “They help build up the stats we have on the wonderful diversity of different worlds that exist.” 

“ They are so unimaginably far away that we’re unlikely to ever be able to reach them” 

LEWIS DARTNELL 

There’s every reason to suspect that they are out there. After all, a huge part of our understanding of space hinges on the fact that there’s nothing special about the part of space we occupy. This so-called Copernican principle – named after the Polish mathematician and astronomer Nicolaus Copernicus – tells us that if there are planets here, there should be planets everywhere. However, as Dartnell says, the distances involved are intimidating. Spotting an Earth-sized planet in the Milky Way is hard enough – it’s the same as trying to see an ant on the Moon. Seeing one in our nearest major galaxy, Andromeda, is a whole new ball game. It’s the equivalent of being able to see a virus on the lunar surface. To photograph one we’d need a telescope a hundred-times wider than the entire Earth. With that clearly out of the question for the foreseeable future, the big question is whether there are other ways to find them.