来自外星的危险

地球已经存在了 46 亿年，在此期间它经受了一些灾难性的事件 ，比如陨石冲击和气候的巨大改变。那么未来又会是怎样呢？系好安全带，让我们一起探讨那些来自外星际对地球可能会造成的危险 。, ,What is the greatest threat to life on our planet? Is it climate change? Shortages of food or water? Or might an altogether bigger danger come from somewhere further away: space?, ,We're not talking about an invasion by little green men here. Instead, how about the prospect ¹ of being hit by a gigantic meteorite ², zapped by lethal ³ cosmic rays or fried by the deadly energy of an erupting star?, ,It wouldn't be the first time. Good old planet Earth has had a rough ride over the last 3.7 billion years, with some spectacularly devastating ⁴ events. The most famous mass extinction ⁵ was 66 million years ago, when it's widely believed a meteorite killed off the dinosaurs ⁶. A 110-mile-wide crater ⁷ in Mexico with the same geological age supports this theory., ,Believe it or not, this wasn't the most brutal ⁸ episode in our planet's history. That was when a staggering 96% of life was wiped out at the end of the Permian period, 252 million years ago. Scientists don't know for sure why this happened, but any potential explanations carry with them the grim possiblity that similar events could happen again., ,For example, some experts believe that our Sun has a very dense ⁹, dim twin star which is too far away to observe directly. This sleeping giant, dubbed ¹⁰ the “Death Star ”, could distort the paths of orbiting chunks ¹¹ of icy rock and hurl ¹² them towards the rest of the Solar System., ,Is this what happened 252 million years ago? Or is there instead, perhaps, a distant, ninth planet in our Solar System which pulls in passing comets and sends them hurtling our way?, ,If that isn't enough to put you off your breakfast, then consider what's happening to our friend, the Sun. The star that has given us the warmth to sustain life is gradually turning into a deadly foe ¹³. Like all stars, it is slowing dying, burning through its energy supplies. As it does so, it expands, and in about 2 billion years it will have grown so much that the heat will make life on planet Earth unbearable ¹⁴., ,While all this sounds a little macabre ¹⁵, take comfort from the fact that the chance of being hit by a giant interstellar projectile ¹⁶ is incredibly slim, and that 2 billion years is a very long time., ,And there's another thing: if the dinosaurs had not snuffed it when they did, it's extremely unlikely that human beings would have had the chance to evolve at all.