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 of being hit by a gigantic meteorite, zapped by lethal 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 events. The most famous mass extinction was 66 million years ago, when it’s widely believed a meteorite killed off the dinosaurs. A 110-mile-wide crater in Mexico with the same geological age supports this theory.

Believe it or not, this wasn’t the most brutal 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, dim twin star which is too far away to observe directly. This sleeping giant, dubbed the “Death Star”, could distort the paths of orbiting chunks of icy rock and hurl 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. 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.

While all this sounds a little macabre, take comfort from the fact that the chance of being hit by a giant interstellar projectile 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.
词汇表
little green men （比喻）外星人
meteorite 陨星、陨石
cosmic ray 宇宙射线
good old （赞美语气）老牌的、靠谱的
a rough ride 一段艰难的历程
mass extinction 物种大灭绝
to kill off 杀死、消灭
crater （火山口式的）圆坑
geological 地质的
to wipe out 彻底灭绝
dense 密度大的
to orbit 沿轨道围绕（行星或恒星）运行
Solar System 太阳系
comet 彗星
to hurtle 猛冲
to put someone off (something) 丧失对某事的兴致
macabre 恐怖的，令人害怕的
interstellar 星际的
to snuff it （俗语）死亡，断气