LET鈥橲 get one thing out of the way: aliens are almost definitely out there. On average, every star in the Milky Way has a planet orbiting it. Fully one-fifth of those stars have a planet that could be temperate and conducive to life as we imagine it. That鈥檚 50 billion potentially habitable planets just in our own galaxy 鈥� which is one of billions in the universe.
鈥淚f you鈥檙e going to say that there鈥檚 no chance we鈥檙e going to find any life elsewhere, you must think there鈥檚 something really miraculous about Earth,鈥� says at the SETI Institute in Mountain View, California. 鈥淎nd that鈥檚 a suspicious point of view, that we鈥檙e just miraculously better than all the other planets.鈥�
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That doesn鈥檛 mean intelligent life is close by. We have been exploring our solar system for a long time, so if it contained intelligent life forms we would probably know about it by now. With simple, microbial life, it is a different story. The best places to look are the icy outer solar system moons Europa, Enceladus and Titan because we know they have liquids that could support life, says , director of the Carl Sagan Institute at Cornell University in New York.
鈥淥ur most basic assumptions about life are from a sample of one planet鈥�
For anything bigger, we must peer further afield 鈥� and, as yet, our technology for spying life at a distance is rudimentary. Our best bet is to study the atmospheres of alien planets for signatures of gases like oxygen and methane that only coexist if some thermodynamically implausible process 鈥� call it life 鈥� is constantly replenishing them. We can鈥檛 do that quite yet, but with the imminent launch of the James Webb Space Telescope and construction of the Extremely Large Telescope in Chile, we should soon be able to.
I don鈥檛 want to be alone
Remotely sensing just one other world with alien life would tell us that we aren鈥檛 alone in the universe, and that life is probably widespread. But it won鈥檛 tell us what that life is like. We naturally tend to think of any advanced life as human-like, but we don鈥檛 even know what future humans will be like. 鈥淚f someone had written this article 100 million years ago and asked what aliens would be like, they probably would have heard from a triceratops or a brontosaurus that aliens are probably small at one end, big in the middle, and small again at the other end,鈥� says Shostak.
Even assumptions such as life being carbon-based and requiring liquid water are based on a sample of one planet. Life on Titan could use liquid hydrocarbons in the way we use water. Some scientists have speculated that life could be silicon-based. Given computing鈥檚 rapid progress, advanced alien life could even consist of artificially intelligent machines, says Shostak.
It is probably just as well not to think of life as one thing. 鈥淲hen I look around the Earth, I see so many forms of life that I could never have imagined,鈥� says Kaltenegger. 鈥淚 think whatever we can imagine, the diversity of life if it exists out there is going to just blow our minds.鈥�
Cutting-edge science throws up all sorts of controversial, nebulous and mind-bending concepts. Here鈥檚 your guide to how to think about some of the fiddliest of them:
- In the quantum world, uncertainty reigns 鈥� or is it all in the mind?
- Think you understand how evolution works? You鈥檙e probably wrong
- Why information could be our route to the universe鈥檚 deepest secrets
- Who do you think you are? Why your sense of self is an illusion
- Homo sapiens? Genetic insights suggest we may not really be a species
- Big bang retold: The weird twists in the story of the universe鈥檚 birth
- Firms and governments use the internet to spy on us. Should we care?
- D鈥檕h! Why human beings aren鈥檛 as intelligent as we think
- Extinction is a fact of life. Could we stop it 鈥� or even reverse it?
- No more goody two shoes: Why true altruism can鈥檛 exist
- Why it鈥檚 time to call time on the 鈥榥ature vs nurture鈥� debate
- Dark energy: Understanding the mystery force that rules the universe
