Human whereabouts revealed-how did the aliens "find" us?

Have you ever heard of crazy plans to send signals to the deep space? What if the evil aliens receive these signals and come to Earth to rob us of our water resources, enslave us, or even eat our own? What can we do to stop these lunatics from broadcasting our presence to the Milky Way? To launch a petition to the United Nations? Or did you make a solemn protest to NASA?

If this is exactly what you and your friends are worried about, I have some bad news to tell you. We have been "broadcasting" ourselves to the universe for hundreds of millions of of years. If aliens want to know where we are, they just have to look at them with their telescopes.

We are in the golden age of the discovery of extrasolar planets, and thanks to the Kepler telescope and other space telescopes, the total number of alien planets has recently crossed 1000. With so many amazing alien planets, our next challenge will be to study the atmospheres of these planets and find evidence of life. Some compounds can occur naturally, such as water and carbon dioxide, but there are some components that must be supplemented by some kind of process to be present for a long time. Methane, for example, can only exist in the atmosphere for only hundreds of years if not because of the process of cow farts and microbial decomposition of carrion.

If we see methane or oxygen in the atmosphere of an extrasolar planet, we can safely speculate that there is life there. If we see by-products of industrial civilization, such as air pollution, we can make it clear that they are at what stage of technological development. This method applies to us, and it should apply to aliens as well.

For life on Earth, oxygen is poisonous in the first billions of years of evolution. But then, at least 2.4 billion years ago, cyanobacteria evolved photosynthesis and learned to coexist peacefully with oxygen. This is known as the great oxidation event.

In the first 10 to hundreds of millions of years, almost all oxygen produced by life is absorbed by the oceans and rocks. Once these places are "filled", oxygen begins to gather in the atmosphere. 500 million years ago, there was enough oxygen in the atmosphere to keep breathing like we did today. Such a large amount of oxygen, for aliens, should be very obvious. They should know that life has evolved on earth, and that they may have sent spaceships to plunder our water and put early animals, such as dinosaurs, into their teeth.

If aliens are willing to wait any longer, we will send them more signs. The Industrial Revolution began in early 18th century. This time, it's our turn to pollute our atmosphere with all kinds of industrial processes. Similarly, aliens looking at our planet with space telescopes will know that we have grown into a technological civilisation at this very moment.

Into the 20th century, we mastered the radio communication capabilities and began sending our signals to outer space. Our radio signals, intentionally or unintentionally, have now spread in space into a bubble of more than 100 light-years. So if there are any aliens that listen to this expanding space bubble, they may all have a chance to hear our voices. They know we're here, and they know that some of us really like Ke$ha (American pop singer and songwriter).

Finally, until recent decades, a handful of astronomers have tried to send our signals directly to other stars using our most powerful radio telescope. The signals haven't gone far, but to be honest, I'm not worried at all. For hundreds of millions of of years, life itself has leaked our place to the universe. And life will also help us find other civilizations if they are really there.

So, what are we going to do now? Should we turn off all the lights, pretend we're not at home, or continue to broadcast our existence actively to the universe?

PS: A picture of @ent in Chinese.