But time and again, serious scientists have thought they're found evidence of extraterrestrial life. As recently as 2016, astronomers proposed that never-before-seen dimming patterns from a star could be evidence of gigantic structure built by an advance civilization to harness the star's energy. It turned to be dust. Scientists feel confident that there is biology beyond Earth. Not because we've found it, we haven't found it. The reason that we think that they're out there is simply, if not, then Earth is some sort of miracle. For most scientists who study the universe, searching for aliens isn't crazy. What's crazy is that we haven't found them.
In a universe so vast, where is everybody?
The fact that we haven't found any evidence of aliens became known as the Fermi paradox. There are about ten to the power of 22 total stars. That's about 10,000 stars for every grain of sand on Earth. A conservative scientific estimate says 5% of those stars are similar to our sun. Which means 500 billion billion suns in the universe. Many scientists are more confident than ever aliens exist because of some game changing discoveries in the last few decades. Nobody could say for sure if there were any planets outside of our solar system, until the 1990s. Now, scientists think one in five sun-like stars is a planet, similar to our own.
We've also discovered life on Earth in environments where nobody expected to find it. We see life all the way deep in the sub-surface of the planet, miles down, in like, gold mines, near volcanic calderas, on nuclear reactors. That actually gives scientists lot of hope for the search for life elsewhere.
Estimates of how many Earth-like planets will develop life vary. So, let's say even with this new scientific confidence, it's just one out of every thousand. That means every tenth grain of sand on Earth represents a planet with life on it. And if just one out of every thousand of those planets develop intelligent life, that's a quadrillion intelligent alien civilizations in the universe. 10,000 just in our galaxy. Extraterrestrial life also has time on its side. With all that time and space, the math seems pretty clear. We should've found aliens by now, or they should have found us.
There's one popular explanation for why we haven't found evidence of aliens. We have found it. Governments have just covered it up. Every so often, something comes out that gives this theory new life, like revelation in 2017 that the US government had spent millions on a secret program to investigate UFO sightings. But as almost any scientist will tell you, looking into UFOs isn't the same as searching for extraterrestrial life. Scientists have their own favorite theories about why we haven't found aliens. But it's important to remember that when Fermi calculated the odds that alien life is out there, it was just an educated guess. With so many stars and planets, he bet at least some of them will develop life, which would evolve and spread out. The trouble with this bet is there's lot we don't know about life.
The great filter theory helps us think about what we don't know. Imagine the evolution of life as a series of hurdles. First, molecules start replicating themselves, which evolves into single-cell life, then multi-cell life and then animals with large brains that can use tools, and then smarter animals that create even better tools. That's us. Ans finally, animals that can figure out how to colonise the galaxy. Given the size and age of the universe, it seems like a lot of alien species should have beat us to that last stage, unless one of those stages is much harder than we think.
We know what the major hurdles are in the evolution of life. Just not how hard they are to get past. Another example of life would help us understand life better. But so far, no aliens have contacted us. So, it's up to find them. When scientists look for intelligent aliens, they look for what are called techno signatures evidence of alien technology. We want to find extraterrestrial intelligence, by finding something that's engineered, something that's artificial, something that nature can't produce. From up close, earth techno signatures in the form of city lights. From further out, aliens might notice the satellites and space station orbiting our planet. From even further, they might pick up radio signals or stumble across the Voyager probes that are hurtling across interstellar space. Sometimes the things we think are techno signatures turn out to be natural phenomena, like that pulsar. But since radio signals are still our most promising leads, scientists do a lot of listening to the sky.
Jill Tarter would know. Jodie Foster's character was based on her. But that's just one way to search for extraterrestrial life. There are also biosignatures. If aliens came to Earth looking for life after we were long gone, they would find biosignatures in the form of fossils ans chemical evidence of the processes. If aliens were observing us from afar, they would see biosignatures in the form of water and the gases in our atmosphere. Oxygen is so reactive that it can only be in our atmosphere if it's being continuously produced. Without life, Earth's atmosphere would have no oxygen, so we're trying to look for gases that don't belong, that might be attributed to life, and we call them biosignature gases ( CH4, N, O2, CO2). Searching for biosignatures on other planets is really hard. We can't even see planets outside our solar system. Stars are so much brighter than plantets. It's like trying to see a firefly in a spotlight.
Today, we have a planet finding technique called the transit technique. When a planet goes in front of its star, the starlight drops by a tiny amount. These drops in light give scientists clues about whether a planet might have life on it. Like the distance from its star. We call the "Goldilocks zone" the distance from the star, where the planet, as heated by the star, is not to hot, not too cold, but just right for life. Researchers have been able to surmise some amazing things about planets just from these light patterns.
Scientists think they found a super Earth with really intense gravity. A planetary system with seven planets all crammed into the Goldilocks zone. Ans even a planet that could have red vegetation, from the different wavelengths of light it receives. We now know of over 3,500 planets outside of our solar system. Most of them were discovered in just the last five years. And tools are only getting better. The next generation of space telescopes will be able to see more distant galaxies. A newly launched satellite will survey the entire sky for possible planets, rather than just small sections. And astronomers are developing new technologies that would let them see distant planets directly.
For all the exciting new ways to search for life in deep space, scientists are also searching a lot closer to home. In the 1970s, we sent two landers to Mars to test the soil for evidence of life. The first and only time we've ever tried. One of the experiments came back negative. But another came back positive for evidence of a process that we only associate with living things. The contradiction could mean an unknown chemical reaction occurred that only looked like a living thing consuming energy. But since the '70s, we've learned that life in extreme environments uses energy differently and leaves different markers on its environment.
The Mars 2020 mission is our next shot. Unlike the Viking experiments, it won't test for currently living things, but it will look for signs that life once did exist in certain Martian environments. A mission is also in the works to look for biosignatures in the frozen oceans of Jupitar's moon Europa. In the search for intelligent life, scientists are also trying to expand their thinking and their search.
Advances in our own technology give us new ideas about what intelligent aliens might be like. Science fiction has shaped our space programs from the very beginning. Consider the volume of all the Earth's oceans. All right, and let's say, that's the volume of search space, where we might find a signal. Well, in 50 years, how much of that ocean have we searched?
It's a pretty disappointing one glass of water. Whether we find extraterrestrial life or learn that we are alone, it will tell us a lot about our civilization and what our future might be.
Think back to that great filter theory. It could be even life rarely gets started or that the universe is teeming with life, but none of it as smart as us. Or maybe the hardest stage is ahead of us. And some unknown challenge awaits humanity. The importance for the search for life elsewhere in the universe is kind of the search in understanding ourselves. It's important to understanding how did we as a planet come here. And how rare are we or how rare are we not? And for humans, it's an understanding of, what's the next big step for us.
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