Welcome to brain Stuff from How Stuff Works, Hey, brain Stuff, Christian Sager Here. The Milky Way is thought to hold at least a hundred billion planets. About seventeen billion of those are roughly the size of Earth. And check this out. One in every six of our galaxy stars have planets revolving around them. As far as confirmed planets go, though, as of this recording, NASA has confirmed nine hundred and
sixty two of those one hundred billion planets. We used to think that they were rare, but since we launched the Kepler Space Telescope, we're finding more planets every day, especially ones with water on them. And what we've also learned is that these planets don't need to be as perfect as we once thought to sustain life here at home. We've learned that life is a lot hardier and exists in some extremely hard environments. All of this adds up to a lot more evidence that we aren't alone in
the galaxy. While Earth is still the only planet that's proven to support life, we're actively searching for other worlds to join us. But to do that, first, we have to define what life actually means. And not everyone agrees on this topic, but let's try this one out. Life requires an organized cellular structure that functions in a relatively unchanging state. It's capable of reproduction, growing, taking in energy from its environment, and responding to stimuli, and it can
adapt to the environment it lives in. Okay, and per Darwinian evolution, we know that similar organisms produce similar organisms, so for instance, a dog reproduces another dog, not a dandelion. And while there's variation from one generation to the next, some are more favorable than there's, leading to natural selection, which leads us to how tough we've discovered life can survive to be. Until around thirty years ago, we thought all life on Earth needed energy from the sun and
couldn't survive in extreme temperatures. But we were so wrong. When we explored hydrothermal vents on the ocean floor, we discovered clams, crabs, and giant tube worms. The latter survived through bacteria in their tissue that helped them derive energy from the water, subsequently feeding the other animals. What this potentially means is that life could exist in similar extreme environments on other planets or moons. In other words, big
things have small beginnings. Now we're taking a huge leap in the likelihood of alien life, and while people like Frank Drake and Carl Sagan have speculated how many civilizations could evolve out there, we still don't know the basics of what extraterrestrials require to exist. Yes, we can work from variable traits were familiar with from natural selection, but would they be bilaterally symmetrical like humans or more like bacteria or viruses. Scientists in the field of astrobiology have
drawn a few guidelines to how aliens might work. They'd be governed by the laws of physics and chemistry. Their bodies would also require solvency, temperature, pressure, and energy. In addition, they would probably be comprised of complex molecules, including informational molecules like DNA that carry their genetic information. So, armed with these assumptions about alien biology, to next find alien life,
we've got to go planet hunting. Traditionally, we've looked for something like Earth, rocky, watery, and orbiting a star and what is called the gold Delock zone because it's not too hot and it's not too cold. It's got to be just right. But we've discovered three things about planets that make it much more likely that there's life out there. First, of all, water is not as rare as we thought. Take Mars, for example, scientists found water there inn or
Jupiter's moon. Europa it's a freezing atmosphere, less mess bombarded by radiation. But we also think it has a deep ocean under its icy exterior that could have hydrothermal vents similar to the ones on Earth, spewing chemicals and nurturing
strange marine life. If we stick with the assumption that water is essential to life, then the necessary materials are all over the galaxy, and Europa could also prove the second thing we know about the possibility of planetary life that life is way more flexible than we originally thought. We found strains of bacteria surviving under miles of ice in Antarctica for thousands of years. Could similar microbes be
on Europa or somewhere outside our solar system. If that's the case, then do we discard the notion of that Goldilocks habitable zone near the Sun. And the third thing we now know about planets is what we said at the top of the episode. There are billions of them. By measuring the light coming from each star and then waiting for it to temporarily dim, we pose it that there's a planet there eclipsing the star's view. Now, how
do we see this? Well? With the Kepler Telescope spacecraft launched in two thousand nine, Kepler orbits our Sun using a photometer light meter to monitor more than a hundred and fifty thousand stars, looking for any changes in their brightness to identify an eclipse. That's where we get the potential number of seventeen billion planets from Scientists then compare Kepler's findings with spectroscopic data from ground based observatories to
confirm planetary candidates. And soon there will be two new space instruments to bolster Kepler's findings. The Transiting Exoplanet Survey Satellite or TESTS will launch in scanning for nearby exoplanets, and the James Web Telescope, scheduled to launch in eighteen, is so powerful it could analyze these planets atmospheres for signs of life. Kepler's already discovered several planets with inhabitable
Goldilocks zones, but they're not at all like Earth. Instead of water and rocks, they seem to be small cores surrounded by atmospheres of hydrogen and helium, which doesn't sound life supporting at all. But don't forget there are still several ways life could exist on planets we used to consider inhabitable. There could be life deep under a planets subsurface, like the bacteria in Antarctica or the nematode worms discovered three point six kilometers below South Africa's deepest gold wine.
Or what if a planet had shifting habitable zones attributing to a wobbling planet. Moving clouds could reflect radiation back into space, or temperatures could rise and fall dramatically, but at certain latitudes heat would dissipate, allowing for liquid water to resist boiling. It's hard to know if photosynthetic life could survive such drastic changes, but maybe it's possible. I mean, then diesel survived on such a planet in the Chronicles
of Riddicks, so you know, I'd say anything's possible. Even moons orbiting exoplanets could be habitable if they were large enough to hold onto an atmosphere. For instance, even if a planet within a habitable zone didn't have the right composition for life, its moon might have rocky, watery settings like Earth kind of like end or in the Return of the Jetta, you know, the one with the Ewoks.
As we've seen, the previously established rules for life sustaining planets are diminishing as we gather more and more information about the universe. However, this brings up the Fermi paradox, established by Enrico Fermi in nineteen fifty. If alien life or even civilizations exist in the Milky Way Galaxy, why haven't we detected them yet? Check out the Brainstuff channel on YouTube, and for more on this and thousands of other topics, visit how stuff works dot com.