Earth Is Probably Not Our Original Home

From the formation of the Solar System until today, Earth's story from stardust to vibrant world is both extremely long and very exciting. And one of the most fascinating things that ever happened on its surface was the appearance of life. The Solar System is nearly 5 billion years old. It began when the Sun formed from gas and dust that rotated toward a central point, eventually turning into a celestial body which could generate energy on its own – a star. The remaining stardust became the star's 8 planets, and as far as we know, only one of them has life on it. The origins of life on Earth are still not 100% proven to this day. Two sources of doubt are the tar paradox and the water paradox. The tar paradox says organic elements combined with energy from the Sun or geothermal heat are not enough to form anything except asphalt. Something else has to kick-start the creation of the first organic molecules. We're all taught that life comes from water, but there are many other worlds where there's even more water than on Earth, yet no life. Besides this, water is actually dangerous for life in its early stages. This is the water paradox. If we could rewind a tape showing the evolution of life right to its beginning, we would see the four major molecules essential for its creation – nucleic acids, that's RNA and DNA, lipids, proteins, and carbohydrates. Scientists believe that the first life forms on the planet were formed from strings of RNA molecules. The problem, though, is that the early planet Earth was a water world, with land making up only 2-3% of its surface. Water would've dissolved the first RNA bonds that tried to form under the surface. The planet also lacked oxygen, which is very important for creating and sustaining new bacteria. 4 billion years ago, another planet had just enough oxygen and water to make the necessary chemical reactions possible – Mars. At that time, there was more oxygen on Mars than on Earth. And our celestial neighbor certainly had some water as well, although it didn't cover the whole surface. These conditions were perfect for long strands of RNA to form in the Martian deserts. But the red planet could not sustain early life forms. Scientists believe some organic molecules from Mars made it to Earth through space after a meteor hit Mars' surface. The quantity of water on Earth then became a great advantage for developing the first forms of life from the newly arrived Martian molecules. We're really lucky to live on Earth. It's frightening to imagine what it would be like to make even a quick visit to some of the other planets. Take Venus, for example. Although it's relatively close to our home planet, the conditions on its surface are unfriendly to say the least. It's actually not Mercury, the first in line, but Venus, the second, that's the hottest planet in the Solar System. The temperature on its surface can reach 860°F – that's hot enough to melt lead. The planet's atmospheric pressure, meanwhile, is 92 times that of Earth's. So what about Mercury then? Hugging the Sun so close, yet with no atmosphere to retain heat, it experiences huge temperature differences between its day and night sides. The side facing the Sun can heat up to 800°F, while the night side gets as cold as minus 290°F. Life on Earth would involve 58 days of scorching heat, followed by 58 days of freezing cold. The gas giant Jupiter was discovered centuries ago. Huge storms cover its entire surface. One of them, that iconic Great Red Spot, has been raging for over 340 years, and is much larger than the whole Earth. Lightning on the gaseous planet is a thousand times more powerful than what we see here. Just about every characteristic of a planet is determined by how far it is from its star. So if we want to find places with similar temperatures and humidity as Earth, we need to look for ones that are about as far from their parent stars as the Earth is from the Sun. The space where a planet is at an optimal distance from its star is called a habitable zone. Planets orbiting there could possibly support liquid water. To date, scientists have discovered at least a dozen such planets in the far-off regions of our galaxy. Let's look at one of them. Kepler-186f is the most Earth-like planet ever discovered in the habitable zone. It's about 40% larger than Earth and has a rocky surface. The star it orbits is a little different from our Sun. It's a red dwarf, which in star terms is small and cold. Because they're less active than the Sun and other stars, red dwarfs can live for longer and be even better candidates for sustaining ecosystems on surrounding planets. The Gliese 581 star system is made up of another set of planets orbiting a red dwarf. It's in the constellation of Libra and is 20 light-years away from our Solar System. One of its planets, Gliese 581 d, isn't far from the habitable zone. The planet's atmosphere creates a greenhouse effect that raises its surface temperatures to a level just warm enough to sustain liquid water. Due to its mass, Gliese 581 d belongs to a group of worlds known as super-Earths as it's about 7 times heavier than our planet. It's possible that Gliese 581 d is covered by a large deep ocean, and who knows what's hiding in those depths. There are billions upon billions of stars similar to the Sun in our vast and unexplored Universe. Most believe the cosmos began with the Big Bang and is still expanding to this day. Others think the Universe has no boundaries and that it has always existed and will continue to exist forever. Whatever is true, there's a huge number of Earth-like worlds out there, and one day we'll know for sure whether life really does exist outside our Solar System.