字幕表 動画を再生する 英語字幕をプリント The Earth sits comfortably in its orbit tilted on its axis at 23 degrees. Knock the planet over - and it wouldn't be the Earth as you know it. This is WHAT IF, and here's what would happen if the Earth's axis was tilted by 90 degrees. Only two planets in the Solar System lie on their sides - Uranus and the dwarf planet Pluto. Now, if something took the Earth off its stand, the blue planet would have no chance of developing any complex life on its surface. But let's start at the beginning. This axial tilt, or obliquity, is what drives the seasons here on Earth. Before the collision that created the Moon, the Earth's axis was slowly wobbling around somewhere between 0 and 85 degrees. Then, our newly formed Moon stabilized it. But if that collision happened at a different time, things would turn out very different here on Earth. It would be a strange new world. As the Earth made its way through orbit, its poles would be pointing straight towards the Sun. One hemisphere would be shrouded in darkness for six months straight, while the other would be getting cooked by the blazing sunlight. One day on Earth would last a whole year. At the North Pole, the daytime temperatures would rise to a broiling 50°C (120°F). A day at the South Pole would be even worse. Because the South Pole would be located away from climate-controlling ocean currents, it would heat up to an almost boiling 80°C (176°F). The poles would soak up so much heat from the Sun, that they wouldn't even freeze during the six-month-long night. You wouldn't recognize the steaming equatorial tropics. With a 90-degree axial tilt, part of the equator would stay encased in ice all year round. At some point in this Earth's existence, our continents would get clumped together around one of the poles. Inland temperatures in the daytime would get truly hellish - reaching the boiling temperature of water. Clouds could help the situation and not let all the water vapor off the planet. But you wouldn't know. You wouldn't stick around that long. In this scorching heat, the best-case scenario would see just a few kinds of bacteria survive. Those bacteria might evolve into more complex life forms, but they wouldn't be anywhere close to how complex we turned out to be. The reason would be a lack of oxygen. Green plants would be having a hard time surviving during the six months of complete darkness. They'd drop their seeds at nightfall to grow after sunrise. Interestingly enough, if the Earth was 60 million km (40 million miles) further away from the Sun, a 90-degree axial tilt wouldn't be so bad. The temperatures at the poles would never raise above 46°C (115°F) during the day. The coldest it would get at night would be 3°C (37°F). The only place covered with ice would be the highest mountains. So you see, simply orbiting a star in the habitable zone doesn't mean that a planet is actually capable of sustaining life. We're the lucky ones living here on Earth. Maybe one day we'll discover another exoplanet, just like our home. But that's a story for another WHAT IF.