Imagine a habitable planet orbiting a distant star. You’re probably picturing a variation of Earth. Maybe it’s a little cloudier, or covered in oceans. Maybe the mountains are a little higher. Maybe the trees are red instead of green. Maybe there are scantily clad natives…OK, let’s stop there.
That image may very well be completely off-base. There is good reason to think that the first potentially life-bearing worlds that are now being detected around other stars (see here for example) probably look very different than Earth. Rather, these planets are more likely to look like giant eyeballs whose gaze is forever fixed on their host stars (which is not something I recommend doing with your own eyeballs).
Let’s take a step back. The easiest planets to find are those that orbit close to their stars. The sweet spot for finding a habitable planet—with the same temperature as Earth—is on a much smaller orbit than Earth’s around a star much fainter than the Sun. But there are consequences of having a smaller orbit. A planet close to its star feels strong tides from its star, like the tides Earth feels from the Moon, but much stronger. Strong tides change how a planet spins. Tides drive the planet’s obliquity to zero, meaning that the planet’s equator is perfectly aligned with its orbit. The planet will also be “tidally locked”: It always shows the same side to the star. It looks something like this:
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