[ INTRO ] Astronomers have confirmed thousands  of planets beyond our solar system, but they’ve yet to identify a definite Earth 2.0.

Sure, they’ve found some rocks that are  about the size and mass of the Earth, but that doesn’t mean they  have an Earth-like climate. After all, Venus has about the same mass and size, but it’s also the definition of “a bad place  to visit, and you wouldn’t want to live there.” So when we try to look at a planet  that’s light years away from us, how can we tell if it’s another  Earth, or another Venus?

It turns out, scientists can get  hints about a planet’s climate without having to send a probe across  those vast interstellar distances. And it involves figuring  out how fast a planet spins. Earth rotates, or spins, around its  axis roughly once every 24 hours.

And the speed of that rotation affects how fluids like our air and water move around the planet. It’s a phenomenon called the Coriolis effect. It helps to create weather systems  like the trade winds and Gulf Stream, which help to redistribute heat from  the equator to higher latitudes.

And it can also help create  powerful storms like cyclones. The Coriolis effect arises because  different parts of the Earth move at different speeds  while the whole planet spins. Over the course of 24 hours, stuff at Earth’s equator has to travel way faster to get back to its starting point  than anything near the poles.

We’re talking 1,600 kilometers an hour  compared to effectively standing still! If a planet is spinning faster, then  the Coriolis effect is stronger. We can see this on Jupiter, which  has a day that’s only 9.8 hours long.

The effect is so powerful it interrupts winds  traveling from the equator to Jupiter’s poles. Instead, these winds are  forced to travel horizontally, and some reach speeds greater  than 600 kilometers per hour. Venus, on the other hand, takes over 5,800 hours to  complete just one full rotation!

With almost no Coriolis  effect on the planet at all, it has much simpler atmospheric circulation. And according to some climate  simulations of Venus-like exoplanets, that might contribute to a  runaway greenhouse effect. So if we’re looking to find a  planet with an Earth-like climate, we’ll want to check how fast it’s spinning.

And that requires getting around the pesky fact that distant planets look like  single points in our telescope data. It’s kinda hard to see a planet rotating  if you can’t make out any actual features! But back in 2014, one team of  scientists found a way around that.

They targeted a young gas giant, bigger  than Jupiter, named Beta Pictoris b. And they looked specifically at  the planet’s overall spectrum, the combination of colors found in that  single point of light as the planet spins. When an object moves away from us, the wavelengths of the light  it emits get stretched out.

In terms of color, that means  that object looks redder to us. And when the object is moving  towards us, the waves are compressed, causing it to look bluer. The redder or bluer the light is, compared to what the colors  would be for a stationary object, the faster that object is moving.

And when you’re looking at a spinning  planet, one edge is moving toward you, while the other moves away. So astronomers can measure how  much the color gets shifted to figure out how long a day on that planet is. When the team looked at Beta Pictoris b, they found its day is only 8 hours long!

But Earth, you may have  noticed, is not a gas giant. It’s a lot smaller and dimmer, so astronomers need to find a method that’s better  for getting the spins of possible Earth 2.0’s. To figure out how fast a small rocky planet spins, some scientists have proposed  measuring reflected star light.

Yeah. Nothing fancy, just the light from  the star that bounces off the planet. If the planet is covered in different features, from oceans to glaciers to mountain ranges, those regions could reflect different  colors and amounts of light.

So as they rotate in and out view, the light  that our telescopes pick up would change. Astronomers could figure out  how long the day is based on how frequently that light  pattern repeats itself, because the same features would keep  coming back around, and around.~~ So in the near future, to  find an Earth-like world, we may be able to look not just for  a body that’s the same size, mass, and right distance from its star, but also one that spins at just the right  speed so its climate could be like our own. The next generation of  bigger and better telescopes, such as the European Southern  Observatory’s upcoming Extremely Large Telescope, will make it way easier for  astronomers to use these methods and track the spin of planets out there.

And maybe one day we’ll get a  sort of galactic weather report, for worlds that are still too far to visit. But for now, we visit those worlds  on the internet through videos like this one! Thank you for exploring the  galaxy with SciShow Space.

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