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A decade or so ago, the title of this video would have sounded ridiculous.

The first planet outside of our solar system was only confirmed in 1992. And when I was in high school and college, we’d only found a handful of exoplanets.

For all we knew, stars with planets around them were super rare. But now, it seems everywhere we look, planets litter star systems. And, indeed, it seems increasingly likely that almost 100% of stars have planets around them.

It’s taken awhile to get a good idea of how common it is for a star to have planets around it, because we just didn’t have enough data. For a long time, we mainly detected exoplanets using the radial velocity method, where the gravitational pull from a planet orbiting a star makes the star wobble slightly, which changes the wavelength of the light we detect from Earth. So far, we’ve detected more than 600 exoplanets this way.

Problem is, most of the planets we can detect using the radial velocity method are hot Jupiters: giant planets orbiting very close to their stars. Since they’re so big and orbit so close, they cause bigger, shorter wobbles that are easier for us to detect. The other issue is that the radial velocity method doesn’t really work for stars farther than about a hundred light-years from Earth.

Beyond that, it’s just too hard to see the wobbles. They don’t have enough of an effect on the wavelength of the light we detect. So the radial velocity method doesn’t tell you much about how common planets are that aren’t hot Jupiters, or about stars farther away from Earth.

Which is why the transit method has been so useful. A transit is when a planet’s orbit takes it between its parent star and Earth. As it passes in front of its star, the light we detect from the star dims a little.

The Kepler space telescope looks for exoplanets using the transit method. Since it launched in 2009, it’s found more than 2,000 confirmed exoplanets. It’s still harder to spot smaller planets with the transit method, but at least it’s easier than using the radial velocity method.

And the transit method works for planets that are much farther away, because even if it’s too far for us to see the effects of the star’s wobble, the planet will still have a measurable dimming effect. We’ve used the transit method to find plenty of planets hundreds of light-years from Earth. So the transit method is a pretty successful way to find exoplanets, but it still has its limitations.

Mainly, if a planet doesn’t pass between its star and Earth, you’re not gonna detect it. For example, astronomers were able to find that system of 7 Earth-sized planets around the star TRAPPIST-1 using the transit method because the planets have relatively close orbits that just so happen to take them between their parent star and Earth. But if they weren’t lined up so perfectly, we never would have detected them.

Now we’ve found so many exoplanets using the transit method that it’s pretty clear that planets around other stars are really common, at least within a few thousand light-years of Earth. When it comes to finding planets around stars that are really far away, like, tens of thousands of light-years, our best bet is to use gravitational microlensing. That’s where the gravity from a star with a planet orbiting it warps the light we detect from a second star behind it.

It doesn’t let us see the planet, but it lets us know it’s there. So far, we’ve only been able to use microlensing to find a few dozen exoplanets, because the chances of two distant stars lining up in just the right way are incredibly low. Still, it’s the best method we have right now to find exoplanets far from Earth, and we might even be able to use it to find some outside of the Milky Way, in the nearby Andromeda galaxy.

With so many limitations to our exoplanet-detection methods, we aren’t going to see exoplanets around every star. But it seems like practically every star could have them. We used to think there were some exceptions.

For example, astronomers once thought that binary star systems weren’t gravitationally stable enough for planets to form and survive. But then we found planets around binary stars. Astronomers also used to think that blue giant stars, which are huge, hot, bright, and don’t stick around for very long, were too intense for planets to form around them.

The matter the planets would form from would be blown apart before it could condense. But then we found protoplanetary disks, the disks of rock and gas and dust that planets form from, around blue giant stars. Whenever we think there’s an environment where planets can’t form, it turns out we’re wrong.

So, are there any stars out there without planets? I mean, probably. It’s a big universe.

But based on all the exoplanets we’ve found in the last couple of decades, it seems like almost every single star could have planets, and probably does. Because of the limitations of the tools we have used thus far, and the relatively short time we’ve had access to those tools, we don’t yet have the most accurate picture of the average star system in the universe. But it has become clear that planets are a natural feature of stellar formation, they are common, they are everywhere, and they are just waiting to be discovered.

Which is why you should subscribe to SciShow Space, because when they are discovered, we will let you know! Also, you might have a thing that you’re thinking about doing, maybe you’re already doing it. Would it be better if it had a website?

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