Planets Around Distant Stars

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  • 0:02 Planets Around Other Stars
  • 1:07 Exoplanets
  • 2:03 How Exoplanets are Observed
  • 3:43 Transits and Microlensing
  • 4:48 Lesson Summary
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Lesson Transcript
Instructor: Artem Cheprasov

Artem has a doctor of veterinary medicine degree.

This lesson will describe how extrasolar planets can be found and if any have been found. Methods like Doppler shifts, microlensing, and transits will be defined.

Planets Around Other Stars

You know, it wasn't all that long ago that we humans thought we are pretty special. Some ancient civilizations, not knowing any better, thought they were at the center of the world. Later, humans learned that no country or civilization is at the center of the world.

Of course, that didn't stop us from thinking we're special. Then we thought that the Earth is in the center of the universe. Now we know that's anything but the case. Not even our sun is at the center of the universe.

And then, as usual, not learning from our high-minded mistakes, many thought that if we're not at the center of the world or the universe, at least we are the only place in the universe where planets exist. We didn't know any better, since we couldn't see planets before.

Not long ago, even this notion was dispelled. Our planets aren't all that unique. We now believe most stars have planets orbiting around them. Of course, the ability, technology, and techniques designed to detect planets around other stars are still in their infancy, so we have much to learn. But this lesson will describe the basics of what we do know.


An exoplanet, or extrasolar planet, is a planet that orbits a star other than our sun. It's important to know that astronomers have observed that many stars either had or, if they're still forming, have disks of matter from which the sun and planets form. It is therefore entirely possible that most stars have at least one planet orbiting around them.

In fact, it could be that there are a trillion planets in our Milky Way Galaxy alone! That would mean there could be potentially hundreds of millions or billions of worlds that can support life. If there are billions of such worlds, then it would seem possible that some life, if not intelligent life, could exist in those planets. Again, that's just in one galaxy. We believe there are over 100 billion galaxies in just the observable universe, let alone the entire universe.

How Exoplanets Are Observed

Of course, we cannot see all the exoplanets. Again, we're still getting started. It was only in 1994 that the first extrasolar planet was truly confirmed by Dr. Alexander Wolszczan of Penn State University. Our technology will one day allow us to detect more and more, but for now we're limited in what we can see.

So, how do we know extrasolar planets are there to begin with? Well, it's actually hard to see one directly because they are faint and the glare of its star would make it even harder to see. Therefore, if you can't beat 'em, join 'em. Instead of fighting the forces of the faraway star, we use our observations of it to our advantage.

I'll show you what I mean in an example you can replicate at home with a friend. Let's have you stand in a spot, holding a rope by one of its ends. You are the star of our show, literally and figuratively.

Your friend can hold the other end of the rope. They can be the planet revolving around a faraway star. OK, now have the friend walk around you and tug gently on the rope as they do so. Not to the point where you fall down but where you wobble a little bit as a result.

This is what happens when a planet orbits around a star. The gravitational forces will tug, ever so slightly, on the star, causing it to wobble. Therefore, even if we can't see the exoplanet, we can see its effects on its more visible mama star. The wobble of the star produces Doppler shifts, which can be detected in the star's spectrum.

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