When you look up at the night sky, and if
you happen to live far from city lights, you can see thousands of stars. It seems like
the sky is crammed shoulder to shoulder with them. And you’re only seeing the tiniest
fraction of stars there are; billions more exist that are too faint to see with just
your eyes. As you ponder this incredible number, a natural
thought arises: Are there planets circling those stars, too? And are any of them like
Earth? People just like you and me have wondered
about this for thousands of years. And right now, today, we can answer that question. And the answer is: yes. Our Sun is orbited by a lovely array of planets.
And they’re wildly diverse — big, small, airless, rocky, gaseous, hot, cold, and more.
That makes you think that maybe forming planets is easy, with so many varieties to choose
from. Even if making planets is hard, there are so many stars in the sky that it’s hard
to believe our Sun is the only one that’s been able to pull this trick off. Astronomers have fretted over this for a long
time, but trying to find such planets is hard. The biggest problem is, any such planets would
be faint, far away, and sitting right on top of their parent star. Being able to see one
in a telescope would be like trying to spot a firefly sitting next to searchlight. So if you can’t spot a planet like that
directly, maybe you can spot it indirectly. Imagine two kids, one big and one small, facing
each other. They clasp hands and start to spin around. As they do, the little kid, who
weighs less, will make a big circle, and the bigger kid will make a small circle. The same would be true of a star and planet.
As the planet orbits the star, it makes a big circle (or ellipse). But the planet has
gravity, too, and it tugs on the star. That means the star will make a small circle—what’s
called reflexive motion. For a long time, astronomers looked really
hard for this motion in nearby stars. But it turns out that indirect effect is also too small to
see. There were a few false alarms, but no real planets. Then, in 1992, everything changed. Astronomers
Aleksander Wolszczan and Dale Frail made a shocking announcement: They found not just
one planet, but two orbiting a pulsar, the dead remnant of a star that had exploded. This was really bizarre: When a star explodes,
it’s a catastrophic event that should destabilize any orbiting planets. It’s the LAST place anyone would
have thought to find them. However, follow up work quickly confirmed that the planets did indeed exist,
and in fact a third one was found a few years later. The first true alien planets had been found.
Officially, we call them exoplanets, which, you have to admit, is pretty cool. While this was an incredible discovery, it
was still a little unsatisfying. For one thing, pulsars are really weird, and for
another it looked like those planets may have formed around the pulsar after the supernova explosion,
from the material left over from the catastrophe. That’s nothing at all like our own solar
system. And that still left the question open: Are there exoplanets orbiting SUN-LIKE stars? We didn’t have to wait long to find out. In 1995, Swiss astronomers Michel Mayor and
Didier Queloz made a big announcement: They had found a planet orbiting the star 51 Peg, a star
very much like the Sun just 50 light years away. How did they do it? Well, remember those two
kids holding hands and circling each other? Even though the wiggling back and forth of
the star is too small to measure, that doesn’t mean the effect is undetectable. As the host star of the exoplanets makes its
little circle, sometimes it’s headed toward us, and sometimes away. That means that its
light will undergo a Doppler shift, and that CAN be detected. It’s not a big shift, and
takes some pretty fancy equipment to see it, but it’s measurable. That’s how Mayor
and Queroz found their planet. And the planet they found, called 51 Peg b,
is weird. For one thing, the orbital period turns out
to be just a little over 4.23 days. That’s right, I said DAYS. That means the planet
is seriously close to its parent star, just 8 million km out. Compare that to Mercury,
which is on average 55 million km from the Sun. Not only that, but the amount of Doppler shift
in the star is related to the mass of the planet; a more massive planet pulls harder
on the star, making it move more quickly. They found the planet was at least half the
mass of Jupiter, and probably more. That was a problem. According to planetary
formation models, that wasn’t possible! You can’t form a planet that big that close
to a star. Well, it turns out the models are probably
right. The planet DIDN’T form that close. It probably formed farther out, just like
Jupiter did. And like Jupiter, it then moved, migrated inward toward the star as it interacted
with the disk of planet-forming material around the star. In our solar system, Jupiter didn’t
get very far in its inward motion – it’s thought that interactions with Saturn put the brakes
on that, and pulled Jupiter out to where it is now. Apparently 51 Peg b didn’t have its own
version of Saturn pulling on it. Its inward spiral continued until it ran out of disk
material to interact with, which was very close indeed to the star. We call planets
like that “hot Jupiters”. Once 51 Peg b was found, other teams began
looking for short-period planets, and within a few years several more had been found, many
of them hot Jupiters just like 51 Peg b. Now mind you, at first there was a LOT of
doubt and skepticism in the community about these exoplanets discoveries. A lot of other
phenomena could masquerade as planets, like starspots, or pulsating stars, or background
stars messing up the measurements. Scientists discussed these possibilities vociferously
— as well they should. Science is all about not fooling ourselves. A good scientist WANTS
other scientists to try to poke holes in their ideas. It’s disappointing to be wrong, but
if we are we want to know. That all changed in 1999. A planet called
HD 209458b had been discovered on a very short orbit around its star, taking just 3.5 days.
As luck would have it, from Earth we see the planet’s orbit edge-on. That means once per
orbit it passes directly in front of its star. This event is called a transit, and when the
planet transits the star it blocks a little bit of the star’s light, and that means
we can detect a dip in the star’s brightness. And sure enough, that dip was found. HD 209458b
was the first independent confirmation of an exoplanet, and pretty much everyone was
on the bandwagon after that. The beauty of transiting exoplanets is that the
amount of starlight blocked tells you how big the planet is; a big planet blocks more
light. If we know the planets’ mass from the star’s Doppler shift, we can use the
planet’s size to calculate its density. This is important: A gas giant like Jupiter
has a low density, and a rocky metallic planet like Earth has a very high density. Without even
being able to see the planet directly, we can already start to determine what it’s
like physically. In 2009, NASA launched a space-based telescope
named Kepler, designed specifically to stare at 150,000 stars to detect that telltale dip
in light indicating exoplanet transits. And oh my, did it work. By early 2015, Kepler
found its 1000th confirmed exoplanet, and there are 500 more confirmed from ground-based
telescopes. That’s more than 1500 planets! And we have well over 3000 more candidates
from Kepler awaiting confirmation. All these planets have been found using indirect
methods. What about actually seeing them, getting photos of them? That’s hard, because planets
are so faint. But it’s not impossible. In 2004, the first picture of an exoplanet
was released: 2M1207b, a planet with five times the mass of Jupiter. It orbits a brown
dwarf, a peculiar kind of low mass star that we’ll learn more about in a future episode.
It’s a young system, which makes it easier to see: The planet is still glowing hot from
its formation, and it appears a lot brighter using a telescope that can see in the infrared. About a dozen other planets have been seen
this way, too. My favorite is the planet orbiting the star Beta Pictoris. It has seven times
the mass of Jupiter, and orbits the star in about 20 years—and we’ve actually seen
it move! Images taken a few years apart actually show the planet in different positions around the
star, confirming its orbital motion. That is incredible. Taking photos of these planets is still a
daunting task, which is why so few have been seen. But we’re getting better at this,
and as new technology comes along we’ll get more pictures of exoplanets and learn
even more about them. The sheer variety of exoplanets is staggering.
Hot Jupiters are the easiest to find, because they’re massive and fast, making their signal
easier to detect. But as the techniques have improved, planets of lower mass have been
seen; the smallest exoplanet found is smaller than Mercury, and not much bigger than Earth’s
Moon. We’ve seen planets bigger than Earth but smaller than Neptune, called “Super
Earths”. About 500 multiple planet systems have been found, too, including one with seven
planets. We’ve found them around every kind of star,
too. Exoplanets have been detected around stars like the Sun as well as tiny, cool red
dwarfs; hot, massive blue stars; and even red giants, stars nearing the ends of their
lives. One exoplanet system announced in 2015 is incredibly old; the host star is 11 billion
years old! When our solar system was just beginning to form, these planets were already over
six billion years old—older than our solar system is now. We’ve even seen planets orbiting binary
stars, making Star Wars seem a lot closer to home than being in a galaxy far, far away. We’ve seen so many exoplanets now that we
can extrapolate a bit and get some numbers. The results are staggering: In our galaxy
alone, there may be hundreds of billions of planets. In fact, planets may outnumber stars
in the sky. Now, if we’ve seen planets as big as Jupiter,
and as small as Mercury, then we must have seen planets around the same size as Earth,
right? Yes. Yes, we have. We’ve actually found
hundreds of them so far, it looks like making planets the same size as ours is pretty easy
for stars to do. But Earth-SIZED is one thing. Earth-LIKE is
another. How many of these planets might actually be habitable? That is, at the right distance
from their star to have Earth-like conditions, where liquid water could exist on their surface? We’re not sure. But from what we’ve seen
so far, it looks like the galaxy may have more than 10 billion Earth-like planets. Ten. Billion. And maybe a lot more than that. Now I want to be careful here: We don’t
know what kind of atmospheres these planets will have, or what they’re composed of.
Do the planets have magnetic fields strong enough to prevent solar wind from eroding
away their atmosphere? Do they even have an atmosphere, let alone liquid water? We don’t know. But still, there are a LOT of
planets out there. There could very well be a twin of Earth orbiting a star not too far away. And over the
whole galaxy? We could be part of a very large family. After all this time, we finally have an answer to one of
the biggest questions we’ve ever asked in astronomy: The sky is filled with planets. Today you learned that planets orbiting other
stars exist and can be detected with a variety of methods. Nearly 2000 have been found so
far. The most successful method is using transits, where a planet physically passes in front
of its parent star, producing a measurable dip in the star’s light. Another way is to measure
the Doppler shift in a star’s light due to reflexive motion as the planet orbits.
Exoplanets appear to orbit nearly every kind of star, and we’ve even found planets that
are the same size as Earth. We think there may be many billions of Earth-like planets
in our galaxy. Crash Course Astronomy is produced in association
with PBS Digital Studios. Head over to their YouTube channel to catch even more awesome
videos. This episode was written by me, Phil Plait. The script was edited by Blake de Pastino,
and our consultant is Dr. Michelle Thaller. It was directed by Nicholas Jenkins, edited
by Nicole Sweeney, the sound designer is Michael Aranda, and the graphics team is Thought Café.
absolutely brilliant