[ ♪ Intro ]
If you’re a fan of astronomy, you probably
think you know the story of the first time
we discovered a planet outside our solar system.
It goes something like this: “Once upon
a time, long ago, it was 1995…
[ fasts forward sounds ]
And by observing changes in a star’s motion,
Swiss astronomers found our very first exoplanet
orbiting the star 51 Pegasi.”
It’s an amazing story, but it has just one
problem: The planet that came to be known
as 51 Pegasi b wasn’t the first planet discovered
around another star.
Or the second.
Or even the third.
It just happened to be the first planet we
found around a star like our Sun.
People often forget about the first true exoplanets
because they orbit something very different:
a pulsar.
That’s right: Between 1992 and ‘94, astronomers
discovered a whole star system around one
of the weirdest objects in the universe.
The pulsar in question is PSR B1257+12, which,
thankfully, some astronomers nicknamed Lich.
It’s about 2300 light-years from Earth.
Like all pulsars, Lich is a special version
of a neutron star, an object with a kind of
misleading name.
They aren’t stars in the normal sense because
they don’t convert hydrogen into helium
in their cores.
Instead, these objects are actually the leftover
cores of other stars.
They form when stars more massive than our
Sun end their lives in powerful supernova
explosions, which are some of the most violent
events in the universe.
As the star explodes outward, its core is
compressed under unimaginable pressure.
So much pressure, in fact, that the electrons
and protons inside its atoms are literally
crushed together into neutrons.
What’s left is basically a ball of solid
neutrons about 20 kilometers across.
And because that ball is a lot smaller than
what the core started as, it also spins a
lot faster -- just like how a dancer does
when they pull in their arms.
Lich, for example, makes an entire rotation
every 6.22 milliseconds!
Most neutron stars have powerful magnetic
fields, which can blast out beams of radiation,
like radio waves.
Depending on the star’s orientation, that
beam can sweep across Earth like a lighthouse
as the star rotates, sending a pulse of radio
waves our way.
If it does, we call it a pulsar!
Timing this beam is how astronomers figure
out a pulsar’s rotation rate, and they’re
some of the most accurate clocks in existence.
I’m not kidding: Lich’s period isn’t
exactly the 6.22 milliseconds I mentioned earlier.
It’s actually… well, this.
And that incredible precision is how the very
first exoplanets were found.
In 1992, astronomers studying this recently-discovered
pulsar noticed something unusual: The timing
of this supposedly super-accurate clock seemed
to be drifting.
It was a tiny change, but it was enough to
alter the exact distance between Lich and
Earth, meaning its pulses sometimes arrived
a little early or a little late.
And since we can normally rely on a pulsar’s
timing to be very steady, these changes must’ve
corresponded to stuff around it, tugging on
the star and affecting its orbit.
By looking for a pattern in the timing variations,
astronomers were able to figure out not only
that there were planets — three of them!
— but also how massive they were.
They started out with some pretty technical
names, but they’ve since been nicknamed
Poltergeist, Phobetor, and Draugr.
And their masses were one of the real surprises:
Not only do two of the planets have masses
only a few times as much as Earth, but one
has a mass similar to our Moon!
This makes them some of the smallest exoplanets
ever detected.
But you probably wouldn’t want to visit
there.
All three orbit their pulsar at least twice
as close as the Earth orbits the Sun — which
is probably a bad place to be with all those
powerful magnetic fields.
Actually, if you think about it, it seems
like these planets really shouldn’t exist at all.
They shouldn’t have been able to survive
the supernova that destroyed their original star.
So how’d they do it?
Easy: They probably didn’t.
It’s much more likely that they formed after
their host star blew up.
From another star.
That was also destroyed.
Seriously, if you see a star about to blow
up, just back away, very, very quickly.
It gets nasty in there.
Many neutron stars also have companion stars
in orbit around them, and Lich may have been no exception.
Sometimes, in systems like this, material
from that companion star gets pulled onto
the neutron star.
It might even be an especially common process
for pulsars.
Eventually, if enough material gets stolen,
the companion star basically disintegrates,
forming a disk of debris around the pulsar.
Now, around regular, young stars, planets
form from disks like this.
So it’s reasonable to say that would happen
around dying stars, too.
As far as we can tell, that’s likely how
Poltergeist and its friends ended up in the universe.
Of course, if that all sound like a pretty
unlikely scenario to you, the data would agree!
While planets seem to be incredibly common
around normal stars, we’ve found them orbiting
less than 1% of known pulsars.
And that’s probably a good thing, because
pulsar planets have to be among the universe’s
most tortured objects.
I mean, how many planets are born from a dying
star, while in orbit about another dying star?
It’s not exactly a field of daisies.
The first exoplanets we ever found might often
get overlooked, but studying them can remind
us that the universe is rarely what we think
it’s supposed to be like.
From the very beginning, we knew exoplanets
were going to be weird.
And since the 1990s, we’ve been proven right
over and over again.
Thanks for watching this episode of SciShow
Space!
If you’d like to learn about some of the
weird, ridiculous exoplanets we’ve found
since 1992, you can watch our episode about
three exoplanets with some seriously extreme weather.
[ ♪ Outro ]
