Can we see planets in other galaxies?

Daniel and Jorge talk about how astronomers are trying to detect planets in far away galaxies!

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Episode Transcript

Speaker 1 (00:08):
Hey, Daniel, do you remember the first time you saw
an X ray of yourself? I do. Actually, I once
broke a tiny little bone in my wrist the first
time I went snowboarding. And were you amazed to get
to see the inside of your body? I was really
excited about it, but then I was kind of underwhelmed.
It was sort of like a big whitewash. It was
hard to actually understand like what was going on inside there,

(00:31):
but the doctor could read it right. Oh. Yeah. To
him it was like crystal clear. He was like, oh,
this bone, that bone, the other bone. He knew exactly
what was happening. He spotted this tiny little break. And
that's pretty amazing, isn't it? How you know the knowledge
of an experience? I how they can pull out data
that other people can't see. Yeah, exactly. It makes me
wonder how X ray astronomers see the universe. Yeah, as

(00:54):
long as they don't go snowboarding, they're probably safe. I
hope the universe doesn't break its wrist. I am or

(01:15):
handmade cartoonists and the creator of PhD comics. I'm Daniel.
I'm a particle physicist, but I'm no longer a snowboarder.
Where you ever, Daniel, It doesn't sound like it went well.
I was a snowboarder for about five minutes and then
I retired. But no, it didn't go well. You achieve
what you wanted to achieve in that area of activity
and then decided to focus on physics. Yeah, exactly, I

(01:36):
crossed it off the list. Well. Welcome to our podcast,
Daniel and Jorge Explain the Universe, a production of I
Heart Radio in which we don't talk about snowboarding and skiing,
but instead we focus our energy on trying to understand
the universe, the vast reaches of space, the crazy explosions
going on inside stars, the weird things planets are doing
whizzing around stars in our galaxy and other galaxies and

(01:59):
in far reaches of the universe, and we try to
explain all of it to you without making too many
banana jokes. Although there is no out there in space, right,
there is galactic snow. Technically there's a lot of ice. Yeah,
we have ice giants in our Solar System. There's a
huge amount of frozen water all over the Solar System
and all over the universe. Water turns out, is not

(02:22):
actually that rare. It's only rare to be liquid on
the surface of a body like it is here on Earth.
Who do you think will be the first person to
snowboard on Neptune? Probably an astronomer, definitely not me. Hopefully
where where a risk guard? That would be my advice,
because there are some crazy mountains out there and some

(02:42):
crazy different kinds of snow, Like I wonder if methane
snow is good for snowboarding on or not. Do you
think aliens have a lot of different words for snow
and like we do here on Earth. I don't know.
Maybe if aliens have a lot of wrists, then they
don't go snowboarding because you know, it's too easy to
break one. They're an increased risk. They have like eight risks,

(03:05):
increased risk risk. Well, there are a lot of interesting
planets and asteroids and comments out there in the universe,
and we've talked about a lot of the ones that
we can find here in our Solar System on our podcast,
and also we talked about finding planets in our galaxy.
But it's a big universe and who knows what's out

(03:25):
there beyond our galaxy? And a deep question we're always
asking about the universe. Is how unusual is our neighborhood?
You know, we spent most of the time on this
planet just looking in our immediate neighborhood, understanding our planet,
our solar system, and then wondering, is this weird or
is this typical? Are other stars out there? Do they

(03:46):
also have planets like ours? Or are we the only
solar system out there with multiple planets? Or maybe other
solar systems have like dozens of planets. And so in
the beginning, we usually just wonder and we speculate, And
now we're in an era where we can actually start looking. Yeah,
because we have spotted planets in other solar systems within
our galaxy. I think you know, right now we not

(04:07):
only know that there are thousands and thousands of them
out there, but we've also started being able to actually
see them and even like check the weather on them. Yeah,
it's really pretty amazing. We are living in an extraordinary
era because for thousands of years people have wondered about
that question, are their planets around other stars? And now
just in the last twenty five years, we know for

(04:29):
a fact the answer to that question, and the answers
are kind of exciting. Ryany tells us that there are
a lot of planets out there, and there are a
lot of them that are probably like Earth, and so
that's exciting to actually like know for a fact the
answer to questions people have been wondering about for thousands
of years. Yeah, but most of the ones we've seen,
or at least have detected so far, we've seen by

(04:51):
looking out into the stars at night, and most of
them are in our galaxy. Right, It's pretty much all
of those thousands of exo planets that we found are
in the Milky Way. Yeah, that's right, because the Milky
Way is sort of our galactic neighborhood. It's about a
hundred thousand light years across, and so it's the best
place to look at other stars because they're the ones
that are nearby. But then, of course our imagination reaches

(05:13):
further and wonders like, well, maybe the Milky Way is unusual,
Maybe the Milky Way is weird, or maybe it's typical,
and it makes us wonder what is it like to
be a planet around a star in a far away galaxy? Yeah,
because the Milky Way is not the only galaxy, right,
There are hundreds of billions of galaxies that we can
see or know about. There might be maybe an infinite

(05:34):
number of them. Yeah, I think the last count is
in the observable universe. There are more than two trillion
galaxies right, each with hundreds of billions of stars. And
as you say, that's just the observable universe, we have
no idea what fraction of the actual universe that is.
It could be literally zero volume fraction, because the universe

(05:54):
could be infinite. Yeah, and it is sort of possible
now that you mentioned it is it would be weird.
But it is maybe possible that maybe the Milky Way
is it is strange, right, Like, maybe our galaxy is
the only one that is stable enough or calm enough
or something enough for it to have stars with planets
with life and them. Yeah. Absolutely, And it's one of

(06:14):
my favorite kinds of questions because any answer to this
question is mind boggling. Either the Milky Way is weird
and it's the only one that has the conditions to
make these kind of planets. WHOA, that would be weird, right,
or it's not. And then all those other galaxies are
also teeming with planets, which makes the number of earthlike
planets in our universe a huge number. That's hard to

(06:37):
hold in your head, and it makes you really suspect
that the odds of life are high. Yeah, but I
guess the question is can we confirm news? Do we
know for sure there are other planets in other galaxies
besides the Milky Way? And so to be on the program,
we'll be asking the question can we find planets in

(07:00):
other galaxies far far away a long time ago? And Daniel,
that's literally true. Every galaxy we look at is far
far away and the light is coming to us from
a long time ago. So they're basically all the setting
for Star Wars. Yeah, you know, that actually did blow
my mind recently. I was watching the Star Wars movie
and you know when those words come out at the beginning,

(07:21):
a long long time ago in a galaxy far far away,
and it did sort of made me think about some
of our conversations where it's like, oh, that means, you know,
it's sort of happening in real time. It's just that
the light is just getting to us now. Yeah, well
it could have happened a long time ago and the
light could still be arriving, So you can imagine that
instead of watching it on your TV screen you're just
looking through a telescope watching these battles play out in

(07:42):
a far away galaxy. Of course, you know the events
have already occurred and you're just watching them. But that's
sort of just like watching a movie, right, and the
movie is totally filmed before you watch it. It's not
like they're acting it live or anything. But it's fun
to imagine. Yeah, but even a movie like Star Wars,
if you think about it, they only hang out in
one galaxy. You know, it's the Galactic Empire, the only

(08:03):
goal around the galaxy. They never go to different galaxies. Yeah,
that's right, And I think they use that sort of
as a mechanism to suggest this is impossibly distant. This
is some where we could never go. This is a
different part of the universe. Because galaxies are crazy far apart.
They're not just really bit right there, like fifty or
hundred thousand or two hundred thousand light years across. The

(08:25):
space between them is much much bigger than the size
of the galaxies, right. Galaxies tend to be millions of
light years apart. So it's like, you know, if your
house was out in the deep woods and the next
house wasn't for miles and miles away. Yeah, yeah, I
think galaxies are like hundreds of thousands of light years wide,
but there are millions of light years apart. Yeah, they're

(08:47):
like little islands. Yeah, they're like little islands. Yeah, there's
a lot of variation in the size of galaxies, but
roughly that's correct. They're like basically ten times further apart
than they are wide. All right, Well, then the question
is are their plan in those other galaxies, and if
there are, how could we ever find them or maybe
even see them. That's right, because we want to move
beyond just speculation. We don't want to just wonder if

(09:10):
they're there. We want actual facts, We want data, we
want observations, We want to know because science is not
just about guessing and speculating. It's about asking nature questions
and hearing the answers. And the best moments are when
those answers are a surprise. Yeah. So, as usual, we
were wondering how many people out there in the public
and in our audience know if we can find planets

(09:31):
in other galaxy. It seems to impossible. I'm gonna put
my money on impossible, but we'll see. So Daniel went
out there and solicited answers from people on the internet,
and so thank you everybody who participated as usual, And
if you would like to give a shot to answering
tough physics questions without any preparation, without any googling, without
any background knowledge, please write to us two questions at

(09:55):
Daniel and Jorge dot com. We would love to put
your baseless speculation on the podcast. So think about it
for a second. If someone as you, if you thought
you could find planets in other galaxies, and what would
you say. Here's what people had to say. I was
under the impression we had already found planets in other galaxies,
but maybe not. That does seem very far away now

(10:18):
that I think about it and say it out loud.
That really depends on the times, because someday we totally
could find those planets. For example, if we launch robotic probes,
maybe with self replicating capabilities, they could go on and
on for billion years and catalog everything they come across.

(10:44):
But since the universe is expanding, I don't think we
will ever be able to reach the other galaxies. Yes
we can. My guess is that we probably don't yet
have any method to tech exoplanets and other galaxies directly.
I would imagine that the only way to detect them

(11:05):
would be indirectly, possibly from gravitational effects on that stars. Sure,
eventually we can, but I don't think we can now.
The distances are just too far for us to see
the wobble of the star with the planet orbiting around it. Well,
I can't think of a way then with the current

(11:26):
techlorldsy we could do that, all right, A lot of
optimism here. Somebody just said, yes, we can see sip
with it was that Obama didn't sound like him? Yeah, exactly.
I think there's a lot of good optimism here, and
also some great ideas, I like, you know, launching robotic
probes to other galaxies, though that would take a long
long time for them to get there and then report back,

(11:46):
so graduate students don't propose that for your PhD. Yeah.
And also it seems like somebody here thought that we
had already found planets in other galaxies. I guess, um, Yeah,
it's sort of hard to remember that distinction between stars
in our galley sea and stars in other galaxies. And
remember that when you look out in the night sky
and you see stars, all of those stars are stars
in our galaxy to the naked eye, a distant galaxy

(12:10):
is too faint for you to make out the galaxy
by itself. It takes like a telescope or a good
camera and you have to like build up that light
over several hours or days in order to see those
galaxies because they are so far away. Yeah, and if
you do see a galaxy on a telescope or on
a photo, it really just looks like a little smudge
from Earth. Unless you have like an amazing super telescope,

(12:31):
it's really almost impossible to make out the individual stars
in them. For most galaxies, that's true because they're really distant.
For the closest galaxies, they are actually quite large in
the night sky, like Andromeda. If you could see it
it was bright enough for you to see, it would
be larger in the sky than the full moon. Oh really, wow,
I didn't know that. Yeah, it's pretty incredible. So if

(12:51):
you look at a picture of Andrameda, it's taken over
many many hours or sometimes many nights just to build
up enough photons for you to see it. It's just
so far away. It's not very bright, but it's huge,
and so it takes up a big fraction of our sky. Wow,
it's pretty cool. All right. Well, let's get into this
topic then of how we would find planets in those
other galaxies. And I guess we should start by maybe

(13:13):
recapping how we know about planets in this galaxy. How
can we possibly know there are planets around us? Yeah,
it's pretty cool, and these are techniques that were developed
again just in the last couple of decades. You know,
for a long time people have wondered about this, But
planets around other stars are hard to see because those
stars are pretty far away. We're talking about light years

(13:35):
and light years away, and then the planets are really
close to their star in comparison to the distance from
here to there. I've heard you say before, and I
like this analogy that it's sort of like looking for
a tennis ball around a street light on the other
side of the country. It's very difficult to see a
very small thing next to a very bright thing. Yeah,

(13:55):
because suns are pretty bright, and planets don't. They don't glow.
Did you reflect light? That's right, they just reflect light,
and so it's very very difficult to see them directly.
So people came up with a few really clever techniques
to try to deduce the presence of planets, and these
days we actually have a few that are pretty successful. Historically, though,
the first one that really worked is something called the

(14:17):
wobble method, and this is based on the idea that
the planet doesn't just orbit the star. The planet in
the star sort of orbit each other because while the
planet is moving around the gravity of the star, the
planet is also a big massive object and it tugs
on the star. So the planet in the star together
actually orbit the center of mass of those two objects.

(14:38):
What this means is that if a star has a
planet around it, it wobbles a little bit, It moves
a little bit, sort of like shakes in the sky.
It's not stationary relative to us, and this is something
we can see, So we can see the gravitational effect
of a planet on its star by watching it wiggle. Yeah,
it's pretty amazing. We tend to think of our Sun,
for example, as being stationary and all the planets are

(15:01):
going around it. But the Sun is actually kind of
wiggling and getting pulled this way and that way by Jupiter,
by Us, a little bit, by Mars. It's not like
fixing space. Yeah, it's mostly by Jupiter though, and you
know this is not a huge effect. It's a subtle
effect because the star is usually most of the mass
of everything in the Solar system, Like in our Solar system,
the Sun is of the mass of the Solar System

(15:23):
and Jupiter is about of the rest of it. So
if you're looking at our Solar system from really really
far away, you could probably detect the effect of Jupiter
as a little wiggle on the location of the Sun.
But it wouldn't be a huge effect. It's not like
the Sun is moving around Jupiter the same way Jupiter
is moving around the Sun. It's a much smaller effect
because the Sun has so much more mass. Yeah, okay,

(15:45):
so you can look at a star and if you
see a wiggle, you know that it has a planet
around it, But that doesn't tell you much about the
planet itself, right, So there are other ways to tell that. Yeah,
you can basically just tell the mass of the planet
by the amount of the wiggle. And you can't see
the wiggle sort of side a side, right, we don't
have enough angular resolution and like see stars moving at
that resolution. But what you can do is see the

(16:06):
wiggle sort of back and forth. Is the star moves
away from us and then comes closer. It changes the
frequency of the light that the star is sending us
is a little bit of a Doppler shift. So that's
how we see it actually wiggling. You see a wiggle
in the color, like the star looks a little blue,
a little red, a little blue, and a little it
and if it looks regular enough, you think, hey, there's
a planet there. Yeah, and that can tell you the

(16:27):
mass of the planet, because you have to know how
heavy the planet is to pull on the star at
that amount, And they'll tell you a little bit about
sort of the rotation of that planet around the star,
or at least the star's rotation around the center of mass,
because that affects like how long it takes to go
back and then forth. But it doesn't tell us something
really key, which is how big is the planet. And

(16:47):
we're interested in knowing like are these planets really hot
and dense? Are they big fluffy blobs? You know, we're
interested in like planets that might have life on them.
So this method can't tell you the radius of the planets.
You don't really know what's going on with the planet.
But yeah, there are other methods yeah, it just gives
you a wobbly estimate. Yeah, exactly. All right, well, let's
get into some of the other ways. We know that

(17:08):
there are planets in other stars in our galaxy and beyond.
But first let's take a quick break. All right, we're
talking about finding planets in other galaxies. We know there

(17:31):
are planets here under our feet and in our Solar system,
and we've seen thousands of planets in our galaxy, in
other stars in our galaxy. But the question is are
there planets in other galaxies? Have we seen them? Can
we see them? Is that impossible? What can we know
about them? Nothing is impossible. What we think is impossible

(17:51):
in a hundred years will be like an undergraduate research project.
You know. That's what I love about the progress of science.
Only the impossible is impossible. Yeah, what's impossible today is boring.
Next week it's an iPhone app and there exactly. Yeah.
And so we're talking about ways to learn more about
these planets around stars in our galaxy. Yes, and and

(18:12):
they all sort of involved looking at the star that
the planet revolves around. Right, that's mostly the the idea,
because you know, the star is so brad it's really
hard to see the actual planet, Yeah, exactly. So mostly
we're looking at the effect on the star of the planet,
and so one effect is that it shakes the star.
The other is that we can actually have a little

(18:33):
bit of an eclipse, like if it's lined up perfectly
so that sometimes the planet passes between us and this
other star, and that just you know, has to be
by chance that the plane of that solar system is
aligned to the planet passes between us and the other star.
It will block some of the light of that star,
and not completely, of course, because the planet is typically

(18:54):
much much smaller than the star, but it will pass
in front of it, and you will see a dip
in the amount of light you're getting from the star.
And you can see this regularly. Can go dip and
then back up, and dip and then back up, and
so that's a really good sign that there's something, some
sort of dark mass orbiting that star. Right. It's kind
of like when you're watching a movie and someone stands
up in front of you, you know, they temporarily kind

(19:15):
of block the light from the screen. That's kind of
how it is. Right, It's like a big source of
light and something moves in front of it you the
overall light from that will sort of go down, and
if they did that, you know, every two minutes or so,
you would get pretty annoyed. But that's the scenario here,
is that we see this regularly. So this is a
really awesome method, not just because it's harder to fake

(19:36):
because you're seeing this thing like happen all the time
you see regularly if you watch the star long enough,
but also because you can tell the size of the planet. Right,
The bigger the planet, the more light it blocks. The
smaller the planet, the less light it blocks. So you
can measure the radius of the planet, which is super
duper cool because if you know the mass of the
planet and it's radius, you can tell it's density and

(19:59):
that gives you a lot of clues about like what
it's made out of. Is it mostly rock? Is it iron?
Is it just a big loose ball of ice? Is
it just a fluffy collection of gas? Like that tells
us so much more about what the planet is, all right,
So that's another way to tell if a star has
planets around it, But that also has some negatives, right,
Like you can only see the planet if it happens

(20:20):
to go in front of you between you and the
sun and the star, and um, there are other things
that could maybe causing this, right, yeah, exactly, other things
could be passing in front of it. Doesn't have to
be a planet, it could be some other weird kind
of star, you know, like a brown dwarf for you know,
some sort of neutron star or something. So you don't
necessarily know, but you can get a lot of information

(20:40):
about the composition of it, so you can rule a
lot of that kind of stuff out. And this is
really our workhorse method. This is the method we've used
to find a lot of planets recently. And even though
you can't see every planet, you can do calculations, you
can extrapolate. You can say, well, if I've seen a
bunch of them, I know how likely it is for
everything to be lined up perfectly right for me to
see it, So I can estimate how many solar systems

(21:02):
are there out there that aren't lined up perfectly, and
you can make guesses about those planets. So it's pretty effective. Yeah,
and there's a big telescope in space that's doing most
of this right exactly. The Kepler Telescope is basically launched
just to do this and so it's just like churning
out these candidates. And you know, in the beginning it
was rare. We had like one or two and they
had like special fantasy names. And now there are thousands

(21:24):
of these things and more discovered every week, and so
now it's like a statistical game. Now we're able to
ask questions like how unusual is Earth? Or how weird
is it to have a hot jupiter that's close to
your star? Or how unusual is it to have nine
planets or eight planets? So that's really fun. Do you
think they will run out of names? Like I know
they use letters and numbers now, like you know, maybe seven, three, nine.

(21:47):
They sort of remind me of license plates, like you know,
all these names are just like random collections of digits,
and so I think they can just keep adding digits
and they're never going to run out of names. Do
you think when they get to like our two D one,
the will skip the number just to avoid you know,
infringing star Wars is right, I'm sure Disney's lawyers have
already written those letters, all right. So those are two

(22:08):
good ways to know if there are planets around other stars.
But there's also kind of a more direct way. Right,
Like I've seen pictures of planets around other stars, like
it is possible to kind of look at them, take
pictures of them. And now we have super powerful space
telescopes and clever techniques, you could actually look at fairly
nearby stars and see light off of those planets directly.

(22:29):
So we have like direct images, actual pictures from those
solar systems, and not very many of them, just a few,
because everything has to be like lined up just right,
and the planet has to be really big and kind
of far away from its star, and the whole star
has to be pretty close to us, and you have
to line up this coronagraph to block the life from
that star just right. But we've done it, and that's

(22:50):
pretty exciting. Yeah, but most of what we've done is
within our galaxy, and it sounds like it's already really
hard to see planets around other stars within our Milky
Way galaxy, which is our neighborhood. And so now the
question is, how could we possibly ever see planets around
other galaxies. I mean, when we look at a galaxy
just kind of looks like a fuzzy cloud, like a

(23:12):
fuzzy collection of stars. Yeah, the problem with all these
methods is that they start to fail as the star
gets further and further away. Right, A star that's further
away wobbles less, and the light that comes from it
is harder to look at and harder to separate from
the nearby stars. And so these things start to fail
as the stars get further away, which is why, for example,

(23:33):
we haven't even seen planets all through our own galaxy.
There are parts of our galaxy where we have not
detected any planets. The furthest planet we've ever seen is
about twenty seven thousand light years away, whereas the whole
Milky Ways a hundred thousand light years across. Right, So
now it seems almost impossible to imagine going to another
galaxy millions of light years away. But there are some

(23:55):
very cool techniques people have come up with recently that
will let us do this way. You said a while
ago that stars wobble less still further they are away.
Why is that wouldn't they wobble the same or is
it just gets lost in the noise. No, you're right, stars,
if they're far away, they actually wobble the same way.
It doesn't really matter how far away we are, and
of course, the sideways wobble is not what we're looking at,

(24:17):
but the back and forth wabble that's the red shift
and the blue shift. We could still see that for
stars that are further away, but they're harder to see.
You're looking through lots of other stars. They're further away,
they're more dim, and so it's just harder to study
these things that are further away. It's a harder to
make out the wiggle. Yeah, and which seemed almost impossible
to do this with stars in another galaxy, just because

(24:37):
it's so far away and you know they're probably drowned
out by all the other stars in that other galaxy.
But you're saying there, it is sort of possible to
look at planets there. Yeah. People have come up with
crazy ideas to do this, and so the sort of
three ideas that I think are pretty awesome. The first
one is called gravitational micro lensing, and it also uses gravity,

(24:59):
but it's not the wa hobble of the star that
it's using. It's looking to amplify the light of a
star by another star passing sort of in front of it.
That's kind of how we look at dark matter, right,
And that holds to like if we look for that
kind of lensing effect exactly because mass doesn't just like
create gravity, it actually bends space. And so if something

(25:20):
passes between us and another object, it will bend the
space between us and that object. So the light from
the object in the background gets distorted, just like if
you had a lens in the sky, but now it's
a gravitational lens. It's bent space, so it changes a
path of light. But you can use all of your
intuition for how a lens works to understand how a

(25:42):
gravitational lens work. The principles the same, even though the
bending mechanism is different. So what happens here is you
have a star in the background, and then some star
in the foreground passes between you and that star, and
as it passes right through that line between you and
the background star, it creates this lensing effect and it
just dorts the background star. That's cool, that's gravitational micro lensing.

(26:04):
But if there happens to be a planet around the
star that's doing the lensing, then as the planets going
around the star, it will change how that lens works,
and so it will sort of like distort the distortion
in a particular way, So it changes how the background
star looks like there'll be a wiggle in the lensing basically, right,

(26:24):
like the lensing effect will be wiggly. Yeah, exactly, Just
like if somebody stands in front of you in the
movie theater and blocks your path, if they have like
a little toddler running around them the whole time, they'll
create a different shadow, right, And so it's the same idea.
And if the parent is like, you know, chasing after
the child, you would you would notice that from the lensing, Yeah,
you would notice that. So it changes the pattern of

(26:46):
the brightening and the fading that you get from gravitational lensing,
and it doesn't in a particular way. You can even
enhance it, right, You can get like a flare from
this planet if it's just in the right spot to
exaggerate and enhance the light of the background are And
so this is pretty cool, yeah, but you still need
a background star to sort of see and be able

(27:06):
to like see the life from it and be able
to tell it a part. Can we do that? And
with stars and other galaxies, like can we see individual
stars in like Andromeda? You can't always see individual stars.
But you're right, you need something in the background. It's
not critical that you have just one star in the background, right,
You just need some source of light. And then you
need to have a model for how that light will

(27:27):
be distorted by a foreground object. And so if you
have some sort of source in the background, you can
mimic you can model how that light would be distorted
by gravitational lensing, even if it doesn't just come from
one star, even if it comes from like a background galaxy. Interesting, okay,
So then the star we want to measure would be
in another galaxy, and now we need like a light
source behind that other galaxy, behind that star in the

(27:49):
other galaxy, or in or outside the galaxy. It doesn't
have to be in that other galaxy. It can be
like in another galaxy, even behind it, just anywhere behind
the star that we want to look at. We need
this perfect lineup of the star we're looking at and
then the star behind it, so the background star can
get lensed by the foreground star. So that's a big disadvantage.
Another big disadvantage is that it usually just happens once.

(28:12):
It's like a chance thing. These two stars are not
like usually in a binary system or anything. So it's
just like by chance that one happens to pass through
the line of sight to the background star, which means
you can't repeat it. You just get like one observation,
and that's kind of hard to like really base the
claim on if you only see something once. And this
works with stars in other galaxies, like we can tell

(28:33):
this wiggling in the lensing for something that far away. Yeah,
at the end of the program, we'll talk about some examples,
but it really can work. And the really cool thing
about it is that it can detect stuff that's pretty
low mass because the gravitational lensing is very sensitive to
the mass of the planet, so you can even work
for like planets down to the size of Mars. Cool.
All right, Well that's one way. What's another way we

(28:55):
can look at planets and other galaxies? Another way it's
more similar to the t ends it method, and that
you're looking for an eclipse. Here, you have a star
you're looking for in the other galaxy, and you try
to find a star that's in a binary system with
something that's producing really bright X rays like a black
hole or a neutron star or something. Then the star

(29:15):
you're looking for sometimes will eclipse that X ray will
like block those X rays. And this is something we
can see in other galaxies because X rays are more
rare than other light and they're really intense, and so
it's possible to see these things in other galaxies. So
this lets us see planets around binaries, meaning like solar

(29:36):
systems with two stars in them. Yeah, exactly, if one
star that maybe has a planet around it, and the
other star is a really strong emitter of X rays
and maybe it's a black hole, maybe it's not a star,
but some really bright source of X rays, and if
those two things line up just right so that the
star you're interested in studying blocks the X rays from
the other one, then you can see that. You can

(29:56):
see this sort of like dip in the X ray pattern,
and we can tell all like you know, if we
if I look at a galaxy and and I send
some X rays, I can tell that it's coming from
a particular star, like we have that resolution. Yeah, because
X ray emitters are more rare. So there's a lot
of stars in the galaxy, butN not that many strong
X ray emitters, and so that makes it less likely
to happen, but it also makes it easier to separate them, right,

(30:18):
so there are fewer of these things. Also, X ray
emitters tend to be really really small. These they are
very compact objects on black hole or neutron star, and
so it's more precise. Right, you can like block the
entire X ray emitter with your star or with your planet. Cool.
All right, So there's one more way and in which
we could detect planets in other galaxies. And then let's

(30:40):
talk about what we've actually found. Have we found planets
in other galaxies? And what can we know about them?
But first let's take another quick break. All right, we're

(31:01):
talking about exo galactic exoplanets. Do I need to repeat
the exo or can I just say at once like
exo galactic planets. I think it's like sergeants at arm right,
it's exo galactic planets instead of planetary exo galactics or
something that planet planets and other galaxies that are not
the Milky Way where we're at. And we talked about

(31:23):
a couple of ways in which we can actually maybe
see these planets, and you're going to talk about the
last one. There's one that involves pulsars. Yeah, this is
my favorite one because it's super crazy in science fiction. Ee,
and this involves pulsars, right, And so pulsars are neutron
stars that give off a really really intense beam of light,
but they're also spinning, and if the beam is not

(31:45):
perfectly aligned with the spin, then it sort of sweeps
across the galaxy and gives us a pings. So every
time it passes across us, we see life from it,
and then it goes dark, and then it passes across
from us and it goes dark. These things are crazy
and amazing, but also because they're super duper precise, Like
they spin at a very precise speeds and they don't
seem to change, so we can see like a ping

(32:07):
from them and then a gap and a ping from them,
and the time between those pings is very very regular,
and so that makes them really awesome clocks. And it
means that we can do things like measure their speed
relative to the Earth, and in particular we can see
whether they're moving back and forth because that will change
how often we get the pin from them. Yeah, so

(32:28):
it's kind of like the Doppler effect, but instead of light,
you're looking at the frequency of the pulse are blinking, Yeah,
exactly when the pulsar is moving away from the Earth
because it's getting wobbled by a planet that's around the
pulse are, then the time between the pulse becomes a
little bit longer. And then when the pulsar swoops around
it's coming towards the Earth, the time between those pulses

(32:50):
gets a little bit shorter because it's sort of closer
to us when it emits the next pulse. So if
you watch the pulsars timing and you see this wiggle
where it's like the pulse as they're getting longer and
shorter and longer and shorter, that tells you that the
pulsar is wobbling. And because pulsars are so precise, you
can measure these from pulsars in other galaxies, right, And

(33:13):
pulsars are very kind of noticeable, right even within the
Big Gus they are. Yeah, they're very noticeable, and they're
sort of rare. And that's the disadvantage of this method
is that, like there aren't that many pulsars and so
you can't really like find all the planets this way.
But you know, if you're just looking to find a planet.
This is one technique. Another disadvantages that pulsars. You know,

(33:34):
they're the remnant of the death of a star. You
had a big star which then collapsed and made a
neutron star. And so it's not always likely that planets
will like survive this process, that they won't just get
like blown up when the star goes red super giant.
And so it's not that common to have pulsars with
planets around them, but it's possible. So it's a dead

(33:54):
star spinning really fast and hopefully it still has a
planet circling around it. That's power all enough to make
it wiggle, yeah, noticeably, And then then we could maybe
tell if there's a planet. But but again we couldn't
tell anything about the planet, could we. Well, we could
tell that if there's life on that planet, it mus
have had a really good sunscreen because it survived a
very traumatic event. Right, So it's not their on their

(34:16):
underground bunkers with their conditioning, yeah exactly, watching TV shows, yeah,
their Netflix and chilling on that planet. Yeah. Well these
are all cool ways. I guess the question now is
do they work? Have they worked? Have we actually found
planets with them? So what do we know, Daniel, have
we found planets in other galaxies? So we actually have
seen planets in other galaxies, which is so much fun

(34:38):
to say, and to know that we've achieved this huge
breakthrough in terms of our like actually factual knowledge about
stuff going on super duper far away. Right, and by way,
you mean like the Royal weed. I mean we've been
me sitting on my couch reading news articles about astronomers
doing the actual work. You're like, I did that, We
did that, we could It's like quantum mechanics, right, what's

(35:02):
the point of doing science if nobody's reading your papers?
And so I'm participating just by reading their papers. Does
the paper exist if nobody ever reads it? Exactly exactly,
I'm collapsing the wave function of these papers. All right,
So we found planets and other galaxy. Yeah, so we
have to that are sort of like preliminary haven't been
confirmed that come from gravitational lensing, and these are tough

(35:23):
because you can't repeat them, and so like it seems
like a planet, but who can really tell and you
can't really do any follow up studies. And one that
seems really pretty solid using the X ray eclipse method. Alright,
so step us through. What's the first one we found.
So the first one we found is from this crazy
system called the twin Quaysar. So remember, gravitational micro lensing

(35:44):
requires you to have something in the background. Right, you're
studying a star. You want to know if there's a
planet around it, but first you have to have something
behind it that's going to get gravitationally lensed by your star.
So it turns out that there's a quasar super duper
far away. Remember, a quays are is basically just a
huge source of light. It's very bright. It's probably the

(36:04):
accretion disk of a black hole, and all that gas
is really hot and giving off a lot of light,
so they're some of the really brightest sources in the galaxy.
And right between us and this quasar is another galaxy.
So if the quays are really far away, and between
us and the quasar is a galaxy that's right between it,
and it's gravitationally lends that quasar into two pieces, so

(36:26):
we see basically two copies of this quasar split. So
it's called the twin quasar because it's already sort of
constantly being gravitationally lens in two bits, Right, and we're
pretty sure it's not two quasars. We're pretty sure that
it's the same, but it's just a lens distortion that
makes it look like they're twins. Yeah, because they're basically identical,

(36:47):
and you can see correlated fluctuations in the two. So
sometimes you'll see something happen in the A part of
it and it will also happen in the B part
at the same time. So you're pretty confident we're seeing
like two images of the same ways are but we
can't see the actual galaxy in between, or can we Well,
what we can do is we can see the effects
of that galaxy, and so this is exactly what happened,

(37:09):
is that we saw a fluctuation in one half of
the twins and not in the other, and we think
that's an effect of the galaxy that's doing the lensing,
because it only appeared in one of them, and what
we saw was like a little dip in the light.
And this is consistent with some big planet in this
foreground galaxy, sort of like changing the lensing of the

(37:31):
quasar behind it. What that seems implausible to me. So
so we have a point source of light that God
destroyed it into two by a whole galaxy, and you're
saying that a tiny little planet in that galaxy can
affect that lensing. Hmm, that's exactly what they're claiming, And

(37:51):
again it's hard to confirm, like what they're seeing is
consistent with that hypothesis, but like it could also be
other things, right, it could be just like you're changing
the arrangement of the stars in that galaxy and that
changes the gravitational lensing, and so it's consistent, you know,
with the planet, but it's not a confirmed observation, right,
because you're saying that the whole galaxy is wiggling because

(38:14):
of this one planet. Is that what you're saying the
whole galaxy is shaking because of this one planet going around.
We're saying that the light from the quaysar is going
through that galaxy in a way that's sensitive to how
that planet is moving. And this would be a big planet,
so it would have to be large to affect this.
But yeah, you know, the photons that we're seeing our
gravitational lens by that galaxy, and we're saying that it

(38:37):
would be changed by the motion of this planet. But
how do you know it is a planet? Couldn't it
be like a little black hole in that galaxy? Or
couldn't it be a star or something wiggling? It could
be yeah, and it could be a rogue planet. Right,
So we don't really know very much. We just know
like something happened in this planet between us and the
quays are so as I said, it's not really like
a very well confirmed detection of an exo elactic planet.

(39:00):
It's just like an early candidate, all right. So and
this was back in ninety six, but we have more
recent events. Yeah, So then people tried to do the
same thing for a closer galaxy. They said, well, let's
look at Andromeda and Drameda is only two million light
years away. And what they did is they said, what
would it look like if we had a gravitational micro
lensing event in Andromeda? Now, Andrameda is pretty close when

(39:23):
it comes to galaxies, and so you can't make out
individual stars very well, but you can like make out
clusters of stars, and you can say, if one of
the stars in that cluster was gravitationally micro lensed by
another star, what would it look like? You can sort
of like calculate what that would look like and say oh,
you would get a dip or a change in the
brightness of the star in a certain way. Then they

(39:44):
looked for that and they saw it in Andromeda. This
is in two thousand nine. They see this sort of
like the thing that looks like a micro lensing event
in Andromeda, and we think it is a planet, Like,
how big of a planet? If it's a planet, then
it would have to be like six or seven time
the size of Jupiter right in order to cause the
effect that they saw, So that would be a big

(40:05):
would be a big planet. Isn't that almost a star?
Like put in something that big collapse into a star. Yes,
something much bigger than that would turn into a brown dwarf,
like about ten times a jupiter with the right composition
would turn to a brown dwarf. About a hundred times
the massive jupiter would turn into a star would start
to fuse. So this could still be a planet, but
we don't really know, and you know, again it can't

(40:26):
be confirmed. It was a one time thing. We saw
this one wiggle. It's characteristic, oh, a planet, but that
doesn't necessarily mean that it definitely was a planet. So
again it's like it's a candidate it's exciting, but you know,
it's not the best evidence that we have, right, But
it turns out that more recently, last year we got
a pretty good candidate. Was a difficult year, but it

(40:48):
was a pretty good year for exo galactic planets. Yeah,
somebody made this X ray eclipse method work. They found
a pair of stars that are binary system. One of
them is giving off a bunch of X rays and
the other one sometimes blocks those X rays, and it
blocks it in this way that you can tell that
there's something else going on with this star. The star

(41:11):
is doing the blocking must have something around it that's
changing how it's doing that eclipse, and we can see that,
and we can see the effect on the eclipse, and
so we're pretty sure that there's a planet around that star.
And this is in the Whirlpool Galaxy, which is like
twenty three million light years away in the constellation URSA Major.

(41:33):
So something is emitting X rays and something is blocking
it regularly, and we can see the eclipse and then
we can see variations in that eclipse. Right, if it
was just an eclipse from another star, you would see
a regular pattern, but we see a pattern on top
of that which means there's something orbiting that star changing
how it's eclipsing it, and from that pattern we can

(41:55):
tell actually some really interesting information about the planet. We
think it's like just about the size of Saturn, maybe
a little bit smaller, and it orbits that other star
around ten times the orbital radius of the Earth around
the Sun, so ten au. Wow, that's crazy precise. It
seems like a lot of detailed information about something so
far away. Yeah. Well, because we can take repeated measurements, right,

(42:18):
so we can study these patterns, we can understand the
period of this thing. We can look at all the
dips and the flips and the wiggles, and so that's
what gives you a lot more confidence that this really
actually is a big object orbiting that star and to
make these kinds of measurements. So this technique is much
better than gravitational micro lensing because it allows for repeated observations.

(42:38):
But I guess these would be planets orbiting weird things
like maybe not a railar star, but like a black
hole or a neutron star. Right, Like the source has
to be something special, Well, it has to be a
little weird because it has to be in a binary system.
You can have a pretty normal star with a planet
around it, but then you have to be in a
binary system with something that's giving off X rays, so
you can then eclipse those X rays. So like our

(43:00):
star wouldn't be visible from the Whirlpool galaxy using this
technique because our star is not in a binary system
with a neutron star or with the black hole giving
off a bunch of X rays right right. So it
feels like a lot of these really distant methods for
other galaxies only seem to work in really strange situations. Yeah,
you know what I mean. Like, we can find planets

(43:20):
in our galaxy pretty much any star we can sort
of check to see if it has planets, But in
other galaxies we have to rely on these weird kind
of phenomenons or arrangements. It is so far we don't
have a way to just check all the stars in
the galaxy. No, we definitely, and even for the ones
in our galaxy, right, And it comes down to coming
up with clever ideas. But that's what I love about

(43:41):
astronomy is that they have to come up with these
clever ideas. They think, well, this seems impossible. What if
there was a really weird configuration and this happened to
be attached to that which would swing around this other thing. Oh,
then maybe we could figure it out, and then you know,
then we bootstrap our way up. We figured that out,
and then we come up with other ways. And so
it's just an opportunity for creativity. I mean, somebody needs

(44:02):
to figure out more ways to see these things, because
there are a lot more planets out there to look at. Yeah,
there's room for improvement or new technology, or room for
us actually going to these other galaxies and looking. Yeah,
of course the direct observation would be fascinating, but that
would take millions of years, we think, unless, of course,
you know, we could just build that worm whole highway

(44:23):
and then we can get to those other galaxies pretty quickly.
But yeah, there are opportunities out there. This is a
young field. We only recently saw the first observation of
a planet around any other star, and so studying planets
around stars and other galaxies is a whole open field
out there. So for you enthusiast thinking about school and
becoming a physicist, this could be your big discovery. There's

(44:45):
lots of exciting stuff left to do. This could be
your PhD. That might take millions of years, but you know,
get a hang in there. Yeah, Unfortunately you can't get
the posthumous Nobel Prize. So well, I'm gonna wait for
a new film starting called The Exo Universe Galactic Planets.
We need for us to be able to type things
in other universes, and then we'll zoom in on those

(45:06):
planets will see people snowboarding down weird slopes filled with
weird kinds of chemical snow. Yeah. All right, Well, I
guess it's kind of interesting to think about planets and
other galaxies because it doesn't seem likely that will ever
visit them, do you know what I mean? Like in
our galaxy when we see a planet now it's like,
you know, twenty thousand or twenty seven light years away,

(45:27):
that's sort of doable for a human colony. But you know,
other gasy that is really far away, like we may
never get to those other galgs. No, you're right, and
it's more about like asking these questions about whether our
galaxy is typical and whether it's usual. Like when we're
studying our galaxy, are we getting misled about how the
universe works? Or is our galaxy like a pretty good
test case for understanding the whole universe. So it's more

(45:49):
about like understanding the broader context than actually like finding
other homes for humanity. Yeah, I guess it would be
pretty cool to know what those plants are like and
how many they are are and if we can pretty
much expect all galaxies to have as many planets as
we have, because that would be pretty mind blowing. That's
a big number to hold in your head. But it
makes a lot more sense than thinking that there are

(46:11):
no planets in other galaxies or fewer planets in other galaxies.
Most likely the Milky Way is pretty typical. It's also
cool to think about maybe there are civilizations in those
other planets in other galaxies and they're trying to look
at us. Yeah, I hope. So. Unfortunately, there's nothing really
weird enough about our start to make it extra visible
from other galaxies. Right, We're not like eclipsing and X

(46:33):
ray source or whatever. But maybe physics students in those
other galaxies have come up with a crazy clever way
to discover planets in our galaxy. Yeah, stay tuned, let's
just listen to their podcasts. Yeah, wait, wait, a few
million years, or maybe it's arriving now like the Star
Wars movie. Al Right, well, hopefully that gives you something

(46:54):
to think about when you look at the night sky
and wonder how many planets there are. Now many people
are in the Thanks for joining us you hope you
enjoyed that, and stay safe on the slopes. See you
next time. Thanks for listening, and remember that Daniel and

(47:15):
Jorge Explain the Universe is a production of I Heart Radio.
Or more podcast from my Heart Radio visit the I
Heart Radio app, Apple Podcasts, or wherever you listen to
your favorite shows.