Can a planet be bigger than its star?

What counts as a planet and is it possible for it to outweigh or outsize its parent star?

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Episode Transcript

Speaker 1 (00:08):
Hey, Daniel, are your kids taller than you? You know
that day is not too far off. I've only got
a few inches on my thirteen year old. Oh man,
that's crazy. How about your parents? Are you taller than
your parents? I'm like one quarter inch taller than my dad.
I like how you've measured that precisely. It's like an
important quarter inch to be taller than your dad. He's

(00:28):
an engineer, so we pulled out the high precision measurement
devices when that happened. I wonder if your dad was
happy that he lost that measurement. But you know, it
kind of makes me wonder. Do you think stars feel
the same way new stars have children? Yeah? I mean
like they have planets, right, Like, do you think they
would feel proud if their planet was bigger than them?
Or would they be jealous? I don't know. I just

(00:49):
hope that one day my own kids escape my orbit
and start their own solar systems. That's confusing, though, who
would be the sun? You or your son? Hi am

(01:12):
or handmade cartoonists and the creator of PhD comics. Hi,
I'm Daniel. I'm a particle physicist, and I hope my
kids never move out of the house, and do they
wish that too? Even after this whole or deal we've
been through, you still want them in the house. You know.
It's pretty typical. When they were five or six and
they learned that kids grow up and move out, they
were like, what, No, we want to live at home forever.

(01:34):
But now that they are teens and tweens, they are
counting the days until they get to move out of
the house. Wow. But welcome to our podcast Daniel and
Jorge Explain the Universe, a production of I Heart Radio,
in which we take you on a tour of everything
that's amazing and crazy. The big things in the universe,
the tiny things in the universe, the things that are
so huge they blow your mind, and the tiny particles

(01:57):
that are hard to wrap our minds around, and we
app your mind around all of it. Yeah, a lot
of wrapping. In this episode, we like to talk about
all the amazing things out there that is wrapping us basically.
I mean, we're surrounded by mystery and wonderful and incredible
things happening in the universe, and we spend a lot
of time as humans looking out there into the universe

(02:19):
and trying to understand the way things work, And one
deep question we have that we're asking all the time
is whether what we see is typical, whether it's usual,
or whether there are rule breaking examples out there, whether
there are other ways the universe could be arranged. Yeah,
because the universe is full of surprises. And so one
thing that we've noticed that or that probably as kids

(02:41):
we know it is about our Solar system, about planets
and moons, is that there's sort of a like a
size hierarchy, right, Like you know, suns and stars are
bigger than planets, and planets are bigger than moons, and
moons are generally bigger than like asteroids, right, and rocks, yeah, exactly,
And it goes all the way down to space dust

(03:01):
that we talked about on a recent episode. It's fascinating
how our Solar system isn't actually filled with just these
like different categories of objects, but this whole spectrum of
objects from the tinies little particles out there all the
way up to the Sun. But there does seem to
be sort of an order their stars and planets and
moons and then all these other little rocks, and so
it's fun to wonder about whether that order could be

(03:24):
inverted It'd be kind of weird to think that all
kids are shorter than their parents, because how would it end.
It would end with particle children, right, particle sized children, Yeah,
cork size exactly. It actually seems to be going to
the other way. A lot of people have kids that
are taller than them, which means, you know, if you
extrapolate that eventually children will be the size of stars.

(03:45):
Oh wow, well my kids are already stars. Daniel in
my twinkling eyes walked right into that. But I guess
a big question about the universe and our Solar system
is does it have to be that way? Do stars
always have to be bigger than planets and through planets
always have to be bigger than their moons? And one
thing that inspired this question in my mind at least,

(04:06):
was reading about a recent discovery of a really, really
strange planetary system. There's a planet out there about four
thousand light years from Earth that was recently discovered and
confirmed by Hubble. And this planet is huge. It's three
thousand times the size of the Earth. It's ten times
the size of Jupiter. This thing is a monster. Wow,

(04:27):
that is huge. I mean, Jupiter is like the biggest
kid in our block. You know, now you're saying there's
a planet that's ten times bigger. Yeah, and it's got
a moon that's ten times the size of Neptune. So like,
this moon in this other solar system is bigger than
most of our planets, because than than us, much bigger
than Earth. Yes, absolutely, so we are overshadowed by this moon.

(04:49):
And that got me thinking about the question like, well,
is it possible to have a planet so big that
could even be bigger than its own star? Yeah, So
to be on the program, we'll be asking the Russian
can planets be bigger than stars? Well? Here about these
huge planets kind of makes me a little house joke.

(05:12):
You know. It's like when you see somebody with a
bigger house, you're like, I wish I had a pool.
Is that what you think? I think? Man, cleaning that
pool must be a pain in the butt. Well, it
would take away time from your that's true, yeah, exactly,
unless you have lots of couches and lots of different rooms.
But then you know that feels like a chore, like,
oh man, I have to sit on that couch and
nobody's on a miscouch in a while. So did you

(05:33):
just get a rolling couch. Then you can just scooch
over to your pool lounge next to the pool while
you clean it. There's always an engineering solution to you know.
I'm a big believer in smaller houses actually, so I
like our cozy little planet. I had some friends actually
who were on track to getting divorced, and the reason
was that their house was too big for real, for real,

(05:54):
because they were always shouting each other from across the
house and they didn't end up spending time together. And
they moved into a smaller house where they have to
share the space and get along, and they actually totally
improved their marriage. So there's an equals of one study. Well, again,
I think some engineering solus might have helped. You know,
there's walking talkies intercoms that could have saved their marriage
as well, and kept their big house that's too high

(06:16):
tech and this is much cheaper anyway, Bigger is not
always better, you know. I like our cozy little planet. Yeah,
we don't have to shout at each other to talk.
We have cell phones, dand he and podcasts. Yeah. Well,
but it's a big question here about bigness and whether
planets can be bigger than stars. Having a hard time
wrapping my head around that question. I mean, how could

(06:37):
have planet be bigger than its star? Or what does
it even mean to be bigger like dancer size or
or what? Yeah, lots of fun stuff we will dig
into in this very episode of our podcast. All right, Well,
as usual, Daniel went out there into the wild to
the Internet to ask people if they thought that planets
could be bigger than stars. And so, if you are
a denizen of the wilds of the Internet and you

(07:00):
are waiting for somebody to ask you tough physics questions
for which you have no time to prepare an answer,
please write to us two questions at Daniel and Jorge
dot com and we will send you something. Yeah, I
think about it for a second. Do you think planets
can be bigger than stars? Here's what people have to say. Well,
I would guess it couldn't be bigger, although I would

(07:20):
also guess that it couldn't be more massive or more
dense star. I'm thinking about the door star they discovered
that has a planet. Some planets can be bigger than
a growth star. So yeah, I think the size of
stars changes over its lifetime and after it's gotten through
its red giant phase. Um. There's a few things that

(07:43):
could happen afterwards, but I think it can ultimately turn
out to be a much smaller size than it was
for the majority of his life, and some large gas
giant far enough away. UM. I think as a chance
of being larger than its star at a point in time.
My guess is that if a planet was bigger than

(08:03):
its star in the sense that its actual mass was
larger than that of its star, the planet's gravity would
be bigger than that of the star, and it wouldn't
really rotate around the star, but would probably form a
kind of binary system. I think that a planet could
be bigger than it's sun, but it would have to

(08:26):
be secluded so that wouldn't pick up other mass. But
I would also wonder if there could be sustainable life
on this planet, and also would there be no nighttime
on this planet? If the Sun is in place of
the Moon, but the Sun is also spitting around the Earth,
would it be eternal daytime? All right? Pretty intelligent answers

(08:50):
here A lot of people digging into the masses of
things and different kinds of stars that people could orbit
or that planets could orbit around. A lot of interesting
answers here. Yeah, a lot of fun speculation. Thank you
especially to Ryan, our nine year old listener for sharing
his speculation about what would be like to live on
such a planet. I love the breadth of ages we

(09:10):
have in our listening group. Yeah, so I guess let's
jump into it, Daniel, And let's start what I guess
with the basic question, which is like, how big could
planets get? Like, is there a size limited planets? Like?
Doesn't does it at some point collapse into a star? Yeah?
This one is not that satisfying because it turns out
that the definition of a planet, the thing that distinguishes

(09:31):
something from being a planet and a star, is really
closely connected to the size, So it's a bit arbitrary.
And what distinguishes a star from a planet is whether
or not there is fusion happening inside. Like you've got
a big blob of stuff but it's just sort of
sitting there and not fusing. You call that a planet.
If it's got enough stuff so the collapses and causes
fusion to happen inside of it, then you call it

(09:54):
a star. And the thing that controls whether that happens
is basically the mass of stuff you have, right, like
the gravity the thing that's compressing all of that mass,
it might trigger fusion or not. It's not dependent on
the size of it, right, the physical volume. It only
depends on the mass. And you do these calculations, and
you can talk about things in terms of like the

(10:15):
mass of Jupiter. So Jupiter, for example, doesn't have enough
stuff to have enough gravity to collapse and cause fusion
to happen at its core. You need like about ten
or thirteen times the massive Jupiter to have a special
kind of star called a brown dwarf, which is a
special kind of fusion. And so anything up to about
ten times the massive Jupiter is definitely a planet because

(10:38):
it can't have fusion inside of it, right, But it doesn't.
It also depends on what it's made out of. Like
Jupiter is made mostly out of hydrogen, right, which would
sort of fuse easily. But if you have something, you know,
like a giant planet made out of iron, you would
need a whole lot more to get anything going if
if you can't all yeah, it does definitely depend on
the material. But most the stuff in the universe is hydrogen, right,

(11:01):
So if you're gonna get a blob of stuff and
cold us it into an object. It's mostly going to
be hydrogen, but you also are sensitive to the kinds
of hydrogen you get, Like you can get a brown
dwarf only under certain conditions. We have the right ratios
of different isotopes of hydrogen to start a particular kind
of fusion. To start the kind of fusion we have
going on in our son, for example, you need basically

(11:22):
pure hydrogen of the simplest isotope, and then you would
even need more of it. So how much stuff you
need to start fusion definitely depends on the amount of
stuff you have. And you're right, if you just start
with a blob of iron, the massive hydrogen, that wouldn't
actually fuse, right, Yeah, because it can. You could have
like a giant planet made out of iron that it
could be. You know, the whole size of the galaxy?

(11:43):
Is that? Is that crazy? Or would that just turn
into like a neutron star at some point? Wow, the
size of the galaxy. Oh my gosh, you took like
all of the iron in the galaxy and blobbed it
up together into a big planet. Would that start to
fuse or do other crazy stuff? Yeah? I don't know.
I don't know anybody's does that calculation. That's really fun.
It definitely wouldn't fuse, right because, as you say, fusion above,
iron actually absorbs energy and so it would cool the object,

(12:06):
and so it wouldn't create fusion. If you get enough
heavy metal in there, then gravity eventually wins. It'll just
compress it further and further until it collapses into a
new drawn star. And if it has even more mass,
it will become a black hole. So a galaxy sized
blob of iron would pretty likely turn into a big
black hole. But in terms of the stuff that we

(12:27):
actually see out there in the galaxy, the materials that
are available, most of the stuff out there in the
galaxy is still hydrogen. You know, we've been slowly cooking
hydrogen into heavier elements and the inside of stars, but
it's still a very tiny fraction of the hydrogen in
the galaxy that's been turned into heavier metals. There's kind
of an upper limit then, on the mass a planet
could have, at least in the universe that we see

(12:49):
not in his imaginary iron field universe. That's right up
to about you know, ten or fifteen times the massive Jupiter.
You can still call it a planet. You can arrange
for other ways to work to get more massive without
actually fusing, but that's the typical limit. I see anything
more than fifteen times a massive Jupiter, if it's made
out of hydrogen, then it's going to start to fuse

(13:10):
and become a son and it would become a star exactly.
All right, Well, that's like the mass limit. But we
we kind of post a question as a bigger can
a plan and be bigger than stars, in which case
it can has more to do with the volume, right,
like the measurement of it the size. Yeah, and this
is really weird. Like if you took Jupiter and you
started adding mass to it, you have like a hydrogen pump,

(13:32):
and you just started dumping hydrogen into Jupiter, then it
would get more massive, but it wouldn't actually get larger
very quickly because it would mostly just get denser, Like
the gravity would get more intense and it would hold
itself together and it would get denser and denser. So
as you pump hydrogen into it, you could actually get
Jupiter up to like seventy times its mass without changing

(13:53):
its volume very much. You're saying Jupiter right now is
actually kind of fluffy. I mean a lot of it
is just like gas right out it is just gas.
And if you added seventy jupiters and put them all
on top of each other, they would just like collapse
to a denser object about the same size as Jupiter. Yeah,
are there any examples of that? Have we seen? Yeah?
Actually there's a bunch of them. There's a star out there,

(14:14):
trappist one A. It's eighty times the massive Jupiter and
it's a star. Right, so this thing is burning, it's
a star. It's the same size as Jupiter. Right, So
right now out there there's a star which is the
same size as a planet. It's just much much more dense, right,
This thing is so dense. It's mostly hydrogen, but it's
like twenty five times as dense as granite. That. Yeah,

(14:36):
that's crazy to think that hydrogen can be that dense. Yeah,
they're really weird phases of hydrogen. Als. So when he
gets so dense, they are these things called like metallic hydrogen,
and we can dig into that one time in some
other episode. But there are a bunch of these things,
Like there's another star out there that's a red dwarf
and it's about the size of Saturn, and so it's
actually a star that's smaller, smaller than Jupiter, right, And

(14:58):
that's just because of these weird effects that as you
add more volume, the gravity gets more intense, and so
the planet doesn't actually grow in size. It only grows
in mass. But it's still a star, meaning it has
fusion at the center. It's just not exploding maybe like
our star. And then there are some other examples. You know,
we say that as you add more mass to the planet,

(15:18):
it doesn't actually grow in size. That's what we expect
and that's what we see most of the time, but
there are some counterexamples that we don't yet understand. Like
there's a planet out there they found called celt eleven B.
This is the one they called the styrofoam planet because
it's one fifth the mass of Jupiter, but it's actually
like larger than Jupiter. So it's like a big fluffy planet,

(15:40):
extra fluff. It's like Jupiter exactly. It's like the whipped
up version. Right. Somebody put a mixer in there and
set it on high and they're gonna fold it into
their angel food cake. But this is not something that
we understand, like it's thought in the models it shouldn't exist,
and so it tells us that there's something about planetary
formation we don't understand, or maybe it's some weird thing
and it just explode, odored and still coalescing. You know,

(16:02):
a lot of questions there, but mostly we expect that
you can't get a planet much larger than Jupiter by volume.
Oh I see, So there is kind of a size
limit of two planets because if you keep adding more,
at some point it will stay the same size, but
at some point it will become a star. So you
can't have a bigger planet. Yeah, exactly. Roughly, Jupiter is

(16:25):
about the biggest planet you can make that we understand.
There's one example out there that the king of planets
currently is this planet out there h D zero five
four six b N, which we think has seven times
the diameter of Jupiter. So this is the largest known planet,
but it still looks like it's forming, like it's in
a young solar system, and so it may actually be

(16:46):
like a brown dwarf that's still sort of coalescing. The
biggest baby in the universe. It's a big, dangerous baby.
So careful what you call it. But I guess the
main point is there's a size limit and a mass
limit to planets if we sort of stick to what
we see in the universe, which is mostly hydrogen. But

(17:08):
I guess the point is that there isn't large enough
concentrations of the heavier elements to make biger planet. Is
that kind of yeah? I mean, there might be some
really big planets out there that fluctuate into having huge
deposits of rock, but you know there'll be a lot
of rock. Is a larger rocky core in the center
of Jupiter than the volume of the Earth, right, So
typically if you get that large a rocky planet, it's

(17:30):
also going to have a huge amount of gas around it,
as you'll end up with the gas planet. All right, well,
let's get into how the other side of the equation,
which are the stars? How small can a star get?
And then we'll talk about the question can a planet
get bigger than its star? First, let's take a quick break.

(18:00):
All right, we're talking about planets and stars and their
relative sizes. Can your kid be bigger than you depends
on what you feed them. I suppose, yeah, who knows
what they put in milk these days? But Yeah, we
talked about planets, and now let's talk about stars, Like,
what's the smallest size star can get? Because if you
can have a tiny star but it's pretty heavy, then

(18:21):
you could imagine it can have a much bigger planet
orbiting around it. Yeah exactly, We're gonna talk about the
biggest planet. Now we need to talk about the smallest
possible star. The thing that's really fascinated about stars is
that their size depends on where they are in their
life cycle. Like a star isn't just born and then
it fizzles out and it always stays the same size.
It actually evolves a lot. So the size of a star,

(18:43):
even like our sun, depends on where it is in
its life cycle. Yeah, it changes size, like it's our
son is going to get bigger, much bigger at some
point in the future and then shrink. Yeah exactly. And
so the way a star is formed is that you
get a huge blob of gas like hydrogen gas, which
move to the stuff when you're forming a solar system,
goes to the Sun because gravity is a runaway process.

(19:05):
You know, the heaviest thing has the most gravity, so
it attracts the most stuff, so you get most of
the hydrogen sort of falling in towards the center of
the Solar system, and that's how it gets big. Right,
that's why it doesn't just turn into a planet. It
turns into a star because it has much more than
ten or a hundred or even a thousand times the
mass of Jupiter. And so the early stages of the
stars that it just gathers all that stuff and then

(19:26):
fusion happens in the center of the star and that
pushes back against the gravity. Right, you have gravity pulling
everything in to form the star and then fusion shining
energy out and keeping it from collapsing any further. Yeah,
So then that does that put a limit then to
how small or how large a star can be. Well,
in the beginning, it just depends on the mass, Like
as you add mass to a star, it gets bigger

(19:49):
and bigger and bigger. And our Sun is actually unusually
large on average, like most of the stars in the
galaxy are smaller than our Sun. But there's no limit
really on the size of a star in this phase,
Like it can get really really big. And we're gonna
do a fun podcast episode next week. I think about
what is the biggest star in the universe. But we're
interested in the smallest star, right, But what happens when

(20:11):
a star burns is that fusion pushes all of this
stuff out. So you get a big star, which causes
this gravity, which collapses stuff in, and then it causes fusion,
which works backwards. It's like a back reaction. It pushes
all the stuff out. It tends to make the star bigger.
So what happens in the life cycle of the stars
that it burns for a long time, like billions of
years depending on its size, and then it grows like,

(20:32):
as you said, our star is gonna get much much bigger,
and not like twice as big or three times as big.
It's gonna be huge. It's gonna grow so much that
it's radius is going to almost encapsulate the Earth. Yeah,
there'll be a nice toasty time for Earth if we're
still here. And this is important to understand in the
question later about whether or not you're gonna have a

(20:54):
planet surrounding a star, because before a star gets small,
it gets really really big, right, And and what happens
is that it collapses right as you say, what happens
in fusion is that you're making these heavier metals, and
so you start out with a blob of hydrogen, but
soon you have a core of helium which is fused
from the hydrogen, and then that helium, if the star
is big enough, burns and the heavier stuff and you

(21:15):
end up with neon and carbon and oxygen and all
of this stuff. And so now you have increasing density
in the center of the star and gravity is sort
of pulling on it. Again. I love the dynamics of
star formation and star life cycles because it's this like
back and forth between fusion that's pushing out on the
star and gravity that's like trying to collapse it, and
each one sort of trips itself up, Like gravity causes fusion,

(21:37):
which pushes stuff out, but then fusion creates denser stuff,
which increases the gravity. And so eventually what happens is
that the star collapses because you get so much heavy
stuff in its center that it can no longer burn. Yeah,
it's a big drama, like a bit of a dysfunctionnel
relationship between fusion and gravity. Maybe they should just get
a smaller house, you know, maybe that would work. A

(21:59):
smaller universe. Hey, we would all be a lot cozier,
be like living in the pandemic all the time, forever,
all right, But I guess it seems like the inevitable
fate of most stars is to shrink. Like theyll might
have some heydays where they're huge, but then eventually they
all shrink because they run out of field. Basically, yeah, exactly,
they shrink, and what you are left with is some

(22:21):
really really dense core. Like they blow out a lot
of the stuff and you get some sort of like
layers of fluff blown out into the Solar system. But
at the core, which you're left with, it is a
few different options. Depending on the mass you started with,
you might end up with a white dwarf, or a
neutron star, or actually a black hole. What's a white boar?
A white dwarf is the future of our Sun. It's

(22:43):
basically just a huge hot blob of heavy metals, and
there's no more fusion happening anymore. It's like not big enough,
there's not enough compression to cause fusion at its core.
But it's still hot. Right. If you like took a
scoop out of the center of the Sun and put
it in space, it still be a big hot blob
of heavy metals and that's what a white dwarf is.

(23:04):
It's not glowing anymore because it's making fusion, but it's
still hot. So they call it a white dwarf because
it glows from its heat, right, But it's not technically
a star anymore, right, because there's no fusion. Man, good question,
is a white dwarf a star? You're right, there's no
fusion happening anymore. It's like a stellar remnant. It's definitely
not fusing. But I think you do still call a
white dwarf a star. Really, but we just said earlier

(23:25):
that we need a fusion. Daniel, you're confusing me. Oh
my god, astronomical names are confusing. What news flash? Well,
it's they call it a star because it's bright and
it's giving of light right in the form of heat,
but it's not fusing. It's kind of it's kind of
somewhere in there. It's somewhere in there. Does it become
a planet eventually? Eventually? Sort of? What happens is that

(23:45):
these things eventually cool and they become a black dwarf.
And a black dwarf is just a white dwarf that's
had enough time to radiate away its heat into the
universe and cool from being white hot to be you know, cooler.
The interesting thing is that there aren't any black dwarks
in the universe yet because they think it would take
like trillions of years for a white dwarf to cool off,

(24:08):
and so there just hasn't been enough time yet to
form any of these things. I guess they sort of
become planets, but everyone's too polite to tell them that
they're no longer a star. You know, it's kind of
like a professor emeritus. You know, they're not really professors anymore,
but you know, you don't want to striper way their title,
yeah exactly, or last decades a list celebrity. They don't
get invited to the parties anymore, but you know, people's

(24:28):
last for their autograph. But the amazing thing is that
you get like six of the mass of the original star,
but now compacted into an area that's about the volume
of the Earth. Our sun will be about the size
of the Earth after all this happens, even though it
will be much much more dense than the is. It
sounds tiny, But then there are other possible fates for
a star, right, not just a wordwarf. That's right, If

(24:50):
the amount of stuff that you end up in the
core is large enough that you still have a lot
of gravitational pressure. You can collapse those heavy metals even
further and you don't get fusion. What you do is
you sort of force the electrons and protons together and
you end up with forming neutrons. It's called electron capture.
You're like push the electron into the proton and you
get this interaction in the core that turns all the

(25:12):
protons and electrons into neutrons, and then you get a
neutron star. Now, but technically that's also not a star
because there's no fusion going on anymore. Hold on, I'm
gonna make sure I have a comprehensive list of your
objections to astronomical categories so we could submit it to
the committee. Let's get this straight out, because you're confusing.
At least one cartoon is here anyway. That's right, Well,

(25:33):
I have the official form here, so I'll make sure
to fill it out and submit it after we'dune. But
you're right, it's a neutron star. It's not fusing, right.
It's probably still hot like we see neutron stars, but
they don't emit light in the same way. Mostly we
see them in the X ray. But these things are
tiny because they're super duper dense. So you have like
one and a half times the mass of the Sun,

(25:55):
and the radius of these things is like ten kilometers.
It's like a whole star in Los Angeles. Wow. But
it's amitting light just from its heat or from from
some kind of process or why are we still giving
it the honorary title of the star. It's definitely not fusing,
and so it's giving off light the same way everything
gives off light that everything with a temperature radiates. It's
called black body radiation. And all matter that has electromagnetic

(26:19):
interactions will give off light at some frequency that's connected
to its temperature. And that's why you know things that
get hot glow, even things that you don't see glowing
or are actually glowing just at wavelength that you cannot tell.
All right, And then a star can also end up
as a black hole, right, Like if you compress you
have more mass, even like stronger gravity, it can collapse

(26:42):
into a black hole. Yeah, exactly. The neutron stars prevented
from collapsing into a black hole because these neutrons don't
want to press against each other even further there like
pushing back. There's some pressure pushing back, but if you
have enough mass, you can also overcome that. And then
you get a total gravitational collapse into a black hole.
And because the things are collapsing right there, even more dense.

(27:02):
And so from white dwarf to neutron star to black hole,
the density of matter at the core at least is increasing,
and so the radius is decreasing, and so these things
get pretty small. And now you're gonna tell me that
the black holes also star, Daniel, it's a black hole star?
Is that the technical term? It's a star of science?
At least, it's a star of mystery. It's a tell

(27:24):
our performer out there in space. But yeah, then at
that point it's no longer star. I mean, come on,
it's a black hole. A black hole is not a star.
I think we can definitely rule on that one here today.
All right, well, let's get into now whether or not
you could have a planet that's bigger than its star.
We talked about how big planets get and how small
stars can get, and how to push the limits of

(27:47):
what astronomers call things in space. But first let's take
a quick break. All right, we've set up these two
topics Daniel, stars and planets, and now we're gonna ask

(28:09):
can a planet be bigger than its star? I guess
the question I just remember what we talked about is
is yes, right. I mean, you can get tiny stars
and you can get big planets. So there must be
some point, someplace in the universe where the two are together.
It certainly seems possible, right, But the key thing is
can the planet survive? Because the stars tend to be
large when they're young, then get even larger before they

(28:32):
get smaller, and so for a planet to outsize its star,
it has to survive that transition. And that transition is
not an easy one to survive, right, Like going to
this red super giant is going to be pretty toasty
for any inner planets, and the collapse to like a
black hole or a neutron star can involve a super nova.
So even if the remnant is smaller than the planet

(28:55):
originally was, to satisfy sort of say, technically you have
to survive that transition. You have to make it to
be bigger than your parents. I guess, I guess you
have to make it through your parents middle age or something.
You have to wait it out, wait for them to shrink,
and then you're taller than your parents. Right exactly when
their life explodes, if you can somehow hold on, then

(29:18):
stay away from them. To Jupiter. Yeah, Jupiter, you should go,
like you know, on a backpacking trip through Europe and
come back when the Sun has become a white spread.
Take that job in Asia for sure, and then come back.
I think that is good parenting advice and good astronomical advice.
Become a rogue planet at some point in your life,

(29:39):
you get that tattoo, you know, maybe that's what the
red spot is on Jupiter. Maybe it's already done his
traveling man it. Maybe it's a tattoo of the sun.
You know how some people get like a mom tattoo.
Maybe that thought it's red, you know, that makes sense.
We are solving deep, deep questions. An old, old, ancient

(29:59):
miss the three is about the universe today on the
podcast Physics and Parenting Podcast. All right, well, um, yeah,
it's tricky to survive, but it's totally possible, right Like
Jupiter in our solar system is going to survive our
sun exploding and becoming a white dwarf right, because it's
so far away. It is, but you know, it's going
to be pretty crazy and we're gona lose some of

(30:20):
the planets, and so it might be that Jupiter doesn't survive.
Like Jupiter itself won't be disintegrated, but it will be disturbed.
And so, for example, if Neptune takes off or Saturn
takes off after the Sun goes white dwarf, and maybe
that it perturbs the orbit of Jupiter so that it
also gets flung out into space. Or you need like
a new stable configuration and things have changed, and some

(30:41):
of these things are a bit fragile, but it's technically possible, though,
write like, isn't it kind of hard to kick a
planet out of Sun's orbit? It's not that easy. But
one thing that can do it is having your star
grow to red super giant and push off outer layers,
and so it totally can happen. And that's because what's
left of the star when it's shed its layers is
much smaller than the original star, so it just doesn't

(31:04):
have the gravity to hold onto the big planets in
the same orbit. So those planets sort of relax and
get bigger orbits, and now those orbits are a little
bit looser and more chaotic, and so they're more susceptible
to tugs from passing stars that can pull them out
of their orbits and even out of the Solar system.
All right, well, it might happen in our Solar system.
But we have examples of this configuration of a bigger

(31:26):
planet than its star that we seem like, do we
have evidence we actually have seen one. Yeah, NASA's telescope tests,
which is excellent at looking for these things, that has
spotted this solar system in the constellation Cancer. It's about
fifteen hundred light years away, and at the center is
a white dwarf, right, And a white dwarf is something

(31:47):
which is a stellar remnant, which means that there used
to be a big, powerful star there which like our Sun,
burned for a long time billions of years and then
collapsed into a white dwarf. And around this white dwarf
they see a planet which is bigger than the white dwarf.
It is. It's about the size of Neptune. And these
white dwarfs, you know, they're about the size of a

(32:07):
small rocky planet like the white dwarf that's in the
future of our star will be about the volume of Earth,
and so this one we think is about the same size.
And so this Earth size star has a planet around
it that's the size of Neptune. Wow. So that's the
situation that we're asking about the star with the planet
that's bigger than flying around. And we've seen these, or

(32:28):
at least we've detected them using gravity, right, Yeah, exactly,
we have detected this one. It's sort of weird. We
don't really understand it. Like the Neptune size planet. It's
pretty close to the star. It's much closer to the
star than you would expect because that star must have
been like a red super giant at some point toasting
any planet that was near it. And so this planet

(32:48):
is like inside the radius of where the star should
have been, and so there must have been some crazy
gymnastics changing orbital radii after that happened. Oh, I see,
it's like us surviving the flare up of our sun, Like,
how could we still be there? Yeah, exactly, And so
probably this planet was somewhere further out and then something

(33:09):
crazy happened and it migrated closer. After the planet became
a white dwarf. So that's sort of like one survival
strategy is like maybe you don't go all the way
to Asia, but you just have like a really distant
orbit and then you come back in closer after then
you reconnect with your parents after life, move into the city,
and then you come back to the suburbs, you know,

(33:29):
when they're ready to retire. All right. So that's one
example that we've seen. Are there more? That's the only
one that we've seen. But you know, the fact that
we've seen this one means that it is possible to
survive your stars transition to white dwarf for neutron star
or even black hole, and so it's possible that there
are a lot of these things out there. It's probably

(33:50):
pretty common. Yeah. Cool. Well, I feel like we kind
of cheated a little bit, though, Daniel, because if I
hold you to the technical definition of a star to
say that they're as to be fusion in it, it
sort of becomes a very different question, right, Like, it
is possible for a white dwarf to have planets that
are bigger than it, but a white twarf doesn't have
fusion in it, that's right. A white dwarf doesn't have

(34:11):
fusion in it, and either does a neutron star. So
if I hold you to the definition of that we
post earlier about a star that it has to have
fusion in it, do you think it's possible still for
a fusing star to have a planet that's bigger than it. No,
I don't think so, because a fusing star would have
to be pretty big, right. You need to be like
a hundred times the massive Jupiter to get that fusion going,

(34:32):
and that would eventually be larger than Jupiter. And if
you had a planet in orbit that was that same size,
it would also turn into a star. And so what
you would get then is a binary star system. And
so it sort of comes just out of this definition
of what we call a star or a planet. If
you're gonna have something that's not fusing orbiting around someone
that is fusing, then the thing that's fusing has got

(34:53):
to have more mass otherwise it wouldn't be fusing. You
just confuse me on that last state. So it doesn't
sound possible, right, because if the star has to be fusing,
they're usually bigger than the largest planet that you can
have without fusion. That's what you're saying. You just defused
the suggestion I diffused this whole podcast apparently, but I

(35:16):
guess I mean, it's still possible. You just have to
invert what you call who's orbiting who? Like, you could
technically have a really heavy planet like it's it's it's
mostly iron or there's a lot of iron in it,
so it's very heavy. You could have a star maybe
that's bigger than it, but it could be that the
star is orbiting the planet. If the planet had more mass,

(35:39):
then you could say the star is orbiting the planet.
You know, in reality, it's not the case that the
planet is orbiting the star. The two things are sort
of orbiting each other, and the point that they orbit
is their center of mass. And typically a star is
much much heavier than the planet, and so the center
of mass is close to the center of the star.
But in a binary star system, the center of masses
between them, and so they're both orbiting this point that's

(36:00):
between the two of them. So where that point is
that their orbiting depends on the relative mass the two objects.
So yeah, in your fantasy system where you get to
build up whatever you like, you could make an enormous
iron planet that has more mass than the star it's using.
Of course, they would collapse into a black hole, but
the star would mostly be in orbit around Jorge's black

(36:21):
hole world. Alright, Well, it sounds like a lot of
this depends on the definition of a star. If we
let stars keep their honorific even after they stopped using,
then it's totally possible for a planet to be bigger
than a star, because then the star kind of shrinks
and becomes a door for a neutron star. Yeah, if
you think professor emeritus are still professors, then a white

(36:43):
divorf is still a star. But if not, then it's
technically possible but not likely. Yeah, but sort of just
by definition, because that's what we call a planet, and
that's what we call a star. Boy all right, well cool,
It sounds like the universe still has a lot of
surprises in store for us, maybe new and interesting configurations
that we didn't think we're possible. Yeah, and there are

(37:04):
lots of solar systems out there that we don't understand,
weird planets made out of styrofoam, and strange stars doing
things that we don't understand. So this is all based
on our current understanding of how planets and stars form
and what's going on inside them. But there are lots
of surprises out there in the universe, and the only
way to learn them is to look. And so we
should all be supporting astronomy and building more space telescopes

(37:27):
so that we can just sort of buy our way
into answers to these questions to revealing what's out there
in the universe. The only way is to look or
to go out there, which you should tell your kids
that's an option if they want to become astronauts and
they can get as far away from you as possible.
But nobody out there should take this podcast as advice
that you should get a red spot tattoo by yourself.

(37:49):
Sure there are people already out there, all right. Well,
thanks for joining us. We hope you enjoyed that. See
you next time. Yea, thanks for listening, and remember that
Daniel and Jorge Explain the Universe is a production of

(38:09):
I Heart Radio or more podcast for my Heart Radio,
visit the I Heart Radio Apple Apple Podcasts, or wherever
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