Has our solar system lost any planets?

Daniel and Jorge talk about how instabilities in the early solar system may have ejected an ancient ice giant.

See omnystudio.com/listener for privacy information.

Episode Transcript

Speaker 1 (00:08):
Hey, Daniel, what pets do you have these days?

Speaker 2 (00:10):
Oh, we just have our rescue dog, Peppito.

Speaker 1 (00:14):
Haven't you had other pets in the past, like rodents.

Speaker 2 (00:17):
We did have rats for a time, and we actually
had cats before that.

Speaker 1 (00:21):
Oh what happened Pepito?

Speaker 2 (00:23):
Ate them? No, we've never had a pet eat another pet.
We've only lost them to old age.

Speaker 1 (00:30):
Old age. All that dark chocolate and big goods in
your house just did them in with the heart attack.

Speaker 2 (00:37):
We don't feed dark chocolate at the dog, but everybody
does eat pretty well at our house.

Speaker 1 (00:42):
Unless you like white chocolate, then your starved.

Speaker 2 (00:44):
You know. If that's the reason you run away from home,
then maybe you never were really a white sun.

Speaker 1 (00:48):
Wait, if you don't like white chocolate, you're not a
white son. It sounds like a white lie.

Speaker 2 (00:53):
I have a dark secret. Hi.

Speaker 1 (01:10):
I am Poor hammy cartoonists and the author of Oliver's
Great Big Universe.

Speaker 2 (01:13):
Hi. I'm Daniel. I'm a particle physicist and a professor
at UC Irvine, and I will not waiver in my
campaign for dark.

Speaker 1 (01:21):
Chocolate, Dark chocolate, dark matter. You're just a very dark physicist.

Speaker 2 (01:27):
I'm trying to bring light to the world at the
same time as exposed all the dark secrets of the universe.

Speaker 1 (01:32):
Oh, you're trying to expose dark matter. I thought you
were all about letting the universe.

Speaker 2 (01:37):
Be Absolutely not. I do not believe in universe privacy.

Speaker 1 (01:40):
You're like the universe paparazzi.

Speaker 2 (01:42):
That's exactly right, except I'm not selling it to the
national inquirer. I'm just publishing papers.

Speaker 1 (01:47):
Well, your buyer is the cosmological inquirer, the human inquirer.

Speaker 2 (01:55):
Inquiring brains want to know.

Speaker 1 (01:57):
Yeah, do you stand outside their home, like, Hey, dark
matter over here, over here snapping pictures?

Speaker 2 (02:03):
If I knew where dark matter lived, I would definitely
go there with my dark matter camera.

Speaker 1 (02:06):
I thought dark matter was all around us. It lives
in us and within us. It surrounds us and binds
the galaxy together.

Speaker 2 (02:14):
You're absolutely right, it's everywhere. We just don't know how
to take a picture of it.

Speaker 1 (02:17):
But anyways, welcome to our podcast, Daniel and Jorge Explain
the Universe, a production of iHeartRadio.

Speaker 2 (02:22):
In which we join our inquiring minds with yours to
wonder together about the nature of the universe, to think
deeply about how everything comes together to make the cosmos
and the night sky that we appreciate to think about
how the tiniest little particles and the most massive black
holes shape the very world we live in, and whether
it has always looked this way.

Speaker 1 (02:43):
That's right. We satisfy our curiosity for stars and what
they're doing with their lives, and we take pictures and
also sound recordings of what's out there in the universe
and what's going on to maybe get a clue about
how it all works.

Speaker 2 (02:55):
We'd like to figure out the fundamental nature of the
universal laws that everything follows. But I also like to
know the story of the universe. What happened, How did
we end up where we are? How long have things
looked this way for? How long can we rely on
things to look this way? Do we live in a
momentary blip of the universe or is this a long
term trend?

Speaker 1 (03:15):
Yeah, looking at our past is a way to look
into our future. We can try to deduce. But the
rules of the universe are and what they might mean
for us in the deep future? What is going to
be the future of humanity here in our solar system?
Can we call this our home for the next few
billion years?

Speaker 2 (03:31):
Are you not planning to move out of that house
for a few billion years, so your kids can always
come home and their kids and their kids and their kids.

Speaker 1 (03:36):
Well, we're kind of just squatting in this solar system,
right like we just popped in here, started living here.
We didn't ask if anyone owned these planets. What if
the real owners come back when they they're like, what
is going on here? Call pest control.

Speaker 2 (03:50):
Don't we have some sort of like solar B and
B contract squatter rights. Maybe exactly after one hundred million years,
we're officially allowed to call ourselves the owners. It's a
good question how long things have looked this way. When
you look by the night guy, you expect to see
basically the same stars as you did a year ago,
and you know that you're looking at roughly the same
stars that Newton looked at, and the Egyptians looked at

(04:12):
and the Sumerians looked at thousands of years ago. But
the Solar system operates on a very different kind of
timescale than your life or even human civilization. And in
fast forward things don't seem so stable. They seem quite
chaotic and dynamic.

Speaker 1 (04:26):
Yeah, when we were all kids, we learned in school
about the different planets in our solar system and how
many of there are. And that's basically the same story
that our kids are learning in school as well, right, Like,
it hasn't really changed so much, except maybe for Pluto.

Speaker 2 (04:39):
We of course change what we mean by a planet
and make up new categories all the time. But you're right,
the stuff that's out there that we're seeing, whatever name
we give it, hasn't changed in our lifetime or in
our grandparents' lifetime.

Speaker 1 (04:52):
Yeah, And so I guess you kind of get the
sense that maybe it will never change, you know, you
sort of memorize these facts and these things and think
that maybe it's going to be like that forever. But actually,
if you look at the grand scale of the Solar
System and the universe in or galaxy, things are rapidly changing.
If you look at it from that point of view.

Speaker 2 (05:08):
If the Solar system changes, do you think we all
have to go back to elementary school to learn a
new mnemonic?

Speaker 1 (05:13):
Oh, there's a mnemonic. I didn't grow up yours. I
don't know what do you use? Obviously the mnemonic did
it work because you don't remember it.

Speaker 2 (05:24):
There's a lot of mnemonics to help you memorize the
order of the planets. One of them is my very
easy method just speeds up nothing. Mercury, Venus, Earth, Mars, Jupiter, Saturn, Urinus, Neptune.

Speaker 1 (05:36):
Whoa, that is so not kit friendly. How many kids
use methods and have methods?

Speaker 2 (05:42):
The history of them is actually really funny. There's ones
from the fifties that go like men very easily make jugs,
serve useful needs.

Speaker 1 (05:48):
Perhaps, Oh man, I wonder why we'd stopped using that one.

Speaker 2 (05:52):
And then a more recent one says, my very energetic
mother jumps skateboards under nana's patio.

Speaker 1 (05:58):
Oh, there you go. That's a pretty good one, and
true as well for some people.

Speaker 2 (06:01):
I'm sure my very educated mother just served us notches.

Speaker 1 (06:06):
Oh that's an even tastier.

Speaker 2 (06:07):
One, exactly. But the point is that though these things
have seemed stable for a long time, it's not necessarily
true that they always will be.

Speaker 1 (06:16):
Yeah, things are always changing, And in fact, you can
ask the question of whether our solar SYSM had more
planets in the past.

Speaker 2 (06:23):
It might be that the planets we know today are
not all the planets that have ever orbited our star.

Speaker 1 (06:29):
And if we had more, what happened to them? So
today on the podcast will be tackling the question has
our Solar System lost any planets? That just seems kind
of irresponsible there. How can you lose a whole planet?

Speaker 2 (06:47):
I mean I had it in my hands and then
I put my keys down and the last place.

Speaker 1 (06:51):
Remember having that planet was on top of the dog.

Speaker 2 (06:56):
Blamed the dog. Huh, that's the first thing.

Speaker 1 (06:58):
The dog ate my planet. Classic excuse.

Speaker 2 (07:02):
Or maybe it just went rogue because it needed to
find a white chocolate friendly Solar system. You know, maybe
it just didn't fit in here.

Speaker 1 (07:09):
It just rebelled against your tyranny of trying to dictate
what kind of chocolate people should eat or feel good
about eating. Doctor, Why.

Speaker 2 (07:17):
You call it tyranny, I call it wisdom. Let's call
the whole thing off.

Speaker 1 (07:22):
Yeah, that's what I'll tyrn say at two. Yeah, So
it's been an interesting story of the Solar System. You
got to wonder if maybef we had more than nine
or eight planets in the past. Well, we definitely had
more planets to pay in the past before Pluto got downgraded.
But that's a separate story and a separate reason.

Speaker 3 (07:37):
Right.

Speaker 2 (07:37):
Yeah, Pluto is still there, it's just not called the
planet anymore. It's called a dwarf planet.

Speaker 1 (07:42):
Right, But we can ask the question of whether our
solar system did really have other giant planets like Jupiter
or Mars or Venus, but maybe they decided they didn't
like it here.

Speaker 2 (07:51):
It's really fun to dig into the history of the
Solar system and understand how we got here, how it
might have been different, and give us a sense for
what other solar systems out there are likely to look like.

Speaker 1 (08:01):
So, as usual, we were wondering how many people had
thought about the question of whether our solar system lost
any planets, or at least misplaced them temporarily. Maybe, So,
as usual, Daniel went out there into the internet to
ask people has our Solar system lost any planets?

Speaker 2 (08:16):
Thanks very much to our group of volunteers. We greatly
appreciate them, but we also would like to add you
to their ranks. Please don't be shy write to me
two questions at Danielanjorge dot com.

Speaker 1 (08:27):
What do people get I know if they sign up.

Speaker 2 (08:29):
The satisfaction of hearing their voice on the podcast and
a weekly injection of hard physics questions.

Speaker 1 (08:35):
And also a monthly supply of white chocolate that gets
kicked out of your house.

Speaker 2 (08:39):
I will send you exactly zero grams of white chocolate.

Speaker 1 (08:44):
Well, think about it for a second. Do you think
our Solar System has lost any planets? Here's what people
had to say.

Speaker 3 (08:50):
I don't know if we can know for sure, maybe
by the orbits of current planets, but I'd have to
assume given the five billion years or so that or
some spent around, that at least one planet has come
in and been kicked out. But maybe it depends on
if we consider those objects planets.

Speaker 4 (09:04):
So I think that there have been planets knocked out
of the Solar System, especially since when the Solar System
was first created there would have been loads of rocks
flying around to form planets. So then there would have
been planets formed and then hit by maybe another planet
which knocked them out of the system.

Speaker 5 (09:20):
Nothing that I know of, they're all accounted for. Some
of them have lost the designation planet, like Pluto. I
think regularly objects get flung out of Solar systems due
to gravitational interactions with.

Speaker 2 (09:32):
Other own jobjects.

Speaker 5 (09:33):
So I can imagine Jupiter getting tired of somesome little
planet and flinging it out. Maybe when we were forming
all the planets performing them. Some of them were close
to the Sun and got gabbled up. I'm curious to
know if there's any record of planets that we're here
in now or not.

Speaker 6 (09:47):
Apart from the reclassification of Pluto as a dwarf planet,
meaning that we've effectively lost one planet, I have heard
rumors about a tenth planet, which would now be a
ninth planet, that potentially all within our inn a soda
system and then could have collided with Earth and created
the Moon and then spun off out into an orbit
way out in our outer Soder system. Other than that,

(10:10):
I'm unaware of any lost planets.

Speaker 1 (10:12):
Interesting answers. It seems to be all over the place.
Some people say yes, some people say no, not really,
some people say poor Pluto.

Speaker 2 (10:21):
There does generally seem to be an appreciation of the
fact that the Solar System might not have always been
an orderly, stately placed that there might have been primordial chaos.

Speaker 1 (10:31):
That's right. It was a big party here in the
Solar System, where we're kind of in the after party
of the Solar System, right.

Speaker 2 (10:37):
We're waking up the next morning going, man, what happened?
And has anybody seen a dog?

Speaker 1 (10:43):
Yeah? Well, why am I waking up next to Venus here?
How did that happen?

Speaker 2 (10:47):
And why is the hot tub filled with white chocolate?

Speaker 1 (10:50):
Yeah, so let's start with the basics, Daniel, is it
even possible for Solar system to lose the planet? I
thought that, you know, once you form, things are kind
of stuck to you gravitationally in orbits, or that at
least that it's hard to escape the gravitational field of
like Sun or all these planets. Wouldn't they either fall

(11:11):
in or go into a stable orbit.

Speaker 2 (11:13):
I think the key idea is the word you used, form, Like,
when do you consider the Solar System to have formed?
The Solar system? Formation is a slow and gradual, constant process.
It's basically always changing. And so you can go all
the way back to the very beginning of the Solar
system to understand the chaos of that formation and understand

(11:33):
that that formation is a constant process, that things are
always potentially bumping into each other and disturbing each other.

Speaker 1 (11:40):
Wait, are you saying that if we leave the window
open for the fact that maybe the Solar System is
still forming, does that mean we technically haven't lost any planets? Like?
Can I use it in my real life.

Speaker 2 (11:51):
No, it just means that during the formation, planets could
form and be lost. There is no final form to
the Solar System. It's a constantly evolving thing. It's not
like at some point somebody says, Okay, the Solar System
is finished, let's package it and ship it and move
on to the next project. I see.

Speaker 1 (12:06):
It's like a Pokemon, is what you're saying. It's always evolving,
it's looking for its final form.

Speaker 2 (12:12):
I don't know enough about Pokemon to know whether that
analogy holds, So I'm just gonna.

Speaker 1 (12:15):
Trust you on that. I don't know either. To be honest,
I just heard final form and it made me think
of Pokemon.

Speaker 2 (12:21):
Well, it's sort of in the same way that animals
never have a final form. Evolution is a constant process.
Things are always changing in response to the environment.

Speaker 1 (12:30):
Except for crocodiles and sharks, they're pretty settled there.

Speaker 2 (12:33):
In the plateau. Yeah that's true.

Speaker 1 (12:35):
Yeah, but let me take us through to some of
the early history of the Solar System. How do we
get planets in the first place.

Speaker 2 (12:40):
So planetary formation is a super fascinating topic, and it
helps us understand like the formation of the Solar System
as a whole. Remember that the Solar system forms from
the collapse of a huge cloud of like gas and dust.
It's mostly hydrogen, which is made in the Big Bang,
and it's also interspersed with a bunch of other heavier
stuff that's made from other solarss where the stars have

(13:01):
already fused heavier elements out of that hydrogen. So you
have this big cloud of mostly hydrogen with a few
heavier bits in it, and it collapses into stars. You
don't just get one solar system. You typically get several
made at the same time. In one of these stellar nurseries,
we have a big blob of this stuff and it
collapses and most of the stuff goes into the center
to make a star, like ninety nine percent of the
stuff goes in to make the star. But you typically

(13:23):
have a disc of gas and dust that's orbiting that star.
It's spinning too fast to collapse in the way the
Moon is orbiting the Earth without falling into the Earth,
and so you get this protoplanetary disc around this new star,
and that disc then coalesces into larger stuff. Gravity is
doing the work there to pull the gas and dust

(13:44):
in the disk into heavier things. And where you have
like little spots of iron or little spots of heavier metals,
those things will use their gravity to form larger objects.

Speaker 1 (13:54):
But I think the Solar System formed into a disc first,
and then the star kind of ignited.

Speaker 2 (13:59):
Right. Moment of ignition depends a little bit on the
mass of the star. I mean, in some cases you
don't even get ignition if there isn't enough mass there.
You have like a subcritical brown dwarf, but it's definitely
collapsing into a disc as it forms. Right, a big
amorphous blob is going to collapse, and it's going to
collapse into a disc shape because of its angular rotation.
So the two things can sort of happen simultaneously, And
when ignition happens depends on the mass of the star.

Speaker 1 (14:22):
Right, And there's also kind of an intermediate step there
where the disc kind of turns into rings for a while.
Right before you get the planets.

Speaker 2 (14:30):
Exactly, you get the seeding of structure and they pull
together into larger and larger objects, and rings are basically
just clusters of larger objects. So you get these gaps emerging,
and then you get those things formed together into planets
or not, depending on the tidal forces. A large object
can form and sort of gather up a lot of
the gas and dust near it, and then it can

(14:51):
also distort the other stuff nearby, preventing it from forming.
So it's a bit of a chaotic process in the beginning,
and it also depends a little bit on your distance
from the st There's a point it's called the snow line,
after which water tends to be ice, tends to be
a solid, and before which it tends to be vapor.
Like if you're close enough to the star, it's warm
enough that the water is vapor and if you're further

(15:11):
from that point, the water is frozen. That helps form
giant planets. So you tend to have these large planets
with ice and rock seeding structure on the outer part
of the Solar System past the snow line, and then
less ice so smaller planets before the snow line in
the inner Solar system.

Speaker 1 (15:27):
Right in the inner Solar system, you get all the
rocky planets.

Speaker 2 (15:29):
Right exactly because the gas there is blown away by
the radiation from the ignition of the Sun. In the
very beginning of the Solar system, the Sun is pumping
out a huge amount of ultraviolet, very high energy photons,
which tends to blast the inner planets clean, which is why,
like our initial atmosphere on Earth was blown off by
this stellar wind in the very early years of the

(15:50):
Solar System. So you get the rocky planets in the core,
and then you get the gas and ice giants out
past the snow line.

Speaker 1 (15:56):
Right, And I think the process is like, yep, these
rings kind of like Saturn has rings right now, and
the rings eventually little by little collapse into planets or
first planet tesimals first, right.

Speaker 2 (16:07):
Yeah, planet testimals a super fun word. They sound like
many cute little planets where they're basically like building blocks
of planets, and they don't always form, right, which is
why you have like the asteroid belt and the Kuiper Belt.
It depends on the tidal forces of the nearby stuff,
so it's not happening in isolation. This is complicated interplay
between all of the objects.

Speaker 1 (16:26):
Right, But it's kind of a bit of a runaway process.
Like once you seed a planet or once more you know,
some planet tesimals moush together, then that becomes kind of
a center of gravity and more and more stuff falls
into it, and that's kind of how you get a planet.

Speaker 2 (16:39):
Right, Yeah, that's kind of how you get a planet exactly.
And in this initial picture, everything forms very orderly, like
they tend to be mostly in the same plane, and
it be mostly circular because you have this big disc
as we say, that collapses in the rings and then
planet ismals and then planets. But once you have these
large objects formed with their own significant gravity, then they

(16:59):
can start to hug on each other pretty hard, and
you can get instabilities, you can get chaos, you can
get resonances, and that's how planets can migrate, and they
can tug on each other and you might even lose.

Speaker 1 (17:10):
One, right, because I guess there's no guarantee that your
orbit is going to be stable. I mean, it's such
a complex and you know, there's so many things moving
around that there's no guarantee that even if you're orbiting
around the Sun, you're going to be there forever, because
something else might come around and knock you off your
orbit or pull you away from your orbit, right.

Speaker 2 (17:28):
And that can be things from outside the Solar System,
like a passing star can nudge something and perturb the
otherwise stable orbits of the Solar System like even just
a little nudge from a star that's coming nearby, it's
not like it has to pass right through the Solar
system can cause a cascade effect of instabilities. But also,
just like the planetismals and the Kuyper Belt or the

(17:48):
asteroid belt can tug on stuff, and enough of that
happening can cause things to go.

Speaker 1 (17:52):
Wonky and wonky. They might have gone in our Solar system,
perhaps wonky enough to lose a couple of planets here
and there. Let's get into that idea and whether or
not we did misplace a couple of planets in our history.
But first let's take a quick break. All right, we're

(18:20):
talking about whether our Solar system has lost any planets.
I feel like that sounds very irresponsible of the Solar system.
Can would you say, like, have any planets escaped our
Solar system?

Speaker 2 (18:30):
Maybe we've grown up and graduated planets. They're like off
into the universe living their best lives.

Speaker 1 (18:36):
Yeah, there you go. You don't want it to live
at home forever.

Speaker 2 (18:39):
Exactly when your kid graduates and goes to college, you
don't consider that you've lost them.

Speaker 1 (18:43):
Yeah, there you go. So have we shepherded planets out
into the larger cosmos is the question of the day.

Speaker 2 (18:50):
Yeah, in this scenario, they would like come back and
visit with their own little moons or something that we
could coop over. Oh look, how cute.

Speaker 1 (18:56):
Oh yeah, yeah, except we already turned their bedroom into
like a workout room or a craft and so now
there's no room for them.

Speaker 2 (19:02):
Sorry about that, your orbits being used?

Speaker 1 (19:05):
Yes, sorry, you'll have to airbnb a nearby apartment or something.
But yeah, So it's possible in the early chaos of
a solar system to lose a planet, right because things
aren't quite settled. Even though we're all bound gravitationally to
the central star, things can get kind of wonky and
maybe wank enough to actually fling a planet out into space.

Speaker 2 (19:27):
Exactly and when this happens is sort of the most
recent question people have been struggling with. There's a classic
model of the formation of the Solar system and how
the planets move around that we'll talk about. It's called
the Nice model because it was developed by researchers in Nice, France,
that has a bit of an issue with when exactly
all this chaos happened that might be solved by a

(19:48):
more recent model with a different picture for how these
instabilities might have been triggered.

Speaker 1 (19:53):
Well, now, I wonder if some listeners out there might
be confused about how you can lose a planet, Like,
there isn't that much around us, right in terms of
other Solar systems or any large stars or galaxy or
you know, very heavy objects. So even if something gets
flung into space, wouldn't it eventually come back?

Speaker 2 (20:09):
You're right that space near the Solar System is pretty empty.
I mean, the closest star is light years away, and
so its gravitational pull is pretty weak. But you can
still have an escape velocity. If you throw something hard
enough off the Earth, it will leave the Earth and
never return. If you throw something out of the Solar
System with enough velocity enough to escape the gravitational well

(20:30):
of the Solar System, then it will not return.

Speaker 1 (20:33):
Mmmmm yeah, I think we talked about that in an
other episode. It's kind of a weird kind of math, right,
Like you need to have enough velocity so that as
you get further and further, the pool of gravity pulling
your back gets weaker and weaker, and so actually you
kind of outrun the pool of gravity exactly.

Speaker 2 (20:49):
It seemed confusing because you know that gravity has an
infinite extent, like no matter how far away you are,
and the Sun is always pulling on you. But just
because there are infinite number of contributions doesn't mean it
adds up to an infinite force. It's just like any
integral or converging series. It can add to a finite
amount of energy. So as long as you have more
energy than the sum of all the tugs the Sun

(21:09):
will ever pull on you, you can escape the Solar System.
So if, for example, you have a planet that gets
a push from another planet and gets flung out into
the deep dark space, it might never return.

Speaker 1 (21:20):
Yeah, I guess sort of like we've done with spacecraft
that we sent out into space right like it left
Earth eventually, it was going so fast it left the
gravity of Earth and through the gravity of maybe other planets.
And now we have some that are going out of
the Solar System exactly.

Speaker 2 (21:33):
That can be a little bit more complicated because they
can have rockets and they can use gravitational assists from
other planets, but the principle is the same. Like Voyager
and Pioneer, they have enough velocity that they're leaving the
Solar System without any more rocket burns or gravitational assists.
It's definitely possible to leave home.

Speaker 1 (21:50):
All right. Well, now the question here today is have
we actually lost any planets? Did the Solar System have
more planets than the eight that we have now, and
have we missed or have any left home?

Speaker 2 (22:01):
But if you look at the pattern of the planets
that we have now and you try to tell a
story about how we got there, it's a hard thing
to do without another planet. The patterns that we see
in the eccentricities of the planets and the structure of
the Kuiper Belt and the asteroids is much easier to
explain if there was at one point another ice giant
like Neptune that was flung out of the Solar System.

Speaker 1 (22:24):
I mean, you sort of look at how the planets
are moving now and their orbits, and you basically hit
the rewind button, kind of like you use math and
a computer to backtrack what the Solar System was doing
millions and millions of years ago, and you're saying that
it doesn't make sense or what.

Speaker 2 (22:40):
It actually usually works. In the forward direction, like you
start from the Protosolar System and try to evolve forward
and see if it matches what we see today. Conceptually
it's the same as backtracking, but the way the simulations
actually work is forwards. You know, we model physics equations
forwards in time, and we try to see if we
can get to the current Solar system and explain everything
we see. And what we find is that doesn't really

(23:02):
work without another planet.

Speaker 1 (23:03):
But are there like a million things that could have
happened in between? Absolutely, how do you make it match
what we have now? Like, what do you start with?
And if you can make it match, how do you
know it's not just you're making it error?

Speaker 2 (23:16):
Absolutely, it's not definitive, right, it's statistical. It's totally possible
that our solar system could have arisen without another planet.
But it's just a question of what's more likely. Like
when you run the simulations of our solar system, how
many times do you get to something like what we
have now with a lost planet and without a lost planet?
And so is it just easier to make this arrangement

(23:38):
with a lost planet or without? It's totally possible to
do it without, but it's just less likely. It happens
in fewer of those simulations.

Speaker 1 (23:45):
And by like what we have now, you don't mean
like exactly what we have now just kind of like
sort of like what we have now.

Speaker 2 (23:51):
If you run enough simulations, you can get essentially a
sense for what's more likely and what's less likely under
various hypotheses. And if you have another planet in your system,
then you get more simulations that are similar to ours. Yeah,
so more like probability gets clustered in the kind of
arrangement that we have now. And the cool thing about
that is that it tells the story. You can look
at those simulations and you can see, oh, what happened

(24:12):
in the inner what happened in the early days of
the Solar System? How did this happen?

Speaker 1 (24:17):
Okay, So then scientists have been running these simulations and
it's kind of hard to get what we have now
without some mystery planet that moved away from the Solar System.
How did scientists think that happen?

Speaker 2 (24:28):
So the original models, called the NIE model, basically blames
it on the Kuiper Belt. So in the original Solar system,
you have Jupiter, Saturn, Urinus, Neptune, all formed in very nice,
neat circular orbits like we talked about, in fairly closely
spaced to each other. But then you have these planetismals
out in the Kuiper Belt that haven't formed into planets,
but they're tugging on Neptune, they're tugging on Urinus, they're

(24:50):
tugging on Saturn, and they get pulled into the inner
Solar System. And when that happens, these big planets get
pushed out a little bit, and then the planetismal falls
further into the Solar System until it reaches Jupiter, and
then Jupiter actually pushes it back out and Jupiter gets
pushed in. The effect of these little planetismals, these little
tugs is to pull out Neptune, Saturn, Uriness and to
push Jupiter in a little bit. So they're disturbing the

(25:12):
Solar System. And you might think, well, what can one
little rock do? And the key is that there's lots
of these rocks, and so over time this can really
have an effect on the orbit of the.

Speaker 1 (25:21):
Planets because we know that at around that space, that
ring of the Solar System, you had a lot of
big rocks, and maybe you have a lot of big
rocks right now exactly.

Speaker 2 (25:30):
The Kuiper Belt is huge. There could be like trillions
of objects out there. We think today it's the source
of comets that fall into the Solar System, the short
period comets. There's an even bigger blob of stuff out
in the Oort Cloud. It might be the source of
long term comments, but it's an enormous, massive stuff out there,
and each of those little bits as they interact with
the Solar System can give a little tug. You have

(25:51):
these nice circular orbits that were formed initially, but now
they're getting perturbed by these tugs from all these rocks
in the Kuiper Belt. Mmm.

Speaker 1 (25:59):
Okay, scientist think that maybe these rocks from the Kuyper
berl maybe cause some planet that we had before to
exit the Solar System.

Speaker 2 (26:06):
Well, essentially leads to some instability because you're pushing Jupiter in,
you're pushing Neptune, Saturn, Urinus out, and then those planets
start to interact like they used to be in a nice,
happy orbit. But now you get instabilities and resonances from
those planets themselves. Jupiter starts to drift inwards, and then
Saturn pulls on Jupiter, and Jupiter pulls on Saturn. You
start to get a regular orbits, and Saturn actually pulling

(26:28):
on Jupiter is what saves it. Saturn pulls on Jupiter
and changes its direction so it migrates back out away
from the Sun. Without Saturn there, it might have been
that Jupiter would have just like plummeted into the Sun
thanks to the influence of the Kuiper Belt.

Speaker 1 (26:41):
You know, it all sounds kind of complicated, So I
wonder why do scientist think that making it more complicated
by adding another planet makes it easier to understand? Like
it's all really complex dynamics, right, Like, so then how
does adding another planet make it easier to predict, like
what's the missing thing that we currently have? That missing
planet would help with.

Speaker 2 (27:01):
Some of the features of our Solar system that we
see today are difficult to explain without adding another planet,
you know, like the irregular orbits of Jupiter and Saturn.
They're not perfect circles. There are sort of ellipses that
have like a five percent eccentricity, And there's this structure
in the material of the Kuiper Belt. A lot of
it seems to have been lost, and a lot of
the rest of it is in resonance with Neptune. And

(27:23):
these things aren't like smoking guns that say like, oh, look,
there has to be another planet here. But when you
run the simulations, you get these kind of features more
often when you add another planet. If you put another
ice giant in around the size of Neptune between Saturn
and Urinus, then the story you get when you run
these simulations more closely resembles the Solar System we have today.

Speaker 1 (27:44):
And I guess scientists just throw this mystery planet in
and all kinds of velocities in all kinds of sizes,
and you sort of see overall, like, hey, it does
kind of shape the Solar System more into what we
have now.

Speaker 2 (27:56):
Yeah. Another way to say it is like they were
running the simulations with just the planets and they were
noticing that they pretty rarely ended up describing the situation
that we see today, Like it was very unlikely to
get Jupiter and Saturn to have these eccentricities, and to
have these asteroid belts along Jupiter's orbit, the Trojans and
the Greek camp of asteroid belts, those things were pretty

(28:16):
rare to get in a Solar system without this additional planet.
But when you put that new planet in, it started
to be less unlikely. It started to be like, oh,
this kind of thing happens pretty frequently, and so it's
just a question of like, how do you explain it.
It's not the only possible story, right, Maybe there are
two other planets. Maybe something else happened that could explain this,
but it does make the story more likely.

Speaker 1 (28:36):
Maybe the planet had a big fight with Jupiter, stormed
out of the house with all their bags, left to
go live with their aunt or their niece. This is
the Nice model, right, This.

Speaker 2 (28:45):
Is the Nie model exactly. They went to live with
their aunt in France.

Speaker 1 (28:49):
So then that's one model you're seeing. One model says
and maybe it was all these big rocks from the
Kuiper Belt that maybe caused a lot of instability out
there in the icy planets, and then maybe it caused
a planet that we used to have to flyway exactly.

Speaker 2 (29:01):
And one of the nice things about this model until
recently was that it lined up with other pieces of
evidence for when this instability happened. And in the Nice model,
this happens like about a billion years after the Solar
system is formed, so you get like the ignition of
the Sun, you get the formation and the gas planets
and the rocky planets. Things cycle around for a little while,
and it takes time for the Kuyper Belt to sort

(29:23):
of drive this because these are tiny little rocks. This
sort of lines up with another piece of evidence from
looking at our Moon. Our moon is a great record
for impacts in the Solar System, like when rocks have
been raining down in the inner Solar System. And when
the astronauts in the Apollo mission went to the Moon,
they gathered a bunch of samples to study, like the
craters and the impacts, to try to get a sense
for like what is the history of the Solar system.

(29:45):
One of them been sort of more or less impacts
when it's been like bad weather and good weather, and
for a long time, there was this evidence for what
we call a late heavy bombardment, that there's this period
of billion years after the Solar system formed when a
lot of rocks were raining down in the the Solar system,
And that kind of lines up with the story of
the nice model that like all these rocks from the
Kuiper Belt were coming in and making trouble and maybe

(30:07):
also some of them were landing on the Moon. So
that was sort of a nice story.

Speaker 1 (30:11):
For a while, meaning like there's a lot of activity
from the Kuiper Bell which may have contributed to us
kicking out an icy planet.

Speaker 2 (30:19):
Exactly, and until a few years ago that all seemed
to kind of hang together. But then there was a
reanalysis of this data from the Moon and it turns
out that it may have been a mistake.

Speaker 1 (30:28):
Well, we lost the theory the dog at theory.

Speaker 2 (30:31):
It turns out of the way the astronauts gathered the
data that may have basically only collected data from one
big impact. So what we thought was a bunch of
impacts that all happened at the same time, like three
and a half billion years ago, might have actually just
been one big impact that the astronauts gathered from. So
it could have been like essentially just a statistical anomaly

(30:51):
in the data that made it look like there was
really bad weather for like a few hundred million years
three and a half billion years ago, But it was
really just one bad day that the astronauts happened to
elect data from.

Speaker 1 (31:00):
Wait what, so we didn't we just had one sample.
We didn't take some samples from all over the Moon
or analyze the craters visually through telescopes, so.

Speaker 2 (31:10):
We don't have samples from all over the Moon. And
they definitely collected a bunch of samples from different locations,
and that's why they thought maybe this was like a
fair sample from everywhere on the Moon. But a reanalysis
of it suggests that a single impact site Imbrium might
be responsible for basically all of the evidence that the
astronauts gathered. I don't know if the astronauts are being
lazy and not following instructions or if it was not

(31:32):
a well organized study, but more recent analysis suggests there
may have been no late heavy bombardment. There may just
be like a gradual decline in the number of impacts
over time.

Speaker 1 (31:42):
And so maybe this idea that kyper Berl maybe caused
us to lose an icy planet maybe didn't really happen,
or could it still have happened without this late heavy bombardment.

Speaker 2 (31:50):
This really causes us to doubt that model. And there's
been another lingering problem with this Nie model that has
never really been answered, which is why the terrestrial planets
kind of survived it. If you have these gas giants
doing this dance a billion years after the Solar System
is formed, when Earth and Mars are also already formed,
then how did the Earth and Mars and Venus survive

(32:12):
all these gravitational tugs. If Jupiter comes into the inner
Solar System basically turns around at the asteroid belt, how
does Mars stay in orbit? How does Earth stay where
it is? So one concerned about the Nie model has
always been how did the terrestrial planets not get disrupted
by the giants. So now there's a news story about
when this instability happened and what the cause was that

(32:33):
doesn't rely on this late heavy bombardment and places the
blame on the instability somewhere else.

Speaker 1 (32:39):
But I guess why do we assume that there was
an instability because we think that maybe we did lose
a planet.

Speaker 2 (32:44):
Because we can't explain the orbits and the eccentricities and
the structure of the Kyper Belt without some kind of
motion of these planets. We know the planets moved around,
we know there was interaction. We know that they did
not form in the order that we see them today.

Speaker 1 (32:57):
All right, So then what's this new model? Not so nice,
the less nice model exactly, not the Nie model, the
Nephew model.

Speaker 2 (33:07):
The nibbling model. So this model is called the rebound model,
and it suggests that this instability happened much much earlier. Actually,
while the rocky planets were forming, or maybe even before
they formed, that was basically an early instability.

Speaker 1 (33:21):
Well, that's a big difference in timescale, But don't your
simulations as a solar system sort of help you pinpoint
when it happened.

Speaker 2 (33:28):
It turns out that the simulations can come in to
either an early instability or a later instability, like the
instability for in the nice model, like a billion years
after the formation of the Solar system, can explain the
orbits that we got if there, in fact was a
bunch of interactions from the Kuiper Belt. But you could
also have an instability much earlier on that could reproduce
the orbits and the eccentricities that we see today.

Speaker 1 (33:51):
All right, so then what does this model say? What
happened according to this model? So this is called the
rebound model, and essentially the instability trigger here, the thing
that can kicked off all this bouncing around was the
interaction of the planets with this gas. Imagine the formation
of the Solar system, as we talked about earlier. You
have these planets forming and they're pulling their stuff together,

(34:11):
but you still have something of a protoplanetary disk. You
still have a bunch of gas sort of in between
the planets. Now we don't have that today, and the
reason is that the Sun has effectively blown all that out.
As the Sun triggered and the ignited and its radiation
grew and grew, it blew out all that gas from
the Solar system. So in the first ten million years
or so, that gas disc is sort of moving out

(34:33):
through the Solar System and it affects the orbits of
those planets. If the planets are passing through the gas,
it slows them down, and if that gas is getting
pushed out by the star, it actually carries those planets
with them a little bit. So as this gas disc
is getting pushed out of the Solar System, it passes
through all of these orbits and it gives them all
a little tweak. So this rebound model suggests that the

(34:55):
interaction of the planets with this gas disc as it's
getting blown out of the Solar System and trigger these
same instabilities and can explain all the features we see
in the Solar system today. But couldn't you kind of
make the same argument as before with the other model,
like Wooden Huck How is it then that we'd the
Earth and Mars and Venus have such a nice even
orbits if we were disturbed and blown out.

Speaker 2 (35:16):
Yeah, great question. It's because this happened much earlier, and
so essentially this happened before those terrestrial planets even form.
The terrestrial planets we think formed after the gas giants.
M how come the gas giants have a lot of
advantages over the rocky planets in the inner solar systems.
Number one, there's ice out there, which is like a
solid that can help seed the formation of planets. To
there's a lot more gas out there because the Sun

(35:38):
hasn't gobbled it up, and you don't have this proto
star messing everything up and heating things. So it's much colder,
which makes it easier for gravity to pull things together.
So the outer Solar system is a much easier place
to form planets. So we think that gas planets formed
before the gas disc actually evaporated, sometimes in like the
two to ten million year range. But rocky planets take
longer because the inner Solar system is much hotter and messier,

(36:01):
is less gas available to form planets and no ice whatsoever,
So those take like thirty to one hundred million years
to form.

Speaker 1 (36:07):
Okay, so I think what you're saying is that this
new model, this rebound model, is saying that we kicked
off a gassy icy planet a long time ago, before
we even had Earth and Venus and Mars and the
rocky planets inside, and that it was due to a
lot of this gas being blown out of the center.

Speaker 2 (36:23):
Exactly, and as the sort of inner radius of that
gas passes through these early ice giants and gas planets,
it triggered that instability. They did their crazy dance with
Jupiter moving in and the other planets moving out and
ejected an ice giant planet that left the Solar System,
And all that happened even before the Earth and Mars
were formed, so they didn't mess up the formation of

(36:44):
the Earth and Mars because it was already done by then.

Speaker 1 (36:47):
All right, well, those are both great stories. Now the
question is can we see the planet that we kicked out?
Are there lonely dejected planets floating out there in space
that we can see and maybe identify and track to
perhaps our Solar System? So let's stick into that, But
first let's take another quick break. All right, we're asking

(37:17):
the question, did our Solar system lose a planet or
I guess shepherd it out peacefully into the cosmos.

Speaker 2 (37:26):
That's the niser model. If anything happened, it sounds like
we can blame it on the gas giants, Like we
weren't even around when all of this went down. It's
just the drama we heard about when we showed up.
Oh I see.

Speaker 1 (37:36):
It's like, yeah, it's like you're the younger sibling and
there's all this drama before you were even born.

Speaker 2 (37:41):
Exactly, Like why is everybody so mad? And who is
this missing sibling everybody's talking about you never met?

Speaker 1 (37:47):
Oh wow, this is just god, this is just turned
into a Korean drama. I feel like super complicated.

Speaker 2 (37:53):
But it is sort of the story. I mean, what
we learned from this is that these unstable giant planets,
the ice giants and the gas basically sculpted the inner
Solar system. I mean, it didn't disrupt the already formed
Earth in Mars and Venus, but it created the gravitational
context for them to form and probably changed how they
did form, the way younger siblings arrive, and family dramas

(38:15):
that have existed for years.

Speaker 1 (38:17):
But again, I guess this is just kind of a model, right,
or sort of a guess to maybe explain some of
what we see. We don't quite know for sure, right.

Speaker 2 (38:24):
We definitely don't know for sure. We've quibbled before about
what a guess means. Scientifically, I think we have a model,
we have some evidence for it, we're never exactly sure.
I mean, we have not identified a planet and said
that's our lost planet. It's just easier to explain what
we see in the universe when you add this to
the story. But that happens for lots of things, like
we don't witness the early years of the Earth's formation,

(38:47):
but we have a pretty detailed story about the formation
of the Earth based on the patterns of evidence that
we see in the rocks beneath our feet. And so
that's a big part of science, is developing a story
to explain the data, even if you don't directly witness
all of those events, right, right.

Speaker 1 (39:01):
But I guess what I'm trying to say is that
we had a pretty good story that seemed to check
out and make sense before, but then it turned out
to be not quite correct.

Speaker 2 (39:09):
Yeah, that's true. These stories are always evolving in they're
getting better, Like we like the nice model, but there
were some dangling questions about how the Earth and Mars
survived it. And now we like this new model, the
rebound model, But there's always going to be dangling questions,
and somebody's going to come along with a better model
and maybe tell a different story in five years. It's
a constantly evolving story.

Speaker 1 (39:27):
I guess. But I wonder if maybe like a smoking
gun or something to definitely be able to say, like, hey,
there used to be a planet here in the Solar
System that we don't have anymore, is to actually maybe
see this planet that we kicked out or that left
Liane its own out there in space. Isn't it possible
that we could see a planet like and track it
to our Solar system out there beyond our Solar System?

Speaker 2 (39:49):
I suppose it's possible, But we're talking about events that
happened four billion or more years ago, so that planet
is pretty far gone by now. If it did leave,
we can do so of more indirect discoveries, though. We
can look out and say, are there any rogue planets?
If this is happening in our Solar System, it should
be happening in other Solar systems. Shouldn't space be filled

(40:10):
with these ejected ice giants when it happened in other families,
not just ours, and we can go and look for those.

Speaker 1 (40:17):
Yeah, we had a whole episode on rogue planets. There
might be a billions of them out there right exactly.

Speaker 2 (40:22):
We can actually spot some of these using what we
call micro lensing. If one of these planets out there
floating between stars, passes in front of a star, like
a little eclipse, then it'll blink out and actually change
the way that light bends around the planet. So we
can use these micro lensing techniques to try to spot them.
We also have infrared telescopes like the Wise telescope that
can try to directly image them. These planets don't glow

(40:45):
in the visible light, but they do have some temperature,
and everything with the temperature glows in some frequency. These
would low in infrared, and so it's possible to see them.
So we have seen a bunch of these rogue planets,
and so we can estimate that there's like one of
these things for every star in the galaxy.

Speaker 1 (41:03):
Well, I mean there's ae hundred billion of them right
in our galaxy.

Speaker 2 (41:06):
Exactly, and we've only spotted a few of them. And
so the calculation of like how many there are is
very uncertain. There's a huge extrapolation there, which means a
huge uncertainty, but we know it's a pretty big number.
We know it's not just like ten in the galaxy.
There's lots of these things out there, and that lends
credence to this story that, like when solar systems form,
there's a period of instability when big planets can get

(41:29):
thrown out.

Speaker 1 (41:30):
So we've actually seen these like if you look at
a picture of the nice guy in the infrared, you
can see these thoughts moving across the sky.

Speaker 2 (41:36):
We can actually see these planets, and we have seen
with direct imaging some of these rogue planets. Again, not
very many, and so we're extrapolating from a handful up
to a big number. We've definitely seen non zero and
very recently James Webb saw some really crazy stuff out there.
They found these Jupiter mass binary objects. They call them jumbos.
These are pairs of planets floating out there in the

(41:58):
galaxy with no star near by.

Speaker 1 (42:00):
Wait what, well, first of all, James Webb, you mean
the telescope right, Yes, not James web from Erie, Pennsylvania.

Speaker 2 (42:07):
We did not exhume the previous NASA administrator and ask
him to look at the night sky and then write
down with what he said. Though that would make a
pretty cool graphic novel. Yeah.

Speaker 1 (42:16):
Yeah, Well, I just don't want to assume everyone knows
what James Webb is.

Speaker 2 (42:19):
No, you're exactly right. The James web Space Telescope a
very powerful device that we launched a couple of years
ago and is an infrared telescope capable of seeing things
that are pretty cold. Spotted forty two pairs of jumbos.

Speaker 1 (42:32):
WHOA and so is there? Is it weird that they're
in pairs? Or does it feel normal that they're in pairs? Meaning?
Does that mean that they were ejective from their Solar
system in pairs like they left together.

Speaker 2 (42:43):
It's a great question. We don't know the answer to that.
Simulations suggest that it's unlikely for big planets to leave
the Solar system together, right. They would have to be
like bound together and then leave together, which means that
their fragile orbits around each other would have survived very
chaotic period. Seems very unlikely. So these are rogue planets

(43:03):
that are out there without their star, but they don't
seem to have been ejected from solar systems. So it
just sort of like adds to the murkiness of what's
going on with rogue planets.

Speaker 1 (43:12):
Whoa wait, wait, so maybe they were ejected and then
they met up with another jumbo out there in space.

Speaker 2 (43:18):
We actually don't have a great story to explain how
these even exist. Like that seems very unlikely for all
these jupiters to like start dancing around each other just
in space.

Speaker 1 (43:27):
Well, maybe they have like a planet dating app or something.

Speaker 2 (43:31):
Maybe they're speed dating there, or they're square dancing or something.
They're all changing partners.

Speaker 1 (43:35):
That's right, they have jumpler under phones.

Speaker 2 (43:39):
Some people suggested maybe they just formed independently, like you
had a solar system and it didn't have enough stuff
to actually have a star. You just got a couple
of jupiters. But we think that there's like a minimum
amount of mass you need to get like your own
solar system, otherwise you just get slurped up in like
a neighboring solar system when that huge stellar nurseries breaking
up into chunks that form solar systems. So we think

(44:00):
that these things are probably too small to have seeded
their own structure and be their own solar system. We
don't think they could have been ejected from other solar systems.
So it's something of a question of where these came from,
you know, just to paint the picture that like, there's
a lot we still don't know. There's a lot of
guessing going on.

Speaker 1 (44:16):
But I guess if they didn't come from a solar system,
then that doesn't really tell us anything about this idea
of how often solar systems kick out planets exactly.

Speaker 2 (44:25):
But it adds doubt to the argument that because we
see a bunch of rogue planets out there that suggest
that planets are lost from solar systems, because there are
planets out there whose formations we just don't understand and
we think don't come from having been lost by a
solar system.

Speaker 1 (44:39):
All right, well, I guess to answer the question of
the episode, has our solar system lost any planets? The
answer is maybe, we guess. So we guess. Maybe we
used to have a brother, an older brother, but now
nobody likes to talk about him or her, and it's
very awkward. It makes all the models break, all the

(45:00):
pets are uncomfortable.

Speaker 2 (45:01):
Almost all of the models we use to explain the
solar system do have an additional planet. It's not absolutely required.
It's possible to explain the Solar System without an additional
planet that got ejected during one of these early instabilities.
But it just makes the story come together more crisply.
It makes our Solar system seem less unlikely.

Speaker 1 (45:19):
Now, how does this match up with I know there
are scientists that think that we have maybe a ninth
planet in our Solar system. We just can see it
planet X, right.

Speaker 2 (45:28):
Yeah, there are some people who look at like gravitational
aberrations in the orbits of our planet to see if
there's something else out there tugging on it. But there's
no conclusive evidence for that. It's like very very gentle,
and there's a lot of disagreement about whether it's just
noise or has other explanations.

Speaker 1 (45:42):
It's almost sort of the same, right, They use simulations
and try to figure out what would best explain what
we have now.

Speaker 2 (45:48):
Yeah, exactly, but the data there are not conclusive.

Speaker 1 (45:50):
All right. Well, another interesting lesson to keep track of
your planets. Don't lose them because once they're gone, they're
gone forever.

Speaker 2 (45:57):
And try to understand where you came from and what's
your contact is, what happened before you showed up on
the scene.

Speaker 1 (46:03):
Yeah, well, not that we have much influence over what happens,
but it's interesting to think about what might happen in
the future, Like, is it possible that the Earth might
get kicked out of the Solar System right in the
future exactly?

Speaker 2 (46:13):
The difference between these models tells a very different story.
If it really is lots of gentle tugs from planet Esimals, well,
that could still happen in the future. We have the
Orc Cloud, we have the Kuyper Belt. There's still tugging
going on. But if it was something that only happened
in the very early formation of the Solar System itself,
as this gas was pushed out, that's not something that's
likely to be reproduced, and so that level of instability

(46:35):
is probably not going to happen again.

Speaker 1 (46:37):
Now, Daniel, if we did have an icy gas planet before,
but it was filled with white chocolate, are you happy
that it's gone or are you sad that we don't
have anymore?

Speaker 2 (46:45):
No, it's bittersweet. I wish it's the best.

Speaker 1 (46:48):
No, it's not bitter, it's sweet. It's white chocolate. That's
the whole point of white chocolate.

Speaker 2 (46:53):
It's oversweetened. That's the problem.

Speaker 1 (46:55):
But maybe it'd be good for you because it would
make all the white chocolate lovers go to this planet
and leave ours exactly.

Speaker 2 (47:00):
Let's arrange transit to the frozen planet of white chocolate.

Speaker 1 (47:04):
Well call it white.

Speaker 2 (47:07):
No son of mine?

Speaker 1 (47:10):
All right, Well, we hope you enjoyed that. Thanks for
joining us, See you next time.

Speaker 2 (47:19):
For more science and curiosity, come find us on social media,
where we answer questions and post videos. We're on Twitter, Discord, Instant,
and now TikTok. Thanks for listening, and remember that Daniel
and Jorge Explain the Universe is a production of iHeartRadio.
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or wherever you listen to your favorite shows.