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[Music]
hello everyone thank you for being here
um now this is a story all about how our
view of the solar system got flipped
turned upside down and if you listen to
my talk for just under an hour I'll tell
you how we think we know how many
planets there
are
so let's get the obvious thing out of
the way first right if you're an old
person like me you grew up in a solar
system that was composed of nine planets
um and of course there were ways to
memorize the names of the planets and uh
English is not my first language but
when I researched what these were I
encountered this data so my very
educated mother just served as nine
pizzas is a popular one but you get
several variations of this the mother
could be eager could be energetic uh and
the food could be different
um but this was a way of memorizing it
now in German there was just one and it
was very
popular plan now obviously both of these
no longer work in a solar system of of
eight planets but also if we want to
literary go to literary criticism um I
don't like the fact that it's always the
father in the German version um and I
don't like that it's just about random
food uh in the English version so um if
we update this we get an English version
that was actually recommended by
astronomers so it's kind of official uh
my very educated mother just served as
nachos I mean I like the fact that it's
now definitely the educated mother but
still it's about random food and the
food is not even healthy um so uh when I
was writing my own astronomy books I
came up with this
one so now we have mom involved and it's
it's about
astronomy
so thank you for coming to my talk about
the solar
system but let's take a step back and
wonder what even is a planet how do we
recognize them and how were they
recognized Through the Ages because this
will help us in uh understanding the
number and and the identity of the
planets that were known historically
here I have a picture of the night sky
taken not quite at random this is about
the time of the next Congress so uh you
can take notes and in one year come to
me and uh tell me if this was correct
but it was a stellarium so I trust the
software um we see two of today's
planets and one of the classical planets
up there in the picture so to the upper
left you have Mars hanging out in the
constellation of cancer you have the
moon between Castor and Pollock in the
Gemini um constellation and then uh
towards the right you have Jupiter uh
between the horns of Taurus um and now I
will advance by I think two months uh so
here comes another view of the sky
and now for the same field of stars the
planets have moved right Mars has moved
from cancer into Gemini it's now where
the moon was before Jupiter is closer to
one of the horns of Taurus and the Moon
is nowhere to be seen um and this is how
we can understand the classical meaning
of the word Planet which comes from the
Greek uh Wanderer uh so it's anything
that moves across the sky in a way
different from the Stars obviously
everything moves right you look at the
sky and the scar and the stars move and
the Sun comes up and down but if
something moves and it moves differently
to the stars or relative to the stars
then that is what you may call a planet
and this is how in Antiquity and through
the Middle Ages the classical planets
were defined it was anything in the sky
that didn't move like the stars so it's
logical to assume that the Moon is one
of them we just saw how it moves across
the sky then we have Mercury and Venus
which are close to the sun the sun
itself was considered a planet because
it moves across the sky in a way
different from the stars and Mars
Jupiter and Saturn were known now this
is the this is the way that people
understood the sky and kind of thought
of our surroundings in the universe in
those days um we have to clear up one
thing and give some credit to people in
Antiquity they did not actually believe
that the Earth was flat this happened a
few Millennia ago but mostly um the idea
that people up until the Middle Ages uh
thought that the Earth was flat is
itself the myth of the Flat Earth which
this true was thought up in the 19th
century by United States American uh
authors just people who were just for a
living making [ __ ] up um and they said
you know what up until Columbus people
thought the Earth was flat and he was
going to sail around it and nobody
believed him until he did so this was
basically to make Columbus look good uh
but it was completely made up um and of
course both of these facts are different
from the fact that today um there are
simply crazy people uh who would tell
you that the Earth is flat and who
refute any kind of
[Music]
evidence and if you want to get a
clearer overview of this I just
recommend these three Wikipedia articles
because they clear up the situation uh
quite nicely
now the people knew that the Earth was a
sphere but they did believe that the
Earth was in the center of everything
and from today's standpoints it's easy
to say well Ah that's arrogant everyone
thought we were in the center of
everything and how could we think that
the universe is so big but think of the
way that we perceive the sky and the
stars and anything that moves we are
standing here and everything seems to
move around us uh let's look at it from
a different way if I were to ask anyone
of you to please describe faces of
people maybe faces of people around you
you would look around and say okay well
there's some long hair there's a beard
there's large eyes and there is brown
eyes you would probably not in the first
moment think of describing your own face
because because you are inside your own
face and you rarely ever see it unless
you you take some measures and you take
care to look at your face and try to
describe it um and in a similar way
people who were standing on the earth
and looking up didn't think about uh
what the position of Earth was relative
to everything else that they saw they
just thought that this was where things
were happening and it was happening all
around them but still this view of the
universe evolved um and they try to make
mathematical descriptions of it um this
is a this is a manuscript script um from
from medieval times and uh I I can't
read the script either but what you can
see is that mathematically or by the
physics that were involved it was a very
very complex explanation of how things
were supposed to move and these circles
on top of circles they're called
epicycles and this was a mathematical
tool that today actually isn't is pretty
much not used for anything in physics
but back then you needed epicycles to
try and explain what you see in the sky
because if the Earth is at the center of
everything and the Sun and the Moon and
all the other things are moving around
us then only by employing these very
complicated epicycles can you try and
explain what you see but of course they
have no physical equivalent in the real
world these epicycles don't exist they
were just uh it was necessary to make
them up to try and explain what people
were seeing and then came the first
Revolution um of our understanding of
the solar system and uh it was about
what's in the center so now comes the
the shift in perspective when it was
realized uh that in fact it's not the
Earth in the center of the solar system
but it's actually the Sun and this is
the meaning of the word heliocentric um
and with the sun in the center uh we
then get a different number of planets
for a start because the moon also was no
longer considered a planet equal to the
others because the mo the moon moves
around the earth and from the beginning
of the 17th century it was also clear
that um also Jupiter had moons you can
see that uh towards the right there's
five little balls
um and that's actually Jupiter and its
four largest
moons um and so it was realized that our
moon is something different other
planets also have moons and that there
are six planets in the solar system and
our Earth is one of them now this
started around uh the middle of the 16th
century the idea was uh first published
by Copernicus uh actually from his
deathbed which is how afraid he was of
the church he said um if I publish this
right before I die then you can't hurt
me anymore which actually says something
about the church right because the
church would say oh no no after death we
still own you but apparently he didn't
think
so um then came Kepler's laws which were
some of the first uh steps towards
understanding mathematically what was
going on Kepler's laws were basically um
the updated version of the epicycles
thing it was now it was now a clear a
much simpler way of mathematically
describing what was going on and then
came Newton's laws and with
understanding gravitation in the the way
that were that was described by Newton's
Laws uh things really moved into a
modern understanding of the solar system
basically the same kind of mathematics
that still mostly allows us uh to steer
space probes even if you send a space
probe to Jupiter and it's going to orbit
one of the moons that's basically the
same mathematics that you had back then
you need to take into account special
effects which they didn't have but
mostly the mathematics was already there
in that time um but things were changing
mostly because uh instruments were
getting better telescopes were getting
better more people were having
telescopes and looking at the sky
Galileo galile was among the very first
to look at the sky through a telescope
uh he pioneered a lot of stuff but more
and more people were doing it and one of
the things that was part of this
Evolution was that in
1781 um William hashel and his sister
karolina hashel discovered the planet
Uranus um and this updated the solar
system to say okay look um everything we
talked about before it's still true but
now there's another one at the end and
this was fine and mathematically and and
from a world view uh point of view it
didn't wasn't much of a problem you just
have the solar system like you had
before and you add Uranus way out on the
outside and it was only possible to see
Uranus once the um once the telescopes
were good enough um I once saw Uranus
through a pair of binoculars uh which is
pretty cool um but of course back in the
day when you only had uh the naked eye
uh it wasn't possible to observe it so
in the age of opes Uranus sort of
entered the picture um and that now
comes the second revolution the things
were happening in the inner solar system
up until now we had the sun in the
middle and a number of planets and some
of the planets have moons but that's all
there's going on they also saw comets
sometimes but I'm not going to talk
about comets today they just come and go
basically um but now something is going
to happen in the inner solar system and
this is the discovery of what is today
known as the asteroids and let's let's
take a a look back I zoomed in the
picture that we just saw so this is from
the from about
1790 um when we were looking at the
solar system and saying okay we've got
all these planets in the middle and
Uranus on the outside but look how
between Mars and Jupiter on the outside
so it's zoomed now in in in this in this
uh crop of the picture Jupiter is is the
outermost thing you see there is a
dotted line like something's missing
like something should be there but isn't
and there's a very good reason why the
astronomers making these charts were
were leaving this dotted line in this
place um and we're going to talk about
why they thought that something was
missing there that should be noted in a
chart um so yeah um seven planets were
known but an eighth was was somehow
necessary they felt and the reason why
they felt that this was necessary is uh
called it taas Bodi law I learned it in
in astronomy we called it the tius B
gazettes it's actually more of a it's
actually more of a play on numbers it's
not not really a physical
law it's more of a thing that happened
to work um this is a modern formulation
of it this is not how it was
historically described but it's it's
kind of easy to understand so if you
look at the distance of some planet from
the Star expressed in and that's in the
denominator of the first of of the left
side 1 Au which is one astronomical unit
okay so imagine something is one
astronomical unit from the Sun this is
the distance of the Earth from the sun
150 million km
and we have a number of planets in the
solar system so you take 40% of that and
you add another 30% of that multip it by
two to the exponent of N and for n you
put in these numbers in the series minus
infinity is the first one okay that's
that's a bit clutched in there and then
it goes from 0o to six which looks kind
of natural and if you do that um you get
a progression of what distance a planet
should be from the sun should be in the
sense of just if we put in the numbers
into this thing right someone uh taas
and Bodi among them but not the only
ones made up this number series and if
you put in the right numbers you almost
exactly get the distances of the planets
as they really are from the Sun so this
looked like magic or as they as people
back then saw it a sort of divine rule
God made the solar system to make sense
and we can see it in that thing because
if we put in the right numbers it's
almost exactly where the planets are in
the solar system and the dotted line
that we just saw um is where where I
blotted out something for the number
three that's where they thought
something was missing that's where they
thought well if the taas bod law works
this well for all the planets but we
haven't seen something for n equals 3
well then it must be there we just need
to look better and that's what they
tried to do uh around the year 1800
shortly before an eminent group of
astronomers met at conferences um and
they call themselves in German the him
poit like they were going to observe the
sky and look for order and make things
right um and they exchanged notes on who
should be part of the him poit uh they
they uh they uh they divided the sky
into sections and say okay you guys from
France you're going to look in that
direction and you guys from Britain
you're going to look in that direction
and we we're going to look really hard
in that direction and when we find
something we write to all the others and
this was the himit polit and they were
going to solve uh the mystery of the
missing planet for TAA bod law n equals
3 and um josepe PTI from Italy was not
part of that group
um he was going to be included
apparently he didn't get the memo uh I
mean the the mail was not as reliable as
kaaos post back in those days I guess um
so in 1801 actually on the night of the
1st of January the very first day of
1801 um he discovered the missing thing
and the missing thing was series a body
that a body that we now call series
which is between Mars and Jupiter um it
plays a a big role in the expanse so you
all know about it uh Siri station um and
so PTI discovered it and everything
seemed to make perfect sense right you
can now put in all these numbers into
the taas bod law and you get where
series is and the number fits really
well this this thing was discovered
right where it was supposed to be so for
a very happy moment the solar system
seemed to make perfect sense we have the
taas body we have the mathematical
description it all looks great and this
state of everything being in perfect
order lasted for about a month or
two in 1801 that's when the solar system
made sense for a bit um because then
more discoveries were made and they were
in orbits very close to where series was
found people looked in the same in the
same plane and in a similar general
direction and they found more bodies uh
so now besides series we have Palace
Juno and Vesta they were discovered um
in the five or six years following ps's
discovery of Series so now we're up to
11 and it's not so great anymore because
at the N equals 3 place in the taas bod
law we now have four bodies okay that's
a little weird maybe it's not quite as
weird as having none at all but still it
wasn't really making that much sense
anymore for another 40 years um until
the next thing
happened uh oh yeah uh so now here we
have another description uh from that
time this is I think from the
1830s um and here you can see that Sirus
Palace U Vesta and Juno were included
among the planets right they were same
as the other one the same uh status as
Jupiter and mercury and all the
classical ones and we have another fun
thing which you can see down towards the
right here Uranus is most is called
hersel why hersel or Uranus well when um
William herel uh he was a guy from
Germany but he lived in uh he lived in
Britain and he was working as an
astronomer for the king of Britain when
he discovered Uranus he was going to
call it after his
employer uh so it was going to be called
gum zidus I think Latin for George star
because George III was the king and why
shouldn't we name something in the sky
for all eternity for centuries to come
after that guy who's paying me right
now and people from France said no
way and people from the United States
where this chart comes from said we're
just going to call it after the guy who
found it because that's that's still
better um and from Germany actually came
actually from the border guy I think uh
came the um proposal of calling it
Uranus which is you know a mythical
figure from Greek mythology and
everything was pretty much back in order
but in some historical uh sources you
can find it called hersel for that
reason because it was not going to be
called George
um okay and then happen then came the
discovery of Neptune what which was then
the 12th Planet because we already had
the four between Mars and Jupiter and
now Neptune now Neptune was found not by
looking through a telescope in the first
place but by looking very closely at the
way that the other bodies in the solar
system moved especially Uranus because
if you look at Uranus then you find that
um its orbit is somehow perturbed if you
apply Newton's laws to describe the
orbit of Uranus then something doesn't
fit quite well and you can use
mathematics and try and find out okay
maybe there's another planet further out
in the solar system and its gravity is
somehow perturbing it's it's disturbing
the orbit of Uranus it's kind of tugging
at the thing and you could use that to
predict where this further out Planet
should be and that's what Urban leier
which is the guy in the picture a French
mathematician did and actually about the
same time John couch Adams a British
mathematician was doing the same thing
and now these two are having the problem
that we also heard about in the talk
before the funding agencies were not
going to help them out because because
these two pretty much unknown
mathematicians were writing to the
directors of the astronomical societies
and of the biggest observatories in
their countries and they were saying
look I did some calculations and here's
all 30 pages of them um I think for
varier was actually 100 Pages Adams
managed to do it in 30 pages but he was
being less precise um and they said
please go look at at somewhere over
there I swear it's it's going to be
there and the directors of the
observatories were saying no we got
nebulas to look at we've got other
things we've got to follow we're not
going to look for some Planet because
some mathematician comes and and makes a
suggestion um and John couch Adams was
kind of he was kind of intimidated and
he said oh well okay maybe maybe later
maybe I I'll write again sometime and he
was forgotten but obier said well if
you're not going to help me I'm going to
write to other people he wrote to
Britain which is how they realized that
wait we we also had one of our guys
making the same prediction right if it's
one of our guys what would you do what
what did we do about him oh we ignored
him right well now we got this French
guyy and he's saying the same thing
let's make it so that the French don't
discover the planet first let's listen
to our guy
now let's listen to our guy and find the
planet so that's how they scrambled uh
in Britain in France nothing was
happening and so aan varier said I'm
through with talking to directors of any
place I'm just going to go and write uh
write letters to astronomers who are
actually using the instruments who are
out there every night in the cold
looking through these things and he
wrote his proposal to um Yan go Gala who
had um who was observing in Berlin and
actually the same night that Gala
received the letter from OB varier he
looked at it and what varier and the
Brits could not know was that in Berlin
at that time in the hands of Gala was
one of the most one of the most
upto-date and precise Star Charts uh
that anyone had at that time so all Gala
had to do was look at the respective
part of the sky and look at all the
stars that he was seeing and find the
one that's not in his list because he's
got the best list of stars in that
Century um and so he looked and he
actually found Neptune he found the
planet that was going to be called
Neptune um and that was a great
discovery and then um the French were
very happy and the Brits were very angry
um and the Germans were all like you
know where the telescope was standing
that made this
discovery and that's how Neptune was
discovered and actually Neptune now
doesn't fit with the taas bod law at all
so the taas bod law is now basically uh
forgotten and
with this with this squabble came
actually a kind of War of the press so
in the British press they were saying Oh
look The French they stole our planet we
had this guy Adams and we listened to
him so well and we were going to find
his Planet uh but the French they stole
it they they kind of found it first and
then they they collaborated with the
Germans and it's an outrage and while
all this was going on in Europe there
was an astronomer um in the United
States um Benjamin Pierce and I mean
look at him how how much fun do you
think it would be to get into an
argument with this guy
exactly he he was a very very difficult
character but he was right in one thing
he noticed that in past observations
Neptune had already been seen but nobody
had recognized that it was Neptune this
is in astronomy this is called a
precovery when something that's going to
be discovered later is already in your
notes there was no photography back then
so it wasn't photographs um it's already
in someone's notes but they didn't
recognize it it was actually recorded as
some star some star among many others
but the Americans figured this out and
the Americans in that time
scientifically astronomically they were
nobodies they they had just gotten
started with their country what what
were they going to tell us but Benjamin
Pierce was actually right about having a
better calculation of Uranus's orbit and
the reason why his calculation sorry
Neptune's orbit the reason why his
calculations of Neptune's orbit were
much better than all the European
calculations was that he wasn't bound to
the taas bod law he had recognized that
we don't need to follow this there is no
rule made by God that the solar system
should follow so the numbers come out
nice he just said look I have better
data I have a better calculation um of
Neptune and the Europeans they were all
fighting with each other right the Brits
and the French and the Germans and they
were looking over and saying who are you
guys we don't believe your calculations
but still the Americans were right now
immediately after that so Neptune was
the 12th Planet immediately in the
months following more asteroids were
discovered and it was getting ridiculous
it was getting out of hand because
between Mars and Jupiter we now had tens
dozens of new asteroids popping up all
the time I found here a really great
example of uh an illustrated astronomy
sort of school book uh sort of thing you
know for the interested public and
saying telling you what the solar system
looks like and you see that we've got a
bunch of asteroids there um inserted
into the inner solar system and you can
actually find different different
editions of this online and if you look
at the fifth edition from 1849 you see
that it says well how many planets are
are there in the solar system well it's
16 you see eight there's the eight big
ones and then there's eight other small
ones and just a couple of years later
15th Edition In 1855 they're already up
to well he doesn't even do the sum
anymore right he doesn't write it's 31
planets he just write okay we got eight
large ones and then 23 and they kind of
updated uh they kind of updated the
chart on the bottom left there but they
didn't do a new drawing the the big
drawing they were not going to change
that to insert new asteroids every month
uh that was tedious um so that was when
the solar system started to actually
make less and less sense because what
what what were we finding all the time
um and here we've got two uh
Publications from Germany and they
basically uh admitted that okay it's
getting ridiculous we can't even assign
symbols anymore so classically all the
planets had these symbols you probably
recognize um probably recognize the
Venus system which which is also uh you
know for femininity um and then on the
right you see okay we're just giving up
on these symbols we're just putting
numbers now because
there's so many we can't be making up
symbols all the time um and that's when
the inner solar system got really
crowded basically and now varier enters
the stage once again and he's got an
idea because at one time when he was
discovering Neptune he was looking at
the orbit of Uranus and he was seeing
perturbations and he was using clever
mathematics to try and find out if there
was another planet tugging there well
there were also perturbations of the
orbit of Mercury right next to the sun
right in the middle of the solar system
and these perturbations you could also
apply the same kind of mathematics and
said well I know how to do that one um
so he hypothesized that a planet called
Vulcan was actually further inside from
the orbit of mercury even closer to the
Sun than Mercury there was this planet
called Vulcan is what he says what he
proposed and made calculations about and
made predictions where you're going to
find it now there are very few charts of
the solar system this is actually the
only one I found um that include Vulcan
because it was always kind of a theor
itical idea but for a time there between
1860 and 1880 many people were taking
this Vulcan idea quite seriously
because well he was the guy who found
Neptune who are we to argue that okay
but this Vulcan thing it probably
doesn't exist people were going out and
making many observations but if you try
to find a Planet that's very close to
the Sun you have to look very close to
the Sun and how do you do that well you
have to have a filter but then the Sun
is going to be too bright anyway you can
look
during a solar eclipse right so the Moon
is blocking out the Sun and at that
moment you can look quite next to the
Sun to see if a planet is there or you
can look for transits where this
hypothetical Vulcan Planet would wander
across the dis of the Sun and quite
quickly but you also have to have
filters and you have to have make very
good um well photography still wasn't
around in that time uh so it was quite
difficult many people were looking for
Vulcan um and during the Great American
eclipse of uh 1878 we see here a picture
of well-known astronomer Maria Mitchell
from the United States uh setting up
observatories there um some people said
they found it most people said they
didn't and there were many many
astronomers looking and then shortly
after the one guy that had announced the
discovery with the most auana varier was
already dead and the one guy that had
said I definitely saw Vulcan he also
died um so now nobody was really alive
anymore to really say that Vulcan
definitely does exist and at that kind
at that time everybody just sort of
forgot about it oh remember when we
thought there was a planet Vulcan next
let's not talk about it it didn't turn
out to be true but there still were the
perturbations of the orbit of mercury
you couldn't discuss those away and they
were solved only a few decades later
when Albert Einstein um developed the
general theory of relativity and there
it is explained that because the because
the orbit of mercury is eccentric it
sometimes goes closer to the Sun and
sometimes a little further away um that
the bending of SpaceTime actually gives
gives a little kick to the orbit of
mercury and this is the explanation and
not the fact that there would be another
planet called Vulcan but I won't give
aan varier too hard a time about that uh
I mean 150 years before Einstein he
really just couldn't have
known okay
um it's getting more and more ridiculous
I mean the books are now saying yeah the
books are mostly saying we've got eight
big planets and then there's a whole
bunch of other thing they they were
getting to be called the minor planets
uh in German Klein plantin um and these
minor planets were not really counted
with the big ones but now another big
one appeared on the scene so now we're
up to well more than a thousand but also
Pluto um and Pluto was discovered by um
Clyde Tomball and actually this was also
part of a campaign a dedicated campaign
of looking for another body Beyond
Neptune where people were saying ah look
the orbits look a little weird and let's
look very close clely at the sky with
the best modern telescopes that we have
right now we might find another planet
and it's actually uh Clyde Tomball
getting the credit for discovering Pluto
is one of the rare instances where you
have um a big Observatory and and an
important director who has all the money
and all the connections just hiring some
young guy I think he was 27 at the time
yo you you go look at these you go look
at these thousands of photographic
plates for 2 years and then tell me if
you find a planet or something um and
cly Tombo actually found Pluto and he
actually got the credit for it so if you
now look up ask anyone who discovered
Pluto well it was Clyde tomar and not
just the director of The Observatory
which is a nice break because normally
as we know um it's the big shots getting
the credit for the discoveries when
others were actually doing the work um
and now we move into the solar system as
we knew it during like I said our
childhood of us old people um but now
the asteroids are excluded and it's hard
to find out when exactly this happened
the best I found out is that during the
the 1950s and
1960s astronomers and and connected
people just kind of stopped referring to
the minor planets as planets they sort
of silently changed the definition to
say okay we got stuff orbiting the
planets these are moons and uh we've got
stuff orbiting the Sun these are planets
but then we've also got this whole bunch
of little stuff and we don't want to
count them all so we just take the
biggest nine and these are now the
planets of the solar system and they
included Pluto this was the view of the
solar system up until the beginning of
the '90s um but actually for Pluto Pluto
had a had a great start people thought
it was was a large planet and everything
uh but already in the end of the 1970s
um so about 30 years after the discovery
of Pluto it was starting to go downhill
H 40 years it was starting to go
downhill for Pluto um and we now come to
the third Revolution uh when things were
happening in the outer solar system that
people were not expecting um and this is
about trans neptun objects Kyper Bel
objects and extreme trans neptunian
objects um well the name says it a trans
neptunian object is just something
that's behind Neptune um and Kyper Bel
objects are named after Gad Kyer which
was um an astronomer uh originally from
the Netherlands and he kind of proposed
that there would be a number of bodies
Beyond Neptune and this is now called
the Kyper belt um and the way down for
Pluto began when its large Moon Sharon
was discovered so Sharon the largest
moon of um the largest moon of uh Pluto
was discovered and you see well it's not
Digital Data obviously it's a
photographic plate so this is chemical
data if you will um but you see the
Bulge there on the left right it's it's
not it's not a it's not a round ball
it's got this thing towards the top and
that's actually the large moon of Pluto
that is Sharon um and when Sharon was
discovered in in 1978 it immediately
allowed a calculation very easily to
determine the mass of Pluto so once you
see something orbiting something else
it's very easy to determine the mass of
the thing that is being orbited um and
for Pluto it turned out that it was
really small way smaller than Mercury um
and really the tiniest planet and
definitely not one of the big ones
actually and this was when it started to
go down for Pluto I see a plushy Pluto
there in the first row and it's being
yeah it's being
comforted
I also have one of these and I like it
very much um but more stuff was being
found and now we can think back to the
years after 1801 right between 1801 and
the 1850s when more and more things were
being found between Mars and Jupiter and
people were saying oh God we got to stop
we got to start counting right are we up
to 16 or is it 18 planets already um and
this was starting again in the outer
solar system this is pictures of the
discovery of 1992 qb1 which was actually
just a technical designation that
encodes when exactly this object was
found um but it came to be known as qb1
that was kind of its nickname and it was
the first trans neptunian object after
Pluto and Sharon to be discovered and so
people were saying uhoh there's more
stuff in the outer solar system than we
knew about um and more and more objects
were being found I think uh by the early
2000s we were up to hundreds uh hundreds
of trans neptunian objects and then
there were some which became really
prominent because they were really weird
this is a discovery picture this is a a
Hubble Telescope picture of sedna and
sedna has a truly extreme orbit if you
look at the little teeny tiny uh rest of
the solar system towards the right there
that's Pluto way on the outside and
setna has got this extremely eccentric
orbit and it takes tens of thousands of
years to go around the Sun and people
were saying well if we got that kind of
stuff out there what else could it be
that we have missed so far that is also
part of of the solar system um and one
of the most prominent ones was Aris now
obviously at a chaos event hail Aris um
it was kind of important and and it was
also named kind of on purpose after the
the goddess of strife and Discord um
because it kind of got the fight really
started now arys was discovered um also
in the outer solar system not quite as
far out um as sna was but it turned out
arys was almost but not quite larger
than Pluto but it was definit a good bit
heavier than Pluto again because there
is a moon and it's possible to observe
the moon going around arys uh the Moon
is called dysnomia and uh well it helps
you determine the mass of arys and it's
definitely heavier than Pluto so now
people were saying oh God not this
again remember 150 years ago when we had
to introduce the 10th and 11th and 12th
and so on planet we're not falling into
that trap again no sir and this is the
context of the 2006 decision of the
international as astronomical Union to
say look we've got the eight planets and
they're the big ones you know them um
but we also have these dwarf planets now
because we keep finding all of these
objects in the outer solar system um and
these objects are all um out there and
we don't want to start introducing the
10th and 11th and 12th Planet because
okay ays would definitely have to be a
planet if Pluto keeps being a planet but
then what about sedna quaa haruma Mak
some of these are too small some of
those are on the on the brink
what what is even a planet anymore and
that's why they said okay we introduce
new rules um you have to be orbiting the
Sun you have to be not a moon and you
have to have cleared your immediate
surroundings you have to be kind of the
only planet on the Block um in order to
be counted as a full planet and this
obviously also um discounted uh anything
in the asteroid belt because the
asteroid belt is just full of stuff um
but siries actually got made into a
dwarf planet so siries is now the new
brother of Pluto um and they're both
dwarf planets and a number of other
objects are as well and this is kind of
the new order that the solar system has
found right we got the eight Big Planets
uh we got a lot of other stuff most much
of it is in the asteroid belt there's
also some stuff um out Beyond Neptune
but these are not the real planets there
dwarf planets or asteroids or some
things we haven't mentioned centaurs or
comets you know little stuff um but of
course astronomy keeps evolving and this
guy uh Mike Brown he calls himself the
Pluto killer that was his Twitter handle
I think most of the time um he wrote
this book how I killed Pluto he's not
even a member of the international
astronomical union he didn't even take
part in the vote but he still got the
international astronomical Union to vote
Pluto out so he's being kind of smug
about it um and he was he was involved
in the discovery of sedna and Aris and
many of these outer solar system bodies
which which is his claim to saying well
Pluto had it coming it's just not the
only body out there and I helped
determine that and then he said what if
we had another planet out
there he called it Planet 9 and it was
all over the news 10 years ago um and
let's look at his proposal of Planet N9
because he's proposing something way
bigger um than Pluto or ays or the other
bodies out there he's proposing
something along the lines of a body
between the size of the Earth and Uranus
mostly um and the idea behind his and
and some collaborators theories of
Planet 99 this is a theory um some
astronomers follow it many don't um it's
not actually oh yeah I forgot um it's
not the same as Planet X now Planet X
was something something of a meme in the
astronomy world in the uh between the
1950s and the 1980s where people were
saying I think there's something out
there between Neptune not talking about
Pluto I'm talking about something big
and this was called Planet X now this
book um is actually a fantastic book and
if you enjoy this talk you will
definitely uh enjoy this book The Hunt
for Planet X um which is about the
history of astronomy and and how we
understand the solar system but this
again is also different from the
crackpot conspiracy theory something is
going to slam into the Earth and there's
going to be an apocalypse or whatever um
kind of thing that is also sometimes
called Planet X um so now we have three
different things right we have the
historical Planet X that people
sometimes suspected back in in uh past
decades we have the crackpot conspiracy
Theory Planet X which definitely doesn't
exist and we have Planet 9 which is what
Mike Brown the Pluto killer is talking
about and I I think this picture sums it
up uh in quite the best way now these
are all extreme TNS so all of the orbits
that you see there are the orbits of
objects that are out Beyond Neptune but
their orbits are extreme in a way that
um they are very elongated they go very
far out from the Sun they are very
inclined right they're not in the same
uh plane as most of the other planets
but they go a totally different way um
and what brown and and his collaborators
are saying it look at how they're all
clustered they all kind of point in
mostly the same direction is what he's
saying and the reason for that has to be
a really big planet which is way out
Beyond Neptune and it's perturbing all
these right it's it's kicking all these
little bodies out into their extreme
orbits and just the fact that their
orbits look like that is evidence for
the fact that this planet should be
there we just haven't found it yet um
and like I said he he makes some some
good arguments other people have good
counterarguments saying for example that
um well we should have seen it by now
there's a lot of observational programs
going on looking at the outer solar
system and even infrared cameras looking
for very dark bodies and we have never
found anything that big I mean if it's
something between the size of Earth and
Uranus we must have seen it by now
another counterargument to this planet 9
hypothesis is to say um yeah you found
all these extreme TNS but there's
probably a bunch more and this is
probably a selection effect it's
probably an effect of where you were
looking at the time that you were
discovering them if you look that way
and you find I don't know 10 objects
whose orbits look weird maybe if you
looked that way you would have found
another 10 objects with equally weird
orbits but they could be equally
distributed throughout space and this is
the counter argument to say that well
you have selection effects you were just
looking you were looking mostly in one
way now Mike Brown was say that this is
not the case but this is kind of where
the debate stands and it's kind of
stalled Planet the idea of Planet 9 made
a big splash 10 years ago and it hasn't
really progressed very far all the
arguments are on the table most
astronomers don't think um that they are
convincing and so this is where the
solar system is um we've got this handy
little diagram of in the talk before we
heard that astronomers are really great
at naming things here you see that
astronomers are also very good at
classifying things uh in a very clear to
understand way um and if Planet 9 or
Planet X were to exist then of course
the blue planets box uh would have to
extend downwards towards the trans
neptunian objects but exclude the dwarf
planets I bet that someone uh on
Wikipedia already has an SVG file ready
uh for when that
happens and this is kind of where we
stand um the solar system is uh is a
weird place there's a lot going on we've
got a bunch of large bodies we've got
many many many small bodies it's uh
sometimes hard to categorize them and
it's sometimes hard to exclude the way
that we humans like to think about the
universe from what is actually out there
and happening and this is how throughout
uh throughout the decades and the
centuries we've had the seven classical
planets but then the first Revolution
happened and we realized that the sun
was actually in the center so that gave
us six planets until Uranus was
discovered and we got seven planets um
right before the taas B law was
gloriously completed in 1801 to include
series and basically 5 minutes later um
more bodies were being discovered in the
inner solar system uh Neptune brought
the number up to 12 in the middle of the
19th century um and by the time Pluto
was found the whole asteroids thing was
getting way out of hand so uh now that
the third revolution of finding more
stuff in the solar system is in full
swing we are back to eight for the
moment and that is where I conclude my
talk and I thank you very much for your
attention
[Music]
okay thank you Mel so
far we have around 15 minutes for
questions so if you have a question in
the room please please come to one of
the microphones and I think this is
microphone number two so uh please go
ahead uh I think mic 2 is not open I'm
still not hearing anything let's
see try again hello hi uh thank you very
much for your talking it was highly
entertaining and very informative um I'm
wondering if I'm imagining things but I
like I have no idea on the S subject
matter whatsoever but I see seem to
remember that the asteroid cluster
around series uh that there is a maybe
crackpot hypothesis that uh all the
asteroids are actually the the
uh of an exploded planet or whatever
yeah um it's it's not too far out there
so we basically have very very bad ideas
about how the uh or very imprecise ideas
about how the bodies in the solar system
actually came came to be how planets
start forming it's getting much better
since we see many other planets uh with
with exoplanet discoveries uh we see
other solar systems forming and this
gives us a much better idea of how a
solar system forms but uh back in the
times when we only knew our own our own
system we had very bad ideas uh um uh
very bad ways of finding out how a solar
system um is actually formed think of um
somewhere where there's only adults
you've only ever known adults everyone
is an adult and you don't know what
children are how do you describe
children what do you think children look
like how do children work you would have
no idea and it was this situation that
we had uh in the solar system um and
there is a go-to theory of when we don't
know how something happened it's
probably that uh which is uh large
collisions large collisions are probably
an explanation of why Uranus is rolling
on its side large collisions are uh
pretty much definitely the reason how
the moon was formed around the earth and
it's relatively large large collisions
are probably the reason why Venus is
turning backward um and so it makes kind
of sense to to posit that a large
Collision may have um created the
asteroid belt but um another a different
theory is actually physically considered
more plausible at the moment which is
that the gravity of Jupiter is so
dominant um and it it even creates uh
sorts of gaps where there is uh
gravitational resonance uh in the inner
solar system um so there are places
there are orbits where almost no
asteroid are because at that point they
would have a resonance and be kicked out
by Jupiter all the time um and so the
the leading theory of how the asteroid
belt came to be is that all the planets
were forming and in that area uh they
just there just wasn't a chance because
gravity was so wild for things to coales
into one body but if we look at uh
single bodies there is actually some
hints at um at asteroid families so this
is where you can look at the light and
you take Spectra and you can see what
metal and what Stones they are made of
and so on um and there are actually a
number of asteroid families where you
can say okay these 20 or 30 asteroids
they probably all come from one which
was then struck by something so we see
small families in the asteroid belt most
of the astronomers don't believe that
they all come from one and the same body
it was probably rather the case that a
large body just didn't form in that
place thank all right let's jump to Mike
one um that was great and super
accessible thank you uh you said that
once a body is identified as having a
satellite it makes it really easy to
work out what its mass is can you
explain like M5 how that works so the
more heavy something is um the faster
you need to be in an orbit around it um
I could orbit a a small asteroid I mean
a small a 10m asteroid I could orbit it
really easily I could I could just be
throwing pebbles and keep myself in an
orbit but but to orbit something that's
very heavy like the Earth for example um
you have to have a speed of nearly 8 km/
second because otherwise the gravity is
so strong that you just fall down and
the trick of being in an orbit around
something is to basically be so fast
that you're kind of falling behind the
Horizon you're so fast that yes you're
falling down all the time you're in free
fall but you don't actually hit the
thing it's what uh Douglas Adams in The
Hitchhiker's Guide called falling down
but forgetting to hit the ground that's
actually from an orbital mechanics
standpoint a very good description of
how you orbit something and so if you
see how fast something is orbiting
around something else then you have a
very good idea of how heavy that thing
is that it's orbiting
around thank all right uh microphone
number four please hey uh yeah have very
similar question to the first one but um
are all of these objects that you've
been talking about spherical and if no
which ones are not spherical question
very good question um in the asteroid
belt there is only one body which is
spherical by its own accord probably
which is series the largest asteroid
with a diameter of about 1,000 km um
there is another one uh which is
considered to be actually a little too
small to be spherical but it still looks
spherical and this is hiia uh it's one
of the first dozen or so asteroids that
was discovered you will automatically be
spherical as a body in the solar system
if your gravity is heavy enough because
you start pulling in all the stones and
and all the you know all the metals and
everything you start pulling it into
yourself with your own gravity such that
a sphere results and if that happens the
body is set to be in hydrostatical
equilibrium uh it's it's so heavy that
it just pulls itself into a ball this is
the case for uh for series like I said
there is hiia which is seen to be
spherical but actually is probably not
heavy enough to actually be that but all
the larger bodies um are spherical of
their own accord just because of their
own
gravity and and the and the smaller ones
actually it's it's fun because if you
look at those which are not spherical we
used to say that they are all mostly
potato shaped uh so like random Stones
ellipsoids kind of um but in in past
years we've seen for example the comet
that was visited by the Rosetta probe
the chum of gimeno
67p um and we saw the aroko the body
that is out Beyond Pluto that was
visited by the New Horizon probe after
it had plast Pluto
and these are Bob baites they look like
rubber duckies right you got a rubber
ducky which is basically like two little
balls stuck together with a with a thin
neck in between and we now know a comet
and a a a um a trans neptunian object
which both have this shape and it's
probably they were probably created by
two smaller objects touching and kind of
growing together but not fusing
completely into one body so you get
these rubber ducky bodies um and some
even smaller ones are kind of diamond
shaped it's as if you took a diamond and
you you you um turned it around to make
it a a 3D object this is for example the
um the ryugu asteroid that was visited
by the Japanese hayabuza probe um or you
have the benu that was visited by the
Osiris Rex space probe from NASA these
have this diamond shape and it's only in
recent years the last 10 or 20 years
that we've started to visit these bodies
with spacecraft because we can't see
their shape with telescopes from Earth
they're just way too small for that and
since we have started visiting these
very small bodies with space propes we
seen that there are all kinds of funny
shapes and it's probably going to get
funnier the more of these bodies we
visit okay let's take a question from
the internet from our signal Angel are
there any uh unexplained differences
between our planet system and other
planet systems oh that's a very good
question yeah we see a bunch of types of
planets in other solar systems that we
don't see in ours one of the big ones is
what's called hot Jupiters hot Jupiters
are large gas planets of the mass of
Jupiter or or even more but they are
very close to their stars um we see
these around small red dwarves which is
kind of which is also due to the way
that we detect um exoplanets so finding
these kinds of planets around these
kinds of stars is is easier than others
but hot Jupiters are one thing that we
don't see in our own solar system uh
another thing that we don't see is super
Earths uh we know U maybe a dozen or two
dozen super Earths in other solar
systems and these are called that
because they're larger than Earth but
they're definitely ohue talking yes um
they are larger than Earth but they are
definitely not um a big gas planet like
Saturn they are somewhere between Earth
and Uranus um and so these um little
confused with the microphone but the the
um the speakers are going to work again
in a minute
um uh so they are between the mass of
Earth and the mass of Uranus and these
are called super Earths and actually we
don't even know if the super Earths have
a thin atmosphere like our Earth does or
if they have a thick atmosphere like
Uranus there's probably um there's
probably things in between
um the universe doesn't want you to hear
this but I'm going to say it
anyway um and uh yeah so we see other
solar systems forming we see them
forming other planets um the key to all
these Mysteries and many of the
differences between our solar system and
others is probably planetary migration
which is the the suspected fact that
after a solar system has formed the
planets start tugging and and sort of
kicking at each other with their gravity
and sorting the whole thing a new so
there's probably almost no solar system
which looks the same way as it did on
the first day after the planets formed
because they all kicked each other
around over the billions of years that
pass afterwards and that's probably what
happened in our solar system and in
others as well um but it's a very active
field of research this question of how
do other planetary systems look
different from our own is one where
there are many exciting answers still to
be
found okay let's uh jump to MC three hi
um on one of your slides you had a
really nice chart of like the SE of like
handdrawn orbits in the 1700s and I
noticed that the orbits weren't like
centered on the same point and I
wondered if you have any knowledge like
why they chose to represent it like that
uh let's see this no this is
circular um we had this storical
ones uh do you mean this one no it was
still early still before
so uh yeah this one okay um what is
represented here is um excentricities so
actually uh the orbit of mercury takes
it uh quite a bit closer to the Sun at
times than at other times and this is
called eccentricity is where you're not
going around another body um in a in a
perfect circle but you are sometimes
closer and when you are closer you're
moving faster and you are sometimes
further away and while you are further
away you are moving slower this is what
Kepler's laws um this is what Kepler's
laws first poured into mathematics and
uh I would say that this is actually
mostly a representation of these
eccentricities uh for the large bodies
like Jupiter uh it's pretty much
circular because with its large gravity
uh gravity Jupiter um experiences a lot
of tidal force between it and the Sun
and tidal forces tend to Circ ciz your
orbits um but if it looks kind of weird
it's probably mostly because of mercury
in the inner solar system which has a
tilted and quite eccentric um uh orbit
yes thank you let's uh go to mik four um
okay um I have a question regarding the
definition of of the planets does it
have to take account take into account
the planal orbit of the planets I mean
or the origins of the planets because um
I think I've read about before when
planets form from
um um the Proto solar system the planets
will form from the same cloud of gas
will be on the same Planet same plane of
orbits and other objects that are part
of the solar system but not on the same
plane are should objects that were
caught by the gravity of the
sun um could be yeah for examp so for
example Pluto I've it's quite inclined
Pluto has I think like a 17 or almost
20% a 20 Dee tilt towards the ecliptic
plane yes so yes uh so it's maybe it's
likely that it's not the object that was
formed from day one from the solar
system so it couldn't be pled for for
this reason or well um there's another
more um there's there's another simpler
explanation um so you're saying that
Pluto's orbit looks weird and it's
tilted and it's quite eccentric
sometimes actually Pluto's orbit takes
it closer to the Sun than Neptune and
other times it's still further away um
but but uh it's more likely that Pluto
was kicked by Neptune at some point uh
so the planets with with orbital
resonances can sort of Shuffle each
other around we have seen as far as I
know two times already bodies that
definitely Came From Another Star and
they were inside our own solar system
but only for a while and then they shot
out again one of these looked very much
like a comet this was two ey borisov and
the other one was on the news a lot this
was one ey mua mua um this was where one
of the astronom
one of the American astronomers said oh
it's probably an alien solar sail or
something it probably really was a rock
that guy always talks about aliens
because he wants to be on the news um
but uh we have seen objects from
definitely somewhere else but their
orbits are super eccentric so for
something to be captured into the solar
system and actually orbit the sun near
where the other planets are it's much
more likely that it's a body that yes it
did form in the solar system and then it
was kicked around a little little um but
to answer the the way way your question
got started the definition for a planet
today as defined by the international
astronomical unit doesn't take into
account the orbit very much it's a
three-step algorithm you have to be
orbiting the Sun you have to be not a
moon of someone else and you have to
have cleared the orbit around you of
other bodies so you don't you're not
allowed to have small neighbors
basically um and for this Pluto was
excluded because there's all sorts of
all manner of Kyper Bel objects around
it ser is excluded because of the
asteroid belt but they they are
basically all considered part of the
solar system and probably formed here
thank
you okay do we have another question
from the internet with our signal Angel
uh no the internet has not asked any
then we'll just continue in the room uh
Mike one please thank you thank you for
the great talk very insightful I have
two very short questions I hope it works
can he because the second question is
funny so the first question is uh we're
learning all the time and I think you
you made a great point of like yes we
had these EP circles and then Kepler and
Newton and Einstein so my question is
what about dark mattera do you think we
will learn something new about the solar
system once you understand dark mattera
um everything that physicists have put
into theories about what Dark Matter
probably is says that its effect is much
too weak on the solar system you see the
effects of Dark Matter probably in how
galaxies rotate which are collections of
hundreds of billions of stars
but the solar system itself should be
way too small to be affected by Dark
Matter except maybe that there could be
a ball of Dark Matter U uh in in the
center of the sun because that's where
it would collect and this could change
the way that the sun vibrates this is
something that people actually look at
they look at the surface of the Sun and
how the surface of the Sun vibrates and
they want to try and see if they can see
a signature of a ball of Dark Matter way
down in the center of the Sun but this
is by no means proven and it definitely
would not have an effect on the movement
of the planets cool thank you and very
quickly why does salasa the asid look
like salami or CIA yes salacia um um you
you are allowed to name so I'm not I
don't know the history of that name
particularly you are allowed to name a
body in the solar system uh if you found
it and if the international astronomical
Union thinks the name that you've chosen
is not too silly um so there's probably
a mythological figure but it ALS Al
works as a joke actually the guy that
found uh the moon of Pluto the guy that
found Sharon had a wife called Sharon um
but he looked it up in uh he looked it
up in an encyclopedia and he found that
Sharon was also I think it's the the
figure that um fairies the dead Souls
across the river sticks in the
Underworld which is where the god Pluto
also exist so it fit perfectly and his
wife Sharon was very happy thank you all
right we're almost at time I think we'll
have time for one more question at Mike
four please uh thanks uh you've
explained quite precisely what makes a
planet right those three um things but
what differentiates a dwarf planet from
an asteroid and why Ser is an a dwarf
planet and not an asteroid that's a good
question and there uh the distinction is
not very sharp um it's actually you
would probably say that um so that
there's not there's not a fixed written
rule not an algorithm that I know of but
the general feeling in the AST
International Asic Union is um that to
be a dwarf planet you have to be in
hydrostatical equilibrium we had a
question before about when is a body
spherical in the form of a ball it's
when its own gravity can pull it into
the form of a ball and so for series we
know that this is the case for hygia
which is not considered a dwarf planet
it could be the case and then it would
have to be included um for many of the
bodies out in the in the Kyper belt and
the uh trans neptunian objects we don't
actually know their shape we don't see
it we can only guess from the mass but
here's a funny thing the international
astronomical Union once defined a small
number of dwarf planets I think actually
six or seven and since then because of
all the drama they just stopped
registering new dwarf
planets bodies in the solar system
continue to be found and many of them
fit the criteria of being a dwarf planet
but the international AST astronomical
Union is just not taking any calls on
the
subject all right this concludes our
talk thank you so much Michelle this was
highly entertaining thank you
[Applause]
[Music]