So yeah I'd really like to
thank the school of physics and

the society physics students in
particular for inviting me so

it's great to have
the opportunity to come out and

talk to people about the kinds of things
that we think about an astronomy I think

particularly as a person who works at an
institution dedicated to public astronomy

at the outlook planetarium one of the
things that strikes us the most is that

people are fascinated by this stuff but
they don't always have access to someone

that they can ask questions that they can
talk about that they can say hey I heard

this what does it really mean and so it's
always a special treat to come out and

talk with people about the kinds of things
that we do every day as astronomers so

I am out to Northwestern and the out
there together my appointment split

you can see my e-mail address right
there that's because invariably at

about three in the morning you're
going to sit bolt upright in bed and

have a question and you may need to ask
me what did you mean when you said and

you can certainly e-mail me and most
most times I'm up until about two AM so

you may get an answer right back for me
but if you don't I'm out with my telescope

and I'll answer you soon as I come in so
I blog about this sort of stuff you can

follow my blog there and I'm not
tweeting right now during the talk but

you can follow me on Twitter and if you
tweet something at me I also will try and

answer it so what I want to talk about
today is about the notion that we

live in an age of discovery and this is
something that as scientists we're kind

of used to we're used to the idea that we
always are making new discoveries about

the world and that as a consequence of
those new discoveries we're transforming

our understanding of how we fit into
the cosmos and what our place in

the universe really is and so
today I want to pick up on that thread and

I want to take that in a particular
direction and talk specifically

about the idea that there are new worlds
to discover out in the universe and

talk about what we're finding
what we're thinking and

why we're in the rested in that.

So generically.

I'll just start by talking about kind of
the way we all learned about the age of

exploration the thing your social safety
chair told you when you were in elementary

school and then we'll use that to segue
into this whole discussion of how do we

find planets where are we finding planets
what are those planets all about both near

to home and around other stars and the one
thing I want you all to kind of take away

and bear in the back of your mind is that
the thing that enables this is technology

discovery and science is enabled by all of
us being curious and asking questions but

the thing that makes new discoveries
really possible is new technology and

all of the things that we're going to talk
about tonight are really being driven and

pushed forward by the advent of technology
that we didn't have ten years twenty years

fifty years ago so that's the outline
that will go through and we'll talk about

that so when I was in elementary school
right we learned about the discovery of

the world the age of exploration began in
terms of western culture shortly after

the initiation of
the resits Renaissance and

the Age of Enlightenment the European
empires were slowly expanding about

the world to end sending fleets around
the world to discover a new planet and

that new planet was our planet it was
the earth and probably one of the most

impressive representations of
that age is in maps like this and

I really like maps like this because
they capture two things simultaneously

one thing they capture is our
state of knowledge and you and

I grow up today in an age where every
corner of the earth has been mapped

to exquisite precision we know to meter
accuracy what the coastlines look like and

where the rivers are and
how high the mountains are and

where all the lakes are but
five hundred years ago they were mapping

out the world by go into every
single place they could go to and

no did it down on a piece of paper and
what's impressive A.

Maps like this is you can see very
recognizably that they knew what the world

looked like you can see quite clearly the
outline of Europe and Africa it's a little

distorted compared to what you and
I are used to seeing satellite images but

it's clearly Europe and
Africa clearly the North American and

the South American coastlines
the other side is less clear but

the other thing this map shows
is a very clearly shows what

the frontiers of exploration are it shows
the places in the world where we really

don't know what's going on the northern
regions the far side of the North and

South American continents the Incognita
reach in the region where we now know and

article life no one has ever been there
all of this is captured on the map and

so Simon simultaneously
the map shows what we know and

what we don't know it shows the boundaries
of our knowledge the frontiers

which are limited only by our ignorance
and we're trying to loom innate that

constantly by pushing farther afield and
exploring more and in this

age technology is what enabled exploration
of the world which is the same message for

today to go and she came in many
forms the probably most recognizable

form of technology was in ships OK we
didn't explore the world in dugout

canoes because we couldn't make it across
the great oceans the way you discover

what the world looks like is you make it
a way to get reliably across the ocean

discover what the world looks like and
get back home to tell the tale and

that meant you had to build ships
that were capable making the voyage

ships that were capable of enduring storms
and whatever they find on the other

side you need to be able to carry crews
and feed them keep them healthy and

most importantly for
things like making this map

you had to be able to accurately know
where you were on the planet and

the technology needed to do that
is quite appropriately astronomy

you need some engineering to you have to
build clocks and that's our stuff but.

Really it's astronomy and as
an astronomer I'll take that conceit and

accept that astronomy is what enabled
the discovery of the world OK so

this was the technology that allowed
us to begin what we often are taught

of as the age of exploration but you and
I live in the New Age of Exploration and

now we have mapped the world
to very high precision and so

when we think about exploration
even though all of us go hiking and

go to places we've never been before we
kind of know what the world looks like and

so when we think about exploration we
think about pushing out from the earth

looking at other worlds in
other places in the cosmos and

that exploration also is enabled by
technology there's all kinds of different

technology thanks so there's clearly big
telescopes some of those big telescopes

some of us can use in our backyards and
make discoveries ourselves in fact

amateurs do this all the time we discover
comets we discover supernovas we discover

asteroids today you and I live in the
future amateurs can discover exoplanets

OK technology enables
discovery to be made and

telescopes are probably the single most
obvious form of technology that we

use to discover the cosmos around us both
in our backyards as well as in gigantic

observatories around the world rockets
are probably the most prominent.

Representation of the space age
that we're all familiar with

rockets are what enable us to make
the voyage from the surface of the earth

out into the shallows of the cosmic
ocean the rocket is the single device

that allows us to get
away from the earth and

go to those places that we've never been
in the same way that ships enabled us to.

Make the journey around the earth and then
probably most importantly robots have been

our emissaries despite the fact that we
have rockets in spite the fact that we can

go on these great voyages to the shallows
of space humans really haven't been.

That far into the cosmos the farthest
any human has been from the earth is

around the moon and there are only twelve
people who have ever made that journey and

all of them have been men Unfortunately
none of them have been women but

that's all the farther and human has ever
been but we can build robot emissaries we

can build machines that act as
our eyes our ears our taste or

smell and they can go exploring
the universe and in fact they have they've

been all over the solar system some of you
may recognize this particular spacecraft

This is very prominent last week in
the News this is Rosetta which visited for

the first time this comet sixty
seven C G And it dropped for

the first time on the surface of
the comet a small probe named feeling

which returned to us for the very first
time samples from the surface of a comet

pictures from the surface the comet
a place that before last week no

human has ever laid eyes on from up
close and I just saw today before

I came over this afternoon they detected
organic molecules on the surface and

I have a reading about that but
they get and so that's cool OK because

we know something about comets that within
the before that we have long suspected So

this is technology that is
in a believing the discovery

of worlds beyond the Earth OK Initially
they enable discovery of the earth

we sent rockets to look back at
the Earth but now we're looking outward.

So let me tell you a story that most of
you're familiar with that will lead us

into this discussion about planets so
astronomers have deep set beliefs

about the way the universe is constructed
and over the centuries particularly since

the invention of the telescope as
we have studied the solar system

we have developed ideas about what the
architecture of the solar system is and

every now and then we get it in our
heads that there is some reason for

whatever it is that we think there's
a planet missing in the sequence and

so when we find that we think there should
be a planet we tell all the astronomers in

the world go get your telescopes and
let's go find the missing planets.

And most of you know this is
happened we went looking for

a planet which we discovered and
we initially called it a planet but

it only survived as a planet for a little
while eventually that planet was demoted

because we were beginning to find
other worlds that were like this but

not like the real planets and
so in the end we have this

world that we used to think is the planet
is no longer a planet but next year for

the first time we're going to visit
with a spacecraft from Earth OK and

it's going to be awesome OK so
what we're talking about here right.

Now I know we're talking
about the asteroid one series

OK in eighteen zero one.

Astronomers had know this that there was
a gap in the sequence between Mars and

Jupiter where they fully expected there to
be a planet and so they sent word around

the world to every astronomer to go
looking for a planet between Mars and

Jupiter and on January first in eighty
no one just Seppi Piazzi an Italian

astronomer discovered a planet precisely
where we thought there should be a planet

he knew it wasn't a star because he
could see it moving he wasn't sure he

wanted to call it a planet so he kind of
told people he thought it was a comet but

he wrote in letters to his
friends I found the planet OK and

he called that planet he named
it Ceres OK Now over the next

decade astronomers started
to find other planets

in the same region that we found series in
the region the solar system that you and

I call the main asteroid belt OK So
they started finding other large bodies

that were not just because other planets
but they were in the same place and

they began to suspect well maybe we
shouldn't call this thing a planet but

for fifty years if you picked up a book in
the mid eighty's hundreds on astronomy it

said that series was a planet and so
finally astronomers demoted it they said

look there's too many things here these
things are all going to be called faster.

Royds And this of course caused enormous
consternation and people really upset and

this is all a very familiar story to you
because you've heard this story before

right OK just in a different context
which we'll talk about in a minute but

next year in March the Dawn spacecraft
which I show you right here

will be the first spacecraft from
Earth ever to visit series and so

this is very interesting because series
is the largest of the asteroids and

there are a lot of questions we
have about the asteroid belt

why there is one large body like
Sirius why there aren't many.

Others just like it why it never
conglomerated into another world what's it

made of there's been long suspicions that
there's a lot of water on Ceres all of

these kinds of questions Dawn
is going to investigate.

But all of you thought I was
talking about a different world so

let's talk about
the discovery of planets so

the discovery of planets really began
in the eighteenth century there were

five planets known since in T twenty
Mercury Venus Earth Mars Jupiter and

Saturn these are the planets that
you can see with your naked eye

that six worlds because normally people
didn't include earth sorry OK but

in seven hundred eighty one William
Herschel observing from his backyard in

Bath in Somerset England with a small
amateur telescope which we have at

the Adler Planetarium you can come
see it he discovered a new planet and

he also called it a comet it looked fuzzy
to him but he called it a comet and

after much study and planetary orbit
analysis eventually people decided

that it was in fact a new world it to
follow the sequence of planets that people

were thought the universe was the solar
system was organized by at that time and

so eventually we called it a planet this
discovery was enabled by the existence

of the telescope you're in this by.

Happenstance is bright enough
to see with your naked eye.

In fact people had seen it with their
naked eye before they didn't even know it

was a planet though right so Flamsteed
had categorized urine this in the early

sixteen hundred as a star he called it
thirty four tour I because when he saw

it it was in the constellation Taurus but
no one thought to watch it for

the next one hundred eighty years and
noticed that it moved OK but

Hershel when he looked at it through his
telescope could see that it wasn't to

pinpoint of light it actually had
structure and that's why he decided it was

a planet Sirius which I just told you
was the next kind of planetary discovery

the discovery of Ceres was really
enabled by mathematics because.

After he had observed Ceres fell ill and.

Series was lost in the haze of the sun it
went on the other side of the sun where

they can see it and then they couldn't
find it again when they came out the other

side and so we didn't really know what the
think the famous mathematician whose name

most of you will recognize call
Frederick Gauss developed a method for

reconstructing orbits
from observations and so

mathematics enabled us to
recover Ceres late that year and

so it was added to the list
of planetary discoveries.

In eight hundred forty six very a was
a mathematician who had been looking

at perturbations to YOU'RE IN THIS IS
orbit he had noticed that there were small

squiggles basically in the way you're in
this was moving and he had predicted based

on mathematical techniques that there
should be another planet out beyond you're

in this and so in eight hundred
forty six galley went looking for

that planet and discovered Neptune right
in the place that we very a had predicted

they should exist and so this was really
the first time that planetary science had

been enabled by two technologies together
by the synergy of mathematics and

observational technology
at the same time and

this is this is I think to my mind one
of the first times in the history.

Of astronomy that you really see
the way modern astronomy works

you have people who think about math in
the kind of theoretical considerations

things that we do on computers today who
were thinking about the universe and

then coupling that with our
friends who can go out and

actually use telescopes which some of
us can and some of us can't right so

OK And then of course the story that all
of you thought I was telling you was

the discovery of Pluto in one nine hundred
thirty Pluto again was a planet that

astronomers thought should exist there was
a massive search under way to discover

Pluto outclassed Well they didn't
know they were looking for Pluto but

they were looking for a planet out past
Neptune and this discovery was enabled by

a new technology in the Strada me namely
photography and so quite discovered

Pluto in the spring of one nine hundred
thirty using photographic plates that he

had taken at the end of January so
all of these discoveries of planets and

let me call them all planets for
the moment were enabled by different modes

of technological expansion
in the Strada me.

Now Pluto is just like a series
about to be visited So

on July fourteenth next year.

New Horizons which is a spacecraft from
planet earth is going to sail by Pluto and

for the first time we will see Pluto
up close every picture you've seen of

Pluto now like this one in the background
is an artist's conception there are some

interesting pictures of Pluto from the
Hubble Space Telescope but nothing really

supremely detailed and so New Horizons
has been found since two thousand and

six it is now traveling at something
like sixteen thousand miles per hour and

it's going to fly right through Pluto
pass the system and go on to Al it

into the area beyond Pluto an area that
astronomers call the caper belt so

we get one shot to do it right and

hopefully we see everything that we can
see if our experience with previous space

probes that go out to the solar system or
any experience flying through and

taking as many pictures as you can will
be nowhere near enough to satisfy your

curiosity you're going to be undoubtedly
many more questions to be answered.

After New Horizons visit then before.

I would suspect on July fifteenth
the Internet will be dead because all of

us are to be at home trying to
download pictures of Pluto so

do whatever shopping you need to
do on Amazon before then OK so.

Let's talk about Pluto here for
a minute so things really went south for

Pluto the same year that New Horizons
was launched in two thousand and

six the astronomers who belong to a large
professional organization known as

the International Astronomical Union
the I.A.U. got together in Europe and

we had the vote and the vote was
what should we call a planet and

the way this is usually framed in
the context of astronomical history is

what should we decide Pluto is should
we vote on Pluto status as a planet and

the outcome of that meeting has gone down
in history as one of the most infamous

days in the straw to me a set of
astronomers in a very empty room about

five percent of all the astronomers on
the planet voted to demote Pluto to

strip it of its status as a planet and
it's now something less than it was

it's a dwarf planet OK the solar
system has eight planets period done

OK Now those of us who grew up in
an age where Pluto was a planet

are greatly offended by this those of us
who are raising our children right our

children are greatly offended by it but
there is a group of children who are being

raised believing that their only planets
in the solar system OK they'll come to

Georgia Tech eventually in the astronomers
will straighten them out OK but

this is what happens OK And
the thing that is important about this

the thing that I think you should remember
is that before this vote happened

astronomers never had a formal definition
of what it meant to be a planet.

So this wasn't really about Pluto
this was really about what we mean

by the word planet and the question
that you should be asking is why.

Now why is it today that we decided

finally we needed to have an official
definition of planet that we as

astronomers agreed on that we
couldn't look up in the dictionary.

What made that happen.

Now as you know pandemonium ensued OK
people were deeply upset by this you had

people supporting Pluto people protesting
Pluto police in between protecting

everyone from the other side many of
us took our own stance my daughter and

I certainly have our opinions about this
as well she's a little more strongly

opinionated about this than I am but
that's because I raised her right and

so you know people really
cared about this and

so I like to come talk about this because
the reason people care about this is based

on this kind of deep seated belief we all
have that our science teachers never lie

to us right science were often
taught is a fact about or

a collection of facts about the way the
universe is organized But as scientists

the way we think about the world is that
science is an evolving body of knowledge

and yes I may have once thought Pluto
is a planet but now I'm not so sure and

I like talking to people about why that
is and assuaging the difficulties you

have coming to emotional terms with the
fact that we may not believe that Pluto's

a planet anymore OK Plutus a planet so
don't worry I'm on your side too OK So

the question is why did this happen OK And
I think this is most.

Easily summarized by a quote
from the Grasse Tyson Neil was

certainly on the forefront of this
battle but he summarized very succinctly

what the problem with Pluto was and
the problem with Pluto was not so

much the problem with Pluto it's a problem
with the fact that we are beginning to

discover watts of things in the solar
system that look like Pluto but

don't look like us don't
look like the earth and so

this is what he said he said
Pluto is mostly ice so it's.

It's a large rocky planetoid
has a lot of ice in it and

the problem most people have with Pluto is
that if you could magically slide Pluto

around in the solar system right so

scientists we do this in our heads all
the time yeah I could take Pluto and

I could drag it in to the orbit of the
earth no problem easy to do right OK if

you could do that the heat from the sun
would evaporates the ice on Pluto and

Pluto would grow a tail just
like a comet and as Neil so

eloquently puts it that is no
kind of behavior for a planet so

this is really the crux of the matter this
is what astronomers were worried about

they were worried about the fact that
we're beginning to discover lots of bodies

in the outer solar system that are mostly
ice lots of bodies in the outer solar

system that have some rock to them they
have gravity even look like planets but

they're mostly ice not mostly rock and
so they really wanted a way to

categorize these objects that separated
them in our minds from the way we

think about what you and I think of as
planets under our feet the rocky worlds

OK So this set up a big argument on one
side of the argument there's a whole

host of planetary scientists these are
the ringleaders Neal as many of you know

is the director of the Hayden Planetarium
and kind of sparked this debate in

the public's mind because he removed Pluto
from the Hayden Planetarium display of

planets and this instantly started getting
him hate mail from third graders and

all kinds of great stuff
OK Mike Brown is a new Tory is

planetary physicist from Cal Tech He's
actually a great guy we just like to tease

him but he is really the ringleader
in terms of planetary scientists for

what's going on here in this great debate
on the other side of the debate you have

people like Alan Stern so Alan Stern is
the P.I. He's the scientist in charge

of the New Horizons missin this is going
to Pluto and you might ask yourself

why does Alan care why does he have
a bone to pick in this fight because his.

Robot's going to Pluto he shouldn't
even care if it's a planet or

not Kate but he does and there's lots
of us who are on this side with Allen

because we don't like the way we're
talking about planets right now and

although it doesn't help him certainly my
daughter and I are both on this side OK So

we have these people who are bad guys but
trying to look cool and

these people who are good guys and so
the question is what are we talking about.

So the thing is is that
planets are very difficult

to study much more difficult than other
things that we study in science most

of you when you learn science in school
you had something you could touch and

something you could experiment with you
know make solutions together in chemistry

you grow tomato seeds in biology you
dissect frogs you roll RAM carts up and

down ramps and physics you do things with
your hand in the laboratory you do them

a million times so that you can see
what the outcome of an experiment is but

astronomy is different you can't always
touch the things that you're dealing with

you usually have to just sit back and
look astronomy

is a spectator sport all you can do for

the most part is look and when all you
can do is look at objects the way you're

trying to decide what they're all about
is through an exercise that we call

taxonomy taxonomy is taking
what you can see and

a very few things that you may
know about an object and trying to

understand how it similar to other
objects by sorting and grouping all those

objects together in the individual groups
OK Now those of you who have experience

in biology will recognize this text the
origin of taxonomical classification but

we use it in astronomy too so
let me go through an example to describe

how we think about taxonomy So
it just so happens that today is a great

day to think about taxonomy because we are
just past the holiday where I discovered.

An amazing cosmic substance called Candy.

OK and all over my house are just
piles and piles of this stuff and

if I had never seen it before I might why
I might want to write down the scientific

description of what this stuff is OK
because someday someone's going to ask me

what's this candy stuff that you talk
about what is it OK And so I sit down and

I look at it and I can sort it into groups
and when you look at this pile of Can day

I have right here there's very clearly
a way to sort this into groups OK

there's clearly Candy that lives in
boxes in candy that lives in wrappers.

That summarizes everything about those
things that you see on the screen

right there right.

Right OK there's no other way to divide
these up OK there's box candy and

Candy this in wrappers and we're good OK
this is the situation we find ourselves in

planets OK We have some things
we know about planets and

we can sort the planets one way based
on the properties they have but

if we're careful and if we're a contrarian
scientists are never contrary we hate to

argue with each other but sometimes we do
there are some of us who think you should

sort of a different way and certainly
there are people who think you should sort

the candy into two word games and into one
word games OK how many of you think that's

the right way to do it no I know OK well
then there must be some of you who think

we should divide it into brown packaging
and not brown bagging OK OK Well I know

really it's chocolate not chocolate isn't
that right yeah I see I knew it right

OK so you can do this over and over and
over again there are enormous numbers of

ways to sort objects into groups
based on just what you can see or

the few things you can learn with
simple experiments about them and

the truth is it's not always clear
that you can sort things into groups

in such a way that you can clearly
identify what their properties should be

based on naming what group they
belong in and this is the situ a.

And we find ourselves in planets and so
multiple pieces is the way I like to sort

it because you get plenty more over
here OK The problem is see there's lots

of ways of doing this this isn't one
of them the problem is that eventually

you're going to divide things into so
many groups.

That there's as many groups as there
are objects that you have OK And so

here I've divided this
into four groups but

eventually you can see I can divide
multiple chewy things into yellows and

greens and reds and I can divide these
into chocolates that are you know

composed in different ways these
are multiple pieces both of them so

they probably their Laffy Taffy if you're
young you can divide as many things as you

want and with planets we can do the exact
same thing how many planets are there

like Saturn one OK those are the Saturn
like planets how many planets are there

like Earth one OK those are the earth like
planets how many planets are there like

Venus one OK and those of us like
planets OK And so we get into this

mode where taxonomy is not the most useful
way to think about the universe and

the reason that's difficult because not
everyone agrees on the taxonomy and that's

the position we find ourselves in now
when we think about planetary science so

let's talk about what the official
definition is when the I

you voted they if I were to be.

Abusive about it they were trying to find
a definition that would exclude Pluto

from Planet hood that
was their goal OK but

nevertheless they set down
the list of rules and

those rules apply to all planets not
just Pluto and so those rules are disks

in order to be a planet you must orbit
the sun OK Well Pluto orbits the Sun So

Pluto should be a planet if that was the
only part of the book OK gravity must make

you round OK now Pluto was round but
at this point people start having a little

bit of this because they don't like things
that are ambiguous and the definition

of round is really really in big use in
astronomy OK so let's consider Jupiter.

Jupiter is still one of the planets But
Jupiter is spinning so

fast that it's actually not perfectly
round it's so blate it's a little bit

wider at the equalizer than it is that
the poles just like when you spin.

Out of pizza dough right it wants to
flatten out because it's spinning but

Jupiter still a planet
even though it's not round

OK What about the earth right you
don't have many of them in Georgia but

on Earth there are these things called
Mountains OK mountain clearly arch part

of the earth being round OK So when do
I ignore mountains and when do I not

ignore realms OK so this idea that
something can be round is a little bit

fuzzy in our minds there are very clearly
things in the solar system that aren't

round the comet that we just landed on is
not round it's shaped like a rubber duck

OK asteroids totally shaped like potatoes
OK but there are things that are more

round than the others and the question
is what is round enough to be a planet.

This is the one that gives most of us
the most consternation in order to be

a planet you must have cleared your orbit
now as a gravitational dynamicists this

really bothers me because clearing
your orbit is a very fuzzy

statement and astronomers will sit
around and they will write pages and

pages of algebra and talk about Hill
radio I gravitate to influence and

come up with some very obtuse definition
of what it means to clear your orbit but

if it takes five pages of algebra to
decide whether or not something is

cleared its orbit maybe it's not the most
useful characteristic to decide whether or

not you are something and so some people
object to it on their grounds but

the real reason to object to this is
that this is a time dependent statement.

If the criterion for being a planet
is that you can clear your orbit and

I simply wait long enough Pluto
will eventually encounter

everything in its orbit and
throw it out of the way and so

what do we do then this Pluto
suddenly magically become a planet.

OK Well most of you will eventually
get your undergraduate degrees and

suddenly magically you'll become
professionals so maybe that's OK but

in astronomy we don't like that we
like things to be what they are and

not change unless something dramatic
happens to the object itself and

Pluto suddenly magically clearing its
orbit isn't going to change what Pluto is

it's going to change
Pluto's environment and

that's what bothers a lot of us it
bothers us that how your environment is

behaving defines what you are that doesn't
really make a lot of sense sometimes.

So what should you think of all this
OK Should I really care OK other than

the fact that we're impugning Pluto's
honor endlessly because we keep telling it

it's not a planet should I care
about this OK And the answer is yes

mostly because it influences the way
we think about the world around us and

because it is influencing the way we think
about the world this is far from home so

let me give you a couple examples at
home in the we'll talk about far away so

let's think about
Jupiter's Jupiter planet.

Yes Are you sure OK It better be right

the solar system is the Sun Jupiter and
a whole bunch of other stuff that wants to

be as cool as Jupiter OK Jupiter is
the single most massive object in

the solar system after the sun it
really ought to be a planet but

as many of you may know in Jupiter's orbit
there are large groups of asteroids that

we call the Trojans and the Greeks these
are asteroids which have been captured by

Jupiter's gravity they follow in front of
Jupiter and behind Jupiter trapped forever

between Jupiter and the sun's
gravitational pull now by and large we

can assume that these things aren't things
that you Peter won't crash into OK Jupiter

is not going to suddenly crash into all
these asteroids so we can say sure it's

clear it's orbit because these things
are never going to crash on to the planet

OK but let's do the magical thing that
Neil de Grasse Tyson said we should do.

Let's take Jupiter and

less magically slided in the solar
system out past the orbit of Pluto and

if I do that there has not been enough
time in the age of the solar system for

Jupiter to have cleared its orbit
if it lived out where Pluto lives.

So if I find a planet out past normal
radius the size of Pluto should

I instantly decide that it's not a planet
even if it's the size of Jupiter.

I know Jupiter is pretty big I think maybe
it should be a planet no matter what's

going on wherever it is in
the solar system OK but

if I could magically flighted out
there it would fail the definition and

we'll talk about why that's
important in the moment.

Here's an example of why
clearing your of it's crazy and

as a notion this is a plot of the inner
solar system inside Jupiter's

orbit these blue asteroids that
you see here are the Trojans and

the Greeks in Jupiter's asteroid
the green are the main belt asteroids

This is the orbit of Mars and all of
these red dots are asteroids that orbit

within the size of Mars orbit they cross
the Earth's orbit they cross Venus orbit

they cross Mercury's orbit
Wait all of those are planets.

So what do we mean when we say we've
cleared your orbit if Earth really

could clear that orbit you know I
wouldn't be worried about an asteroid so

big not even Bruce Willis could
save us hitting the Earth OK but

it's going to happen I guarantee
you it's going to happen someday

OK because there's still stuff floating
around that there is going to hit and so

how do I reconcile that with the idea
that our orbit isn't clear OK So

these are difficulties that we have with
the definition of what it means to be

a planet and so as an astronomer
I began to think about this and

I think about in the following
terms I think maybe the problem

isn't that we don't know enough about
planets maybe the problem is that we're

just too fond of the planets that we
can see up close maybe the problem.

Is that we've spent so
much time with these worlds that we

think they're the ones that should define
what we mean when we say the word planet

OK the question that we really should
be asking is is our solar system

normal is that what solar systems
are normally designed like or

is it unique is there no other
solar system in all of the cosmos

that has the architecture of the design
the composition of planets and or planets

if we want to not call them planets
that we see in our own solar system and

so if we really want to reconcile this
question this is the case with all science

we need more examples we can't base all of
our knowledge of the cosmos based on one

instantiation of what the data looks
like we need to find more examples and

so since we live in the solar system and
we've found all the quote unquote planets

in the solar system the only thing we
can do is look to other stars OK So

we have started next year will be
the twentieth anniversary of the discovery

of the first planet around
another star you and

I are living through the first time in
human history where we are absolutely

assured that there are planets
around other star systems

we've known it right plenty of his have
seen Star Trek plenty of us have seen

Star Wars we know there's planets
elsewhere in the cosmos but

the scientists we didn't know and
in one thousand nine hundred five we

started discovering planets around other
stars and there has been a steady and

continuous stream of discoveries
ever since I just looked it up and

as of yesterday there are one thousand
eight hundred forty nine known

planets around other
stars in the universe.

There are five thousand nine hundred
seventy three candidates these

are detections which may be a planet which
we haven't gone back in confirmed yet but

they have all the possible characteristics
of being a planet we need to.

I'm We need telescopes we need people
to go and confirm these things but

if they were all planets you and I would
be talking today not in the universe

that has eight planets but in a universe
that has almost eight thousand worlds and

that's a lot of data that's a lot of what
makes a planet a planet to think about.

OK So let's talk about one of those
things that we've discovered so

the largest planet finding mission
has been a satellite in orbit called

Kepler Kepler is mission was to sit up
in the sky and watch one particular

place in the sky off the wing of Cygnus
right in the Milky Way for as long as it

was able and there are about ten
thousand stars in that field of view and

it would watch those stars over and over
and over again and what it's watching for

is a planet passing right in front of
the star what we call a transit kind of

like a mini eclipse and making the light
from that star get a little bit dimmer and

then getting bright again when the planet
goes out from in front of the star This is

called the Kepler Henri it's several years
old now but it that time is shows all

of the planets that Kepler had discovered
up to I think this is the two thousand and

twelve version all of these planets
are to the scale of our solar system and

we're zooming in but
let me read it here so I can.

Show you that so up here in the corner
is our solar system to scale OK So

this is the orbit of Mercury Venus there's
the earth and there's Mars so you can

see that most of the systems that we
have discovered and they get smaller and

smaller as you go around the spiral into
the middle our solar system scales that

are smaller than the inner solar
system of our solar system.

The size of the dot in the Ori is the size
of the planet we've discovered OK so

you can see the size of earth here so that
is something like the size of Jupiter or

many times the size of
Jupiter OK there are all.

So as we zoom in here many instances
of what we call Kepler multis systems

where just like our solar system
we see more than one planet and

the thing that should freak you out that
freaks us out this is stronger mergers

is we see more than one planet
zooming around this is a good example

in an orbit smaller than
the orbit of Mercury.

And many of those planets
there's what five or

six right in there
are the size of Jupiter.

So what do you do when you take
planets the size of Jupiter and

you compact them into a space
smaller than the size of Mercury and

the answer is we don't
know we have no idea

how to make planetary systems like
the ones we're seen in the Kepler data

we have no idea how to get planets
the size of Jupiter let alone multiple

planets the size of Jupiter that
close to their parents start.

We have no idea how to make
solar systems this small.

But we see them and they don't look like
our solar system and so the question in

our mind right now is which ones normal
are we normal or are these normal.

And this is a question that
is being driven by data but

it's also a question that's being
driven by computer simulations and

those of us who are theoreticians that's
what I do I work with math and numbers and

computers every day we spend lots of
time trying to simulate what the solar

system like this looks like and
how long it might survive.

So let's talk about some of the things we
see in some of these crazy solar systems

so the first example when we see lots of
these in the Kepler data I just showed you

some examples of these is something called
a hot Jupiter OK so this was the very

first solar system found around the star
like the sun it was unexpected butt.

We did find it we found a planet
that was half the mass of Jupiter.

Orbiting in an orbit one tenth
the size of Mercury's orbit around

this planet star So these are sometimes
called roasters because they're so

close to the sun or to the to the parent
star that is baking the atmosphere

off the planet slowly roasting it
alive and the big mystery is how

do you get the plant this close before it
actually Rosol away we have no idea OK So

these are called the hot Jupiters and
we see lots and

lots of these in the Kepler data before we
start discovering exoplanets there is not

a planetary scientist alive who would
have told you you can get a gas giant

that close to a parent star because that's
not what's true in our solar system OK

we only see the gas giants far out so
we don't know how this happens.

Now with all those planets close together
or there is a lot of gravitational

interaction gravitational interaction
means that those planets are in each

other's way they're in each other's orbits
they haven't cleared their orbits and

sometimes the gravitational interaction
is so strong that one of the planets E.J.

X. one of the other planets out of
the solar system it unbind it from

its parent star do you mean that planet
to roam forever in the dark between

the stars these are called rogue planets
and this is one of the first examples that

we found of this is
a world that is six times.

The mass of Jupiter.

By any definition of planet that
should be a planet because it's so

enormously big but that's not what I think
is awesome about this planet what's also

about this planet is it six times the mass
of Jupiter which means to give the jet did

it was probably in a fight with the planet
that was much bigger than this.

That was really super close to it.

And so
we had two planets bigger than any play.

And that we have ever seen in our own
solar system interacting strongly enough

that one of them gets objected from
the solar system voted off the island for

all eternity right it's crazy we have
no idea how this happens today but

we'd like to know.

But this is the holy grail this
is the thing we're really after

this is Kepler one eighty six
out this is the first time ever

we have seen a rocky planet
that's the size of the earth

at an appropriate distant from its parent
star that the temperature on the surface

of the world is enough that
you could find liquid water.

OK astronomers call this the habitable
zone and much as we argue about

the definition of planet we also argue
about the definition of habitable zone but

you Nerika only if you had a planet
like the earth in the habitable zone

liquid water could survive on
the surface and the reason you and

I care about that is because in all
our prejudice about the structure

of life in the universe we think water
is one of these sensuous because here

on Earth we don't see life exist without
water and so all of these searches for

other planets all of this trying to
understand with other worlds is a like

is the attempt to find a world
like our own to find a world

where liquid water may exist to find
a world where we can imagine life

like we see here on Earth might exist this
is the first world that we've ever seen

that approximately fits that bill but
it's not quite the same of as Earth and

the reason has little to do with the
planet we don't know anything about this

planet yet
we don't know if he has clouds will I

know if it has water as we just know
that is the size and mass of Earth but

the reason has to do with its parent star

OK this is a multi-system there
are five planets in the system but

its parent star is much much smaller
than the Sun It's what we call a red.

Dwarf it's cooler than the sun it will
live much much longer than our own sun.

But as a consequence of the fact that it's
cooler that means Kepler one hundred six

F. is much closer to the star
than the Earth is to the sun and

so again we're back to that same conundrum
we have of how do you build solar systems

with planets lots of planets in
close to the planet star but

nonetheless it is a planet that is much
like our own in terms of its size and

we expect its composition.

So in the end this is what the question
has always been about the reason we care

about what we mean when we say the word
planet is because we're really in arrested

in this planet we're trying to understand
what this planet is all about we want

to know what is the Earth unique in
all of the cosmos if I just look at

all the planets in the solar system even
if I just look at the dwarf planets no two

of them are the same they're all unique
in very interestingly in odd ways and

the earth in particular is unlike any
other world in the solar system and

what we're trying to understand is is
that unlike any other world in all of

the Galaxy and as humans the.

Conceit that we bring to that question is
we really want to know if these worlds

are habitable not because any of us
hope to ever go there we could we can

imagine ways that we could get to these
worlds but it seems unlikely that you or

I or probably our descendants will ever
visit one of these habitable worlds but

we'd like to know if they're habitable
because it makes us feel safe inside that

there's someplace we can go right OK But
really this is what the question is

are we alone is there life
elsewhere in the cosmos or

is Earth the only place and

we didn't talk about extraterrestrial
life tonight we could have an entire

conversation about your stressful life if
you wanted to but really ultimately do.

This is what the question is all about
is Earth the only place that there's

this stuff called Life Ok so this is what
the whole planetary discussions about and

that's the last thing I'm going to say
I'll leave you with a couple of places you

can go learn more if you want to all of
the exoplanet data is available online

if you'd like to explore it there are two
main archives the exoplanet Cyclopedia and

the exoplanet data Explorer
if you like making graphs and

looking at graphs these are the things for
you there are very nice cell phone apps

on both i Phones and
Androids that will access the data so

while you're writing Martha you can sit
around and make graphs of exoplanets and

show the people writing next to you how
cool you are OK if you want to actually

find planets at the Adler Planetarium
we run a program called the Zooniverse

which is a citizen science
project it's a project that uses

people like you to look at science
data that is very difficult for

a computer to look at and planetary data
from Kepler is one example of that and

so if you go to Planet Hunters start
org You can do it on your phone you

can do it on your computer you can start
looking for exoplanet data and all

you have to do is look at the data click
yes there's a planet no it's a planet and

just go through those on your commute
every day and it helps us a lot and

it helps you understand
a little bit this science data.

Planet Hunters Planet Hunters dot org OK

it's a community project we call this real
science online people powered science and

there's lots of different projects in
the universe if you don't want to look for

planets you can look for
animals on Serengeti you can look for

crabs on the bottom of the ocean you can
look at galaxies you can the zoo zoo

is looking for animals in urban areas
there's all kinds of stuff you can do at

this university it's a good project if you
want to read about Pluto but I recommend

these three books this is Mike's book How
I Killed Pluto and Why It Had It Coming.

It's all about why Pluto shouldn't be a
planet Alan Boyle has written a very nice

book called The Case for Pluto which is
all about why police should be a plant.

So I recommend you read this one and not
this one and Neil of course has his own

take on this he has a very nice book
called The Pluto Files that explores

the whole history of this question might
give a very nice talk at the kek Institute

which you can see streaming online so if
you go to that U.R.L. I think it's a two

part video you can listen to Mike tell you
all about why put those on the planet and

why we think it belongs with the outer
part of the solar system none

of us disagree with that we just disagree
with the final conclusion and I've given

the text talk about this which I know many
of the students in the room have seen and

you can go watch me talk about Pluto
as well if you want to so that's all

I'm going to say I'd like to say thank you
for listening and I'm certainly happy to

chat and take questions afterwards
I don't know how you want to travel

to.

If you have a question just raise your
hand and we'll pass the microphones around

so that everyone in the room
can hear your question.

There was great lecture so.

You know I'm kind of indifferent on
this on this question of Pluto but

I think the issue is not so much wanting
to Pluto but come up with a definition

rather let me ask you could you come
up with a definition that would

include Pluto but still exclude all
the Kiper built objects Yeah so

so I actually don't think
you should go down that path

because I agree with Mike's principle
argument which is Pluto looks

like the objects in the copper belt
OK What I disagree with is the philosophy

that we can't live in the solar system
that has fifty planets OK and that that's

really right this is the thing that you
hear people say right how are we going to

memorize the names of fifty planets where
you don't have to have a computer and

a magical device in my pocket they can
look at the names of the plans for

me right what we care about this
astronomers is how do you characterize

these objects such that when I say to
you planet you know what I mean and.

Yes precedence for this in astronomy So
let me give you an example which I didn't

talk about but one which is only recently
been resolved there is an object in

astronomy called a brown dwarf OK have
maybe you've heard around or same by OK so

a brown dwarf is an object bigger
than Jupiter but smaller than the sun

OK it is infrared bright they're easy
to discover in the infrared and for

a long long long time we argued about
how do I know you're a brown dwarf and

not a star.

OK And astronomers back and forth about
this incessantly meetings over and

over again why did you call that a ground
or why didn't you call it down and

eventually we zeroed in on the one
property that we thought was most

important and that property is you have
to be fusing hydrogen in your core and

when that happens you cross
the boundary from being a brown dwarf

into being a star and once we decided that

we all look at that definition we're
like aha that makes sense OK And

when I look at an object I can decide
this is a brown dwarf this is a star and

then big you asleep because that single
data point captures what we think is

important and the problem here is that we
can't agree on what's important right and

Mike and his crew are absolutely right
Pluto looks like the belt objects and

what we can't decide is what
does it mean to be a planet what

do we think is important to characterize
you as a planet or not a planet and

that's where the argument is and I don't
think any of us have a good feeling it for

what the right answer is So that's why
we're still having the argument so.

As I mentioned to you earlier I'm
looking at a minor in astronomy and

I was fascinated with your Planet Hunters
dot org And what they have to offer

what experience can I learn just from
doing the Planet Hunters dot org or

any of the other community science so
I can throw them into my resume right

so so community science
particularly through the unit.

As has many levels of participation and
this is true of all of the projects

that you work on lots of
people participate in this

universe by simply doing categorization
they just click a few They work for

five minutes every day on their
commute to work and they click through

their other citizens who become more
deeply involved in the science and

they see interesting things they know
these things there's a discussion section

board where they can interact with
scientists they ask questions and

there have been astronomers who
are astronomers I call them astronomers

because that's what they're becoming
They have been citizens who have

become more scientifically active simply
by interacting a lot based on what they

see in the data and there's many famous
examples of this that I could give you but

what you get out of this universe
is what you put into it and

certainly people people do come work with
us as a consequence of these activities.

You can send me an email and
I will sit quietly answer it.

Right here yes or question you gave us.

A very nice.

Talk about how BIG used
a different planet so

my question is what different do
you want to find planets right so

so so it's said this on the side I
guess I didn't say it out loud but

every exoplanet known fails the I
you definition of planet because and

this is very curious culturally
to me because when we voted on

the definition planet the first
definition said you must orbit a star and

then the final definition that says you
must orbit the sun and so by definition

every exoplanet fails the definition
of planet the current definition of

planet is unique to our solar system and
as an astronomer who likes

to have as much data as I can in my hands
that seems kind of crazy because I really

should be using what the architecture
of every solar system in the galaxy is

to help inform me about how planetary
systems form what planetary systems look.

Like how they evolved what the
distribution of planets looks like except

us so so this is probably the single
most important reason why we absolutely

have to go back to this definition because
it was made after we started discovering

exoplanets But before we had eight
thousand of them in our pocket.

And more to come right this
is not going to stop right.

Have we been able to do any spectroscopic
analysis of Kepler one eighty six S Yeah

so this is the next holy grail
in planetary science is being

able to spectroscopic only observe
the planets themselves and

the difficulty with that is getting
a telescope that is capable of separating

the star from the planet itself Now the
reason we care about that is because we

want to understand the atmosphere and
the atmosphere's themselves will

tell us questions like Is
there water are there other.

Bio molecule markers people people have
this kind of idea that if there's life on

those worlds it's like our life that the
atmosphere will look like the atmosphere

on Earth and you'll be able to tell that
they're burning all their carbon fuels and

all that sort of stuff but we'll see but
really it's we're limited right now by

technology technology will enable that and
so if you listen to your friends who study

planets they will talk about building
thirty meter telescopes in space or

you know all kinds of crazy ideas about
what it would take to do this and

it's not outside ever reach we can do
it we know exactly what we need to do.

Cost a bit money right.

How do you go about.

Understanding a systematic sufficiently
well detection processes to make confident

estimates about the actual population
parameters you mean so when you say

the actual population parameters you mean
the programmers the Describe the total

population plants in the galaxy Yeah so
so this is.

So so let me just talk.

About one parameter OK and the one
parameter that astronomers planetary

astronomers really care about is something
called Ada earth which is the fraction

of planets in the in the galaxy that
have planets that are roughly your size

terrestrial worlds rocky planets like
Mercury Venus Earth and Mars and so

when we set out to start
measuring planets we knew

that we would have to categorize enough
stars that we could accurately predict

based on our sample how many of those
stars have small rocky worlds and right

now the earth the exact value I don't know
off the top of my head is something like

point six point seven but the astronomers
are of a mind that our statistics are so

good that we're willing to just assume the
earth equals one because in all of these

planets where we see multis and we have
no reason to believe that systems don't

have multiple planets we do see small
rocky many of them are super Earth at this

point you know things larger than Earth
but but we're pushing the technology is

allowing us to reach down to those things
that are exactly the size of Earth and

we're beginning to suspect
that any time you have a multi

you're going to have rocky worlds and
heuristically in terms of theory and

our understanding of how plants form
that kind of makes sense are theoretical

prejudices that yeah rocky rocky worlds
are going to form because the stuff

that forms the Terri Nebula the gas and

dust that forms the star and then
the planets is silicates and rocks and

stuff that we're made of and so
it theoretically makes sense but

it's good to see in the data that it
happens and so really the question about

a the Earth early on was just two stars
have planets we see stars have planets

just uniformly and so we're we're we're
willing many astronomers You will hear say

it Earth equals one but in the actual data
you know we just haven't detected an Earth

like planet around everyone but we think
it's one because we've just seen planets

around every single freakin
star we look at right now.

Or.
Let's think or speak or one more time and

if you have questions everybody's
going to speak with them thank.

You.