[MUSIC PLAYING]
Too many stars, too little time.
A couple of years
ago in April 2013,
the MIT-led NASA mission TESS
launched on SpaceX Falcon
9 rocket out of Cape Canaveral.
It was a spectacular launch.
And TESS is a mission
that is looking
for planets around other stars.
Here's an image taken by TESS.
This actually is just a tiny
fraction of what TESS sees.
TESS is essentially four
glorified telephoto lenses.
They're only about 10
centimeters in diameter.
But they have a giant baffle
to block out stray light.
And these four cameras
are wide field.
And they're all bolted on
a platform on a spacecraft
in a big orbit around Earth.
And they stare at one patch
of the sky for a month,
a giant strip of the sky.
I'm one of the leaders
of the TESS mission.
And we actually meet
every Tuesday and Thursday
to find planets, actually.
Our computers look at all
this data, look at millions
of stars, and flag planets.
And we search through them.
Here's how TESS finds planets.
You can see the planet
going in front of the star.
We don't see that.
That's an artist's conception.
We see what you're seeing on
the graph, where a planet goes
in front of-- we measure the
brightness of stars, millions
of stars as a function of time.
And we see a tiny
drop in brightness
that may be due to a planet.
And when this planet finishes
going in front of the star,
the light curve, or the
brightness of the star,
returns to normal.
And each time the
planet orbits the star,
that same drop in
brightness will
happen over and over again.
And actually, thousands of
planets around other stars,
called exoplanets, have
been found this way.
And we meet every
Tuesday and Thursday
to review planet
candidates that have
come from our computer searching
the brightnesses of millions
of stars.
And there's kind of a long
story about the field.
It's quite mature now.
It's even somewhat tedious.
That's hopefully
not being recorded.
But we find these
planets all the time.
There's thousands of them.
So 20 years earlier,
I was a postdoc
at an iconic, idyllic
place called the Institute
for Advanced Study.
It's not at
Princeton University.
It's in Princeton
about a mile away.
Back in those days, we
didn't have thousands
of planets, about 30 planets.
And I put this chart
for you, showing you
the number of planets
as a function of year.
So you can see on the
far right right now,
there's lots of planets,
hundreds of them.
But earlier on, when this takes
place, there's about 30 or 40.
And there's no planets that
go in front of the star.
Actually, for a planet to
go in front of the star,
as seen from our
telescopes, the orbit has
to be very specially aligned,
like literally just edge on.
So we only see some
small fraction of them.
One had been found but had
been an already known planet
that people had followed
up and seen to transit.
There's a lot of ways to find
planets listed there in colors.
You don't have to
worry about those.
But the transit
technique that we
use with TESS that's so standard
today, it wasn't in use then.
When I was at Princeton--
when I was at the Institute
for Advanced Study,
I met Gabriela, also a postdoc.
She was a postdoc at Princeton.
And it was just so fun.
We met.
We just really hit it off.
She was just so funny.
She was so brilliant at math.
And it was the first time I
had a best friend in science.
Now, what was great about
Gabriella was, in addition
to being a postdoc
at Princeton, she
had an affiliation in Chile.
And in Chile, there are
telescopes on the mountains,
because it's so dry,
and the air is clear.
And you can do great astronomy.
But each country that
owns a telescope there,
they give 10% to 15% of their
telescope time to Chileans.
So she counted as a Chilean.
So together, we sat
down and figured
out what could we do to apply
for this telescope time.
And we decided we would
search for planets
by the transit technique,
which wasn't a thing then.
There were less than five
groups working on it.
But it was still competition.
And we were postdocs.
It wasn't our
specialty or anything.
So we proposed to use
this telescope in Chile.
It was a 4 meter telescope,
wide field of view.
And we were successful.
And Gabriela was an observer.
And she was really great
at math and algorithms.
I was good with strategy.
And I worked on exoplanets.
I was-- I won't say I was.
Or maybe I should say I am or
I was great at programming.
So she would dictate algorithms.
And I would literally
just program them.
And we worked so hard.
We were working nonstop.
I literally could program
without making a mistake
in those days.
Yeah, so we worked a
lot on this project.
And so other people
were working on it too.
It was amazing.
It's like going
to the North Pole.
It's like exploring.
There was this wide open field
ahead of us, so many stars,
so many planets.
Someone was going to be
the first one to find it.
And we wanted it to be us.
It was way ambitious
for two postdocs.
We actually added some more
people to the team later.
We'd tell people
about the project.
And they would say,
well, who's the PI?
It's like, I'm the
PI and Gabriella.
And they'd be, no, no, no, wait.
Who's in charge?
It's like, no, we are.
OK, so here's a small tiny
area of the star field.
It's just for show.
Our field was very wide.
We had lots and lots of stars.
It was 1,400 times
bigger than this.
It's just to show you some
stars and what they were like.
We worked hard.
We were having a great time.
We would work.
And then she'd come
to see my family.
She'd hand out with
me at my apartment.
We'd take my dog for a walk.
We'd go up for lunch.
We'd go out for dinner.
We were just working nonstop
trying to get this job done.
Let's see.
OK, so here's some data.
Each one of these
is a target star.
So I'm just showing you three
stars out of tens of thousands
of stars.
On the y-axis, it's showing
you brightness of stars.
Don't worry about the unit.
It's this arcane kind
of astronomy unit.
And on the bottom
is time in days.
So each one of these rows is
one night, essentially 10 hours.
And so it's like
reading a book almost,
like read from the
bottom up to the top.
We had 11 nights.
One's missing, because
it was raining.
It's not supposed to
rain there actually,
but we wanted a lot
of telescope time.
And they gave us time
in Chilean winter, which
is when it rains, actually.
It rains there.
It's bad for astronomy.
And some of the data is
crummy, because it was cloudy,
or the air was messy.
And so on the left here, you
can see it's a variable star.
It's a star that's changing
and brightness on and off,
on and off, on and off.
It's called the
delti scudi star.
And there's lots of stars out
there that are just varying.
In the middle, you see
a drop in brightness,
like the cartoon
thing I showed you.
It turns out that
one is too deep
and it lasts too
long to be a planet.
It's some kind of binary star.
This next one, we
actually thought
that we found the first planet
ever by this new method.
And we were really
excited about this planet.
We told the other
members of our team.
We started out writing our
paper that would go in Nature,
of course.
And we actually had some
other telescope time
to follow up this planet,
because this is just
showing you a little
drop in brightness.
But it turns out lots of
things can cause a drop
in brightness, two stars.
There's other kinds
of false positives.
And we had to get time on
a bigger telescope using
a different technique that--
to make a long
story short, we just
needed it to follow up
and get a planet mass
and figure out what it was.
So we got the time.
And it was queue observing
so the data came to us.
Gabriella analyzed the data.
We knew what we should see
with this other technique.
A planet's a planet.
And some stuff
were known by then.
It was supposed to show almost
like a sinusoidal variation
that match the period
of this object.
And what did Gabriella see?
Absolutely nothing in the data.
It turned out that
what had happened was
we had discovered
this false positive.
We hadn't found a planet.
We'd found a false positive.
It's called a blend.
And what happened was,
we had seen, presumably,
a star, instead of a planet
orbiting a star, a star
orbiting, transiting a star.
And that would have a giant
drop in brightness, so, so huge.
But there was another
star on the same pixel.
It must have been like
a triple star system.
And that other star, shown
here, was throwing light
into the transit signal,
making it super shallow.
This is extremely common now.
Back then, we didn't know.
We thought we had a planet.
We spent all its telescope
time following it up.
This is extremely common.
No problem, back to work,
back to working nearly
around the clock and not doing
anything else on the project.
So one day, Gabriella came
into work inconsolable.
I couldn't get
anything out of her.
She was incredibly upset.
But she'd come to
tell me something.
It's like, did someone die?
I don't know what this was.
And eventually, she was able to
kind of spill it out that all
that summer-- and this was
towards the end of the summer
when we were rushing
to get our work done--
she had been talking to a
famous old male astronomer
at Princeton.
And he had been
asking her almost
on a daily basis about the
project, technical details,
really nitty gritties.
She had just found
out he was working
on the same project,
different mountain
also in Chile,
different telescope.
And honestly, if he's such
a super famous astronomer,
why did he need to do this?
He didn't, right?
He could have figured
this out with his team.
Well, Gabriella went
into a mental dive
and never really recovered
enthusiasm for the project.
They didn't have mind heart
and all that back then.
But the problem was my bad.
I didn't have the skill to pick
up her part of the project.
And so it just
kind of languished.
When you make an experiment--
because before that,
I had just been a
programmer, like a modeler.
So at least I got
to understand data.
That's what my
postdoc advisor said.
But if you do an experiment,
it's like a layer cake.
Every layer needs to
be just right, perfect.
We had a problem, because
one of our layers was bad.
And that was our telescope
was so big, 4 meters.
We had to look at
very, very faint stars.
They were really too faint to
do the kind of follow up work
that we needed.
Remember I told you TESS
has these 10-centimeter--
I'm talking about four meter.
Well, that was too bad,
because we couldn't really--
it was kind of a
mistake, looking back.
But this other professor's
team, meanwhile, they
were also looking at
really faint stars.
And they put out 20
planet candidates.
And a different team followed
them up to get the masses,
see what they were.
And later on that same
year, someone else
found the first planet
by the transit technique.
I'd invested a lot in this.
And it was my best
friend who, at the time,
was kind of in the
process of losing.
I was just crushed.
And I don't seem
that upset about it
now, because it's
a long time ago.
I was just so crushed.
And I remember, I went to
a conference out in Seattle
and, with a really good
friend, went on a hike
on Vancouver Island.
And I saw a bald eagle
fly low across the river.
And I just thought,
you know, the world
is still a beautiful place.
So one thing, that blended
object I told you about,
that false positive, we
noticed that the transit light
curve was really suspicious.
When a planet
transits a star, it
crosses the limb of the star.
And the light just drops.
It's like a box.
But this wasn't.
It was very, very slanted.
And so Gabriella and
I started working out
the equations that
describe a light curve,
looking at them more closely.
It was just algebra.
We were writing on blank paper.
We were in her small office.
There was paper everywhere.
We went out for lunch.
And then we went out for dinner.
It was nearing midnight.
It struck midnight.
And we just started seeing
something really familiar
in all this crazy algebra.
And at the very same moment,
we both looked at each other,
burst into laughter,
and said, density.
We had discovered that the light
curve of the star, the light
curve from the planet transit,
it can actually tell you
about the density of the star.
Now, I don't have
time to explain why.
But I know most of you would
understand it if I had time.
But we actually know the density
of stars because of the star
type, the type of stars.
There's a grid library that
you can associate it with.
And you know the mass and
size, therefore the density.
So if the density
from the light curve
does not absolutely match
the density of the star
just from what you know
about stars regularly,
then there's no possible way
that that light curve can
be due to a transiting planet.
So we had discovered that the
precise shape of the transit
can tell you whether it's a
blended false positive or not.
So I kind of realized
this is the only thing
I'm going to get
out of the project,
despite all those
years of working.
So I worked really hard.
We worked really hard.
We got our paper out.
And even though it's
not my favorite paper,
I'm still proud to tell
you that this paper
gets cited a few
dozen times per year
and has hundreds and hundreds
and hundreds of citations.
It was a paper I wrote at
the beginning of my career.
But it still has really
stood the test of time.
Now, I actually know there's a
planet in that data somewhere
I'm convinced, just
because of the numbers.
And I still do feel
sad about it, actually.
And all the things
that went wrong,
I still carry those
with me today.
And they guide my decisions
in how I go about my work
and choose my projects today.
Thank you.
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