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So when you look out
at the stars at night,

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it's amazing what you can see.

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It's beautiful.

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But what's more amazing
is what you can't see,

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because what we know now

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is that around every star
or almost every star,

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there's a planet,

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or probably a few.

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So what this picture isn't showing you

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are all the planets that we know about

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out there in space.

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But when we think about planets,
we tend to think of faraway things

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that are very different from our own.

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But here we are on a planet,

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and there are so many things
that are amazing about Earth

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that we're searching far and wide
to find things that are like that.

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And when we're searching,
we're finding amazing things.

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But I want to tell you
about an amazing thing here on Earth.

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And that is that every minute,

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400 pounds of hydrogen

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and almost seven pounds of helium

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escape from Earth into space.

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And this is gas that is going off
and never coming back.

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So hydrogen, helium and many other things

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make up what's known
as the Earth's atmosphere.

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The atmosphere is just these gases
that form a thin blue line

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that's seen here from
the International Space Station,

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a photograph that some astronauts took.

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And this tenuous veneer around our planet

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is what allows life to flourish.

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It protects our planet
from too many impacts,

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from meteorites and the like.

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And it's such an amazing phenomenon

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that the fact that it's disappearing

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should frighten you,
at least a little bit.

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So this process is something that I study

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and it's called atmospheric escape.

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So atmospheric escape
is not specific to planet Earth.

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It's part of what it means
to be a planet, if you ask me,

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because planets, not just here on Earth
but throughout the universe,

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can undergo atmospheric escape.

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And the way it happens actually tells us
about planets themselves.

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Because when you think
about the solar system,

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you might think about this picture here.

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And you would say, well,
there are eight planets, maybe nine.

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So for those of you
who are stressed by this picture,

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I will add somebody for you.

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(Laughter)

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Courtesy of New Horizons,
we're including Pluto.

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And the thing here is,

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for the purposes of this talk
and atmospheric escape,

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Pluto is a planet in my mind,

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in the same way that planets
around other stars that we can't see

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are also planets.

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So fundamental characteristics of planets

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include the fact that they are bodies

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that are bound together by gravity.

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So it's a lot of material
just stuck together

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with this attractive force.

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And these bodies are so big
and have so much gravity.

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That's why they're round.

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So when you look at all of these,

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including Pluto,

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they're round.

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So you can see that gravity
is really at play here.

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But another fundamental
characteristic about planets

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is what you don't see here,

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and that's the star, the Sun,

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that all of the planets
in the solar system are orbiting around.

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And that's fundamentally driving
atmospheric escape.

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The reason that fundamentally stars
drive atmospheric escape from planets

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is because stars offer planets
particles and light and heat

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that can cause the atmospheres to go away.

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So if you think of a hot-air balloon,

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or you look at this picture
of lanterns in Thailand at a festival,

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you can see that hot air
can propel gasses upward.

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And if you have enough energy and heating,

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which our Sun does,

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that gas, which is so light
and only bound by gravity,

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it can escape into space.

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And so this is what's actually
causing atmospheric escape

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here on Earth and also on other planets --

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that interplay
between heating from the star

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and overcoming the force
of gravity on the planet.

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So I've told you that it happens

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at the rate of 400 pounds
a minute for hydrogen

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and almost seven pounds for helium.

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But what does that look like?

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Well, even in the '80s,

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we took pictures of the Earth

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in the ultraviolet

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using NASA's Dynamic Explorer spacecraft.

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So these two images of the Earth

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show you what that glow
of escaping hydrogen looks like,

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shown in red.

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And you can also see other features
like oxygen and nitrogen

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in that white glimmer

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in the circle showing you the auroras

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and also some wisps around the tropics.

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So these are pictures
that conclusively show us

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that our atmosphere isn't just
tightly bound to us here on Earth

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but it's actually
reaching out far into space,

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and at an alarming rate, I might add.

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But the Earth is not alone
in undergoing atmospheric escape.

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Mars, our nearest neighbor,
is much smaller than Earth,

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so it has much less gravity
with which to hold on to its atmosphere.

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And so even though Mars has an atmosphere,

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we can see it's much thinner
than the Earth's.

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Just look at the surface.

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You see craters indicating
that it didn't have an atmosphere

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that could stop those impacts.

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Also, we see that it's the "red planet,"

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and atmospheric escape plays a role

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in Mars being red.

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That's because we think
Mars used to have a wetter past,

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and when water had enough energy,
it broke up into hydrogen and oxygen,

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and hydrogen being so light,
it escaped into space,

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and the oxygen that was left

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oxidized or rusted the ground,

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making that familiar
rusty red color that we see.

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So it's fine to look at pictures of Mars

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and say that atmospheric escape
probably happened,

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but NASA has a probe that's currently
at Mars called the MAVEN satellite,

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and its actual job
is to study atmospheric escape.

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It's the Mars Atmosphere
and Volatile Evolution spacecraft.

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And results from it have already
shown pictures very similar

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to what you've seen here on Earth.

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We've long known that Mars
was losing its atmosphere,

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but we have some stunning pictures.

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Here, for example,
you can see in the red circle

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is the size of Mars,

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and in blue you can see the hydrogen
escaping away from the planet.

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So it's reaching out more than 10 times
the size of the planet,

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far enough away that it's
no longer bound to that planet.

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It's escaping off into space.

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And this helps us confirm ideas,

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like why Mars is red,
from that lost hydrogen.

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But hydrogen isn't
the only gas that's lost.

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I mentioned helium on Earth
and some oxygen and nitrogen,

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and from MAVEN we can also look
at the oxygen being lost from Mars.

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And you can see
that because oxygen is heavier,

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it can't get as far as the hydrogen,

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but it's still escaping
away from the planet.

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You don't see it all confined
into that red circle.

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So the fact that we not only see
atmospheric escape on our own planet

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but we can study it elsewhere
and send spacecraft

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allows us to learn
about the past of planets

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but also about planets in general

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and Earth's future.

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So one way we actually
can learn about the future

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is by planets so far away
that we can't see.

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And I should just note though,
before I go on to that,

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I'm not going to show you
photos like this of Pluto,

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which might be disappointing,

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but that's because we don't have them yet.

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But the New Horizons mission
is currently studying atmospheric escape

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being lost from the planet.

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So stay tuned and look out for that.

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But the planets
that I did want to talk about

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are known as transiting exoplanets.

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So any planet orbiting a star
that's not our Sun

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is called an exoplanet,
or extrasolar planet.

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And these planets that we call transiting

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have the special feature

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that if you look
at that star in the middle,

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you'll see that actually it's blinking.

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And the reason that it's blinking

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is because there are planets
that are going past it all the time,

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and it's that special orientation

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where the planets are blocking
the light from the star

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that allows us to see that light blinking.

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And by surveying the stars
in the night sky

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for this blinking motion,

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we are able to find planets.

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This is how we've now been able
to detect over 5,000 planets

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in our own Milky Way,

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and we know there are
many more out there, like I mentioned.

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So when we look at the light
from these stars,

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what we see, like I said,
is not the planet itself,

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but you actually see
a dimming of the light

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that we can record in time.

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So the light drops as the planet
decreases in front of the star,

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and that's that blinking
that you saw before.

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So not only do we detect the planets

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but we can look at this light
in different wavelengths.

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So I mentioned looking at the Earth
and Mars in ultraviolet light.

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If we look at transiting exoplanets
with the Hubble Space Telescope,

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we find that in the ultraviolet,

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you see much bigger blinking,
much less light from the star,

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when the planet is passing in front.

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And we think this is because you have
an extended atmosphere of hydrogen

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all around the planet

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that's making it look puffier

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and thus blocking
more of the light that you see.

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So using this technique,
we've actually been able to discover

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a few transiting exoplanets
that are undergoing atmospheric escape.

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And these planets
can be called hot Jupiters,

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for some of the ones we've found.

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And that's because
they're gas planets like Jupiter,

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but they're so close to their star,

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about a hundred times closer than Jupiter.

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And because there's all this
lightweight gas that's ready to escape,

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and all this heating from the star,

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you have completely catastrophic rates
of atmospheric escape.

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So unlike our 400 pounds per minute
of hydrogen being lost on Earth,

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for these planets,

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you're losing 1.3 billion
pounds of hydrogen every minute.

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So you might think, well,
does this make the planet cease to exist?

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And this is a question
that people wondered

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when they looked at our solar system,

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because planets
closer to the Sun are rocky,

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and planets further away
are bigger and more gaseous.

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Could you have started
with something like Jupiter

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that was actually close to the Sun,

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and get rid of all the gas in it?

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We now think that if you start
with something like a hot Jupiter,

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you actually can't end up
with Mercury or the Earth.

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But if you started with something smaller,

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it's possible that enough gas
would have gotten away

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that it would have
significantly impacted it

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and left you with something very different
than what you started with.

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So all of this sounds sort of general,

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and we might think about the solar system,

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but what does this have to do
with us here on Earth?

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Well, in the far future,

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the Sun is going to get brighter.

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And as that happens,

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the heating that we find from the Sun
is going to become very intense.

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In the same way that you see
gas streaming off from a hot Jupiter,

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gas is going to stream off from the Earth.

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And so what we can look forward to,

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or at least prepare for,

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is the fact that in the far future,

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the Earth is going to look more like Mars.

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Our hydrogen, from water
that is broken down,

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is going to escape
into space more rapidly,

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and we're going to be left
with this dry, reddish planet.

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So don't fear, it's not
for a few billion years,

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so there's some time to prepare.

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00:11:05,080 --> 00:11:06,136
(Laughter)

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00:11:06,160 --> 00:11:08,976
But I wanted you
to be aware of what's going on,

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not just in the future,

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but atmospheric escape
is happening as we speak.

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So there's a lot of amazing science
that you hear about happening in space

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and planets that are far away,

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and we are studying these planets
to learn about these worlds.

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But as we learn about Mars
or exoplanets like hot Jupiters,

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we find things like atmospheric escape

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that tell us a lot more
about our planet here on Earth.

250
00:11:33,760 --> 00:11:37,816
So consider that the next time
you think that space is far away.

251
00:11:37,840 --> 00:11:39,056
Thank you.

252
00:11:39,080 --> 00:11:42,120
(Applause)