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So when we think about ocean worlds, it's
good to compare them to what

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we know about Earth. In total proportion, Earth
is about point-one-percent water.

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An ocean world is a body that has, in
proportion, about ten times more water

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than Earth does.

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And when we think of the TRAPPIST planets,
those planets have about fifty times

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more water in proportion to what Earth does.

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Ocean worlds do appear to be common in our
galaxy. As far back as the early 2000's,

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We had astronomers, some of them still here at
NASA Goddard that suggested that we would

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have ocean worlds orbiting low-mass stars.

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Recently we've looked at about fifty-two
exoplanets, and these are low-mass exoplanets

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and what we've found is, of these fifty-two
planets one out of every four

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may be an ocean planet.

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And when it comes to these ocean planets,
over half of them may be ice-covered

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ocean worlds, and so Enceladus and Europa
may serve as small-scale analogues

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of these planets.

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So there are a number of different ways to
search for life on planets around other stars,

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but the key method is the study of the atmospheres.

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We can search for signs of life - biosignatures,
we call them, things like oxygen,

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water vapor, carbon dioxide, even more
unusual biosignatures -

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things like chlorofluorocarbons, or other things
that are only produced by intelligent life.

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By looking for these key constituents of
planetary atmospheres that signal life,

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We can discover lifeforms on other planets
that we could never actually visit in our lifetime.

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So this is very analogous to how we study the
atmospheres of moons and planets

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in our own solar system, and really makes the
connection between studying the plumes

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of Europa, and the atmospheres of planets
around other stars.

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What I would like to see is the definition
of a habitable zone expanded

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We don't want to keep thinking too narrow
about liquid on the surface, broaden the scope

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and really try to embrace other worlds that
might seem too far from their host star,

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and frozen out, when they really aren't frozen
at all. At great depths, they harbor

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a warm, hydrothermal-driven, liquid water
environment.

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