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If
an
exoplanet
happens
to
transit
its
star,
we
can
often
determine
both
its
mass
and
its
radius,
and
hence
estimate
its
density
and
possibly
even
its
composition.
More
than
200
transiting
hot
Jupiters
have
been
measured
in
this
way.
But
when
astronomers
calculate
their
densities
the
numbers
they
obtain
are
generally
much
lower
than
predicted
by
theory.
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