____________________
history
we
have
wondered
about
the
possibility
of
life
beyond
the
Earth
.
It
is
only
in
recent
years
,
however
,
that
advances
in
technology
____________________
revealed
the
existence
of
extrasolar
planets
(
or
"
exoplanets
"
)
,
____________________
is
to
say
,
planets
which
orbit
not
our
own
Sun
,
but
other
stars
in
the
universe
.
So
____________________
,
astronomers
have
identified
a
few
thousand
exoplanets
,
but
believe
that
billions
more
exist
.
Although
many
astronomers
believe
that
a
____________________
number
of
planets
in
the
universe
are
capable
____________________
supporting
some
kind
of
living
organism
,
____________________
or
not
life
has
developed
on
any
of
them
is
not
yet
known
.
An
essential
requirement
for
life
is
liquid
water
.
If
a
planet
is
to
have
liquid
water
on
its
surface
,
its
temperature
must
be
____________________
too
hot
nor
too
cold
.
However
,
such
a
planet
,
other
____________________
the
Earth
,
has
yet
to
be
discovered
.
It
may
be
found
in
the
near
future
,
____________________
.
Move
on
to
your
next
mission
if
you
really
want
to
know
more
.
