There is actually a lot more out there than we
know! For a long time, scientists knew of 9
planets within our solar system. This number
included a planet called Pluto, which was the
farthest from the sun. As telescopes got better,
astronomers discovered that there are many
thousands of rocky chunks out by Pluto — and that
Pluto is basically one of the largest. So why
call that a planet and not the rest? As a
result, astronomers decided to call Pluto — and
the rest of those bodies — “plutoids”. So now
there are officially 8 planets in our solar
system, and a great many plutoids.
But beyond that — in the last 20 years,
astronomers have begun to discover that there are
planets around other stars, which are outside of
our solar system. These are called
“exoplanets” (exo means “outside”, so
they’re planets outside our solar system).
Since 1988, a few thousand new exoplanets have
been discovered orbiting other stars! These
are very far away, and are very hard to see.
More are being discovered every year, and
astronomers are getting closer and closer to
finding some that are even “earth-like” — which
have the right size, distance from the star (so
they’re not too hot and not too cold) and may even
have water. If (or when) such earth-like planets
are discovered, then the conditions would be right
for life as we know it, and it would be very
exciting question would be whether something may
be living there...
Planets that are large or close in to the solar
system are noticeable, so I think I can say
confidently that we've found all of them. The
Kuiper objects - Pluto, Eris, and many others
- are far from the sun, faint, and small,
which makes them difficult to detect. There could
easily be many more Pluto/Eris-sized Kuiper
objects far out enough that we haven't seen them
yet. And then, there are other stars, and they
have planets, too.
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