Video summary
In 2006, the International Astronomical Union established a formal definition for what constitutes a planet, which fundamentally changed our understanding of the solar system's classification. To be considered a planet, an object must orbit the Sun, possess sufficient mass to assume a spherical shape under its own gravity, and have cleared its orbital path of other debris. Pluto failed to meet the third criterion because it resides in the Kuiper Belt, a region crowded with numerous objects of comparable size, preventing it from dominating its orbit. Consequently, a new category called "dwarf planet" was created to classify these bodies, resulting in the reclassification of Pluto and leading to the identification of four other dwarf planets: Ceres, Eris, Haumea, and Makemake.
Ceres, discovered in 1801, holds the unique distinction of being the only dwarf planet located within the asteroid belt between Mars and Jupiter. Originally mistaken for a new planet, it was later reclassified as an asteroid before finally becoming a dwarf planet. Its surface is heavily cratered and geologically inactive, though recent imagery reveals bright patches likely composed of salty deposits rather than ice. Alongside Pluto, Ceres is one of only two dwarf planets that have been visited by spacecraft; the New Horizons mission flew by Pluto in 2015, revealing an icy world with methane and nitrogen ices, a thin nitrogen atmosphere, and surprisingly young terrain in some areas where geological activity has wiped out craters.
The remaining three dwarf planets—Eris, Haumea, and Makemake—are all located in the Kuiper Belt and have not yet been visited by spacecraft, making detailed study difficult. Eris is comparable in size to Pluto and possesses a moon named Dysnomia, which allows astronomers to calculate its mass. Haumea is distinct because it is not spherical but rather an ellipsoid or flattened shape, a stable configuration for such a massive object, and it has two known moons. Makemake is another large object in the Kuiper Belt that appears as a bright star from Earth's perspective due to the Sun's diminished appearance at such distances. While future missions may eventually explore these distant worlds, current technology limits our knowledge primarily to observations made from Earth.
In summary, the discovery of numerous large objects in the outer solar system necessitated a refinement of planetary definitions, leading to the demotion of Pluto from major planet status to dwarf planet. This shift highlights the dynamic nature of astronomical classification and underscores the importance of spacecraft missions like New Horizons in providing the first detailed views of these distant worlds. As we continue to study Pluto's complex surface, atmosphere, and its tidally locked moon Charon, our understanding of the Kuiper Belt expands, offering insights into the formation and evolution of the solar system's outer regions.
Read the full video transcript
Greetings and welcome to the
introduction to astronomy.
In this lecture, we are going to talk
about Pluto as well as the other dwarf
planets in the solar system. So, this is
a relatively new object at least in
terms of definitions as to what type of
planet that had not been classified
previously.
So, let's look a little bit about the
definition of a planet. What is a
planet? Well, really was never formally
defined until 2006.
Now, in 2006, the International
Astronomical Union decided that a planet
must do two three things. First of all,
it must orbit the sun. That removes
moons from being planets even if they
are larger than some of the planets and
we do have at least one moon that is
larger than Mercury. But, if it doesn't
orbit the sun, it's not a planet.
It must be massive enough to pull itself
into a spherical or an ellipsoidal shape
under gravity. So, it's gravity must be
dominant that it pulls it in
to that compact shape. It cannot have an
irregular shape.
And it must be able to clear its orbit
of debris.
So, things like the asteroid belt and
the Kuiper belt that we'll look at would
objects there would not be able to be
classified as planets because there is
still a lot of debris, lots of material
in those in those regions.
Now, the discover what really
necessitated this is that we began
discovering a large number of large
objects in the Kuiper belt that really
necessitated this change. The asteroid
belt, there's only one large object. Uh
so, it really wasn't as big of a deal
there. However, when we started finding
more objects, especially those similar
in size to Pluto, this started to become
a problem.
So, is Pluto a planet? Well, Pluto does
orbit the Sun, so it meets the first
criteria.
It is massive enough to pull itself in a
spherical
or ellipsoidal shape. That meets the
second criteria. However, it does not
meet the criteria to be able to clear
its orbit of debris. It orbits in the
Kuiper Belt, which has lots of other
objects, many small, but some that are
comparable in size to Pluto.
So, what was done was to make a new
classification of object, and that was a
dwarf planet. And there are now five
dwarf planets in the solar system that
have been confirmed, and we will look at
those in turn.
So, let's look at overview here. What
are the dwarf planets? Well, the first
dwarf planet was actually discovered in
1801, and that was Ceres. This was the
largest object in the asteroid belt, and
is the only dwarf planet in the asteroid
belt.
When Ceres was discovered in 1801, it
was thought it might be a new planet.
So, it was originally classified as a
planet existing between Mars and
Jupiter. However, we noted shortly
thereafter discovery of other objects in
that area, and therefore it was
reclassified to an asteroid, and then
reclassified again to a dwarf planet.
So, Ceres has had many classifications
over the little over 200 years we have
known of it.
Pluto was discovered in 1930, and was
visited by the New Horizons craft in
2015.
Only Ceres and Pluto have been visited
by spacecraft.
And then we've discovered Haumea in
2004, Eris in 2005, Makemake in 2005.
Eris of these is the one that is
comparable in size to Pluto. Maybe a a
bit bigger, maybe a little bit smaller,
about the same same size though. And
it's difficult because we don't have
direct measurements of it. We have seen
it from Earth, but we have not visited
Eris.
So, let's look at each of these in a
little bit more detail. First of all,
Ceres.
Ceres is the largest object in the
asteroid belt and was is only about a
thousand kilometers across.
It was discovered in 1801 and as I've
mentioned, it was originally thought to
be a new planet. Then it became an
asteroid. Now it is a dwarf planet. So,
it's gone through a number of different
changes in its
uh classification over the years. Now,
we'll look at this in a little bit more
detail actually when we talk about the
asteroids, but it is one of those craft
that has been studied one of those
objects that has been studied close up
by spacecraft and we can see it is
heavily cratered, although we do see
some very bright areas here. Not ice as
we saw in the moons of the outer
planets, but maybe some kind of salty
deposit that is dug up in these impacts.
So,
very heavily cratered, very old surface
that's been around for a long time. No
sign of geological activity.
Now, of course, the best known of the
dwarf planets is Pluto and Pluto was
discovered in 1930 by Clyde Tombaugh and
he was searching for a planet that was
disturbing the disturbing the orbit of
Neptune. Now, what that is is that there
was Neptune didn't seem to be orbiting
exactly the way we thought. However,
what was
what was known was we had used this
method to discover Neptune previously
because of irregularities in the orbit
of Uranus and that led to the discovery
of Neptune, so it was thought to do the
same thing. However, it took an
exhaustive study of many regions of the
sky to actually find Pluto.
When we looked at this from earlier
studies from Earth, we get images like
this. So, here is looking for some
companions stars
companion moons and we can see several
moons there in the image.
And of course, some of the best images
would be from Hubble Space Telescope and
we can see some lighter and darker
terrain. So, until about 2015, this was
our best image of Pluto. We knew there
were some lighter and darker areas, but
we really didn't have any idea of the
detail.
Now, in 2015, it was visited by the New
Horizon spacecraft.
And we found that it had an icy surface
like the large moons. That was pretty
much expected. It had ices of methane
nitrogen. So, those were things that
would be expected in that part of the
solar system. What was not expected was
that there were some regions with very
few craters. So, it's an active object
because some certain areas have had
regions that have been wiped out of
craters.
And the darker areas, which are more
heavily cratered. So, in a way like our
moon except in reverse. On the moon, the
darker areas are the lighter lightly
cratered ones.
On
Pluto,
the dark areas are more heavily
cratered. And we saw a variety of
different types of terrain on the
surface. So, we can look at some of
those images.
Here we see some of those regions and uh
various different type fault lines, very
light planes, very low-lying areas,
heavily cratered areas. So, it was very
interesting surface that we saw and not
quite what was expected.
We saw signs of some of the things some
of the close-up areas very, very
flat. So, we can see how nice and smooth
some of these regions are and that they
have been flooded. And in fact, some of
this jumbled material could be leftover
material from craters that did not get
filled in when the material flowed. So,
this could have been the higher-lying
areas that were left behind when some
icy material flowed through.
Now, we also saw that there was a very
big variety just in a very short range.
Here we have some of this region very,
very flat and smooth with some pieces
sticking through where lava had flooded.
Here a not much higher region kind of on
the edge of this that was left behind.
And then we go into this more jumbled
region and then we have a more heavily
cratered region here. So, just in a
relatively small region, we do see a
number of different types of terrain.
And astronomers uh even nearly a decade
later are still studying the images New
Horizons sent back from Pluto to learn
more about this distant dwarf planet.
Now, one thing we do know is that Pluto
does have a very thin atmosphere of
nitrogen gas. This can be seen in an
image like this one from behind actually
when New Horizons was leaving Pluto, it
looked back towards the sun and could
see Pluto's atmosphere in relief. It's
primarily nitrogen gas because nitrogen
gas freezes out at a very, very low
temperature. And at this point, Pluto
was still warm enough to have a thin
atmosphere. Pluto is currently in an
elliptical orbit and moving further away
from the sun, so that atmosphere will
likely freeze out onto the surface for
hundreds of years until Pluto comes in
closer again.
Now, Pluto does have a number of moons
including the large moon Charon,
uh which was discovered in 1978 and
looks in many ways a lot like Pluto.
We see definitely signs of activity on
the on the surface, some heavily
cratered regions, but some cracking on
the surface as well and maybe some
flatter areas as well with fewer
craters.
So, it's a very interesting image and it
is tidally locked to Pluto and Pluto is
tidally locked to Charon. Those two are
locked together and means they always
keep the same side facing each other.
So, one side of of Charon always faces
Pluto, just like our moon does with
Earth, but also one side of Pluto points
toward Charon. So, that means if you're
on one part of Pluto, you can see Charon
in the sky and it will just sit there.
However, if you're on the other side,
you never see the moon. It will never
rise or set. It is locked in what we'd
call a geo for Earth, a geosynchronous
orbit. It is locked and will stay in the
same point in the sky much as we do with
geosynchronous satellites.
There are also other moons that have
been discovered. Four smaller moons we
can take a quick look at here. Nix and
Hydra were known prior to New Horizons
leaving
Earth. Styx and Kerberos were actually
discovered on route, not by the New
Horizons craft, but by imaging from
Earth
and were were studied were studied not
imaged very well because the orbit had
already been pretty much set, so we
couldn't get close to all of these and
you note that some of them
have not less of detailed images. Hydra
has a much more blurry image than Nix.
It just happens to be where they were
relative to the spacecraft when it flew
by Pluto.
Now, these two are the best studied
dwarf planets, Ceres and Pluto. However,
there are other dwarf planets as well
and that includes Eris, which is
comparable in size
to Pluto in have a moon, Dysnomia, which
helps us to determine its mass from its
orbit. And we can see both of those here
here as seen from Earth, but again, as
with Pluto, we can't see much from
Earth. This is even further away, so
it's very difficult to get any kind of
imaging of it. In fact, essentially
impossible to get any imaging of the
surface.
We also have Haumea.
And we can picture that here. Now, this
is not an image an image of it. This is
actually an artist's conception based on
our understanding of Haumea from
observations from Earth. We see that it
has a couple of moons that we do know
know. It is one of the objects that is
not spherical, but is ellipsoidal. So,
it's in a flattened shape. This is also
a stable configuration under gravity for
a massive object. And if you recall,
that was one of the requirements to be a
planet or a dwarf planet was to have
enough mass to pull it into one of these
shapes.
And then there is Makemake.
Again, an artist's conception image of
what it might look like from around that
that dwarf planet looking back toward
this relatively bright star in the sky.
A very bright star in the sky, but that
is our sun. So, at this depth in the
solar system, the sun is just an
extremely bright star, but does not
stand out the way it does here on Earth.
So, as of right now, these other three
objects have not been studied and are
not current plans to be able to go visit
them to see, but of course, future
missions may change that and may get us
a chance to learn more about these dwarf
planets. So, let's go ahead and finish
up with our summary. And what we've
looked at is that in 2006, a planet was
defined, and that meant Pluto was
removed from the planet classification
and reclassified as a dwarf planet. We
now know five of these dwarf planets,
two of which have been explored by
spacecraft,
and the studies of Pluto will help us
understanding these other objects in the
Kuiper Belt.
So, that concludes this lecture on Pluto
and the dwarf planets.
We'll be back again next time for
another topic in astronomy.
So, until then, have a great day,
everyone, and I will see you in class.