Many of you spotted that last week’s “guess the planet” was
actually a moon. Let’s take a look at the definitions of the different objects
of the solar system, and how they relate to one another. In many cases the
category into which we put an object tells us a lot about it, In others it can
seem almost arbitrary. For example the “demotion” of Pluto from planet to dwarf
planet status in 2006, proved unpopular with the public, but was an important
change which allows us to better classify Pluto like objects.
So what are the different objects in the solar system and
how are they classified? Clearly we have the Sun at the centre. It is our only
star, and everything else revolves around it. We then have the planets and dwarf
planets with which most of us probably have at least a passing familiarity. In
terms of mass the contents of the solar system is mostly Jupiter. This figure,from Wikipedia shows how it dwarfs everything else, including Saturn which is
our next most massive object.
Jupiter and Saturn are both gas giants. They are composed primarily
of hydrogen and helium. These are the lightest elements of the periodic table,
so the fact that Jupiter and Saturn are so much more massive than everything
else drives home how huge they really are.
Uranus and Neptune are also gas planets, but they have a
somewhat different composition, and so are classified as Ice Giants. They still
have a lot of hydrogen and helium in them, but are mostly composed of heavier
elements, including oxygen, nitrogen, and carbon. These were incorporated into
the planets in the form of ices, although they don’t remain ice like today. These
planets were originally classified as gas giants, but were separated into their
own class once it became apparent that they were substantially different to
Jupiter and Saturn.
Although they are fascinating, we probably won’t be looking
at the gas planets much in this blog. They kind of lack the solid surface
needed for geomorphology. That said I might throw them into the guess the
planet column every so often...
The giant planets might have most of the mass of the solar
system, but it is the smaller objects which introduce most of the variety.
There are four terrestrial planets; Mercury, Venus, Earth and Mars. For much of
the history of astronomy “planet” had been fairly loosely defined and so Pluto was also part of that list. However as more
Pluto like objects were discovered in the outer solar system it became clear
that a more rigorous classification scheme was needed. The new definition of a
planet is that must orbit the sun, be large enough to be roughly spherical, and
have “cleared the neighbourhood around its orbit” of smaller bodies.
Let’s look
at each of these statements in turn.
Orbiting the sun is fairly straight forwards; this is what separates
planets from satellites or moons. We usually think of moons as being smaller
than the planets, but this doesn’t mean that they are small objects, just
smaller than the object they orbit. Some, such as Ganymede and Titan are
actually larger than Mercury, and vastly bigger than Pluto, if they orbited the
sun they would be considered planets on a par with anything in the inner solar
system. Pluto’s moon Charon is almost as large as the planet it orbits, and in
some ways they could be considered to be a binary planetary system. The solar system
has 181 known moons, orbiting most of the main planets and a few of the dwarf
planets.
The second criteria in the definition of a planet is that it
“has sufficient mass for its self-gravity to overcome rigid body forces so that
it assumes a hydrostatic equilibrium (nearly round) shape” this sounds complicated,
but just means that when the planet was forming, its gravity was strong enough
to pull it into a spherical shape, rather than remaining irregular like many of
the smaller moons and asteroids. Planets are never perfectly spherical; they
are actually what is termed an “oblate sphereoid”.
The dwarf planets meet these first two criteria, they are
roughly spherical, and they are in orbit around the sun. However they don’t
meet the last of the three, which is that they have cleared the neighbourhood
of their orbit. This means that they have swept up all of the other material in
the same part of space as them, with it either impacting as meteorites, or
going into orbit as satellites.
The eight planets have largely cleared their orbits,
although there is so much very small material floating around that it is quite
hard to be entirely certain. The dwarf planets have not, and share their
regions of space with numerous asteroids and planetesimals.
There are currently five dwarf planets; Ceres, Pluto, Haumea,
Makemake and Eris. Four of these are transneptunian objects, while Ceres is the
largest body in the asteroid belt. As many as 200 other transneptunian objects
are likely to meet the criteria to be dwarf planets, but assessing these is a
long and arduous process, so relatively few have so far been accepted as such. This
is the reason for Pluto being reclassified. If we were to continue to consider
it a planet then there are a great many objects which also qualify.
The roster of planets has never been set in stone; it has
been continually expanded as new objects were discovered. It is worth noting
that Pluto wasn’t always the ninth planet, and is not the first planet to be
reclassified as something else. Ceres is the largest body in the asteroid belt.
When first discovered, prior to Pluto, it was classified as a planet. However it
was later demoted to an asteroid. It is now considered a dwarf planet again.
Pluto may have been considered a planet for a long time, but
it is very different to the other terrestrial planets and never quite fit into
that classification. It is much smaller, it is much icier and it is located in
the outer solar system. This gives it dramatically different environmental
conditions to the rocky planets and moons. The current classification makes it the
largest of an interesting new class of objects, which we are only beginning to
explore. A lot of people dislike this, because for cultural reasons we are used
to thinking of it as part of a select club. But I would argue that Pluto deserves
its own category.
So what about the objects that don’t meet the second or
third criteria? The remainder of objects in the solar system are officially
classified as “small solar system bodies”. They are much smaller than even the
littlest of the dwarf planets, and there are a vast number of them. There are 552,894
known asteroids and 3,083 known comets. Like many of the smaller moons they don’t
have sufficient mass to become spherical. They are frequently found in regions
of space such as the asteroid belt, where numerous other small objects occur.
Over the course of this blog I’ll be trying to show you
interesting things from as many of these worlds as possible, be they planets,
moons or not. We have an amazing amount of instruments out in the solar system
studying these bodies. So this shouldn’t be too difficult to achieve.
References:
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