As occasionally happens, a science-related development is one of the leading stories in the news, at the moment. I'm referring, of course, to the "demotion" of Pluto from the rank of planet.
What happened is that the International Astronomical Union (IAU) adopted a new definition of what constitutes a planet, which Pluto no longer meets. According to the Wikipedia summary of this matter, the following definition was adopted:
...A planet is a celestial body that (a) is in orbit around the Sun, (b) has sufficient mass for its self-gravity to overcome rigid body forces so that it assumes a hydrostatic equilibrium (nearly round) shape, and (c) has cleared the neighbourhood around its orbit.
As seen in this list of objects in the solar system ranked by mass, Pluto is pretty far down.
The references to mass and gravity, of course, implicate Newtonian physics (many universities have a "Department of Physics and Astronomy," thus illustrating the interconnectedness of the two disciplines). As conveyed on the Wikipedia's gravity page:
Newton’s law of gravitation states that: every particle in the universe attracts every other particle with a force that is directly proportional to the product of their masses and inversely proportional to the square of the distance between them.
Further:
Every planetary body, including the Earth, is surrounded by its own gravitational field, which exerts an attractive force on any object that comes under its influence. This field is proportional to the body's mass and varies inversely with the square of distance from the body.
A couple of areas warrant further discussion, in my view. First, I had never heard of the term "self-gravity" until seeing it in the new definition of planet (above). Professor Dave Hanes of Queen's University of Canada has a website that addresses several aspects of gravity. Most relevant to the aforementioned definition is a section of his site entitled "Self-Gravity Can Determine Form," excerpts of which follow:
So far my discussion of scales and function has focussed on things like ants and elephants, which are affected by the gravity of some nearby immense object like the Earth... Moreover, the gravitational force of nearby human-scale objects, even fairly large ones, can safely be ignored. For instance, when you walk past a city bus, you don't have to worry about its gravitational attraction pulling you off the sidewalk and into the road...
In astronomy, by contrast, we are dealing with very large bodies, like planets and stars. They are dominated by self-gravity, by which I mean that the shape and proportions of any object this big are determined by the way in which the various parts of it feel and respond to the gravitational forces exerted by its own other parts. This consideration explains, among other things, why the Earth and all sufficiently large objects must be spherical (or nearly so).
Why should this be? Why couldn't the Earth take on any shape into which we might choose to sculpt it? After all, you are used to the idea that the rocks of the Earth are very rigid and inflexible, at least the ones immediately beneath our feet. Couldn't you mould the Earth into any shape you desired, given sufficiently powerful earth-movers and other equipment?
The answer is no. Suppose, for instance, you tried to use enormous bulldozers to raise mountains to a height of even just a few tens of kilometers. You would not succeed: the sheer weight of material pressing down on the lower layers would overwhelm the structural strength provided by the rocks, and they would slump and flow sideways. The new-built mountain would collapse...
From the known strengths of materials, in fact, you can calculate just about how big a mountain could ever be, here on the Earth: it works out to be roughly the size of Mount Everest. On Mars, which is a smaller planet, the gravity is less, and mountains can be about three times larger. Remarkably, this was known before any spacecraft ever visited Mars, and Carl Sagan, an astronomer at Cornell, predicted that we might find such a huge mountain on Mars - which we did! It is called Olympus Mons (Mount Olympus); it is the largest mountain in the Solar System.
In astronomy, therefore, any body of sufficiently large size, bigger than a few hundred kilometers in diameter, MUST be spherical (although to be absolutely correct I should point out that if it is spinning very rapidly, it can be somewhat flattened out, although still possessing a very smooth and regular surface). The important thing to remember is that the apparently very irregular surface of the Earth, with features like hills, mountains, and valleys, is quite misleading: these features are big only compared to us. Relative to the size of the Earth itself, these features are tiny and easily understood in terms of the limited strengths of rocks and minerals, as I have noted. Bigger features simply could not survive.
By the way, if you were to build a scale model of the Earth about a meter in diameter, like a very large beach ball, even the very highest mountains on it would be represented by little bumps about half a millimeter in height, scarcely detectable to the touch. The surface of the Earth is actually remarkably smooth.
Another part of the new definition of planet is that the entity must "clear the neighborhood." The Wikipedia's page on that subject states the following:
The phrase refers to an orbiting body (a planet or protoplanet) "sweeping out" its orbital region over time, by gravitationally interacting with smaller bodies nearby. Over many orbital cycles, a large body will tend to cause small bodies either to accrete with it, or to be disturbed to another orbit. As a consequence it does not then share its orbital region with other bodies of significant size, except for its own satellites, or those governed by its own gravitational influence.
The bottom line:
Pluto has not cleared the neighborhood of its orbit (vis-à-vis Neptune and Kuiper Belt Objects such as the Plutinos)...
Indeed, failure to clear the neighborhood appears to be Pluto's primary deficiency. According to the following BBC article:
Charon was discovered in 1978... Until now, it has been regarded as a moon of Pluto. But both Pluto and Charon have enough mass to be spherical - both bodies independently satisfy the proposed definition of a planet.
I highly recommend this BBC document, written in Q & A format, for readers who want to investigate the Pluto story in greater detail.
