The New Solar System
Many will be relieved that the long drawn out debate over
nomenclature of a few objects of the Solar System is moving forward in a clear
cut direction.
First thing to be happy about, is that there is now a physical definition
of what a Planet is. Historically, any object that was seen to move anomalously
against the background was initially called a "wanderer" or a
"planet". There were five such objects that could be seen by naked
eyes - Mercury, Venus, Mars, Jupiter and Saturn. With the telescopes came
observations of other bodies of the Solar System - all of which would actually
be similar "wanderers".
When Ceres was found in 1801, it was initially called a Planet. So were a few
other objects found by 1807 - all of which were later termed Asteroids when
their masses were found to be very much lower than the masses of the classical
planets.
The odd case of Pluto took many years to evolve - it was found in 1930 and was
thought initially to be about as big as the Earth. It was only by late 1970s
that a better estimate of its mass could be found and we now find that
it is only about 1/25 the mass of Mercury!
We then found several other objects beyond
So, a big question mark on Pluto, that led to a physical definition of the word
"planet". We should all cheer for it and give Pluto its special
status that helped us to understand the Solar system better.
Keats, in early 1800s, wrote of the wonder that would be felt by someone
"when a new planet swims into his ken". Well, that wonder need not be
confined to the 8 classical planets of the Sun, we just extend that wonder to
newer and smaller bodies of the Solar system and look at their physical
distributions in mass, orbit dimensions and inclinations.
What is
this new physical definition of the word “Planet”?
A planet, according to
this new definition, is an object that orbits the parent star (the Sun, in case
of the Solar System), is massive enough that its self gravity has pulled its
mass in, into a spherical shape, and is also massive enough that it has pulled
in most of the matter from its neighbourhood and hence cleared this
neighbourhood of matter.
What does clearing a neighbourhood mean? Are we to expect no
matter at all, in the vicinity of a planet? Afterall, there are the near Earth
objects and the Trojan Asteroids which cross Jupiter’s orbit. The operative
thing here is that, by clearing its neighbourhood, a planet becomes the dominant
object in that neighbourhood. One could get a feel for what this means by
looking at a graph drawn between the mass of any solar system object versus the
total mass in its neighbourhood. The graph clearly shows all the planets on one
side – the total mass of small objects in their neighbourhood is negligible
compared to the planet in this neighbourhood.
Written
by Dr. N. Rathnasree, Director, Nehru Planetarium, New Delhi.
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