Download Star or planet, or what?

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David W Hughes
Star or planet, or what?
Anthony Whitworth (A&G 41
6.18) is worried about differentiating between a star and a planet and
suggests that we look inside. He
defines a planet as being an object
that is “severely fractionated chemically, with the fractionation being
caused by gravitational settling
rather than nuclear fusion”.
Isn’t it dangerous to base such a
fundamental definition on something we know so little about? This
is not to say that geophysicists
haven’t discovered a great deal
about the interior of Earth over the
last century, but when it comes to
other planets our knowledge of
what is inside them is skimpy to say
the least. And isn’t “gravitational
settling” something of a red herring
too? Even with stars and nuclear
fusion, the end-product is an object
with the heaviest element in the
centre, where they have fallen, and
with the lighter elements progressively floating on top of each other.
Maybe the best thing to do is to
approach the star/planet boundary
problem artistically, zoologically
and historically.
Artistically we admit defeat before
we start. Here a planet is like “good
art”. You know good art when you
see it, but it is extremely difficult to
define. And like poetry and prose,
you can easily point to good examples of each, but infrequently one
comes across something that is neither one thing nor the other.
The zoological approach leans
heavily of the work of Martin Harwit, as expounded in his excellent
book Cosmic Discovery, the search,
scope and heritage of astronomy
(The Harvester Press, Brighton,
1981). Harwit talks of an astronomical zoo which gets larger and larger
as the subject develops. Each cage in
the zoo contains a different kind of
celestial animal. Initially the bijou
zoo had five cages. The most populous contained the fixed stars; then
there was one for wandering stars
(planets?), another for hairy stars
(comets) and finally two lonely
cages for the Sun (which was not
yet recognized as a star) and the
Moon (which had clear bodily
markings). Modern knowledge
inflation now decrees that today’s
potential occupants of the planetary
cage should be dispersed into a row
of separate cages, one each for terrestrial planets, gas giant planets,
Pluto-like objects in 2, 3 resonances
with Neptune, large satellites of gas
1.8
giant planets, large differentiated
asteroids and so on. All these are, to
use Whitworth’s words, “severely
fractionated chemically, with the
fractionation being caused by gravitational settling”. The zoological
analogy is easier if we know how an
object/animal was formed, where it
is found, and its specific characteristics, such as mass, temperature, size,
energy output and composition.
I was also surprised that Whitworth seemed to give the impression
that the problem of defining the
term “planet” arose sometime in the
mid-1990s. Actually it is as old as
astronomical history. A restricted
trawl through some handy books
revealed the following, which I list
in order of increasing age.
Planet: An astronomical object
which is in orbit around a star, but
does not have enough mass to
become a star itself, and shines only
by reflected light… upper limit mass
is 50 times that of Jupiter… at the
lower end mass range it is just less
than the size of Mercury. (John Gribbin 1996 Companion to the Cosmos
Weidenfeld and Nicolson, London.)
Planet: A solid (or partially liquid)
body orbiting around a star but too
small to generate energy by nuclear
reaction. (William K Hartmann and
Chris Impey 1994 Astronomy the
Cosmic Journey 5th edition,
Wadsworth Pub. Co., California.)
Planet: From the Greek for wanderer; any of the nine (so far
known) large bodies that revolve
around the sun, traditionally, an
heavenly object that moved with
respect to the stars (in this sense the
sun and moon were also considered
planets by ancient astronomers).
(Michael Zeilik 1993 Conceptual
Astronomy; A Journey of Ideas John
Wiley & Sons Inc., New York.)
Planet: A body that orbits the Sun
or another star and shines only by
the light that it reflects. (Valerie
Illingworth [ed.] 1979 A Dictionary
of Astronomy Macmillan, London.)
Planet: Small body having no
light or heat of its own which
moves round sun. (David S Evans
1946 Frontiers of Astronomy,
Sigma Books Ltd, London.)
Planets: …dark bodies shining
only by reflected sunlight… revolving around the Sun in orbits nearly
circular, moving all in the same
direction, and nearly in a common
plane of the ecliptic and the sun’s
equator… there are also at present
known nearly 300 little planets,
which probably represent a single
one, somehow “spoiled in the making” so to speak, or burst into fragments. (Charles A Young 1895 A
Textbook of General Astronomy
for colleges and scientific schools
Ginn & Co., Boston.)
Planètes: On donne ce nom aux
corps célestes qui n’étant pas
lumineux par eux-mêmes, empruntent dans notre systèm éclat du
soleil; on les distingue des étoiles en
ce qu’ils scintillent moins à vue simple que ces astres, surtout à une
certaine hauteur. (1856 Dictionnaire
D’Astronomie.)
Planet: Heavenly bodies, which
move around another, as their centre
of motion. Primary planets are such
as move around the Sun, as a centre:
secondary planets are moon, which
move around their primary planet.
(Lewis Tomlinson 1840 Recreations
in Astronomy Parker, London.)
Planet: They are those stars which
do not always remain in the same
place in the heavens, but move
round the sun and receive their
light from him. (1828 First Steps to
Astronomy and Geography,
Hatchard & Sons, London.)
Planet: From πλανητηζ, wanderer,
in opposition to a star which
remains fixed. (Jehoshaphat Aspin
1825 A Familiar Treatise on Astronomy Samuel; Leigh, London.)
Planet: A celestial body revolving
about the Sun. The planets may be
known from the fixed stars by their
change of situation in the heavens.
(1820 A Popular Grammar of the
Elements of Astronomy Thomas
Squires.)
It is clear from the above that
there can be no watertight definition. Astronomy is not like that.
There are stars that are obviously
stars, and planets that are obviously
planets. There is also a small subset
of objects that fall between the two
extremes. This is often the case.
Think of the trouble one has trying
to distinguish between asteroids and
comets at a time when they are in
the outer solar system, and the heated discussion as to whether Pluto is
a planet or merely the largest object
in the Edgeworth–Kuiper belt.
It is difficult to omit extrinsic characteristics. Modern cosmogony indicates that all planets are closely associated with stars. Planets are in orbit
around stars and vice-versa. Also
there are no known “orphan” planets wandering through the galactic
disk. And the formation of stars by
the condensation of those fragments
of an interstellar gas cloud that happen to obey the Jeans Criterion, differs drastically from the formation
of planets, by gentle, slow, accretion
of small planetesimals in a flattening
circumstellar nebula.
Intrinsic characteristics, however,
are best. My choice, in order of preference, would be mass, luminosity,
composition, energy generation
mechanism, and density distribution.
What are needed are two punchy
definitions that can sit happily in
astronomical glossaries. How about:
Star: An independent, hot, radiating, near-spherical astronomical
body with an initial mass greater
than about 0.03 solar masses, and
an initial composition close to the
universal cosmic composition. The
median stellar mass is around 1⁄7 the
solar mass and the majority of stars
have, at some time in their lives,
generated energy internally by
nuclear fusion processes.
Planet: A secondary, accreted, cool,
near-spherical astronomical body in
orbit around a star, having a mass
between 30 Jovian masses and 1⁄6000
Jovian masses, shining in the main
by reflecting radiation and having a
composition that is metal-rich in
comparison to cosmic composition.
David W Hughes, Reader in
Astronomy in the Dept of Physics and
Astronomy, University of Sheffield.
February 2001 Vol 42