A simple question until the twentieth century has pitted astronomers
and planetary scientists in an intense debate about the definition of
a planet in recent years. Until 1781 when Uranus was discovered, there
were only five planets known to humanity. The word planets means "wanderer" in
Greek and indicates the recognition of planets as wandering points of
light against a background of fixed stars when observed over months.
The ancient civilizations recognized these objects as distinct from
the stars and gave them special status - for example naming the days
of the week for them. The intellectual exchange of ideas in the ancient
cultures (Hindu, Arabic, Greek) resulted in a seven day week with days
named consistently after the celestial objects then known - Sun (Sunday),
Moon (Monday), Mars (Tuesday), Mercury (Wednesday), Jupiter (Thursday),
Venus (Friday) and Saturn (Saturday). No single person discovered these
ancient planets.
With the use of the telescopes, the precision of astronomical observations
(position measurements) of planets improved, leading to a problem leading
to the accurate prediction of the planetary positions based on laws
of planetary motion postulated by Kepler. The difficulties led to the
postulation of a planet beyond the orbit of Saturn that would influence
the motion of Jupiter and Saturn through its gravitational influence.
This led to the discovery of Uranus by Sir William Herschel in 1781.
Observations of the position of Uranus over the next few decades led
to a similar predicament in understanding its motion in orbit according
to the laws of planetary motion and another massive planet was postulated
beyond the orbit of Uranus. This led to the discovery of Neptune in
1845 by John Galle. SInce the orbital periods of these planets are long
compared to those of the other planets (84 and 162 years), the uncertainty
and the lack of of sufficient precision and short duration led the astronomers
to seek another large planet beyond the orbit of Neptune to explain
the orbital motions of Uranus and Neptune, which led to the discovery
of Pluto by a young astronomer, Clyde Tombaugh in 1930 using the Lowell
telescope in Flagstaff, Arizona.
In 1976 a moon of Pluto was discovered and two smaller moons were discovered
in 2005. At about the same time it was recognized that Pluto was too
small to exert any appreciable gravitational influence on the motion
of the giant planets and for a while the quest for the "Tenth Planet" continued
until about 1980s, but at about the same time, it was recognized with
the accumulated observations that the motions of the planets did not
require any additional large planet beyond the orbit of Pluto. The motions
of the spacecraft cruising out of the solar system such as Pioneers
10 and 11 also suggested the lack of any massive object that would have
influenced their trajectories, and the quest for a large planet was
pretty much over.
Improvements in telescopes and detectors (both CCD and infrared) led
to the detection of many small objects beyond the orbit of Pluto, but
most of them were found to be smaller than Pluto with cold, dark surfaces.
A few however were larger, but much further away with quixotic orbits
such as Quaoror and Sedna, and although initially called planets, their
elliptic and inclined orbits with very long periods did not earn them
the right to be recognized as planets and they remained identified as
prominent members of the Kuiper Belt Objects (KBOs). The International
Astronomical Union could not come to any agreement on the definition
of a planet in 2003 when some tried to demote Pluto from its planet
status and relegate it to a membership in the KBO club.
The situation changed in 2005 when Michael Brown of California Institute
of Technology announced the discovery of an object nearly twice as large
as Pluto with a moon half as large and in orbit around the Sun at nearly
twice the distance as Pluto's from the Sun. Was this object a planet?
The IAU was thrown into turmoil again and a committee was named to come
up with the definition of a planet which issued its recommendation prior
to the meeting of its General Assembly in Prague in August 2006. This
proposal would have recognized not only Pluto and "Xena" as
planets, but also Charon and Ceres, creating an even dozen planets in
our Solar System. This proposal however was not received well by the
assembled members of the IAU in Prgague (a small fraction of its membership)
and quickly a competing proposal was put forth by a small group that
was eventually adopted, stripping Pluto of its planet status and relegating
it as an asteroid.
The adopted definition however is self contradictory and if applied
with equal attention would strip every planet except Mercury of its
planethood. Clearly many planetary scientists are not prepared with
the definition adopted by the IAU which is dominated by non-planetary
scientists and the debate continues and perhaps will lead to a new definition
of a planet at the next general assembly. Since definitions are made
relevant through their use, it is also likely that the planetary scientists
will render the definition irrelevant through lack of use and the cultural
acceptance of Pluto as planet will prevail. Only the future will tell.
What is not at all controversial is that our solar system has a variety
of planets, broadly classified into three groups - inner planets that
have solid surfaces and are close to the Sun, the gas giants and dwarf
planets further away from the Sun which also have cold, solid surfaces
often covered with icy material.
- Sanjay S. Limaye
