Hi friends

This is to appear in the magazine 'Dream 2047' publihsed by Vigyan
prasr (www.vigyanprasar.gov.in)
T V Venkateswaran
Scientist D, Vigyan Prasar
New Delhi

Why Poor Pluto is no longer a Planet?

It is just about 75 years since the discovery of Pluto; hitherto cute
little baby planet of our solar system. With the raising of quite a
few yellow cards in Prague, on August 24th, 2006, by 3000 and odd
astronomers, Pluto was demoted from full-fledged planet to "dwarf
planet." Moreover, under the revised classification, the object 2003
UB313 which in many ways precipitated this final debate, sometimes
called Xena, and much touted as the "10th planet, becomes the largest
known dwarf planet. Indeed it is an irony that Pluto was a `planet'
when the New Horizon space mission was launched in January 2006; but
will no longer be: as it would reach Pluto-Charon in 2015.

Not the first time

Demotion of stellar bodies from the initial status assigned to it is
not new to astronomy; or for that matter science. In fact when
Galileo Galilee famously turned his telescope towards the heaven he
was blessed with the enchanting sight of rings of Saturn, mountains
on the moon-landscape and extraordinary sight of `stars' rotating
about the planet Jupiter. Yes, indeed Galileo unhesitatingly asserted
that "that there are three stars in the heavens moving about Jupiter,
as Venus and Mercury around the Sun" and named them
initially "medicean stars", in honour of Medic family who provided
patronage to him during his years of trial and tribulations. However
they were subsequently reclassified as "medicean planets"; for the
naming them, as `stars' were deemed inappropriate. In those
days `moon' was the proper name for the natural satellite of Earth
and many other satellites of planets such as Rhea of Saturn were also
called as `planets'. It was only around 1700s that astronomers
commenced the practice of calling the satellites of planets
as `moons'.

1781 March 13, William Herschel along with his sister Caroline
discovered the planet Uranus. After careful study to rule out the
possibility that this object could be comet, Herschel concluded that
he has indeed discovered a `new' planet; a planet about which no
scared text had any inkling. Obviously such momentous discovery
inspired many other astronomers to search the skies for other planets
that may be lurking around.

Meanwhile, Johann Elert Bode, a German astronomer observed that, if
you divide the distance from the Sun to Saturn into 100 lengths,
then; Mercury is at 4 lengths; Venus is at 7 lengths (4+3); Earth is
at 10 lengths (4+6) ; Mars is at 16 lengths (4+12) ; Nothing was seen
at 28 lengths (4+24) ; Jupiter is at 52 lengths (4+48) ; Saturn is at
100 lengths (4+96). Discovery of Uranus too seemed to provide
credence to this claim; as per the above rule the next planet beyond
Saturn was to be at 196 (4+192); although Uranus was actually at 192
lengths. It was so near, it was thought to verify the law. Encouraged
by this, in 1800 Hungarian Baron, Franz Xavier von Zach, set up a
group of astronomers called the Celestial Police, to find
the `missing planet'. They divided the Zodiac into zones and
allocated different areas to different astronomers in the group.

The Lady luck had a different plan; she smiled on Rev Father Giuseppe
Piazzi, an Italian Astronomer. Piazzi was compiling a star catalogue
at Palermo Observatory in Sicily. He accidentally discovered
a `planet' between the orbits of Mars and Jupiter on January 1, 1801
and named it after Sicilian God of harvest `Ceres'. Thus at the
beginning of the 19th century there were 8 planets, including Ceres
and Uranus. However Zach's Celestial Police was not in vain, Heinrich
Olbers, a member of the Celestial Police discovered Pallas, which too
was in the same orbital area of Ceres, on March 28th 1802. Karl
Harding, another member of the Celestial Police, found Juno during
September 1804. Heinrich Olbers discovered Vesta in March 1807. All
these were classified as Planets, and the number of planets rose to
11 including Vesta. However there was something fishy in the
discoveries, all these `planets' seemed to occupy the same orbital
area as that of Ceres.

Things appeared to cool off; the fifth planet in the same orbital
area Astraea was discovered only in 1845, nearly 39 years after the
discovery of Vesta. However, soon the floodgate of discovery of
planets burst at its seam in 1847 with the discovery of three new
asteroids. By the end of 1851 there were 15 asteroids, although
astronomers sere disquieted, they braved themselves and still listed
all of them as planets- solar system then consisted of 23 planets
including Neptune (discovered in 1845), Uranus and 15 `planets' in
the orbital region of Ceres. By 1668 more than 100 stellar objects
were discovered in that region. Surely things were unseemly; in fact
due their starlike appearance, Herschel had declared even as early as
1802 that "from their asteroidal appearance, I shall take my name,
and call them Asteroids; reserving for myself, however, the liberty
of changing that name, if another, more expressive of their nature
should occur." Astronomers who had not heeded to this perceptive
counsel at that time had to but agree; at one go 15 `planets' were
reclassified as `minor planets' or `asteroids' around 1850s.

Discovery of Pluto

Discovery of Pluto is intimately linked to that of Neptune. Once
Uranus was discovered astronomers observed that there was
perturbations in its path; as if there is another massive body beyond
Uranus giving it a jab. Convinced that there is a planet beyond
Uranus search began; Adams in England and Levarrier in France
spearheaded the search. They calculated the possible position of
Neptune and with much drama the planet indeed was discovered by Galle
and D'Arrest of Berlin Observatory on September 23, 1845.

Neptune accounted for the observed perturbations in the path of
Uranus, nonetheless Neptune itself showed perturbations as if being
shoved around by a planet beyond it. `Ah' said astronomers `lets go
for the Planet X'. Nonetheless it was soon evident to astronomers
that treading the same path will not lead them to the Planet X.
Calculations after calculations were made and yet the elusive planet
was not to be seen. Where calculations and prediction failed;
perseverance triumphed. With the invention of photography it was
possible to recorded the position of stellar objects weeks apart and
compare them. Clyde Tombaugh in fact did exactly that; and on 13th
March 1930 Lowell Observatory announced the discovery of the ninth
planet of the Solar system thereby bringing an end a search, which
went on for about 25 years. When Tombaugh discovered Pluto
astronomers welcomed it as the long sought "Planet X", which would
account for residual perturbations in the orbit of Neptune. In a
curious twist to the tale those perturbations proved to be illusory,
and the discovery of Pluto was fortuitous.

Pluto an odd ball

Right from the day of its discovery Pluto has been an odd ball.
Pluto's orbit deviates significantly from a perfect circle while the
major planets have quasi-circular orbits. It is so elongated that it
crosses the orbit of Neptune. Due to this, from 1979 to 1999 for
twenty years in fact Pluto was rather closer than Neptune. Pluto's
orbit is also considerably tilted – whooping 17 degrees- compared to
the orbits of the major planets. At the time of its discovery Pluto
was estimated to be the size of Earth; later downgraded to size of
Mars. Further, in 1978, Pluto's companion, Charon was discovered.
Chaorn was estimated to be half its size; earlier the size of Pluto
was estimated by including the size of Charon as it was not possible
to resolve them separately. This led to further downsizing of Pluto;
now we know that Pluto is much smaller than the major planets- just
about 2320 km across- smaller than seven moon of Solar system
including our moon. Its mass is only 0.2% of the Earth's mass, and
100,000 times less than the mass of Jupiter. Pluto rotates in the
opposite direction from most of the other planets. Pluto has been an
irritant as a planet for astronomers quite long; aastronomers were
indeed puzzled what to make of this small, frigid world.

Why now?

2005 witnessed the 75th anniversary celebrations of the discovery of
Pluto; Clyde Tombaugh was hailed. Nevertheless even before the din
and dust settled down poor Pluto has been striped of its Planethood.
As a matter of fact, even at an earlier occasion astronomers had
suggested that Pluto does not really belong to the category of
Planets. In 1999, an inadvertent suggestion, that Pluto may be termed
the 10000th minor planet giving it "dual citizenship" of sorts as
both a major and a minor planet was made by Brian Marsden of the
Minor Planet Center of IAU. This stormed into a major debate that
spread into streets; passionate editorials were written and email
campaign was conducted fearing that Pluto might be "demoted" to non-
planet status, in particular in USA. Taken aback, putting rest the
rumors IAU then emphatically declared that there were no plans to
change Pluto's planetary status.

However, for astronomers it was evident for long that the fate of
Pluto was sealed; sooner or later it will lose its status as Planet.
The need for a strict definition was deemed necessary after new
telescope technologies, especially Hubble Telescope and use of
Computer technologies to compare photographs, began to reveal far-off
objects in the region of Pluto. The first Kuiper belt object (KBOs)
1992 QB1 was discovered in 1992. 2001 KX76 an icy, reddish world over
a thousand kilometres across was soon discovered. Two is a company,
but surely three is a crowd; soon many more objects were discovered
in the Kuiper Belt area going around the sun in the region of Pluto.
In 2002, Quaoar (1280 km diameter) was discovered, making it a bit
more than half the size of Pluto. Another discovery, Orcus, is
probably even larger. In 2004 Sedna, an extremely distant object
beyond the Kuiper belt was discovered with estimated 1800 km
diameter, close to Pluto's 2320 km. As of now more than 783 objects
have been discovered in the same region as that of Pluto. The
proverbial last straw on the camel was the discovery of Trans-
Neptunian object 2003 UB313 (popularly called as Xena). Announced on
July 29, 2005 it rekindled the debate as to whether to classify Pluto
as a planet or not. It is estimated to be at least as large as Pluto;
and is the largest object yet discovered in the solar system since
Neptune in 1846 and has caused some to refer to it as the "10th
planet" of the solar system. It was clear that Pluto is embedded in
a vast swarm of small bodies, just like the asteroid Ceres in the
asteroid belt between Mars and Jupiter. Pluto has many friends
orbiting nearby and at least one of them is larger than Pluto. There
have been another important developments in our knowledge
of "planetary systems" in the last decade or so: the discovery of
celestial bodies orbiting stars – exoplanets- other than the sun.
These recent developments make it pressing to arrive at a proper
definition for the word "planet".

What is in a name?

In popular culture, whenever something new is discovered in the outer
solar system, the first question asked is "Is it bigger than Pluto?"
When it is understood to be only half the size of Pluto (2002
discovery of Quaoar) or maybe three forth the size of Pluto (2004
discovery of Sedna) there is a little disappointment and it is said
that " Ok , well, it is not a planet'. But when it is said "Well
rather this one bigger than Pluto" (2005 discovery of 2003 UB313) it
is remarked "Hurrah! The 10th planet has been found!" In popular
imagination only objects larger than Pluto are to be called planets.
However from the point of view of science it is arbitrary and
whimsical. For the uninitiated sure enough world of science appears
to be weird; Bats are able to fly but it is not a bird; Whales waddle
in ocean yet they are not fish but are mammals. Penguins can swim and
toddle but can not fly, yet it is a bird! However such
classifications allude to deep relationships; Penguins are closely
related to birds from the point of view of evolution; and bats
possesses organs specific to mammals. If Pluto is included as a
planet, we have no physical basis for excluding UB313, dozens of
other large spherical KBOs, and Ceres. The term "planet" would then
lose any taxonomic utility. But an important function of scientific
nomenclature is to reflect natural relationships, not to obscure them.

IAU's Resolution

International Astronomical Union (IAU) an international scientific
body established in 1919, currently with 9000 members, organizes once
in three year a general body to take stock of the development in
Astronomy. XXVI th Congress held at Prague Czech Republic between
August 14-25, 2006 adopted a resolution given first-ever scientific
definition to `Planet' by a vote of the International Astronomical
Union.

In light of our improved understanding of the solar system
astronomers have revised their classification. An initial proposal
put forward criteria that kept Pluto's status and brought the club to
12 - adding 2003 UB313, the asteroid Ceres, and Pluto's largest moon,
Charon to the nine planets. But this scheme met with considerable
opposition from astronomers at the assembly who felt that very soon
many members of the KBOs could lay claim to membership of planetary
club under this classification. Revised resolution that was adopted
defines three distinct classes of objects in the solar system:
planets, dwarf planets, and small solar-system bodies. There are 8
planets in the solar system: Mercury, Venus, Earth, Mars, Jupiter,
Saturn, Uranus, and Neptune. A dwarf planet is not a planet.

The resolution adopted states "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. Further it clarifies that "A "dwarf
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, (c) has not cleared the neighbourhood around its orbit, and
(d) is not a satellite. In addition the resolution states that "All
other objects except satellites orbiting the Sun shall be referred to
collectively as "Small Solar-System Bodies"."

That the planet should go around the Sun is but natural, so is the
second criteria, which essentially mean that the object should be
massive enough to be crushed on its own weight into a spherical shape
(or near spherical shape). This too is comprehensible; you do not
want any and every lump of rock going around the sun to be
termed `planet'. But how do we make sense of the third commandment?

The Third Commandment

The third norm is linked to the dynamics of planet formation. Solar
system includes several distinct populations; the planets,
satellites, asteroid belt, Kuiper Belt, Oort Cloud, etc. These
distinct populations reflect different pathways in the evolution of
the solar nebula, which in the first place coalesced in to central
star (Sun) and solar system objects. In Safronov' s model of
planetesimals and planetary evolution, a planet is an end product of
disk accretion around a primary star that sweeps up or scatters most
of the mass from its orbital zone in the accretion disk around a
central star. Further dynamics of planet formation is linked to the
degree to which a body dominates the other masses that share its
orbital zone. The disk evolution in a mature systems tend to produce
a small number of relatively large bodies (planets) in non-
intersecting or resonant orbits, which prevent collisions between
them. According to this model the inner region of the accretion disc
coalesced into terrestrial planets; outer regions formed into gas
giants. On the other hand objects like Ceres and Pluto remain in an
arrested state of development, unlike mature planets and hence are
rightly classified as `dwarf planets'.

Further in course the evolution of solar system planets come to
dominate their orbital region such that it is more massive than the
total mass of all of the other bodies in a similar orbit. For
example, the planet Neptune has 8600 times the mass of Pluto, the
largest body that crosses its orbit. Likewise, the planet Earth has 2
x 108 times the mass of the asteroid (1036) Ganymed, the largest body
that crosses its orbit. The major planets have accumulated, captured,
or ejected all the mass in their immediate proximity. They are the
dominant bodies in their regions of space. In contrast, the asteroids
and KBOs are members of populations with a shared orbital space, in
which no member so dominates the others by mass. The two largest
asteroids, Ceres and Pallas, differ in mass by a factor of about 4
and the largest known KBO (UB313) has only about twice the mass of
Pluto. Our solar system has no intermediate cases between solitary
bodies (planets) and members of populations, defined in this way.

Asteroids and comets, including KBOs, differ from planets in that
they can collide with each other and with planets. It is postulated
that about 3.8 billion years ago, after a period known as heavy
bombardment, most of the planetesimals, which did not coalesce into
planets within the solar system had either been ejected from the
Solar system entirely, into distant eccentric orbits such as the Oort
cloud, or had collided with larger objects due to the regular
gravitational nudges from the Jovian planets. Debris leftover from
this evolutionary phase of solar system remains as asteroid belt, the
Kuiper Belt, and the Oort cloud. Moons such as Phobos, Deimos,
Triton, and many of the small high-inclination moons of the Jovian
planets are considered to be `captured' planetesimals.

Moreover Neptune totally dominates Pluto's region of the Kuiper belt.
While much of the material in the Kuiper belt has indeed been tossed
aside or accumulated by Neptune, a very special region has actually
been captured by Neptune. We now know that Neptune formed much closer
to the sun than where it was today, and, as Neptune moved out, it
pushed objects in this special region out with it while forcing them
into a peculation orbit where they orbit the sun precisely twice for
every three orbits of Neptune. Pluto is the largest of this class,
and it and the others only exist where they do because of the
dominance of Neptune. Same is the case with so called `Trojan
asteroids' of Jupiter, they are swept by Jupiter's gravity to remain
60 degrees ahead or 60 degrees behind the planet, in what are known
as the Lagrange points.

In fact in the light of our modern understanding conventional list
of "nine planets"- four terrestrial planets, four giant planets, and
Pluto – has lost any scientific rationale, and is now merely
historical. While it is quite understandable that in 1930 astronomers
felt that Pluto was an exceptional object and decided to call it a
planet; on the other hand majority of astronomers today recognize
Pluto as a large member of a vast population of small bodies beyond
Neptune.

Science is all about recognizing that earlier ideas may have been
wrong. For a long time biologists thought that all microbes were
germs causing diseases in humans. Today we are more enlightened we
are aware that some bacteria are beneficial; moreover we have
realized that there is another class of microbe- viruses. Sure enough
we have changed our ideas about which bug was what. We are all better
off now as the new classification has clarified meaning and has
allowed researchers and health professionals go about their jobs
better. It is no wonder that replying to a question as to how Clyde
Tombaugh would feel about the Pluto's demotion, his wife Patricia
Tombaugh stated, "He was a scientist. He would understand they had a
real problem when they start finding several of these things flying
around the place"

BOX
Victor Safronov model of how planets evolved

The model proposed by Victor Safronov (1917-1999), Soviet astronomer
for evolution of planets around a star from an initial interstellar
cloud informs our modern understanding of the dynamics of planet
formation. His theory was in contrast to then prevailing theory that
conceived planets to be end product of a process of gravitational
fragmentation of the proto-planetary disc directly into planets.
Safronov theorized that planets form from aggregation of countless
smaller bodies: dust grains and debris left over in the solar system
after formation of the Sun. Initially the dust or small grain move
about due to Brownian motion, or turbulent motions in the gas to
cause the collisions that can lead to coalescing. However, when the
bodies reach sizes of approximately one kilometer, then they can
attract each other directly through their mutual gravity, aiding
further growth into moon-sized protoplanets enormously. Many such
planetesimals are indeed formed in the accretion disc, many
eventually break apart during violent collisions, but a few of the
largest planetesimals survive such encounters and continue to grow
into protoplanets and later planets. In this model Planets are end
product of secondary accretion in a disk around a primary star.
Planets in this sense occur only in highly evolved (old) systems,
which have reached the final cleanup phase of accretion, with the
major bodies in stable non-intersecting orbits. In this model Planets
are the solitary bodies that prevail in the creative-destructive
evolution of a disk, and are dynamically distinct from the
populations of leftover debris - mainly asteroids and comets.