Why Poor Pluto Is No Longer A Planet? – II

T V Venkateswaran

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-fourth 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, organises once in three years a general body to take stock of the development in Astronomy. XXVIth Congress held at Prague Czech Republic between August 14-25, 2006 adopted a resolution giving 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 into 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 system 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, the 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 recognise Pluto as a large member of a vast population of small bodies beyond Neptune. 

Science is all about recognising 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 realised 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”.

[Concluded]

(Source: ‘Dream 2047’, a publication of Vigyan Prasar)