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On the Irrelevance of Being a PLUTO! Size Scale of Stars and Planets

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On the irrelevance of being a PLUTO! Mayank Vahia DAA, TIFR Irrelevance of being Pluto 1 Size Scale of Stars and Planets Irrelevance of being Pluto 2 1 1 AU 700 Dsun Irrelevance of being Pluto 3 16 Dsun Irrelevance of being Pluto 4 2 Solar System 109 DEarth Irrelevance of being Pluto 5 11 DEarth Venus Irrelevance of being Pluto 6 3 Irrelevance of being Pluto 7 Solar System visible to unaided eye Irrelevance of being Pluto 8 4 Solar System at the beginning of 20 th Century Irrelevance of being Pluto 9 Solar System of my text book (30 years ago) Irrelevance of being Pluto 10 5 Asteroid Belt (Discovered in 1977) Irrelevance of being Pluto 11 The ‘Planet’ Pluto • Pluto is a 14 th magnitude object. • It is NOT visible to naked eye (neither are Uranus and Neptune). • It was discovered by American astronomer Clyde Tombaugh in 1930. Irrelevance of being Pluto 12 6 Prediction of Pluto • Percival Lowell and William H. Pickering are credited with the theoretical work on Pluto’s orbit done in 1909 based on data of Neptune’s orbital changes. • Venkatesh Ketakar had predicted it in May 1911 issue of Bulletin of the Astronomical Society of France. • He modelled his computations after those of Pierre-Simon Laplace who had analysed the motions of the satellites of Jupiter. • His location was within 1 o of its correct location. • He had predicted ts orbital period was 242.28 (248) years and a distance of 38.95 (39.53) A.U. • He had also predicted another planet at 59.573 A.U. which was not found. • Ketakar did not compute inclination of the planet's orbit to the ecliptic. Pickering and Lowell estimated it to be 21 and 10 degrees respectively. Irrelevance of being Pluto 13 But… • All the calculations of Pickering, Lowell, Ketakar and others were based on wrong data! • After the discovery of Pluto's satellite Charon in 1978, the planet is now known to be too small to influence Neptune's orbit. • Thus, the accuracy of Ketakar's predictions must be regarded as sheer coincidence! Irrelevance of being Pluto 14 7 Pluto • A planet of the Solar system (1930 to 2006) • Demoted to a non-planet in 2006 • The demotion was left incomplete by making it a “Pluton”, that is – too unimportant to be a planet but not so irrelevant as to be a boulder or meteor! Irrelevance of being Pluto 15 Orbit of Pluto • Pluto takes 248 years to orbit the Sun. Pluto came closest to the Sun in 1989. • Between 1979 and 1999, Pluto was closer to the Sun than Neptune. In 1978 its Moon Charon was discovered. • In 2003 it was discovered to have (at least) 2 more moons. Irrelevance of being Pluto 16 8 Problems of Pluto • From the beginning, it was clear that Pluto was very different. • Pluto’s moon Charon was almost half the size of Pluto itself. More were discovered recently. • By Nineties, it was realised that there is a big belt of medium to small size objects beyond Neptune. Irrelevance of being Pluto 17 Irrelevance of being Pluto 18 9 Planet Mass Dia. Rotn. Dist. Rev. Eccn. Inclin. (* M E) (km) (A.U.) (deg) Mercury 0.0553 4880 58.81 d 0.39 87.97 d 0.211 7.0 Venus 0.815 12,104 -243.69 d 0.72 224.70 d 0.007 3.4 Earth 1.000 12,742 23.93 h 1.00 365.26 d 0.017 0.00 Mars 0.107 6780 24.62 h 1.52 686.98 d 0.093 1.85 Jupiter 317.83 139,822 9.93 h 5.20 11.86 y 0.048 1.305 Saturn 95.162 116,464 10.50 h 9.54 29.46 y 0.056 2.489 Uranus 14.536 50,724 17.24 h 19.18 84.01 y 0.047 0.773 Neptune 17.147 49,248 16.11 h 30.06 164.79 y 0.009 1.773 Pluto 0.0021 2274 6.41 d 39.53 247.68 y 0.2482 17.15 # # # Moon 0.0123 1738 27.322Irrelevanced 1.000of being Pluto 365.26 0.0554 5.145 19 Irrelevance of being Pluto 20 10 Pluto Pluto (bottom right) compared in size to the largest moons in the solar system: Ganymede (Jupiter), Titan (Saturn),Irrelevance Callisto of (Jupiter) being Pluto, Io (Jupiter), Moon (Earth), 21 Europa (Jupiter) and Triton. Irrelevance of being Pluto 22 11 Irrelevance of being Pluto 23 Irrelevance of being Pluto 24 12 Kuiper Belt Objects • By late 2005 at least a dozen objects KB objects needed to be accommodated in solar system. • They were of size comparable to Pluto. • More were being discovered at an increasing rate. Irrelevance of being Pluto 25 Irrelevance of being Pluto 26 13 Irrelevance of being Pluto 27 Irrelevance of being Pluto 28 14 Irrelevance of being Pluto 29 Why bother? • Scientists wanted to decide once and for all whether Pluto is a planet or not. • The very basis of scientific process is accuracy. • Part of the job of scientists is to reduce the mass of facts to a smaller number of concepts. • This facilitates understanding and predictions. Irrelevance of being Pluto 30 15 So the matter went before IAU • International Astronomical Union is the largest International body of professional astronomers. • IAU is therefore a legitimate body to decide on the issue. • IAU formed a “Planet Definition Committee” in 2004 under the leadership of Prof. Owen Gingerich (Harvard-Smithsonian Centre for Astrophysics) . • IAU had its general body meeting in Prague in August 2006. • The matter was therefore taken up in the general body of IAU. Irrelevance of being Pluto 31 Irrelevance of being Pluto 32 16 Irrelevance of being Pluto 33 Semimajor Designation Diameter (km) axis (AU) Date found Pluto 2320 39.4 1930 2003 UB313 2400 ± 100 67.7 2003 136472 1800 ± 200 45.7 2005 136108 ~1500 43.3 2005 Charon 1205 39.4 1978 (90482) Orcus ~1500 39.4 2004 (50000) Quaoar 1260 ± 190 43.5 2002 (28978) Ixion 400 – 550 39.6 2001 55636 < 709 43.1 2002 55565 650 – 750 47.4 2002 55637 ~910 42.5 2002 (20000) Varuna 450 – 750 43 2000 Irrelevance of being Pluto 34 17 Irrelevance of being Pluto 35 Irrelevance of being Pluto 36 18 IAU attempt 1 • IAU resolution implicitly assumed that a planet is: – in orbit around the star – not big enough to be a star (brown dwarf) – not satellite to other planets. • The real defining part was: – It should be big enough to achieve a nearly spherical shape. – Mass of at least 5x10 20 Kg and radius of 800 km (for rocky bodies) is required for this. – Pluto seemed to fit these criteria and was branded a planet. This property was based more on physics, nostalgia, history (politics?) of the planet formation than on other criteria. Irrelevance of being Pluto 37 What it meant • All agreed that these were “necessary condition ”. But there were doubts if these are “sufficient condition ”. • 2003 UB 313 (Xena) would become a planet. • Ceres would become a “Giant Asteroid ”. • Pluto-Charon would be considered as binary planet system . • 12 other objects would be analysed further. • IAU proposed a new sub-category of planets of Pluto like objects called “Plutinos”. New “planets” beyond Neptune eccentric orbits would be called Plutinos. Irrelevance of being Pluto 38 19 Irrelevance of being Pluto 39 Objections • The proposal was severely criticised and a counter proposal was produced by a group of scientists. • It was argued that definition is insufficient on the physical grounds and not simple enough for the common public. • Charon and Ceres being promoted to the planet status did not go down well. • It was proposed the planet should also be "a dominant object in its local neighbourhood". • The bodies which satisfy IAU definition but are not the dominant bodies in their local neighbourhood would be called “dwarf planets". • In the IAU definition, Plutinos was a sub-category of planets where as the counter-proposal was clear on the issue that "a dwarf-planet is not a planet“ In the meeting IAU’s proposed definition was rejected. Irrelevance of being Pluto 40 20 IAU Second Resolution • IAU General body met a second time in view of the objection from so many objections. • The original single proposal was divided in 3 different proposals. – Planets should be nearly circular. – Being the dominant object in the local population was relegated to a secondary criterion. – the "dwarf planets" were defined as a sub-category of planets and not a distinct class. All three IAU proposals were defeated again Irrelevance of being Pluto 41 Irrelevance of being Pluto 42 21 IAU Third Resolution • Prof. Burns as a member of IAU resolutions committee chaired the third session. • The draft presented now was more like the counter proposal and sailed through the test vote. • Pluto like objects, were given the name of Dwarf Planets and classified to be outside the regime of planets. • The rest were called “minor solar system bodies” Irrelevance of being Pluto 43 Irrelevance of being Pluto 44 22 Final status • Planet : A planet must meet the following conditions: – (a) It has sufficient mass for its self-gravity to overcome rigid body forces so that it assumes a hydrostatic equilibrium (nearly spherical) shape, – (b) It is in orbit around a star, and is neither a star nor a satellite of a planet. – (c) it should be the dominant object in their region – There are 2 kinds: Gas Giants (Jupiter, Saturn, Uranus, Neptune) and Classical (Mercury, Venus, Earth, Mars) Only 8 objects belong to this class.
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