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 D sun Irrelevance of being Pluto 3 16 D sun Irrelevance of being Pluto 4 2
Solar System 109 D Earth Irrelevance of being Pluto 5 11 D Earth 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 27.322 # d 1.000 365.26 0.0554 # 5.145 # Irrelevance of being Pluto 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), Callisto (Jupiter), Io (Jupiter), Moon (Earth), Europa (Jupiter) and Triton. Irrelevance of being Pluto 21 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 Designation Diameter (km) Semimajor 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 UB313 (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. Dwarfs or Pluton: Dwarf planets are all medium size objects. They satisfy the necessary conditions (a and b) to be planets. Those beyond Neptune, near Pluto are called Plutons. Plutons includes Pluto and Xena (2003 UB 313 ) now renamed Eris and its moon is called Dysnomia. 3 dwarf planets are now known, probably another 12 or 13 need to be added. Satellite: Anything orbiting a planet. Centre of gravity does not fall outside the planet. Includes several bodies much larger than many planets, such as Jupiter's moon Ganymede (diameter: 5262 kilometres). More than 150 are known. Small solar system body: Anything orbiting the Sun that's not a planet or a satellite. Most asteroids and comets would Irrelevance be SSSBs. of being More Pluto than 130,000 are known. 45 Name Diameter Mass* Radius* (AU) period*(yrs) Moons Atmos. Mercury 0.39 0.06 0.39 0.24 none None Venus 0.95 0.82 0.72 0.62 none 96%CO 2 4%N 2 Earth** 1.00 1.00 1.00 1.00 1 78%N 2 21%O 2 Mars 0.53 0.11 1.52 1.88 2 95%CO2 3%N2 Jupiter 11.21 318 5.20 11.86 63 86%H2 14%He Saturn 9.41 95.2 9.54 29.46 56 93%H2 5%He Uranus 3.98 14.6 19.2 84.01 27 83%H2 15%He Neptune 3.81 17.2 30.1 164.8 13 80%H2 19%He Irrelevance of being Pluto 46 23
Dwarf Planets Name Ceres Pluto Eris (Xena, 2003UB 313 ) God of: growing plants Underworld Discord (no 134340) (no 136 199) Region of SS Asteroid belt Kuiper belt Scattered object Category Asteroid Pluton Scattered object Diameter 975 909 km 2306±20 km 2400±100 km Mass (M E ) 0.00016 0.0022 ~0.0025 Radius in km 471 1148.07 ~1,200 Density (in g/cm 3 ) 2.08 2 Rotation period (d) 0.378-6.39 (ret) Orbital radius (AU) 2.5-2.9 39.5 37.77-97.56 mean 2.766 39.48168677 67.6681 Orbital period (y) 4.599 248.09 557 Orbital Eccentricity 0.08 0.24880766 0.44177 Orbital inclination 10.587 17.14175 44.187 Axial tilt (deg) 4 119.61 surface temp (in K) 167 40 30 No of satellites 0 3 1 Irrelevance of being Pluto 47 Possible dwarf planets Name Category Diameter (km) Mass 1 2005 FY9 (Easterbunny) Kuiper belt 1600 2000? unknown 2 Orcus Pluton 840-1880 6.2-7.0 10 20 kg 3 Sedna Scattered obj. 1180 1800 1.7-6.1 10 21 kg 4 2003 EL61 (Santa) Kuiper belt ~ 1500 ~4.2 10 21 kg 5 Quaoar Kuiper belt 989-1346? 1.0-2.6 10 21 kg 6 Charon (sat. of Pluto) Pluton 1207 km ± 3 (1.52±0.06) 10 21 kg 7 2002 TC302 Scattered obj. 1200 unknown 8 Varuna Kuiper belt ~936 ~5.9 10 20 kg 9 2002 UX25 Kuiper belt ~910 ~7.9 10 20 kg 10 2002 TX300 Kuiper belt <900 unknown 11 Ixion Pluton <822 unknown 12 2002 AW197 Kuiper belt 700±50 unknown Irrelevance of being Pluto 48 24
Irrelevance of being Pluto 49 FINAL SOLAR SYSTEM Irrelevance of being Pluto 50 25
And the word went out Irrelevance of being Pluto 51 Irrelevance of being Pluto 52 26
Pluto and its residents were quite upset! Irrelevance of being Pluto 53 Irrelevance of being Pluto 54 27
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Irrelevance of being Pluto 57 Why Study Kuiper Belt? Other primitive solar systems are much bigger than our own solar system. Kuiper belt is the outer most part of the Solar System known to us. (Oort cloud of SS residue at 50,000 to 100,000 AU is suspected to exist but there are doubts on its stability). It was discovered in 1992 and has an estimated 35,000 objects greater than 100 km in diameter. It holds information on the farthest reaches of the Solar System and a memory of the events that shaped our Solar System. It is highly perturbed. Irrelevance of being Pluto 58 29
Irrelevance of being Pluto 59 Why bother? Pluto is an important member of the Kuiper belt. The belt has 3 classes of objects classical objects (in nearly circular orbit) Resonant objects (In 2:3 resonance with Neptune- Pluto belongs to this class) Scattered objects Kuiper belt carries information on: possible lost giant objects long period planets and possible binary companion of the Sun. Other data on the origin of Solar system About 15% of the objects are in binary indicating a dense cloud with a total mass of about 10 Earths. So where are the unseen 90% objects (are they lost or too small)? Kuiper belt is the source of short period (few tens of years) comets. Irrelevance of being Pluto 60 30
Study of the Solar system Modelling Solar System requires understanding of Kuiper belt. Kuiper belt is probably the last stable location of the solar system where the escape velocity (~ 7 km/s) is much higher than rotation velocity (~ 5 km/s). Beyond this, the Sun s gravity is too weak. The Kuiper belt has to be studied in detail for information on early solar system and its edge. It may also hold clues to Dark matter and its role in formation and stability of Solar System. This classification provides a proper framework for people working in the field. Irrelevance of being Pluto 61 Some even think that there are more planets than astronomers can see. Because a lot of our leaders seem to be living on a planet that could not be the Earth of you and me! Irrelevance of being Pluto 62 31
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Some people are of course more worried about the Earth. Irrelevance of being Pluto 65 Irrelevance of being Pluto 66 33
Irrelevance of being Pluto 67 To whom does it matter? Not to anyone studying objects in the Solar System. Not to Astrologers (just in case you ask). Not to people in any other discipline. Not to Pluto. It matters only to people interested in the origin of the Solar System as a whole. Irrelevance of being Pluto 68 34
The only positive outcome of the debate, as far as most of us are concerned, is that we all get a free cup of tea (and biscuits) courtesy Satyanarayan! Irrelevance of being Pluto 69 Acknowledgement My thanks to the organisers of the ASET Colloquium. My special thanks to Aniket Sule for providing me with the first hand account of the proceedings of IAU. My thanks to the authors of various cartoons that have been used in this lecture. My thanks to unknown persons who left a lot of resources on internet, especially WIKIPEDIA which I have used extensively in this lecture. My thanks to all the friends, who nudged me to give this lecture. Irrelevance of being Pluto 70 35