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Home , 20000 Varuna, 28978 Ixion, 90482 Orcus, Axial tilt, Ceres (dwarf planet), Charon (moon)

On the irrelevance of being a !

Mayank Vahia DAA, TIFR

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Size Scale of and

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1 1 AU

700 Dsun

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16 Dsun

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2

109 DEarth

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11 DEarth

Venus

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Solar System visible to unaided eye

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4 Solar System at the beginning of 20 th Century

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Solar System of my text book (30 ago)

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5 Belt (Discovered in 1977)

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The ‘’ Pluto

• Pluto is a 14 th object.

• It is NOT visible to (neither are and ).

• It was discovered by American in 1930.

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6 Prediction of Pluto • and William H. Pickering are credited with the theoretical work on Pluto’s 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 .

• His location was within 1 o of its correct location.

• He had predicted ts 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 . 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 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!

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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!

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Orbit of Pluto • Pluto takes 248 years to orbit the . Pluto came closest to the Sun in 1989.

• Between 1979 and 1999, Pluto was closer to the Sun than Neptune. In 1978 its Charon was discovered.

• In 2003 it was discovered to have (at least) 2 more .

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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.

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9 Planet . 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

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10 Pluto

Pluto (bottom right) compared in size to the largest moons in the solar system: (Jupiter), (),Irrelevance of (Jupiter) being Pluto, (Jupiter), Moon (), 21 (Jupiter) and .

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12 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.

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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.

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15 So the matter went before IAU

• International Astronomical Union is the largest International body of professional .

• IAU is therefore a legitimate body to decide on the issue.

• IAU formed a “Planet Definition Committee” in 2004 under the leadership of Prof. (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.

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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) ~1500 39.4 2004 (50000) Quaoar 1260 ± 190 43.5 2002 (28978) 400 – 550 39.6 2001 55636 < 709 43.1 2002 55565 650 – 750 47.4 2002 55637 ~910 42.5 2002 (20000) 450 – 750 43 2000

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18 IAU attempt 1 • IAU resolution implicitly assumed that a planet is: – in orbit around the – not big enough to be a star () – 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.

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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. • 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 “”. New “planets” beyond Neptune eccentric would be called Plutinos. Irrelevance of being Pluto 38

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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.

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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

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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”

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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 (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 (Jupiter, Saturn, Uranus, Neptune) and Classical (, Venus, Earth, ) 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 and its moon is called . 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.

• : Anything orbiting the Sun that's not a planet or a satellite. Most and wouldIrrelevance be SSSBs. of being More Pluto than 130,000 are known. 45

Name Diameter Mass* Radius* period*(yrs) Moons Atmos. (AU) 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%CO 2 3%N2

Jupiter 11.21 318 5.20 11.86 63 86%H 2 14%He

Saturn 9.41 95.2 9.54 29.46 56 93%H 2 5%He

Uranus 3.98 14.6 19.2 84.01 27 83%H 2 15%He

Neptune 3.81 17.2 30.1 164.8 13 80%H 2 19%He

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23 Dwarf Planets

• Name Ceres Pluto Eris(Xena, 2003UB 313 ) God of: growing plants Underworld Discord (no 134340) (no 136 199) • Region of SS Kuiperbelt 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 • (in g/cm 3) 2.08 2 • 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 • 0.08 0.24880766 0.44177 • 10.587 17.14175 44.187 • (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

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FINAL SOLAR SYSTEM

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25 And the word went out

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26 Pluto and its residents were quite upset!

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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. ( 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

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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 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 . 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.

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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 (~ 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.

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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!

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32 Some people are of course more worried about the Earth.

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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.

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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!

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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

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