On the irrelevance of being a PLUTO compressedvahia/Pluto.pdf · On the irrelevance of being a PLUTO! MayankVahia DAA, TIFR Irrelevance of being Pluto 2 Size Scale of Stars and Planets

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Irrelevance of being Pluto 1

On the irrelevance of being a

PLUTO!

Mayank Vahia

DAA, TIFR


Size Scale of Stars and Planets

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700 Dsun

1 AU


16 Dsun

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Solar System

109 DEarth


Venus

11 DEarth

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Solar System visible to unaided eye

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Solar System at the beginning of 20th Century


Solar System of my text book (30 years ago)

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Asteroid Belt

(Discovered in 1977)


The ‘Planet’ Pluto

• Pluto is a 14th magnitude object.

• It is NOT visible to naked eye (neither are Uranus and Neptune).

• It was discovered by American astronomer

Clyde Tombaugh in 1930.

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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 1o 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.


But…

• All the calculations of Pickering, Lowell, Ketakarand 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!

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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!


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.

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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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17.150.2482247.68 y39.536.41 d22740.0021Pluto

1.7730.009164.79 y30.0616.11 h49,24817.147Neptune

0.7730.04784.01 y19.1817.24 h50,72414.536Uranus

2.4890.05629.46 y9.5410.50 h116,46495.162Saturn

1.3050.04811.86 y5.209.93 h139,822317.83Jupiter

1.850.093686.98 d1.5224.62 h67800.107Mars

0.000.017365.26 d1.0023.93 h12,7421.000Earth

3.40.007224.70 d0.72-243.69 d12,1040.815Venus

7.00.21187.97 d0.3958.81 d48800.0553Mercury

(deg)(A.U.)(km)(* ME)

Inclin.Eccn.Rev.Dist.Rotn.Dia.MassPlanet

Moon 0.0123 1738 27.322#d 1.000 365.26 0.0554# 5.145#


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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.


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• 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.

Kuiper Belt Objects


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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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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.


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200043450 – 750 (20000) Varuna

200242.5~910 55637

200247.4650 – 750 55565

200243.1< 709 55636

200139.6400 – 550 (28978) Ixion

200243.51260 ± 190 (50000) Quaoar

200439.4~1500 (90482) Orcus

197839.41205Charon

200543.3~1500136108

200545.71800 ± 200 136472

200367.72400 ± 1002003 UB313

193039.42320Pluto

Date found

Semimajor

axis (AU)Diameter (km)Designation

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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 5x1020 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.


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.

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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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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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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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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 UB313) 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 be SSSBs. More than 130,000 are known.


80%H2

19%He13 164.8 30.1 17.2 3.81 Neptune

83%H2

15%He27 84.01 19.214.6 3.98 Uranus

93%H2

5%He56 29.46 9.54 95.2 9.41 Saturn

86%H2

14%He63 11.86 5.20 318 11.21 Jupiter

95%CO2

3%N221.88 1.52 0.11 0.53 Mars

78%N221%O2

1 1.00 1.00 1.00 1.00 Earth**

96%CO24%N2

none0.62 0.72 0.82 0.95 Venus

Nonenone 0.24 0.39 0.06 0.39 Mercury

Atmos.Moonsperiod*(yrs)Radius*

(AU)

Mass*DiameterName

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Dwarf Planets• Name Ceres Pluto Eris (Xena, 2003UB313)

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 (ME) 0.00016 0.0022 ~0.0025

• Radius in km 471 1148.07 ~1,200

• Density (in g/cm3) 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


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 × 1020 kg

3 Sedna Scattered obj. 1180–1800 1.7-6.1 × 1021 kg

4 2003 EL61 (Santa) Kuiper belt ~ 1500 ~4.2 × 1021 kg

5 Quaoar Kuiper belt 989 - 1346? 1.0-2.6 × 1021 kg

6 Charon (sat. of Pluto) Pluton 1207 km ± 3 (1.52±0.06)×1021 kg

7 2002 TC302 Scattered obj. ≤ 1200 unknown

8 Varuna Kuiper belt ~936 ~5.9 × 1020 kg

9 2002 UX25 Kuiper belt ~910 ~7.9 × 1020 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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And the word went out


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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. (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.

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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 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.

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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.


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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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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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!


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.

Documents

On the irrelevance of being a PLUTO compressedvahia/Pluto.pdf · On the irrelevance of being a PLUTO! MayankVahia DAA, TIFR Irrelevance of being Pluto 2 Size Scale of Stars and Planets