Download On the irrelevance of being a PLUTO! Size Scale of Stars and Planets

On the irrelevance of being a
PLUTO!
Mayank Vahia
DAA, TIFR
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Size Scale of Stars and Planets
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1
1 AU
700 Dsun
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16 Dsun
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2
Solar System
109 DEarth
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11 DEarth
Venus
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3
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Solar System visible to unaided eye
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Solar System at the beginning of 20th Century
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Solar System of my text book (30 years ago)
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Asteroid Belt
(Discovered in 1977)
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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.
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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!
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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!
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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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Planet
Mass
Dia.
(* ME)
(km)
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
0.0123
1738
0.0554#
5.145#
Moon
Rotn.
Dist.
Rev.
Eccn.
(A.U.)
Inclin.
(deg)
#d
27.322
1.000 365.26
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Pluto
Pluto (bottom right) compared in size to the largest moons in the solar system:
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being PlutoIo (Jupiter), Moon (Earth),
Ganymede (Jupiter), Titan (Saturn),
Callistoof(Jupiter),
Europa (Jupiter) and Triton.
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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.
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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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Diameter (km)
Semimajor
axis (AU)
Date found
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
450 – 750
43
2000
Designation
Pluto
(20000) Varuna
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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.
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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. 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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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.
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of beingMore
Pluto than 130,000 are known. 45
Most asteroids and comets would
be SSSBs.
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%CO2
4%N2
Earth**
1.00
1.00
1.00
1.00
1
78%N2
21%O2
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
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Dwarf Planets
•
Name
God of:
Ceres
growing plants
•
•
•
•
•
•
•
•
Region of SS
Category
Diameter
Mass (ME)
Radius in km
Density (in g/cm3)
Rotation period (d)
Orbital radius (AU)
– mean
•
•
•
•
•
•
Orbital period (y)
Orbital Eccentricity
Orbital inclination
Axial tilt (deg)
surface temp (in K)
No of satellites
Asteroid belt
Asteroid
975×909 km
0.00016
471
2.08
0.378
2.5-2.9
Pluto
Underworld
(no 134340)
Kuiper belt
Pluton
2306±20 km
0.0022
1148.07
2
-6.39 (ret)
39.5
Eris (Xena, 2003UB313)
Discord
(no 136 199)
Scattered object
Scattered object
2400±100 km
~0.0025
~1,200
2.766
39.48168677
67.6681
4.599
0.08
10.587
4
167
0
248.09
0.24880766
17.14175
119.61
40
3
557
0.44177
44.187
37.77-97.56
30
1
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Possible dwarf planets
Name
Category
1 2005 FY9 (Easterbunny) Kuiper belt
2 Orcus
Pluton
3 Sedna
Scattered obj.
4 2003 EL61 (Santa)
Kuiper belt
5 Quaoar
Kuiper belt
6 Charon (sat. of Pluto) Pluton
7 2002 TC302
Scattered obj.
8 Varuna
Kuiper belt
9 2002 UX25
Kuiper belt
10 2002 TX300
Kuiper belt
11 Ixion
Pluton
12 2002 AW197
Kuiper belt
Diameter (km) Mass
1600 – 2000?
840 - 1880
1180–1800
~ 1500
989 - 1346?
1207 km ± 3
≤ 1200
~936
~910
<900
<822
700±50
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unknown
6.2 - 7.0 × 1020 kg
1.7-6.1 × 1021 kg
~4.2 × 1021 kg
1.0-2.6 × 1021 kg
(1.52±0.06)×1021 kg
unknown
~5.9 × 1020 kg
~7.9 × 1020 kg
unknown
unknown
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.
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• 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.
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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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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!
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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.
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