What happened to Pluto?

1
What happened to Pluto?

• Jeremy P. Carlo
• Columbia University

2
August 2006 IAU decision

• (Short version) Pluto is no longer a planet.
• Public Reaction Not good...
• What happened?!

3
August 2006 IAU decision

• (Long version)
• Were going to go back and look at the basics, to
  try and understand the rationale behind the
  decision.

4
What is a planet?

• To start lets consider this simple question.
• Well look at the question as a function of
  history, and how it has been answered in the past.

5
The Ancients (before 1500 AD)

• The Ancients had no telescopes (all naked-eye
  observing).
• They noted that the stars remained fixed
  relative to each other, though they all went
  around the sky every 24 hours...
• ...except for exactly seven, the wanderers
  (planets).
• (There were also random objects like comets and
  novae, but thats another story for another
  talk...)

• Planets Objects which wandered across the sky.
• Identified with gods.
• The Seven Ancient Planets
• The Moon
• The Sun
• Mercury
• Venus
• Mars
• Jupiter
• Saturn

6
The Ancient Solar System

• Geocentric - Earth at center (pretty reasonable
  to assume)
• The seven planets orbit the earth in circles,
  surrounded by the fixed stars.
• Appeared to explain the observed motion of the
  planets... fairly well.
• But Mercury and Venus followed strange paths in
  the sky, and the Sun and Moon just looked
  different from the others.

7
The first modern astronomers (1500-1600)

• By the 1500s, Tycho Brahes naked-eye
  observations made it clear the old model wasnt
  working too well.
• Copernicus proposes a Sun-centered (heliocentric)
  model.
• Still the same seven planets (plus the Earth),
  only rearranged.
• Earth is the third planet, with the Moon going
  around it. Everything else orbits the Sun.
• Orbits not circles, but ellipses (Keplers
  modification to better fit Tychos data)

8
The Solar System 1600

• Sun Dist. from Sun (AU)
• 1. Mercury 0.3
• 2. Venus 0.7
• 3. Earth 1.0
• Moon
• 4. Mars 1.5
• 5. Jupiter 5
• 6. Saturn 10

• Planets - the six known objects that orbit the
  sun.
• If it goes around a planet, then its a moon.
• Officially six planets plus one moon, all known
  since ancient times.
• Planets move in elliptical orbits around the sun
  according to Keplers Laws.

9
Mercury

• The Known
• Planets,
• 1600

10
The Known Planets, 1600

• Venus

11
The Known Planets, 1600

• Earth and Moon

12
The Known Planets, 1600

• Mars

13
The Known Planets, 1600

• Jupiter

14
The Known Planets, 1600

• Saturn

15
The First Telescopes (1610-1700)

• Telescope invented by Hans Lippershey in 1607,
  mostly for military use.
• Galileo was the first to observe the sky with a
  telescope.
• Galileo
• Galilei

• Craters on the moon
• Sunspots
• He later became blind...
• Phases of Venus
• The planets show discs like the Sun and Moon
• Four moons around Jupiter!
• Io, Europa, Ganymede Callisto
• First discovery of solar system bodies besides
  the seven known to ancients.
• First objects to orbit something other than the
  Sun or the Earth.
• Orbited Jupiter in agreement with Keplers Laws!

16
The Solar System 1700

• Sun Dist. from Sun (AU)
• 1. Mercury 0.3
• 2. Venus 0.7
• 3. Earth 1.0
• Moon
• 4. Mars 1.5
• 5. Jupiter 5
• 4 moons
• 6. Saturn 10
• 5 moons

• Still six planets.
• Galileo discovers 4 moons of Jupiter in 1610 -
  the first objects found to orbit something other
  than the earth or sun.
• In 1655 Christiaan Huygens discovers rings around
  Saturn as well as its largest moon, Titan
• In the 1670s and 1680s, four more moons around
  Saturn are found - Tethys, Dione, Rhea and
  Iapetus.

17
Io
Europa
Ganymede
Callisto

• Jupiters Moons (to scale)

18

• Saturn and its largest moons (not to scale)

19
The 1700s...A dry spell and then a shocking
discovery!

• After about 1690, there were no new solar system
  discoveries (other than comets, etc.)
• But all telescopes up to this point were rather
  small, until William Hershel came around. He
  liked to think BIG...
• Herschels
• 40-foot
• telescope

• Many discoveries - comets, nebulas, star
  clusters...
• But in 1781 he found an object that moved in the
  sky, a new wanderer...
• Keplers laws placed it beyond the edge of the
  solar system, twice as far out as Saturn
• A new planet! Uranus...
• For the first time in recorded history!
• And in 1787, two moons were found orbiting Uranus
  - Titania and Oberon.

20

• Uranus
• First planet discovered since ancient times (1781)

21
The Solar System 1800

• Sun Dist. from Sun (AU)
• 1. Mercury 0.3
• 2. Venus 0.7
• 3. Earth 1.0
• Moon
• 4. Mars 1.5
• 5. Jupiter 5
• 4 moons
• 6. Saturn 10
• 7 moons
• 7. Uranus 20
• 2 moons

• The family is growing. Seven planets!
• Two new moons around Saturn - Mimas Enceladus
• The solar system is doubled in size, very nearly
  overnight!
• So a new planet was discovered. Can we perhaps
  find others? Were about to find out...

22
1801 Deja vu all over again?

• In 1801 Giuseppe Piazzi finds yet another
  wanderer.
• This time Keplers Laws place it between Mars and
  Jupiter
• Ok, not as exciting as finding something beyond
  the known edge of the solar system, but well
  take what we can get.
• Two problems
• Ceres is rather dim. Really dim for how close it
  is, actually.
• It doesnt show a disc like all the other
  planets, but appears starlike, asteroidal at
  all achievable magnifications

• This new find, named Ceres, must be really small.
• But an even bigger problem soon became apparent
  Three more similar objects were found over the
  next six years Pallas, Juno and Vesta.
• One small planet, maybe, but four?
• Luckily, no more asteroids followed, at least
  for a while...

23
The Solar System 1810

• Sun Dist. from Sun (AU)
• 1. Mercury 0.3
• 2. Venus 0.7
• 3. Earth 1.0
• Moon
• 4. Mars 1.5
• 5-8. Ceres, Pallas, 2-3
• Juno Vesta
• 9. Jupiter 5
• 4 moons
• 10. Saturn 10
• 7 moons
• 11. Uranus 20
• 4 moons

• Getting kind of crowded - eleven planets?
• Of course this all hinges on counting the four
  asteroids as planets, despite their small size.
• It all came crashing down in 1845 when a fifth
  asteroid (Astraea) was found, and more soon
  followed.
• These asteroids arent really planets at all,
  but instead form a class of different objects.

24
The Solar System 1845

• Sun Dist. from Sun (AU)
• 1. Mercury 0.3
• 2. Venus 0.7
• 3. Earth 1.0
• Moon
• 4. Mars 1.5
• The Asteroid Belt 2-3
• 5. Jupiter 5
• 4 moons
• 6. Saturn 10
• 7 moons
• 7. Uranus 20
• 4 moons

• Back to seven planets. Whew!
• But we must set some limit - too small, youre
  not a planet.
• But how small is too small?
• Perhaps this asteroid belt consists of objects
  that failed to completely coalesce into a
  full-blown planet?
• Meanwhile, work continued toward discovering more
  planets, and a surprise was in immediate store...

25
1846 The year of the mathematicians

• Up to now, every discovery was made by accident -
  somebody looking in the right place at the right
  time.
• The 1801 discovery of Ceres proved to be
  disappointing.
• But some scientists noted that there were
  anomalies in the orbit of Uranus (which by this
  time had completed nearly one orbit)
• Could there be an eighth planet causing these
  perturbations?

• John C. Adams in England and Urbain Leverrier in
  France, both independently proposed the existence
  of a planet beyond Uranus.
• Johann Galle of Germany looked at the predicted
  location in 1846, and sure enough a new planet
  was there!
• International collaboration!
• Neptune, the first object to be discovered on
  purpose, in a triumph for the predictive power
  of science.

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• Neptune
• First planet found on purpose (1846)

27
The Solar System 1846

• Sun Dist. from Sun (AU)
• 1. Mercury 0.3
• 2. Venus 0.7
• 3. Earth 1.0
• Moon
• 4. Mars 1.5
• The Asteroid Belt 2-3
• 5. Jupiter 5
• 4 moons
• 6. Saturn 10
• 7 moons
• 7. Uranus 20
• 4 moons
• 8. Neptune 30
• 1 moon

• Eight planets, and this time for real!
• Of course, two weeks after Neptune was
  discovered, its largest moon Triton was also
  discovered.
• In the same spirit, by the late 1800s it
  appeared there were further orbital perturbations
  as well...
• Can we do it again?

28
Early 1900s The Search for Planet X

• It became apparent that there were further
  perturbations in Uranus orbit. Could there be
  another planet, beyond Neptune? Percival Lowell
  thought so, and initiated a search for Planet
  X.
• Although Lowells search failed, a far more
  comprehensive search was taken up by Clyde
  Tombaugh.

• Clyde Tombaugh in 1930

29
Paydirt!

• In January 1930, Tombaugh found what he was
  looking for.
• Above Tombaugh and blink comparator
• Right Discovery images of Pluto (arrow),
  January 23 and 29, 1930

30
The Solar System 1930

• Nine Planets! (Finally!)
• But Pluto clearly was smaller than Uranus and
  Neptune.
• Originally it was guessed it was about the size
  of the Earth.
• But even that figure proved too optimistic as new
  data came in.

• Sun Dist. from Sun (AU)
• 1. Mercury 0.3
• 2. Venus 0.7
• 3. Earth 1.0
• 1 moon
• 4. Mars 1.5
• 2 moons
• The Asteroid Belt 2-3
• 5. Jupiter 5
• 9 moons
• 6. Saturn 10
• 9 moons
• 7. Uranus 20
• 4 moons
• 8. Neptune 30
• 1 moon
• 9. Pluto 36

31

• Pluto
• Planet X at last? (1930)

32
The distinctions get blurred

• Up to now everything has been tidy, neat and
  orderly.
• Clear demarcations between planets, moons,
  asteroids comets.
• But the distinctions were about to get blurry...

33
Just how small is Pluto?

• Pluto and Charon

• Originally Pluto was thought to be about the size
  of the earth.
• But Plutos mass was continually lowered from its
  discovery in 1930 until the 1970s.
• The last straw came when James Christy discovered
  Plutos moon Charon in 1978, enabling an exact
  mass determination
• Pluto 1/500 Earth mass
• Not only was Pluto (by far) the smallest planet,
  its also smaller than at least seven known
  moons!
• But still bigger than the asteroids...

34
And it isnt alone either...

• In 1992, a new trans-Neptunian object (TNO) was
  found, 1992 QB1.
• 1992 QB1 was smaller than Pluto, but still rather
  large, and presumably similar to Pluto in origin
  and composition.
• It was believed that Pluto and 1992 QB1 were the
  prototypes of a new class of objects filling the
  trans-Neptunian Kuiper Belt.
• Indeed, 1992 QB1 was merely the first of many
  TNOs to be discovered, although Pluto remained
  the largest.

• Attractive suggestion Pluto is not a planet,
  but merely the largest of the Trans-Neptunian
  Objects (TNOs), as Ceres is the largest of the
  asteroids.
• This creates a new class of objects, the TNOs,
  analogous to the asteroids.
• But Pluto loses its long-held planetary status...
  Very unpopular with the public!

35

• My Very Educated
• Mother Just Served Us
• Nine Pizzas

36

• My Very Educated
• Mother Just Served Us
• NOTHING!
• What happened to Pluto!!!

37

• My Very Educated
• Mother Just Served Us
• NOTHING!
• What happened to Pluto!!!
• This proposal proved very unpopular, and didnt
  get far...

38
Pluto and the largest TNOs, 2003

• Plutos still the biggest TNO(but the gap is
  closing...)
• (artists conceptions of Sedna and Quaoar)

39
Enter Xena (2003 UB313)

• Discovered by Michael Brown of Caltech in 2003,
  but not realized until 2005. Named 2003 UB313,
  unoficially nicknamed Xena. (X Tenth Planet?)
• Subsequently found that its at least as big as
  Pluto!
• And it has its own moon, Gabrielle.
• Clearly, if Pluto qualifies as a planet, then so
  does Xena.
• But there could be hundreds of other objects
  bigger than Xena.
• Do all those get to be planets too?
• What do we do?

40
A matter of size...The Nine Planets, to
scalePlutos the smallest (by far!)
41

• Pluto vs. the largest moons
• even the moons are bigger!

42

• Pluto vs. Xena, 2005
• strike three for old Pluto?
• (artists conceptions)

43
What do we do?

• Pluto is the smallest planet (by a wide margin)
• Its also smaller than at least six moons.
• And its not even the largest trans-Neptunian
  object anymore!
• And there could be hundreds, maybe thousands,
  more of those!
• Dilemma
• Either remove Pluto from the list of planets, or
• Xena and every one of the hundreds of other
  large TNOs get to be planets too.

44
Toward a scientific definition

• So far weve gotten by on a wing and a prayer,
  without a formal definition of what a planet was.
• It was obvious what a planet was, for the most
  part.
• But now we need a scientific definition.
• The IAU convened in Prague in August 2006 to
  tackle this topic.

45
Setting a minimum size

• The simplest proposal is to set a minimum
  diameter for planethood.
• But how much? 1000 miles? Kilometers? Fathoms?
  Cubits? Or 500, 750, 2000, 200, 5000?
• Getting astronomers to agree is like herding
  cats...
• ..especially upon such an arbitrary definition.
• Does Nature provide a yardstick?
• It turns out She does...

46
Its all about the shape

• One simple idea
• An object large enough to form into a sphere
  under gravity, is a planet.
• This is actually a lot more significant than it
  sounds.
• Sphericity implies that the object is dominated
  by gravity, rather than microscopic
  intermolecular forces.
• It is also associated with stratification of the
  interior, which leads to the possibility of
  geological activity.

47
Roundness

• Roundness also makes it more likely a planet will
  hold an atmosphere, which is necessary for liquid
  water... which is necessary for life and for all
  sorts of interesting chemistry.
• So roundness is really a significant
  characteristic!

48
Whos round?

• All nine planets are round.
• Ceres (the largest asteroid) is round. But Vesta
  (the second largest) is significantly elliptical.
  Presumably the rest of the asteroids are
  elliptical as well.
• Many moons are also round.
• Although we havent seen them up close, a large
  number of KBOs are almost certainly round as
  well.

49
Typical small asteroidswith irregular shapes
50
A Typical Comet

• Halleys Comet
• 16 km largest dimension
• Also irregularly shaped!

51

• The largest moons (plus 2 planets)
• All round!
• How do intermediately-sized objects look?

52

• Asteroid 4179 Toutatis
• 4 km across
• Clearly not round!
• Jupiters moon Amalthea
• 170 km across
• Clearly not round!

• Asteroid 243 Ida ( Dactyl)
• 30 km across
• Clearly not round!

53

• Saturns moon Hyperion
• 280 km across
• Still not round!
• Saturns moon Mimas
• 400 km across
• Round!

• Neptunes moon Proteus
• 400 km diameter
• Getting there, but not quite.

54

• Saturns moon Enceladus
• 500 km across
• Round!

• Uranus moon Miranda
• 470 km across
• Round!
• Saturns Moon Tethys
• 1000 km across
• Round!

55
What about the large asteroids?

• Ceres - the largest asteroid.
• 950 km diameter
• Round!

• Vesta - the second- largest asteroid.
• 600 km largest dimension
• Not round, but somewhat elliptical.

56
How big is big enough?

• While there is some dependence on composition, it
  appears the minimum size for roundness is
  somewhere near 500 km.
• In particular, rocky objects probably have to be
  a little bigger to become round than do icy
  objects.
• 400-600km covers the transition region.

57
And the TNOs?

• Unfortunately we have no good photographs of any
  TNOs - the best we have are the ground-based and
  Hubble images of Pluto.
• Diameters are generally based upon visual
  magnitude and assumed surface reflectivity
  (albedo).
• It appears at least one large TNO, 2003 EL61, is
  not round (likely due to its very rapid
  rotation), but almost all the others almost
  certainly are.

58
The avalanche of round objects

• Not only do we have the nine existing planets,
  but we pick up at least twenty moons, at least
  one asteroid (Ceres), and at least a half-dozen
  known (and who knows how many unknown) TNOs.
• Thats 40 planets (and counting..)
• We must come up with a stricter definition...

59
Eliminating the moons

• Lets add a second condition
• Any round object which orbits a planet, is a moon
  and not a planet.
• This was a definition proposed (and rejected) by
  the IAU.
• Under this definition we have the nine existing
  planets, plus asteroid Ceres, Plutos moon
  Charon, Xena and some unknown number (but
  likely at least half a dozen) number of
  trans-Neptunian objects.
• Ill explain...

60
The Third Condition

• Subsequent IAU proposal
• In addition to the first two criteria, a planet
  must also have cleared the neighbourhood around
  its orbit.
• Anything that meets the first two criteria but
  not the third is termed a dwarf planet.
• This proposal was voted upon and accepted.
• End result Pluto, Charon, Ceres and Xena
  (now officially named Eris) become dwarf planets,
  and were back down to 8 full-fledged planets.

61
A definition, finally!

• A planet is a celestial body that
• (a) is in orbit around the Sun
• (b) has sufficient mass for its self-gravity to
  overcome rigid body forces so that it assumes a
  hydrostatic equilibrium (nearly round) shape
• (c) has cleared the neighbourhood around its
  orbit.
• Planet satisfies all three of (a), (b), (c)
• Dwarf Planet satisfies only (a), (b)
• Small Solar System Body only (a)

62

• The Solar System Today

63

• Pluto, the Planet (1930-2006)

64

• Pluto, the Planet (1930-2006)
• 134340 Pluto, the Dwarf Planet (2006-??)

65
Addendum Alphabet Soup

• TNOs, KBOs, SDOs...
• What are they?
• Classifications of Pluto-like objects (PLOs?) in
  the outer solar system.
• Primary Subdivisions
• Cis-Neptunian Objects
• Neptunes largest moon Triton (special case)
• Trans-Neptunian Objects

66

• Cis-Neptunian Objects
• Neptune Trojans - captured KBOs?
• Centaurs - scattered KBOs?
• Trans-Neptunian Objects
• Kuiper Belt Objects
• Resonant KBOs
• Non-resonant KBOs
• Scattered Disc Objects
• Oort Cloud Objects
• Triton - captured KBO?
• Relationship to comets?

67
Cis-Neptunian Object

• Cometlike objects (but much larger) orbiting
  within or at the orbit of Neptune.
• Neptune Trojans - analogous to Jupiters Trojans
  - follow or lead Neptune in its orbit by 60
  degrees.
• Centaurs - large TNO-like objects orbiting
  between Saturn and Neptune. Prototype 2060
  Chiron
• Probably have same or similar origin to TNOs.

68
Special Case Triton

• Triton is the largest moon of Neptune.
• It orbits Neptune retrograde, extremely unusual
  (unique) for such a large object.
• Theory is that it is a captured object similar to
  Pluto.
• If so, it is thus far the only such object seen
  up-close.

69
TNO Trans-Neptunian Object

• An object which orbits the sun with an average
  distance greater than Neptune.
• Pluto was the first (and still prototype) TNO.
• TNOs can be subdivided into several groups,
  depending on orbit.
• Kuiper Belt Objects (KBOs)
• Scattered Disc Objects (SDOs)
• Oort Cloud Objects

70
KBOs Kuiper Belt Objects

• Members of a class of objects existing just past
  Neptune, average distance 30-50 AU.
• Prototype Pluto.
• Orbits are dominated by Neptunes effects.
• Sharp cutoff after about 48 AU.
• Generally low inclination orbits (lt30º)
• KBOs can be further subdivided
• Resonant KBOs
• Non-resonant KBOs

71
Resonant KBOs

• Orbital periods are a rational fraction of
  Neptunes
• 32 (e.g. 3 Neptune orbits to 2 of the objects
  orbit)
• Orbital period 165 years x 3/2 248 years
• Prototype Pluto
• These objects are called Plutinos
• Other resonances 21 (twotinos, 52, etc.)
• These occur because of Neptunes repeated
  gravitational influence.
• The orbits are usually somewhat elliptical and
  moderately inclined.

72
Non-resonant KBOs

• No relation between orbital period and Neptunes.
• Prototype 1992 QB1 (cubewanos)
• Tend to have more closely circular orbits than
  their resonant counterparts.

73
Scattered Disc Objects (SDOs)

• More distant members of the TNO class, past the
  Kuiper Belt edge.
• Beyond about 50 AU, Neptunes gravitational
  effects are minimal.
• Prototype members Sedna, Eris (both gt75 AU).
• Often have very elliptical orbits (Sedna goes out
  to 1000 AU)
• Tend to have high inclinations (Eris is 45?)

74
The Pluto-like Objects
75
Oort Cloud Objects

• The Oort cloud is a hypothetical reservior for
  comets, etc., gt10,000 AU away.
• No objects in the Oort cloud have been directly
  observed.
• Could the scattered disc and Kuiper belt be
  inward extensions of the Oort cloud?

76
Relationship to Comets?

• The Pluto-like objects have very similar
  composition to comets, although theyre much
  bigger.
• Could comets simply be the smaller members of
  this class of objects (presumably there must be
  oodles of them, as-yet undiscovered), pushed into
  the inner solar system by gravitational
  interactions?
• Two basic types of comets
• Short-Period Comets (50-100 years)
• Long-Period Comets (1000 years)

77
Short-Period Comets

• Orbit of Comet Halley, a typical short-period
  comet.
• Its aphelion right about at the Kuiper belt!
• Of course we only know about it because it
  periodically comes close to the earth.
• Many other comets share these properties!

78
Long-Period Comets

• There are also long-period comets (such as
  Hale-Bopp and Hyakutake) which have orbits that
  take them well beyond the known reaches of the
  solar system.
• Could these objects come from the Oort cloud?

79
Outer Solar System Objects

• All these objects - comets, KBOs, SDOs, centaurs,
  etc. - are related to one another.
• All share the same origin in the birth of the
  solar system, and have similar compositions.
• Clearly what weve seen so far is only the tip of
  the iceberg, and much more is to be discovered!