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Jeremy P. Carlo Columbia University

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1 Jeremy P. Carlo Columbia University
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)
We’re going to go back and look at the basics, to try and understand the rationale behind the decision.

4 To start let’s consider this simple question.
What is a planet? To start let’s consider this simple question. We’ll 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 that’s 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 1500’s, Tycho Brahe’s naked-eye observations made it clear the old model wasn’t 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 (Kepler’s modification to better fit Tycho’s 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 it’s a moon. Officially six planets plus one moon, all known since ancient times. Planets move in elliptical orbits around the sun according to Kepler’s Laws.

9 The Known Planets, 1600 Mercury

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 Kepler’s Laws!

16 The Solar System ~1700 Sun Dist. from Sun (AU) Still six planets.
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 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 1670’s and 1680’s, four more moons around Saturn are found - Tethys, Dione, Rhea and Iapetus.

17 Io Europa Ganymede Callisto
Jupiter’s Moons (to scale)

18 Saturn and its largest moons (not to scale)

19 The 1700’s... 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... Herschel’s “40-foot” telescope Many discoveries - comets, nebulas, star clusters... But in 1781 he found an object that moved in the sky, a new “wanderer...” Kepler’s 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? We’re about to find out...

22 1801: Deja vu all over again? In 1801 Giuseppe Piazzi finds yet another “wanderer.” This time Kepler’s Laws place it between Mars and Jupiter Ok, not as exciting as finding something beyond the known edge of the solar system, but we’ll take what we can get. Two problems: Ceres is rather dim. Really dim for how close it is, actually. It doesn’t 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 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” aren’t 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 Back to seven planets. Whew! But we must set some limit - too small, you’re 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.

26 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 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 1800’s it appeared there were further orbital perturbations as well... Can we do it again?

28 Early 1900’s: 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 Lowell’s search failed, a far more comprehensive search was taken up by Clyde Tombaugh. Clyde Tombaugh in 1930

29 Above: Tombaugh and blink comparator
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 7. Uranus 20 +4 moons 8. Neptune 30 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 Pluto’s mass was continually lowered from its discovery in 1930 until the 1970’s. The last straw came when James Christy discovered Pluto’s moon Charon in 1978, enabling an exact mass determination: Pluto = 1/500 Earth mass Not only was Pluto (by far) the smallest planet, it’s also smaller than at least seven known moons! But still bigger than the asteroids...

34 And it isn’t 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 This proposal proved very unpopular, and didn’t get far...
My Very Educated Mother Just Served Us NOTHING! What happened to Pluto!!! This proposal proved very unpopular, and didn’t get far...

38 Pluto and the largest TNOs, 2003
Pluto’s still the biggest TNO (but the gap is closing...) (artist’s conceptions of Sedna and Quaoar)

39 Enter “Xena” (2003 UB313) Discovered by Michael Brown of Caltech in 2003, but not realized until Named 2003 UB313, unoficially nicknamed “Xena.” (X: Tenth Planet?) Subsequently found that it’s 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 scale Pluto’s 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?
(artist’s conceptions)

43 What do we do? Pluto is the smallest planet (by a wide margin)
It’s also smaller than at least six moons. And it’s 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 we’ve 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? 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 It’s 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 Who’s 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 haven’t seen them up close, a large number of KBOs are almost certainly round as well.

49 Typical small asteroids with irregular shapes

50 A Typical Comet Halley’s 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! Jupiter’s moon Amalthea ~170 km across Asteroid 243 Ida (& Dactyl) ~30 km across Clearly not round!

53 Saturn’s moon Hyperion
~280 km across Still not round! Saturn’s moon Mimas ~400 km across Round! Neptune’s moon Proteus ~400 km diameter Getting there, but not quite.

54 Saturn’s moon Enceladus
~500 km across Round! Uranus’ moon Miranda ~470 km across Round! Saturn’s Moon Tethys ~1000 km across

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. km 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, 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. That’s 40 planets (and counting..) We must come up with a stricter definition...

59 Eliminating the moons Let’s 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, Pluto’s moon Charon**, “Xena” and some unknown number (but likely at least half a dozen) number of trans-Neptunian objects. **I’ll 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 we’re 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 ( )

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 (PLO’s?) in the outer solar system. Primary Subdivisions: Cis-Neptunian Objects Neptune’s 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 Jupiter’s Trojans - follow or lead Neptune in its orbit by 60 degrees. Centaurs - large TNO-like objects orbiting between Saturn and Neptune. Prototype: 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 AU. Prototype: Pluto. Orbits are dominated by Neptune’s effects. Sharp cutoff after about 48 AU. Generally low inclination orbits (<30º) KBOs can be further subdivided: Resonant KBOs Non-resonant KBOs

71 Resonant KBOs Orbital periods are a rational fraction of Neptune’s:
3:2 (e.g. 3 Neptune orbits to 2 of the object’s orbit) Orbital period = 165 years x 3/2 = 248 years Prototype: Pluto These objects are called “Plutinos” Other resonances: 2:1 (“twotinos,” 5:2, etc.) These occur because of Neptune’s repeated gravitational influence. The orbits are usually somewhat elliptical and moderately inclined.

72 Non-resonant KBOs No relation between orbital period and Neptune’s.
Prototype: 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, Neptune’s gravitational effects are minimal. Prototype members: Sedna, Eris (both >75 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., >10,000 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 they’re 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 ( 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 we’ve seen so far is only the tip of the iceberg, and much more is to be discovered!


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