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What happened to Pluto?

What happened to Pluto?. Jeremy P. Carlo Columbia University. August 2006 IAU decision:. (Short version) Pluto is no longer a planet. Public Reaction: Not good... What happened?!. August 2006 IAU decision:. (Long version)

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What happened to Pluto?

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  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) • We’re 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 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 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 The Ancients (before ~1500 AD)

  6. 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. The Ancient Solar System

  7. 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) The first modern astronomers (~1500-1600)

  8. 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. The Solar System ~1600

  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. 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! The First Telescopes (1610-1700)

  16. 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 1670’s and 1680’s, four more moons around Saturn are found - Tethys, Dione, Rhea and Iapetus. The Solar System ~1700

  17. IoEuropa GanymedeCallisto Jupiter’s Moons (to scale)

  18. Saturn and its largest moons (not to scale)

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

  20. Uranus First planet discovered since ancient times (1781)

  21. 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... The Solar System 1800

  22. 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... 1801: Deja vu all over again?

  23. 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” aren’t really planets at all, but instead form a class of different objects. The Solar System 1810

  24. 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, 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... The Solar System 1845

  25. 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. 1846: The year of the mathematicians

  26. Neptune First planet found “on purpose” (1846)

  27. 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 1800’s it appeared there were further orbital perturbations as well... Can we do it again? The Solar System 1846

  28. 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 Early 1900’s: The Search for Planet X

  29. 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 Paydirt!

  30. 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 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. The Solar System 1930

  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. 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... Pluto and Charon Just how small is Pluto?

  34. 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! And it isn’t alone either...

  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 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 2005. 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 scalePluto’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? 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. 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 asteroidswith irregular shapes

  50. A Typical Comet • Halley’s Comet • ~16 km largest dimension • Also irregularly shaped!

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