PYTS/ASTR 206 – Pluto and the Rest of the Kuiper Belt 1 Announcements HW5 due today for 50% late creditPYTS/ASTR 206 – Pluto and the Rest of the Kuiper Belt 2 PTYS/ASTR 206 – The Golden Age of Planetary Exploration Shane Byrne – [email protected] Pluto and the rest of the Kuiper BeltPYTS/ASTR 206 – Pluto and the Rest of the Kuiper Belt 3 In this lecture… Discovering Uranus and Neptune Planet X still to be found Pluto Discovery of Pluto Pluto’s strange orbit Pluto’s interior, surface and atmosphere Moons of Pluto and Formation The Kuiper Belt Different groups of objects Properties and sizes of KBOs History of the Kuiper Belt Extra-solar Kuiper Belts Why Pluto isn’t a planet New Horizons at Pluto – 2015PYTS/ASTR 206 – Pluto and the Rest of the Kuiper Belt 4 Something was wrong with the orbit of Uranus… Its rate of motion didn’t match that expected from Newton’s law of gravitation Either… Newton’s laws were wrong OR There was another planet perturbing things An extra planet was independently predicted by John Adams – 1843 – but both he and English Astronomers weren’t that interested Urbain LeVerrier – 1846 Looked for and found by Johann Galle Considered a triumph for modern mathematicsPYTS/ASTR 206 – Pluto and the Rest of the Kuiper Belt 5 …Uranus wasn’t the only planet with unexpected motions Mercury’s orbit also couldn’t be explained Again, either… Newton’s laws were wrong OR There was another planet perturbing things Massive hunt for the planet ‘Vulcan’ between Mercury and the Sun In this case there was no planet… It turns out that Newton’s laws are slightly wrong… Einstein’s theory of general relativity can explain Mercury’s motion without an extra planetPYTS/ASTR 206 – Pluto and the Rest of the Kuiper Belt 6 Vulcan was a dead end, but not the only dead end Something was still wrong with the orbit of Uranus (and Neptune) Speculation about a 10th planet builds Planet ‘X’ Main Proponent was Perceval Lowell The ‘canals on Mars’ guy Searches for planet X Late 19th and early 20th century Lowell dies but this work continuesPYTS/ASTR 206 – Pluto and the Rest of the Kuiper Belt 7 A new technology makes the search for planet X possible Photographic plates were first attached to telescopes in the 1890s Weren’t superseded by CCDs until the 1980s Lead to discovery of many asteroids, new moons etc…PYTS/ASTR 206 – Pluto and the Rest of the Kuiper Belt 8 A ‘blink-comparator’ shows up moving objects Stars don’t move enough to notice Asteroids move very fast Outer solar system objects move very slowly Clyde Tombaugh found a new outer solar system object in 1930 Working from Lowell observatory He looked at millions of stars to find this moving pointPYTS/ASTR 206 – Pluto and the Rest of the Kuiper Belt 9 With hindsight Planet X was found in previous plates Orbit was well determined and was larger than that of Neptune Named PLuto partly in honor of Perceval Lowell Did Pluto fit the bill ? Quirks in the orbit of Uranus Explained by uncertainty in Neptune’s mass Pluto had no resolvable disk It must be very small – nothing like another Neptune Pluto was dim Either very small and bright or large and dark Detection of methane ices in 1970s meant it was small and bright Much smaller than any other planet Really a planet? Looked more like Triton than anything elsePYTS/ASTR 206 – Pluto and the Rest of the Kuiper Belt 10 Semi-major axis 39.5 AU Period is 248 years Neptune is 30 AU The orbit of Pluto is also odd… Highly inclined: i = 17° Eccentricity is high: e = 0.25 Pluto’s orbit looks more like an asteroid’s orbit than a planet’s It’s so eccentric that it crosses Neptune’s orbit Pluto ranges from 29.6 to 49.3 AU from the sun. Neptune is more or less constant at 30 AUPYTS/ASTR 206 – Pluto and the Rest of the Kuiper Belt 11 Pluto is in a 3:2 resonance with Neptune Neptune orbits 1 times for every 1 of Pluto’s orbits Neptune is always far away from Pluto when Pluto crosses inside its orbit This is a very stable arrangement! Why doesn’t Pluto collide with Neptune?PYTS/ASTR 206 – Pluto and the Rest of the Kuiper Belt 12 Pluto characteristics Small: 0.18 Earth Diameter ~ 2300 km Low Mass: 0.002 Earth Density of 2000 kg m-3 50-70% Rock 30-50% Water Ice Surface composition Frozen ices Mostly nitrogen Methane And a little derived ethane Carbon MonoxidePYTS/ASTR 206 – Pluto and the Rest of the Kuiper Belt 13 Maps of Pluto’s surface remain poor Spacecraft encounter scheduled for 2015PYTS/ASTR 206 – Pluto and the Rest of the Kuiper Belt 14 Pluto’s Atmosphere Detected with a stellar occultation Very thin atmosphere of mostly Nitrogen Like Triton Pluto is currently receding from the Sun This atmosphere might freeze out as surface ice soon (decades)PYTS/ASTR 206 – Pluto and the Rest of the Kuiper Belt 15 Pluto isn’t alone Large moon Charon discovered in 1978 1200km across (half of Pluto) 17,500km distant, 6.4 day orbit Very circular orbit Sometimes called a double planet Both Pluto and Charon are tidally locked to each other The same face of each body points toward the other all the time No tidal effectsPYTS/ASTR 206 – Pluto and the Rest of the Kuiper Belt 16 Charon less dense than Pluto (1600 Kg m-3 vs. 2000) Less rocky material Less geologic activity? Mass of 12% of Pluto Lower mass means no atmosphere Also no atmospheric ices like nitrogen Surface dominated by water-ice Evidence for very recent cryo-volcanism Crystalline Ammonia hydrates and water Loses crystal structure in ~30,000 years We’ll know a lot more in 6 years Spacecraft flyby
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