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UA PTYS 206 - Discovering Uranus and Neptune

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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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UA PTYS 206 - Discovering Uranus and Neptune

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