Is there anything wrong with this picture? 1 What is a planet? Why? Does it matter what you call an object? Does the public care so much? Were scientists made fun of, but not HP Computer Company? How? International Astronomical Union Coming to a consensus 2 1
Come up with your own definition of a planet based on what you know Test your definition against new discoveries Keep in mind those questions about science as you do this: Bias? Definition open to debate and being changed? 3 Why? Because you will see that the argument in the summer of 06 was really a continuation of an argument that started back in 1801 4 2
Look up into the sky, what do you see? Stars Sun Moon Some planets (Mercury, Venus, Mars, Jupiter, Saturn) All move around the Earth but: Stars stay in fixed positions compared to each other The Sun moves around compared to the stars The Moon moves around compared to the stars The planets move around compared to the stars 5 So have two groups: Planets meaning: Sun Moon The bright planets (Mercury, Venus, Mars, Jupiter, Saturn) Stars meaning: Everything else Happy Happy? 6 3
What makes the Sun different from the planets? What makes the Moon different from the planets? OK, so make that four groups: Sun Moon Stars The Bright Planets Based upon a combination of how they move and what they look like 7 Galileo saw things in the solar system never seen before Four large moons of Jupiter First seen by Galileo He named them the Medici Stars Who the heck are the Medici? These Medici Stars don t go around the Earth! Do we now need another group? Note your technology drives your questions 8 4
Sun Planets Moons Stars Based upon a combination of where they are, how they move, 9 Let the average distance between the Earth and the Sun equal 1, then Planet Distance Series Mercury 0.4 (0 + 4) / 10 = 0.4 Venus 0.7 (3 + 4) / 10 = 0.7 Earth 1.0 (6 + 4) / 10 = 1.0 Mars 1.5 (12 + 4) / 10 = 1.6??? (24 + 4) / 10 = 2.8 Jupiter 5.2 (48 + 4) / 10 = 5.2 Saturn 9.5 (96 + 4) / 10 = 10.0??? (192 + 4) / 10 = 19.6 The series was found in 1766 Called the Titus Bode Law 10 5
William Herschel, an English astronomer was doing an all sky survey Found the planet, Uranus He called it George s Star Who the heck was George? 11 Let the average distance between the Earth and the Sun equal 1, then Planet Distance Series Mercury 0.4 (0 + 4) / 10 = 0.4 Venus 0.7 (3 + 4)) / 10 = 0.7 Earth 1.0 (6 + 4) / 10 = 1.0 Mars 1.5 (12 + 4) / 10 = 1.6??? (24 + 4) / 10 = 2.8 Jupiter 5.2 (48 + 4) / 10 = 5.2 Saturn 9.5 (96 + 4) / 10 = 10.0 Uranus 19.2 (192 + 4) / 10 = 19.6 12 6
??? = (24 + 4) / 10 = 2.8 The asteroid Ceres was found in 1801, its average distance from the Sun is 2.8 The asteroid Juno was found in 1804, its average distance from the sun is 2.7 The asteroid Vesta was found in 1807, its average distance from the Sun is 2.4 13 Herschel questioned are these objects really planets. Note he was the only other person to have found a planet Could this bias his opinion? As more and more asteroids were discovered, people wondered can you really have hundreds of planets. Scientists decided that these weren t planets but called them asteroids. But for 50 years Ceres was called a planet! 14 7
Start with a big cloud of gas and dust Starts to collapse under its own weight Heats up and spin s up as it collapses Stuff in center forms star Stuff in orbit around star clumps together to form larger and larger objects -> planets 15 They are thought to be failed small (Earth-like) planets Total mass 0.0006 Earth Mases Either Never got big enough to become a real planet Or got all the way up to Mars sized Big enough iron core forms Water on the surface Life? Then a massive collision breaks up the planet down to its core 16 8
Ceres largest main belt asteroid, It is the only one big enough to be round 17 18 9
19 http://www.youtube.com/watch?v=z0tj1yi6dss 20 10
People noticed that they could not predict accurately where Uranus should be in the night sky John Adams and Urbain Le Verrier, independent of each other came up with the idea that maybe there was another planet beyond Uranus that was pulling on Uranus and changing its path. Scientists looked where they predicted this planet to be and Neptune was found. 21 Let the average distance between the Earth and the Sun equal 1, then Planet Distance Series Mercury 0.4 (0 + 4) / 10 = 0.4 Venus 0.7 (3 + 4)) / 10 = 0.7 Earth 1.0 (6 + 4) / 10 = 1.0 Mars 1.5 (12 + 4) / 10 = 1.6 Asteroids 2.1 3.5 (24 + 4) / 10 = 2.8 Jupiter 5.2 (48 + 4) / 10 = 5.2 Saturn 9.5 (96 + 4) / 10 = 10.0 Uranus 19.2 (192 + 4) / 10 = 19.6 Neptune 30.1 (384 + 4) / 10 = 38.8?? 22 11
Scientists noticed that they could not predict accurately where Neptune should be in the night sky So scientists made a prediction that there should be a planet beyond Neptune. Scientists looked for another planet and Pluto was found. 23 Scientists noticed that Pluto did not have enough mass to affect Neptune s orbit Went back and checked and found that there was no problem with Neptune s orbit 24 12
Another object was found near Pluto s orbit and a little bit bigger (maybe) than Pluto! Is this a planet? Or is this the problem with the asteroids all over again? Pluto is one of the largest members of another belt of objects called the Kuiper belt. ½ rock, ½ ice (by volume) Think long orbital period comets that never get close to the Sun 25 26 13
Astronomers decided they needed to decide what a planet is. International Astronomical Union formed a committee International, so no more George or Medici Meets every two years Committee report was in 2006 (IAU meets every two years) 27 A planet is a celestial body that: Self-gravity is strong enough for it to be nearly round in shape Which is in orbit around a star but: Itself is not a star Not a satellite of a planet Advantages Simple Covers high and low mass objects Works well for other solar systems Pluto stays in But the riff-raff comes in 28 14
29 Ceres largest main belt asteroid, it is the only one big enough to be round Eris and its moon Dysnomia. Eris was formally known as 2003 UB 313 or Xena. Dysnomia was formally known as Gabrielle. 30 15
A planet is a celestial body that: Is in orbit around the Sun Self-gravity is strong enough for it to be nearly round in shape Has cleared the zone around its orbit A dwarf planet is a celestial body that: Is in orbit around the Sun Self-gravity is strong enough for it to be nearly round in shape Is not a satellite of a planet Has not cleared the zone around its orbit 31 32 16
IAU met again in 2008. What is a planet was back on the agenda. Special class of dwarf planet: Plutoid In orbit around the Sun at a semimajor axis greater than that of Neptune Near-spherical shape Not cleared the neighborhood around their orbit. Satellites of plutoids are not plutoids themselves, even if they are near-spherical in shape 33 34 17
Upper limit to the mass of a planet Can an object change from being a star to being a planet? Lower limit to the mass/size of a planets Planets in orbits around other stars Free floating planets or can an object stop being a planet? 35 Or why is the Sun not a planet? Stars shine (give of visible light) but planets do not Fusion! OK, so what is fusion? Lighter atomic weight nuclei are fused into heavier atomic weight nuclei What makes fusion hard to achieve & What is needed for fusion? Like charges repel High pressure & high temperature overcome this With enough mass can meet these two conditions 36 18
Elements do NOT all start to fuse at the same temperature and pressure Most common element in the universe is hydrogen Normal hydrogen nucleus has one proton Heavy Hydrogen or deuterium one proton plus one neutron Deuterium fuses at a lower temperature & pressure than normal hydrogen 37 Brown dwarfs Massive enough that fusion started for deuterium But not massive enough for normal hydrogen fusion Fusion takes place only over a short time period for a brown dwarf Brown Dwarfs start at about 13 x Jupiter mass If a brown dwarf is in orbit around another star and fusion is not actively occurring, is it a planet? Does your definition cover this case? 38 19
Has to be round? Why are planets round? So the composition of an object will determine in part if it is a planet, icy objects will become round at a smaller size than rocky objects 39 2005 FY9 Makemake 2003 EL61 Haumea Hiʻiaka Namaka 40 20
Use a minimum size or radius? Jupiter s moon Ganymede is larger than Mercury Atmosphere? Mercury does not have an atmosphere But Titan, a large moon of Saturn, has an atmosphere thicker than either the Earth s or Mars 41 Currently (as of last night) http://exoplanet.eu/ 861Exoplanets 677 Planetary systems 128 Multiple planet systems Current methods work best for finding large planets (Jupiter or larger) that orbit close to the star 42 21
How do astronomers find extra-solar planets? http://science.howstuffworks.com /planet-hunting1.htm 44 22
45 How do astronomers find extra-solar planets? Kalas et al. (2000) Direct Imaging 23
47 Finding objects larger than Jupiter but closer to the star than Mercury caused some problems Don t think they formed there Formed in outer part of their solar system and moved inwards. A planet in the way could be thrown out So if a planet sized object formed near a star but no longer orbits a star is it a planet? 48 24
http://ciera.northwestern.edu/rasio/ UpsAndPR/JPG/UpsAnd.mov 50 25
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