The Trouble with 'Planets' The history of the 'planet' debate The scientific context The future Brett Gladman UBC, Dept. of Physics and Astronomy Institute of Planetary Science
It all seemed so easy... J M V E M S
The five 'wanderers'
The only other known denizens: comets Appeared brilliantly in the sky and vanished Halley was the first to identify (in 1705) any comet as periodic No-one considered calling them planets... Were physically very different (tails, not disks)
1781 : The discovery of Uranus J M V E M S U
1781 : The discovery of Uranus While hunting for comets, William Herschel discovered Uranus Moved slowly night to night Disk No tail The first 'planet' discovered in recorded history!
1801 : the first asteroid, Ceres J M V E M Ceres S U
Ceres was rapidly joined by other asteroids... Although Ceres was originally referred to as a planet, the rapid discovery of other asteroids in the main asteroid belt along with realization that all were D < 1000 km stopped this quickly
1846 : the discovery of Neptune J M V E M Ceres S U N
Hmnnn...the motion of Uranus Le Verrier and Adams predict the position of Neptune based on residuals in the orbit fitting of the observations of Uranus ==> another planet
The start of the hunt for Planet X J M V E M S U N? Ceres
Remnant residuals in the motion? Tough to do for Neptune But there were claims for systematics in the motion of Uranus... Residuals of Uranus (Standish 1993)
Remnant residuals in the motion? Tough to do for Neptune But there were claims for systematics in the motion of Uranus... Neptune had been tens of arcseconds Was there a transneptunian planet? Called 'Planet X' Lowell : 7 Earth-mass! Residuals of Uranus (Standish 1993)
The hunt for planet X Tombaugh finds Pluto in 1930 (after a year of effort) Searched much of the sky to 16th magnitude over the next decade... No other comparable objects
The mass of Pluto How do you measure the mass of a planet? It's just a point of light after all... Neptune's mass was estimated to be roughly that needed to perturb Uranus
The mass of Pluto Pluto's mass remained high for decades... Hoyt (1930) : 0.7 Earth-masses Kuiper (1950) : 0.1 Earth-masses Sagan and Leonard (1966) : 0.8 Earth-masses
The mass of Pluto Pluto's mass remained high for decades... Hoyt (1930) : 0.7 Earth-masses Kuiper (1950) : 0.1 Earth-masses Sagan and Leonard (1966) : 0.8 Earth-masses But then in 1978 J. Christy discovered Charon, Pluto's moon Period = 6 days at 20,000 km
Gives mass immediately Mass of Pluto+Charon: ~1.5 x 1022 kg ~0.002 Earth-masses! Less massive than our Moon (0.012 Earthmasses) Is Pluto a planet??? But it's the only one there...
The Kuiper Belt The structure that had to exist Theoretically from planet-formation To explain the Jupiter-family comets
Search programs Need to hunt large areas of sky (near the ecliptic...?) Hundreds to thousands of square degrees Need to observe 'rather faint' Kowal did mag~18 in 1980s CFHT/CFEPS : 400 square degrees to mag~23 Oschin Schmidt : 10,000 square degrees to mag~20 Spacewatch: all sky to ~18
Mosaic CCD cameras Take multiple images of the same patch of sky at ~hour intervals Trans-neptunian objects move at ~150/d (in /hour) Varuna's discovery, 2000 Note: limited to probe <~100 AU at many sites
Pluto wasn't unique anymore It's been clear in the late 1990s that while Pluto was still 'King of the Kuiper Belt', it was going to have serious contenders for the throne From our understanding of Solar System formation, it is clear that there were (and still are) lots of Plutos...and probably larger...
Big Kuiper Belt objects Pluto has friends In analogy with the asteroid belt, many thought Pluto's time had come Buffy 2003 UB313 was the nail... Slightly larger than Pluto
2003 UB313 : Fact Sheet Now 97 AU from the Sun Mag ~ 19 Within one mag of the limit of the Schmidt survey D=2400 +/- 1000 km (Pluto is D=2310 km) Very high albedo (0.9!) Nickname Xena Has a moon, not followed...
IAU 2006 : Definition of a 'planet' 1) 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, and (c) has cleared the neighbourhood around its orbit.
The currents planets and dwarf planets
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Planetary Satellites Are (by defintion...) smaller than their primary But many are extremely large bodies These are clearly hydrostatic...
Planetary Satellites In fact, some of the largest satellites of the jovian planets are the size of Mercury. How do you decide what's a moon?
Hydrostatic Equilibrium Pressure forces balance the gravitational contraction of the body (rather than material strength) Such objects flow to become spheres (or, at least, they become approximately figures of revolution)
Hydrostatic Equilibrium The initial proposal had a footnote guideline for hydrostatic equilibrium (for normal materials): M > 5 x 1020 kg and D>800 km This should BECOME the definition! At least it is subject to verification! 800 km
What does THIS mean? 1) A "planet" is a celestial body that:... (c) has cleared the neighbourhood around its orbit. Certainly fits with our historical desire that planets dominate their surroundings...
Planetary accretion Planets (or most) form via bottom-up accretion At some point a planet would dominate, but when? Should it be total mass in 'crossing orbits'? Should it be second-biggest? What about extra-solar planets?
The 'demotion' of Pluto to a dwarf A fascinating exercise in human psycology Absolutely nothing scientifically interesting...
Reclassification in science We realize that some aspects of our definition don't fit new understanding...you get transitional forms... Bearing live young is no longer on the 'mammal' list!
Reclassification in science When I was young... CAR
Reclassification in science When I was young... TRUCK CAR
Reclassification in science But now...
Reclassification in science But now...
The future of planet finding What might we yet find lurking out there???
Remnant residuals in the motion? Residuals of Uranus (Standish 1993) CORRECTED post-voyager
The future of planet finding What might we yet find lurking out there??? There IS NO LARGE planet within ~100 AU of the Sun (That is, a few Earth masses) There is VERY LITTLE constraint on distant objects in the 100-1000 AU range... - There are probably tens of Plutos - I think there could be Mars-massed...
IAU 2006 : Definition of a 'dwarf planet' 2) A "dwarf 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, and (c) has NOT cleared the neighbourhood around its orbit (d) is NOT a satellite PROBLEMS: Definitions b+c are - not rigorous - not quantitative