Pluto is (still) not a planet.
Prof. Richard Pogge OSU Department of Astronomy Science Café – 2013 Sept 11 Fool: The reason the seven stars are seven is a pretty reason... Lear: Because they are not eight? Fool: Aye, indeed, thou wouldst make a good fool. William Shakespeare King Lear (1605) The Solar System in 1766
Saturn Mars
Earth Jupiter Venus
Mercury In 1766, Johann Titius noted that planet distances follow a simple geometric progression with a gap between Mars & Jupiter. = 0.4 + 0.3×2 m = 0 1 2 3 푚 4 5 푎
m V E M J S
Johann Bode published what we now call the Titius-Bode Law in 1772.
Johann Daniel Titius Johann Elert Bode (1747-1826) (1729-1796) William Herschel discovered the planet Uranus on 1781 March 13.
NASA
First new planet since antiquity, but it is not a naked-eye planet. Uranus fits the Titius-Bode law!
m V E M J S U
Where was the missing planet between Mars & Jupiter? In 1799, Baron Franz Xaver von Zach organized 24 astronomers into the “Celestial Police”.
Their charge:
To make a systematic search for the "missing" 5th planet between Mars and Jupiter predicted by the Titus-Bode Law.
Baron Franz Xaver von Zach (1754-1832) Giuseppe Piazzi discovered Ceres on Jan 1, 1801 while looking for the "missing" star Mayer 87.
Hubble Space Telescope Giuseppe Piazzi (1746-1826) Ceres was right where the Titius-Bode Law said it should be.
m V E M c J S U
But, things soon got complicated... Heinrich Olbers discovered Pallas on May 28, 1802 while observing Ceres.
Hubble Space Telescope
New problem: Pallas’ and Heinrich Wilhelm Matthäus Olbers Ceres’ orbits crossed! (1758-1840) In 1802, William Herschel coins the term "asteroid" ("star-like"), rejecting the idea these are new planets.
They are very much smaller than the other planets.
Their orbits are more tilted and elliptical than the other planets.
Ceres & Pallas cross each other’s orbits.
Bristol City Museum & Gallery By 1807, 4 new "planets" had been discovered in the space between Mars & Jupiter
Vesta: 1807 (Olbers)
Vesta (NASA/Dawn) Juno: 1807 Ceres: 1801 (Piazzi) (Harding)
Then nothing for 38 years... Pallas: 1802 (Olbers) The discovery of Neptune in 1846 was a factor in reclassifying Ceres, Pallas, Juno, and Vesta as "Minor Planets“.
Urban Leverrier Johann Galle
Predicted mathematically by Leverrier and Adams from the orbital wobble of Uranus.
Found by Galle at Leverrier’s predicted position.
Violates the Titius-Bode law by 30% (closer than “predicted”). Wealthy amateur Percival Lowell began a search for a planet beyond Neptune in 1909.
It was expected from residual wobbles in Uranus’ and Neptune’s orbits.
Built a private observatory in Flagstaff, AZ.
Actually photographed Pluto twice but didn't notice it.
Percival Lowell (1855-1916) Pluto was discovered photographically by Clyde Tombaugh on February 18, 1930.
Clyde Tombaugh at the Lowell Blink Comparator
Lowell "Pluto" Astrograph
The new planet was named Pluto, and became immediately controversial.
It is too small to cause orbit wobbles in Uranus or Neptune.
Its orbit is highly tilted and elliptical.
Its orbit crosses the orbit of Neptune.
But, nobody questioned that it was a planet... Pluto Neptune Pluto is a tiny, cold, icy world orbiting far from the Sun.
Temperatures: −378 to −396º F
Rocky core over an icy mantle.
Surface is covered with frozen gases.
Has a thin nitrogen atmosphere. Pluto has 1 large moon (Charon) and 4 smaller moons.
Hubble Space Telescope Pluto is smaller than our Moon.
Pluto and its large moon Charon have the largest parent-to-moon mass ratio in the Solar System. In the 1950s, Edgeworth and Kuiper predicted the existence of a large population of small icy bodies beyond the orbit of Neptune.
The first discovered in 1992 by Jewitt & Luu.
Hundreds more were found by the early 2000s.
All are smaller than Pluto, but many have similar orbital characteristics. We now call this the “Kuiper Belt”
JHU/APL
Flattened belt of icy objects between 30 and 50 AU from the Sun (from Neptune’s orbit to beyond Pluto). In January 2005, Brown, Trujillo, and Rabinowitz discover 2003UB313 and set off a controversy.
It is about the same size as Pluto.
It has a very elliptical & tilted orbit
It has a Pluto-like composition.
It has 1 moon, resulting in a mass a little larger than Pluto’s.
Was it the 10th planet?
Mike Brown Chad Trujillo David Rabinowitz The group's codename for the new object was "Xena" The official name chosen was “Eris”
The goddess of discord…
In August 2006, the IAU took up the question of how to define a “planet”.
1.Orbits around the Sun and is not a satellite.
2.Shaped by Gravity: sufficient mass to assume a spheroidal shape.
3.Has cleared its orbital neighborhood ("orbital dominance")
Three Dwarf Planets: Ceres, Pluto, & Eris The Solar System in September 2013
Gas Giants
Terrestrials Ice Giants
Dwarf Planets The Terrestrial Planets are the rocky planets found in the inner Solar System 0.4–1.5 AU from the Sun.
Mercury Mars (0.055 ME) (0.11 ME) Venus Earth (0.82 ME) (1 ME)
Composed of mostly Silicates and Iron with solid surfaces All are High Density: 3.9 – 5.5 g/cc (rock & metal) Jupiter & Saturn are Gas Giants with thick Hydrogen & Helium atmospheres over rock & ice cores
Saturn:
95.2 ME 9.44 RE Jupiter:
318 ME 11.2 RE Uranus and Neptune are Ice Giants made mostly of ices with thin Hydrogen & Helium atmospheres.
Uranus Neptune
14.5 ME 17.1 ME 4.01 RE 3.88 RE The Dwarf Planets
Pluto Eris
Makemake Ceres
Haumea
40+ other candidates… Moon Earth
The most controversial and least well-defined criterion is “orbital dominance”. The "One planet per orbit" rule. Examples: 1. Anything near Earth has its orbital altered by the gravitational influence of Earth.
2. Ceres and Pallas have orbits that cross, so neither "dominates" its orbit.
3. Pluto is in a 3:2 resonance with Neptune, and its motion is dominated by Neptune.
Clears its orbit quickly compared to age of the Solar System Planet
Not enough time to have cleared its orbit Dwarf Pluto and Eris are the largest of a new family of Trans-Neptunian Objects.
More than 1000 known today.
Could be as many as ~70,000 more than 100 km in diameter.
The largest are >1000 km in diameter.
Total combined mass could be 2% to 4% the mass of Earth.
Icy material leftover from the formation of the Solar System
The New Horizons mission was launched in 2006 to study Pluto and the Kuiper Belt up close.
Launch: January 19, 2006
Pluto fly-by: July 14, 2015
Explore Kuiper Belt 2016-2020
Pluto & Charon, July 2013 New Horizons at Pluto Further Reading:
The Pluto Files How I Killed Pluto Neil deGrasse Tyson and Why it Had it Coming Mike Brown (see pages 165-175)