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)

Saturn

Jupiter

Mars

Earth Venus

Mercury

The Solar System in 1766

In 1766, Johann Titius noted that planet distances follow a simple geometric progression with a gap between Mars & Jupiter.

Johann Daniel Titius (1729-1796)

Johann Elert Bode (1747-1826)

Johann Bode published what we now call the Titius-Bode Law in 1772.

m V E M J S

𝑎 = 0.4 + 0.3×2𝑚

m = 0 1 2 3 4 5

William Herschel discovered the planet Uranus on 1781 March 13.

NASA

First new planet since antiquity, but it is not a naked-eye planet.

m V E M J S U

Where was the missing planet between Mars & Jupiter?

Uranus fits the Titius-Bode law!

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)

In 1799, Baron Franz Xaver von Zach organized 24 astronomers into the “Celestial Police”.

Giuseppe Piazzi discovered Ceres on Jan 1, 1801 while looking for the "missing" star Mayer 87.

Giuseppe Piazzi (1746-1826) Hubble Space Telescope

Ceres was right where the Titius-Bode Law said it should be.

m V E M J S U c

But, things soon got complicated...

Heinrich Wilhelm Matthäus Olbers (1758-1840)

Heinrich Olbers discovered Pallas on May 28, 1802 while observing Ceres.

Hubble Space Telescope

New problem: Pallas’ and Ceres’ orbits crossed!

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

Then nothing for 38 years...

Vesta (NASA/Dawn)

Ceres: 1801 (Piazzi)

Vesta: 1807 (Olbers)

Pallas: 1802 (Olbers)

Juno: 1807 (Harding)

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”).

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.

Wealthy amateur Percival Lowell began a search for a planet beyond Neptune in 1909.

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

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.

The new planet was named Pluto, and became immediately controversial.

But, nobody questioned that it was a planet...

Pluto Neptune

Temperatures: −378 to −396º F Rocky core over an icy mantle. Surface is covered with frozen gases. Has a thin nitrogen atmosphere.

Pluto is a tiny, cold, icy world orbiting far from the Sun.

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.

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.

In the 1950s, Edgeworth and Kuiper predicted the existence of a large population of small icy bodies beyond the orbit of Neptune.

We now call this the “Kuiper Belt”

Flattened belt of icy objects between 30 and 50 AU from the Sun (from Neptune’s orbit to beyond Pluto).

JHU/APL

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…

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

In August 2006, the IAU took up the question of how to define a “planet”.

Terrestrials

Gas Giants

Ice Giants

Dwarf Planets

The Solar System in September 2013

The Terrestrial Planets are the rocky planets found in the inner Solar System 0.4–1.5 AU from the Sun.

Earth (1 ME)

Mars (0.11 ME)

Venus (0.82 ME)

Mercury (0.055 ME)

Composed of mostly Silicates and Iron with solid surfaces All are High Density: 3.9 – 5.5 g/cc (rock & metal)

Saturn: 95.2 ME 9.44 RE

Jupiter: 318 ME 11.2 RE

Jupiter & Saturn are Gas Giants with thick Hydrogen & Helium atmospheres over rock & ice cores

14.5 ME 4.01 RE

17.1 ME 3.88 RE

Uranus Neptune

Uranus and Neptune are Ice Giants made mostly of ices with thin Hydrogen & Helium atmospheres.

The Dwarf Planets

Eris Pluto

Ceres

Earth

Makemake

Haumea

40+ other candidates… Moon

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.

The most controversial and least well-defined criterion is “orbital dominance”.

Clears its orbit quickly compared to age of the Solar System

Not enough time to have cleared its orbit

Pla

net

Dw

arf

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

Pluto and Eris are the largest of a new family of Trans-Neptunian Objects.

Launch: January 19, 2006 Pluto fly-by: July 14, 2015 Explore Kuiper Belt 2016-2020

The New Horizons mission was launched in 2006 to study Pluto and the Kuiper Belt up close.

Pluto & Charon, July 2013

New Horizons at Pluto

Further Reading:

The Pluto Files Neil deGrasse Tyson

How I Killed Pluto and Why it Had it Coming

Mike Brown (see pages 165-175)