What Happened to Pluto?!!

Scale in the Solar System, New Discoveries, and the Nature of Science

Mary L. Urquhart, Ph.D.

Department of Science/Mathematics Education

Marc Hairston, Ph.D.

William B. Hanson Center for Space Sciences

From Nine to Eight?

On August 24th Pluto was reclassified by the International Astronomical Union (IAU) as a “dwarf planet”.

So what happens to “My Very Educated Mother Just Served Us Nine Pizzas”?

Official IAU Definition

A planet:

(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 neighborhood around its orbit.

A dwarf planet must satisfy only the first two criteria.

What is Science?

National Science Education Standards

(National Research Council, 1996)

“…science reflects its history and is an ongoing,

changing enterprise.”

Beyond Mnemonics

Science is “ not a collection of facts but an

ongoing process, with continual revisions and refinements of concepts necessary in order to arrive at the best current views of the Universe.”

- American Astronomical Society

A Bit of History

• How have planets been historically defined?

• Has a planet ever been demoted before?

Planet (from Greek “planetes” meaning wanderer)

This was the first definition of “planet”

planet Latin English Spanish Italian

Sun Solis Sunday domingo domenica dimanche

Moon Lunae Monday lunes   lunedì lundi

French

Mars Martis Tuesday martes martedì marti Mercury Mercurii Wednesday miércoles mercoledì mercredi

Jupiter Jovis Thursday jueves giovedì

Venus Veneris Friday viernes venerdì   vendredi

Saturn Saturni Saturday sábado sabato samedi

jeudi

Copernican Revolution
(16th Century)

“Planet” now means “any celestial body orbiting the Sun.” The second time we’ve defined the word “planet.”

Copernicus
Ptolemy

Find these images and learn more about the Universe models of Ptolemy and Copernicus at

http://www.pbs.org/wnet/hawking/universes/html/univ_coper.html

In 1801, 20 years after the discovery of Uranus, Ceres was discovered by Giuseppe Piazzi.

Cerese was demoted to asteroid after the discovery of many other objects in similar orbits.

“Planet” became defined as “any celestial body orbiting the Sun that is ‘big enough,’ in other words Mercury-sized or bigger.” The third time we’ve defined the word “planet.”
The hunt for Planet X (1903-1916)

Clyde Tombaugh 1904 - 1997

Problems with Pluto’s mass

(this is the only scary mathematical plot that we’ll use here, we promise!)

from Dessler and Russell (1980)

Discovery of Charon in 1978

Images courtesy of the U.S. Navel Observatory and NASA/HST/ESA

Pluto and Charon

Pluto and Charon as imaged by the Hubble Space Telescope in 1994

Image Credit: Dr. R. Albrecht, ESA/ESO Space Telescope European Coordinating Facility; NASA

The
Kuiper
Belt

Image from http://www.ap.stmarys.ca/~ishort/Images/SolSystem/

1992 QB1

The first Kuiper Belt Object (KBO) was discovered in 1992
Diameter: 200 km

A 1992 discovery image of 1992 QB1 (indicated by the arrow) captured by Jewitt and Luu using the University of Hawaii's

2.2 m telescope on Mauna Kea, from http://science.nasa.gov/headlines/y2001/ast13sep_1.htm

More Kuiper Belt Objects (KBOs)

More than 1000 KBOs have now been found.

100,000 may be detectable from Earth.

1000 km

Image Credit: NASA JPL-Calech

Neptune’s Moon Triton:
A Captured KBO

Image credit: Paul M. Schenk, Lunar and Planetary Institute

• Is Pluto Just a KBO?
• UB313: Bigger than Pluto!

An NASA artist’s conception (left) and an actual Hubble Space Telescope image (right) of a 2400 km diameter Kuiper Belt Object orginally called UB313, discovered by Michael Brown.

Credit: NASA, ESA, and M. Brown (California Institute of Technology)

So, What is a Planet?

Dysnomia

Eris

Image Credit: NASA, ESA, and A. Feild (STScI)

Scale in the Solar System

We’ll use a scale model of the solar system using a 1:10 billion scale factor, and including 8 planets and the first 3 dwarf planets:

1. Ceres

2. Pluto

3. Eris

Size?

“... anything bigger than TEXAS is certainly not minor, not small, and not a dwarf,

Therefore, a planet must be any star-orbiting, nonfusing celestial body larger than the smallest sphere containing TEXAS.”

-Bob Johnson
Graduate Student
UT Dallas

http://www.johnstonsarchive.net/astro/planettexas.htm

• Smaller than Many Moons
• Ida and Dactyl

10 km

Roundness Criteria

Should everything round be a planet?

Saturn’s moon, Mimas Diameter: 394 km