IS PLUTO A PLANET?JOONHYUK KWON

IS PLUTO A PLANET?

OUTLINES

▸ Composition of the solar system

▸ The definition of the “planet”

▸ History of the definition of the planet

▸ Debates to classify the objects in Solar System

▸ IAU resolution

▸ Other things about the Pluto

OBJECTS IN UNIVERSE (1)

OBJECTS IN UNIVERSE

▸ Star

▸ • Upper mass limit: about 120 M¤

above that radiation pressure blows the star apart

▸ • Lower mass limit: 0.076 M¤

below that core temperatures are too low for fusion

▸ Braun Dwarf

▸ Below 0.076 M¤, H cannot undergo stable nuclear fusion

▸ burn their D quickly slowly cool

OBJECTS IN UNIVERSE (2)

PLANETS (CLASSICAL PROPERTY)▸ Less massive than Braun Dwarfs

▸ Massive enough to be rounded by its gravity

▸ Form a disk surrounding stars

▸ Terrestrial planet (Mercury, Venus, Earth and Mars)

▸ similar to Earth, with bodies largely composed of rock

▸ Giant planet (Jupiter, Saturn, Uranus, Neptune)

▸ Gas giants : Jupiter and Saturn

▸ Ice giants : Uranus and Neptune

PLANETS

HISTORY OF PLANETS

NUMBER OF PLANETS ON TIME PERIOD

KUIPER BELT

OBSERVATION OF KUIPER BELT

▸ a region of the Solar System beyond the planets, extending from the orbit of Neptune. (from 30AU to 50AU)

▸ KBO (Kuiper Belt Objects) or TNO (Trans-Neptunian Objects)

▸ In 1992, ‘1992 QB’ was discovered, the first KBO since Pluto. Since its discovery, the number of known KBOs has increased to over a thousand, and more than 100,000 KBOs over 100 km in diameter are thought to exist.

IAU COMPROMISE 1ST

DEFINITION OR PLANET - REVISITED▸ First draft proposal of IAU compromise (in 2006)

▸ Orbits a star

▸ Large enough for its gravity to make it round

▸ neither a star nor a satellite of a planet

IAU COMPROMISE 1ST

12 CANDIDATES FOR PLANET

IAU COMPROMISE 1ST

THE IAU COMPROMISE - TERMS

▸ dwarf planet

▸ Describe all planets smaller than the eight "classical planets" in orbit around the Sun, though would not have been an official IAU classification

▸ Small solar system body (SSSB)

▸ objects underneath the "spherical" threshold

▸ Plutons

▸ planets with highly inclined orbits, large eccentricities and an orbital period of more than 200 earth years (that is, those orbiting beyond Neptune)

IAU COMPROMISE 1ST

CRITICISMS AND REVISED PROPOSAL

▸ ambiguity for the planets ejected from its star system

▸ double planet issue :

▸ at present the Moon is defined as a satellite of the Earth, but over time the Earth-Moon barycenter will drift outwards and could eventually become situated outside of both bodies. This development would then upgrade the Moon to planetary status at that time, according to the redefinition.

IAU COMPROMISE 2ND

ALTERNATIVE PROPOSAL

A planet is a celestial body that

(a) is by far the largest object in its local population,

(b) has sufficient mass for its self-gravity to overcome rigid body forces so that it assumes a hydrostatic equilibrium (nearly round) shape,

(c) does not produce energy by any nuclear fusion mechanism.

* objects satisfies (b) and (c) but not (a) is defined as “dwarf planet”.

IAU COMPROMISE FINAL

FINAL DEFINITIONS FROM IAU

(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 neighborhood around its orbit.

(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, (c) has not cleared the neighborhood around its orbit, and (d) is not a satellite.

(3) All other objects [3], except satellites, orbiting the Sun shall be referred to collectively as “Small Solar System Body".

IAU COMPROMISE FINAL

FINAL DEFINITIONS FROM IAU

(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 neighborhood around its orbit.

(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, (c) has not cleared the neighborhood around its orbit, and (d) is not a satellite.

(3) All other objects [3], except satellites, orbiting the Sun shall be referred to collectively as “Small Solar System Body".

IAU COMPROMISE FINAL

CONDITIONS TO BE A PLANET - CLEARED THE NEIGHBORHOOD (1)

▸ The planet has become gravitationally dominant, and there are no other bodies of comparable size other than its own satellites, or those otherwise under its gravitational influence.

Stern and Levison: ”planetary bodies control the region surrounding them"

Λ : a measure of a body's ability to scatter smaller masses out of its orbital region over a long period of time.

⇤ =M2

a32

k

IAU COMPROMISE FINAL

CONDITIONS TO BE A PLANET - CLEARED THE NEIGHBORHOOD (2)

ref. Wikipedia

IAU COMPROMISE FINAL

SO… IS PLUTO A PLANET?

▸ The answer is obviously NO, since it has not cleared the neighborhood around its orbit.

▸ IAU said that

▸ Pluto is a "dwarf planet" by the above definition and is recognized as the prototype of a new category of Trans-Neptune Objects.

JOURNEY TO PLUTO

SOME FEATURES OF PLUTO - NEW HORIZON

Spherical mosaic of New Horizons images (September 10, 2015)

Pluto as viewed by New Horizons during flyby (July 14, 2015)

Pluto and Charon, to scale. (July 8, 2015)

ref. Wikipedia

REFERENCES

REFERENCES

IAU - http://www.iau.org/news/pressreleases/detail/iau0603/

Prof. Alan Calder (StonyBrook University) "What did New Horizons teach us about the system formerly known as Planet Pluto?"

Stony Brook AST248 : www.astro.sunysb.edu/fwalter/AST248/planets.ppt.pdf

Andrew Fraknoi What’s a Planet and Why is Pluto Not in the Planet Club Anymore?

Swinburne university astronomy.swinburne.edu.au/sao/downloads/HET616-M03A03

Joseph StrombergNew Horizons' Pluto mission has already taught us 6 amazing things http://www.vox.com/2015/7/16/8978451/new-horizons-pluto-facts

THANK YOUJOONHYUK KWON

ADDITIONAL 1

CONDITIONS TO BE A PLANET

▸ a Hydrostatic equilibrium

▸ external force (gravity) is balanced with a pressure gradient force

In any given layer of a star, there is a hydrostatic equilibrium between the outward thermal pressure from below and the weight of the material above pressing inward.

The isotropic gravitational field compresses the star into the most compact shape possible. A rotating star in hydrostatic equilibrium is an oblate spheroid up to a certain (critical) angular velocity.

An extreme example of this phenomenon is the star Vega, which has a rotation period of 12.5 hours. Consequently, Vega is about 20% larger at the equator than at the poles. A star with an angular velocity above the critical angular velocity becomes a Jacobi ellipsoid, and at still faster rotation it is no longer ellipsoidal but oviform, with yet other shapes beyond that, though shapes beyond scalene are not stable.