The document discusses the criteria for defining planets in our solar system and beyond. It explores size, shape, orbit characteristics, historical classifications, and membership in populations like the Kuiper Belt. While size and orbit were traditionally the main criteria, the discovery of objects like Ceres, Pluto, and exoplanets require considering other factors. The International Astronomical Union is working to develop a definition that accounts for findings both within and beyond our solar system.

1. The Origin of
Stars, Planets, and
Life in the Universe
Dr. Natalie Batalha
nbatalha@science.sjsu.edu

2. How many planets
are there in our
Solar System?
Poll Question:

3. Taking an Inventory of the Solar System
Pluto
Neptune Comet
UranusMars
Earth
Saturn
Mercury
Sun
Venus
Jupiter

4. http://www.cfa.harvard.edu/iau/lists/InnerPlot.html

6. Solar System Table of Contents:
I. Sun
II. Mercury, Venus, Earth, Mars
III. Asteroid Belt
IV.Jupiter, Saturn, Uranus, Neptune
V. Kuiper Belt (icy asteroids)
VI.Oort Cloud (comets)

7. SIZE SHAPE ORBIT TEMP COMP
Which of the following criteria are
important for defining a planet?

8. Size and Shape: Earth is spherical,
asteroid Gaspra is not.
12,800 kilometers
(avout 8000 miles)
15 kilometers
(about 9 miles)

9. Asteroid Ceres is spherical and about
the size of Texas. Is it a planet?

10. Io Europa Ganymede Callisto
Moon
Mercury
Mars
Pluto

11. Definition so far:
a) Orbits the Sun;
b) Is at least as large as Pluto.
Ceres Ganymede
YES NO YES NO

12. Orbital Characteristics
Mercury Earth Mars
7° Venus
Saturn
Jupiter
Uranus
Neptune
Pluto
17°

14. Poll Question:
Why is Pluto’s orbit
different from those of
the other 8 planets?

15. Discovery of Kuiper Belt Objects
• 1992, Jane Luu and David Jewitt
• More than 1,000 KBO’s now known
• 11 Pluto-sized KBO’s
• Pluto is a Kuiper Belt Object

16. How to Find Kuiper Belt Objects

18. The Orbits of Some Kuiper Belt Objects

19. Artist’s Depiction of UB313
Credit: Robert Hurt (IPAC)

20. UB313
(1500 miles)

21. Should UB313 be the 10th
Planet in the Solar System?
Yes No

22. Size Orbit History Population
Which of the following criteria are
important for defining a planet?

23. 193 planets orbiting other
stars (a total of 155 systems)
have been identified to date.
What do they tell us?

24. Most are in elliptical orbits

25. Gas giants are
seen orbiting
very close to
their parent
star!
Copyright © 1998-2006 Lynette R. Cook

26. Three planets all within 0.2 AU
(Mercury orbits at 0.4 AU!)

27. Giant planets with 10x’s the mass of
Jupiter.
Copyright © 1998-2006 Lynette R. Cook, All Rights Reserved

28. The International
Astronomical Union is
working to craft a definition
that satisfies both Solar
System and Extra-Solar
System findings.

29. Summary
• Object size and orbit main criteria
• Historical context a consideration
(Pluto is already on all of the plastic placemats!)
• Membership in a larger population
will also be a consideration
• Object must be smaller than about
13 x’s Jupiter’s mass

30. For more information:
Discovery of UB 313:
http://www.gps.caltech.edu/~mbrown/
First Discovery of Kuiper Belt Objects:
http://www.ifa.hawaii.edu/faculty/jewitt/kb.html
List of planets orbiting other stars:
http://exoplanet.eu