Download Chapter 2 Astronomy Notes

Astronomy Notes
Planets, Dwarf Planets, Pluto, Eris & Ceres
Unit 3 Chapter 2 Activities 11, 12 & 13
Purpose
Scientists are constantly revising and updating their theories and definitions as they
gain new evidence and information. This update has to do with the International
Astronomical Union’s (IAU) new definition of a planet, along with the category of
dwarf planet, which resulted in the “demotion” of Pluto from a planet to a dwarf
planet.
There are three sections to this update. The first section reviews the historical usage of
the word planet. The second section reviews the IAU’s new definitions. You should use
the material in these two sections in class discussions about the meaning of planet. The
third section suggests some points you might raise when discussing Pluto’s change in
status and the other dwarf planets, Eris and Ceres.
Historical Background
For millennia, the word planet was used to designate objects that wandered across the
sky. That is, they did not stay in a “fixed” place on the celestial sphere like stars
appeared to do. Thus, planet not only included the “wandering stars” Mercury, Venus,
Mars, Jupiter and Saturn, but also included the Sun and the Moon. Only after people
firmly accepted the fact that Earth revolved around the Sun along with the five
wanderers did planet come to mean a body that orbited the Sun. Now Earth was
counted as a planet, but not the Sun or the Moon.
The number of planets rose from six to seven when Uranus was discovered by William
Herschel in 1781. Astronomers counted eight planets after Ceres was discovered in
1801; nine, when Pallas was found in 1802; ten, with the discovery of Juno in 1804; and
eleven with Vesta’s discovery in 1807. All four of the “planets” discovered between
1801 and 1807 were small bodies that orbited between Mars and Jupiter in similar
orbits. Although some astronomers (notably Herschel) suggested that these small
bodies be re-classified as asteroids (Greek for “star-like”), most astronomers resisted
“demoting” them until after 1845, when the first asteroid since Vesta was found and
several more soon followed. In 1851, astronomers began designating asteroids as
“minor planets” and gave them numeric designations such as “1 Ceres,” “2 Pallas,” etc.
Ceres and its companions were effectively demoted as planets. Not until the mid 1860s
were Ceres, Pallas, Juno and Vesta dropped from most lists of planets.
Neptune, discovered in 1846, now became the eighth planet. The Solar System
remained a system of eight planets until Clyde Tombaugh discovered Pluto in 1930 out
©2007 InterActions Project
1
Astronomy Notes: Planets, Dwarf Planets, Pluto, Eris & Ceres
beyond the orbit of Neptune. Originally believed to be a cold, icy planet around the size
and mass of Mars, in 1978 Pluto was found to have a large moon, Charon, orbiting close
to Pluto. That allowed scientists to get a firm grasp on Pluto’s mass because Charon’s
orbital period is a function of the Pluto-Charon combined mass. They discovered that
Pluto’s mass was only about 20% of the Moon’s mass. Though it continued to be
classified as a planet, Pluto was now ranked as the smallest planet by far.
Then in 1992, history began to repeat itself. A new icy body smaller than Pluto,
provisionally designated 1992 QB1 (it still has not been named), was found beyond
Neptune’s orbit. This “snowball’s” discovery was followed rapidly by the discovery of
hundreds of other icy bodies with similar orbits, including scores of “plutinos” with
orbits similar to Pluto’s own (see the last couple pages of Activity 13). Soon
astronomers realized that Pluto was just part of a larger group of objects, a “belt” of icy
bodies beyond Neptune’s orbit that was roughly analogous to the Asteroid Belt. They
named this belt the Kuiper Belt, after a scientist who predicted its existance back in
1951.
As the years passed and the number of KBOs and similar “trans-Neptunian objects”
(TNOs) discovered increased, astronomers found more and more bodies that
approached Pluto in size. Many began to question the continued inclusion of Pluto as a
planet. If Pluto was a planet, it seemed inevitable that a number of TNOs would
eventually be counted as planets as well, and the Solar System could increase in size to
dozens of planets. Distant Sedna, discovered in 2002 far beyond the Kuiper Belt and
which is probably not much smaller than Pluto, was hailed by some in the press as the
“tenth planet.” When Eris (originally designated 2003 UB313 and nicknamed Xena) was
discovered in 2005, even NASA hailed the new discovery as the “tenth planet” because
it was estimated to be larger than Pluto. Two other bodies, discovered at the same time,
were also nearly Pluto’s size.
Rather than immediately accepting Eris as a new planet in 2005, the IAU decided to
consider a new definition for “planet.” One year later, after much debate, the IAU
accepted a new definition for planets in the Solar System that excluded both Pluto and
Eris. The IAU simultaneously defined a new category of “dwarf planet” that included
Pluto and Eris. Another former planet, Ceres, the largest asteroid, was also counted as a
dwarf planet—effectively getting “promoted” while Pluto got “demoted.” Another
twelve bodies (asteroids and KBOs/TNOs alike) were candidates for dwarf planet
status. The Solar System is back to eight planets and an unknown number of dwarf
planets.
While some astronomers were exploring the icy frontier of the Solar System, other
astronomers were using new techniques to search beyond the Solar System for planets
around other stars. Since 1992, astronomers have found over 200 such planets, and they
are discovering new ones at the rate of 25-30 each year. Most of these planets have been
the size of Jupiter or larger. Indeed, some have been so massive that they fell just short
of having enough mass (about 13 Jupiter masses) to become stars. These discoveries
2
Astronomy Notes: Planets, Dwarf Planets, Pluto, Eris & Ceres
prompted the International Astronomical Union to issue guidelines in 2003 on what
differentiated planets from stars and related objects (most notably “brown dwarfs”).
See the Teacher Resource for on-line references on this topic.
Defined by the IAU: Planets and Dwarf Planets
On August 24, 2006, the International Astronomical Union adopted the following
resolution that defined the terms “planet,” “dwarf planet,” and “small Solar System
objects” for bodies in the Solar System:
The IAU therefore resolves that planets and other bodies in our Solar System, except
satellites, be defined into three distinct categories in the following way:
(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 neighbourhood 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 neighbourhood
around its orbit, and (d) is not a satellite.
(3) All other objects except satellites orbiting the Sun shall be referred to collectively as
"Small Solar System Bodies".
Criterion (b) in the definitions of both “planet” and “dwarf planet,” the rounding of a
body due to its self-gravity, is the topic of “How Gravity Determines Shapes of Planets
and Stars” in Activity 13 (page 278 in the Student Edition).
In Criterion (c), the phrase “cleared the neighborhood” does not mean that no other
bodies share a planet’s orbit or cross its orbital path. It does mean is that a planet has
“cleared the neighborhood” of any competing masses (see the discussion of planet
formation in Activity 13, pages 276-277), and that it largely controls the orbital
dynamics of masses that remain. For example, Neptune’s gravity determines the orbits
of its moons. Its gravity also keeps Pluto, which crosses its orbit, and hundreds of other
plutinos (see Activity 13, pages 281-282) in orbits around the Sun that are resonant with
Neptune’s own. Neptune is also much more massive than the combined mass of its
moons, the plutinos, and all other objects close to its orbit.
In 2003, the IAU had also released an official statement that differentiated planets from
stars and other massive objects. This is the part of the statement that pertains to planets:
Objects with true masses below the limiting mass for thermonuclear fusion of deuterium
(currently calculated to be 13 Jupiter masses for objects of solar metallicity) that orbit
stars or stellar remnants are "planets" (no matter how they formed). The minimum
3
Astronomy Notes: Planets, Dwarf Planets, Pluto, Eris & Ceres
mass/size required for an extrasolar object to be considered a planet should be the same as
that used in our Solar System.
Note that deuterium is a heavy isotope of hydrogen, and a “stellar remnant” are bodies
like black holes, neutron stars and white dwarfs that are left over after a star “dies.”
The IAU definitions from 2003 and 2006 were incorporated into the definitions of planet
and dwarf planet used in the InterActions text:
planet: a body smaller than a star that has been made round by gravity, orbits a
star such as the Sun, and dominates its orbital neighborhood
dwarf planet: a small body made round by gravity that orbits the Sun and does
not dominate its orbital neighborhood; dwarf planets include Pluto, Eris and
Ceres
Notes for Activities 11-13
Activity 11:
•
Copy and distribute Astronomy Handout 1, which is available on this web site. It
is designed to be printed on both sides of a single sheet of paper. The information
on this handout is always more current than information in the text.
•
Discuss the decision of the IAU in August 2006. Note that, because of that
redefinition:
o Planets are now that dominate their orbital neighborhoods. That means
that planets are much more massive than the combined mass of other
bodies in their orbital neighborhood.
o Pluto’s mass is only a fraction of the combined mass of other objects in its
orbital neighborhood. Therefore, it is no longer considered a planet. That
means the Solar System has only eight planets.
•
Dwarf planets are a new category of Solar System objects. They orbit the Sun
and are round due to self-gravity, but do not dominate their orbital
neighborhoods like planets do. They are parts of larger populations, like
asteroids or KBOs. The first dwarf planets identified by the IAU are:
o Ceres, the largest asteroid and from 1801-1860 or so the “eighth planet”
o Pluto, a large KBO and from 1930-2006 the “ninth planet”
o Eris (nicknamed “Xena”), a KBO a bit larger than Pluto, which upon
discovery in 2005 was billed as the “tenth planet”
4
Astronomy Notes: Planets, Dwarf Planets, Pluto, Eris & Ceres
Activity 12:
•
Copy and distribute Astronomy Handout 2, which is available on this web site. It
is designed to be printed on both sides of a single sheet of paper. The information
on this handout is always more current than information in the text.
Activity 13:
In this activity, stress how the role of gravity helps distinguish planets from dwarf
planets, and planets and dwarf planets from smaller bodies. Specifically:
•
Formation of Stars and Solar Systems: Emphasize that one difference between
planets and dwarf planets is that planets have the gravity necessary to sweep up
the rock and gas in their orbits, while the gravity of dwarf planets (and smaller
objects) is too weak to do so.
•
How Gravity Determines the Shapes of Planets and Stars: Emphasize that the
rounding of a body by its own gravity distinguishes planets and dwarf planets
from smaller bodies.
5