Download Unit 1 Cycle 2: Interactions and Energy

Astronomy Update
Planets, Dwarf Planets, Pluto, Eris & Ceres
Unit 3 Cycle 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 changes to Activities 11 through 13 and Scientists’ Ideas: Earth
and Space Science Part 1 that reflect the new IAU definition of planet and the addition of
dwarf planet as a type of Solar System object. Two handouts for Activities 11 and 12
have also been prepared for students and are available at the on-line Teacher Resource.
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.
©2006 InterActions Project
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Astronomy Update: Planets, Dwarf Planets, Pluto, Eris & Ceres
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
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
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Astronomy Update: Planets, Dwarf Planets, Pluto, Eris & Ceres
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
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:
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Astronomy Update: Planets, Dwarf Planets, Pluto, Eris & Ceres
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
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.”
Changes to Activities 11-13
The IAU’s definitions have been incorporated into two new “Science Words” for
InterActions in Physical Science, planet and dwarf planet, as well as an added Scientists’
Idea. In addition, to complete the set of astronomy Science Words, definitions have
been added for star, solar system, and galaxy. These changes and other changes are
indicated below.
Changes for 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.
•
New Science Words: star, solar system, planet, dwarf planet.
star: a celestial body consisting of hot gases that shines due to the energy
produced by nuclear fusion reactions in its core; the Sun is a small star
solar system: one star (or more), all the bodies that orbit the star, and their
satellites (moons); our Solar System is one of hundreds that we know of
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
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•
Planets are now objects that orbit the Sun or another star, are round due to selfgravity and dominate their orbital neighborhoods. That means that planets are
much more massive than the combined mass of other bodies in their orbital
neighborhood.
•
Because it does not dominate its orbital neighborhood, Pluto 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. The first dwarf planets identified by the IAU are:
Astronomy Update: Planets, Dwarf Planets, Pluto, Eris & Ceres
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”
•
Slide Show: There is a new slide, after the gas giants slide, featuring the first
three dwarf planets. See the slide show posted at the Teacher’s Edition web site.
Also, the KBO formerly designated 2004 DW now has a name: Orcus.
•
Question 2: Omit Question 2f. The answer was “Pluto,” but since this text was
written two more small moons have been discovered around Pluto.
•
Visiting Our Neighbors: Although no spacecraft have yet visited the distant Solar
System, it is worth noting that the New Horizons spacecraft, launched in January
2006, is on its way and will reach Pluto in July 2015.
•
Key Question 1: Add “dwarf planets” to the list of objects found in the Solar
System.
Changes for 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.
•
New Science Word: galaxy. Introduce during “Distances between Stars and
Galaxies.”
galaxy: a giant cluster of stars bound by gravity; galaxies can contain between
hundreds of millions and hundreds of billions of stars
•
Elliptical orbits: The most elliptical orbit among the planets now belongs to
Mercury, whose distance from the Sun varies between 0.31 AU and 0.47 AU.
That means that Mercury is 50% further away from the Sun at its furthest than it
is at its closest.
•
Slide Show: The slides for the Solar System proper have been modified.
Terrestrial planets now have green orbits, gas giants have blue orbits, and the
orbits of dwarf planets (including Sedna, a dwarf planet candidate in late 2006)
are brown. The orbits for Ceres and Eris are new.
Changes for Activity 13:
•
Formation of Stars and Solar Systems: Toward the end of this section (ideally,
right after the first paragraph following Question 2), note the following:
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Astronomy Update: Planets, Dwarf Planets, Pluto, Eris & Ceres
Unlike planets, asteroids and KBOs, including dwarf planets like Ceres, Pluto and Eris,
are too small and lack the gravity to sweep up the rock and gas in their orbits, and
therefore they do not dominate their orbital neighborhoods like planets do.
•
How Gravity Determines the Shapes of Planets and Stars: Before Question 4,
note the following:
For the Solar System, the International Astronomical Union considered the rounding of a
body by its own gravity to be important enough that they included it in their definitions
for planet and dwarf planet. Thus, any body in the Solar System that became round by
its own gravity and orbits the Sun is either one of the eight planets or a dwarf planet.
•
Little Plutos and Wandering Planets: Again, the plutino formerly designated
“2004 DW” is now called Orcus. You might note the number of planets found
orbiting other stars as of late 2006 exceeded 200, with a new planet being
discovered approximately once every 12 days. These planets range in mass from
several times the mass of Earth to several times the mass of Jupiter.
Scientists’ Ideas: Earth and Space Sciences Part 1
•
“Dwarf planets” should be added to the list of Solar System objects in Idea 1.
Specifically, amend Idea 1a to say:
1. The Solar System…consists of :
a) the Sun, eight planets, and several dwarf planets
•
Add the following idea:
14.
Both planets and dwarf planets are bodies that orbit the Sun and have been
made round due to gravity. They differ in one important respect:
a) Planets dominate their orbital neighborhoods, which means that they are
much more massive than combined mass of all other objects in their
orbits, and have much stronger gravity.
b) Dwarf planets do not dominate their orbital neighborhoods and usually
belong to other groups that have similar orbits. For example, Ceres is an
asteroid, and both Pluto and Eris are KBOs.
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