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Tom Van Flandern / Meta Research / tomvf@metaresearch.org
Abstract. Recent events and
discoveries mandated that the International Astronomical Union (IAU) attempt to
define a planet to distinguish planets from the myriad of other types of bodies
in the solar system. A preliminary proposal for this definition proved too
controversial, and was replaced during the IAU’s August 2006 meeting in
Jim Christy’s discovery of Charon, a moon of Pluto, at
the U.S. Naval Observatory in 1978, seemed on the one hand to guarantee Pluto’s
status as a major planet because, back then, only major planets had moons. At
the same time, Charon’s orbit gave us the first reliable estimate of Pluto’s
mass, which we learned was only about 1/400 of the mass of the Earth. That made
Pluto too small to be the source of perturbations on the other outer planets.
And over time, astronomers began to question whether Pluto was deserving of
being called a planet, or whether it was more like Ceres, the largest asteroid
in the main asteroid belt between Mars and Jupiter.
All through the 1990s, we were
treated to discoveries of moons of asteroids and other minor bodies, and the
discovery of a new asteroid belt beyond
Two events brought this
simmering problem to a head.
(1)
In 2000, the
(2)
In 2005, Mike Brown discovered that a TNO first imaged in 2003 was larger than
Pluto. But if Pluto was a planet, then it seemed that the new body must also be
one. So where was the line between planet and asteroid (minor planet) to be
drawn?
A committee of planetary
astronomers began meeting in 2005 to try to draw up an official definition of a
planet for the International Astronomical Union (IAU), which has the authority
to make such decisions for the field of astronomy, whether or not anyone else follows
them. However, the committee was unable to reach agreement on any definition.
So a subset of the committee and a few specialists, seven astronomers in all,
tried again to agree on something they could recommend to the IAU. The
underlying problem is that there is a continuum of masses of bodies with no
major gaps or steps where we might draw a clear line. We call these bodies
“planets”, “moons”, “asteroids”, “comets”, and “meteoroids”, which take the
mass continuum down to dust size particles. None of these categories are
well-enough defined to avoid ambiguous cases arising.
On the high end of the mass range, we also have no reliable distinction between
planets and stars. The old idea was simple: Stars give off their own light,
planets shine by reflected light. However, that too has become a murky
distinction. We have only the weakest of theoretical reasons for suggesting
that about 13 Jupiter masses is the minimum necessary mass to ignite
thermonuclear reactions inside a star and start it shining. However, we now
know that Jupiter, Saturn, and Neptune radiate about twice as much heat back
into space as they take in from the Sun. So in that sense, they can radiate
more than they reflect. Does that make them mini-stars? Until we understand
where that heat is coming from, efforts to draw a theoretical line between
planets and stars will remain conjecture.
Moons used to be simply bodies
that orbited planets. Then we discovered asteroids and even the occasional
comet orbiting major planets, and found that both asteroids and comets can have
satellites. The definition became even murkier when we started launching
artificial satellites and interplanetary spacecraft.
We have yet to find any single
criterion that is unique to comets or asteroids, and have seen several examples
of comets becoming asteroids and vice versa. Recently, when a small body
transitioned from a solar orbit close to Earth’s into a temporary
Earth-satellite orbit, we could not quickly determine if it was a natural
object or the return of a man-made spacecraft. And even meteoroids the size of
a dust grain can make fairly bright streaks as they burn up in Earth’s
atmosphere.
So the IAU committee faced the
impossible task of being compelled to come up with a definition of a planet
when there was no clear scientific basis yet for making one. The result was
necessarily somewhat arbitrary.
The 7-member committee’s initial
recommendation following its deliberations was to define a “planet” as a body
orbiting the Sun, too small to be a star, and large enough to be approximately
in hydrostatic equilibrium (i.e., have a mostly round shape). This seemed a
rather clever compromise intended to please or satisfy as many interests as
possible. By this definition, there were to be 12 planets now, the original
eight majors plus four new “dwarf planets”, with Pluto included in this last
group. This recognizes both Pluto’s importance, planet-wise; and also its
status as a lesser to the other major planets. Despite the proposal’s flaws, I
could not help but admire the compromising spirit and ingenious way to “cut the
Gordian knot” recommended by the committee. Even though my own writings have
contributed to blurring the meaning of “planet” by producing strong evidence
that Mercury, Mars, and Pluto are not original planets but rather escaped moons
of present or former (exploded) planets, the compromise proposal was one I
could have voted for.
The four members of the new
dwarf planet category in the proposal were:
My late colleague, Robert
Harrington, and I were both at U.S. Naval Observatory in 1978 when Jim Christy
discovered Charon. We were called upon to verify the discovery and develop its
implications before a discovery announcement was made. The story of the
discovery and confirmation is detailed in chapter 19 of my book. [[2]] This association with the discovery might
be construed as biasing me toward classifying Charon as a planet. But my
support for the proposal is limited to considering it the best available
compromise we now have. Any such pro-planet bias is mild and easily overcome by
any good reasons to do otherwise.
This draft definition raises
other questions. The center of mass of Jupiter and the Sun lies outside
Jupiter. Should this argue that Jupiter and the Sun are a double star? Or if
the Sun collapsed to a neutron star, driving the center of mass of Sun and
planets outside the Sun for all planets, would that promote all the major
planets to mini-stars? And does Earth’s large Moon make Earth part of a double
planet too? By the definition, the latter case is not yet a double because the
Moon-Earth center of mass is 1000 miles below Earth’s surface. However, the
Moon is evolving outward through tidal friction, and would therefore someday
become part of double planet (by this definition) billions of years from now.
Or with a slightly different definition, we could argue that the Moon is
already part of a double planet because the Sun’s gravitational pull on the
Moon is greater (by a factor of two) than Earth’s gravitational pull on the
Moon.
Ceres is the largest main belt asteroid, and was thought to be a planet when
first discovered. It was the first planet to be demoted when asteroids became
numerous. But a case for planethood can be made because Ceres is the leader of
its class of bodies, having more mass than all other main belt asteroids put
together. It also keeps a round shape and orbits the Sun, so it meets the
criteria of the draft definition.
Eris is significantly larger than Pluto according to recent determinations, and
is now the largest known TNO. Its orbit ranges from 38-98 au (as compared with
29-49 au for Pluto). In fission theory for solar system origin [[3]], Eris was very likely a moon of exploded
hypothetical parent body “Planet X”. Fission theory further predicts that, if
Planet X was a gaseous planet, as seems likely, Eris will have a
yet-to-be-discovered twin with 20% less mass. [[4]]
Other relatively large TNOs such as Sedna (~1600 km), Quaoar (~1250 km), Varuna
(~950 km), 2003 EL61 (~1500 km), and 2005 FY9 (~1800 km) may also have been
former moons of exploded Planets T or X, and might qualify as dwarf planets
after we learn more about their size and shape. [[5]]
Eris is pronounced with a long
“E” as in “eerie”. In light of the controversy stirred up by demoting Pluto
from the ranks of the planets by the adopted definition (below), the naming of
Eris was especially appropriate. Eris is the Greek goddess of discord and
strife. She stirs up jealousy and envy to cause fighting and anger among men.
At the wedding of Peleus and Thetis, the parents of the Greek hero Achilles,
all the gods with the exception of Eris were invited, and, enraged at her
exclusion, she spitefully caused a quarrel among the goddesses that led to the
Trojan war. Eris’s moon has been named Dysnomia after the Daemon spirit of
lawlessness. She is the daughter of Eris. [1]
The recommended definition discussed
above was rejected by the IAU and replaced with a hastily reworded new
definition that passed an IAU vote on the last day of the General Assembly. [[6]] There was not adequate time to consider
the implications of the new wording, and that has turned out to be unfortunate.
The definition passed by the
IAU and now in effect is as follows. 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. The
problem is obviously with the last of these criteria because the other two were
common to the draft resolution. The idea that bodies clear the space near their
orbits arose from the accretion theory of solar system formation, which
probably has no correspondence to reality. The basic problem with “clearing the
space” is that it is undefined, and applies to masses of all sizes if taken
literally. This is a frequently overlooked or misunderstood property of
dynamics, and therefore bears a bit of explanation.
If two masses of any size are placed into the same or very similar orbits
around a single dominant central mass, and gravitation is the only significant
force acting, the two masses cannot collide. Small differences in orbital
period may cause them to approach one another. But as soon as either mass
starts to approach the other mass, the relative velocity between the two masses
will start to increase by slowing the forward orbital motion of the leading
mass and speeding up the forward orbital motion of the trailing mass. That
velocity change immediately begins to decrease the orbital period of the
leading mass and increase that of the trailing mass. These imposed changes
continue until inevitably the leading body’s shorter period causes it to start
pulling farther ahead, and the trailing body’s longer period causes it to start
falling farther behind. The two masses then continue to separate until the
reverse phenomenon starts a new cycle, or until the leading body catches up to
the trailing body from behind and starts a new cycle.
These dynamical behaviors are
well known, and are the reason why “Lagrange points” exist. For example, Trojan
asteroids exist in Jupiter’s orbit and stay near the points 60° ahead of (L4)
and 60° behind (L5) Jupiter. But even particles far from Lagrange points suffer
the same phenomenon if they try to approach the planet’s gravitational sphere
of influence with a small relative velocity.
So collisions of bodies in the
same orbit are forbidden by the laws of dynamics. The same applies to bodies in
similar orbits, where “similar” is a function of the size of the sphere of
influence of the larger orbiting body. Typically, the radius of an orbiting
body’s gravitational sphere of influence increases linearly with its distance
from the dominant central mass around which it is orbiting. For bodies of
ordinary density near Earth’s orbit, the radius of the sphere of influence is
roughly 100 times the body’s own radius. (For comparison, our Moon’s orbit is
about 60 Earth-radii away, and is therefore permanently inside Earth’s sphere
of influence. The Moon cannot escape Earth’s sphere of influence on its own,
nor can anything outside the sphere of influence get captured inside the sphere
of influence without help.) If the two bodies are dust particles, the zone of
forbidden collisions will be very small indeed, being only 100 times the radius
of a dust particle. But the zone exists just the same for masses of any size.
So when do collisions become
possible? At least one of the two bodies must have enough orbital eccentricity
so that it crosses the orbit of the other at a substantial angle. Then the
relative speed of the two bodies can be much larger. When the relative velocity
becomes large enough that one body can approach the other and traverse its
sphere of influence before the slow alteration in its orbital period has a
chance to propel it away again, a collision can happen. But the orbits must
always be dissimilar and the relative velocities must be high for this to
happen, so such collisions tend to be destructive rather than accretive unless
the two bodies are very dissimilar in mass as well.
The consequence of this
dynamical principle is that no body can accrete other material in or near its
own orbit; so no body can “clear out a zone” near its own orbit. A body cannot
draw in other bodies, so other bodies can collide only if already on a
potential collision orbit before the encounter. Another corollary is that
planetary rings are always the result of a body breaking up rather than one
that “failed to accrete”. We can immediately see many applications of this
dynamical principle throughout the solar system.
One of the proponents of the new definition explained
the reasoning behind it this way: “The next and perhaps most important,
transition occurs between objects which are or are not massive enough to clear
residual planetesimals through accretion or scattering. Empirically we can see
that Ceres, for example, is not massive enough to clear the asteroid belt of
its many remaining planetesimals, while the other terrestrial planets have
certainly been successful in clearing their regions of influence. As a cultural
matter, collecting a few small bodies into orbital resonances has not
disqualified an object from being a planet (e.g. the Trojans for Jupiter or the
Plutinos for
Yet even the same proponent
points out problems with this “cultural” definition: “An interesting situation
would arise if, for example, a Mars-sized object were found in the inner Oort
cloud. By the dynamical definition this object would not be considered a
planet, yet most people would instinctively feel that a body that size should
indeed be called a planet.”
Our Members and subscribers
will mostly be familiar with the pervasive evidence for the exploded planet
hypothesis (EPH) as explaining the origin of asteroids and comets in general,
and the main asteroid belt, Kuiper belt, and Oort cloud in particular, in a
much simpler way than the several mainstream theories the EPH would replace.
From this perspective, large bodies remain in the two asteroid belts because
the planets that exploded there left behind major moons in similar orbits, just
as Mars was apparently left in its orbit when the original “Planet V” occupying
that orbit exploded. And “Trojan asteroids” in the same orbit as a planet
usually arise when a moon of that planet explodes.
So the circumstance that larger bodies reside within asteroid fields is normal
and expectable, and leads to conclusions that are “instinctively” wrong, as in
the proponent’s counterexample. One might still argue that Jupiter’s gravity
does allow Jupiter to accrete many asteroids and scatter others that come too
close, thereby clearing a narrow zone off to either side of its own orbit.
However, any body orbiting the Sun can clear a similar zone scaled down proportionally
in size.
These dynamical and practical
considerations make the adopted definition of a planet meaningless because the
described effect is proportionally the same for all bodies regardless of size. It
would likely not be possible to show a meaningful difference between any two
bodies in regard to their ability to do this “zone clearing”.
Immediately following the IAU
General Assembly, when the resolution was announced, many planetary astronomers
worldwide voiced their objections. The following is the content of a petition
to the IAU to reconsider the resolution at its next General Assembly in 2009:
On August 24th, a session of the International Astronomical Union (IAU),
meeting in
Just after the August 24th vote, serious technical and pedagogical flaws were
pointed out in the IAU’s definition of planets. As a consequence of these
flaws, a grass roots petition stating, “We, as planetary scientists and
astronomers, do not agree with the IAU's definition of a planet, nor will we
use it. A better definition is needed” was placed on the web at http://www.ipetitions.com/petition/planetprotest
and circulated by email to a small fraction of the world’s astronomical
research community. In less than five days, the petition was signed by 300
professional planetary scientists and astronomers. The list of signatories
includes researchers who have studied every kind of planet in the solar system,
as well as asteroids, comets, the Kuiper Belt, and planet interactions with
space environment. They have been involved in the robotic exploration of the
solar system from some of the earliest missions to Cassini/Huygens, the
missions to Mars, ongoing missions to the innermost and outermost reaches of
our solar system, and are leading missions preparing to be launched. The list
includes prominent experts in the field of planet formation and evolution,
planetary atmospheres, planetary surfaces and interiors, and includes
international prize winning researchers.
“This petition gives substantial weight to argument that the IAU definition of
planet does not meet fundamental scientific standards and should be set aside,”
states petition organizer Dr. Mark Sykes, director of the Planetary Science
Institute in
“I believe more planetary experts signed the petition than were involved in the
vote on the IAU’s petition. From the number of signatories that the petition
received in a few days, it’s clear that there is significant unhappiness among
scientists with the IAU’s planet definition, and that it will not be
universally adopted by scientists and text book writers. To achieve a good
planet definition that achieves scientific consensus will require more work.”
added co-sponsor Dr. Alan Stern, Executive Director of the Space Science and
Engineering Division of the Southwest Research Institute.
The
IAU executive committee sent a brief response saying they are considering the
petition.
At the moment, Pluto is classified as a “small solar system body”, to wit, a
dwarf planet. It seems not unreasonable that more enlightened consideration
will eventually recognize dwarf planets as a class of true planets, much as
“gas giant planets” and “terrestrial planets” are now, but “minor planets” are
not. Pluto fans should not give up hope that Pluto will be returned to the
ranks of the planets at the next IAU meeting in August 2009, perhaps to be
joined by other notable bodies such as Ceres, Charon, and Eris.
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“When an
idea is wanting a word can always be found to take its place.” – Johann W. von
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