It isn’t, as the
Celestron website one-sidedly states on its page about observing Pluto, a “fun
holiday to celebrate.” What today is is the anniversary of a terrible day for
science and knowledge, a day in which a very political vote was taken, which
has done tremendous harm to science education and communication over the last
18 years.
“It” is the 18th
anniversary of the highly controversial vote by four percent of the IAU on a
very flawed planet definition that excluded Pluto and all small planets that do
not “clear their orbits,’ or, in other words, gravitationally dominate their
orbits.
That definition is
problematic because it puts primacy on a celestial object’s location rather
than on its intrinsic properties. This runs contrary to the use of taxonomy and
definitions in science. It was also a violation of the IAU’s own bylaws, which
require a resolution to be put to the appropriate IAU committee before being
put to the floor of a General Assembly. This was not done with the resolution
adopted by the IAU in 2006.
The passage of 18
years without this travesty being corrected is a disservice to science and to
all who are interested in it. During this time, New Horizons revealed Pluto to
the world as a geologically active world with some complex processes seen elsewhere
in the solar system only on Earth and Mars, yet the IAU and supporters of its
definition have completely ignored this new information and doubled down on the
“orbit clearing” requirement.
A baby born when
the IAU vote took place is now old enough to vote. This means an entire
generation has grown up with confusion and misinformation, as the media continue
to present the IAU view as gospel truth rather than what it really is—one side
in an ongoing debate.
And this year, scientist
Jean Luc Margot, who was once a student of Mike Brown, the astronomer who unprofessionally
brags that he “killed” Pluto, has proposed to remedy only part of the IAU
definition to include exoplanets.
By stating that planets
must orbit the Sun rather than a star, the IAU definition precludes any
exoplanets from being classed as planets.
The problem with
Margot’s proposal, which was presented at this year’s IAU General Assembly but
not put to a vote, is that it goes out of its way to preserve orbit clearing,
or as more accurately stated, gravitational dominance, as a criterion for
planethood.
The proposal establishes
a minimum mass for planet status that includes Mercury but does not include
Pluto or Ceres.
The paper uses
circular reasoning to justify this. It states, “Dynamical dominance looms large
in planetary taxonomy because both Ceres and Pluto lost their status as planets
once they were found to belong to a belt of small bodies.”
This is circular reasoning because it essentially says, Ceres and Pluto lost
their status as planets not because of any change they underwent but because
the IAU created a requirement of dynamical dominance that excluded them.
Contrary to media
reports and this article, Pluto and Ceres never lost their status as planets.
Instead, what happened is that the IAU stopped considering them planets. That
doesn’t mean they stopped being planets or that we have to stop viewing them as
planets. This is an important distinction.
Pluto and Ceres
are far more than objects in belts. They are significantly different from the overwhelming
majority of the objects in those belts because they are rounded by their own
gravity, a state known as hydrostatic equilibrium. This is crucial because it
is the threshold when active geology begins, making these objects significantly
different than tiny, shapeless asteroids and comets, which are shaped only by
their chemical bonds.
Classifying these
objects merely as inhabitants of belts completely ignores their unique
characteristics, shared with the terrestrial planets, and blurs the distinction
between them and the majority of objects in these belts.
And again, it amounts
to putting an object’s location above its intrinsic properties, which is a
major problem.
It also precludes
any rogue planets, which do not orbit any star but float freely in space, from
being considered planets. Many of these objects have been found over the last
18 years. If they aren’t planets, then what are they?
If rogue planets were planets when they orbited their parent stars but stopped
being planets when they were ejected from those orbits, then we again have the
fallacy that an object’s location is put above its intrinsic properties. The
same object can be a planet in one location but not a planet in another
location.
The same is true
for binary planet systems like Pluto-Charon, as neither object in such a binary
“clears its orbit.” As for gravitational dominance in a binary system, that
would require one of the companions to be significantly more massive than the
other one, which is not the case for Pluto-Charon.
Adherents of the
geophysical definition, which does not require orbit clearing or dynamical
dominance for planethood, do not discount orbital dynamics or think they aren’t
important. There is no reason why, in accordance with a study published in
2000, we cannot distinguish dynamically dominant planets from non-dynamically
dominant planets via the use of subcategories.
With the geophysical
definition, some planets are gravitationally dominant while some are not. This and
its importance are acknowledged by placing these two types of planets in
different subcategories. Dwarf planets are simply a subclass of planets that
are not gravitationally dominant. This does not mean they aren’t planets at
all, just that they are a different subclass of planets.
In fact, the IAU itself had a resolution in 2006 establishing dwarf planets as
a subclass of planets. This resolution was voted down in a vote of 186-183, in
a group with a membership of 10,000.
So dynamical
dominance, to which Margot gives such priority, has this status due to 186 people
out of 10,000, a difference of just three votes among the 424 IAU members
present at that time.
This belies Margot’s
claim that “A dispassionate analysis of the features of solar system bodies
yielded a distinct group of eight bodies that have been historically referred
to as planets.”
It isn’t a
dispassionate analysis that yields this. It is a vote of 186-183 on a
particular day in 2006, nine years before the New Horizons Pluto flyby.
Margot’s paper
doesn’t miss the chance to push the fallacy that opposition to making gravitational
dominance a criterion for planethood is based on emotion. In a highly
patronizing and demeaning statement, it says, “Readers who are chagrined that
smaller bodies are not recognized as planets should take comfort in the fact
that these bodies are no less worthy of exploration.”
Readers and others
who oppose keeping dynamical dominance as a criterion for planethood are not “chagrined.”
We dissent and do so because we reject this premise and prefer the geophysical definition,
which, while not discounting an object’s location, does not put primacy on that
as a requirement for planethood.
In other words,
our objection is scientific, not emotional.
Margot’s proposal
was not put to a vote at this year’s IAU General Assembly but that could be
done at its next one in 2027.
At the same time,
most planetary scientists believe the IAU is the wrong venue for planet
definition. Most IAU members are not planetary scientists but other types of
astronomers. It makes no sense that those who study galaxies, black holes,
cosmology, etc., should decide what a planet is. That decision should be made
by a different organization, a group composed of planetary scientists, and
should be done not in a vote but through evolution over time as more knowledge
about these worlds becomes available.
Eighteen years is
a long time for a bad definition to stick around, but at the same time, the
passage of time does not make that definition any more “official” or “legitimate.”
Those of us who oppose the IAU definition and recognize its harm to planetary
science and public education will continue to fight it and advocate for a better
definition. We will never give up, and we will never accept what we know is
wrong. And in the long term, I strongly believe, we will prevail.
