Hear Ye, Hear Ye! Carolyn Porco and Scott Tremaine are Wrong about Pluto!

In a Facebook announcement dated February 14, 2023, planetary scientist Carolyn Porco takes on the role of planetary town crier, stating, “Hear Ye, Hear Ye! Scott Tremaine, renowned celestial dynamicist & a founder (along w/ my thesis advisor Peter Goldreich) of the field of planetary rings, has written a textbook on The Dynamics of Planetary Systems. At my request of some time ago, he included in his book a discussion of the definition of a planet...And what did he find? Pluto is NOT a planet !! Reason has prevailed. Rejoice!

Porco is admittedly an accomplished scientist best known for her work on NASA’s Cassini mission to Saturn. But in this case, she is wrong.

Reason does NOT prevail when a person with an agenda specifically requests that a fellow scientist push that agenda in his book while refusing to acknowledge the ongoing debate and both sides in that debate.

Porco’s position on the planet definition issue is actually more extreme than that of the IAU. She opposes the use of the term dwarf planet because she claims Pluto and Ceres are both asteroids! She states in the comments, “Actually, I don't ever use, and won't, the name 'dwarf planet'. It's inconsistent. How can something be a dwarf member of a category it doesn't belong to?”

Regardless of their own position on this issue, most scientists acknowledge the ongoing debate between dynamicists and geophysicists over the question of defining a planet. Most show respect for the opposing position even if they don’t agree with it.

In my writings and public presentations, I always make the effort to present both views. While I am clear on the view I hold, I believe it would be a disservice to readers and/or audience members to pretend my view is the only one and that all others have been discredited when this is not the case. I would rather members of the public hear both sides of this issue and then make up their own minds than feed them my viewpoint alone.

Porco goes on in subsequent comments to denigrate the geophysical planet definition by claiming roundness means nothing, adding, that the term dwarf planet “actually has no information in it besides 'sort of roundish'. On the other hand, ‘Pluto is a large Kuiper Belt Object’ tells you composition, location, relative size, approximate level of solar illumination, etc. Far more useful.”

To her, there is apparently no difference between tiny, shapeless asteroids and KBOs and objects large enough and massive enough to be rounded by their own gravity, in spite of the fact that the latter have complex processes not seen on asteroids and in some cases, seen elsewhere in the solar system only on Earth and Mars.

Just calling Pluto a large Kuiper Belt Object tells us nothing about its structure, its geology, its atmosphere, the interaction between its atmosphere and surface, its many varied terrains, its cryovolcanoes, its likely subsurface ocean, and its interactions with companion Charon as the solar system’s only binary planet system.

Pluto actually has far more in common with some of the larger, spherical moons in the solar system, considered by many planetary scientists to be secondary or satellite planets, than it does with tiny, shapeless Kuiper Belt Objects like Arrokoth. Today, planets such as Ceres; Jupiter’s moons Europa, Callisto, and Ganymede; Saturn’s moons Enceladus and Titan; Neptune’s moon Triton, and Pluto are the solar system’s top contenders for hosting microbial life due to the growing evidence that they harbor subsurface oceans.

Porco insists that gravitational dominance alone determines what is a planet without ever addressing the fact that this dominance depends on location and can lead to the same object being considered a planet in one location and not a planet in another one. If Earth were in Pluto’s location, it would not gravitationally dominate or clear that orbit.

Furthermore, she states, “But Mercury, Venus, Earth and Mars dominate their orbital corridors. If they hadn't, you wouldn't see them, you'd see a cloud of debris.

Does Mercury actually dominate its orbit, or does the Sun clear out debris there? The answer is unclear. Furthermore, there is no cloud of debris around Pluto. If there were, New Horizons would have had no trouble finding a second and even a third flyby target. The reality is the Kuiper Belt is vast, and most KBOs are nowhere near Pluto but much further out and very scattered.

She goes on to say, “'Round' is a perfectly useless criterion. It doesn't work for those distant bodies for which we don't, and won't for a very long time to come, if at all, have shape information for. So it fails as a metric for categorization...I mention ‘spherical’ but merely because it’s a side effect of (or proxy for) planetary properties and processes I am interested in with categorization in mind. E.g: a degree of planetary differentiation.”

But another commenter points out that gravitational dominance is much more difficult to detect at great distances than is roundness, and that time and the technological advancements that go with it will eventually answer these questions for distant objects, both in our solar system and others.

Additionally, planetary differentiation is very much related to roundness, as she admits in the sentence I bolded above. Active geology begins happening when objects reach the threshold of being rounded by their own gravity. Categorizing Ceres and Pluto as asteroids is bad science because it blurs the distinction between complex objects shaped by their own gravity and tiny ones that are often little more than rubble piles, held together only by their chemical bonds.

One commenter even repeats the false claim that Mike Brown discovered dwarf planets larger than Pluto despite the fact that no such dwarf planets have been found. Eris was initially thought to be larger than Pluto, but in November 2010, a team of astronomers led by Bruno Sicardy observed it occult a star and found it to be marginally smaller than Pluto though slightly more massive.

The persistence of this misconception after more than a decade is an example of why the IAU definition has harmed science by widely spreading confusion and incorrect information.

Unfortunately, the tone of the discussion in the comments takes a turn for the worse when Porco resorts to personal attacks against New Horizons Principal Investigator Alan Stern and other supporters of the geophysical planet definition, demeaning them by calling them “Pluto fanatics.”

Here are some examples of her inappropriate personal attacks:

“The Pluto fanatics were desperate to get a Pluto mission, and it was deemed of vital importance to maintain Pluto as a planet, so they would have more justification. They were practicing politics, not science.”

This is blatantly false. New Horizons was already approved and had already launched when the IAU vote took place. Most New Horizons scientists view Pluto as a planet because they favor the geophysical definition over the dynamical one. It is a purely scientific disagreement.

As New Horizons planetary scientist Cathy Olkin noted, "I naturally refer to Pluto as a planet because that seems like the right moniker. It has an atmosphere; it has interesting geology; it orbits the sun; it has moons. 'Planet' just seems right to me."

Porco went on to comment, “Stern has admitted he wanted Pluto to be a planet because he was afraid New Horizons wouldn’t be chosen if it wasn’t. He DOES appeal to emotions. You want to compare our respective domains of expertise? Stern did a mission to Pluto. I ran an experiment and published papers that required knowledge on my part in atmospheric meteorology, the kinematics and dynamics of planetary rings including faint rings of tiny micron-sized particles, the geology, geophysics and geodesy of planetary satellites, the tidal interactions between planetary bodies, the geysering eruptions of Enceladus, and a lot more. So, a body like Pluto is NOT outside my field.”

Again, her first statement is false. And while Porco did accomplish everything she states, Stern has also published numerous papers on many of these same subjects and is the world’s leading Pluto scholar. And his advocacy and leadership of the New Horizons mission essentially unveiled Pluto to the entire world.

Porco should feel secure enough in her own accomplishments to not belittle another scientist who clearly made extensive contributions to planetary science just because he disagrees with her on planet definition.

In one remark, she responded to a commenter by saying, “You are clearly an ignoramus and should keep your mouth closed.”

She even goes as far as comparing opponents of the IAU definition to climate deniers!

Ad hominem, or personal attacks, are a sign that someone is losing the debate.

Significantly, Porco blocks me on Twitter, and I could not comment on her Facebook page because it only allows those she permits to comment.

But I will not keep my mouth closed. I will not be silenced, and neither will advocates of the geophysical planet definition.

If someone has to resort to censorship to force their view on the world and disallow any comments that disagree with their view, they are NOT causing reason to prevail. They are enforcing tyranny. And that is anything but a reason to “rejoice.”