Opposing Pluto's Demotion Does Not Equal "Whining"

    The distant Planet Pluto

Adam Frank’s April 28 Forbes column, “Quit Whining: Pluto’s Not a Planet” is not just written in a very condescending, patronizing tone. It also includes outdated information and lacks important data based on which many planetary scientists consider Pluto and all dwarf planets to be a subclass of full planets.

Words matter, and the words Frank uses are unprofessional, disrespectful, and downright insulting to those who don’t share his view and support a geophysical planet definition over the IAU’s dynamical one.

Using terms like “wailing, “whining,” and “crying” to describe the reaction of those who oppose the IAU vote sets the wrong tone for the article and amounts to putting down those with whom he disagrees with defamatory language.

Frank invokes the strawman argument that accuses Pluto supporters of being motivated by emotion. This completely dismisses the genuine objections by some of the world’s leading planetary scientists to the IAU definition that excludes Pluto, adopted back in 2006.

Just four percent of the IAU voted on that controversial resolution, and most weren’t planetary scientists but other types of astronomers. Their decision was immediately opposed in a formal petition signed by an equal number of planetary scientists, led by New Horizons principal investigator Alan Stern.

Unfortunately, the media has barely covered this side of the ongoing debate.

Opposition to the IAU definition is centered on preference for a geophysical planet definition over a dynamical one. The geophysical definition puts primacy on a celestial object’s intrinsic properties; in contrast, the IAU definition, which is a dynamical definition, puts primacy on an object’s location.

Frank completely ignores the fact that the IAU vote was held in violation of the IAU’s own bylaws, which require any resolution put to the floor of the General Assembly to first be vetted by the appropriate IAU committee.

This was not done in 2006. The resolution adopted was thrown together the last day of the two-week conference, after most attendees, including the chair of the organization’s Planet Definition Committee, Owen Gingerich, had already gone home. No absentee voting was permitted.

In addition to objecting to a very flawed planet definition, opponents of the IAU decision also object to the problematic way in which the vote was conducted.

Alongside his article, Frank includes an image of the largest known Kuiper Belt Objects that is outdated by 15 years. That image includes a caption that says, “Pluto isn’t even the largest Kuiper Belt Object,” and portrays Eris, aka Xena, as bigger.

While Eris was initially thought to be bigger than Pluto, a team led by scientist Bruno Sicardy in 2010 measured Eris’s size when it occulted a star and determined it is marginally smaller than Pluto though 27 percent more massive. In 2015, New Horizons confirmed that Pluto is bigger.  Why is Frank making an argument using outdated information?

The biggest problem with Frank’s article is his blurring of the distinction between tiny, shapeless Kuiper Belt Objects and those large enough to be rounded by their own gravity. The latter threshold matters because this is when active geology begins.

Frank lumps all worlds beyond Neptune as “construction debris left over from the assembly of the solar system.” This fails to acknowledge the very important distinction between objects large enough to be rounded by their own gravity and the majority of tiny KBOs that are shaped only by their chemical bonds.

When celestial objects are rounded by their gravity, they begin to develop complex geology, which New Horizons found on Pluto. Most KBOs are tiny and shaped only by their chemical bonds. In contrast, Pluto is likely layered into core, mantle, and crust, and has weather, varied terrains, active geology, and a likely subsurface ocean. Frank never mentions any of this. In fact, he never says anything about Pluto’s intrinsic properties.

Pluto and Ceres are actually two of a growing number of solar system ocean worlds, worlds that have subsurface oceans that could potentially host microbial life. Other such worlds include Europa, Ganymede, Enceladus, Titan, and possibly Triton.

Pluto being smaller than Earth’s moon does not negate its planet status according to the geophysical definition, which considers dwarf planets to be a subclass of full planets, and does not require orbit clearing as a criterion for planethood. Any celestial object that meets the critical threshold of being rounded by its own gravity is a planet according to this definition.

This is very similar to the situation with stars, where variety in size and mass is even greater than it is among planets. The Sun is tiny compared to massive, giant stars. Red dwarf stars, whose planets may be a top target in the search for extraterrestrial life, are even smaller and less massive than the Sun. Yet no one claims they aren’t stars, just like no one claims dwarf stars are not stars, or dwarf galaxies are not galaxies.

By saying Pluto is not a planet but a Kuiper Belt Object, Frank presents an incomplete view of both Pluto and the solar system. There is no reason objects cannot be dually classed. Pluto is both a planet and a Kuiper Belt Object. The first tells us what it is; the second tells us where it is. Haumea, Makemake, and Eris are similarly dually classed.

The problem is not with the term dwarf planet but with the claim that dwarf planets aren’t planets at all but another type of object. This claim is contradicted by the Dawn mission’s findings at Ceres and the New Horizons mission’s findings at Pluto, both of which of which found these worlds to contain complex planetary processes.

Why should Earth and Jupiter be put in the same class but Pluto excluded? Earth has more in common with Pluto than it does with Jupiter. Jupiter has no solid surface, is composed of mostly hydrogen and helium like the Sun, and has its own mini-solar system of moons. In contrast, both Earth and Pluto have solid surfaces, active geology, nitrogen atmospheres, and interaction between those atmospheres and their surfaces.

The scientist who first coined the term dwarf planet, Alan Stern, intended it to designate a new subclass of small planets, not to designate a class of non-planets. The four percent of the IAU who voted in 2006 misused his term, stating in a vote of 186-183 that dwarf planets are not planets. They never revisited the issue in spite of the Dawn and New Horizons findings.

Yes, letters from children may be emotional, but as someone who has written and spoken publicly on this issue, I believe they reflect an inherent recognition of Pluto’s nature. Children look at its roundness and complexity and see a planet. They are not wrong.

Frank’s support of a dynamical planet definition is a legitimate scientific position. However, his mischaracterization of those who oppose it and why they oppose it, as well as his denial of the ongoing debate, detract from the credibility of his article.

Opposition to the IAU definition is a scientifically legitimate position, not “whining.” It will continue until a better definition is reached. If NASA Administrator Jared Isaacman wants to lend his support to this view, more power to him!

After all, it is NASA, through the New Horizons mission, not the IAU, to whom we owe practically all the knowledge we have about the Pluto system.