Blue Dot 167 Apple Podcast: What is a Planet? December 13, 2019

In this episode, inspired by our good friend Alan Stern, the Principal Investigator for the New Horizons Spacecraft, we examine a question that has vexed astronomers, planetary scientists and 5th graders, for decades: " What exactly is a Planet ?" And more importantly, why should we care about definitions like this in science.

Queen guitarist Brian May weighs in on Pluto as a planet

Support for Pluto's planet status is growing! First NASA Administrator Jim Bridenstine, now astrophysicist and Queen guitarist Brian May join the Pluto-is-a-planet team, which already has the support of numerous planetary scientists as well as that of former NASA engineer Homer Hickam and the Star Trek's original Captain Kirk, William Shatner.


Reprinted from SpaceFlight Insider

Pluto 2

A comment by NASA Administrator Jim Bridenstine affirming he still considers Pluto to be a planet has drawn criticism from some media outlets that may be motivated more by politics than science. It is also a potential game changer in the decades-long planet debate.

On Friday, August 23, Bridenstine publicly stated his support for Pluto’s planet status while taking a tour of the University of Colorado at Boulder‘s Aerospace Engineering Sciences Building. A video of Bridenstine’s statement was posted on Twitter by Cory Rappenhagen, a meteorologist at Colorado’s 9News TV station.

“Just so you know, in my view, Pluto is a planet. You can write that the NASA Administrator declared Pluto a planet once again. I’m sticking by that; it’s the way I learned it, and I’m committed to it,” Bridenstine said.

Thirteen years ago, the International Astronomical Union (IAU) adopted a planet definition that excluded Pluto. The definition required planets orbit the Sun, be large enough to be rounded by their own gravity, and clear the neighborhood of their orbits.  Pluto does not meet the last condition because it orbits in the Kuiper Belt, a region of tiny icy bodies and was therefore labeled a dwarf planet.

The move was prompted by the recent discovery of several small objects in the Kuiper Belt, especially Eris, which is slightly smaller though more massive than Pluto.  Rather than class them all as planets, the IAU chose to place them in this separate category under the justification that the solar system cannot have too many planets because children will not be able to memorize all their names.

However, scientists who oppose the definition noted memorization is an archaic method of teaching the solar system and pointed out there is no scientific basis to the argument that the solar system cannot have too many planets.

The definition was controversial for many reasons, including the fact that it was enacted by just four percent of the IAU‘s membership, with many planetary scientists signing a petition rejecting.

Alan Stern of the University of Colorado at Boulder, principal investigator of NASA’s New Horizons mission to Pluto and longtime opponent of the IAU definition, emphasized no solar system planet fully clears its orbit of asteroids and comets. Jupiter orbits with a host of Trojan asteroids, and even Earth has asteroids in its orbital path.

Also problematic is the fact that the IAU definition gives primacy to an object’s location over its intrinsic properties. Some scientists noted that if Earth were in Pluto’s orbit, it would not clear that orbit of Kuiper Belt Objects. This means the IAU definition could result in the same object being labeled a planet in one location and not a planet in another location. It is also biased against objects in more distant orbits, which have larger orbits to “clear.”

And in spite of the discovery of numerous exoplanets orbiting stars other than the Sun, the IAU definition requires a planet to orbit the Sun rather than a star.

Ironically, the term “dwarf planet” was first coined by Stern in a 1991 paper as the proposed name for a class of small planets large enough to be rounded by their own gravity but not large enough to gravitationally dominate their orbits. In a 2000 paper, Stern and Hal Levison of the Southwest Research Institute in Boulder (SwRI) distinguished between planets that gravitationally dominate their orbits, which they called “uber planets,” and those that do not, which they called “unter planets.”

However, they never proposed that “unter planets” or dwarf planets not be considered planets at all, which runs counter to the use of the term “dwarf” in astronomy, as dwarf stars are still considered stars, and dwarf galaxies are still considered galaxies.

These papers became the basis of the geophysical planet definition crafted by Stern and favored by many planetary scientists. The geophysical definition rejects the notion of orbit clearing, instead requiring an object to orbit a star, not be a star itself, and be large enough to be rounded by its own gravity to attain planet status. Bridenstine’s declaration puts him firmly on the side of those advocating the geophysical definition,

In 2015, NASA’s New Horizons spacecraft flew by Pluto and found it to be far different from the dead rock many expected. Active geologically and possibly host to a subsurface ocean, Pluto has a layered atmosphere, varied terrains, interaction between its surface and atmosphere, and cryovolcanoes. Some processes seen on Pluto have been found elsewhere in the solar system only on Earth and Mars. Yet the spacecraft’s findings did not prompt the IAU to revisit the issue.

Bridenstine’s public rejection of the IAU definition generated controversy. On Tuesday, August 27, Business Insider‘s Tom Porter  questioned Bridenstine’s qualifications for the job of NASA Administrator in an article titled, “Trump’s NASA chief, who has no scientific background, says Pluto is a planet.”

Bridenstine was criticized after his nomination for the NASA administrator position in 2018 because he is not a professional scientist and was not chosen from within NASA ranks.  Some critics painted him as a climate change denier though Bridenstine has since publicly declared his acceptance of the fact that climate change is driven by human activity.

A former Congressman from Oklahoma, Bridenstine served as a Navy pilot and and previously ran the Air and Space Museum in Tulsa, Oklahoma. He is the primary mover of NASA’s Artemis program, which aims to return humans to the Moon in 2024. In contrast to the Apollo program, Artemis is the first step toward a long-term human presence on the Moon.

According to planetary scientist Will Grundy of the Lowell Observatory, who serves as a New Horizons co-investigator, Bridenstine’s view is very much in line with that of many planetary scientists.

“He’s just using the word the way planetary scientists use it, nothing unusual there,” Grundy said regarding Bridenstine.  “Sure, there was that controversial IAU vote back in 2006, but it’s important to realize that’s not how planetary scientists use the word, and there’s no reason for the public to use it that way, either.  Planetary scientists are interested in planets’ diverse compositions, structures, and the processes that operate on them.  Those things don’t go away or become uninteresting for a planet that’s in the wrong orbit according to the IAU definition.”

Porter’s article made no mention of the fact that Pluto’s status remains a matter of debate in the science community. A list of articles he recently wrote for Business Insider reveals he is a frequent critic of US President Donald Trump, raising the question of whether his article might be motivated more by politics than by science.

Britain’s Mirror chided Bridenstine for calling Pluto a planet 13 years after it was “declassified,” also without any acknowledgment of the ongoing debate among scientists.

Planetary scientist Phil Metzger of the University of Central Florida (UCF), who earlier this year published an article on the history of planetary taxonomy in the journal Earth and Planetary Astrophysics, commended Bridenstine for taking a stand in favor of the geophysical definition.

“I think Jim Bridenstine knows that the public’s thoughts about Pluto aren’t just sentimentality. People understand that a planet is a special thing in nature, the just-right type of place between cold lifeless asteroids and hot lifeless stars. Planets are where complex geology and possibly even life can exist. This is deeply important to science and deeply important to who we are as human beings. People look at Pluto and they see this flourishing of emergent complexity which makes planets important — more in Pluto than in any other planet besides Earth. I think Jim has tapped into the public’s instinct about this.”

While the IAU has shown no interest in revisiting the issue, the debate has continued for more than a decade. In 2017, planetary scientist Kirby Runyon of the Johns Hopkins University Applied Physics Laboratory (JHUAPL) presented the geophysical planet definition first proposed by Stern at that year’s Lunar and Planetary Sciences Conference as an alternative to the IAU definition.

At the heart of the planet debate is the question of who decides what a planet is and whose definition is deemed “official.” While much of the mainstream media has granted official status to the IAU, Stern and other scientists reject this along with the notion that science can be done by voting rather than through an organic process.

“The NASA Administrator is right in his views,” Runyon emphasized. “Professional planetary scientists also refer to Pluto and the 120+ similarly sized (solar system) planets as full planets.”

Thirteen Years Later, Pluto is Still A Planet

Pluto's Heart

At July’s “Pluto System After New Horizons” conference, held at the Johns Hopkins University Applied Physics Laboratory (JHUAPL) in Laurel, Maryland, professional papers presented by scientists drove home the fact that 13 years after the controversial IAU planet definition vote, more and more people view Pluto as a planet.

While many presenters used the term “planet” in reference to Pluto, that was hardly necessary, as the data spoke for itself—varied, complex surface terrains, the makeup of the layers that comprise Pluto’s atmosphere, and the origin of the solar system. I was fortunate to attend some of the sessions and hope to provide a more detailed discussion of each topic in future blog entries.

Meanwhile, Astronomy magazine’s September issue features Pluto, along with Jupiter’s moon Europa, Saturn’s moons Titan and Enceladus, Neptune’s moon Triton, and Mars, on the front cover as prime solar system locations that could host microbial life.

Over time, more and more people are viewing these images and reading articles like this one and coming to realize on their own that the planet definition adopted 13 years ago today by the IAU is highly flawed and blatantly wrong in considering dwarf planets to not be planets at all. While the IAU continues to refuse to revisit the issue, its definition is clearly losing ground.

Even media coverage of the debate is improving. While still problematic, an article published today by CNN at least makes an attempt to acknowledge that this is an ongoing debate with two legitimate sides, as is evident from its title, “Why Pluto is no longer a planet (or is it?).” Putting the question “is it” in parentheses diminishes the pro-Pluto side, implying just a small number of scientists reject the IAU definition, when this is far from the case. To really be fair, the title should give equal standing to both positions.

Several sections of the article are problematic in terms of words and phrases used. The writer continues to push the erroneous view that only the IAU has the right to determine what is and is not a planet. This can be seen from sentences such as "Pluto was relegated in 2006 when the International Astronomical Union (IAU) created a new definition for planets and decided Pluto did not fit the bill."

She goes on to say that August 24 has been dubbed “Pluto Demotion Day,” a term I have never heard used and one likely to come from Mike Brown, whom she erroneously describes as the sole discoverer of Eris. She may be repeating his statement without questioning it, something a good journalist should do.

The Brown-friendly wording continues with statements such as “The final blow came in 2003,” “And then there were eight,” and “Brown is dubbed the man who killed Pluto” complete with a link to his website. The truth is that Brown is not "dubbed the man who killed Pluto." He dubbed himself that; the sentence erroneously makes it sound like everyone else did this when that is not the case. He did this as a gimmick, most likely to sell books and pursue money and fame.

Just as important are the things she does not say. Nowhere does she acknowledge that that only four percent of the IAU voted on this; most were not planetary scientists but other types of astronomers, and the fact that an equal number of planetary scientists signed a petition rejecting the IAU decision within three days. She also fails to mention that Brown did not discover Eris alone but as part of a team of three. Significantly, the other two members of Eris's discovery team, Chad Trujillo and David Rabinowitz, reject the IAU definition—another fact the article never states. Neither does it ever note that Pluto-Charon is actually a binary planet system.

Additionally, the writer states that Pluto does not “clear its orbit” because its orbit overlaps with that of Neptune. This was not the reasoning used by the IAU. As pointed out by Margo Schulter on Facebook's Society of Unapologetic Pluto Huggers (SUPH) page, the IAU’s issue was that Pluto orbits with many Kuiper Belt Objects, including some small planets, in the Kuiper Belt. Whether these count as being in Pluto’s “neighborhood” is questionable, as they are scattered in vast expanses of space nowhere near Pluto.

The writer states that debate over Pluto’s planet status began several decades after the planet’s discovery, which is wrong. The debate actually started almost immediately after its discovery because telescopes in 1930 could not resolve Pluto into a disk. Some scientists thought it was a moon of a larger planet, but such a planet was never found.

The article never mentions that Alan Stern is the person who initially coined the term "dwarf planet" and that he intended it to refer to a third class of planets in addition to terrestrials and jovians. The four percent of the IAU who voted on this misused his term. Nor does it acknowledge the post-New Horizons Pluto conference held a month ago.

And contrary to the writer’s claim, the IAU decision did not prompt "a wave of science textbook reprints." It did prompt some, but those are problematic because doing this represents an unquestioned acceptance of science by dictate of "authority." Journalists should be able to recognize the inherent problem in this. The better textbooks teach the controversy and present both sides of this ongoing debate. They also have companion websites they can update regularly.

This is better than many previous articles but still has a long way to go. There should be much more discussion of New Horizons' findings, its discovery of planetary processes seen elsewhere in the solar system only on Earth and Mars, and its likely subsurface ocean. Instead, we get what appear to be attempts at conversational language, with statements such as “how the mighty have fallen,” “Pluto planetary days are remembered fondly,” and “it's still arguably the coolest (non) planet to learn about -- literally speaking.” That phrase “non-planet” should not be there, even in parentheses, as it amounts to favoring one side.

To the writer’s credit, she does quote Alan Stern about Pluto’s complexity and mysteries, acknowledge Phil Metzger’s study on the history of planet definition, and report on last year’s public debate held at the Harvard-Smithsonian Center for Astrophysics, where participants voted overwhelmingly in Pluto’s favor.

Unfortunately, the article does not have a comments section, which is a decision by CNN but one that makes it impossible for people to respond and have an open conversation of their own.

At last month’s conference, Kirby Runyon addressed the issue of pedgagogy of the solar system. He emphasized that whether done through the schools or the media, teaching the solar system has to change based not only on discoveries in our solar system, but also on the 4,000 exoplanets discovered in the last 25 years. Instead of asking children to memorize a list of names, a 19th-century method, 21st century teachers need to focus on the large, whole solar system, complete with inner, middle, and outer zones. Runyon plans to reach out to educators and textbook publishers and encourage them to focus on a large, diverse solar system with many planets, including spherical moons of giant planets, some of which could have subsurface oceans capable of harboring life.

Change is happening slowly, but it is happening, and in the direction of moving away from the limited IAU view of eight planets to the reality of many in our solar system alone. We need to keep reaching out to the media, correct their errors, and inform reporters of facts they likely never previously heard.

Spread the word—planet Pluto lives!

Pluto Lives