My Account

New to Astromart?

Register an account...

Need Help?

If you have any questions or experiencing an issue please report to

Yes Virginia, Pluto Really is a Planet

Posted by Guy Pirro   09/12/2018 12:33AM

Yes Virginia, Pluto Really is a Planet


Is Pluto a Planet? In 2006, only 474 members of the International Astronomical Union's 10,000 members (at that time) took part in the vote that stripped Pluto of its status as a planet. In the ensuing years, the other 95% of astronomers have been speaking out and the consensus seems to be that the new definition of a planet is confusing. A number of prominent astronomers have described the decision to strip Pluto of its planetary status as a “muddled” ruling that is unlikely to settle ongoing debates over how to define a planet. Now, a recent study by the University of Central Florida concludes that the reason Pluto lost its status as a planet is not valid and that a proper definition of a planet should be based on its intrinsic properties rather than ones that can change, such as the dynamics of a planet’s orbit -- the rationale that was used by the IAU to strip Pluto of its planetary status. (Credit: Robert H. Wells, University of Central Florida) (Image Credit: NASA)



Yes Virginia, Pluto Really is a Planet

What is a planet? For generations the answer was easy -- A big ball of rock or gas that orbited the Sun. And there were nine of them in our Solar System. But then astronomers started finding more Pluto-sized objects orbiting beyond Neptune. Then they found Jupiter-sized objects circling distant stars -- First by the handful and then by the hundreds. Suddenly the answer wasn't so easy. Were all these newly found things planets?

The International Astronomical Union (IAU), the organization that is in charge of naming newly discovered worlds, tackled the question at their 2006 meeting. They tried to come up with a definition of a planet that everyone could agree on. But the astronomers couldn't agree, so they voted and picked a definition that they thought would work. The results have been mixed. In the end, the IAU did accomplish one thing -- They figured out a way to turn something simple into something complex.

The current, official IAU definition says that a planet is a celestial body that:

Is in orbit around the Sun

Is round or nearly round, and

Has "cleared the neighborhood" around its orbit (Whatever that means).

But this definition baffled students in classrooms and the public in general. For one thing, it only applied to planets in our Solar System. What about all those Exo-planets orbiting other stars? They don't orbit the Sun, so are they planets?

And Pluto was booted from the planet club and called a dwarf planet. Is a dwarf planet a small planet? Not according to the IAU. Even though a dwarf fruit tree is still a small fruit tree, and a dwarf hamster is still a small hamster, a dwarf planet is not just a little planet. In fact, according to the IAU it is not a planet at all.

The reason Pluto lost its status as a planet is not valid, according to a recent study led by planetary scientist Philip Metzger of the University of Central Florida (UCF).

In 2006, the International Astronomical Union established a definition of a planet that required it to “clear” its orbit, or in other words, be the largest gravitational force in its orbit.

Since Neptune’s gravity influences its neighboring planet Pluto, and Pluto shares its orbit with frozen gases and objects in the Kuiper belt, that meant Pluto could no longer sustain its planetary status.

However, in a new study, UCF planetary scientist Philip Metzger, who is with the university’s Florida Space Institute, reported that this standard for classifying planets is not supported in the research literature.

Metzger, who is lead author on the study, reviewed scientific literature from the past 200 years and found only one publication – from 1802 – that used the clearing-orbit requirement to classify planets, and it was based on since-disproven reasoning.

He said moons such as Saturn’s Titan and Jupiter’s Europa have been routinely called planets by planetary scientists since the time of Galileo.

“The IAU definition would say that the fundamental object of planetary science, the planet, is supposed to be a defined on the basis of a concept that nobody uses in their research,” Metzger says. “And it would leave out the second-most complex, interesting planet in our solar system.”

“We now have a list of well over 100 recent examples of planetary scientists using the word planet in a way that violates the IAU definition, but they are doing it because it’s functionally useful,” he says.

“It’s a sloppy definition,” Metzger says of the IAU’s definition. “They didn’t say what they meant by clearing their orbit. If you take that literally, then there are no planets, because no planet clears its orbit.”

The planetary scientist says that the literature review showed that the real division between planets and other celestial bodies, such as asteroids, occurred in the early 1950s when Gerard Kuiper published a paper that made the distinction based on how they were formed.

However, even this reason is no longer considered a factor that determines if a celestial body is a planet, Metzger says.

Study co-author Kirby Runyon, with Johns Hopkins University Applied Physics Laboratory in Laurel, Maryland, says the IAU’s definition was erroneous since the literature review showed that clearing orbit is not a standard that is used for distinguishing asteroids from planets, as the IAU claimed when crafting the 2006 definition of planets.

“We showed that this is a false historical claim,” Runyon says. “It is therefore fallacious to apply the same reasoning to Pluto.”

Metzger says that the definition of a planet should be based on its intrinsic properties, rather than ones that can change, such as the dynamics of a planet’s orbit.

“Dynamics are not constant, they are constantly changing,” Metzger says. “So, they are not the fundamental description of a body, they are just the occupation of a body at a current era.”

Instead, Metzger recommends classifying a planet based on if it is large enough that its gravity allows it to become spherical in shape.

“And that’s not just an arbitrary definition,” Metzger says. “It turns out this is an important milestone in the evolution of a planetary body, because apparently when it happens, it initiates active geology in the body.”

Pluto, for instance, has an underground ocean, a multilayer atmosphere, organic compounds, evidence of ancient lakes and multiple moons, he says.

“It’s more dynamic and alive than Mars,” Metzger says. “The only planet that has more complex geology is the Earth.”

Co-authors on the research included Mark Sykes (of the Planetary Science Institute), Alan Stern (of the Southwest Research Institute), and Kirby Runyon (of Johns Hopkins University Applied Physics Laboratory). Before joining UCF, Metzger worked at NASA’s Kennedy Space Center from 1985 to 2014.






Pluto: The Other 95% of IAU Astronomers Speak Out

In 2006, only 474 members of the International Astronomical Union's 10,000 members (at that time) took part in the vote that stripped Pluto of its status as a planet. In the ensuing years, the other 95% of astronomers have been speaking out and the consensus seems to be that the new definition of a planet is confusing-- especially the requirement that a planet must “clear its immediate neighborhood.” If taken literally, under that definition Neptune is at risk of losing its status as a planet too.

Several Johns Hopkins University astronomers described a decision to strip Pluto of its planetary status as a “muddled” ruling that is unlikely to settle ongoing debates over how to define a planet and whether the term should apply to Pluto.

Their reactions came after the vote by the International Astronomical Union in 2006 at their meeting in Prague that defined a planet as “a celestial body that is in orbit around the sun, has sufficient mass for its self-gravity to overcome rigid body forces so that it assumes a … nearly round shape, and has cleared the neighborhood around its orbit.” Because Pluto does not meet the last criterion, the IAU demoted it to “dwarf planet” status.

The decision left the solar system with only eight planets. Since its discovery in 1930, Pluto had been considered the system’s ninth planet.

In 2008, top scientists and educators convened at a Johns Hopkins University conference to explore the basic, but controversial, question: What is a planet?

Dr. Mark Sykes, a conference organizer and director of the Planetary Science Institute in Tucson, Arizona, said at the time, “The time is ripe to hold a scientific conference to examine how planets form and evolve, both within our solar system and around other stars, as well as their physical characteristics. This event provides researchers with a unique opportunity to examine all sides of this issue and talk about it, face to face. They can also sit down with educators to discuss how to broach the planet definition controversy in the classroom.”

“No votes will be taken at this conference to put specific objects in or out of the family of planets,” said APL’s Dr. Hal Weaver, a conference organizer. “But we will have advocates of the IAU definition and proponents of alternative definitions presenting their cases.”

“This topic provides the perfect opportunity to teach science as a process, not a collection of facts,” says organizing committee member Dr. Keith Noll of the Space Telescope Science Institute in Baltimore. “We also need to stress the importance of incorporating new discoveries to continually improve our understanding of the diverse objects within planetary systems.”

"We're going to do something that the IAU did not, which is discuss what we know about planetary bodies in the solar system and around other stars, and discuss the value of different ways of defining objects as planets and what that means," said Mark V. Sykes.

"If a new consensus emerges it will easily overturn the IAU. This is not an issue," said Neil deGrasse Tyson, "If not, they'll stick with what they've got until something better comes along."

At the time of the IAU decision, Andy Cheng, Johns Hopkins University Applied Physics Laboratory stated “I think the IAU vote is a muddled compromise that will not settle the question of ‘What is a planet?’ Pluto is not a ‘planet’ according to resolution 1, but it is a ‘dwarf planet’ by resolution 2. So is it a ‘planet?’ I thought so before and still think so now -- but those who did not think so before can now point to the IAU definition and say that Pluto is really not a planet but a sort of second-class citizen. “Actually, that is the same situation that has prevailed with Ceres, other asteroids, and comets for many years (centuries in the case of some of these objects). Those objects were known as minor planets before, but now a few of the minor planets have been promoted to ‘dwarf planets.’ “So I suppose I should be happy that Pluto wasn't demoted all the way into the minor planet category.”

Harold (Hal) Weaver, Johns Hopkins University Applied Physics Laboratory: “I don't expect the hoopla over the demotion of Pluto from the realm of ‘classical planets’ to have any effect on the conduct of the New Horizons mission. The scientific investigation of Pluto remains an important component of our effort to understand the processes that shaped the outer solar system, even if some of the objects in that region defy our efforts to categorize them. The New Horizons mission remains as viable as ever because it will provide the initial reconnaissance of one of the solar system's newly discovered frontiers. “Regarding the resolution itself, I'm with Andy Cheng in concluding that the situation is still somewhat muddled. What exactly is meant by a planet ‘clearing its neighborhood?' Since many ‘plutinos’ … (including Pluto) … cross Neptune’s orbit, I'd say Neptune's neighborhood still needs some clearing! … It just seems a bit risky to me to base a definition on a theoretical construct (‘dynamically cleared regions’) that's only approximate at best and may change significantly as our understanding of planet formation evolves over time. “I further note that there have been particularly large swings in the theories of outer solar system dynamical evolution during the past decade. What was ‘conventional wisdom’ five years ago has been replaced with the latest fad, and I don't expect that situation to change any time soon."

Karl Glazebrook, professor in the Department of Physics and Astronomy at Johns Hopkins added “What is meant by ‘clearing its orbit?’ How does this relate to having an orbit overlapping Neptune? Clearly Neptune has not cleared its orbit. “They should have gone with something clean like a size criterion. Seems to me like a muddled compromise which will just cause more problems and the issue will have to be revisited again.”

William P. Blair, research professor in the Department of Physics and Astronomy and chief of observatory operations for NASA's Far Ultraviolet Spectroscopic Explorer Satellite, operated by JHU: “I think the demotion of Pluto into the realm of other minor objects outside the orbit of Neptune is the most consistent thing to do to straighten out the nomenclature of our solar system. However, I don't find the wording of the official planet definition to be very clear, and hence it will continue to be open to interpretation. “I find it comforting to know, though, that Pluto hasn’t changed just because of our nomenclature. It is the same today as it was yesterday, and as it has been for thousands of years. It is still the most accessible of the objects beyond Neptune that we can study, and studying it should reveal much new information about the outer solar system.”

Richard Conn Henry, professor in the Department of Physics and Astronomy: “I am delighted that rationality has prevailed! Keep in mind that our own Sun is a dwarf star ... and Pluto is now a dwarf planet! Pluto is an extremely interesting and important object, and I am overjoyed that NASA's New Horizons mission is on its way to Pluto! Hurrah for Pluto, first dwarf planet to be visited by a NASA mission!”






In 2014, the Harvard-Smithsonian Center for Astrophysics (CfA) also decided to revisit the question of "what is a planet?" The CfA hosted a debate among three leading experts in planetary science, each of whom presented their case as to what a planet is or isn't. The goal was to find a definition that the eager public audience could agree on.

Science historian Dr. Owen Gingerich, who chaired the IAU planet definition committee, presented the historical viewpoint. Dr. Gareth Williams, associate director of the Minor Planet Center, presented the IAU's viewpoint. And Dr. Dimitar Sasselov, director of the Harvard Origins of Life Initiative, presented the Exo-planet scientist's viewpoint.

Gingerich argued that "a planet is a culturally defined word that changes over time," and that Pluto is a planet. Williams defended the IAU definition, which declares that Pluto is not a planet. And Sasselov defined a planet as "the smallest spherical lump of matter that formed around stars or stellar remnants," which means Pluto is a planet.

After these experts made their best case, the audience got to vote on what a planet is or isn't and whether Pluto was in or out. The results are in.

According to the audience, Sasselov's definition won the day, and Pluto is a planet.

Pluto may be a little planet, but it is a planet nonetheless.






Is Jupiter Really a Star? It Depends on How One Defines a Star

So now the IAU has informed us that poor little Pluto is really not a planet after all. I guess in the back of our minds we always knew that there was something just a little peculiar about Pluto. The "truth" has finally come out and our suspicions have been justified. Thanks to the deliberations of the IAU we now know that Pluto has been an imposter all along.

But how about Jupiter, that other strange planet at the other extreme? Is it really a planet? Maybe not. Instead, could it really be a very small star or possibly a brown dwarf?

If we can change the definition of a planet, then maybe we should revisit the definition of a star too. It turns out that Jupiter shares more similarities with the Sun than with the other planets. So, is Jupiter really a star? It depends on how we define a star. Let's look at the evidence.

Jupiter is a gaseous ball with no surface. The composition of Jupiter is very much like that of the Sun.

The Sun is 78 percent hydrogen and 20 percent helium. Jupiter is 89 percent hydrogen and 11 percent helium.

Jupiter radiates twice as much energy as it receives from the Sun. It is the only planet that puts out more energy than it takes in.

Jupiter has a very intense magnetic field like the Sun

At last count, Jupiter has a system of 79 moons -- It is kind of a miniature Solar System unto itself.

The Sun produces its energy through nuclear fusion. Jupiter does not. Well, five out of six is not bad.

So why isn't Jupiter a star? Well, maybe it is and we just don't know it yet.






Is It Time for the IAU to Define a Moon too?

Scientists have recently discovered that the planet Jupiter is turning 79… And Saturn is turning 62 -- not in years, but in moons. Actually, the number for Saturn is over 150 if you count “moonlets.” (There you go -- Another term for the IAU to define).

"When the Cassini mission launched back in 1997, we knew of only 18 moons orbiting Saturn," said Carl Murray, a Cassini imaging team scientist from Queen Mary University of London. "Now, between Earth-based telescopes and Cassini we have more than tripled that number -- and each and every new discovery adds another piece to the puzzle and becomes another new world to explore."





It’s Time for the IAU to Get to Work on New Definitions

In 2019, the International Astronomical Union celebrates its 100th anniversary.

Here’s an idea: Celebrate the occasion by coming up with a plan to clearly and unambiguously define planets, stars, and moons… And this time involve all 13611 members in the final voting. Let’s see what the results are then. Certainly, this would be an interesting and immensely useful topic for discussion at the next get-together of the IAU Officers in Paris in January 2019.



For more information:




Astromart News Archives:


Do you enjoy reading these News Items?

Then click here and buy the Astromart staff a cup of coffee (and maybe even some donuts):



Flag Counter