May 19, 2018 : Could We Put Pluto, Ganymede Or Titan Into The Category Of Planets ?

Today, there are officially eight planets in the Solar System. Prior to the major change of 2006 in the classification of planetary bodies, there were officially nine planets in the Solar System. The nine planets, from the Sun to the Outer Solar System, were Mercury, Venus, The Earth, Mars, Jupiter, Saturn, Uranus, Neptune and Pluto. Since the reclassification of planetary bodies performed by the International Astronomical Union (IAU) in 2006, Pluto is not part of the category of planets anymore. Yet, Pluto is quite big and is not the moon of another planetary body. Pluto is now part of the category of Dwarf Planets. From the discovery of Pluto by Clyde Tombaugh in 1930 to the revolution of 2006, Pluto had been considered a planet. Why is Pluto part of another category of planetary bodies ? Why aren't Pluto, Ganymede, Titan or Ceres planets ? To a certain extent, the current classification can be misleading for the general public since there are more than eight planetary bodies in the Solar System. Some moons like Io, Titan or Triton can be more interesting to study or explore than planets like Mercury, for instance.

Since the revolution of 2006, a celestial body must satisfy three criteria to be considered a planet. First, the planetary body must turn around the Sun and it must not turn around any other celestial body than the Sun. In other words, if the planetary body is a moon or a natural satellite, it is not a planet by definition. Second, the celestial body must be large enough to be round, spherical or spheroid or the celestial body must have enough gravity to be round, spherical or spheroid and the celestial body must have a gravity which is too low to make fusion reactions possible in its interior. In other words, the gravity of the celestial or planetary body must be high enough to make the planetary body round, spherical or spheroid but it must be lower than that of a star like the Sun or Proxima Centauri. Third, the celestial body or planetary body must have  cleared its neighborhood  of debris, asteroids, comets or planetary bodies. Pluto doesn't meet the third criterion since Pluto is part of a family of planetary bodies, asteroids or comets evolving beyond Neptune in a ring of icy bodies known as the Kuiper Belt. Pluto is in fact a Kuiper Belt Object or KBO like Charon, Makemake and Haumea. Further collisions or aggregation and accretion processes are needed in the Kuiper Belt to turn the future Pluto into a planet according to the new definition.

A fraction of the general public and of the scientific community criticizes the fact that Pluto is not considered a planet anymore since it seems to have all the ingredients of a typical planet. Pluto is part of a complex system of celestial bodies with a major moon known as Charon and with four other moons which look like asteroids or comets. The four other moons are Styx, Nix, Kerberos and Hydra. The center of gravity of the system is outside Pluto which is unusual in the Solar System. The distance between Pluto and Charon is particularly low compared to the distance between the Earth and our grey moon. If we analyze the new definition of a planet, we can conclude that exoplanets are not planets since they don't turn around the Sun. We have already found a multitude of exoplanets in our galaxy, the Milky Way. But have they cleared their neighborhood of asteroids, comets or planetary bodies ? Are they the only celestial bodies present in their orbital path ? The famous scientist Alan Stern who was the principal investigator of the New Horizons mission proposed by NASA is among the researchers who criticize the new definition of a planet and who regard Pluto as a planet. Everybody remembers the historic flyby of Pluto and Charon performed on July 14, 2015 by the New Horizons spacecraft.

Alan Stern considers that the requirement regarding the clearing of the neighborhood is ridiculous. Beyond the Gas Giant or Ice Giant Neptune, there are numerous planetary bodies which could be regarded as planets and planetologists are likely to discover new worlds in that enigmatic area of the Solar System. Quaoar, Haumea, Makemake, Sedna or Eris are among those worlds which have been discovered recently. For instance, Eris is the most massive planetary body identified beyond Neptune. Eris is a little smaller than Pluto but the planetary body is heavier than Pluto due to a higher mean density. In fact, some researchers believe that the new definition of planets is based on a nonscientific will to make the galaxy of planets of our Solar System manageable because we are likely to find many more planetary bodies of limited size beyond Neptune in the future. Today, we have eight planets in the Solar System according to the new definition and there may be a hidden planet known as Planet 9. The existence of that hypothetical planet has been deduced from orbital observations of several Trans-Neptunian Objects or TNOs.

The debate is far from being over and a new spark is coming with a new book of the specialist of Pluto Alan Stern and of the planetary scientist or exobiologist David Grinspoon regarding the Pluto flyby. The book of both planetary scientists is entitled  Chasing New Horizons: Inside the Epic First Mission to Pluto  (Picador, 2018). The authors also published a  Perspectives  piece entitled  Yes, Pluto Is a Planet  in the newspaper The Washington Post on May 7, 2018. The researchers consider the new definition imprecise and they criticize in particular the  clearing your neighborhood  criterion. The planetologists pointed out :  This criterion is imprecise and leaves many borderline cases, but what's worse is that they chose a definition that discounts the actual physical properties of a potential planet, electing instead to define 'planet' in terms of the other objects that are or are not orbiting nearby.  The definition may imply that the early Earth was not a planet since it evolved in an ocean of debris, dust, gases, asteroids or comets. Alan Stern and David Grinspoon added :  This leads to many bizarre and absurd conclusions. For example, it would mean that Earth was not a planet for its first 500 million years of history, because it orbited among a swarm of debris until that time, and also that if you took Earth today and moved it somewhere else, say out to the asteroid belt, it would cease being a planet. 

Alan Stern and David Grinspoon champion a better definition of planets which is easier to understand. The researchers regard the  geophysical planet definition  as a better definition since it is particularly simple and easy to understand for the general public. This definition which was unveiled last spring at a planetary science conference in Texas points out that planets are  round objects in space that are smaller than stars . Obviously, one can wonder what is the threshold from which a celestial body is round and what is the boundary between the planet and the star. With the geophysical definition of a planet, the number of planets in the Solar System would be largely higher than the current number of official planets which is only eight. In fact, it would easily top 100 planets since the definition incorporates all the celestial bodies which appear round. That's the case for Enceladus. That's also the case for moons like The Moon, Io, Europa, Ganymede, Callisto, Mimas, Tethys, Dione, Rhea, Titan, Iapetus, Titania, Oberon, Triton or Charon. The Dwarf Planet Ceres, located in the Asteroid Belt between Mars and Jupiter as well as the Dwarf Planet or Scattered Disk Object Eris would also be considered planets with the simple geophysical definition.

The struggle for a new definition of planets and for a new classification of celestial bodies or planetary bodies may last for a long time since the new definition is based on the updated wealth of data regarding our Solar System. The astrophysicist and author Ethan Siegel argued recently in a piece for Forbes on May 8, 2018 that we have to correctly analyze the environmental context of a celestial body in order to understand the nature of the celestial body or planetary body. He pointed out :  The simple fact is that Pluto was misclassified when it was first discovered; it was never on the same footing as the other eight worlds. The 2006 move by the IAU was an incomplete attempt to repair that mistake. Let's note that when Pluto was discovered, nobody knew the exact diameter of Pluto. Some specialists had advanced that its size was of the same order as that of the Earth. He argued that the geophysical definition  is a step in the opposite direction: It's a step towards making a larger, more confusing mistake that will render a definition meaningless to the majority of people who use it.  Many of us don't know a lot of things about astronomy and a lot of people are not aware that there are more than eight interesting worlds to study or explore in our Solar System.

The famous astronomer Mike Brown, from the California Institute of Technology, is a key player who led to the new classification of planetary bodies in our Solar System in 2006. He had discovered several planetary bodies beyond Neptune and in particular Eris, a bright planetary body which appears to be more massive than Pluto. He pointed out via Twitter :  So, hey, Pluto is still not a planet. Actually, never was. We just misunderstood it for 50 years. Now, we know better. Nostalgia for Pluto is really not a very good planet argument, but that's basically all there is. Now, let's get on with reality.  Mike Brown who also advanced the Planet Nine Hypothesis had written a book regarding the revolution in the classification of planetary bodies entitled  How I Killed Pluto and Why It Had It Coming  (Spiegel & Grau, 2010). Even if Pluto is not a planet by definition today, the planetary body or Dwarf Planet appears to be one of the most captivating worlds explored in the Solar System. The general public tends to be more interested in Pluto than in a planet like Mercury. Some moons like Io, Europa, Titan or Triton also draw the attention of researchers and the general public. We also find active worlds far from the Sun beyond Earth.

The image above reveals the Earth, Ganymede, Titan and Pluto at scale. Ganymede, which is the largest moon of Jupiter, and Titan, which is the largest moon of Saturn, are the biggest moons in the Solar System. Pluto is the largest Dwarf Planet identified in the Solar System and the second most massive Dwarf Planet identified in the Solar System behind Eris. Pluto used to be considered a planet until the famous discovery of Mike Brown in 2005. Could we regard those planetary bodies as planets like the Earth, Mercury or Mars, due to their significant size ?

Credit for the original view of the Earth: DSCOVR.
Source of the original view of Ganymede: Wikipedia.
Source of the original view of Titan: Wikipedia.
Source of the original view of Pluto: Wikipedia.
Montage Credit: Marc Lafferre, 2018.

- To get further information on that news, go to:



Back to Main Page