June 6, 2020

Are you a Planet? A humble proposal.

Here’s a fun proposal. Next time you find yoursef bored, go into the science department of your local university and just casually pose the question “gee, maybe Pluto should/shouldn’t be a planet.” Then stand back and watch the slide rules fly (yes real science geeks still pack slide rules, for when the apocalyspe comes!)

Sooner or later, the International Astronomical Union (IAU) will have to face a cold reality; the universe is a harsh mistress, and frequently denies us neat pigeon holing. This isn’t your father’s solar system; clearly some categories that are derived from some real physical perimeters are warranted. After all, the rest of science operates this way; it’s kind of embarrassing that such “soft” sciences such as psychology and biology can come up with meaningful categories for zoological phyla or psychiatric disorders, but a room full of astronomers can’t seem to top “round thingy” or “an object must clear its orbit!” It’s only a matter of time before new discoveries further complicate things. A brave new solar system is now upon us!

But first, some astro-background. The term planet comes from the Greek planos, which means wanderer. To the ancient eye, certain objects were seen to move against the starry background over the course of days and weeks; under this simple definition, the sun and moon were considered planets, but the earth was not!

This definition doesn’t help us with the modern situation very much; your inebriated brother-in-law who occasionally wanders upon your door step may fit the bill. But this vague definition survived almost right up to the present day, only being tweaked twice before the 20th Century. When a heliocentric, or sun centered universe, came into vogue during the European Renaissance, the term planet came to mean an object wandering about the sun; hence, the earth joined the club. The sun and moon were out. The second hint that something may be amiss came in centuries that followed as the club swelled to unacceptable size. Apparently, there is an unwritten law that states that the Planetary Club must be a select one! Anyway, in 1800 on New Years’ Day, astronomer Giuseppe Piazzi discovered Ceres; the subsequent discoveries of Pallas, Vesta, and Juno soon followed. As the ranks quickly swelled, it was realized something needed to be done with the tiny, wandering, star like bodies, orbiting mostly between Jupiter and Mars. They were termed planetesimals, or asteroids, from the Greek asteroeid?s which means “star-like”, a pretty good description of how they appear at the eyepiece. Hence the technical translation of Astroguyz, which means “star-like -guyz,” complete with the deliberate cute misspelling! In any event, Astronomers toasted their cleverness as the exclusivity of their club was maintained, or at least they wouldn’t have to deal with it in their lifetimes! Also, with the coining of the term asteroid, they knew that they could market a cool video game and hence one day profoundly change the face of nerd-dom.

But nagging problems persisted. Uranus and Neptune were accepted as post telescopic planets without question; now finding new planets became the sexiest thing in observational astronomy. The illusive Vulcan crept temporarily on the list in the late 19th century; likewise, Pluto also followed suit and was eagerly embraced when discovered in 1930.

But it wasn’t long before problems became evident with Pluto’s qualifications for planet-hood.  It had a see-saw ride throughout the late 20th century; first, its orbit was calculated to be highly inclined and eccentric; very un-planet like. Then in 1978 a moon, Charon was discovered, a very planet-like thing to have. However, said moon also enabled astronomers to calculate Pluto’s mass to a higher degree of accuracy; at 0.0021 earth masses, it was found wanting. The Pluto and Charon system was also realized to have a close primary to mass ratio; at 9 to 1, it’s the highest in the solar system. At 81 to 1, the Earth-Moon is only a distant runner up! Calls went out to classify Pluto-Charon as a binary or double planet.

Then in 1993 the first binary asteroid, Ida and Dactyl was discovered. Clearly, asteroids could have moons as well.  Another nail in the “Pluto-isn’t-a-Planet” coffin. In the closing years of the 20th century the debate heated up slightly; a tenuous atmosphere of methane ice was discovered; the “Pluto-Planet-hood mafia,” gained slight ground. It was also realized that it may be now or never NASA to send a mission to explore the last world to be photographed up close. If the deed was put off much longer, the atmosphere may have refrozen to the surface as Pluto recedes from the sun. And on a 248 year orbit, that wait may be long indeed! Hence, New Horizons was launched in 2006. We should finally have Pluto pics to add to the Solar System family portrait and our desktop backgrounds in 2015. Be sure to check this space for all of the exclusives…. Hopefully we’ll all be neuro-blogging in our sleep by then!

Then, after the turn of the millennium (the REAL turn, in 2001!), all hell broke loose. Not Y2K, in which the computers were scheduled to rise up and enslave us, but the discovery of a “trans-Neptunian” rather large entity. In 2003, astronomers spotted a distant object and innocuously named it UB 313 . As many scientists are fans of large breasted, dominating super-heroines, the name Xena was proposed.

(Incidentally, I’ve researched this; there is no classical mythological reference, not a single obscure Amazon princess, for the TV swashbuckler! Apparently, she is a modern construct. That kinda shot down her prospects of getting by the curmudgeonly IAU! Ah, but alas, Callisto, a moon of Jupiter, is real!)  An examination of its orbit suggested that it was over twice as distant as Pluto. Therefore, its brightness hinted that it most also be slightly more massive. Soon, a tiny moon in was discovered in orbit around Xena, and aptly named Gabrielle. This seemed to clinch it; we had a new planet on our hands.

Soon Academia was torn asunder. IM’s flew around the globe; RSS readers crashed operating systems as camps were drawn. You either talked dreamily of the plurality of worlds, or stated “its nine planets… end of story!” the way (insert higher power) meant it to be. Finding yourself on the wrong side of the debate could mean big trouble. Meanwhile, the socially adept wondered what all of the fuss was about, and grandma wondered when her shiftless son-in-law you would quit this darned blogging thing and get a real job! Desperately, the IAU tried to stem the tide by giving UB313 the much more mythologically acceptable names of Eris and Dysnomia, but to no avail.  Then in 2006 a break though; the IAU released what was to be a provisional definition of a planet, you had to be basically round (achieve hydrostatic equilibrium), directly orbit your parent star, and not be capable of undergoing nuclear fusion yourself. The ranks swelled; for a brief period, it looked like we would have 13 planets! However, elation from the Pluto-Eris camp was short lived; a later proclamation from the IAU added the very confusing caveat of “the object must clear its orbit” Pluto and Eris were now on the outs. One could argue that Jupiter, with its retinue of Trojan asteroids, also doesn’t fit the bill! Conspiracy theories abounded. Was a European based IAU attempting to backdoor the only planet discovered by an American? Had aliens, intent on setting up shop on Pluto as a stepping stone to galactic conquest and therefore not keen on drawing attention to them selves, infiltrated the IAU? Again, e-mails and slide rules flew. Then, in a bid to quell the panic, the IAU released the Plutoid definition last month.

Certainly, this was a step in the right direction; if planetary science is to remain relevant, we’ll need some sub categories of planets with meaningful definitions. At very least, psychologists won’t point and laugh at astronomers at cocktail parties. Still, the “Plutoid” definition only works in the Kuiper Belt of our own solar system; since the discovery of the first extra-solar planet in 1992, we now know of more potential planets outside of our solar system than in. “Hot Jupiters” also press the limit, and stranger objects are still to come.

Here then, is an Astroguyz proposal. This is totally unofficial, yet it gives us an easy to use method to classify worlds by some meaningful characteristics. It’s also as free of human cultural bias as an ape descendant like myself can make it, so don’t take it too personally!  I float this only because many science educators have approached me with concern, afraid that their science text books with grainy Saturn pics may no longer valid. I usually have to gently break it to them; those books weren’t valid on the day they were printed! Really, astronomy today is changing that fast. But still, it’s all about the children; they’ll be the ones filling out the “Greetings from the Pluto Hyatt!” postcards, so they need to be armed and ready with some relevant definitions:

Definitely not a Planet: Are you large enough to maintain hydrogen fusion? Did you once maintain, or are you the remnant of said process? Then you are decidedly a star, black hole, or pulsar. Are you potato-shaped, i.e. not basically round? Out again. Asteroids like Toutatis fit the bill. Do you not directly orbit a star, but instead orbit a larger host world? You’re a moon, not a planet. Think Titan, Ganymede, etc.

Brown Dwarf: Hey, we already do use the “dwarf” definition! Are you at about 80 Jupiter masses and large enough to begin deuterium fusion? Your status is shaky. Few brown dwarfs are known, although undoubtedly many are out there. This class just misses the planetary status, although we’ll probably find many that blur the line!

Gas Giant: And now, I give to you heftiest class of planets. Paradoxically, although they’re the largest, they have no defined solid surface. Think Jupiter or Saturn. I’ve heard it said that our Solar System is “Jupiter plus debris…” we may find that to be the case around other stars as well.

Terrestrial Planet: OK, this one has a bit of Earth bias in it…say “rocky”, or “stony” if you please. These worlds are basically round, have a solid surface, and have an appreciable atmosphere.  Note that Pluto world fit this description! Mars, Venus, and of course good ol’ planet Earth also do nicely.

Micro-planets: A catchier and less offensive term than “dwarf”, “micro” brings to mind small batch breweries, beer-bearing maidens and grudge rock.  These objects fit none of the above criteria, but are still noticeably round. Eris, Ceres, and Mercury come to mind.

So, there you have it… our two cents in a universe that sometimes defies description. I know, by tomorrow, some far off world will be discovered that turns the paradigm on its head. The universe is indeed funny that way. How about a several Jupiter mass, carbon planet? Such bizarre worlds have been postulated. Or how about planets that have been ejected from their parent stars and are now adrift? Doubtless, there will always be objects that stubbornly refuse to be pigeon-holed. Perhaps, as stated recently in this space, we simply need to approach planetary science like descriptive geology and simply note what we see, without the blinders of categories. Ahhh, its certainly a brave new universe out there!

Next week: the final post(s) in our invasion of the Plutoids saga! We tour the revised Maine Solar System Model…and a chance to spot a denizen of the Kuiper Belt!


  1. [...] definition, whatever it may be. Perhaps the term “planet” will remain a cultural one; perhaps sub-classifications will arise where we can safely say “Captain, we’re approaching a Class M planet…” ala Star [...]

Speak Your Mind