Selected Narrative

This month, the Inter­na­tional Astro­nom­i­cal Union (IAU) is going to have its 2009 gen­eral assem­bly. One topic that may come up for dis­cus­sion is Pluto’s right to be called a planet. Which is really the topic of how to define a planet. And as we shall see below, although Pluto has been the dri­ver for this rede­f­i­n­i­tion, no mat­ter which way it goes this year, if it is dis­cussed at all, Pluto will never again have the same rev­er­ence it once had.

In 2006, the IAU defined a planet to be a celes­tial body that is:

1. In orbit around the Sun.
2. Has suf­fi­cient mass for its self-gravity to over­come rigid body forces so that it assumes a hydro­sta­tic equi­lib­rium (nearly round) shape.
3. Has cleared the neigh­bour­hood around its orbit.

All three of these rules are con­tentious to some degree or other. Not the least of which Rule Num­ber 1 implies that this def­i­n­i­tion only applies to our Solar Sys­tem. Are the increas­ing num­ber of exoso­lar “plan­ets” being found not really plan­ets after all?

Rule Num­ber 3, how­ever, has drawn the most ire, as it is that rule which has seen Pluto demoted from planet sta­tus to the newly defined (in 2006) dwarf planet sta­tus. In fact, it has been asserted that Rule Num­ber 3 was brought in to do exactly that: dump Pluto. Some peo­ple weren’t happy about it, but then some peo­ple don’t like change. But then again, there would still have had to be change were Pluto kept as a planet.

If Rule Num­ber 3 was to be retracted, and Pluto was to be re-reclassified in the big league, then there are at least another four bod­ies in the same boat. Tak­ing the plan­e­tary total up to 13 (and requir­ing another reprint of astron­omy books). The plan­ets would then be Mer­cury, Venus, Earth, Ceres, Mars, Jupiter, Sat­urn, Uranus, Nep­tune, Pluto, Haumea, Make­make and Eris. Ceres? Haumea? Make­make? Eris? Who? What? Where? When?

It turns out that these bod­ies also com­ply with Rule Num­ber 1 and Rule Num­ber 2. In fact, Eris is even big­ger than Pluto. And it’s sus­pected that there could be another 200 bod­ies in the Solar Sys­tem of sim­i­lar ilk. Our Solar Sys­tem could there­fore hold well in excess of 200 “plan­ets”. I’d like to see the mnemonic stu­dents would need to remem­ber all of their names.

And that is only count­ing the bod­ies directly in orbit around the Sun. There are a num­ber of other bod­ies in the Solar Sys­tem that could lay claim to being of plan­e­tary sta­tus, but they are being ignored as they hap­pen to orbit a body that is orbit­ing the Sun. These, of course, are the satel­lites of the major plan­ets. Many of these satel­lites sat­isfy Rule Num­ber 2, our own Moon amongst them. But because of the prej­u­dice of Rule Num­ber 1, they can­not be classed as plan­ets. Which must be galling for a large num­ber of these so-called satel­lites, because, as we shall see below, if they orbited the Sun in their own right, there would be no doubt what­so­ever to their status.

I give to you now the sev­en­teen largest bod­ies orbit­ing the Sun, in order of decreas­ing size (and all shown to scale here). Why sev­en­teen? It may make more sense when you get to num­ber seventeen.

1. Jupiter Planet, Diam­e­ter: 139,822 km

Named for the Roman god Jupiter, the king of the gods.

Eleven planet Earths could fit across its equa­tor. Recently Jupiter was impacted by a comet or aster­oid, leav­ing a black scar. In 1994, Jupiter was again scarred by Comet Shoemaker-Levy 9.

2. Sat­urn Planet, Diam­e­ter: 116,464 km

Named for the Roman god Sat­urn, who was the father of Zeus.

If there were a sea big enough, Sat­urn would float. For all of its size, Sat­urn is less dense on aver­age than water. And if you think F5 tor­na­dos are scary, just be thank­ful wind speeds here don’t reach Sat­urn­ian lev­els: winds have been recorded up to 1,800km/hr.

3. Uranus Planet, Diam­e­ter: 50,724 km

Named for Uranus, the Greek sky god. It is unusual in that all plan­ets up until Uranus’ dis­cov­ery in 1781 by William Her­schel are named for the Roman god equiv­a­lents. If that had been the case here, Uranus would have been named Caelus.

Although it is a gas giant, and fif­teen times more mas­sive than the Earth, if you were able to stand on the “sur­face” of Uranus (essen­tially on top of the top­most of clouds) you would weigh less than what you weigh on the Earth[1].

4. Nep­tune Planet, Diam­e­ter: 49,244 km

Named for Nep­tune, the Roman god of the sea.

If you thought Saturn’s wind speeds were high, just be thank­ful we don’t get Nep­tun­ian lev­els here: winds have been recorded up to 2,100km/hr.

5. Earth Planet, Diam­e­ter: 12,742 km

The name Earth comes from the Anglo-Saxon word erda, which means ground or soil. This in turn became eorthe in Old Eng­lish and then erthe in Mid­dle English.

Our home planet, as if you didn’t know. Although Mt Ever­est is the high­est point above sea-level, the point fur­thest from the cen­tre of the Earth is actu­ally Mount Chimb­o­razo in Ecuador. This is because the Earth is not a per­fect sphere, but an oblate spher­oid: the equa­tor bulges slightly as a result of the Earth’s spin.

6. Venus Planet, Diam­e­ter: 12,104 km

Named for the Roman god­dess of love, Venus. Had peo­ple known about its hell­ish coun­te­nance when it was named, Venus might not have been the name cho­sen. Pluto might well have been a more apt title.

Our twin planet, but only in size. Its atmos­phere con­tains clouds of sul­phuric acid, the pres­sure at ground level is 92 times nor­mal sea-level here on Earth and the tem­per­a­ture at the sur­face (460°C) is eas­ily hot enough to melt lead (melt­ing point 327.6°C), zinc (melt­ing point 419.73°C) and tel­lurium (449.65°C). Yes, even tel­lurium would suc­cumb on the sur­face of Venus.

7. Mars Planet, Diam­e­ter: 6,780 km

Named for the Roman god of war, Mars.

The Red Planet. Home of more fic­tional aliens than any­where else in the uni­verse. Apart from per­haps the Mos Eis­ley Can­tina. In 50 mil­lion years time, Mars may get its own ring sys­tem: one of its satel­lites, the tiny Pho­bos, is slowly spi­ral­ing in to the planet, and will likely break up because of tidal forces. I guess if it doesn’t break apart, there will an even more spec­tac­u­lar, albeit short-lived, end for Pho­bos when it impacts on Mars.

8. Ganymede Satel­lite of Jupiter, Diam­e­ter: 5,262 km

Named for Ganymede, cup­bearer of the Greek gods and Zeus’ beloved. The name was sug­gested by astronomer Simon Mar­ius some­time between 1610 (when it was dis­cov­ered by Galileo, and pos­si­bly Mar­ius him­self inde­pen­dently) and 1614. Obvi­ously peo­ple in the 17^th and 18^th cen­turies didn’t mind mix­ing Roman and Greek deities.

If this were in orbit directly around the Sun instead of indi­rectly via Jupiter, it would be the eighth largest planet. So why can’t it be called a planet too? Ganymede is even expected to host a salt­wa­ter ocean. It might prove a bit hard to get to, unfor­tu­nately: if it really does exist, it’s prob­a­bly about 200 km below the surface.

9. Titan Satel­lite of Sat­urn, Diam­e­ter: 5,152 km

Named for the group of Greek gods known as the Titans. Its orig­i­nal name had sim­ply been Sat­urni Luna (Saturn’s Moon), coined by its dis­cov­erer Chris­ti­aan Huy­gens. That pro­saic name lasted almost 200 hun­dred years before Titan was cho­sen by John Her­schel in 1847.

Another giant moon, this one of Sat­urn, blocks out the eighth offi­cial planet. Titan has a sub­stan­tial methane atmos­phere, and is unique amongst all the satel­lites in the Solar Sys­tem in this regard. Sci­en­tists think that the atmos­phere on Titan is very sim­i­lar to what the Earth’s might have been like, except that it is far colder: about –180°C. Because of this, it was tar­geted by the Cassini space­craft: specif­i­cally the Huy­gens probe, which touched down on Titan’s sur­face on Jan­u­ary 15, 2004.

10. Mer­cury Planet, Diam­e­ter: 4,880 km

Named for the Roman mes­sen­ger god Mer­cury. The Greeks had observed this planet as far back as the 1000 BC, but had believed it to be two sep­a­rate objects, which they named Apollo and Hermes.

In 1974, the space­craft Mariner 10 became the first probe to visit the mes­sen­ger planet. Just before its first flyby of the planet, Mariner 10 picked up some anom­alous ultra­vi­o­let radi­a­tion com­ing from around Mars. The next day it had dis­ap­peared, before reap­pear­ing three days later. Its appear­ance was con­sis­tent with a satel­lite orbit­ing Mer­cury. Even­tu­ally, how­ever, the “moon” started to move away from the planet — some­thing that moons aren’t really sup­posed to do. It was then found to be a star. Mercury’s moon was not meant to be. A new probe, called MESSENGER, is now observ­ing the planet. MESSENGER is due to set­tle in to orbit around Mer­cury in March 2011 after three fly­bys dur­ing the pre­ced­ing few years.

11. Cal­listo Satel­lite of Jupiter, Diam­e­ter: 4,820 km

Named for one of Jupiter’s many lovers in Greek mythol­ogy. Cal­listo was a nymph asso­ci­ated with the god­dess of the hunt, Artemis.

Cal­listo is the sec­ond largest of Jupiter’s moons, and is thought to be about 50% rock and 50% ice. And it’s only just smaller than Mer­cury. Cal­listo served as the basis for a NASA con­cep­tual study called Human Outer Planet Explo­ration (HOPE) (pdf file, 3.1MB) in 2003.

12. Io Satel­lite of Jupiter, Diam­e­ter: 3,643 km

The name was again from Simon Mar­ius. Io (mythol­ogy) was a priest­ess of Hera in Argos, and one of the many lovers of Zeus.

Along with Earth, Io is the most geo­log­i­cally active body in the Solar Sys­tem. Io has over 400 active vol­ca­noes. While Earth has many more (1,500 that we know about, with pos­si­bly many thou­sands more under the ocean), it seems the ones on Io are far more active, con­stantly spew­ing out plumes of sul­phur and sul­phur diox­ide, some­times to over 500 km in height.

13. Moon Satel­lite of Earth, Diam­e­ter: 3,474 km

Moon is a Ger­manic word ulti­mately derived from the Proto-Indo-European root me–, which relates to mea­sure­ments. This under­scores the use­ful­ness of the Moon as a time keeper of the ancients. And not so ancient.

I think it’s time for the human race to head back to the Moon. And do you know what, we’re down to the 13^th largest body in the Solar Sys­tem, and no sign of Pluto, the ninth “planet”?

14. Europa Satel­lite of Jupiter, Diam­e­ter: 3,122 km

Simon Mar­quis named all four of the Galilean moons. Europa was a myth­i­cal Phoeni­cian noble­woman who was courted by Zeus.

All these worlds are yours except Europa. Attempt no land­ing there.

So says HAL at the end of 2010: Odyssey Two. The mak­ers of the giant black mono­liths of Arthur C Clarke’s influ­en­tial series have deemed that Europa is an apt place to nur­ture life, and have warned humans away via the arti­fi­cially intel­li­gent HAL. Well, it’s 2009, and there is no sign of any giant black mono­liths, but Arthur C Clarke was right: Europa is a prime can­di­date for extrater­res­trial life (albeit prob­a­bly only of the bac­te­r­ial equiv­a­lent kind). Being so close to Jupiter induces tidal flex­ing that may allow a warm water ocean to exist under about 20 km of ice. Should this be the case, there may be poten­tial for organ­isms, such as extremeophiles, to exist there.

15. Tri­ton Satel­lite of Nep­tune, Diam­e­ter: 2,707 km

Tri­ton is the son of the Greek god Posei­don (the Greek equiv­a­lent of Nep­tune). It was sug­gested by Camille Flam­mar­ion in 1880, about 34 years after the satel­lite was dis­cov­ered by William Las­sell. But it was almost 70 years before the name Tri­ton caught on. Until 1949, when a sec­ond Nep­tun­ian satel­lite, Nereid, was dis­cov­ered Tri­ton was sim­ply known as “the satel­lite of Neptune”.

Tri­ton is the only major satel­lite to orbit in a ret­ro­grade motion around its par­ent planet. Ret­ro­grade means against the par­ent planet’s direc­tion of rota­tion. (This is prob­a­bly because Tri­ton is a cap­tured Kuiper Belt object, a group of objects that orig­i­nated in the far reaches of our Solar Sys­tem. Pluto is one of these objects as well.) Most other satel­lites in the Solar Sys­tem are thought to have co-originated with their par­ent plan­ets, or at least been formed out of the same pile of rub­ble that was spin­ning in vaguely the same direc­tion. If Tri­ton had never been cap­tured by Nep­tune, it would likely have been the ninth planet as opposed to Pluto. Which reminds me, we’re down to the 15^th largest body in the Solar Sys­tem (not count­ing the Sun), and still no Pluto.

16. Eris Dwarf Planet, Diam­e­ter: ~2,600 km

Named for the Greek god­dess Eris, a per­son­i­fi­ca­tion of strife and dis­cord. The nam­ing of this object was event­ful, and the final name rep­re­sents the chal­lenges had in nam­ing it. Dis­cov­ered in 2005, it was given the nick­name Xena (after the TV pro­gramme Xena: War­rior Princess). The leader of the dis­cov­ery team, Mike Brown, then sug­gested Lila (from Hindu mythol­ogy) and Perse­phone (the wife of Pluto) before finally agree­ing with the astron­omy com­mu­nity on Eris. Eris was offi­cially known as 2003 UB₃₁₃ from its dis­cov­ery until Eris became its sanc­tioned name.

Eris is the first sub­stan­tial object to have been found in the Solar Sys­tem since Clyde Tombaugh dis­cov­ered Pluto in 1930. Eris is one of a num­ber of so-called trans-Neptunian objects to have been found recently. Pluto was the first, although it wasn’t really known as such for many years, at least out­side of the arcane inner sanc­tum of the plan­e­tary sci­en­tists. Other notable TNOs include Make­make, Haumea, Sedna and the tongue-twisting Quaoar. There are many, many other TNOs that have been found in the past few years, and many more will likely be found. Some may even be larger than Eris.

17. Pluto Dwarf Planet, Diam­e­ter: 2,390 km

Named for the Roman god of the Under­world, Pluto. The Dis­ney char­ac­ter Pluto was named in hon­our of the planet.

Finally, we are here. The so-called ninth planet. The 17^th largest body in the Solar Sys­tem. Per­son­ally, I don’t really care that Pluto has been demoted to the ranks of the lesser bod­ies. It’s just a reflec­tion of real­ity, which is the ulti­mate goal of sci­ence. Nos­tal­gia for the old view of nine plan­ets in our Solar Sys­tem has no place here.

How­ever, while I think it would be wrong for Pluto to be returned to the pan­theon of plan­ets (unless we were to go the whole hog and ini­ti­ate tens, if not hun­dreds, of bod­ies into that def­i­n­i­tion), I think that Pluto should always be regarded affec­tion­ately, and we should never for­get the con­tri­bu­tions of Per­ci­val Low­ell (whose quest for the elu­sive Planet X directly caused Pluto to be dis­cov­ered, even though the need for a Planet X was even­tu­ally found to be lack­ing) and Clyde Tombaugh (who spent uncounted hours glued to his blink com­para­tor to find said elu­sive Planet X). And it will be a thrill to see the results of NASA’s New Hori­zons space­craft, due to ren­dezvous with this frozen out­post of the solar fam­ily in 2015. Here’s hop­ing this lit­tle dwarf planet throws up some more curve balls for sci­ence to grap­ple with.

[1] This seems counter-intuitive, but is due to the nature of grav­ity, ably approx­i­mated by Isaac Newton’s Law of Uni­ver­sal Grav­i­ta­tion. (Well, this “law” is tech­ni­cally a sci­en­tific the­ory; and yes, it has been super­seded by Albert Einstein’s The­ory of Gen­eral Rel­a­tiv­ity when it comes to our mod­ern under­stand­ing of grav­ity, but it is a per­fectly fine approx­i­ma­tion for what we need.) Two out­comes from Newton’s law are (1) the mass of a uni­formly dense spher­i­cal object can be con­sid­ered to be con­cen­trated at the exact cen­tre of that sphere (a con­se­quence of the Shell The­o­rem, if you must know) for all bod­ies at or beyond the sur­face of that object, and (2) the grav­ity expe­ri­enced at the sur­face of such a body is pro­por­tional to its mass and inversely pro­por­tional to the square of its radius. This sec­ond out­come means that for a given body, if you were to increase the radius of that body by, for exam­ple, a fac­tor of three whilst keep­ing the mass the same, the grav­i­ta­tional force on the sur­face will go down by a fac­tor of nine. Tak­ing out­comes (1) and (2), and assum­ing for our pur­poses that Uranus is a uni­formly dense spher­i­cal object (close enough to be good enough), it can be deduced that the force of grav­ity felt at its sur­face is a lot less than you might think: about 90% of the force you feel here on Earth. Which means if you weigh 75kg here on Earth, you would feel like you weighed 67.5kg on Uranus. Physics can be strange.

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