OK, so THEY decided that Pluto wasn’t a REAL planet as it didn’t ‘clear the neighborhood’ of other bodies (and fudged the rule by adding something like ‘of object it did not gravitationally control’ so Jupiter could stay a planet, or those Trojans would be a problem…) and ignore things like “Earth Crossers” in our neighborhood (don’t know if that’s because we ‘gravitationally control them’ or if there is a fuzzy “count” threshold… the world gets SO muddy when you first start down the wrong track…)

At any rate, the category “Dwarf Planet” was whomped up out of whole cloth and they stuck into it all the inconvenient planets. While this has been a minor irritant to me for a while, I think I’ve hit on a simple solution. “Planets”, unadorned, is ambiguous as to “dwarf”, “minor” or “major”… So, one could assert that saying “Planets” includes them all. While, to have a group listing without Pluto and friends, would require the modified form “Major Planet”. Simple. It’s the normal rule of things in English.

OK, they made these rules, not me, so:

What ARE the planets in our Solar System?

Major Planets

The usual suspects (outward from the Sun):

Mercury
Venus
Earth (potential issue in that the moon never goes retrograde so we’re really a double planet system… mass of 7.3477 × 10^22 which we’ll come back to later.)
Mars
Jupiter
Saturn
Uranus
Neptune

Historical Planets

Pluto:

The first in my list of 5 historical planets. Bodies that were for a very long time defined as planets, then were set aside after the death of their discoverers and the folks who agreed with them that it was a planet. A dish best served cold? Or just not thinking it through? One is left to wonder.

Ceres:

In the asteroid “belt” and with 32% of the total asteroid belt mass. Was, for 50 years, classed as a full sized planet.

From the wiki:

The classification of Ceres has changed more than once and has been the subject of some disagreement. Johann Elert Bode believed Ceres to be the “missing planet” he had proposed to exist between Mars and Jupiter, at a distance of 419 million km (2.8 AU) from the Sun. Ceres was assigned a planetary symbol, and remained listed as a planet in astronomy books and tables (along with 2 Pallas, 3 Juno and 4 Vesta) for about half a century.

So 1/2 Century of being a planet. Pallas, Juno and Vesta too. Then an asteroid. Now back as “Dwarf” or sub type of non-major planet. OK, so it’s a “Planet” (unadorned) now too. Nice to have you back, Ceres.

However, as other objects were discovered in the area it was realised that Ceres represented the first of a class of many similar bodies. In 1802 Sir William Herschel coined the term asteroid (“star-like”) for such bodies, writing “they resemble small stars so much as hardly to be distinguished from them, even by very good telescopes”. As the first such body to be discovered, it was given the designation 1 Ceres under the modern system of asteroid numbering.

So things have been a bit “ersatz” for a while now… Oh, and when you see an asteroid with a name that starts with a 4 or 5 digit number it gives you a clue how many have been found and cataloged…

The 2006 debate surrounding Pluto and what constitutes a ‘planet’ led to Ceres being considered for reclassification as a planet. A proposal before the International Astronomical Union for the definition of a planet would have defined a planet as “a celestial body that (a) has sufficient mass for its self-gravity to overcome rigid body forces so that it assumes a hydrostatic equilibrium (nearly round) shape, and (b) is in orbit around a star, and is neither a star nor a satellite of a planet”. Had this resolution been adopted, it would have made Ceres the fifth planet in order from the Sun.

And, IMHO, they ought to have stopped there. It would have put Ceres back where it belonged (and was at one time) and would have validated the “rule” about orbital spacing that led to finding it in the first place.

However, it was not accepted, and in its place an alternate definition came into effect as of 24 August 2006, carrying the additional requirement that a “planet” must have “cleared the neighborhood around its orbit.” By this definition, Ceres is not a planet because it shares its orbit with the thousands of other asteroids in the main belt. Instead it is classified as a “dwarf planet” within the asteroid belt rather than being considered the largest asteroid. However, dual classifications such as main-belt comets do exist, and being a dwarf planet does not preclude Ceres from having other designations. The issue of whether Ceres remains an asteroid was not fully addressed.

So we have “cleared the neighborhood” but not all of it (Trojans are ok… and orbital crossers… and comets… and moons… and WHEN is undefined… and does this imply that since the “moons” in the rings of Saturn have not “cleared their neighborhood” they ought not to be moons any more???) Their definition just keeps finding more and more ‘issues’…

But now, on to Pallas, Vesta and Juno … (All, now, counted as ‘minor planets’ and not ‘dwarf planets’, so still a ‘planet’ (unadorned) but my God the system these folks have created is a mess.)

Pallas:

Pallas, formally designated 2 Pallas, is the second asteroid to have been discovered, by astronomer Heinrich Wilhelm Matthäus Olbers on March 28, 1802. It was at first considered a planet, as were other early asteroids, until the discovery of many more led to their re-classification. Pallas appears to be the largest irregularly shaped body in the Solar System (that is, the largest body not rounded under its own gravity), and a remnant protoplanet.

With a mass estimated to be 7% of the total mass of the asteroid belt, Pallas is one of the largest asteroids. Its diameter is some 530–565 km, comparable to or slightly larger than that of 4 Vesta, but it is 20% less massive, placing it third among the asteroids. The Palladian surface appears to be a silicate material; the surface spectrum and estimated density resemble carbonaceous chondrite meteorites. The Palladian orbit, at 34.8°, is unusually highly inclined to the plane of the main asteroid belt, and the orbital eccentricity is nearly as large as that of Pluto, making Pallas relatively inaccessible to spacecraft.

So, as a ‘first cut’ for “Smith’s Planets Rule” I’d make the ‘cut off’ somewhere larger than Pallas since it looks to be a non-rounding for rocks limit. I’d also count Ceres “in” as it clearly is self rounding.

Juno:

Juno (pronounced /ˈdʒuːnoʊ/, or as in Latin: Iūno), formal designation 3 Juno in the Minor Planet Center catalogue system, was the third asteroid to be discovered and is one of the larger main-belt asteroids, being one of the two largest stony (S-type) asteroids, along with 15 Eunomia. Juno is estimated to contain 1% of the total mass of the asteroid belt. Juno was discovered on September 1, 1804, by German astronomer Karl L. Harding and named after the mythological figure Juno, the highest Roman goddess.

Juno is one of the larger asteroids, perhaps tenth by size and containing approximately 1.0% the mass of the entire Main asteroid belt. It is the second most massive S-type asteroid after 15 Eunomia. Though one of the most massive asteroids, Juno has only 3% the mass of Ceres.

Size comparison: the first 10 asteroids discovered, profiled against Earth’s Moon. Juno is third from the left.

Amongst S-type asteroids, Juno is unusually reflective, which may be indicative of distinct surface properties. This high albedo explains its relatively high apparent magnitude for a small object not near the inner edge of the asteroid belt. Juno can reach +7.5 at a favourable opposition, which is brighter than Neptune or Titan, and is the reason for it being discovered before the larger asteroids Hygiea, Europa, Davida, and Interamnia. At most oppositions, however, Juno only reaches a magnitude of around +8.7—only just visible with binoculars—and at smaller elongations a 3-inch (76 mm) telescope will be required to resolve it. It is the main body in the Juno family.

Juno was originally considered a planet, along with 1 Ceres, 2 Pallas, and 4 Vesta. In 1811, Schröter estimated Juno to be as large as 2290 km in diameter. All four were re-classified as asteroids as additional asteroids were discovered. Juno’s small size and irregular shape preclude it from being designated a dwarf planet.

Juno orbits at a slightly closer mean distance to the Sun than Ceres or Pallas. Its orbit is moderately inclined at around 12° to the ecliptic, but has an extreme eccentricity, greater than that of Pluto. This high eccentricity brings Juno closer to the Sun at perihelion than Vesta and further out at aphelion than Ceres. Juno had the most eccentric orbit of any known body until 33 Polyhymnia was discovered in 1854, and of asteroids over 200 km in diameter only 324 Bamberga has a more eccentric orbit.

Vesta:

Vesta, formal designation 4 Vesta, is an asteroid, thought to be a remnant protoplanet with a differentiated interior, and a mean diameter of about 530 km. Comprising an estimated 9% of the mass of the entire asteroid belt, it is the second most massive object in the belt (the largest being the dwarf planet Ceres). It was discovered by the German astronomer Heinrich Wilhelm Olbers on March 29, 1807 and named after the Roman virgin goddess of home and hearth, Vesta.

Vesta is the brightest asteroid. Its greatest distance from the Sun is slightly more than the minimum distance of Ceres from the Sun, and its orbit is entirely within the orbit of Ceres. Vesta lost some 1% of its mass in a collision less than one billion years ago. Many fragments of this event have fallen to Earth as Howardite-Eucrite-Diogenite (HED) meteorites, a rich source of evidence about the asteroid.

OK, with Ceres and Vesta in a co-orbiting position, it looks to me like something of a binary planet that’s just not as tightly bound as the Earth / Moon system.

Vesta is the second-most massive body in the asteroid belt, though only 28% as massive as Ceres. It lies in the Inner Main Belt interior to the Kirkwood gap at 2.50 AU. It has a differentiated interior, and is similar to 2 Pallas in volume (to within uncertainty) but about 25% more massive.

Vesta’s shape is relatively close to a gravitationally relaxed oblate spheroid, but the large concavity and protrusion at the pole (see ‘Surface features’ below) combined with a mass less than 5×10^20 kg precluded Vesta from automatically being considered a dwarf planet under International Astronomical Union (IAU) Resolution XXVI 5. Vesta may be listed as a dwarf planet in the future, if it is convincingly determined that its shape, other than the large impact basin at the southern pole, is due to hydrostatic equilibrium, as currently believed.

Ouch, that “compositionally dependent shape” issue is coming back to bite Vesta… Messy messy messy…

When is a Minor a Dwarf?

The Dwarf Planet group is a bunch of ‘larger minor sized planets’… and if that is sounding lumpy to you, well, that’s what happens when folks making the rules don’t keep a tidy mind… In animal husbandry the dwarf types are smaller than the miniatures that are smaller than the standards, but here, dwarves are larger than the small ones. Go figure.

Dwarf planet
From Wikipedia, the free encyclopedia

Not to be confused with Minor planet.

If you have to warn against confusion, you already have a poor system… but we’ll press on… secure in the knowing that someone, somewhere, thinks dwarfs are larger than minors and ‘clearing the neighborhood’ is, er, definable? And that lack of a “when” the neighborhood cleared… does that mean Earth was not a planet back when we were being more heavily bombarded with Rocks From Space? So we were NOT planets for the first billion years, then at some arbitrary bombardment rate cut off (as we are still having ‘incoming’…) we suddenly WERE a planet? Oh, and that ‘self rounding’ does not depend on composition (water self rounding at about the size of a raindrop or less and steel or stone at much larger sizes… but temperature dependent and, oh God, more messy loose ends…)

A dwarf planet, as defined by the International Astronomical Union (IAU), is a celestial body orbiting the Sun that is massive enough to be spherical as a result of its own gravity but has not cleared its neighbouring region of planetesimals and is not a satellite. More explicitly, it has to have sufficient mass to overcome its compressive strength and achieve hydrostatic equilibrium.

The term dwarf planet was adopted in 2006 as part of a three-way categorization of bodies orbiting the Sun, brought about by an increase in discoveries of trans-Neptunian objects that rivaled Pluto in size, and finally precipitated by the discovery of an even more massive object, Eris. This classification states that bodies large enough to have cleared the neighbourhood of their orbit are defined as planets, while those that are not massive enough to be rounded by their own gravity are defined as small solar system bodies. Dwarf planets come in between. The definition officially adopted by the IAU in 2006 has been both praised and criticized, and has been disputed by scientists such as Alan Stern.

The IAU currently recognizes five dwarf planets—Ceres, Pluto, Haumea, Makemake, and Eris. However, only two of these bodies, Ceres and Pluto, have been observed in enough detail to demonstrate that they fit the definition.

So, we’re going to be real stickers about things exactly fitting our definition, except when we don’t feel like it… Way to go, guys. IMHO, you ought to have made a simple “mass cutoff” at about the size where things ‘self round’ if made of stone and metal. (Or Ice or whatever standard you like. Yes, it’s an ‘arbitrary’ rule. Rules are arbitrary. I’d rather have a usable arbitrary and precise one than an unusable vague one.) You know, like maybe just a little smaller than Pluto. Heck I’d even be happy if you made it about Ceres or Vesta sized. Nice easy rule. Even gets rid of that loose end of a water “planet” self rounding at a much different size than an iron one; you know, that compositional loose end you ignored… Easy to observe for any object with gravitationally influenced things near it (as you can find it’s mass). But no, you make a rule that you can’t apply. Brilliant… /sarcoff>.

Eris has been accepted as a dwarf planet because it is more massive than Pluto.

Oh look, a clue! Laying right in front of you. MASS matters. MASS can be measured. Clue Alert, Clue Alert, Clue Alert!!! (Some folks can’t find a clue if you hit them over the head with it… and even if they themselves have used it against their own rule.

The IAU subsequently decided that unnamed trans-Neptunian objects with an absolute magnitude brighter than +1 (and hence a mathematically delimited minimum diameter of 838 km) are to be named under the assumption that they are dwarf planets. The only two such objects known at the time, Makemake and Haumea, went through this naming procedure and were declared to be dwarf planets.

It is suspected that at least another 40 known objects in the Solar System are dwarf planets, and estimates are that up to 200 dwarf planets may be found when the entire region known as the Kuiper belt is explored, and that the number might be as high as 2,000 when objects scattered outside the Kuiper belt are considered.

The classification of bodies in other planetary systems with the characteristics of dwarf planets has not been addressed, although if they were detectable they would not be considered planets.

So now we’ve got a messy rule that can not be applied, being sort of applied, except when a better system that isn’t the rule is applied, despite not fitting the rules… Can you say “ad hoc”? How about “messy”? How about “more loose ends than a Rastafarian?”

Top 10 Asteroids compared to The Moon

Original Image

Sizes of the first ten Asteroids to be discovered compared to the Earth’s Moon, all to scale. The dwarf planet Ceres is on the far left. The remaining objects, left to right are: 2 Pallas, 3 Juno, 4 Vesta, 5 Astraea, 6 Hebe, 7 Iris, 8 Flora, 9 Metis and 10 Hygiea. The scale is 10 km/px on the original image, though not necessarily on the repro here.

From the Vesta wiki:

Vesta became the first asteroid to have its mass determined. Every 18 years, the asteroid 197 Arete approaches within 0.04 AU of Vesta. In 1966, based upon observations of Vesta’s gravitational perturbations of Arete, Hans G. Hertz estimated the mass of Vesta as (1.20 ± 0.08) × 10^-10 solar masses. More refined estimates followed, and in 2001 the perturbations of 17 Thetis were used to estimate the mass of Vesta as (1.31 ± 0.02) × 10−10 solar masses.

So one “easy” rule I could see is just define planet as anything orbiting the sun with a mass more than 1.0 x 10^-10 solar masses. Vesta and Ceres in, other asteroids out. The sun is 1.9891 x 10^30 kg so that would be 1.9891 x 10^20 kg. Heck, I’d even be happy if folks wanted to put the cutoff at a nice round 2.0 x 10^20 kg just to make it easy to remember.

(That would put Pallas maybe “in” and maybe not at 2.11 (±0.26)×10^20 kg depending on where the +/- ended up landing… but Juno “out” at 2.67 ×10^19 kg; Vesta clearly “in” at 2.67 (± 0.02)×10^20 kg and Ceres too easily makes it at 9.43 (± 0.07)×10^20 kg. That would make Vesta our planet number 11.

Minor Planets

The wiki for minor planets says:

Not to be confused with Dwarf planet.

There’s that Clue Stick again… Whack Whack!!!! MU! MUUUU!! Damn It, MU! Whack WHACK!!

A minor planet is an astronomical object in direct orbit around the Sun that is neither a dominant planet nor a comet. The first minor planet discovered was Ceres in 1801. The orbits of more than 540,000 objects have been archived at The Minor Planet Center. Most of these are in the main asteroid belt.

Looks kind of like an “else” clause to me, but one that is going to attract a lot of its own “else clauses”…

(In programming you often have a list of well defined things: If A do A-stuff, if B do B-stuff, … if Z do Z-stuff. This is typically followed by an “else clause” that says if anything hits this bucket, well, deal with it somehow: ELSE do OMG-WTF-Print Error. Or sometimes ELSE do [report Heinz 57 mutts…].

The term “minor planet” has been used since the 19th century to describe these objects. The term planetoid has also been used, especially for larger objects. Historically, the terms asteroid, minor planet, and planetoid have been more or less synonymous, but the issue has been complicated by the discovery of numerous minor planets beyond the orbit of Jupiter and especially Neptune that are not universally considered asteroids.

Before 2006 the International Astronomical Union had officially used the term minor planet. During its 2006 meeting, the Union reclassified minor planets and comets into dwarf planets and small solar system bodies. Objects are called dwarf planets if their self-gravity is sufficient to achieve hydrostatic equilibrium, that is, an ellipsoidal shape, with all other minor planets and comets called “small solar system bodies”. The IAU states: “the term ‘minor planet’ may still be used, but generally the term ‘small solar system body’ will be preferred.” However, for purposes of numbering and naming, the traditional distinction between minor planet and comet is still followed.

The Saturnian moon Mimas is the smallest body known to be in hydrostatic equilibrium (though not eligible to be a dwarf planet since it does not directly orbit the Sun), while the asteroid Pallas may be the largest that is not. The IAU has so far officially classified five objects as dwarf planets. In order both of discovery and distance from the Sun, they are Ceres, Pluto, Haumea, Makemake, and Eris.

Oh Dear. Even “minor planet” is being mucked around with. And that “you can still use it, but not quite” is just sooo dysfunctional… then another ‘exception rule’ of still using it when it makes more sense (which is going to be most of the time, IMHO…)

But, we have Mimas as an interesting footnote.

http://en.wikipedia.org/wiki/Mimas_(moon)

has it at 3.749 3 (± 0.003 1)×10^19 kg. So somewhere between 4 10^19 and 2 10^20 is where we have self rounding happen and it varies by composition so picking an arbitrary number is simply picking a compositional average type. OK, I’m “good with that”. Heck, if it made Astro folks feel better (even though it’s a bit less ‘pure’ to remember), I’d be happy with a 2.5 x 10^20 kg cut off and that would clearly put Pallas ‘out’, irregular shape, orbit, and all.

So, I hereby promulgate Smith’s Planets Rule: Over 2.5 x 10^20 kg, orbiting the Sun, It’s a planet.

Ceres and Vesta are planets. Pallas and Juno, not. Pluto a planet. That’s 11 planets in the main body of the Solar system. Anything further out? Oh, yeah, those other “Dwarf Planets”…

Send In The Dwarves, there must be Dwarves…

In addition to Ceres (already covered above) we have the “official” list of: Pluto, Haumea, Makemake, and Eris

As I’ve promulgated a rule that makes Pluto a planet (and as most folks are already familiar with it, will just mention in passing that it’s mass is 1.305 (± 0.007)×10^22 kg and that’s about 2 orders of magnitude bigger than needed. Pluto, he’s a planet (at least in a rational system). Welcome home, my boy, welcome home.

Eris

With a mass of 1.67 (±0.02)×10^22 kg is clearly “in”.

Eris was discovered by the team of Mike Brown, Chad Trujillo, and David Rabinowitz on January 5, 2005, from images taken on October 21, 2003. The discovery was announced on July 29, 2005, the same day as Makemake and two days after Haumea. The search team had been systematically scanning for large outer solar system bodies for several years, and had been involved in the discovery of several other large TNOs, including 50000 Quaoar, 90482 Orcus, and 90377 Sedna.

Because Eris may be larger than Pluto, it was initially described as the “tenth planet” by NASA and in media reports of its discovery. In response to the uncertainty over its status, and because of ongoing debate over whether Pluto should be classified as a planet, the IAU delegated a group of astronomers to develop a sufficiently precise definition of the term planet to decide the issue. This was announced as the IAU’s Definition of a Planet in the Solar System, adopted on August 24, 2006. At this time, both Eris and Pluto were classified as dwarf planets, a category distinct from the new definition of planet. Brown has since stated his approval of the “dwarf planet” label. The IAU subsequently added Eris to its Minor Planet Catalogue, designating it (136199) Eris.

Thus did the error begin… Rather than either A) Recognizing past mistakes in kicking Ceres out or B) making a different and maybe acceptably rational rule that would keep Pluto in and give these folks credit for finding a 10th planet (and opening the possibility of more planets), they came up with a warped, distorted, convoluted, unworkable, and essentially useless rule that looks more like a political machination than anything reasonable.

OK, I hearby give the Great Big RASPBERRY Award to the fools who came up with the bastardized rule. Besides, I like mine better. Much easier for keeping a tidy mind… Oh, and I like their first name better too:

Xena

Due to uncertainty over whether the object would be classified as a planet or a minor planet, as different nomenclature procedures apply to these different classes of objects, the decision on what to name the object had to wait until after the August 24, 2006 IAU ruling. As a result, for a time the object became known to the wider public as Xena.

“Xena” was an informal name used internally by the discovery team. It was inspired by the eponymous heroine of the television series Xena: Warrior Princess. The discovery team had reportedly saved the nickname “Xena” for the first body they discovered that was larger than Pluto. According to Brown,

We chose it since it started with an X (planet “X”), it sounds mythological (OK, so it’s TV mythology, but Pluto is named after a cartoon, right?),[b] and (this part is actually true) we’ve been working to get more female deities out there (i.e. Sedna). Also, at the time, the TV show was still on TV, which shows you how long we’ve been searching!

So, “Eris” Xena is in. Planet 12.

Makemake

Mass of ~3 × 10^21 kg (assumed) OK, maybe a bit of an issue on that one as the mass is “assumed” (per the wiki) but probably based on something decent. At an order of magnitude over the limit, I’d call that a Planet.

Makemake (dwarf planet)

Makemake, formally designated (136472) Makemake, is the third-largest known dwarf planet in the Solar System and one of the two largest Kuiper belt objects (KBO) in the classical KBO population.[b] Its diameter is roughly three-quarters that of Pluto. Makemake has no known satellites, which makes it unique among the largest KBOs. It’s extremely low average temperature, about 30 K (−243.2 °C), means its surface is covered with methane, ethane, and possibly nitrogen ices.

Initially known as 2005 FY9 and later given the minor planet number 136472, it was discovered on March 31, 2005, by a team led by Michael Brown, and announced on July 29, 2005. Its name derives from the Rapanui god Makemake. On June 11, 2008, the International Astronomical Union (IAU) included Makemake in its list of potential candidates to be given “plutoid” status, a term for dwarf planets beyond the orbit of Neptune that would place the object alongside Pluto, Haumea and Eris. Makemake was formally classified as a plutoid in July 2008.

In the world of Linux and Unix there is a common command for creating a “make file” that is used to “make” other programs. That command is “make make”… So I hearby dub this The Official Planet God of Unix Programmers ;-) and since they tend not to be bothered by superstitions, it will get planet number 13 designation in my system…

Haumea

With a mass of 4.006 (± 0.040)×10^21 kg it too meets the Smith Planet Test.

Haumea, formal designation 136108 Haumea, is a dwarf planet in the Kuiper belt. Its mass is one-third the mass of Pluto. It was discovered in 2004 by a team headed by Mike Brown of Caltech at the Palomar Observatory in the United States and, in 2005, by a team headed by J. L. Ortiz at the Sierra Nevada Observatory in Spain, though the latter claim has been contested. On September 17, 2008, it was designated a dwarf planet by the International Astronomical Union (IAU) and named after Haumea, the Hawaiian goddess of childbirth.

Haumea’s extreme elongation makes it unique among known dwarf planets. Although its shape has not been directly observed, calculations from its light curve suggest it is an ellipsoid, with its greatest axis twice as long as its shortest. Nonetheless, its gravity is believed sufficient for it to have relaxed into hydrostatic equilibrium, thereby meeting the definition of a dwarf planet. This elongation, along with its unusually rapid rotation, high density, and high albedo (from a surface of crystalline water ice), are thought to be the results of a giant collision, which left Haumea the largest member of a collisional family that includes several large TNOs and its two known moons.

So their definition has a problem due to observational requirements, mine works fine. Hmmm….

Until it was given a permanent name, the Caltech discovery team used the nickname “Santa” among themselves, as they had discovered Haumea on December 28, 2004, just after Christmas.

Personally, I think they ought to have kept “Santa”… ;-) But we will call it Planet 14, at least until someone shows it is too small a mass.

Other Planets?

What about Sedna and the others?

Other TNO Planet Candidates

Original Image

Comparison of the eight largest TNOs. Five of these TNOs are known to have moon(s). 2007 OR10, Orcus, and Quaoar are all estimated as about the same volume. Since Quaoar and Orcus have moons, it is known that Quaoar is much more massive than Orcus. The top 4 are official dwarf planets while the bottom 4 are dwarf planet candidates.

Sedna

90377 Sedna is a trans-Neptunian object discovered in 2003, which currently lies about three times as far from the Sun as Neptune. However, its farthest orbital distance from the Sun is estimated to be 960 astronomical units (32 times Neptune’s distance) and thus it is, for most of its orbit, one of the most distant known objects in the Solar System after long-period comets.

Roughly two-thirds the size of Pluto, Sedna is theoretically large enough to be rounded by its own gravity, and thus would qualify as a dwarf planet under current definitions. However, its distance from the Sun makes determining its shape difficult. Spectroscopy has revealed that Sedna’s surface composition is similar to that of some other trans-Neptunian objects, being largely a mixture of water, methane and nitrogen ices with tholins. Its surface is one of the reddest in the Solar System

There’s that nasty observationally limited definition biting again. But with The Smith Planet definition, it’s not problem. At a mass of 1.8 to 4.3 x 10^21 kg it is in at either range of the estimates. Planet 15, welcome aboard!

Orcus

Mass of 6.32 (± 0.05)×10^20 puts it clearly “in” with an ‘over double’ the mass limit. Planet 16.

90482 Orcus is a large Kuiper belt object (KBO) with a large moon and is probably a dwarf planet. It was discovered on February 17, 2004 by Michael Brown of Caltech, Chad Trujillo of the Gemini Observatory, and David Rabinowitz of Yale University. Precovery images as early as November 8, 1951 were later identified. Orcus is a plutino, locked in a 3:2 resonance with Neptune, making two revolutions around the Sun, while Neptune makes three. This is much like Pluto, except that its orbit is constrained to always be in the opposite phase of its orbit from Pluto: Orcus is at aphelion when Pluto is at perihelion and vice versa. Because of this, along with its large moon Vanth that recalls Pluto’s large moon Charon, Orcus has been seen as the anti-Pluto. This was a major consideration in selecting its name, as the deity Orcus was the Etruscan equivalent of the Roman Pluto, and later became an alternate name for Pluto.

Interesting orbital mechanics, that one…

Quaoar

With a mass of 1.6 (± 0.3) ×10^21 kg it’s a ‘keeper’. Welcome to planet 17.

Quaoar is named for the Tongva creator god, following International Astronomical Union naming conventions for non-resonant Kuiper belt objects. The Tongva are the native people of the area around Los Angeles, where the discovery of Quaoar was made. Prior to IAU approval of the name, Quaoar went by the provisional designation 2002 LM60. The minor planet number 50000 was not coincidence, but chosen to commemorate a particularly large object found in the search for a Pluto-sized object in the Kuiper belt, parallel to the similarly numbered 20000 Varuna. However, later even larger discoveries were simply numbered according to the order in which their orbits were confirmed.

Interesting story, but a lot of playing with the naming / numbering system going on…

Dwarf planet?

Since Quaoar is a binary object, the mass of system can be calculated from the orbit of the secondary. Quaoar’s estimated density of around 4.2 g/cm3 and estimated size of 890 km suggests that it should qualify as a dwarf planet if the mass required for hydrostatic equilibrium is proven. Mike Brown estimates that rocky bodies around 900 km in diameter relax into hydrostatic equilibrium, and that icy bodies relax into hydrostatic equilibrium around 400 km. With an estimated mass greater than 1.3×10^21 kg, Quaoar probably has the mass required (5×10^20 kg) for being considered a dwarf planet under the 2006 IAU draft definition of a planet.

Oh, that lousy definition getting in the way again. Is it rocky or is it ice or is it a dirty slush ball or… And I wonder where that 5 x 10^20 reference comes from… but if they were considering an arbitrary mass cut off of 5, then why not 2.5? At any rate, I’m counting it as Planet 17 in my system. (Probably ought to order the numbers by discovery date, but not caring enough to do that right now, maybe some other day…)

2007_OR10

OK, no mass given in the wiki, but other evidence that it’s probably “big enough” so it’s a provisional Planet 18 (and the last on the list, other than holding number 19 for Pallas and number 20 for Juno, should those be deemed ‘planets’ too via a lower rounding size limit).

(225088) 2007 OR10 is a trans-Neptunian object with an absolute magnitude of 1.7. This qualifies it as one of the largest dwarf planet candidates: Mike Brown estimates that it is between Sedna and Quaoar in size.
[…]
Mike Brown’s discovery team has nicknamed the object “Snow White”, as it would have to be very large or very bright to be detected by their survey. According to Brown, “We have not yet proposed a real name for it, as we don’t know enough to have a name with meaning.” It is currently the largest known Solar System object without an official name.

Distance

2007 OR10 came to perihelion around 1856. It is currently 86 AU from the Sun. This makes it the 3rd farthest known large body from the Sun, after Eris (97 AU) and Sedna (88 AU). It will be further from the Sun than Sedna in 2013.

Absolute magnitude

The size of an object depends on its absolute magnitude (H) and the albedo (the amount of light it reflects). 2007 OR10 has an absolute magnitude (H) of 1.7. This makes it a little less bright than Sedna (H=1.6; <1600km) and brighter than Orcus (H=2.3; ~950km). It is likely that OR10 has a size somewhere between Sedna and 50000 Quaoar (H=2.6).

Personally, I’d rather they kept it named Snow White. Having Santa and Snow White as planets has a certain charm to it ;-)

In Conclusion

So, there you have it. That is how I would (and do) classify the planets. There are 18 of them (20 if a bit looser on the low end of mass). It is based on mass with a ‘reasonable’ cutoff arbitrarily set at 2.5 x 10^20 kg as a reasonably representative compositional point where a solid body will ‘self round’ in space. By this definition, the Moon would be a planet too, if it did not orbit the Earth. Here we have an interesting choice. The moon never goes retrograde, but the barycenter of the Earth Moon system is inside the Earth. I’m going with the barycenter definition and that leaves the Moon a moon, but I can see the case for the fact that the Moon and the Earth both always go forward in the joint solar orbit and just swap sides from time to time… though there is the interesting ‘loose end’ of just when in the lunar retreat process the orbit is about a barycenter inside vs outside the Earth…

This is also a good example of what I call “keeping a tidy mind”. I could just let in the trash definition (and all the attendant crap that comes with it and the redefinitional confusions and…) or I can make a much more stable, usable, and reasonable system. I choose to do the latter. Now I do need a ‘translation table’ from “Smith’s Planets” to “their planets”, and that will change from time to time as they muck about with things. BUT, it’s only that translation table that needs to change. My internal order is stable and maintained…

Welcome to my world… and my planets…

One minor postscript: I’d also be happy with a mass definition lower limit slightly smaller than Pluto. That would keep Pluto in, add Eris, and leave just about all the rest out. Doesn’t have the ‘self rounding’ feature, but hey, nothing’s perfect, and if it let the folks who don’t want 20 planets be happy but without the bogus system they’ve dreamed up, well then, I’m OK with that. But pick one, folks. 10 planets, 18 by Smith’s Planets System, or 20 if you include Pallas and Juno. Not the halfway house not-quite-a-solution you have now.

Some Candidates With Issues

There is an interesting picture of these guys, but remember it’s a bit of an artists rendering…

http://en.wikipedia.org/wiki/(55565)_2002_AW197

With a mass of ≈4.1×10^20? kg it has potential, but the “question mark” on the mass needs to be resolved.

http://en.wikipedia.org/wiki/(55636)_2002_TX300

With a mass of ~1.2×10^19 kg (assumed) it has potential but so far misses the mark, but the “assumed” on the mass needs to be resolved.

http://en.wikipedia.org/wiki/20000_Varuna

With a mass of ≈3.7×10^20? kg it has potential depending on where the mark is set, but the “?” on the mass needs to be resolved.

Though Varuna looks like it has real potential to me.

http://en.wikipedia.org/wiki/28978_Ixion

With a mass of ≈3×10^20? kg it has potential but that is based on estimation so the “?” on the mass needs to be resolved.

http://en.wikipedia.org/wiki/10_Hygiea

This one is not a Trans Neptune Object (TNO) but a regular old asteroid.

With a mass of 8.85 ×10^19 kg it misses my mark. I’m mostly putting it here as it was under consideration during the whole Dwarf Minded Planet Process as a candidate. It also has some interesting features like a mix of hydrocarbons and water (potentially…):

Hygiea’s surface is composed of primitive carbonaceous material similar to the chondrite meteorites. Aqueous alteration products have been detected on its surface, which could indicate the presence of water ice in the past which was heated sufficiently to melt. The primitive present surface composition would indicate that Hygiea had not been melted during the early period of Solar system formation, in contrast to other large planetesimals like 4 Vesta.

It is the main member of the Hygiea family and contains almost all the mass in this family (well over 90%). It is the largest of the class of dark C-type asteroids with a carbonaceous surface that are dominant in the outer main belt—which lie beyond the Kirkwood gap at 2.82 AU. Hygiea appears to have a noticeably oblate spheroid shape, with an average diameter of 444 ± 35 km and a semimajor axis ratio of 1.11. This is much more than for the other objects in the “big four”—the dwarf planet Ceres and the asteroids 2 Pallas and 4 Vesta. Aside from being the smallest of the four, another important difference from the other three is that Hygiea has a relatively low density, which is comparable to the icy satellites of Jupiter or Saturn more than to the terrestrial planets or the stony asteroids.

All of which make Hygiea somewhat interesting…

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