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Headshot of Emily Lakdawalla

The round worlds in the solar system: An updated graphic

Posted By Emily Lakdawalla

02-11-2015 16:06 CST

Topics: New Horizons, pretty pictures, Pluto, scale comparisons, amateur image processing, Charon, Dawn, dwarf planets beyond Neptune, asteroid 1 Ceres

I have a newly updated scale comparison graphic to share. In March of 2014, I exhorted people to get excited about the Year of the Dwarf Planet. As part of that educational effort, I made a scale comparison of every round object in the solar system smaller than 10,000 kilometers in diameter. That's Mars and everything smaller, including nearly 90 likely round objects beyond Neptune. Now that the Year of the Dwarf Planet is nearly over, I can add three more bodies to this graphic: Pluto, Charon, and Ceres. Here is the updated version:

Every round object in the solar system under 10,000 kilometers in diameter, to scale

Montage by Emily Lakdawalla. Data from NASA / JPL, JHUAPL/SwRI, SSI, and UCLA / MPS / DLR / IDA, processed by Gordan Ugarkovic, Ted Stryk, Bjorn Jonsson, Roman Tkachenko, and Emily Lakdawalla.

Every round object in the solar system under 10,000 kilometers in diameter, to scale
Under a size cutoff of 10,000 kilometers, there are two planets, 18 or 19 moons, 1 or 2 asteroids, and 87 trans-Neptunian objects, most of which do not yet have names. All are shown to scale, keeping in mind that for most of the trans-Neptunian objects, their sizes are only approximately known.

If you visit the Image Library page for this graphic, you'll find other versions without text and/or without the not-yet-visited worlds. My update was made possible by Roman Tkachenko, who produced simulated views of Pluto and Charon lit at half-phase from the New Horizons images, which only show the two worlds at nearly full phase. Here are his simulated views, in case they are useful to anyone else:

Simulated view of a half-phase Pluto

NASA / JHUAPL / SwRI / Roman Tkachenko

Simulated view of a half-phase Pluto
This view of Pluto was generated from images captured by New Horizons' Multispectral Visible Imaging Camera (MVIC).
Simulated view of a half-phase Charon

NASA / JHUAPL / SwRI / Roman Tkachenko

Simulated view of a half-phase Charon
This view of Charon was generated from images captured by New Horizons' Multispectral Visible Imaging Camera (MVIC).

While I was adding the three newly photographed round worlds, I also took the opportunity to update the representation of likely trans-Neptunian dwarf planets, using Mike Brown's frequently-updated dwarf planet list. I didn't keep exact count, but there were half a dozen newly-added likely dwarf planets, and a similar number for which there were substantial (on the order of 10%) changes in estimated diameter; only one such change dropped a world off the list of likely round worlds.

On this graphic, I don't show the designations of any of the trans-Neptunian objects that have not been formally named except for the few very largest, because it would just turn into an alphabet soup. However, each of the little half-disks on this graphic represents a specific trans-Neptunian object. If you are planning to give a presentation on some of those non-formally-named worlds, feel free to ask me to generate a version of this diagram that calls out the specific ones you're interested in. More generally, if you need a specific montage of solar system worlds for a presentation or publication, email me! I'm happy to help out.

I recently gave a presentation to the California Map Society about how we map solar system worlds, and used this graphic to show how many maps we have yet to make. I wonder how long it will be before we finally map more than just a couple of the members of the Kuiper belt!

See other posts from November 2015


Read more blog entries about: New Horizons, pretty pictures, Pluto, scale comparisons, amateur image processing, Charon, Dawn, dwarf planets beyond Neptune, asteroid 1 Ceres


Mewo: 11/03/2015 01:13 CST

Do you have a criterion for deciding when an object is "round enough"? If I were compiling this chart I would not include Vesta or Proteus, but that's just a subjective judgment. I assume you also exclude small asteroids that are just coincidentally roundish.

Arbitrary: 11/03/2015 06:48 CST

To Mewo: I think it is reasonable to assume roundness for almost all objects larger than the smallest known round moons, like Mimas or Enceladus if you want in this scale. How could an object "coincidentally" become round? How many round objects do you find when you walk through the forest or other nature? It doesn't happen without a systematic physical mechanism.

stfletch: 11/03/2015 09:21 CST

I can see bodies like Methone being a bit of an issue for this chart. It would appear to fit the definition of round (our best images seem to suggest so anyway), yet it is only around 3km in diameter.

Mewo: 11/03/2015 09:47 CST

@Arbitrary- The asteroid Ida's little moon Dactyl is only about a mile in radius, but is nearer to being spherical than Vesta is. stfletch makes a good point about Methone as well. Doubtless there are many small, coincidentally roundish asteroids and moons out there.

Ion: 11/03/2015 10:04 CST

Relatively new posting in Wikipedia shows the mean diameter of Vesta (525 km) is slightly larger than Pallas (515 km).

laurelkornfeld: 11/03/2015 01:10 CST

According to the geophysical planet definition, which is the preferred definition for many planetary scientists, including many on the New Horizons team, these round objects are all planets. The question isn't how round they need to be but whether they are in hydrostatic equilibrium, meaning squeezed into a round or nearly round shape by their own gravity. Some are of the dwarf planet subclass of planet while others are satellite planets or moons of planets large enough to be in hydrostatic equilibrium. Vesta and Pallas are borderline objects, which appear to have been in hydrostatic equilibrium only to have had a part of themselves lobbed off by an impacting body. The Dawn mission has revealed Vesta to be very planet-like in its composition and processes, leading many scientists to consider it part of an intermediate class of objects between asteroids and dwarf planets, called "protoplanets."

Josh: 11/03/2015 02:40 CST

Let's throw more chaos into this definition! :) Cassini's measurements indicate that in spite of their roundness, all the round moons of Saturn smaller than Rhea - most conspicuously Iapetus - are not actually in hydrostatic equilibrium for their current rotation; their round shapes are reflections of their past history, similar to 4 Vesta and Saturn's moon Phoebe. That also implies that all the dwarf planets except Pluto and Eris are not in equilibrium either, as Iapetus is larger (though not necessarily more massive) than Makemake, Haumea, 2007OR10, Quaoar, and all the other dwarf planets/ candidates. In my opinion, 'planet' should be used to embrace obviously spheroidal objects without significant porosity (thus excluding Methone and Dactyl, both of whom lack the complex geology a true planet can display).

KRUGER: 11/03/2015 11:05 CST

greetings from los robles astronomy club !!! we have members from 14 diferent schools here, and this picture is just perfect for classroom activities, !! also ver beautifull.thank you and congratulatios Mrs regards. Patrick

Eltodesukane: 11/04/2015 10:48 CST

The ISS has a price tag of more than $150 billion. ( while New Horizons cost about $0.7 billion. Which has done more for science, for space exploration, for astronomy, for knowledge? How many other "New Horizons" could we have sent for the price of the ISS?

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