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Emily Lakdawalla • Nov 02, 2015

The round worlds in the solar system: An updated graphic

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 (2015 version, update available)
Every round object in the solar system under 10,000 kilometers in diameter, to scale (2015 version, update available) 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.Image: 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.

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
Simulated view of a half-phase Pluto This view of Pluto was generated from images captured by New Horizons' Multispectral Visible Imaging Camera (MVIC).Image: NASA / JHUAPL / SwRI / Roman Tkachenko
Simulated view of a half-phase Charon
Simulated view of a half-phase Charon This view of Charon was generated from images captured by New Horizons' Multispectral Visible Imaging Camera (MVIC).Image: NASA / JHUAPL / SwRI / Roman Tkachenko

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!

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