A Big Year for Dwarf Planets

Liz: For us, one of the most powerful features of the Wolfram Language is the way it understands real-world (or, in this case, real-solar-system) objects, making it easy to incorporate data on all kinds of things into your projects. In this rather lovely instance from the good people at Wolfram, you can calculate and visualise the relative size of a number of those things on your Raspberry Pi, including the state of Texas, the dwarf planet Ceres, the former dwarf planet Pluto, and the Moon.

2015 is shaping up to be an interesting year in space exploration. For the first time, we will get up-close views of a dwarf planet. In fact, two different spacecraft will visit two different dwarf planets. The Dawn spacecraft is nearing its second primary target, Ceres, later this week. Later this year, the New Horizons spacecraft will visit Pluto.



Of course, related to all of this is the public controversy over the International Astronomical Union’s (IAU) “demotion” of Pluto from planet status. Regardless of your view on the matter, Pluto is still there, just as it always was, and nothing has changed concerning its existence. It doesn’t really matter what we call it. I won’t go into great detail to explain why Pluto was demoted, but we can use the Wolfram Language to explore some of the primary reasons.

One of the requirements for being labeled a planet according to the IAU definition is that the object must have cleared its orbit of other bodies. Planets have typically either absorbed or thrown out intruders so that they dominate their orbital zones. In the case of Ceres and Pluto, both bodies violate this requirement. Here is a graphic showing the orbit paths of Jupiter and Mars in orange, several large asteroids in blue, and the orbit of Ceres in red. As you can see, Ceres lies in the asteroid belt between Mars and Jupiter, along with many other objects, all classified as dwarf planets, asteroids, minor planets, or small solar system bodies, depending on your preference (or in the case of Ceres, by IAU definition).



A similar analysis can be done for Pluto. In the following example, you can see that all of the planets’ orbits (in orange) are relatively nice and concentric until you get to Pluto (in red), which crosses Neptune’s orbit. In addition, there are a number of other known “Plutoids” (in blue) that cross orbits with Pluto. And in fact, there are many more such objects. So if Pluto is a planet, then all of these objects, and many more, could potentially be declared planets, and that would be a nightmare for educational books to keep up with. The traditional question of “How many planets are there in the solar system?” becomes a large number that keeps growing as more objects are discovered.


Something else that is interesting to explore is size. Many people don’t really comprehend the sizes of Ceres and Pluto. The only size restriction provided by the IAU definition for a planet is that the body must be large enough for gravity to have pulled it into a spherical shape. This seems to be the case for both Ceres and Pluto, but this alone doesn’t make them planets. But it’s still interesting to visualize the sizes of these bodies by comparing them to something we are more familiar with. We can make use of GeoGraphics to put the size of these bodies into perspective. Here is a 2D map of the United States, with Texas highlighted in red. The inner disk represents the size of Ceres projected against Texas, the middle disk represents the size of Pluto, and the outer disk represents the size of our Moon. So the cross-section of Ceres is about the same size as Texas. Both Ceres and Pluto are noticeably smaller than our Moon.


With a bit more exploration, we can move this visualization into three dimensions by using texture mapping to move the above map onto a sphere.

First, we define a couple of geographic entities we will need:


Next, we use GeoGraphics to construct our 2D map and then convert it to an image:



Then we obtain several radius values we need, making sure they are all using the same units:



For positioning things, it’s useful to determine a center point for our map:



We need to position Ceres, Pluto, and the Moon near the center point of the map and offset them to appear as if they are sitting on the surface of Earth at that point:





We can apply the map as a texture on a sphere that represents the Earth using ParametricPlot3D:



Finally, we can combine the pieces and compare the sizes of Ceres, Pluto, and the Moon to the Earth:



In the coming days and weeks, the Dawn spacecraft will provide us with the first close-up views of the dwarf planet Ceres. Later this year when New Horizons passes Pluto, we will get our first close-up views of the more controversial member of this group of objects. Nothing we discover will result in Pluto being reinstated as a planet, as the reasons for its demotion are still there. But we will obtain more data on these small objects than has ever been gathered before, and it will give us a new understanding of these often under-appreciated members of our solar system. Dwarf planets are worthy of study regardless of what they are called.

Download this post as a Computable Document Format (CDF) file.


Nuex* Luke Castle avatar


Tom West avatar

You’ve combined two of my favorite things: New Horizons and Raspberry Pi :-)

Jim Manley avatar

A starfield slowly appears out of sheer blackness … a spacecraft flits into view from overhead while an icy spheroid gradually expands from the right-of-center of the starfield … New Horizons is approaching the goal it was launched toward nine years earlier, after being slingshotted by Jupiter eight years ago … at least that’s how Steven Spielberg would probably introduce it :)

Some fun facts about New Horizons: the data transmission rate from the spacecraft is now about 1 kilobit/second and it will take 45 to 90 days to transmit the approximately 9 gigabytes of data to be collected during the closest-point-of-approach on July 14th (so, don’t complain about how few megabytes per second you’re getting on your smart phone, OK? ;) The one-way signal propagation time from Pluto to Earth is now about four hours and 20 minutes, so no quick adjustments can be made if something winds up out-of-focus … that we won’t see for months anyway due to the low data rate. Students will be witnessing reception of images and other data as they are received into the Fall, which I’m incorporating into my lessons.

New Horizons is so cool … -387 to -369 degrees Fahrenheit (-233 to -223 Celsius) on the surface of Pluto, in fact! :D

Kevin avatar

Thanks for this information, I’ve downloaded the CDF and am playing with it at the moment. I’m going to have to learn some more about Woolfram.
Thanks very much,

Tobias K avatar

Great graphics! Having worked previously for Lowell Observatory, I still am not pleased with the IAU’s decision on the classification criteria of what constitutes a planet, but that aside, it will be exciting to get these close-up images from New Horizon next month. You know, if our own planet were as small as Pluto, I seriously doubt we would be calling Earth a “dwarf planet”.

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