Pluto’s Place in the Solar System

When New Horizons​ launched, its mission in the public imagination was to complete the set of the nine planets. No doubt this helped the mission get funded, and I respect that.

Nine Planets with Pluto

“Nine Planets” Collection with new image of Pluto, created by Ben Gross ( bhgross144@twitter ).

By the time it arrived, though, we had accumulated enough evidence to establish that Pluto is not best thought of as one of the “major planets” of our star system. We now think there are only 8 of those, or as I prefer to think of them, 4 “terrestrial planets” and 4 “gas giant planets” (with their accompanying moon-systems, containing more than a dozen major worlds among them):

Solar System Worlds

Worlds of the solar system to same scale, dominated by four gas giant planets, but also showing the four terrestrial planets and many of the larger moons. I think this picture also shows adequately why I think it’s a little silly to lump the “terrestrial planets” (Mercury, Venus, Earth, and Mars) with the “gas giant planets” (Jupiter, Saturn, Uranus, and Neptune) — they are entirely different types of objects!

Now, Pluto is better seen as the first Kuiper Belt Object to be explored. And thus it is the beginning of a whole NEW set of objects in our solar system. There are a lot of small worlds out there in the space beyond Neptune. The Kuiper Belt is like a second, icier asteroid belt around our Sun. And there are scattered worlds beyond those (collectively called “Trans-Neptunian Objects”, which include Kuiper Belt Objects, Scattered Kuiper Belt Objects, and more — we’re just beginning to learn about this part of the solar system, because we’re only now getting sensitive enough telescopes to see these objects).

8 TNOs

The eight largest Trans-Neptunian Objects in the solar system, including Pluto, Eris, and Sedna. ( Credit: Lexicon@Wikipedia / CC By-SA )

Eris is almost exactly the same diameter as Pluto and quite a bit heavier. It is currently about three-times further from the Sun, though in the middle of the 23rd century it will swing back to a distance similar to Pluto’s in its 558 year orbit.

Eris and Dysnomia

Eris and its moon, Dysnomia seen in a Hubble Space Telescope image from Earth orbit.


Orbit of Eris

Eris is currently just past its 1977 aphelion (furthest point from the Sun), and thus is about 95 AU from the Sun. Its next perihelion (closest point) is in about 2256.

The highly elliptical orbit of Sedna

Sedna’s orbit carries it far out into the dark on an orbit with a period of more than 11,000 years, which takes it out to nearly 1000 AU.

Sedna is currently just a little bit closer, and will be making its closest approach late in this century — only to swing back out into the dark for over 11,000 years, in a ellipse that carries it nearly to 1000 AU, where it spends most of its time.

Artist's Concept of Sedna

For now, Sedna is only a tiny blip in our best telescopic images, so we have to imagine what it might look like.

There may be other worlds out there. Some may even be larger than these. Certainly there are many smaller ones.

There will always be more to explore.


About Terry Hancock

Terry Hancock is the producer and director of "Lunatics" ( ). He is also a regular columnist for Free Software Magazine ( ), and a lifelong advocate for space, science, and technology. More at
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