Sol Sun

MercuryMercury ____________

VenusVenus crescent

Earth &
Earth's Moon

Mars Mars: Spirit 
       Sol 30 ____________

Asteroids Vesta

Jupiter Jupiter storm 
       north of Great Red Spot ____________

Saturn Saturn's Rings

Uranus Uranus' Rings

Neptune Neptune

Pluto & Pluto Kuiper Belt

Comets &
Oort Cloud

Comments & Submissions ____________

Figure 1. Dwarf planets Pluto and Ceres. (c) Wikimedia

Is the Earth a Dwarf Planet?

By F. E. Harris

October 31, 2011


With the discovery of over 1000 NEOs, it's getting hard to say Earth fulfills the third part of the definition of a planet, that it has cleared its orbit of debris. Same for Mars. To be ridiculously logical about it, this means that Earth and Mars ought to be demoted to dwarf planet status.

Actually, this article is just an excuse for showing some cool pictures of near-Earth objects. Happy Halloween!

Asteroid_2004_FH animation
Figure 2 : On March 18, 2004, a 30-meter asteroid, 2004 FH, passed by the Earth only 42,600 km (26,500 mi) away, about one-tenth the distance to the Moon. The rapid streak in the picture is a meteor. (c) Wikimedia


Section 1: Definitions

The IAU (International Astronomical Union, 2006) definition for a planet is, in the Solar System, a planet is a celestial body which:1
1. is in orbit around the Sun,
2. has sufficient mass to assume hydrostatic equilibrium (a nearly round shape), and
3. has "cleared the neighbourhood" around its orbit.

The IAU definition for a dwarf planet is:2
1. a celestial body orbiting the Sun
2. that is massive enough to be spherical as a result of its own gravity
3. but has not cleared its neighboring region of planetesimals and
4. is not a satellite.


Asteroid 4 Vesta
Figure 3. Asteroid 4 Vesta. It is not quite round enough to be a dwarf planet. Pluto may have the same problem. We won't know until New Horizons gets to Pluto in 2015.


Section 2:

I don't think they will demote Earth and Mars to dwarf planets because of this. Instead, I think they will have to change the definition again soon. Perhaps the IAU will go with the degree of roundness or the surface gravity, which, depending on the number, might make Ceres, and possibly Pluto, eligible for planet status again. Ceres was considered a planet in the early 1800s, shortly after it was discovered.

Asteroid 4179 Toutatis
Figure 3 : Asteroid 4179 Toutatis. This is a high resolution radar image. Toutatis is roughly 4.5 km by 2.4 km by 1.9 km, and has a mass of roughly 5.0 x 1013 kg. (c) NASA / Goldstone observatory.8


Section 3: Getting Back in the Club

It's pretty much impossible for Pluto to regain its planet status, unless you also let Ceres back into the club.4,5 If degree of roundness is used to set membership, then Pluto and Ceres will have to wait for photos from the New Horizons and Dawn probes, before their status can be determined.

Comet Encke
Figure 3 : Comet Encke. Another example of the junk Earth has not cleared from its orbit. Encke is a close-orbit comet with a period of only 3 years. Its orbit crosses Earth's. (c) Wikimedia.7


Section 3: The Simplest Fudge

Or, one could fudge this in the simplest way, and say that nothing below the diameter of Mercury is a planet. Probably they would want to set the definition about 10 km below the present diameter of Mercury, just in case Mercury is shrinking.6

There is some evidence for tectonic forces shaping the surface of Mercury at this time, besides the effects of meteor impacts. It is not clear yet what these forces are, or how they operate, only that they are not identical to Earth's plate tectonics.

Figure 4. Tectonic forces on Mercury.
The blocks and gaps in this photo are similar to the Basin and Range area of Utah, USA, Earth, and may indicate the crust of Mercury is stretching or contracting.


Section 4: Conclusions

This is just my Goofy prediction (Sorry, Pluto!). I think that once the New Horizons probe reports back with detailed pictures of the surface of Pluto, there will be tremendous pressure to let it back into the club of planets. I think the IAU will cave, and re-admit Pluto.

But I also think there is no way to re-admit Pluto, and keep Ceres out of the club. So I think the textboks after 2018 or 2020 will say the Solar System is a 10-planet system.

Asteroid Steins movie
Figure 5. Rosetta Encounter with Asteroid 2867 Steins. Rosetta came within 800 km of asteroid Steins. The direction of Rosetta's OSIRIS camera changes during this video, to keep Steins in view. Credit: ESA 2007 MPS for OSIRIS Team MPS/UPD/LAM/IAA/RSSD/INTA/UPM/ DASP/IDA9


NASA will maneuver Dawn away from Vesta, and over to Ceres, next year. We will get our first good look at Ceres in 2012. The New Horizons probe will fly past Pluto in 2015. What New Horizons will see could hold plenty of surprizes. I think the IAU will not reconsider the definitions of 'planet,' and 'dwarf planet,' until they have some more solid data on Pluto.

Pluto may have some tricks up its sleeve, but if the space probes come through, we will all be in for a treat.



1. "IAU definition of planet" Wikipedia, 27 September 2011.
2. "Dwarf planet," Wikipedia, 13 October 2011.
3. "4 Vesta," Wikepedia, 3 July, 2011, with later edits.
4. "Ceres (dwarf planet)," Wikepedia, 31 October 2011, latest edit.
5. "http://en.wikipedia.org/wiki/Pluto," Wikepedia, 31 October 2011, latest edit.
6. "Mercury - Then and Now: Mariner 10 Revisited,"
Harris, F. Solar System Science, October 15, 2011, http://solarsystemscience.com/articles/Mercury/2011.10.16a/MercuryThenAndNow.html
7. "Comet Encke" Wikipedia, 12 September 2011,
8. "First Images of Near-Earth Asteroid TU24," January 29, 2008, The Planetary Society.
9. "Results from the Rosetta Encounter with Asteroid 2867 Steins ," Emily Lakdawalla,
January 11, 2010, The Planetary Society. http://planetary.org/news/2010/0111_Results_from_the_Rosetta_Encounter_with.html
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