Home   < DPPH List Object Query Links & Contact


What is a Dwarf Planet ?

According to new definitions adopted by the IAU, the astronomical community around the world, guives birth to a new category of planetary bodies. This new category is called "Dwarf Planet".



Criteria to classify a body as a “dwarf planet”


1)       The diameter of the body should be D >450 km for icy objects and D >800km for rocky ones. These limits are not precisely determined and they depend on factors like the composition of the material and the ambient temperature.


2)       If there is a direct measurement of the relative roughness with values < 1% and the shape correspond to a figure of equilibrium, the candidate is accepted (Case I).


3)       If not, we analyze the observed lightcurve amplitude (Dm). ([1])


4)       If Dm < 0.15 mag, the candidate is accepted as a small departure from a sphere or MacLaurin spheroid with small albedo spots (Case II).


5)       If Dm ≥ 0.15 mag, the lightcurve (the intensity square) is fitted to a Fourier series of order two and the ratio (b) between the quadratic sums of the coefficients of order 1 and 2 is computed.


6)       If b <0.25, the lightcurve can be fitted to a triaxial ellipsoid. We then analyze if this ellipsoid corresponds to the Jacobi family. We compute the range of possible densities as a function of the assumed aspect angle of the observed lightcurve.


7)       If there are solutions with r ≥1, the candidate is accepted as a Jacobi ellipsoid (Case III).


8)       If all the solutions correspond to r <1, the candidate is not accepted. The size might be overestimated due to

an assumption of a low albedo (pv>>0.1) (Case IV).


9)       If b >0.25, the candidate is not accepted, the lightcurve departs from an ellipsoidal figure possibly due to important contributions of albedo spots or there is an overestimation of the size due to an assumption of a low albedo (pv>>0.1) (Case V).



[1] Since we do not have any information of the viewing geometry, we will assume that the observed amplitude corresponds to the maximum possible amplitude for the object.


Learn more about IAU resolutions

Definition of a planet in the Solar System

Plutoid chosen as name of Solar System objects like Pluto (press release)

In the following article you will find the theoretical and observational basis
of the criteria to classify an object as a "dwarf planet":

Which are the dwarfs in the Solar System?
Tancredi, Gonzalo; Favre, Sofía
Icarus, Volume 195, Issue 2, p. 851-862

Send and email to the author to request a personal copy of the article

Home   |   DPPH List   |   Object Query   |   Links & Contact