Re: "Pluto Now Called a Plutoid"

In message <nI2dnQje4vHry_3VnZ2dnUVZ_srinZ2d@xxxxxxxxxxxxx>, Erik Max
Francis <max@xxxxxxxxxxx> writes
Brett Paul Dunbar wrote:

In message <4b025605-faeb-465b-ab4f-70647fb2e05e@xxxxxxxxxxxxxxxxxxx
roups.com>, Quadibloc <jsavard@xxxxxxxxx> writes

As for Pluto, however, since it _is_ much larger than any other
Plutino, it _has_ cleared its own orbit of everything but debris, even
if there are similar bodies in orbits only a little further out, so it
could be left as a planet (but then its diameter, rather than
Mercury's, would mark the dividing line between a QPC and a PC). That
would also mean that Eris would become a planet after we knew that it
was significantly bigger than all the Erisinos out there...
Pluto isn't a planet because it hasn't cleared its orbit Neptune
has. Neptune has cleared that region of space of debris, what's left
either orbits Neptune (e.g. Triton the second largest object in that
region), is in a resonance with Neptune (e.g. Pluto, the third
largest object) or is a transient (e.g. various Comets).

More accurately and succinctly: Pluto is no longer a planet because
the IAU said so. Their "objective" definition was so vague that they
had to include a list of planets to make sure people knew what
modifications that entailed, and Pluto wasn't on it.


No that is actually totally wrong. The definition is entirely
unambiguous, given the sheer size of the gap. On either the
Stern-Levinson parameter or Soter's Planetary discriminant, the gap is
about five orders of magnitude.

Name Stern-Levinson Planetary Mass (kg)
parameter discriminant
(Earth=1)

Earth 1.00 1.7×10^6 (5.9736×10^24 kg)
Venus 1.08 1.35×10^6 (4.8685×10^24 kg)
Jupiter 8510 6.25×10^5 (1.899×10^27 kg)
Saturn 308 1.9×10^5 (5.6846×10^26 kg)
Mars 0.0061 1.8×10^5 (6.4185×10^23 kg)
Mercury 0.0126 9.1×10^4 (3.302×10^23 kg)
Uranus 2.51 2.9×10^4 (8.6832×10^25 kg)
Neptune 1.79 2.4×10^4 (1.0243×10^26 kg)
Ceres 8.7×10^-9 0.33 (9.5×10^20 kg)
Eris 3.5×10^-8 0.10 (1.66×10^22 kg)
Pluto 1.95×10^-8 0.077 (1.29×10^22 ± 10% kg)


The number of objects classified as planets was small enough that it was
possible to list all the known objects that meet the criteria, listing
examples can be useful and there are only eight known so it was entirely
practical to list all of them. The definition of a Dwarf planet on the
other hand is far less clear for example Vesta appears to have become
spherical, solidified then been distorted by as massive impact, it is
unclear whether that is a dwarf planet or a small solar system body.

Stern's objections are more terminology than content, the IAU definition
matches what he calls an überplanet, clearly he finds the class itself
useful even if he isn't happy about the name.

In fact this postulated Kuiper wall object would also clearly be a
planet, indeed its existence is postulated as there is evidence that the
region of space just outside the Kuiper belt has been cleared, an object
which has cleared its region of space is a planet therefore if it is
responsible for the Kuiper wall then it is a planet.
--
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Brett Paul Dunbar
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Relevant Pages

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