Originally posted by BiggerPicture
which one is it...
That's a good question and, coincidentally, there was an article on the
NYT space and cosmos section
Confirmed: Vesta, the second largest of the asteroids, is a runt planet.
Observations by NASA’s Dawn spacecraft show that Vesta formed within the first few million years of the solar system, that its surface is in places
as bright as snow and in others as dark as coal, and that it contains an iron core that may have even briefly generated a magnetic field — all very
Both Vesta and Ceres, the largest asteroid and the Dawn’s next destination, are also heavy enough that gravity has made them round.
“The Dawn team finds itself calling these things planets all the time,” Christopher T. Russell, the mission’s principal investigator, said in an
In the current parlance of astronomers, however, Vesta and Ceres, like Pluto, are dwarf planets, not planets.
So yes, seems like they finally agreed to define Vesta as a dwarf planet. In the NYT paper, they define the word "planet" as follow:
...set by the International Astronomical Union in 2006, not only takes into account size and appearance, but also requires that a planet be the
gravitational bully in its orbit. While Vesta and Ceres are round, they have not vacuumed up or pushed away thousands of other rocks in similar
Interestingly, I have found the report of the 2006 IAU session
about this very
question where members gathered at the 2006 General Assembly agreed that a "dwarf planet" is a celestial body that:
(a) is in orbit around the Sun,
(b) has sufficient mass for its self-gravity to overcome rigid body forces so that it assumes a hydrostatic equilibrium (nearly round) shape,
(c) has not cleared the neighborhood around its orbit, and
(d) is not a satellite.
(a), (c) and (d) were OK, but not (b).
They also said:
A new distinct class of objects called "dwarf planets" was also decided. It was agreed that "planets" and "dwarf planets" are two distinct classes
of objects. The first members of the "dwarf planet" category are Ceres, Pluto and 2003 UB313 (temporary name [Note by me: name is Eris as for
now]). More "dwarf planets" are expected to be announced by the IAU in the coming months and years. Currently a dozen candidate "dwarf planets" are
listed on IAU's "dwarf planet" watchlist, which keeps changing as new objects are found and the physics of the existing candidates becomes better
The "dwarf planet" Pluto is recognized as an important proto-type of a new class of trans-Neptunian objects. The IAU will set up a process to name
Oddly, not a word at this time about Vesta on the IAU session, even considered as an asteroid, but
provides an more thorough explanation for this:
Vesta's shape is relatively close to a gravitationally relaxed oblate spheroid, but the large concavity and protrusion at the pole combined with a
mass less than 5×1020 kg precluded Vesta from automatically being considered a dwarf planet under International Astronomical Union (IAU) Resolution
XXVI 5. Vesta may be listed as a dwarf planet in the future, if it is convincingly determined that its shape, other than the large impact basin at the
southern pole, is due to hydrostatic equilibrium
.... that reproduce the (b) point of the definition of a dwarf planet in the 2006 IAU session.
However, in the NYT paper, they do not really
explain exactly why Vesta should be considered as a dwarf planet for now, do they succeed in
proving that its shape is finally due to hydrostatic equilibrium?
Maybe in the yesterday's "Science" article there was a more thorough explanation...
edit on 12-5-2012 by elevenaugust because: (no reason