Slashdot Mirror
Defining "Planet"
beardoc writes "The Sydney Morning Herald is running a story today about a controversial proposal to define what size a planet might be - depending on what the final definition of how big a planet is, we could end up losing Pluto (at 2300 kilometres) to the status of "asteroid" or gaining three more planets - Quaoar, Varuna and Ceres."
Eros is the same deity as Cupid...one is Greek, the other Roman. Eros is actually the Greek name, whereas Cupid is the Roman. However, since at the time the moon Eros was named Cupid was already defined in our culture as a fat little fairy with wings, they went with Eros.
This message brought to you by the Council of People Who Are Sick of Seeing More People.
Because the traditional designation of what made up a planet was anything that we could find that orbits the sun. We didn't include comets because they looked different from planets and we didn't include asteroids because we couldn't resolve them. Now that we continue to find many large objects that are really little different from Pluto it has suddenly become important to have a real definition of which are planets and which are just big asteroids.
Also, as we find bodies orbiting other stars, the traditional designations for planets is obviously useless.
We hope your rules and wisdom choke you / Now we are one in everlasting peace
Because some moons are larger than Pluto... would they be considered planets?
t s/nineplanets.html )
Diameters:
Pluto: 2274km
Charon: 1172km
Ganymede (orbits Jupiter): 5262km
Callisto (same): 4800km
Titan (orbits Saturn): 5150km
Triton (orbits Neptune): 2700km
Earth: 12756km
Moon: 3476km (Yes, our Moon is larger than Pluto)
Mars: 6794km
Deimos (orbits Mars): 12.6km
Phobos (same): 22km
(all figures courtesy http://seds.lpl.arizona.edu/nineplanets/nineplane
In other words, simple definitions based on size are inadequate. Also, since they're debating whether or not Pluto is a planet, the criteria that it orbits the sun may also be inadequate.
A planet is something which: orbits a star AND is round AND is larger than an arbitrary size AND.. what? The above criteria still allows for a lot of things to be planets that aren't.
We know so little about massive, non-solar bodies outside our solar system. Let's do a little more research on them before we start redefining things.
"In the end they will lay their freedom at our feet and say to us, 'Make us your slaves, but feed us.'" -Dostoevsky
This is sort of handled(here):
Since the COM is inside the Earth, I think it's fair to say that the Moon orbits the Earth (and not vice versa).
The Mongrel Dogs Who Teach
The distinction between a planet-moon and binary planet system is usually the common centre of mutual orbit. If it resides in one body, that body is the planet, and the other a moon, however if it lies between them, in space, then it is a binary planet system.
But doesn't it [Ceres] have a satellite? -- and -- What would we qualifty that as, because a satellite must orbit a planet.
It doesn't appear that Ceres has any satellites. But, there are 31 asteroids that do! That doesn't make them planets though...they're just small asteroids with really small moons.
Can anyone remind me what that sequence of numbers is called that vaguely predicts the distances of planets from the Sun?
Yep, its the Titius-Bode Law. Ceres does fit into this. But the reason we don't have a planet in between Mars and Jupiter is because "many astronomers think the asteroid belt is where a planet tried to form, but was pulled apart before it could solidify, caught between the strong opposing tugs of Jupiter and the sun's gravity." Quote taken from here.
Why does a planet _have_ to be a shpere...How perfect a sphere?
Well.... Ceres's shape is too distorted. Its shape is not spherical enough to be like regular planets. And, to get really technical, no planet is really a sphere. Due to rotation, all planets have a slightly distorted shape.
Let's rephrase that: there *might* be hundreds/thousands of Pluto-sized objects. But we certainly haven't found any yet!
- Pluto - 2300 km
- Quaoar - 1300km
- Varuna - 900km
- Ceres - 479km
- Chiron - ~175km
Note that Quaoar, the largest of the bunch, is half Pluto's size and barely larger than Pluto's moon, Charon.As long as Pluto is substantially larger than any other known transneptunian object, it doesn't seem like we would need to worry about planetary definitions.
Yours truly,
Jeffrey Boulier
The most common Latin word for "Earth" is Terra, the name of the goddess of the Earth. That's right, Terra. She is I believe almost exactly analogous to Gaia.
Gaia is Greek; another Greek form of the name is "Ge." She is a major early goddess (early meaning pre-Olympian).
"Tellus" is Latin for "land" or "earth," including the concept of Earth as a planet. The name is used for a goddess; that -us ending is not the same one you know from "alumnus," but is feminine 3d declension, and forms its plural as "Tellures." I don't know how it relates to "Terra" or "Gaia" (most educated Romans knew Greek as a second language).
Quaoar, Ceres, and Varuna are all the names of gods or goddesses. Varuna is a Hindu god, of rain, I believe, and so a type of creator god; Quaoar, a native American creator god (IIRC); Ceres is the goddess of agriculture in Roman mythology (she is called Demeter in Greek; the long Homeric poem Hymn to Demeter is the centerpiece of her myth; her daughter Persephone might be familiar to SF fans).
Ceres is also the patron goddess of Sicily, and her discoverer was G. Piazzi, a Sicilian scientist. It was given such an important name (Ceres was a major goddess) because it was assumed, from the application of Bode's "Law," that there must be a planet between Jupiter and Mars, and when Ceres was found, it was at first trumpeted as a planet. However, when the asteroids named after Juno (=Hera, the queen of the Gods), Pallas (=Athena, the goddess of wisdom, warfare, etc.), and Vesta (~Hestia, the goddess of the hearth and home, more important to the Romans than to the Greeks - you've probably heard of the Vestal Virgins, the priestesses of Vesta who kept the eternal flame going in her temple and took an oath of chastity they were executed for violating) were all found in roughly similar orbits, they were reclassified as not "planets" but "asteroids."