planet X found

[Irbis]

Whilst it is true that Jupiter is pretty much a too-small dwarf star, it and Sol have one major distinction; namely, Sol is actually fusing hydrogen and generating energy and Jupiter is not.

Actually, Jupiter and Saturn generate energy too, in K-H process. Border between gas planets, dwarfs and stars is really fluid, if astronomers could pick one defining trait they would have already done so.

[Eternal_Freedom]

As interesting as the K-H mechanism is, your sorta ignored the earlier part of that sentence where I said "Sol is fusing hydrogen" whilst Jupiter et al. aren't.

[Irbis]

As interesting as the K-H mechanism is, your sorta ignored the earlier part of that sentence where I said "Sol is fusing hydrogen" whilst Jupiter et al. aren't.

Which would be meaningful, but in the end, it's useless criterion of differentiation of stars from gas giants, as one, not every star use the same process of stellar nucleosynthesis (Sun for example uses PP process, but smaller stars burn more deuterium and larger ones use CNO cycle). Two, from what I remember from astrophysics from my university times, Jupiter does fuse hydrogen, just does it on comparatively very slow rate compared to star. Still, the point remains the same, there is nothing solid allowing you to divide gas giants, dwarfs and stars into concrete units.

[Eternal_Freedom]

Well I could add that a star is generally a star when it's either a) the centre of a solar system or b) part of the common centre of mass of a solar system (for binary/multi-star systems).

Or we could go with "a star is a large body of hydrogen (any isotope) that is fusing it to helium to generate energy and has/or will make it onto the main sequence."

Plus I really don't see why you brought up different fusion processes since no matter what process they are using, it's still using hydrogen (the element not the isotope) into helium. Plus, the pp1 chain does fuse deuterium, it just has to make it from H-1 first).

[Irbis]

Well I could add that a star is generally a star when it's either a) the centre of a solar system or b) part of the common centre of mass of a solar system (for binary/multi-star systems).

Or we could go with "a star is a large body of hydrogen (any isotope) that is fusing it to helium to generate energy and has/or will make it onto the main sequence."

Ok, though what I suspect astronomer's problem with definition such as this is that the object can be multiple things due to its placement. Consider: by above definition, Jupiter would be a star if we removed it from solar system and placed it alone, having moons big enough to be considered planets in their own right.

The same is true for Pluto: if we consider it a planet, we run into Triton, Neptune's moon virtually identical to Pluto (except even larger) that is now thought to have had similar orbit until Neptune captured it. Hence, the need for a new category of objects that don't change status according to place they're in. Astronomers are just people, wishing to have simple neat grades is just human nature

[Borgholio]

Stars can be rogue stars, wandering without solar systems. Generally a star is considered a star if it shines (or used to shine). So our sun is a star. Jupiter is not because it never shined. A white dwarf is a star because it still shines (or used to, if it's really old). A brown dwarf is a category all it's own which is less than a full-on star but more than a large planet like Jupiter.

[Irbis]

Jupiter is not because it never shined. A white dwarf is a star because it still shines (or used to, if it's really old).

Uh, except for the fact all planets shined at one time, even Earth. Look at furnace filled with molten steel, then extrapolate it to whole Earth's surface for about first 250 to 500 million years of its existence. We're living on a star?

A brown dwarf is a category all it's own which is less than a full-on star but more than a large planet like Jupiter.

Problem is, there is no border between gas giant and brown dwarf that wouldn't be completely arbitrary and would apply to one but not the other.

[Borgholio]

Uh, except for the fact all planets shined at one time, even Earth. Look at furnace filled with molten steel, then extrapolate it to whole Earth's surface for about first 250 to 500 million years of its existence. We're living on a star?

Let me rephrase, a star is an object that is generating most of it's energy through the process of nuclear fusion. Smartass.

Problem is, there is no border between gas giant and brown dwarf that wouldn't be completely arbitrary and would apply to one but not the other.

Quite right. In fact, the arbitrary rule of thumb is 13 Jupiter masses. But the truth is there really is no clear-cut line between big gas giants and small brown dwarfs. The only actual dividing line between brown dwarf and a normal star is whether or not there is a sustained fusion reaction in the core or not.


http://en.wikipedia.org/wiki/Brown_dwar ... ss_planets

[Eternal_Freedom]

Well I could add that a star is generally a star when it's either a) the centre of a solar system or b) part of the common centre of mass of a solar system (for binary/multi-star systems).

Or we could go with "a star is a large body of hydrogen (any isotope) that is fusing it to helium to generate energy and has/or will make it onto the main sequence."

Ok, though what I suspect astronomer's problem with definition such as this is that the object can be multiple things due to its placement. Consider: by above definition, Jupiter would be a star if we removed it from solar system and placed it alone, having moons big enough to be considered planets in their own right.

The same is true for Pluto: if we consider it a planet, we run into Triton, Neptune's moon virtually identical to Pluto (except even larger) that is now thought to have had similar orbit until Neptune captured it. Hence, the need for a new category of objects that don't change status according to place they're in. Astronomers are just people, wishing to have simple neat grades is just human nature

As an astronomy graduate, this is pretty much accurate. Clearly definitions change as we get more information, the original definition of a "planet" was a wandering star, a bright spot that moved across the sky.

So, the Sun is a star, since it shines by means of nuclear fusion. Jupiter is a planet because it does not shine by sustained fusion, it orbits a star, is spherical due to it's own gravity and has cleared it's orbit. Pluto et al are "dwarf planets" because they haven't cleared their orbits.

And that's the way we think planets work today. Check back in a month's time, it will probably have changed by then.

[Purple]

An analogy would be, as mentioned, taxonomy. There's subspecies, divisions within a species that show enough differences to have some distinction but not enough to say it isn't the same species. Let's not go into why species is a messy as hell classification as it is... The whole planet/dwarf planet thing is basically trying to avoid the problems biology faces when it's categorizing life forms.

What exactly is bad about the classification used by biology? It always seemed quite nice to me.

[Napoleon the Clown]

It isn't bad so much as, say, species is fuzzy as hell. The closest we have to a definition right now, as I understand it, is if the critters can interbreed and produce viable offspring. Nevermind shit that doesn't have sexual reproduction, things like ring species mess with that to hell and back. Because of how biology is, it's damn hard to get a good definition for stuff.

So yeah, I'm basically saying that they're trying to avoid the weakness found in biology, where it's so damn hard to classify things. The dwarf planet category is astronomers' attempt at making things a little less haphazard and a little more precise, with less room for "maybe, maybe not." Biology is doing the best we can with categorizing things, astronomy is simply trying to keep things precise. Planet/dwarf planet is one of those cases that makes things a little easier. I guess it could be considered akin to how we define snakes vs lizards, too. There are legless lizards out there, so just saying any cold-blooded creature with scales that breathes air instead of water is a snake wouldn't work. (This is something of an oversimplification, I'm sure.)