Comets, Planets, Stars, what's in a title?

[halfeye]

Most of the solar system objects beyond Neptune are comets, some of them very big for comets, but they'd have tails if the came close to the sun.

Ceres is a comet, not as big as Pluto, but big.

Some of the exo-planets found recently are hot Jupiters, but aren't they almost certainly rocky and not like Jupiter at all?

If there was a rocky object with the mass of a red dwarf star (but insignificant amounts of hydrogen or even helium), would it light up?

[Mechalich]

Ceres is a comet, not as big as Pluto, but big.

No, Ceres and Pluto are dwarf planets, as are many other similar bodies in the outer solar system. Comets are a class of small solar system bodies, defined primarily by the characteristics of their orbit. The International Astronomical Union is kind of...lousy when it comes to proper terminology, partly because the study of the various non-planet non-star things in the universe is actually still quite new and they haven't finished fighting out the nomenclature battles yet (this is similar to the situation in biology pre-Linnaeus), or Geology in the mid 19th century.

Some of the exo-planets found recently are hot Jupiters, but aren't they almost certainly rocky and not like Jupiter at all?

If there was a rocky object with the mass of a red dwarf star (but insignificant amounts of hydrogen or even helium), would it light up?

Actually no. It is possible to determine the gross chemical composition of many exoplanets (in the best case one that can be directly imaged can have spectroscopy data taken). The largest known rocky planet has ~40 times the mass of Earth, which is larger than models predict should exist - under normal planet formation models a planet that breaches certain size thresholds will become a gas giant due to the way material is acquired - and may be a bizarre gas giant core that lost or was somehow prevented from acquiring large amounts of gaseous material.

[Palanan]

Originally Posted by halfeye
Some of the exo-planets found recently are hot Jupiters, but aren't they almost certainly rocky and not like Jupiter at all?

No.

Hot Jupiters by definition are Jupiter-like planets orbiting extremely close to their stars, sometimes with an orbital period (year) of a few dozen hours or less. Apart from their extremely close orbits, they are very much like Jupiter in terms of size, mass, and composition. Hot Jupiters are an entirely new class of planet which, obviously, doesn’t exist in our solar system.

“Rocky” planets, better known as terrestrial worlds, are typically one to several Earth-masses, with the larger ones known as superterrans or “super-Earths.” Superterrans are another new class of planet, and given their greater mass and gravity, some of them may have denser hydrogen-helium atmospheres. But they are still fundamentally terrestrial planets.

I encourage you to take a look at the Periodic Table of Exoplanets, which breaks down observed exoplanets into categories based on mass and distance from star. Be aware that the apparent predominance of Hot Jupiters, Hot Neptunes and other tightly orbiting planets is a selection effect stemming from the methods used to discover these planets (spectroscopy and transit) rather than their actual proportions in the general population of exoplanets.

.

[halfeye]

No, Ceres and Pluto are dwarf planets, as are many other similar bodies in the outer solar system. Comets are a class of small solar system bodies, defined primarily by the characteristics of their orbit. The International Astronomical Union is kind of...lousy when it comes to proper terminology, partly because the study of the various non-planet non-star things in the universe is actually still quite new and they haven't finished fighting out the nomenclature battles yet (this is similar to the situation in biology pre-Linnaeus), or Geology in the mid 19th century.

Orbits are a singularly silly way to define objects. There are asteroids that cross from at least extra Martian to inner Venusian orbits, the difference being their composition, asteroids are rocky, comets are gassy when they get close to the sun. Ceres and Pluto are made largely of ices, and would become gassy if they were closer to the sun. They are rounded by their own gravity.

Actually no. It is possible to determine the gross chemical composition of many exoplanets (in the best case one that can be directly imaged can have spectroscopy data taken). The largest known rocky planet has ~40 times the mass of Earth, which is larger than models predict should exist - under normal planet formation models a planet that breaches certain size thresholds will become a gas giant due to the way material is acquired - and may be a bizarre gas giant core that lost or was somehow prevented from acquiring large amounts of gaseous material.

Hot Jupiters are close enough to their stars that it would seem necessary for their gases to boil off, if that's not the case, I'd like confirmation of that, but it would take more than a solitary page with some small pictures and smaller text that I can't read.

[Palanan]

Originally Posted by halfeye
Hot Jupiters are close enough to their stars that it would seem necessary for their gases to boil off, if that's not the case, I'd like confirmation of that, but it would take more than a solitary page with some small pictures and smaller text that I can't read.

Envisioning Exoplanets is an excellent overview of our current knowledge, and if you want a solid introduction to exoplanetary science, this is a good place to start.

If you’re feeling bold, you can jump straight to the Exoplanet Handbook, although it’s a longer read.

[hamishspence]

Orbits are a singularly silly way to define objects. There are asteroids that cross from at least extra Martian to inner Venusian orbits, the difference being their composition, asteroids are rocky, comets are gassy when they get close to the sun.

Comets are also low density - average 0.6 grams per cubic cm. Ceres's density is 2.16 grams per cubic cm. Much denser than comets. It has its own geological history, as well:

https://en.wikipedia.org/wiki/Ceres_(dwarf_planet)

Origin and evolution
Ceres has become considerably less geologically active over time, with a surface dominated by impact craters; nevertheless, evidence from Dawn reveals that internal processes have continued to sculpt Ceres's surface to a significant extent, in stark contrast to Vesta[111] and to previous expectations that Ceres would have become geologically dead early in its history due to its small size.[112]

[Yora]

Orbits are a singularly silly way to define objects.

Astronomical classifications are based on what we see, not what they are. And when we see something we have not seen before, we need a name to talk about it now, long before we have a lot of information what they are like.

Which is why the old star type system of A, B, C, D, E, ... is now O, B, A, F, G, K, M. They were originally sorted only by a single trait that was observable at the time, but as new traits were identified, that old system was no longer useful.

Most astronomers I've heard talk about planet classification seem to not really bothered by it. They accept that there isn't really anything fundamentally different between a planet and a dwarf planet. Dwarf planet was just introduced because some people thought it feels wrong if the solar system has 40 planet or so.

[halfeye]

Comets are also low density - average 0.6 grams per cubic cm. Ceres's density is 2.16 grams per cubic cm. Much denser than comets. It has its own geological history, as well:

https://en.wikipedia.org/wiki/Ceres_(dwarf_planet)

Origin and evolution
Ceres has become considerably less geologically active over time, with a surface dominated by impact craters; nevertheless, evidence from Dawn reveals that internal processes have continued to sculpt Ceres's surface to a significant extent, in stark contrast to Vesta[111] and to previous expectations that Ceres would have become geologically dead early in its history due to its small size.[112]

I do note that Wikipedia says that 30% of the mass is rocky crust, however part of the reason comets are so low in density is their low mass, if they were bigger their own gravity would make them denser. I would be surprised if Ceres isn't close to the expected density of a comet of that mass.

Astronomical classifications are based on what we see, not what they are. And when we see something we have not seen before, we need a name to talk about it now, long before we have a lot of information what they are like.

So long as the classifications can be changed as needed that is fine.

Which is why the old star type system of A, B, C, D, E, ... is now O, B, A, F, G, K, M. They were originally sorted only by a single trait that was observable at the time, but as new traits were identified, that old system was no longer useful.

Most astronomers I've heard talk about planet classification seem to not really bothered by it. They accept that there isn't really anything fundamentally different between a planet and a dwarf planet. Dwarf planet was just introduced because some people thought it feels wrong if the solar system has 40 planet or so.

I don't see anything wrong with 400 planets myself, though it does seem as if almost all the new ones are also cometary in nature.

[hamishspence]

I do note that Wikipedia says that 30% of the mass is rocky crust,

Where?

https://en.wikipedia.org/wiki/Ceres_(dwarf_planet)

Geology
Ceres has a mean diameter of 939.4 km (583.7 mi)[4] and a mass of 9.39×1020 kg as determined from the Dawn spacecraft.[57] This gives it a density of 2.162±0.008 g/cm3,[4] suggesting a quarter of its mass is composed of water.

The implication here is that around 75% of the body's mass is not water - it is rock in various forms.
The section on Internal Structure also supports this:

Internal structure
The active geology of Ceres is driven by ice and brines. Water leached from rock is estimated to possess salinity of around 5%. Altogether, Ceres is approximately 50% water by volume, compared to 0.1% for Earth, and 73% rock by mass.

If you compare Triton to Ceres - much bigger, but comparable in density:

https://en.wikipedia.org/wiki/Triton_(moon)

Triton has a surface of mostly frozen nitrogen, a mostly water-ice crust,[14] an icy mantle and a substantial core of rock and metal. The core makes up two thirds of its total mass. The mean density is 2.061 g/cm3,[6] reflecting a composition of approximately 15–35% water ice.

it showcases how densities of around 2 grams per cubic cm, guarantee that the body must be mostly rock/metal - 2/3 or so in Triton's case, and Ceres is slightly denser than that.

Tethys is slightly bigger than Ceres in size at 1,060 km in diameter - yet is less than half the density - 0.98 grams per cubic cm.

https://en.wikipedia.org/wiki/Tethys_(moon)

That is a much better example of a "mostly ice" body. And it's still much denser than a comet - because there's little or no empty space in it.

Comets are so low-density because they are porous - even more so than rubble-pile asteroids.

[halfeye]

Where?

I misread this:

Dawn found Ceres's surface to be a mixture of water ice and hydrated minerals such as carbonates and clay. Gravity data suggest Ceres to be partially differentiated into a muddy (ice-rock) mantle/core and a less-dense but stronger crust that is at most 30% ice by volume.

That's where I got thirty percent. Unless it's been edited, and if so why they kept a random figure like 30% while changing the text around it would be a puzzle, I misread that as saying the crust was rocky with a total volume of 30%, implying that the middle was water.

Tethys is slightly bigger than Ceres in size at 1,060 km in diameter - yet is less than half the density - 0.98 grams per cubic cm.

https://en.wikipedia.org/wiki/Tethys_(moon)

That is a much better example of a "mostly ice" body. And it's still much denser than a comet - because there's little or no empty space in it.

Comets are so low-density because they are porous - even more so than rubble-pile asteroids.

What I am saying is that comets are less dense because they are low mass, they are essentially snow, ice is much denser than snow, but it's not essentially different, snowballs are halfway to being ice, and that is what more massive ice bodies are like.

[Palanan]

OP, what’s your goal here?

You seem to want to invent different definitions from the ones that professional astronomers use.

Why?

[hamishspence]

What I am saying is that comets are less dense because they are low mass, they are essentially snow,

That's an old theory - the "dirty snowball" theory. Modern theory about comets has them as much heavier on the rock and carbon compounds.

https://en.wikipedia.org/wiki/Comet_nucleus

It was once thought that water-ice was the predominant constituent of the nucleus.[61] In the dirty snowball model, dust is ejected when the ice retreats.[62] Based on this, about 80% of the Halley's Comet nucleus would be water ice, and frozen carbon monoxide (CO) makes up another 15%. Much of the remainder is frozen carbon dioxide, methane, and ammonia.[49] Scientists think that other comets are chemically similar to Halley's Comet. The nucleus of Halley's Comet is also an extremely dark black. Scientists think that the surface of the comet, and perhaps most other comets, is covered with a black crust of dust and rock that covers most of the ice. These comets release gas only when holes in this crust rotate toward the Sun, exposing the interior ice to the warming sunlight.

This assumption was shown to be naive, starting at Halley. Coma composition does not represent nucleus composition, as activity selects for volatiles, and against refractories, including heavy organic fractions.[63][64] Our understanding has evolved more toward mostly rock;[65] recent estimates show that water is perhaps only 20-30% of the mass in typical nuclei.[66][67][62] Instead, comets are predominantly organic materials and minerals.[68]

[halfeye]

OP, what’s your goal here?

You seem to want to invent different definitions from the ones that professional astronomers use.

Why?

Because some professional astronomers are silly and wrong. This is very early in space exploration, mistakes that don't kill anybody aren't a big deal, but it would be nice to fix them anyway.

[hamishspence]

IMO "dwarf planet" is a more useful term than "comet" in this context.

[halfeye]

IMO "dwarf planet" is a more useful term than "comet" in this context.

I am not that averse to "dwarf planet", I'm just saying that "comet" or something like it seems to be right for a bunch of bodies, from small to gas giant to stars that are mainly gaseous (or, technically, composed of elements that are gases at STP), and there are at least potentially rocky objects in the same mass range that are not mainly gaseous (there are undoubtedly potential subcategories within these major categories, but it is probably best (in my opinion) to get the main categories sorted before sorting out the subcategories).

[hamishspence]

A normal comet can have a clearly visible coma and tail, at distances comparable to Ceres's distance. Hale-Bopp, for example, had a clear coma even when it was way out past Jupiter.

Ceres does not have a coma like that. Instead it has a tenuous atmosphere - far less visible than a cometary coma would be.

[Palanan]

Originally Posted by halfeye
…I’m just saying that "comet" or something like it seems to be right for a bunch of bodies, from small to gas giant to stars that are mainly gaseous….

Stars by definition burn fuel in self-sustaining nuclear reactions. Comets very obviously do not.

[Lord Torath]

There's not much difference between comets and asteroids. A comet is an asteroid with a fair bit of ice and frozen gas mixed in with it. An asteroid can be a comet that has lost all (or most) of its embedded ice and gas.

Here's an article about it: When is an asteroid not an asteroid? When it's a chunk of a comet!

So your position is that any body with lots of gas should be called a comet, regardless of size? That definition is so broad as to be useless. About the only things that don't fit it are galaxies, nebulae (possibly, depending on your exact definition of 'body'), asteroids (or comets that have lost all their ice and gas), and bits of space dust. Even black holes are frequently surrounded by discs of gas and dust.

I'd want to know what precisely you feel is wrong with the current official definitions, how you would redefine them, and how your redefinition would be better/more useful.

[halfeye]

There's not much difference between comets and asteroids. A comet is an asteroid with a fair bit of ice and frozen gas mixed in with it. An asteroid can be a comet that has lost all (or most) of its embedded ice and gas.

Here's an article about it: When is an asteroid not an asteroid? When it's a chunk of a comet!

So your position is that any body with lots of gas should be called a comet, regardless of size? That definition is so broad as to be useless. About the only things that don't fit it are galaxies, nebulae (possibly, depending on your exact definition of 'body'), asteroids (or comets that have lost all their ice and gas), and bits of space dust. Even black holes are frequently surrounded by discs of gas and dust.

I'd want to know what precisely you feel is wrong with the current official definitions, how you would redefine them, and how your redefinition would be better/more accurate.

I agree that asteroids to comets is a spectrum, and ones that get close to a star tend to progress toward being asteroids.

There are a lot of things that aren't mainly gas, black holes may have gas around them, but they are not themselves gas, neutron stars are not gas, dust in space isn't gas. Galaxies are too big to be considered I think, they are sort of mainly empty, sometimes with gas as a component but not necessarily.

[Bohandas]

Orbits are a singularly silly way to define objects.

I have to agree with this. I've always suspected that the IAU's current definition of a planet came from a place of pure laziness. Like they realized that there were a lot more of these out there, said "[CENSORED], I'm not remembering all that!" and then came up with a definition that ensured that they wouldn't have to.

It would be much more sensible for the definition to be based on mass, diameter, or geological characteristics (And IIRC some other sciences (such as planetary geology) do indeed define a planet this way so it's not actually all that necessary to lean to heavily on the IAU's definition).

ALSO the term "dwarf planet" implies that it is indeed a planet. And if it's not meant to be that raises some questions about the people doing the naming. I know there's often some animosity between the sciences and the humanities but they ought to know how to use adjectives.

[Yora]

The current definition of planet was tailored so that Pluto isn't one.

Though going by that definition, Jupiter is not a planet because it has not cleared its orbit of asteroids. And Saturn is not a planet because it's not very spherical.

Most astronomers I've seen talk about it really don't seem to care though. There is a continuous spectrum of aggregated particles in space. Wherever you want to draw lines to define different categories is as arbitrary as any other.
Same thing with main sequence stars really. The only really useful definition is that any ball of hydrogen that does not have hydrogen fusion in its core is a brown dwarf, and every one with hydrogen fusion is a star. But whether its class M, K, G, F, or whatever is a completely arbitrary definition that just hung around because nobody has come up with anything that would be meaningfully better.

[Tyndmyr]

The current definition of planet was tailored so that Pluto isn't one.

Though going by that definition, Jupiter is not a planet because it has not cleared its orbit of asteroids. And Saturn is not a planet because it's not very spherical.

Also Neptune, because it crosses the orbit of Pluto. If Pluto hasn't cleared its orbit, then, well...same same?

Definitionally, it seems pretty fuzzy.

[DavidSh]

And Saturn is not a planet because it's not very spherical.

The requirement isn't being spherical, it's being in hydrostatic equilibrium. Which can be rather oblate for a spinning object. We don't count the rings as part of its shape for this criterion.

[hamishspence]

Also Neptune, because it crosses the orbit of Pluto. If Pluto hasn't cleared its orbit, then, well...same same?

Definitionally, it seems pretty fuzzy.

The idea is that every object "of comparable size" is gone.


https://en.wikipedia.org/wiki/Cleari..._neighbourhood

There's no objects close to Neptune's size in Neptune's orbit, so it's cleared. There's no objects close to Jupiter's size in Jupiter's orbit, so it's cleared. And so on.

[Excession]

The current definition of planet was tailored so that Pluto isn't one.

The previous fuzzy definition had already been tailored so that Ceres isn't one, but a lot less scientifically. It pretty much just said "but not Ceres" in there. This made it a poor definition IMO.

There isn't really a good scientific definition of a planet that includes Pluto but doesn't include Ceres, Haumea, Makemake, Haumea, and Eris. If Ceres isn't a planet because it's an asteroid, then Pluto isn't a planet because it's a Kuiper belt object.

[Bohandas]

The idea is that every object "of comparable size" is gone.

Which is untimately the crux of the problem with the definition. An object qualifying in one place might not qualify in another. It's a completely useless definition that tells you nothing about the body in question.

They should pick a size or a mass and have that be the definition.

EDIT:
Basically I would strike off points 1 and 3 in the current IAU definition and reword point two (sufficient mass to maintain hydrostatic equilibrium) to make it clear that it doesn't have to be in hydrostatic equilbrium, it just has to have sufficient mass that it could be. The definition would use the cutoff mass for hydrostatic equilibrium as nearly as it is known and changed accordingly when knowledge of the cutoff is refined.
(Also, I would add new criteria that it may not be undergoing nuclear fusion and may not be a black hole have density high enough to undergo nuclear fusion.)

The current definition of planet was tailored so that Pluto isn't one.

What the heck was the point of that? The only explanations I can think of for why they would do that are laziness and trolling. There's really no reason to exclude Pluto unless they either want to make trouble or to limit the number of official planets.

[hamishspence]

Which is untimately the crux of the problem with the definition. An object qualifying in one place might not qualify in another. It's a completely useless definition that tells you nothing about the body in question.

They should pick a size or a mass and have that be the definition.

That would mean that every large moon is a planet as well. Mercury is smaller than several of the large moons. It would not qualify as a planet if it was orbiting Jupiter.

"Must not be orbiting a much larger planet" is a useful criterion to have.

It's worth noting that even one of the most outspoken "Pluto ought to be a planet" astronomers, Stern, still makes a distinction between "objects that have a large degree of ability to clear their orbit" and objects that have much less ability to do so.

https://en.wikipedia.org/wiki/Cleari..._neighbourhood

Stern–Levison's Λ

If Λ > 1, then the body will likely clear out the small bodies in its orbital zone. Stern and Levison used this discriminant to separate the gravitionally rounded, Sun-orbiting bodies into überplanets, which are "dynamically important enough to have cleared its neighboring planetesimals", and unterplanets. The überplanets are the eight most massive solar orbiters (i.e. the IAU planets), and the unterplanets are the rest (i.e. the IAU dwarf planets).

"IAU version of planet" is equivalent to Stern's "überplanet". "Dwarf planet" is equivalent to Stern's "unterplanet".

[Yora]

The real crime is that Charon is still treated as a moon.

If the center of gravity lies outside of either body, neither of them is a moon. It's a double dwarf planet.

What the heck was the point of that? The only explanations I can think of for why they would do that are laziness and trolling. There's really no reason to exclude Pluto unless they either want to make trouble or to limit the number of official planets.

Well, the real reason was that they didn't want Ceres, Eris, Haumea, Makemake, Quaoar, Orcus, and who knows how many more similarly sized balls of rocks that are out there to be planets. And there just is no way to make a definition in which Pluto is a planet but Eris is not. (That's why they called it Eris. It was going to start some real arguments.)

[crayzz]

The current definition of planet was tailored so that Pluto isn't one.

This isn't quite correct. The current definition is tailored so that Ceres, among a handful of other celestial bodies, is not a planet. Pluto just couldn't make the new cut. It wasn't a choice between 9 planets or 8 planets, it was a choice between 8 planets or 13 planets and counting, and astronomers just generally agreed that the 8 planet definition was more useful in the end.

[Rockphed]

Astronomical classifications are based on what we see, not what they are. And when we see something we have not seen before, we need a name to talk about it now, long before we have a lot of information what they are like.

Which is why the old star type system of A, B, C, D, E, ... is now O, B, A, F, G, K, M. They were originally sorted only by a single trait that was observable at the time, but as new traits were identified, that old system was no longer useful.

Most astronomers I've heard talk about planet classification seem to not really bothered by it. They accept that there isn't really anything fundamentally different between a planet and a dwarf planet. Dwarf planet was just introduced because some people thought it feels wrong if the solar system has 40 planet or so.

I think the current limit is about 13. And while we keep finding more of them, I am not convinced that even all of those are gravitationally round.

This isn't quite correct. The current definition is tailored so that Ceres, among a handful of other celestial bodies, is not a planet. Pluto just couldn't make the new cut. It wasn't a choice between 9 planets or 8 planets, it was a choice between 8 planets or 13 planets and counting, and astronomers just generally agreed that the 8 planet definition was more useful in the end.

As I understand it, the guy who pushed hardest for the definition change essentially used it as a cheap method to push his prestige and research budgets. Since Pluto got redefined as a dwarf planet it came under his research umbrella so he could capitalize on pluto's fame. I am of the opinion that we should have at least 10 planets (Mercury, Venus, Earth, Mars, Ceres, Jupiter, Saturn, Uranus, Neptune, Pluto), with things like Eris and Haumea being on the "possible" list until it is shown that they are round and not orbiting something else. Also, we should teach people about Vulcan, the planet that was hypothesized to be inside the orbit of Mercury.