[Q] summet about planets

[Donovandunne6]

HEY! i was wondering why that guy de-classified pluto from planet status, i mean it has a core, it has a moon and it orbits the sun, so that makes it a planet, im pretty angry at this! i demand justice!

 

so please please we should make a petition to put pluto back to planet status screw the plutoid status :)

[Moontanman]

Well if Pluto is a planet then that means there are several more "planets" that would have to be included, some of which are bigger than pluto and have moons as well. are you down with that?

[Tormod]

There was no "that guy" who declassified Pluto. It was the International Astronomical Union who did it at one of their conventions. Astronomers from all over the world participated in this.

 

Pluto and the Changing Landscape | IAU

[Phebe]

For a short answer, a planet has to be big enough to be round, has to orbit the sun and not another planet or celestial body, and lastly, it has to clear the space around it, meaning there should be no other objects in the path of its orbit. Pluto doesn't fit the last criterion, so it falls into dwarf planet classification.

The re-evaluation was needed upon the discovery of kuiper belt bodies the same size or bigger than pluto.

[Moontanman]

I wonder what they will do when they find an earth sized or even bigger object out there? Not supposed to be there but we all know how that can change with better data. I really have no problem with keeping Pluto as a planet but that means there are dozens of other planets out there too maybe hundreds, I can see it now, kids in school trying to memorize 157 planets. :) I think the idea of limiting planets to objects that clear out their orbits will at lest simplify things if nothing else. but where does that leave planetary sized bodies that do not orbit stars? What do we call them? Are the satellites of sub stellar objects moons or planets? Oh the potential for debate is glorious :)

[Phebe]

That does give us something or some things new to think about.

[CraigD]

I wonder what they will do when they find an earth sized or even bigger object out there?

We (all this “they” talk – we’re all humans, right?) probably won’t find such an object. Best models of the solar system in general and the Kuiper belt specifically put the upper limit of its total mass at about 0.1 Earth masses, with a likely estimate of its actual mass of about 0.033 Earths. That’s plenty of mass for a few dozen as-yet undiscovered Pluto-mass (0.0022 Earths) bodies, but if anything Earth-mass or larger is out there, it likely got there in some freakish way, not due to normal star system formation dynamics.

 

One can make a pretty strong argument that there’s no such body out there, freakish origin or not, because if there were, it would clear an observationally noticeable band of the KB, and likely have done something profound to the orbits of Pluto and Eris (another plutoid, about 1.28 Pluto masses and further out (Peri/Aphelion 37.77/97.56 AUs vs. Pluto’s 29.66/49.31)

I think the idea of limiting planets to objects that clear out their orbits will at lest simplify things if nothing else. but where does that leave planetary sized bodies that do not orbit stars? What do we call them?

The term I’ve heard most often is “rogue planet”. I think it serves well, as almost anyone with a bit of serious astronomy study behind her or him recognizes the term. Most astrophysicists believe there are lots of them, possibly more than there are planets, but with anything close to our current level of astronomical technology, we’re unlikely to ever observe one.

Are the satellites of sub stellar objects moons or planets?

I think the term moon is pretty well pinned down, since the largest moon in the solar system (Ganymede, 0.025 Earths), is almost half as massive as the smallest non-dwarf planet, (Mercury, 0.055 Earths), yet nobody’s much bothered by this.

Oh the potential for debate is glorious :hyper:

It’s fun, and can be educational, since to follow the debate, you have to learn a lot of useful astronomical concepts, but ultimately, IMHO, a quibble over semantics, and a vague form of stereotyping. No matter what we call them, stars, planets, dwarf planets, and moons remain what they are.

 

While on the subject of Pluto and what to call it, I think it deserves another astronomical call-out – one might considered it a consolation prize for its demotion from planet to dwarf planet, or perhaps further insult: Pluto and Charon (0.11 Pluto masses, nearly 10 times the Moon/Earth mass ratio) are arguably not dwarf planet and moon, but a double dwarf planet pair, since their barycenter is not beneath the surface of Pluto but about 15 planetary radii above it. Other than some binary asteroids and even smaller bodies many times ([math]10^{10}+[/math]) smaller than them, this seems to be a unique configuration in all the solar system.

[Moontanman]

We (all this “they” talk – we’re all humans, right?) probably won’t find such an object. Best models of the solar system in general and the Kuiper belt specifically put the upper limit of its total mass at about 0.1 Earth masses, with a likely estimate of its actual mass of about 0.033 Earths. That’s plenty of mass for a few dozen as-yet undiscovered Pluto-mass (0.0022 Earths) bodies, but if anything Earth-mass or larger is out there, it likely got there in some freakish way, not due to normal star system formation dynamics.

 

Spoil sport:doh:

 

One can make a pretty strong argument that there’s no such body out there, freakish origin or not, because if there were, it would clear an observationally noticeable band of the KB, and likely have done something profound to the orbits of Pluto and Eris (another plutoid, about 1.28 Pluto masses and further out (Peri/Aphelion 37.77/97.56 AUs vs. Pluto’s 29.66/49.31) The term I’ve heard most often is “rogue planet”. I think it serves well, as almost anyone with a bit of serious astronomy study behind her or him recognizes the term. Most astrophysicists believe there are lots of them, possibly more than there are planets, but with anything close to our current level of astronomical technology, we’re unlikely to ever observe one. I think the term moon is pretty well pinned down, since the largest moon in the solar system (Ganymede, 0.025 Earths), is almost half as massive as the smallest non-dwarf planet, (Mercury, 0.055 Earths), yet nobody’s much bothered by this. It’s fun, and can be educational, since to follow the debate, you have to learn a lot of useful astronomical concepts, but ultimately, IMHO, a quibble over semantics, and a vague form of stereotyping. No matter what we call them, stars, planets, dwarf planets, and moons remain what they are.

 

I understand about rouge planet but what do you call the satellites of a brown dwarf or other sub-stellar object that is too big to be a planet but too small to be a star? Moons or planets?

 

While on the subject of Pluto and what to call it, I think it deserves another astronomical call-out – one might considered it a consolation prize for its demotion from planet to dwarf planet, or perhaps further insult: Pluto and Charon (0.11 Pluto masses, nearly 10 times the Moon/Earth mass ratio) are arguably not dwarf planet and moon, but a double dwarf planet pair, since their barycenter is not beneath the surface of Pluto but about 15 planetary radii above it. Other than some binary asteroids and even smaller bodies many times ([math]10^{10}+[/math]) smaller than them, this seems to be a unique configuration in all the solar system.

 

I thought they had found another plutiod with that configuration, and doesn't pluto have another moon as well that orbits both Pluto and Charon? To me, once you get past the possibility of life Pluto is the most interesting object in the solar system.

[CraigD]

I understand about rouge planet but what do you call the satellites of a brown dwarf or other sub-stellar object that is too big to be a planet but too small to be a star? Moons or planets?

Every reference I’ve seen to them call them either simply planets, or occasionally “planet mass objects”.

 

According to the wikipedia article “brown dwarf”, only 2 such objects have yet been observed. Of these 2, one, 2M1207b, could reasonably be argued to be one of brown dwarf binary pair, as it’s big (about 4 Jupiter masses, vs. its companion [math]2M1207[/math]‘s 25), with at 41 AUs distance, places the pair's barycenter far from either of them ([math]4 /29 \cdot 41 \dot= 5.7 \,\mbox{AU}[/math] from 2M1207).

I thought they had found another plutiod with that configuration, …

This doesn’t seem to be true of the 3 other recognized Plutoids.

 

Hamuea’s 2 moons are about 0.002 and 0.02 its mass. [wiki=http://en.wikipedia.org/wiki/Eris_(dwarf_planet)]Eris[/wiki]’s moon Dysnomia is only about 0.006 its mass. (IAU rules be damned, I’ll always remember these two as Xena and Gabrielle :))

 

… and doesn't pluto have another moon as well that orbits both Pluto and Charon?

It has 2: Nix and Hydra. They do indeed orbit the Pluto-Charon pair at about 2.8 and 3.7 times Charon’s distance from its barycenter, but are tiny – no more than about 0.00015 Pluto masses, compared to Charon’s 0.12.

 

All these little satellites of the plutoids and other KBOs – and there’s reasonable suspicion that closer observation will reveal many small satellites – suggest that it makes sense to think of them as just the visible parts of ring systems.

To me, once you get past the possibility of life Pluto is the most interesting object in the solar system.

Like many of my generation (b 1960), I’ve been especially fascinated by Pluto since childhood. Though present day descriptions of the solar system may be more complicated and ambiguous than 40 years ago, the growing view of the Kuiper belt as having many cold little worldlets is, IMHO, even more fascinating than the view common in the 1960s.