New Horizons

PopePalpatine wrote: »

I'm just wondering - how was Pluto's rotational period calculated way before New Horizons got there?

Hubble, it can pick out very rough features and wait till it turns up again.

As an aside the ashes of Clyde Tombaugh who discovered Pluto were on board for today's flyby.

Found this nice size comparison of solar system moons and dwarf planets. Just shows how small Pluto is versus some of the moons with 7 being larger than it. It's definitely a dwarf planet! *runs and hides*

LINKY

mg1982 wrote: »

I thought once it arrived at pluto its work was done. But i hear it will continue on to the kuiper belt and send back new information from there. Exciting times.

Yes it is planned to fly by a Kuiper belt object in 2019.

Maximus Alexander wrote: »

If Mercury was included in that image the flaw in that logic would become apparent. :P

Only 2 very large moons bigger than Mercury!

Maximus Alexander wrote: »

So where's the cut off? Seven is too many, but two is not? That's even woolier than 'clearing the neighbourhood'. :P

Pluto is even small relative to its own moon with the centre of gravity with Charon sitting outside Pluto. It also has a highly inclined orbit relative to all the other planets. The definition is problematic but I just don't see Pluto as a planet. On the other hand I don't think anyone would suggest Mercury is not a planet.

namloc1980 wrote: »

Pluto is even small relative to its own moon with the centre of gravity with Charon sitting outside Pluto. It also has a highly inclined orbit relative to all the other planets. The definition is problematic but I just don't see Pluto as a planet. On the other hand I don't think anyone would suggest Mercury is not a planet.

You could argue that Earth is small relative to its own moon - it would be the next lowest on the planet / moon ratio scale after Pluto. That doesn't seem like a good reason to discount it as a planet though.

The problem with the IAU's definition of a planet is that it's not definitive. It's open to interpretation, or debate. Has 'cleared the neighbourhood around its orbit' is imprecise and undefined and, as a result, it doesn't sit well as a basis for ejecting one of the classical 9 planets.

Equally, measuring the size in comparison to other planets' moons doesn't seem like a good basis either. Nor does comparing against the planet's own moon, since a planet is not even required to have one. However, at least if a specific ratio of radius or mass was chosen for one of those measurements, it would be clearly defined. That's not the case at the moment, which is a bit of a shambles.

Finally, Mercury looks more moonish than Pluto.

ZeRoY wrote: »

LORRI FULLFRAME COLOR 13.07.2015 Reprocessed in Registax 6 & CS6
Original HERE

:eek: That's one seriously impressive image ZeRoY! Thanks.

namloc1980 wrote: »

Earth Moon centre of mass is inside the Earth. Clearly Earth is the dominant body in that system. Pluto-Charon centre of mass sits well outside the surface of Pluto so it's more like a binary system than a planet-moon.

Then you'd have to ask - is it not possible to have binary planets? What if we found two Earth-like worlds orbiting one another in another system, would you be reluctant to call them planets on that basis?

Although under the IAU's current definition, there are no planets at all outside the solar system...

namloc1980 wrote: »

Pluto isn't really a classical planet. The classical planets are the ones known since antiquity: Mercury, Venus, Earth, Mars, Jupiter and Saturn. Either way if you want to argue that Pluto is a planet then Eris, Makemake, 2007 OR10, Haumea, Quaoar, Sedna, Ceres and Orcus must also be planets. I don't see anyone seriously arguing that there should be 17+ planets in the solar system.

Point taken about the term 'classical planet'. A misnomer on my part, although it is a semantic difference that doesn't detract from the point I was making.

I don't necessarily have a problem with the idea that there could be 17 or more planets in the solar system. Much as I don't have a problem with there being hundreds, or thousands of asteroids. Mainly, my problem is with hazy definitions.

namloc1980 wrote: »

So how a body looks is a good basis for deciding if it's a planet or not? Mercury has 25 times the mass of Pluto. It even has more mass than Ganymede and Titan combined even though they are marginally wider than it. Redefining Mercury due to some resemblance to the Moon seems like a crazier suggestion than the IAUs definition.

I was being somewhat facetious here, but actually also making a point. Think about what makes a planet look like the moon? A dry, rocky, heavily cratered, predominantly ancient surface. Little or no recent geological or tectonic activity. Little or no atmosphere.

If we are to define a planet on a tenuous basis such as 'clearing the neighbourhood', or barycenters, then in that light these could be equally valid criteria to add to the discussion.

Either a planet is a thing, or it isn't. If it's a thing, then it ought to be defined. The IAU's definition was thought up not because they had found a new class of object that needed a name, but because they had eight objects that they wanted to separate into their own class, and they needed to find a justification for it.

It's putting the cart before the horse and it strikes me as unscientific. It would have been more reasonable to simply define a planet as one of Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus or Neptune. At least it would be unambiguous.

If we're being honest, the planets are too different to share a classification in the first place. Mercury and Jupiter have absolutely nothing in common.

Yeah NH phoned home safe and sound. Full data recorded and all systems green! Ready for those pictures now!