Pluto. It's a planet, then it's not. This week we review Pluto's history, from discovery to demotion by the International Astronomical Union. Learn the 3 characteristics that make up a planet, and why Pluto now fails to make the grade. ...

Read the full blog entry

It was a good show. But Pamela was incorrect when she was commenting on 'Dwarf Planet' Ceres (8:43 into the show).

Ceres has been classified as a Spheroid (in hydrostatic equilibrium) or it would not qualify as a 'Dwarf Planet'.

Ceres is not a (major) Planet because it has not "dominated the neighborhood around its orbit clearing it of comparable objects".

Asteroid Vesta fails as a 'Dwarf Planet' because it is not spherical.

I did enjoy listening to the show.

-- Kevin Heider

How about

"My very errant mathematician just solved us nothing."

or

"May victorious Eggbert make justice safe under nightfall."

or, simple and back to basics...

"My very earnest mother just served us nachos."

What ideas do you have?

Poor Pluto? Ceres was stripped of the title Planet and demoted all the way back to mere asteroid over a hundred years ago. I am glad to see Ceres finally getting some respect.

One of our follow Bautforum members, Vilkata, put up a Ceres website at: Ceres: The Dwarf Planet

You can read about the "Ceres website Bautforum thread" at Ceres - A dedicated website maybe?

-- Kevin Heider

Nice site that. Well worth a visit.

__________________
Of all the things I've ever lost, I miss my mind the most!

My daughter came up with "My Very Evil Mother Just Served Us Nothing"

So, how come Neptune is a planet when it has all these little Plutinos hanging out in its orbit? They can't all be living in Neptune's lagrange points.

I thought that was the definition of Plutinos? They're in some 3:2 resonance with Neptune, so that when they're close to the orbit, Neptune is far away.

Just wanted to jump in here to say I loved the first podcast. Can't wait for the next one.

Right, but aren't they still in Neptune's orbit? Or does it not matter because Neptune is so far away when they are close? When are you far enough from the local planet to not be counted when astronomers are trying to determine if it's cleared its orbit?

In this sense, they are like objects at lagrange points--they are in positions in space/time (as opposed to space or spacetime) such that the planet cannot be expected to have cleared them

That makes sense. Thanks.

I thought saying teachers had a "vested interest" in keeping Pluto a planet was a bit disingenuous. Imagine the windfall text book writers and education materials suppliers are set to reap thanks to this. Or the researchers who can now use Pluto as an easy target to study the "new" field of dwarf planets. Yet it's teachers who are defending Pluto just so they don't have to come up with a new neumonic? This whole Pluto thing just reinforces the stereotype of the cold, uncaring, inflexible scientist that cares more about creating and following a rigid set of rules than educating the general public.

BobK

Thanks for the show, Fraser and Pamela.

I enjoyed being invited to share the friendly, informative conversation.

BobK, I took the comment of <i>"vested interest"</i> to mean that my teachers are warm, caring and flexible scientists. This is what I've discovered in my last 6 years of being a student. As an online and distant education student, I'm hit by the double whammy of costs - the textbooks plus postage. Thank you to my warm and caring teachers.

In all the old 1950s sci-fi movies, the astronauts find an "alternate Earth" in the same orbit as Earth, moving at the same speed, but always on the opposite side of the Sun, so no one ever saw it until the astronauts went out into space.

Under the new definition of "Planet" would a system with these two planets no longer be called "planets" just because one of them didn't clear the other out.

In reality that is impossible. Even if the (2) Earths were exact copies of one another, the gravitional attraction of the other Planets would cause one Earth to wander into a larger & longer orbit thus allowing the other Earth to over take it. This would result in a collision between the (2) Earths.

You might be able to briefly have (2) identical Jupiters at L3 Lagrange points because Saturn is not really massive enough to have a major affect on a very well balanced "Jupiter(1)-Sun-Jupiter(2)" pendulum.

-- Kevin Heider

Not just the other planets--that particular lagrange point is unstable, any perturbation will knock it away

Okay, so I'll buy that everything I saw in 1950s sci-fi movies might not be physically possible.

But what about a "binary planet" (like Pluto and Charon) that orbit a central point -- external to both -- as they both orbit around a star, but are the only two items in the orbit, and are spherical.

What would be the grounds for excluding a "binary planet" from being considered a planet? "We're sorry, but your moon is too big"?

Or is there a reason that two Earth-like planets couldn't be in orbit around each other?

Binary planets will occur. Pluto and Charon are Binary 'Dwarf Planets' since their barycenter (center of mass) is external to both of them. They are not binary 'Planets' because their mass is not great enough to clear their orbit of comparable objects.

Daphnis (a 7 kilometer-wide shepard moonlet; 136,500 km from Saturn) is orbiting in the Saturn Ring System. Since it is one of the largest "ice cubes", "it is clearing it's orbit of debris."

But Daphnis will never get big because the rings are inside the orbits of Saturn's moons and the tidal forces (Roche limit) will prevent the material in the rings from coalescing gravitationally to form moons.

Getting back to your question, Planetesimals (and then later Planets) form from large clumps that are orbiting inside a relatively flat gas disk that is orbiting a young star.

Theory suggests that the Earth-moon system was created when a Mars sized Planetesimal collided with the Earth. It is believed that the Pluto-Charon system was also created when a large object hit Pluto.