2nd Season Dr. Who - The Impossible Planet - 5 star poll

"The Impossible Planet"
11-17-06 Dr. Who

5 stars (Clyde Tombaugh)

0-1 stars (International Astronomical Union)

Tim Bruening...

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In "The Creature In The Pit", a creature from a pit neutralized the
gravity of a neutron star by wrapping it in aluminum webbing. Could
that work with the black hole?

Tim Bruening...

I have read that Pluto was demoted on the technicallity that it had failed
to sweep Neptune out of its orbit. Since Neptune failed to sweep Pluto out
of its orbit, should we demote Neptune too?

pbowles...

It wouldn't be the reason; the reason was that, with the discovery of a

Patty Winter (patty1...

It depends whether you're defining a "reason" as a motivation or a
criterion. Sweeping the orbit is indeed one of the new IAU *criteria*:

(1) A planet is a celestial body that
(a) is in orbit around the Sun,
(b) has sufficient mass for its self-gravity to overcome
that it assumes a hydrostatic equilibrium (nearly round)
(c) has cleared the neighbourhood around its orbit.

Kuiper Belt Object (KBOs) larger than Pluto, now named Eris, a new,
consistent definition of a planet was needed that would either
encompass many more than the 9-10 recognised candidates or which would
have to exclude Pluto and leave the traditional eight. The solution was
to plump for the latter, which ultimately makes things less of a
headache. The result is that Pluto, Eris and Ceres (which was, for a

Patty Winter (patty1...

Yes, that was the *motivation* for finding a way to exclude Pluto: they
realized that many other bodies should be considered planets if Pluto
were left in. But the *criteria* they developed for excluding Pluto should,
by rights, also exclude Neptune.

Brian Thorn...

...and Jupiter, and Earth...

very brief period after its discovery in the 17th Century, a planet -
Pluto isn't the first time an object has been incorrectly designated a
planet and then demoted, it's just the most persistent and the only one
to have entered popular consciousness as a planet) are now consigned to
a new category of dwarf planets nicknamed 'plutons'.

However, the point you mentioned may have been an argument against
designating Pluto a planet on historical grounds, as it alludes to the
reason Pluto was adopted as a planet in the first place. Since it was
discovered, Neptune's erratic orbit was unexplained in the absence of a
modern understanding of gravitation and solar system formation, and so
at the time it was thought that the only way to explain its orbit was
the invoke a mystery 'Planet X' (yes, it should have been Planet IX,
but the X stands for a mystery rather than 'Planet 10'). Pluto was
ultimately the result of this search, but it was known as soon as it
was found that it couldn't be the hoped-for Planet X because it
couldn't have affected Neptune's orbit. Therefore, there are no valid

Patty Winter (patty1...

Nonetheless, the recent IAU resolution doesn't say anything about
whether the nearby stuff has to *affect* a planet's orbit. It just
says that there shouldn't be anything left in the neighbo(u)rhood.
Granted, Neptune doesn't *share* an orbit with Pluto, but they
overlap, so clearly, Neptune has done a poor job of clearing the
area.

Stan Brown...

By that criterion, the earth isn't a planet either, since numerous
asteroids cross its orbit.

I think the "clears its orbit" criterion means only that no other
bodies share the same or nearly the same orbit. And even so, there
has to be an exception for the Trojan points, 60 degrees ahead and 60
degrees behind.

historical grounds for designating Pluto a planet.

Undeterred, for no very good reason people decided to call it a planet
anyway and, although new models of gravitation had made an extra planet
unnecessary to explain Neptune's orbit (and Mercury's - for a while
before Einstein, oddities in that planet's orbit had led to a search
for a mythical 'first planet' nicknamed Vulcan), the idea of a 'real'
Planet X (which this time actually would be the tenth planet) did the
rounds in science fiction for a while. It's come back recently among
serious astronomers, but not as a large object affecting Neptune's
orbit, and number 10 (well, 9 again) won't be anything special if it's
found. There was a New Scientist article recently suggesting that there
could well be Mars-sized KBOs, and possibly planet-sized objects
outside the Kuiper Belt, too far for us to detect at present but still
within the solar system, that a back-of-the-envelope calculation (or at
least a guess) suggests may number a dozen or more.

Anyway, with Bellerophon, Methuselah and the legion of so-far-unnamed
planets outside our solar system, there are more than enough planets to
go around without adding any more in our system. At this rate we'll be
in danger of running out of Greek and Roman deities - there are only a
couple left, with a lot of the asteroids named after them (the most
prominent
being Quirinus, the patron god of the city of Rome, the only major
Roman deity not represented by a planet, satellite or asteroid).

Tim Bruening...

Why not name a planet after Julius Caesar?

Tim Bruening...

Is the IAU really that incompetant at the telescope?

Telescope: Mouth wash at a great distance.

nemo...

Observatory: Where people keep an eye on David Cameron from.

When they try to look intense, some people look as if they're in pain. This
man is one of them!

2nd Season Dr. Who - The Impossible Planet - 5 star poll - SPOILERS AHOY !