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Hope for Pluto—Should We Re-Redefine Planets?

Pluto in enhanced color

Air Date: 03/08/2017
Run Time: 33:51

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Topics: commercial spaceflight, naming things, World View, Pluto, Planetary Radio

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Planetary geologist Kirby Runyon is lead author of an abstract that proposes a new, geophysical definition of what a planet is. It would grant our solar system 110 planets, including Pluto and the moon. Up, up and away with Digital Editor Jason Davis on World View’s balloon ride to just below the edge of space. It’s the asteroid of love and it’s the destination of this week’s space trivia contest. Bill Nye has the week off.

High-resolution enhanced-color global MVIC portrait of Pluto


High-resolution enhanced-color global MVIC portrait of Pluto
This beautiful high-resolution image of Pluto is from a single observation with the MVIC imager on the Ralph instrument. It is an enhanced-color view made of three images captured through infrared, red, and blue filters. The three individual images were denoised, deconvolved, and enlarged by a factor of 2 before being combined into this stunning portrait.

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Trivia Contest

This week's prizes are a Planetary Radio t-shirt, now available in both men’s and women’s styles. Also, a 200-point astronomy account, and a Planetary Society rubber asteroid.

This week's question:

What is the diameter of the primary mirror on the great observatory on the Spitzer Space Telescope?

To submit your answer:

Complete the contest entry form at or write to us at no later than Wednesday, March 15th at 8am Pacific Time. Be sure to include your name and mailing address.

Last week's question:

Several spacecraft landing sites on Mars are designated as memorials. What is the designation of the Viking One memorial site?


The answer will be revealed next week.

Question from the week before:

Where in the solar system would you find a crater named Valentine, after Saint Valentine?


The only crater officially designated with the name Valentine is on asteroid 433 Eros, visited by the NEAR Shoemaker probe.


No trivia contest spoilers please!

sepiae: 03/12/2017 10:40 CDT

Being so late this week, I can't believe that I'm the 1st comment here; I'd have expected a raging debate by now. Right, I'm one for respectfully disagreeing. The problem with categorizing is that each way of doing it will have its problems. That's not only so with planets. I'd happily concede that one such problem with the way things are now is, for instance, the difference between the inner and the joval planets. While I can see where a geologist must be coming from, that particular set of troubles is still there, and I'd say it'd be multiplied. How about this: you're welcome in our midst, little thing, but if you're caught up in an orbit around a planet proper we'll keep calling you moon, especially when you're originating *from* material of this planet. Look at the latest (stunning) images of Pan. It looks like one of Saturn's kids, who's been in the workshed, we've heard some hammering, we've heard some tinkering, and it emerges with this pitiful make-shift version of a Saturn-ring we won't call pitiful before its ears because it's a kid, and it shouts, 'look, I'm mommy!' Let's keep it a little simpler. I'd be happy to reconsider Pluto.

Tim: 03/13/2017 06:01 CDT

@sepiae, I agree and I also have my doubts about a solar system comprising 100+ planets especially as most of these supposed "planets" would actually be moons or very far flung small bits of ice. While I'm OK with the current set of 8 planets, I still wouldn't mind a definition that somehow included other significant worlds like Pluto, Eris and Planet Nine (whenever it's found).

am25544: 03/14/2017 03:21 CDT

My arbitrary definition of a planet is an object orbiting the sun which is substantially similar to what the ancients called planets. Astronomy went through the "What is a planet?" in early 1800's when asteroids were discovered. When several dozen were discovered and more being discovered every year it was decided with little fanfare that these are a different class of objects not planets. We look silly continually arguing point.

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