r/IAmA • u/NewHorizons_Pluto NASA New Horizons • Jul 14 '15

We're scientists on the NASA New Horizons team, which is at Pluto. Ask us anything about the mission & Pluto! Science

UPDATE: It's time for us to sign off for now. Thanks for all the great questions. Keep following along for updates from New Horizons over the coming hours, days and months. We will monitor and try to answer a few more questions later.

NASA’s New Horizons spacecraft is at Pluto. After a decade-long journey through our solar system, New Horizons made its closest approach to Pluto Tuesday, about 7,750 miles above the surface -- making it the first-ever space mission to explore a world so far from Earth.

For background, here's the NASA New Horizons website with the latest: http://www.nasa.gov/newhorizons

Answering your questions today are:

  • Curt Niebur, NASA Program Scientist
  • Jillian Redfern, Senior Research Analyst, New Horizons Science Operations
  • Kelsi Singer, Post-Doc, New Horizons Science Team
  • Amanda Zangari, Post-Doc, New Horizons Science Team
  • Stuart Robbins, Research Scientist, New Horizons Science Team

Proof: https://twitter.com/NASASocial/status/620986926867288064

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u/reddy_prabhat Jul 14 '15

True. The discovery of Kuiper belt objects similar to Pluto is what caused the debate in the first place.

But the reason Pluto doesn't fit definition of a planet is because it hasn't "cleared its neighborhood". It meets the other two requirements.

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u/RCiancimino Jul 14 '15

What does cleared its neighborhood mean?

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u/reddy_prabhat Jul 14 '15

It means that it is the gravitating body for objects within a certain distance of it.

In this case, Pluto has counterparts like Eris, Sedna, etc, which are at a similar distance from the sun. Pluto is not the object that largely dictates the orbits of those objects. It is the sun instead. In fact the orbit of Pluto is very similar to these objects.

A counterexample would be Jupiter. It has many objects near it, but the orbits of those objects are influenced primarily by Jupiter, and not the sun. The earth is another example of an object that has cleared it's neighborhood.

If Pluto was massive enough, it would've caused the nearby objects to fall into it, and eventually, there wouldn't be any objects in Pluto's orbit of significant size.

Here's an analogy: You have a bunch of play-doh on a table, randomly distributed. You roll the largest clump around the table until it's one big clump, and there's no play-doh left on the table. Just add gravity.

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u/RCiancimino Jul 14 '15

So its large enough that it orbits the sun and not something smaller and closer....but too small to influence what's around it?

Is that the gist?

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u/ZappyKins Jul 14 '15

That it would collect all the other objects in it's orbital path. For example Earth had only one major objects orbiting the sun with it. The Moon, but with Eris and Pluto there are many other smaller object in the same orbit.

A planet - like Jupiter or Earth would have absorbed and collected all this debris. Dwarf planets have not cleared these fellow objects.

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u/antiqua_lumina Jul 14 '15

Actually, Earth may no longer be a "planet" if its orbit was as far out as Pluto's. From Phil Plait:

A current definition of "planet", handed down by the International Astronomical Union, is that a planet can sweep up most or all of the material that orbits the Sun near it.

This definition, though, is silly. If the Earth were out at the distance of Pluto, it would have a hard time sweeping clear the material out there, too. The volume of space that far out from the Sun is vast, and the Earth tiny. It would be like trying to sweep your house with a tiny paintbrush.

So the IAU's definition for planet is largely a function of how close it orbits the sun, which strikes me as arbitrary and irrelevant to a meaningful definition of the word.

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u/ZappyKins Jul 14 '15

Well, that's speculation. But yea, a definition is just something people set for particular reason. Earth is MUCH larger than Pluto, and as it absorbed other asteroids it would get more mass and gravity, and start absorbing more and more. Considering it's been around over 4 billion years it would have much time to absorb things. Remember Earth probably came from a disk of stuff around the Sun, and unlike Pluto, absorbed most all of it.

Most of the mass in Earth's orbit is Earth. The same can be said for Mercury, Venus, Mars, Jupiter, etc. The same can not be said for Pluto, or Eris.

PS As a kid I always though Pluto should not have been called a planet. It was too different and subjective, in the same way people called all dinosaurs reptiles and that just didn't seem accurate.

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u/God_Damnit_Nappa Jul 14 '15

I'm still not getting it. Saturn has rings around it and a ton of moons. Shouldn't it technically not be a planet since it hasn't cleared its orbit and has debris around it?

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u/ZappyKins Jul 14 '15

Because they orbit Saturn, and not the Sun.

There isn't a bunch of asteroids and such orbiting the Sun in line with Saturn's orbit.

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u/OllieMarmot Jul 14 '15

Capturing those objects as satellites counts as clearing the orbit.

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u/God_Damnit_Nappa Jul 15 '15

Ah got it. Thanks!

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u/honeybager Jul 14 '15

Basically that it has to float through other space debris. If you look at the other planets they have satellites, but otherwise have clear paths around the sun.

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u/RCiancimino Jul 14 '15

And what is pluto's path as opposed to this that makes it different?

(I'm sorry btw, space noob)

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u/honeybager Jul 14 '15

Auburn666 is correct. I just wanted to add that Pluto is in what is called the Kuiper belt, an area of space riddled with other trans-Neptune objects. Pluto's orbit is through this debris and since there is debris in its orbital path, it has not cleared its neighborhood. I wonder if Earth or Mercury would have cleared their orbits if they were that far out.

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u/RCiancimino Jul 14 '15

Hmm thanks

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u/stoicsilence Jul 14 '15

That its gravity is strong enough to either collect interplanetray debris to itself or hurl it away.

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u/BeefyBeaumont Jul 14 '15

What does 'clearing its neighbourhood mena?'

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u/geek180 Jul 14 '15

What does "cleared its orbit" mean?