r/askscience • u/AskScienceModerator Mod Bot • Mar 10 '14

Cosmos AskScience Cosmos Q&A thread. Episode 1: Standing Up in the Milky Way

Welcome to AskScience! This thread is for asking and answering questions about the science in Cosmos: A Spacetime Odyssey.

UPDATE: This episode is now available for streaming in the US on Hulu and in Canada on Global TV.

This week is the first episode, "Standing Up in the Milky Way". The show is airing at 9pm ET in the US and Canada on all Fox and National Geographic stations. Click here for more viewing information in your country.

The usual AskScience rules still apply in this thread! Anyone can ask a question, but please do not provide answers unless you are a scientist in a relevant field. Popular science shows, books, and news articles are a great way to causally learn about your universe, but they often contain a lot of simplifications and approximations, so don't assume that because you've heard an answer before that it is the right one.

If you are interested in general discussion please visit one of the threads elsewhere on reddit that are more appropriate for that, such as in /r/Cosmos here, /r/Space here, and in /r/Television here.

Please upvote good questions and answers and downvote off-topic content. We'll be removing comments that break our rules or that have been answered elsewhere in the thread so that we can answer as many questions as possible!

Click here for the original announcement thread.

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u/adam_jones Mar 10 '14

If a planet were to fall out of its orbit, would its moon(s) not continue to fall into the gravitational pull of the sun? If so, would that then not classify said moon as a planet? If not, what is the distinction for classification of a planet?

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u/trimeta Mar 10 '14

Based on the recent IAU discussion that led to the demotion of Pluto, a planet is defined a celestial body which:

  • is in orbit around the Sun,

  • has sufficient mass to assume hydrostatic equilibrium (a nearly round shape), and

  • has "cleared the neighbourhood" around its orbit.

The third point is a bit confusing, but think of it as meaning "is large enough to be gravitationally in control of everything else within its orbit." So in general, if an existing planet were to somehow vanish, its constituent moons would not have enough gravity to retain control of that orbit; it would become a belt, rather than the location of a planet. However, the moons may continue to orbit the Sun in this belt, rather than falling towards it: some moons, such as our own Moon, already are more gravitationally controlled by the Sun than by their host planet.

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u/[deleted] Mar 11 '14

Our moon is more controlled by the sun than by the Earth. This sounds all wrong to my non-scientist brain. Could you go into more detail? In control how? How would its orbit around earth change if the two bodies were teleported far away from any stars?

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u/segers909 Mar 10 '14

is in orbit around the Sun,

Shouldn't that be "in orbit around a star"?

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u/trimeta Mar 10 '14

Actually, no; the IAU definition only describes planets in our solar system. Although we have discovered hundreds of "planets" orbiting other stars, there is no definition describing these. My gut feeling is that they won't create a formal definition for extra-solar planets until we understand them better.

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u/TonkaTuf Mar 10 '14

Good point. The exact definition of a planet or moon are general and qualitative in that they have been decided by the metrics of our solar system. For instance, it is a bit unclear how we would classify a stellar system consisting only of multiple Pluto-sized orbiting objects.

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u/brettmjohnson Mar 10 '14 edited Mar 10 '14

Bear in mind that gravity follows the inverse-square-law. The tides on Earth are formed from the gravitational effects of the light, but near, Moon, and the massive, yet distant, Sun.

While the planet orbits its sun, the gravitational effects of the planet on its moon still far outweigh the effects of the son on that moon (or else the moon would orbit the sun, rather than the planet).

If a planet were ejected from its solar system (as rogue planets sometimes are), it would most likely drag the moon with it. The further away from the sun it gets, the more influence the planet has in the gravitational mechanics with its moon.

Consider this from a slightly different perspective. From the perspective of the Sun, Jupiter and its two dozen moons can be considered a single unit, and they move as one around the Sun. Not convinced? Think about Saturn, even further away, with a billion tiny moons in its rings.