# Some astronomy questions for ya

Talking with some friends last night, and we had two things that we couldn’t really resolve. I did some googling and couldn’t really find an answer. So, I thought I’d throw them out there and see what I get.

A> For gas planets like Jupiter or even Stars for that matter-- what creates the gravity that holds it together. I mean, if I had a beaker full of gas that i could see, it would take no form, and would in no way be bound to itself. Mass is what creates gravity, so are we to assume that there is some sort of solid mass in the center?

B> Neptune and Pluto have orbits that cross each other. What would happen if they collided? Would there be any effect here on earth? Would we just be in danger of debris, or would it send massive shockwaves of death and destruction towards us? Or would we just get (at most) a cool nighttime light show?

I’m gonna give my WAG for question A. In a beaker full of gas, the gas particles are attracted to each other, just not enough to bring them together and form a sphere. A really huge mass of gas would have more particles toward the center and the particles toward the edges of this large mas would be attracted enough to the center to move toward it and form a sphere.

A> All matter has mass, regardless of its state. the equation for gravitational attracton between two bodies is F = (G x m x m’) / r[sup]2[/sup] where G is the universal gravitational constant equal to 6.672[sup]-11[/sup] Nm[sup]2[/sup]/kg, m and m’ are the respective masses of the badies, and r is the distance between their mass centers. Gas has mass, ergo a gas planet has gravitational pull regardless of whether it has a solid core or not. That said, it is believed that Jupiter’s core is solid, from the extreme pressure, if nothing else.

B> Neptune and pluto will never collide, since their orbits are tilted with respect to one another, and never actually cross.

X-ray vision is correct. To expand on his post, below a certain amount of mass, the electromagnetic effect dominates and small bodies like humans and icebergs are held together, not gravitationally, but electromagnetically. The mass at which gravitation takes over I’ll leave as an excercise to the reader. (hint: knowing the relative strengths of the two forces is the key here).

Gas has mass. There are many balls of gas in the Universe that are held together by their own gravity–commonly known as stars.

The gas giants formed in the outer part of the Solar System, where it’s cold enough for ice to be in solid form. Because of the extra building material available, the outer planets could grow much larger. Once they were about four times the mass of the Earth, they had a strong enough gravitational pull to attract and retain hydrogen and helium from the solar nebula. The icy and rocky cores remain, but there’s so much gas piled on top of them that the materials are at incredibly high pressures and temperatures. At some points in the interior, the cores are probably in the solid state, but it’s nothing like a terrestrial planet that has a solid surface with atmosphere above. Instead, the planet just gets hotter and denser as you go down.

If Pluto plunged into Neptune, it probably wouldn’t create a flash visible from Earth without a telescope. I haven’t worked out any numbers, so this is a WAG, but Pluto is small, and Pluto and Neptune very far away from Earth. Because of the distance (and the fact that Pluto wouldn’t make much of a splash). Space is empty; there’s nothing to carry any shockwaves of death or destruction.

Now, the orbits of Pluto and Neptune don’t actually cross. Pluto’s orbit is inclined (tilted), so at the points where Pluto’s orbit and Neptune’s orbit appear to cross in a two-dimensional picture, the two paths are actually safely separated.

A Yes, Jupiter does have a very dense core- large and rocky,no doubt molten like our own and very hot, overlain by a deep ocean of metallic hydrogen
http://www.solarviews.com/cap/jup/jupint.htm

The solid matter that would be in the cores of stars is too hot to be solid or even liquid, but instead is distributed through the star…

the core of most stars is a ball of hot, dense plasma, which is tightly compressed by the weight of the star above it, so is more dense than a solid core would be.

B/ no, the orbits are a different shape, and tilted, so that the two planets Neptune and Pluto can never get near each other.

SF worldbuilding at
http://www.orionsarm.com/main.html

Ah, but if you had a balloon, the size of Jupiter, full of gas,
the total mass would be so low that the gas would not collapse, and you would just have a big balloon full of gas- make the skin of the ballon translucent,
and the gas inside the balloon air,
and you could have a giant habitat, with only a small amount of gravity to worry about.
A giant Bernal sphere.
SF worldbuilding at
http://www.orionsarm.com/main.html