Creating A Planet

[Alpha_Geek]

Being nearly all hydrogen, isn't it technically 2/3rds water?

No. Water is a molecule consisting of two hydrogen atoms and one oxygen atom. Take away the oxygen and it isn't water anymore, it's just hydrogen.

Right. So all we need to do is add one oxygen atom for every two atoms of hydrogen in the sun and we're all set. Since it's a two to one ratio, the sun is 2/3rds water!

 

[Christopher]

Wouldn't any large bodies (I mean planet sized or approaching planet-sized) bodies just about ANYWHERE in the solar system reveal themselves based on their gravitational influence if they were rocky or even much more than just balls of loosely congealed slush?

Gravity is by far the weakest fundamental force in the universe. At the distances we're talking about, the gravitational influence of a merely terrestrial-mass planet (tiny on the cosmic scale of things) would be minuscule and extremely difficult to detect. For one thing, bodies at that distance take centuries to complete a single orbit. So it would take an extremely long time to gather enough orbital data to verify the presence of an unseen massive body from its gravitational influence alone. Uranus was discovered in 1781, but it wasn't until the 1840s that it was determined that its orbit was being gravitationally influenced by another planet (Neptune, discovered in 1846). And it wasn't until decades later that astronomers began to suspect a ninth "planet" perturbing Uranus's orbit as well -- and it took them until 1930 to actually locate it.

As I said, the first trans-Neptunian object other than Pluto wasn't discovered until 1992. We've found over a thousand in the past 17 years, but we've only been able to observe a few years' motion of bodies that take hundreds of years at least to complete a single orbit. We haven't had time to gather enough orbital data from them to verify the gravitational influence of other bodies. We're having far more luck detecting them visually.

 

[Asbo Zaprudder]

I read a couple of papers several years ago arguing that there may even be a Jupiter-mass or larger body in the Oort cloud that nudges long-period comets into the inner Solar System. I think the hypothesis was based on the observed arrival directions and frequencies of such comets.

http://www.zetatalk.com/theword/tworx089.htm

Perhaps it's a passing planemo or a brown dwarf similar to the proposed solar companion named Nemesis?

http://muller.lbl.gov/pages/lbl-nem.htm

The following reference has a similar conclusion based on observations of trans-Neptunian objects:
http://adsabs.harvard.edu/abs/2006Icar..184..589G

In this case, the postulated body would be less massive and closer to the Sun, but still beyond the Kuiper belt.

 

[Christopher]

^It's possible. This is a heady time for Solar System exploration, because we're only just beginning to discover a whole new region of the system that we didn't even know existed until recently. Twenty years ago, we thought we'd discovered every significant body in the system, except maybe one or two in some people's conjectures. Now we know that the significant bodies we don't know about considerably outnumber the ones we do. What we thought was the entire Solar System is really just the inner part of it.

 

[Trekker4747]

I read a couple of papers several years ago arguing that there may even be a Jupiter-mass or larger body in the Oort cloud that nudges long-period comets into the inner Solar System. I think the hypothesis was based on the observed arrival directions and frequencies of such comets.

Nibiru!

 

[Asbo Zaprudder]

I read a couple of papers several years ago arguing that there may even be a Jupiter-mass or larger body in the Oort cloud that nudges long-period comets into the inner Solar System. I think the hypothesis was based on the observed arrival directions and frequencies of such comets.

...
Nibiru!

Shh! It might hear you, and suddenly dive at us for no good reason.

 

[hyzmarca]

Addressing the original post, what you want is a variant of a Dyson shell. Simple throwing mass onto a star won't make it habitable, but building a half-spherical shell in orbit of the star at the appropriate distance would (a full sphere might be doable, but would be infeasible due to the lack of net gravitational interaction with the englobed object). More reasonably, do this around a gas giant that is in the habitable zone of a less exotic sort of star.

 

[Christopher]

^I don't think a hemispherical shell in orbit of the neutron star would be particularly stable. If the neutron star were at the geometric center of the hemisphere, then the center of mass of the shell -- the thing that would actually be defining the orbit -- would be at half the distance from the center to the shell proper. That means that the entire mass of the hemisphere would be located outside its orbit, actually twice as far out. So it would be moving too fast for that orbital distance (because it's orbiting at the speed of its center of mass), and would therefore fly outward into space.

 

[sojourner]

then the center of mass of the shell -- the thing that would actually be defining the orbit -- would be at half the distance from the center to the shell proper. That means that the entire mass of the hemisphere would be located outside its orbit

Aren't these two sentences mutually exclusive?

 

[Christopher]

No, because the hemisphere is a holllow shell of radius R, and the orbital path of its center of mass (assuming zero eccentricity) would be a circle of radius R/2. That means that entire circle would be inside the shell.

Although I think I see what you're saying. It's 3-dimensional, so there would be parts of the shell above and below the orbital plane that would be inside the orbit. Still, they would constitute a minority of its mass, and wouldn't be enough to cancel the instability.