Is there a theoretical size limit for a planet?

[fresh_42]

With respect, if a new member had suggested that we include hypothetical iron planets with no known formation mechanism, they probably would have been ridiculed and possibly thread banned if they had kept insisting it needed to be included.

What do you mean by theoretical? We live on one.

Earth is an example of an iron planet. You mentioned that 1.4 sun masses would probably be too heavy. There is a wide range between them. I'm just interested in the betweens.

 

[Drakkith]

What do you mean by theoretical? We live on one.

Well what do you mean by an 'iron planet'? Earth is certainly a very, very dirty iron planet if you're going to call it an iron planet. It's less than 1/3rd iron by mass after all, and only beats out oxygen by about 2%. By number of atoms, oxygen easily beats iron by a huge margin. I think Sophie's reply in post #6 took you to mean a ball of pure or near-pure iron, not an Earth-like planet. Hence his talk about the chemical makeup of potential formation regions.

 

[Algr]

This is pure speculation. I am asking for facts.

Given a certain amount of hydrogen or helium determines whether fusion ignites or not. This defines mass and size.

Sometimes the facts are grey. If an object is heavy enough to fuse deuterium, then it is a brown dwarf, and not a planet. But does the fusion have to be continuous? What if the fusion reaction has a tendency to stall out and then kick in again later? What if the density is such that added shock waves from meteor strikes push the planet into fusion, but only for a while? What if a brown dwarf just runs out of deuterium? Do we call that a planet?

Iron stars may come into existence someday: https://en.wikipedia.org/wiki/Iron_star. The problem with them existing now is that there is nothing to "purify" the iron and prevent it from attracting lighter elements. If a big mass of iron formed somewhere, it would be surrounded by other elements, attract them, and soon just be a conventional planet with a typical iron core.

Clearly there was a process separating iron and rock from hydrogen. [for Earth and smaller planets]
What are the limits of that process?

The process was that the objects were small enough that the accumulated hydrogen could escape over time. The larger a planet is, the more lighter gasses it will hold on to, and the less iron will dominate.

The estimated mass of Jupiter's core is 10-40 Earth Masses but (from magnetic measurements, I assume) there doesn't appear much Iron is in there.

Is this relative quantity or absolute?

 

[Feynstein100]

The upper bound is probably the Chandrasekhar limit, which is about 1.4 solar masses. Beyond this the iron can't hold itself up under the immense amount of pressure and it collapses into a neutron star.

Hmm but what if it were made purely of Manganese instead of iron? Oh wait. That would just be a stellar core undergoing fusion to iron. Never mind

 

[Drakkith]

Hmm but what if it were made purely of Manganese instead of iron? Oh wait. That would just be a stellar core undergoing fusion to iron. Never mind

I'm not sure what it would do. The Silicon burning process in stars seems to skip Manganese, instead adding an alpha particle to Chromium-48 to make Iron-52.

 

[Feynstein100]

I'm not sure what it would do. The Silicon burning process in stars seems to skip Manganese, instead adding an alpha particle to Chromium-48 to make Iron-52.

Wait then where does manganese come from?

 

[PAllen]

Wait then where does manganese come from?

Google helps:

https://www.aanda.org/articles/aa/full_html/2013/11/aa22599-13/aa22599-13.html

 

[swampwiz]

Just speculating, but wouldn't it be possible to find large concentrations of iron, up to whatever the size limit before gravitational collapse. near the centers of galaxies, where the ejecta from multiple supernovae might find its way into a stable orbit around another star or black hole? the farthest explanets found are around 17K LY, about 2/3rds the distance to the center of the Milky Way, so we have no idea what weird cold objects may be floating around there?

My understanding is that once a star runs its course to end up with an iron core, it becomes a black hole in less than an hour. An astronomer would have better luck looking for an Aricebo-type message from ET.

 

[swampwiz]

Hopefully, some of Webb's time is devoted to scanning the sky for brown dwarfs, which emit most of their energy in the IR band. (Can Webb even do such a scan?)

 

[Drakkith]

Hopefully, some of Webb's time is devoted to scanning the sky for brown dwarfs, which emit most of their energy in the IR band. (Can Webb even do such a scan?)

Depends on what you mean by 'scan'. It can take a series of images in a session, move over, and start a new series, but I'm not sure it can do 'drift scanning' like some telescopes on Earth do.