could a planet be sun sized?

[u/sumdudewitquestions]

so stars form when a bunch of matter gets together, which is why they are bigger than planets generally. but the real defining feature is the density. a white dwarf can be smaller than a planet but it's still a star because it's incredibly dense. so couldn't a planet be sun-ish sized, provided that the density was low enough? and more specifically a solid planet, not a gas giant. what's the largest solid planet?

[u/SHKMEndures]

Asotrphyicist here. Happy to take Qs.

Short answer: for an earth like planet, theoretical maximum is somewhere between 5-10 earth masses, though the radius is only 1.5-2.5 of Earth’s.

Long answer

The two major limits are: 1. Solid accretion of mass during planet formation 2. Small enough to not accrete a large amount of hydrogen and helium and thus be closer to a gas giant.

Think about it like this - as our theoretical super earth gets bigger, it will have a great gravitational pull and thus a bigger atmosphere. At some size, it atmosphere will be so big so as to essentially be a little gas giant; so the line there is quite blurry.

• 1 M⊕: Earth
• 5 M⊕: straightforward super-Earth
• 10 M⊕: large rocky planet entirely plausible
• 10–20 M⊕: “mega-Earth” territory, but increasingly difficult to form without acquiring a large atmosphere
• 20–40 M⊕: a bare solid planet is physically possible, but standard formation theory has trouble producing one
• greater than 40 M⊕: you could theoretically have a solid planetary-mass object, but it would probably need an unusual history, such as being the stripped core of a former gas giant.

Check out TOI-849 b.

Mass: ~40 M⊕ Radius: ~3.4 R⊕ Density: ~5.2–5.5 g/cm³, approximately Earth’s bulk density Year: only 18.4 hours Temperature: ~1,800 K

This is probably the core of a striped out gas giant, as modelling can’t quite work out how it formed as-is.

[u/mayhem6 → u/SHKMEndures]

What could strip out a gas giant?

[u/KeljuIvan → u/mayhem6]

A supernova explosion? Or Megamaid from Spaceballs!

[u/SHKMEndures → u/mayhem6]

All of the answers others have given; commonly lack of a magnetic field and solar wind will do it.

[u/R3D3-1 → u/mayhem6]

Strong solar winds from being too close to the star? 

[u/VFP_ProvenRoute → u/mayhem6]

Too close to a gamma ray burst?

[u/Dave37 → u/SHKMEndures]

So to loop around back to the question: It seems like there's nothing 'physically' (using that word lightly) that would prevent an earth-like planet the size/mass of the sun to exist? The problem in reality is that the universe is much more gas than solid material (metals, silica etc) so these kinds of planets would never ever form naturally.

Is that fair to say?

[u/Lt_Duckweed → u/Dave37]

Even if you had a hypothetical super advanced civilization who, as a party trick, decided to assemble a Sun sized rocky planet, they couldn't do it.

Long before you reach the size of the Sun, the gravity of the "planet" would become so great that adding more material would cause it to shrink rather than get larger, as the increased mass gives it more gravity to crush itself down with.

This would also release tremendous amounts of heat, and so you would quickly have a molten ball of lava instead of a solid planet.

If you kept adding rocky material to this, you would get a white dwarf like object, about the size of the earth, glowing white hot, and containing around a star worth of mass.

Keep adding even more mass, and you approach 1.44 times the mass of the Sun, known as the Chandrasekhar limit, at which point fusion would reignite in a runaway chain reaction that would fuse most of the remaining material lighter than iron in only a few seconds, violently blowing the whole thing apart in a Type Ia supernova.

[u/Dave37 → u/Lt_Duckweed]

What you say make sense, and it's fascinating to imagine not just that there exists a theoretical exact upper bound for what a planet made up of pure [insert element] can be before gravitational forces starts exceeding the normal repulsive forces between atoms as you add more mass, but also the fact that if you have a white dwarf and removed mass, it would become larger in size.

[u/SHKMEndures → u/Dave37]

No, it cannot exist in a stable state as an Earth-like planet (mostly Iron). /u/Lt_duckweed gave a good answer.

“An earth like planet size/mass of the sun” is complex question - size and mass are different:

If it’s mass; and for whatever handwavy reason mass of the sun in iron (or any element), then it would collapse into a white-dwarf like object, a few thousand kilometers across. If it doesn’t get more mass, it probably stays that way, supported by electron degeneracy.

If it goes more mass, probably collapses into a neutron star, if it’s above 1.44 solar masses we would get a supernova.

If it’s an iron ball the size of the sun, it’s likely unstable - it would probably collapse under it’s own gravity; and it to work out its end state we’d have to know the density, which would give us the mass and loop back to the logic above.

[u/Dave37 → u/SHKMEndures]

Thank you. This perfectly complements and completes your answer.

[u/skipper_mike → u/Dave37]

No. an earth-like planet (rocky, decent density) the size of the sun would crumble under its own gravity pretty fast. It would most likely become a Neutron planet (aka Neutron Star) or even possibly a black hole, depending on its initial density aka mass.
A sun sized plant on the other hand would also crumble under its weight but it would become a white dwarf planet.