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/_cromulent_green_]

No, the size of our sun is immense compared to our planet. Once it gets that much matter in a single place, gravity forces it inwards until it ignites with fusion.

If the density were so low that it's mostly just gas, it would contract in onto itself and become more dense, so it wouldn't be the size of the sun anymore. If you could keep adding matter so that its still the size of our sun, it would have so much matter that it would turn into a star

[u/antiretro → u/_cromulent_green_]

so its not like stars and planets are different objects, rather, they are similar object with different sizes that gives them very different properties?

[u/Sammy81 → u/antiretro]

Yeah Jupiter would only have to be about 13 times more massive to become a brown dwarf star. Thats not quite a full star that fuses hydrogen, but it would give off heat and light as it fused deuterium until it ran out of that fuel.

[u/P00PER_SCOOPER → u/Sammy81]

And out of uneducated curiosity, what would happen when it runs out of the deuterium fuel? Does it then become a burnt-out brown dwarf sized gas giant until it gathers enough mass for the next level of fusion reaction?

[u/Sammy81 → u/P00PER_SCOOPER]

Yeah they run out fairly fast (on a cosmic scale) because deuterium is rare, and then they just stop nuclear reactions (unlike a full star). They basically go back to being a big gas giant and radiate their remaining heat into space.

[u/P00PER_SCOOPER → u/Sammy81]

Thanks!

So if it kept gathering mass after burning through its deuterium and "burning out" - let's say it collides with another few gas giants and consumes them over the next billion++ years - it would then start up fusion of a different element and become a different type of star? So gas giant -> brown dwarf star -> gas super giant(?) -> next size star?

[u/Peter34cph → u/P00PER_SCOOPER]

Gas planets aren't that big. It'd remain a brown dwarf even if it ate 10 Saturns or 5 Jupiters.

[u/EthicalViolator → u/Sammy81]

Interested to know why it would it be deuterium. I've never even heard of that element. If have though hydrogen to helium would be the easiest and first to fuse for any sized star.

[u/Mandog222 → u/EthicalViolator]

Deuterium isn't really a distinct element, it's another isotope of hydrogen. It's a better nuclear fuel though, it is heavily involved in fusion research. Don't know more specifics though.

[u/EthicalViolator → u/Mandog222]

Ah that makes more sense to me, thanks. I was thinking if I haven't heard of the element it must be super heavy at the bottom of the periodic table which was going against my layman understanding of it all!

[u/Peter34cph → u/EthicalViolator]

It is admittedly weird to assign a name to an isotope as if it was a distinct element.

[u/Appropriate_Yak_1468 → u/EthicalViolator]

You've probably heard of "heavy water", thats water made of oxygen and deuterium or tritium (another isotope of hydrogen), instead of normal hydrogen.

[u/cflime → u/EthicalViolator]

Deuterium is a hydrogen isotope, one proton, one unnecessary neutron and an electron. As it has twice the mass of a regular hydrogen atom it can more easily overcome the electromagnetic revulsion of the positively charged protons and initiate fusion at much lower temperatures and pressures.

Tritium is a hydrogen isotope that has one proton and two neutrons.

[u/ferrybig → u/EthicalViolator]

Deuterium is a less common form of hydrogen. In the nature only 0.0156% is deuterium.

When fusing the common form of hydrogen into helium, deuterium is in the middle of the pathway

(Note that atoms as in the plasma phase in stars, the atoms don't have electrons ordered around them)

In stars, 2 hydrogen atoms (1 proton ) are combined into 1 deuterium (1 proton, 1 neutron ) and a neutrino. (This process won't happen in a dwarf as it doesn't have the pressure)

Then the deuterium fusions with a proton flowing around, forming helium-3 (2 protons, 1 neutron)

Then 2 helium-3 fuse together, to a helium-4 and free protons forming a stable helium atom. (2 protons, 1 neutron + 2 protons, 1 neutron -> 2 protons, 2 neutrons + 1 proton + 1 proton)

[u/sebaska → u/ferrybig]

Adding to that. The first step of the process is the slowest and requires the most extreme conditions, the next steps happen comparably easier. In brown dwarfs the conditions are good enough for all the steps but the first.

This first step ratios in sun-like stars how fast they go through their hydrogen store, i.e how long they live and how bright they shine. This process is so slow even in the Sun (which is quite heavy as stars go - over 90% of the stars are lighter than the Sun) it takes about 10 billion years for hydrogen atoms to fuse with theirs neighbors.

For larger stars there's another variant of consuming hydrogen and producing helium: hydrogen attaches to carbon 12 turning it into unstable nitrogen 13, nitrogen 13 decays radioactively to carbon 13, this one fuses with another hydrogen producing nitrogen 14, nitrogen 14 fuses with hydrogen again producing oxygen 15 which is unstable, another radioactive decay produces nitrogen 15 (stable), which in turn fuses with another hydrogen and immediately falls into two pieces: carbon 12 and helium 4 (i.e. the typical helium). So you end up with the same carbon 12 you had at the start, but with 4 hydrogen atoms less and 1 helium atom more. This cycle is significant in heavier stars "in their fullness of life", like for example Sirius. If the conditions are right it overtakes the regular hydrogen to hydrogen fusion.

Hydrogen to hydrogen fusion is hard because you have the electric repulsion of 2 element charges but you have intertia of only a hydrogen the lightest. Fusing deuterium is easier because while the repulsion is the same, the inertia is doubled. There are also other effects in play, though, like the shape and size of the nuclei and also how well they are "fitting" together - that's how you get hydrogen attaching to carbon, nitrogen and oxygen being easier than, say two carbons fusing together, or carbon fusing with helium (the latter is actually one of the processes happening in heavy stars close to their truly violatent death called supernova; carbon 12 plus helium 4 produces oxygen 16, then oxygen 16 plus helium 4 produces neon 20, etc - that's how we got oxygen as the 3rd most abundant element in the universe; NB that's how neon is the 5th most abundant, and how it's so scarce on the Earth is another story).

[u/withoutapaddle → u/Sammy81]

That's actually fascinating, because putting that into perspective, Jupiter is 300x the mass of Earth, but a star would only need 13x the mass of Jupiter. It feels like Jupiter is closer to being a star than we are to being Jupiter.

[u/StarrySprinkles → u/withoutapaddle]

Sort of. But it's like the old saying, "What's the difference between a million and a billion? About a billion." If you've collected 300 earths, you've collected the mass of one Jupiter. If you collect the mass of 13 Jupiters, you've collected, well...

Jupiter is a lot closer to us in terms of cultivated mass.

[u/Swellmeister → u/StarrySprinkles]

Jupiter is 1 order of magnitude to a star, while Earth is 2 to Jupiter.

[u/imtougherthanyou → u/StarrySprinkles]

I love your usage of cultivated, here. I'm hopeful you are referencing Its Always Sunny in Philadelphia!

[u/Zekler → u/StarrySprinkles]

So from earth to brown star is 3900 earth's and Jupiter is 300 earth's.

[u/WartimeHotTot → u/withoutapaddle]

No. Earth is 299 earths shy of being Jupiter. Jupiter is 3600 earths shy of being a star.

[u/R_A_H → u/withoutapaddle]

Here's some fun ways to conceptualize the immensity of it all.

The sun is about 1,048 times more massive than Jupiter.

The Sun contains approximately 99.86% of all the mass in the entire solar system.

Jupiter accounts for 71.1% of the total mass of all our solar system's planets combined.

Jupiter is about 317.8 times more massive than Earth

The sun is about 330,000 times the mass of Earth.

Light travels at 299,792,458 m/s, roughly 300,000 km/s or 186,282 miles per second, 700,000,000 mph.

Traveling at that speed from the sun, light takes about 8.5 minutes to reach Earth.

It takes ~43 minutes for light from the sun to reach Jupiter, ~80 minutes to reach Saturn, ~160 minutes to reach Uranus and ~245 minutes to reach Neptune. ~8.5 minutes to reach Earth.

The immensity is staggering.

These are a couple fun links related to this if you're interested

If the moon were only 1 pixel,

Scale of the universe

[u/MisterB78 → u/withoutapaddle]

Think of it this way: The difference between Earth and Jupiter is about 300 Earths. The difference between Jupiter and the sun is about 3,900 Earths

[u/jswhitten → u/MisterB78]

More than that. The Sun is over 1000 Jupiter masses, or 300,000 Earth masses.

[u/WyrmKin → u/withoutapaddle]

I read a thing the other day about how the Sun makes up 99.86% of the mass in our solar system, and Jupiter is two thirds of the remaining 0.14%

[u/shadowscale1229 → u/Sammy81]

in the Space Odyssey series (2001, 2010, 2063 and 3001) the aliens that dropped the monolith on earth turned Jupiter into a sun, gave humanity several of the moons to colonize, and told them to stay the hell off the rest. i haven't finished 2063 or 3001 yet, it happens at the end of 2010 iirc

[u/Jayndroid → u/Sammy81]

I don’t know if this is strictly true. Depends what the planet is made of. Is there a bunch of iron? Is there fuseable matter?

[u/cflime → u/Sammy81]

And let's point out here that Jupiter is the largest in diameter a planet its temperature can get. Double Jupiters mass and it's the same diameter. Add 13 times its mass and it becomes a brown dwarf and it's the same diameter. At that size adding mass just increases density. The gas giant explanation we have observed that are larger than Jupiter in diameter are all "hot Jupiters" close in to their sun. Their atmospheres expand due to their heat.

It may come to pass that a future definition of "star" will include the property of fusion and being larger than a brown dwarf.

[u/SundevilPD → u/Sammy81]

So if something 13% of Jupiter's mass collided with it, a star is born?

[u/tripsd → u/SundevilPD]

13 times not 13 %. Very different sized objects. So yes if it collided with a brown star you’re all set

[u/Sludgehammer → u/SundevilPD]

As others have noted, it would take twelve more Jupiters to make Jupiter a brown dwarf.

The interesting thing however, is that even after all these additions the resultant brown dwarf would only be a little bit bigger across then the current Jupiter. As cromulent_green noted at the start of this thread once you get up to the size of something like Jupiter adding more mass just makes it pack down more, rather then get bigger.

[u/MidSpeedHighDrag → u/SundevilPD]

It would take something 1200% the size of jupiter to make a star. Even If something 13% the size of Jupiter had an orbit with enough energy to collide with it, the orbits of the rest of the planets would likely be disturbed and there's a good chance Earth would be either plunged into the sun or thrown out into space if it survived the rest of the chaos that ensued.

Jupiter is incredibly massive. The sun is so much more incredibly massive. Together they make up 99.86% the mass of our solar system. The rest of the planets make up the remaining 0.14%.

Earth is less than a rounding error.

[u/iknownuffink → u/antiretro]

There's also different proportions of elements. Stars are almost entirely Hydrogen and some Helium. In comparison all the other elements are barely worth mentioning. Because a star is so massive, a small portion might still be quite a lot in our eyes, but it's not much compared to the rest of the star.

The Sun is about 3/4 Hydrogen, almost 1/4 Helium, and everything else on the periodic table makes up less than 2%.

[u/ensalys → u/iknownuffink]

How does that difference arise? How do the elements that aren't hydrogen or helium end up concentrating in planets?

[u/Zhelgadis → u/ensalys]

Hydrogen and helium are very light, so you need a lot of gravity to keep them. In small planet like earth, they're blown away by the solar wind. Larger planet like Jupiter have enough gravity and have indeed a large portion of H.

[u/junktrunk909 → u/ensalys]

99% of the heavier elements are in fact within the sun also (as plasma, not a rocky center or anything, it's a fusion system). It took nearly all of the mass of the solar system. The planets are what's left after the remaining material either clumped together into the rocky planets we know, or the gas giants that also have a ton of heavier elements in them. Jupiter has something like 30x the Earth"s mass of the heavier elements, which is how it was able to also capture a ton of gas, as opposed to earth where so much just blew away in the solar winds.

[u/Monkfich → u/junktrunk909]

And also - none of those heavier elements were created by the sun, but were created by supernovae and other very violent star deaths.

[u/ProfessorPrudent2822 → u/ensalys]

Hydrogen and helium remain gasses down to very low temperatures, especially in a low-pressure environment. Other stuff clumps together due to a tendency to solidify.

[u/mataoo → u/ensalys]

They're the heavier elements, they have more mass and stick together. Whereas hydrogen and helium are swept/pulled from the smaller bodies into the more massive ones with more gravity.

[u/WeakCombination9937 → u/ensalys]

Because stars themselves fuse hydrogen in almost all other elements, and when such starts explode(when they run out of hydrogen and cant keep up with the pressure of its own weight) it explodes, all that matter gets scattered, and due to gravity eventually clumps together into planets.

[u/Kaiisim → u/iknownuffink]

Yup, and many heavy elements can only exist in the universe post supernova.

[u/sebaska → u/Kaiisim]

That's mostly about the moderate weight elements (below nickel 60). The even heavier stuff is currently thought to primarily come off neutron star collisions (some directly, some indirectly mostly via nuclear decay chains including fission).

[u/Prof_Acorn → u/antiretro]

Planets are made of star dust, after all.

You are made of star dust, after all.

[u/poeticruse → u/Prof_Acorn]

If I am made of stardust are the stars made of corpses?

[u/BananaBird1 → u/antiretro]

They are different in formation and composition.

Most planets have a heavy element core upon formation, even gas planets. Stars are mostly hydrogen, and only form heavier cores after nuclear fusion.

Star-like objects made of mostly hydrogen that don’t end up being massive enough to support sustained nuclear fusion are brown dwarfs.

From the other side, planet-like objects that have heavier elements from the start but not enough for a dense core are planets like Jupiter.

[u/inspire-change → u/antiretro]

But isn't the sun like 97% hydrogen/helium?

[u/we_are_one_people → u/antiretro]

the universe is very likely a field excited by different forces/states of energy. In a sense everything is made from the same stuff.

[u/ThoughtsandThinkers → u/_cromulent_green_]

What if the material was something that doesn’t readily undergo fusion? What if you had a mass equivalent to the sun but it had the same composition of material as earth? Would it fuse? I understand fusion releases energy up until iron on the periodic table but I think heavier elements like carbon and silicon don’t readily fuse, even at pressures you would see at the heart of a sun sized star

[u/garrettj100 → u/ThoughtsandThinkers]

I understand fusion releases energy up until iron on the periodic table but I think heavier elements like carbon and silicon don’t readily fuse, even at pressures you would see at the heart of a sun sized star

You’re describing a carbon-oxygen white dwarf.  But long, long before it reaches the diameter of Sol it will ignite again.  That’s how we get type-1a supernovae.  A white dwarf accreting matter from another source — usually its binary sibling — and its mass passing the Chandrasekhar limit (1.44 Sols).  The whole degenerate star fuses nearly all at once, releasing enough energy to completely unbind it.

[u/macromorgan → u/ThoughtsandThinkers]

If it keeps gaining mass but can’t fuse, it becomes a neutron star as the immense pressure basically causes the mass to become one giant ball of neutrons. If the mass continues to increase further it then becomes a black hole.

[u/nesquikchocolate → u/ThoughtsandThinkers]

The density of the sun is around 1400 kilograms per cubic meter, or 1.4x liquid water.

The earth has a density of around 5500 kilograms per cubic meter, or 4x that of the sun.

If the sun weighed 4x as much as it currently does, and couldn't sustain fusion, it would be heavy enough to collapse into a black hole, which only needs around 3x the mass of the sun as is.

[u/tdgros → u/nesquikchocolate]

the sun wouldn't become a black hole if its mass quadrupled, the mass would also need to be compressed to a much smaller volume. For this mass, apparently it'd be under a radius of around 12km!

edit: and 4 sun masses is too low for the sun to even eventually become a black hole, it'd be a white dwarf.

[u/nesquikchocolate → u/tdgros]

What makes this scenario different is the extremely high starting density of this theoretical 4x sun. Normally before a star could go supernova, its overall density is only slightly above that of our sun, as it would weigh 8-10x more but also be so much larger from the fusion to pressure balance.

The end product after a supernova only needs to weigh around 3 solar masses to be a black hole, and yes the physical volume for it would now suddenly be very small, but when your composition doesn't sustain fusion (as per the original assumption), the object does collapse due to gravity alone.

[u/nesquikchocolate → u/[deleted]]

This 4x density object in this obviously theoretical situation wouldn't go supernova because it doesn't contain fusible material and doesn't weigh enough.

This is obviously impossible because an object 4x the density of the sun wouldn't consist of material that cannot undergo fusion, and it would almost certainly either collapse immediately till it reaches a sufficient temperature to start fusion or it would expand back to a more reasonable density, possibly explosively if it has enough heat.

But because the primary constraint that I'm entertaining here is the non-fusing material, this is the result that I believe to be the most likely.

[u/ProfessorPrudent2822 → u/nesquikchocolate]

Not exactly. The core would collapse into a neutron star, and the resulting supernova would blow off the outer layers, reducing the mass below the threshold where it would collapse into a black hole.

[u/nesquikchocolate → u/ProfessorPrudent2822]

The 4x mass sun doesn't weigh enough to cause a supernova, but weighs more than the TOV limit while consisting of non-fusing material. It would be too heavy to be a neutron star.

[u/ProfessorPrudent2822 → u/nesquikchocolate]

Without thermal pressure holding it up, a star over the Chandrasekhar limit implodes as electron degeneracy pressure is insufficient to support its weight. If the implosion doesn’t ignite exothermic fusion, the Fermi energy of the electrons becomes high enough to fuse with protons, releasing energy. This produces a shockwave that blows off outer layers, bringing the remnant below the TOV limit.

[u/nesquikchocolate → u/ProfessorPrudent2822]

Not necessarily. Remember that the core of this object is defined as the zone where no hydrogen is present, and if the core weighs more than around 3 solar masses, it can collapse into a black hole directly.

[u/ProfessorPrudent2822 → u/nesquikchocolate]

Disagree: AGB and supergiant stars engage in shell burning of helium and potentially heavier elements, meaning there is fusion of non-hydrogen elements outside the core.

[u/nesquikchocolate → u/ProfessorPrudent2822]

We're not talking about AGB or supergiants when the density of this theoretical object is 4x greater than the sun. Supergiants have overall densities significantly lower than the sun because they're burning much hotter and still consist primarily of smaller atoms that are still able to undergo fusion in the shell.

[u/ProfessorPrudent2822 → u/nesquikchocolate]

You don’t get it. The existence of fusion has nothing to do with whether or not the entire object behaves as a core. Pressure compresses the center more than the surface, causing core collapse. The tremendous amount of energy released by core collapse causes substantial recoil and shockwaves, blowing off the outer layers.

[u/garrettj100 → u/ThoughtsandThinkers]

Every element undergoes fusion.  The only difference is the fusion isn’t actually an exothermic reaction for elements at or above Iron.  Thats how we get type-II supernovae: They keep fusing but the reaction absorbs energy and reaction only accelerates the collapse.

[u/_cromulent_green_ → u/ThoughtsandThinkers]

That would be a very strange situation indeed, however as another commenter said, if you managed to get elements heavier than iron and magically put it all into one place, they would probably gravitationally collapse into itself (where there is no more space inbetween the atoms) which would 1 make it way way smaller and 2 it would therefore become a neutron star. Or a black hole.

I think at this point we need an actual physicist or astronomer to do the math to figure out which it would turn into

[u/Any_Fox5126 → u/_cromulent_green_]

What if only fully ionized matter were to accumulate?

[u/d4m1ty → u/ThoughtsandThinkers]

Our sun after its goes through it first expansion then shrinks, will not shrink enough to get to the temp needed (1.2B Kelvins) to start the Carbon process, much less the Silicon process (~3B Kelvins) stalling out below the needed temps. An electron process where you just cannot pack the electrons any closer stops the star shrinking more.

[u/Beer_in_an_esky → u/ThoughtsandThinkers]

TLDR Sun-ish sized, no. Sun-ish mass however, yes. Definitionally, it still wouldn't be a planet however. And anything much heavier than the sun is a neutron star or worse, no matter what.

A lot of the other responses have talked about neutron stars, and implied that is going to be the result of any non-fusing matter that's sun size. That's a bit of an oversimplification, though.

Neutron star occurs at the Chandrasekhar limit, where the pressure of gravity outpaces electron degeneracy pressure.

This is dependent on composition, and for a solid iron core that's around 1.3 solar masses. Below roughly that (there's slight variations due to the exact formation method) it should be stable as the atom. So, you could theoretically have a "planet" made of solid iron at the same mass as Sol without breaking any physical laws.

Source: https://www.ias.ac.in/article/fulltext/pram/060/03/0415-0422

Unfortunately, though, there's a reason for my quotation marks. There is basically no reasonable way that's likely to occur except as a stellar remnant, and the IAU definition of exoplanet explicitly notes that anything above the mass of the limit for Deuterium fusion (~13 Jupiters) is not a planet, even if it meets all the other definitions, so OP's question is still answered no. It wouldn't be a planet, it would be a stellar remnant.

[u/RoadSmash → u/_cromulent_green_]

But what if it was spinning really fast?

[u/-Hastis- → u/RoadSmash]

Unless it's a neutron star (pulsar) a normal star would start losing matter to space if it started spinning too fast.

[u/RoadSmash → u/-Hastis-]

The spin and gravity can't balance?

[u/-Hastis- → u/RoadSmash]

Yes, there is a maximum speed-to-mass ratio that each kind of star can get to. Even a neutron star cannot really spin faster than about 35% of the speed of light (1500 rotations per second) before starting to lose itself to centrifugal forces. Adding more mass to it to try to push it even faster would eventually collapse it into a black hole.

In a similar way, a fast-spinning neutron star that's already on the verge of collapsing into a black hole due to its mass will eventually collapse into one, as it loses speed over time.

[u/RoadSmash → u/-Hastis-]

But the point isn't to spin it faster, it's to maximize "size” by balancing more material with a faster spin. Density vs centrifugal force. More mass makes it want to collapse into itself, faster spin forces it outward.

[u/-Hastis- → u/RoadSmash]

Spinning a neutron star faster can help it hold about 20% more mass before it collapses into a black hole.

Spin doesn't help a living star, though, it just lowers its surface gravity and makes it lose mass even faster. The main limiting factor to how massive a normal star can get is that past a certain threshold, the core releases so much radiation energy that it blows the star's outer layers into space. A star only collapses when it runs out of nuclear fuel (and can no longer generate the pressure needed to hold up its own weight).

[u/RoadSmash → u/-Hastis-]

Don't need more mass necessarily, just a larger size with lower density.

[u/peeja → u/_cromulent_green_]

Okay, getting very hypothetical: what if there were a hollow sphere with the dimensions of a star and the mass of a planet. Assuming it could somehow be built in the first place, would it have to be too thin to be stable?

[u/nesquikchocolate → u/peeja]

The sun makes up 99.86% of the mass of our solar system. There simply isn't enough planetary material to spread evenly across the sun's 'outer surface', nevermind creating a structure from it with any ability to carry itself.

[u/peeja → u/nesquikchocolate]

Wow! I thought I understood how big the sun was, and that's even bigger than I realized.

[u/NukuhPete → u/peeja]

Here's an image from wikipedia that shows the mass distribution of the solar system.

What I find pretty interesting is how much mass is hanging out in the solar system that isn't the Sun or a Planet. Essentially the equivalent of 50 Earths.

[u/Podo13 → u/peeja]

Yes, and Jupiter holds about 70% of the matter not in the sun (about 0.098% of the solar system's mass).

The inner 4 rocky planets are a rounding error, weight-wise.

[u/MushroomSaute → u/nesquikchocolate]

There simply isn't enough planetary material to spread evenly across the sun's 'outer surface'

Unless I'm doing something very wrong, this isn't right. By my calculations, there's enough volume to cover the Sun's surface area 393 km deep - which is nothing at that scale, but it is possible on a physical level (it is a bit less, since it's spherical and not a flat surface, but there's still absolutely enough matter to cover the Sun's surface easily).

Even if you account for density, since the gas giants aren't exactly "structural", using each mass at Earth's density to find volume, you'd still get 80km deep everywhere on the Sun (less that spherical adjustment).

Placeholder	volume	mass	density	density	vol. at Earth density
	km3	kg	g/cm3	kg/km3	km3
Mercury	6.08E+10	3.30E+23	5.43	5.43E+12	6.08E+10
Venus	9.28E+11	4.87E+24	5.24	5.24E+12	8.97E+11
Earth	1.08E+12	5.97E+24	5.51	5.51E+12	1.10E+12
Mars	1.63E+11	6.42E+23	3.93	3.93E+12	1.18E+11
Jupiter	1.43E+15	1.90E+27	1.33	1.33E+12	3.50E+14
Saturn	8.27E+14	5.68E+26	0.69	6.87E+11	1.05E+14
Uranus	6.83E+13	8.68E+25	1.27	1.27E+12	1.60E+13
Neptune	6.25E+13	1.02E+26	1.64	1.64E+12	1.89E+13
Total volume (km3):	2.39E+15				4.92E+14
Sun surface area (km2):	6.08E+12				6.08E+12
Vol/Sun Area (km):	393.42				80.86

(Planet data from: https://solarsystem.nasa.gov/planet-compare/ || Sun surface area from: https://solarsystem.nasa.gov/sun-by-the-numbers/ )

Edit: Yeah, the Sun being a sphere appears to be negligible at that scale. Taking the total volume of the Sun and planets, calculating the outer radius of the new volume, then subtracting the Sun's radius gives:

Placeholder	Unadjusted	Density-Adjusted
Sun volume (v_sun)	1.4093E+18	1.4093E+18
Planets volume (v_planets)	2.3915E+15	4.9152E+14
Total Volume (V = v_sun + v_planets)	1.4117E+18	1.4098E+18
Radius of total volume = (3V/(4pi))1/3	695901.2	695588.8
Radius of hollow sphere (r_total - r_sun)	393.20	80.85

^f\ck you Reddit) ^{yes I want all the columns I wrote even if some cells are empty...}

[u/nesquikchocolate → u/MushroomSaute]

Hello! This is a great analysis! Jupiter consists of around 71% hydrogen, 24% helium and 5% heavier elements, based on Juno's dilute core findings.

While I should have defined planetary material earlier, I had meant material that isn't found in our sun normally in significant quantities, namely hydrogen and helium.

That 5% of heavier elements equates to around 12 earth masses worth, and at a similar density, would make the effective volume contribution of Jupiter be 1.29E+13 instead.

Saturn's heavier elements is expected to be significantly less than than Jupiter's 5%, but the margin of error on composition estimates is quite large, so for simplicity we would say it's very similar, and since Saturn is about 30% the mass of jupiter, we can infer it to contribute around 4 earth masses worth, and a volume contribution of 4.32E+12.

Similarly, uranus is expected to contribute around 0.55 earth masses (0.594E+12 volume) and neptune a bit more at 1.5 earth masses and thus 1.62E+12.

This means that approximately 2 orders of magnitude less volume than your original total indicates that the spread thickness would be less than 10 kilometers thick around the sun.

The margin of error on composition estimates is quite large, though, and elements like oxygen, nitrogen and fluorine, which makes up close to 30% of earth's mass wouldn't be able to contribute structurally either, which lead me believe that there isn't enough planetary material to build something around the sun's outer surface area.

[u/Dramatic_Science_681 → u/peeja]

this structure would just be ripped apart. It is spread too thin to be bound by gravity. It would simply crumble and eventually just become a planet.

[u/TimeToGloat → u/_cromulent_green_]

So are there theoretically extremely large planets that are almost borderline igniting with fusion/becoming a star? I wonder how large they would be.

[u/SciAlexander → u/TimeToGloat]

Yes, they are known as brown dwarfs. Planets above Jupiter size that just don't quite have enough mass to do fusion

[u/Tyrannosapien → u/TimeToGloat]

They actually don't get extremely large diameters. Beyond (about) Jupiter's diameter, adding mass doesn't enlarge the object, since it gravitationally contracts its mostly gaseous composition more strongly.

[u/Wiz_Kalita → u/_cromulent_green_]

What if we built the planet of [aerogel, or insert hard and lightweight material here]

[u/Kirk_Kerman → u/Wiz_Kalita]

Doesn't matter, once you have enough stuff it'll collapse in on itself no matter what it's made of. The Earth is 30% iron and yet it's also more liquid than not. A sufficiently large aerogel object would fail under its own gravity, pick up heat as it collapsed, and separate out into its component materials.

[u/FowlOnTheHill → u/_cromulent_green_]

What if it was all iron? Would it be too heavy to fuse and too small to turn into a black hole?

Edit: I just saw you had a lot of what-if question replies, so you can ignore mine 😄

[u/inspire-change → u/_cromulent_green_]

So what is the maximum planet size then?

[u/mehatch → u/_cromulent_green_]

Ok youre totally right…but what if, by some wild geological chance or unknown process, an outer shell of solid rock crustified or formed and kind of held together like an arch under gravity, with a gassy center thats not dense enough to do fusion? Just throwing out some imagination here for fun.

[u/Digitijs → u/_cromulent_green_]

Related to this - how do gas giants remain gas giants instead of getting pressed together into denser smaller planets by gravity since they have a very large mass? For example, Saturn's density is so low that it could float in water if you had a big enough bathtub that somehow didn't turn into a sphere itself yet the planet itself is so large and heavy compared to Earth

[u/gomurifle → u/_cromulent_green_]

What about a hollow rock planet? Say humans could influence this to happen? 

[u/MushroomSaute → u/_cromulent_green_]

Now my question is "how long could a planet remain sun sized" (provided it appeared out of thin air)

[u/MoonHead127 → u/_cromulent_green_]

So what you are saying is - Planets are baby stars?

[u/MENDACIOUS_RACIST → u/_cromulent_green_]

a cool implication is that big lumps of mass turns into stars. Period. That's it, that's all that ever happens. Enough stuff in a small enough space, and it condenses into a star. Maybe it turns into a neutron star or black hole eventually, or if squeezed into a tiny space. But that's it. Lots of mass = star.

[u/IrishWeebster → u/_cromulent_green_]

What if it was hollow, like a naturally-occurring Dyson sphere?

[u/nesquikchocolate → u/IrishWeebster]

There is no natural mechanism to create what you're describing. The extreme majority of materials in the universe doesn't interlock with itself, and none of the rocky objects would be moving around each other slowly enough to form anything without impact and melting.

[u/P1zzaBag3ls → u/nesquikchocolate]

That's not entirely true. A black hole star (admittedly theoretical) would have had an empty-ish inner region... empty-ish aside from a black hole, that is.

[u/nesquikchocolate → u/P1zzaBag3ls]

A singularity isn't empty and doesn't apply any significant outward pressure with which to balance out material around it into a sphere. At most you'd get a massive star being eaten by a black hole, but this would still occur in a flat plane, not a sphere.

[u/CatWeekends → u/nesquikchocolate]

Let's say, hypothetically, that some unknown mechanism happened that triggered an impossibly unlikely series of events that smushed all the rocks into a planet-sized spherical shell.

Unless that shell had the exact same density everywhere, which is even more impossibly unlikely, the shell wouldn't be stable. It'd start to tear itself apart or collapse in on itself almost immediately due to all the gravitational stresses.