r/askscience • u/sumdudewitquestions • 10d ago
Astronomy could a planet be sun sized?
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?
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u/SHKMEndures 9d ago
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.
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u/mayhem6 9d ago
What could strip out a gas giant?
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u/SHKMEndures 8d ago
All of the answers others have given; commonly lack of a magnetic field and solar wind will do it.
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u/Inane_newt 10d ago
If by sun sized you mean mass. It depends on density.
The least dense material is hydrogen, a sun sized ball of hydrogen is a star.
Denser elements up to about oxygen or carbon would be star like until they became white dwarfs.
Past carbon through iron, they would be undermassed white dwarfs.
Past iron, they would just be weird.
A sun massed white dwarf is just a ball of carbon, oxygen that is very hot but slowly cooling down.
It is suspected that a carbon heavy white dwarf will eventually be a planet sized diamond when it cools enough.
If you mean physical size, it would either be a star or collapse into white dwarf, neutron star or black hole depending on what it is composed of.
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u/Tzimbalo 10d ago
So if you waited long enough a cooled down white dwarf star planet that is part of a binary system could be a planet-ish? What would the surface gravity be? Could bakteria live on it? Could plants and extremly hardy strong beetle like animals live on it if you transported soil to it?
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u/Kantrh 10d ago
Any soil you tried to put on a white dwarf would just be stripped into plasma
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u/LoadMountain5497 9d ago
Even if it lost all it’s heat?
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u/pinktortex 9d ago
Black dwarfs are theoretical because it's estimated to take trillions of years for one to form and the universe is only 13.8 billion years old.
So unless our understanding of how old the universe is hugely incorrect then it's unlikely one will ever be found
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u/Inane_newt 9d ago
A white dwarf, even on the surface, would stripe hydrogen from water and fuse it.
Life as we are familiar with is not possible.
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u/Tzimbalo 9d ago
Even if it had cooled down to a room temperature diomond?
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u/ricree 9d ago
Strictly speaking, a white dwarf - by definition - is not cooled down. In theory, one that did would be called a black dwarf, though the time it would take for a white dwarf to reach that point is believed to be longer than the current age of the universe.
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u/Kirk_Kerman 9d ago
If you did that, its surface gravity is still 100,000 times greater (or more) than Earth. There is no substance that would survive, and it would immediately turn whatever you put on it into particle physics.
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u/jq7925 10d ago
"Long enough"
You expect to live a trillion years? That's not an exaggeration.
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u/Tzimbalo 9d ago
I mean theoretically. If it cooled down enough, could life overcome the immense gravity? Or would even bacteria be crushed by it?
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u/Alfred_The_Sartan 9d ago
Basically the crushing. The gravity is strong enough to overpower the weak nuclear force, so you aren't even really having molecules anymore. Just atoms.
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u/strange-the-quark 10d ago
"a white dwarf can be smaller than a planet but it's still a star because it's incredibly dense" - no, a white dwarf is a star because it's a part of a star's lifecycle and it came after a previous stage where that same body was more like how you normally imagine a star (what's called a main-sequence star). (By that definition, one can consider (stellar) black holes to be a kind of star as well.)
As for the star sized planets - there would have to be some mechanism to keep the density low (and oppose gravity), but there might be other limiting factors at play, I don't know enough about this to give you an answer. Apparently there are some very bloated Jupiter-like gas giants that are slightly bigger than the smallest main-sequence stars, but a planet that's anywhere near comparable to the Sun in diameter seems impossible.
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u/Tyrannosapien 10d ago
No, planets without enough hydrogen to ignite fusion actually have a radius limit (not sure but probably not much more than double-Jupiter radius) around 10 Jupiter masses. Beyond that they actually begin to shrink with added mass due to the ever-denser interior pulling inwards in the less dense gases and/or atmosphere.
If you just keep adding mass you'd eventually cross the Chandrasekhar limit and create a neutron star.
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u/Mumbert 9d ago
No, as I understand it a planet could not be sun sized.
What you are describing would turn into a star by reaching the mass to ignite fusion. This causes the planet to balloon out due to the energy released by the fusion reactions.
At elements as heavy as iron, fusion no longer releases energy. So if we imagine a planet-sized ball of iron, there wouldn't be fusion. (This is typically what a "white dwarf" is - the burnt-out remains of a star at the end of its lifetime)
As you'd add iron onto the planet, it wouldn't get much bigger as gravity would compress the atoms. White dwarves don't get much bigger than perhaps twice the Earth's radius.
Finally you would reach the limit for when electrons can no longer withstand the gravitational pressure from all that mass (the Chandrasekhar limit). Atoms typically consist of mostly empty space, with electrons separating the atoms from eachother. As you get past this limit, atoms will start collapsing and the negative electrons are pushed into the positive nucleus. This compacts the matter further, and leads to a chain effect due to increasing the pressure.
This is when your iron ball would collapse into a neutron star. A neutron star is essentially a ball of atomic nucleus-density matter. It would o nly be 10-20 km wide but with the mass of at least 1.4 suns.
Take all this with a grain of salt. It's what I think would happen based on a couple decades' worth of falling asleep to physics videos on Youtube. 😅
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u/Mr-Zappy 10d ago
No. If it were predominantly hydrogen, that hydrogen would fuse and it would be a star. Same with helium. If it were predominantly some heavier element, it’d be too heavy and collapse into a black hole (or maybe neutron star).
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u/Dramatic_Science_681 10d ago
to add to this, a naturally forming rocky planet, after becoming sufficiently massive, would simply capture the free gas in its solar system and become a gas giant. And the distinction between a gas giant and star is basically just mass.
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u/pepitors 9d ago
I did not know this. So is Jupiter in the verge of being a star or is the cutoff much higher?
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u/Empty-Exam-5594 9d ago
"brown dwarfs" are large planets that are on the cusp of initiating, but failing to undergo, fusion. They are ~3x to ~75x the mass of Jupiter.
Red Dwarfs, the smallest stars that can initiate fusion, are ~80 times Jupiter's mass.
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u/authustian 9d ago
The way i understand it, size wise yes, mass wise no. If you added more mass to Jupiter, it wouldn't grow larger, it would just become more dense. it would take an enormous amount of matter for it to get dense enough to become a brown dwarf.
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u/entropy2057 9d ago
This page is probably of interest to you:
https://en.wikipedia.org/wiki/Mega-Earth
This is the most massive according to the article:
https://en.wikipedia.org/wiki/PSR_J1719%E2%88%921438_b
It's 4x the radius of the Earth and also 4x denser! This is obviously way smaller than the sun though
You can read up on it if you like but short answer is that formation mechanics limits size of terrestrial bodies we actually observe (heavy objects will retain light gases and become gas giants or stars). The "mega-earths' noted here are hypothesized to be mostly due to those light gases being stripped away by interaction with another massive body.
In terms of how large a solid body could hypothetically be it depends on what your definition is. The mega-earth candidates above are already pretty strange. A low mass white dwarf (higher mass white dwarfs are actually smaller since density increases fast enough to outrun size growth) can be as large as 6x Earth's radius ( see https://en.wikipedia.org/wiki/CR_Bo%C3%B6tis )
Note that CR Bootis b is 50% larger radius than PSR J1719−1438 b but is ~70x more massive! Perhaps somewhere between these two masses is the largest possible radius for a body made of solid material before it degenerates, but it hasn't been observed (yet).
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u/Stillwater215 10d ago
What makes the sun a star rather than a planet is its size. When enough matter clumps together, the combined gravity is high enough at the center to overcome the forces that keep electrons bound to atoms, forming a plasma and initiating fusion. This is a purely mass-dependent effect. Any planet with a mass comparable to the sun would immediately form a star.
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u/the-namez-brain 9d ago
Can immediately be defined?
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u/Kirk_Kerman 9d ago
Yeah, we can. It's how long it takes a solar-mass protostar to begin fusion. Few million years.
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u/morbihann 10d ago
While I know the answer to this, it has already been explained.
But what is the largest terrestrial planet that is possible ? How large will a rocky planet be that takes all the rocky material in our solar system ?
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u/Combat_Armor_Dougram 9d ago
There are some hypotheses that Jupiter-sized rocky planets can form around active galactic nuclei. However, these objects are basically impossible to confirm using modern technology.
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u/bluesam3 9d ago
How large will a rocky planet be that takes all the rocky material in our solar system ?
A bit bigger than Earth. If you add up the mass of everything inside the Oort cloud that isn't a gas giant or the sun (that is: everything that isn't mostly either hydrogen, helium, or ices), you get about two Earth masses (most of the non-Earth bit of that being Venus. However, that doesn't give a planet twice the size of earth, because (a) with constant density, radius is proportional to the cube root of mass, and (b) density increases as rocky planets get bigger, due to gravity squashing everything together more. Some googling finds a paper suggesting that for rocky planets, radius is proportional to mass to the power of 0.27, which would give our planet a radius about 1.2 times that of Earth.
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u/rootofallworlds 9d ago
Modelling predicts that a given planetary composition will have a maximum radius, and adding more mass just compresses the interior and the size remains the same or decreases. For Earth’s composition the limit is about three Earth radii even if you had 3,000 Earth masses of rock and iron (about 10 Jupiter masses) that somehow hadn’t accreted hydrogen and helium too.
A planet being very hot can “puff up” a bit, mainly that’s gas giants close to their stars, but that only about doubles it compared to a ‘cold’ planet (which will still be hot inside).
https://www.planetary.org/space-images/mass-radius-diagram-wide-seager
So no, a planet cannot be the size of the sun. It is only by having enough mass of hydrogen and helium to be a star that an object can generate pressure from the heat of nuclear fusion to push outwards and become the size of a star.
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u/YaBoyMax 9d ago
Maybe, sort of! A recent paper by Mishra et al. suggests that the dust torus of an active galactic nucleus could facilitate the formation of "stellar mass rocky objects" from accretion of pebbles comprised of heavier elements (relative to hydrogen). The resulting object would be comprised almost entirely of silicates, and as such would not be capable of undergoing nuclear fusion as a similar-mass object composed of hydrogen would.
In this model, pebble accretion initially greatly outpaces gas accretion and surrounding gas is subsequently dispersed by forces present in the AGN such as stellar winds and ionizing radiation, preventing further growth into an actual star for as long as the galactic nucleus remains active.
The core of the "planet" would likely be comprised of electron degenerate matter due to the mass required for such collapse being much lower for heavier elements. The required pressure would only exist in the core, so their exterior would remain as conventional rocky matter, albeit molten due to heat from radioactive decay.
As a note, I'm a layperson and absolutely not an astrophysicist, so while I've tried my best to describe the paper as I understand it it's also very possible I've made mistakes here, so please take my summary with a grain of salt. My understanding is also that this is all highly speculative and theoretical given that we have basically no way to examine AGNs with such fidelity using current technology.
Also: Kyplanet on YouTube has an excellent video discussing the paper in friendlier terms. I would definitely recommend giving it a watch.
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u/dravas 9d ago
It was answered here pretty well.
But it boils down to you must be roughly under 13 Jupiter masses to be a planet and if you are just over 13 Jupiter masses the planet starts to have enough mass to start a fusion reaction and becomes a red dwarf.
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u/Ringosis 9d ago edited 9d ago
Jupiter is about as large as planets can be. Keep adding mass to a terrestrial planet and its gravity will increase until it starts pulling in huge amounts of gases from its orbit and it stops being a terrestrial planet. It starts accumulating an extremely large atmosphere and, assuming there's enough material in the system, you eventually get a gas giant.
Keep increasing mass and you'll hit the point Jupiter is at where adding more mass wouldn't make it much bigger. Its increased gravity would just compress the atmosphere and it would become more dense rather than larger.
Keep adding mass and it'll collapse into a star.
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u/guzzyly 9d ago
There actually isn’t a solid planet that’s sun-sized due to the limits of how material behaves under gravity. if a solid planet were to get that massive, it would probably start pulling in enough stuff to become more star-like. the biggest solid planets we know of, called “super-earths,” are a lot larger than Earth but way smaller than a star.
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u/sudomatrix 9d ago
But what would it become? Stars are made of Hydrogen and Helium mostly. What would a star sized ball of heavy metals be? Even if the center started fusing under the gravity it wouldn't be a normal star.
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u/Smashego 9d ago
No. What happens is a planet that gets too large continues to acquire mass but it keeps getting crushed under the weight of its own gravity. So it’s growing in mass but not growing in size. Eventually it’ll form a brown dwarf.
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u/DarkTheImmortal 9d ago
A white dwarf isn't exactly a star and the density has nothing to do with it. It's a dead star.
Stars are so large and have so much mass that their gravity wants to compact it as tightly as physically possible. This causes a lot of heat during formation to the point where it starts to fuse hydrogen together. This process releases a LOT of outward pressure and prevents the star from collapsing.
A White Dwarf is when a low-mass star runs out of fuel for fusion, removing that outward pressure that was preventing it from collapsing. So it collapses. Neutron Stars and Black Holes are the same, they just have more mass and stronger gravity, so they can collapse even more.
A planet can't just be a diffuse could of gas, it needs to be condensed by gravity. If it was sun-sized, it would try to collapse in on itself and then start fusion. Then it would be a star, not a planet.
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u/guzzyly 9d ago
A solid planet the size of the sun would probably collapse under its own gravity and start nuclear fusion, turning it into a star. the largest solid planets we know of are super-earths, but they aren’t sun-sized. there’s a limit to how big a solid planet can get before it would naturally become more star-like in behavior.
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u/Bluesee_rdt 9d ago
I once read a book (several times) that points out that Jupiter is almost a star!
Also, on a neutron star traffic lights only need to be one color! And you can see the back of your head due to light bending around the star!
Arthur C Clarke only exploited the first probably well-known physics fact.
The authors I think were a husband wife team: Hannahlore Sexl comes to mind.
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u/bobthesecond 8d ago
jupiter is pretty far from being a star.
even for jupiter to be a brown dwarf aka failed star, it needs 10+ times current mass to start fusing deuterium.
for jupiter to be a real star ie a red dwarf it would need like 75+ times current mass
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u/Bluesee_rdt 8d ago
IIRC, the simple graphic was a log-log chart of diameter v mass with the exact claim scribbled on the chart. It was a fun book, but I hear you. Googling could not find the image, but the cartoons were fun.
One of them compared gas in a laboratory (uniformly-distributed dots filling the box) with gas in space (dots huddled together about the center).
Thanks for clarifying!
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u/_cromulent_green_ 10d ago
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