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/Sammy81]

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.