The concept of gas giant stellification has always interested me. I would however like to know: if a gas giant, (let’s say Neptune) was stellified by increasing its mass to trigger a temporary fusion reaction, then what spectral type would it be? It’s no good stellifying a gas giant only to end up with a flare star or brown dwarf instead of a nice, miniature yellow sun.

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    $\begingroup$ Increasing it's mass to what? $\endgroup$
    – Tardy
    Nov 19, 2022 at 17:21
  • $\begingroup$ What is the worldbuilding problem you're trying to solve? Are you trying to do this with some kind of technology, or are you just enamoured of planets becoming stars? $\endgroup$
    – elemtilas
    Nov 19, 2022 at 22:41
  • $\begingroup$ Jupiter already has intermittent fusion reactions, that's why it is so warm, its not quite a brown dwarf but the line is blurry. $\endgroup$
    – John
    Nov 21, 2022 at 0:57

2 Answers 2


/by increasing its mass to trigger a temporary fusion reaction,/

"Temporary fusion reaction" reminded me of my proposed sputtering nuclear storms on a near-stellar gas giant. That could be one scenario for your stellified giant.

I recycled my answer from here.

Giant planet renders one side of its moon deadly, the 'shadow side' habitable. How?

Thermonuclear storms


Stellification is a theoretical process by which a brown dwarf star or Jovian-class planet is turned into a star, or by which the luminosity of dim stars is greatly magnified.

For your scenario I pick the 3d of three scenarios offered: sputtering deuterium fusion.

Thermonuclear ignition. It is well established that Jovian-class planets consist mostly of hydrogen and helium.[2] It is theorised that concentrations of hydrogen and helium isotopes at certain depths of a gas-giant planet may be sufficient to support a fusion chain reaction, if sufficient energy can be delivered to ignite the reaction. If a gas giant has a layer with a large concentration of deuterium (>0.3%), ultra-high-speed (2×10⁷ m/s collision of a sufficiently large asteroid (diameter > 100 m) could ignite a thermonuclear reaction.[3]

Your giant with stellar aspirations sometimes undergoes extensive thermonuclear reactions in the deuterium level of its atmosphere. Maybe triggered by asteroids? Maybe by endogenous lightning, or events taking place farther down. They are atomic storms, propagating in spreading circles across the gas giant surface. The reaction and consequent heat expands the atmosphere and then the lights go out. But while the storm is going on, tremendous amounts of radiation, ionizing and otherwise, shine from the planet as it tries to become a star. You want to be on the shady side of the moon when that happens.

Sometimes a couple of days go by between thermonuclear storms. Maybe you can get out on the bright side and back before one comes? Shake a leg!


How can a gas giant planet become a star? By increasing its mass to a mass sufficient for a star, a mass high enough to produce the pressures and temperatures necessary for fusion at the core.

There is a class of objects which are considered to be separate from planets, and separate from stars, intermediate in mass, called brown dwarfs.

If some or all brown dwarfs were reclassified as planets, or as stars, or maybe some brown dwarfs were reclassified as planets and others as stars, there might a dividing mass between planets and stars. Then only a comparatively minor mass increase would be necessary to change a very massive planet by that definition into a very low mass star by that definition.

But at the present time the boundary between the highest mass planets and the lowest mass brown dwarfs is believed to be approximately 13 times the mass of Jupiter or about 4,131.4 the mass of Earth, and the boundary between the highest mass brown dwarfs and the lowest mass stars is believed to be approximately 75 times the mass of Jupiter, or about 23,835 the mass of Earth.

Thus the lower mass limit for a star is about 5.769 times the upper mass limit for a planet.

So if you merged two objects, one a very massive planet and the other a very massive brown dwarf together, to form an object with the mass of a very low mass star, it would be more accurate to say that the brown dwarf was transformed into a star by adding a planet, than to say that the planet was transformed into a star by adding a brown dwarf.

Thus a planet with less than 13 times the mass of Jupiter can only be transformed into a star with at least 75 times the mass of Jupiter by dropping a lot of objects which weigh much less than the planet onto the planet.

So turning a planet into to a star is a problem of collecting atoms, and molecules, and dust grains, and rocks, and asteroids, and (smaller) planets and moving them to collide with the target planet. Which is likely to be a big project, to say the least.

Finding enough matter within a star system after the stars and planets have formed to turn a planet into another star is improbable. That would involve finding at least 60 Jupiter masses in the form of planets and lesser mass objects to load onto one of the planets there to make it a star.

Thus the project probably involves moving most of the mass necessary to make a low mass star over interstellar distances.

However, there may be ways to cheat a bit which I will discuss later.

Cheat Method One:

One) Find a star system still forming from clouds of dust and gas condensing, and with a collapsing cloud of gas and dust which clearly will become a star, based on its mass, but without any planet sized solid objects formed yet.

Two) Find a star system with a large gas giant planet already formed and having the correct velocity through space.

Three) Open a space/time portal ahead of the orbiting planet so that the planet's orbit will take it into the portal.

Four) Open the other mouth of the space/time portal inside the condensing mass of gas and dust which will eventually form a star. The planet will emerge from the portal inside the gas and dust. Being by far the most massive object inside the condensation it will attract the gas and dust. And thus the planet will be the nucleus that the new star will form around, and th star will form sooner than it otherwise would have.

Thus someone could stretch the truth a bit and claim that the planet became a star, instead of a cloud of gas and dust becoming a star around the planet, which would be more accurate.

Cheat Method Two: Hot Merged Planet.

One can claim that an object which is not really a star is similar enough to a star to call a star for some purposes.

And of course the idea of moving a planet over interstellar distances via space/time portals reminds me of how the planet Jarnevon was turned into a sort of fake star in E.E. Smith's Gray Lensman:

He was wrong. Grand Fleet did not stay there long enough to suffer serious losses. For even while the cylinder was forming Kinnison was in rapid but careful consultation with Thorndyke, checking intrinsic velocities, directions, and speeds. "QX, Verne, cut!" he yelled.

Two planets, one well within each end of the combat cylinder, went inert at the word; resuming instantaneously their diametrically opposed intrinsic velocities of some thirty miles per second. And it was these two very ordinary, but utterly irresistible planets, instead of the negative-matter bombs with which the Eich were prepared to cope, which hurtled then along the axis of the immense tube of warships toward Jarnevon. Whether or not the Eich could make their planet inertialess has never been found out. Free or inert, the end would have been the same.

"Every Y14M officer of every ship of the Patrol, attention!" Haynes ordered. "Don't get all tensed up. Take it easy, there's lots of time. Any time within a second after I give the word will be p-l-e-n-t-y o-f t-i-m-e... CUT!"

The two worlds rushed together, doomed Jarnevon squarely between them. Haynes snapped out his order as the three were within two seconds of contact; and as he spoke all the pressors and all the tractors were released. The ships of the Patrol were already free--none had been inert since leaving Jalte's ex-planet--and thus could not be harmed by flying debris.

The planets touched. They coalesced, squishingly at first, the encircling warships drifting lightly away before a cosmically violent blast of superheated atmosphere. Jarnevon burst open, all the way around, and spattered; billions upon billions of tons of hot core-magma being hurled afar in gouts and streamers. The two planets, crashing through what had been a world, met, crunched, crushed together in all the unimaginable momentum of their masses and velocities. They subsided, crashingly. Not merely mountains, but entire halves of worlds disrupted and fell, in such Gargantuan paroxysms as the eye of man had never elsewhere beheld. And every motion generated heat. The kinetic energy of translation of two worlds became heat. Heat added to heat, piling up ragingly, frantically, unable to escape!

The masses, still falling upon and through and past themselves and each other melted--boiled--vaporized incandescently. The entire mass, the mass of three fused worlds, began to equilibrate; growing hotter and hotter as more and more of its terrific motion was converted into pure heat. Hotter! Hotter! HOTTER!

And as the Grand Fleet of the Galactic Patrol blasted through inter-galactic space toward the First Galaxy and home, there glowed behind it a new, small, comparatively cool, and probably short-lived companion to an old and long-established star.


A collision and merger between two large planets should make their surfaces as hot as the surfaces of most stars. Thus each square unit of the surface of the combined world would emit about as much radiation as the same square unit of the surface of most stars. Of course even the largest possible planet would have a much smaller surface area than most stars, smaller than all but the very smallest stars, so the total radiation from the new formed planet would be a tiny fraction of the total radiation of most stars and equal to or greater than the total radiation of only the least luminous stars.

So some people might say that a planet formed by the collision and merger of two or more planets, and glowing with a total luminosity equal to that of the very, very, least luminous stars might count as a sort of a star, going by its appearance and not by the method of producing the radiation.

And it might take the glowing planet many millions of years to gradually cool off and stop emitting light. So it might be another "star" in the system for millions of years, lighting and heating neighboring planets enough to affect their climate and habitability.

Cheat Method Three: Stellar Remnants.

Suppose that a large gas giant planet had a collision and merger with a stellar remnant, a white dwarf star or a neutron.

A main sequence star produces energy by fusion of lighter elements to form heavier elements (up to iron) and excess energy. To oversimplify, the fusing is mostly done by atoms of hydrogen merging to form helium. And the fusing is caused by the mass of all the matter in the star, no matter what elements are in the star and how capable of energy producing fusion they may. The gravitational attraction of the matter in the star forces extremely high pressures and temperatures in the core of the star, pressures and temperatures high enough to force the light elements in the core to fuse.

So two things are necessary for energy producing fusion: Light elements to be fused to produce energy, and high gravity of the object to force the fusion. And the two factors can be separated more than they are in main sequence stars and produce fusion.

Because white dwarfs and neutron stars no longer contain much of the lighter elements than produce energy through fusion, they don't produce new energy from fusion. And so nothing stops their gravity from compressing them more and more until finally internal pressures much greater than in main sequence stars stop their compression. Thus they have extreme densities and surface gravities.

So if light fusible elements like hydrogen fall on the surface of a white dwarf or a neutron star the intense surface gravity might compress the hydrogen so much it starts to fuse and produce energy.

And that is the process which creates novae.

Evolution of potential novae begins with two main sequence stars in a binary system. One of the two evolves into a red giant, leaving its remnant white dwarf core in orbit with the remaining star. The second star—which may be either a main sequence star or an aging giant—begins to shed its envelope onto its white dwarf companion when it overflows its Roche lobe. As a result, the white dwarf steadily captures matter from the companion's outer atmosphere in an accretion disk, and in turn, the accreted matter falls into the atmosphere. As the white dwarf consists of degenerate matter, the accreted hydrogen does not inflate, but its temperature increases. Runaway fusion occurs when the temperature of this atmospheric layer reaches ~20 million K, initiating nuclear burning, via the CNO cycle.[3]

Hydrogen fusion may occur in a stable manner on the surface of the white dwarf for a narrow range of accretion rates, giving rise to a super soft X-ray source, but for most binary system parameters, the hydrogen burning is unstable thermally and rapidly converts a large amount of the hydrogen into other, heavier chemical elements in a runaway reaction,2 liberating an enormous amount of energy. This blows the remaining gases away from the surface of the white dwarf surface and produces an extremely bright outburst of light.


So if there was a collision and merger between a gas giant planet and a white dwarf or neutron star, the matter of the gas giant planet including its hydrogen, helium, & other fusible elements, would wind up on the surface of the stellar remnant, compressed to pressure and heat great enough to start fusion.

In such a situation people might claim:

One) that a planet and a stellar remnant had merged to form a new star.


Two) that a stellar remnant had been rejuvenated and reborn as a star by the influx of new fusible matter from the planet, which seems to be the most correct interpretation.


Three) that the hydrogen and other fusible elements in the planet had started to fuse because of becoming stuck in the intense gravity of the stellar remnant, and thus that the planet had become a star. That seems like the least sensible way of looking at it, but has at least a tiny little bit of validity.

Cheat Method Four: Primordial Black Holes.

Another form of stellar remnant is a stellar mass black hole, formed from a massive enough collapsed star.

Obviously if a gas giant planet merged with a stellar mass black hole the hydrogen from the planet would be compressed to super density & heat on the surface of the black hole and start to fuse.

But of course a black hole doesn't have a solid surface, but an event horizon that would swallow up matter that touched the event horizon. Thus depending on various factors the hydrogen from the gas giant planet might fuse and glow above the event horizon for a longer or short period of time before falling into the event horizon.

And of course the gravity of the black hole accelerates nearby matter as it pulls it in, superheating the matter and making it emit hard radiation.

And if that sort of "star" created that way glowed for the amount of time necessary for a story (an amount of time which can vary greatly depending on the story), it would always be true that the black hole had many times the mass of the planet, and so it would always be somewhat more accurate to say that the black hole had become a star again due to the merger with the planet instead of saying that the planet had become a star.

But if the black hole was not a stellar remnant and had less mass than the planet, and thus less than about 13 times the mass of Jupiter or about 4,131.4 the mass of earth, there would be more justification to say that the planet had become a star instead of the black hole becoming a star.

Primordial black holes might possibly have formed during the Big Bang. And they might possibly have formed with very small masses.

A stellar black hole of 1 M☉ has a Hawking temperature of 62 nanokelvins.[141] This is far less than the 2.7 K temperature of the cosmic microwave background radiation. Stellar-mass or larger black holes receive more mass from the cosmic microwave background than they emit through Hawking radiation and thus will grow instead of shrinking.[142] To have a Hawking temperature larger than 2.7 K (and be able to evaporate), a black hole would need a mass less than the Moon. Such a black hole would have a diameter of less than a tenth of a millimeter.[143]

The mass of the Moon is 1.3472 times 10n to the 22th power kilograms, or 0.0123 the mass of Earth. So the most massive planet, with about 13 times the mass of Jupiter or about 4,131.4 times the mass of Earth, would have about 335,886.17 times the mass of least massive possible primordial black hole.

That gives a large range of possible masses, and thus escape horizon radii, for a primordial black hole that mergers with a gas giant planet. And thus the possible effects of the black hole on the planet may vary greatly with the mass of the primordial black hole.

And possibly someone at this site might be able to do the calculations necessary to find the hypothetical mass of a primordial black hole which would be right (if there is a right mass) to turn a gas giant planet into sort of a star like object.

  • $\begingroup$ 30 miles/sec should leave something hotter than the surface of a typical star. Jarvenon's closest relatives will be white dwarfs, but Jarvenon will have nowhere near the gravity nor heat capacity, it will cool much faster. $\endgroup$ Nov 21, 2022 at 5:38
  • $\begingroup$ @Loren Pechtel Great comment. Do yuo have any idea of how long a hypothetical Jarnevon turned glowing orb might possibly have a fairly steady luminosity to heat some nearby worlds? If it took thusands of years, for example, to terraform a nearby world to be habitable, would the heat from the former Jarnevon last long enough to make the terraformngworthwhile? $\endgroup$ Nov 21, 2022 at 19:24
  • $\begingroup$ Any world that would be close enough to be warmed would have to be in orbit about it and I think any such world would be scoured clean by the smack Jarvenon took. 30 mi/sec is above the escape velocity of any reasonable mass for Jarvenon (if it had that kind of mass it would be a neptune, not a rocky world) and thus a lot of debris went flying. $\endgroup$ Nov 22, 2022 at 2:29

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