Jupiter is a gas giant containing more than two and a half times the mass of all other planets combined. This immense world acts as a gravitational anchor, dictating the dynamics of countless objects from the asteroid belt outward. The question of what would happen if this giant suddenly possessed twice its current mass is a thought experiment. A doubling of Jupiter’s mass would trigger a cascade of physical changes, fundamentally altering its size, internal structure, and influence over the solar system.
The Mass-Radius Paradox
The most counter-intuitive change in a doubled-mass Jupiter would be its external size. For gas giants, adding more mass does not cause the planet to expand; instead, it leads to greater gravitational compression. This phenomenon, called the mass-radius paradox, means a planet with two Jupiter masses would not be twice as large in volume.
The current radius is maintained by a balance between the inward pull of gravity and the outward pressure from its internal material. As mass is added, the increased gravity crushes the interior, raising the density but preventing the radius from growing. Theoretical models suggest that increasing the mass causes the radius to slightly decrease.
A two-Jupiter-mass planet would be a much denser object, occupying a volume only marginally smaller than the current Jupiter. The material deep within would be subject to significantly higher pressure, increasing the planet’s average density from 1.33 grams per cubic centimeter to approximately 2.7 grams per cubic centimeter. The physical support against gravitational collapse would transition toward electron degeneracy pressure, where electrons resist being squeezed into the same quantum state.
Internal Transformation and Core Conditions
The doubled mass would induce a significant increase in pressure and temperature deep within the planet’s interior. This immense pressure would compress the hydrogen and helium gas into a state far denser and hotter than its present condition. The central pressure, already estimated to be in the tens of millions of bars, would rise even further.
The layer of metallic hydrogen forms when hydrogen atoms are squeezed so tightly that their electrons become delocalized. This electrically conductive fluid would expand and intensify under the greater mass. Since metallic hydrogen is responsible for Jupiter’s powerful magnetic field, its increased compression would likely generate a much stronger magnetosphere.
The planet’s core, currently believed to be a dense, rocky, and icy mass several times the size of Earth, would be subjected to crushing forces. The increased gravitational energy from the collapse and compression would also result in a significant increase in the planet’s internal heat generation.
This heightened thermal energy would be radiated outward, making the doubled-mass Jupiter glow brighter in the infrared spectrum. This additional heat would drive more vigorous atmospheric dynamics, leading to more energetic storms and wind patterns.
Approaching Stellar Ignition
Doubling Jupiter’s mass would move it closer to the boundary between giant planets and brown dwarfs, a category sometimes called “failed stars.” A brown dwarf is a substellar object that lacks the mass to sustain the hydrogen fusion that powers true stars. However, it is massive enough to ignite a brief period of deuterium fusion, which requires lower temperature and pressure than regular hydrogen.
To cross the threshold into brown dwarf status, Jupiter would need approximately 13 times its current mass to initiate deuterium burning. A doubled-mass Jupiter is still far short of this mark. The fusion of deuterium, while not a sustained stellar process, would mark the point at which the object begins to generate its own energy through nuclear reactions.
For Jupiter to become a true, sustained star—specifically, the smallest type of red dwarf—it would require roughly 80 times its present mass. This mass is necessary to achieve the core temperature and pressure needed to begin stable, long-term hydrogen fusion. Therefore, even at twice its current mass, the planet would remain a non-fusing gas giant.
Gravitational Influence on the Solar System
The most immediate consequence of a doubled-mass Jupiter would be the alteration of the solar system’s gravitational dynamics. The planet’s gravitational influence scales directly with its mass, meaning the force exerted on every other body would double. This change would immediately shift the Solar System’s barycenter, the center of mass around which the Sun and all planets orbit, moving it further away from the Sun’s center.
The Galilean moons—Io, Europa, Ganymede, and Callisto—would experience a significant increase in tidal forces. The energy deposited by these stronger forces would intensify volcanic activity on Io and increase tidal heating within the subsurface oceans of Europa and Ganymede. This would accelerate geological activity and potentially enhance the liquid state of these oceans.
The increased gravitational pull would also exert a stronger perturbing effect on the orbits of the asteroid belt and the comets in the Kuiper Belt. While the orbits of the inner terrestrial planets would be affected, the most significant long-term perturbations would be seen in the objects closest to Jupiter. These objects could experience significant orbital migration or ejection from the solar system over extended periods.