Quick Answer
Jupiter cannot become a Sun-like star simply by heating up. Sustained hydrogen fusion requires much more mass; objects below roughly 13 Jupiter masses cannot even sustain deuterium fusion, while hydrogen-burning stars require around 75–80 Jupiter masses depending on composition. If Jupiter were somehow replaced by a low-mass star, its gravity and radiation would change the outer solar system and could alter planetary climates, but the exact outcome would depend on the new star's mass and orbit.
Jupiter contains more mass than all the other planets combined, yet it is still nowhere near the mass needed for ordinary hydrogen fusion. Calling it a 'failed star' is useful as a comparison, but it should not be taken to mean that Jupiter is almost capable of igniting like the Sun.
How much mass is needed?
Brown dwarfs occupy the territory between planets and stars. Around 13 Jupiter masses is a commonly used threshold for deuterium burning, while sustained hydrogen fusion begins around 0.075 to 0.08 solar masses, roughly 75–80 Jupiter masses for typical compositions. The exact boundary depends on composition and stellar models.
Suppose a star appeared at Jupiter's orbit
For a useful thought experiment, imagine Jupiter were magically replaced by a small red dwarf star without instantly disturbing Earth's orbit. Earth would then receive additional light and infrared radiation from the outer solar system. The amount would depend enormously on the star's luminosity and distance, so there is no single temperature increase that applies to every version of the scenario.
Would Earth have two suns?
From Earth, the second star could look like a very bright point rather than a second Sun-sized disk. Its apparent size would be small because of the enormous distance. What matters for climate is total received energy, not how large the star looks in the sky.
Established scienceJupiter is about five times farther from the Sun than Earth is. Even a stellar object at Jupiter's current distance would illuminate Earth much less strongly than the Sun unless it were sufficiently luminous.
The moons would be transformed
Jupiter's moons would experience a radically different environment. Some could receive substantial additional radiation and light, changing their surface temperatures and chemistry. A star replacing Jupiter would also have a very different mass distribution and radiation output, altering the long-term dynamics of the Jovian system.
The solar system's architecture
A second star would make the solar system a multiple-star system. Planetary orbits could remain stable in some configurations, but long-term orbital resonances and perturbations would change. The outer planets and small-body populations would be especially sensitive to the added gravitational influence.
The bottom line
Jupiter cannot simply 'turn on' and become the Sun. But if a stellar-mass object somehow occupied Jupiter's orbit, the solar system would become a different gravitational and radiation environment. The most important variables would be the new object's mass, luminosity and orbital configuration.
A note on our approach: Every article on WhatIfLab separates what current science establishes from what remains genuinely speculative. Where we cite a figure or finding, it reflects published, mainstream research at the time of writing — not a prediction dressed up as fact.