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Jupiter remains the largest planet in our Solar System, boasting an equatorial diameter of 88,846 miles and a mass of 1.898 × 10²⁴ kg. For comparison, Earth spans roughly 7,926 miles across its equator and has a mass of 5.97 × 10²⁴ kg—making Jupiter nearly 318 times heavier than our home planet. Astronomers discovered the first exoplanets in 1992, and many now dwarf Jupiter, raising the question: Is there a cosmic ceiling to planetary size?
Indeed there is. When an object surpasses about 13 Jupiter masses, it no longer fits the strict definition of a planet. Instead, it becomes a sub‑stellar brown dwarf—a “failed star” that never ignites sustained hydrogen fusion in its core. If the mass climbs beyond roughly 80 Jupiter masses, the object can sustain fusion and qualifies as a true star. These thresholds explain why the Sun and Moon cannot be classified as planets.
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Planetary size can be measured in different ways, but the largest diameter yet recorded is that of HAT‑P‑67 b. This exoplanet, found in 2017 at a distance of about 1,212 light‑years, has a diameter roughly twice that of Jupiter. Its mass, however, is only about one‑third of Jupiter’s. Similar size giants include WASP‑17 b (1,324 light‑years away) and KELT‑9b (667 light‑years away).
When we look at mass, HD 39091 b stands out as the most massive confirmed planet. Discovered in 2001 at 60 light‑years from Earth, it carries 12.3 Jupiter masses while maintaining a diameter comparable to Jupiter’s. Other hefty planets include HD 106906 b (11 Jupiter masses, 336 light‑years away) and ROXs 42B b (9 Jupiter masses, 468 light‑years away).
Brown dwarfs, the natural extension of massive planets, also challenge our perception of size. The most massive brown dwarf identified to date is SDSS J0104+1535. First spotted in 1992 at about 750 light‑years, new analyses from 2017 reclassified it as a brown dwarf with a mass 90 times that of Jupiter. Its composition is almost entirely hydrogen and helium—about 250 times purer than the Sun.
