Space · Foundations
A planet roughly 21 Earth diameters across and a star with a radius of approximately 2,000 solar radii: cosmic size records exceed our everyday experience. But “larger” can refer to radius, volume, or mass—and rankings are by no means definitive.
Three Different Measures of Size
In everyday life, we often conflate diameter, volume, and weight. For celestial bodies, these quantities should be kept separate. If a sphere's radius doubles, its diameter also doubles, but its volume increases by a factor of eight. Its mass also depends on its density. A highly inflated gas planet can therefore appear very large without being particularly massive. Earth has a mean radius of approximately 6,371 kilometers. The Sun has a radius of about 109 Earth radii, and thus a diameter of about 109 Earth diameters as well. Even our own star is therefore far larger than our home planet. A rounded figure is usually sufficient for size comparisons; many seemingly exact rankings suggest a level of precision that measurements of distant objects cannot support. In such comparisons, “largest sun” usually means the largest known star by radius. However, Sun is the proper name of our own star. The star with the largest radius is not necessarily the hottest, most luminous, or most massive.
WASP-17 b: Approximately 21 Earth Diameters
WASP-17 b is a particularly illustrative example of an extremely large known exoplanet. The NASA catalog lists its radius as approximately 1.87 Jupiter radii. Taking one Jupiter radius as roughly eleven Earth radii gives around 21 Earth radii—and the same ratio for the diameter. Its diameter is therefore on the order of 260,000 kilometers, compared with approximately 12,742 kilometers for Earth. Assuming an idealized sphere, its volume would be approximately 9,000 times Earth's, because the linear factor is multiplied by itself three times. This rounded calculation of our own describes spatial volume, not mass, habitable surface area, or room for thousands of Earth-like landscapes. WASP-17 b is a gas giant, not an enlarged solid Earth. Inflated gas giants show how strongly irradiation and planetary structure can influence radius. Measurement methods also favor the detection of large planets: a large planet passing in front of its star causes a more easily detected dip in brightness. A large discovered planet is therefore not automatically the largest planet in existence.
Sources: [3]
Why the Absolute Planetary Size Record Is Harder to Establish
An honest answer to the search for the largest known planet begins with a qualification: there is no permanently undisputed ranking. Published radii can change as new measurements of the host star become available. Young objects can be particularly large and still developing. Some candidates also lie near the boundary between planets and brown dwarfs. WASP-17 b is therefore used here as a well-documented example of a very large exoplanet, not as a certain current record holder. The question of the greatest mass would need to be addressed separately. An enormously massive object can have a comparatively modest increase in radius, because matter under intense pressure behaves differently from a simple sphere of uniform-density material. For a comparison with Earth, radius is intuitive and straightforward to understand. A ranking, by contrast, would require a consistent definition, a common dataset, and the relevant measurement uncertainties. The more cautious statement is not less fascinating; it is more informative.
WOH G64: A Stellar Radius Reaching Roughly to Saturn's Orbit
In 2024, ESO described WOH G64 as a red supergiant approximately 2,000 times the size of the Sun. As a rounded radius comparison, that corresponds to about 218,000 Earth radii; its diameter would likewise be about 218,000 times Earth's diameter. Its radius would therefore be roughly 1.4 billion kilometers, or just over nine astronomical units. This size comparison for the star must be distinguished from the surrounding dust cocoon imaged by the ESO observations. If we imagined placing a star with this radius where the Sun is, it would extend roughly to the vicinity of Saturn's orbit. This is a geometric comparison, not a statement about a planetary system that actually exists there. Nor should its enormous extent be mistaken for a mass of 2,000 Suns. Even the “surface” of a red supergiant is not as clearly defined as that of a solid ball. Temperature, distance, variability, atmospheric layers, and dust influence the inferred radius. The ESO observation specifically reveals a complex environment and changes in the star. Frequently mentioned candidates such as Stephenson 2-18 should therefore not be declared undisputed winners without scrutiny either. WOH G64 illustrates an order of magnitude; the radius quoted here is an approximate figure associated with a particular date, not a guaranteed, unchanging record.
Sources & further reading
- [1] NASA: Earth Factsscience.nasa.gov
- [2] NASA: Sun Factsscience.nasa.gov
- [3] NASA Exoplanet Catalog: WASP-17 bscience.nasa.gov
- [4] NASA: Exoplanet Factsscience.nasa.gov
- [5] ESO: First Close-up Picture of a Star Outside Our Galaxy (2024)eso.org
Written in our own words with AI assistance, based on the sources listed above. Numbers are rounded; estimates, model assumptions, and open research questions are explicitly put into context. A foundational article, not a daily news report.