How Big Is the Sun? Every Planet Fits Inside — With Room for 500 More

The one-fifth-of-a-percent bombshell

Start with the headline claim, because it sounds like exaggeration and isn't. The Sun's radius is R☉ = 695,700 km — the modern nominal value adopted by the International Astronomical Union in 2015. Earth's mean radius is 6,371 km. Divide them and you get 109.2: the Sun is about 109 times wider than Earth. But volume scales as radius cubed, so that factor of 109 becomes 109³ ≈ 1.3 million. That is how many Earth-sized balls you could, in principle, pack into the Sun's volume of roughly 1.41×10¹⁸ km³.

Now add up the volumes of all eight planets. Jupiter dominates the total — it alone is about 1,321 Earth-volumes, and Saturn adds another ~764. The four gas and ice giants plus the four small rocky worlds sum to roughly 2,200 Earth-volumes. Set that against the Sun's 1.3 million and you find the Sun's volume is about 590 times the combined volume of every planet. Put every planet inside the Sun and you have filled about 0.17% of it. There is room for roughly 500 more solar systems' worth of planets in the leftover space.

Mass tells an even more lopsided story than volume, because the Sun is denser on average than the puffy giants. The Sun weighs 1.989×10³⁰ kg; all eight planets together weigh about 2.67×10²⁷ kg. The ratio is roughly 746 to 1. Stated the way astronomers like to: the Sun is 99.86% of the solar system's mass, and Jupiter is roughly 70% of the tiny remainder. Everything you have ever seen or stood on — every planet, moon, asteroid, and comet — is rounding error on the Sun.

Why a ball of gas is only 1.4× denser than water

Here is the counterintuitive part. The Sun is stupendously massive and enormous, yet its mean density is just 1.41 g/cm³ — only about 40% denser than liquid water, and less than a third the density of rocky Earth (5.51 g/cm³). If you could scoop out an average cupful of Sun, it would weigh a little more than a cupful of water. That is because the Sun is plasma — mostly hydrogen (~73% by mass) and helium (~25%) — not rock and metal.

But mean density hides an extreme gradient. The Sun is not uniform; it is crushed into a steep pressure structure by its own gravity:

• Core: ~150 g/cm³ — over 13 times denser than lead — and about 15 million K. This is where fusion happens.
• Radiative and convective zones: density and temperature fall steadily outward across the bulk of the interior.
• Photosphere (the visible "surface"): density drops to a wispy ~2×10⁻⁷ g/cm³ — a better vacuum than most laboratories can produce on Earth — at a temperature of about 5,772 K.

So the Sun is simultaneously denser than lead at its heart and thinner than air at its face. It is held together not by a solid shell but by the balance between gravity pulling in and the pressure of hot, radiating plasma pushing out — a standoff called hydrostatic equilibrium that the Sun has maintained for about 4.6 billion years and will hold for roughly 5 billion more.

Seeing the scale: marbles, beach balls, and a two-mile walk

Raw numbers slide off the mind, so build the model. Shrink the Sun to a beach ball 1 meter across. On that scale:

• Earth is a peppercorn about 9 mm wide — you'd need to squint.
• Jupiter is a large gumball roughly 10 cm across.
• Earth would orbit that beach-ball-Sun at a distance of about 107 meters — a full football field away — and Neptune would be over 3 kilometers out.

The distances are the real shock. The Sun is 109 Earths wide, but it is about 11,700 Earths away — 149.6 million km, one Astronomical Unit. That is why the Sun and the far-larger reality of its size feel abstract: it fills only half a degree of our sky (about 0.53°), the same angular width as the full Moon. Sunlight takes 8 minutes and 20 seconds to cross that gap at 299,792 km/s, so you always see the Sun as it was over eight minutes ago.

A second model captures the volume claim viscerally. If the Sun were a hollow sphere, you could pour in Jupiter about 1,000 times over and still have room, or drop in all eight planets and fill less than a fifth of a percent. And even the Sun, for all this, is an ordinary star. Betelgeuse, a red supergiant in Orion about 550 light-years away (estimates vary), has a radius of roughly 700–900 R☉ — put it where the Sun is and its surface would engulf the orbit of Mars. The Sun is not big for a star; it is a perfectly average main-sequence dwarf that only looks colossal because it is right next door.

The engine that makes it that big: fusion versus gravity

What sets the Sun's size? A star's radius is the equilibrium point where outward pressure exactly balances the crushing weight of its own overlying layers. Deep in the core, at about 15 million K and 150 g/cm³, hydrogen nuclei fuse into helium through the proton–proton chain, converting about 4.3 million tonnes of mass into energy every second (E = mc²). That energy, streaming outward as radiation and convection, provides the pressure that props the whole star up against collapse.

The Sun fuses roughly 600 million tonnes of hydrogen into helium each second. It is astonishingly economical per unit volume, though — the core generates only about 276 W/m³, comparable to a compost heap; the Sun is luminous because it is enormous, not because any given cubic meter is intensely powerful. Its total output, the luminosity L☉ ≈ 3.828×10²⁶ W, is simply that modest volumetric rate multiplied across an unimaginable volume.

This balance also dictates the Sun's future size. When the core hydrogen runs low in about 5 billion years, fusion will migrate to a shell, the core will contract and heat, and the outer envelope will swell enormously. The Sun will become a red giant perhaps 100–200 times its current radius, likely engulfing Mercury and Venus and scorching Earth. After shedding its outer layers as a planetary nebula, the exposed core will settle into a white dwarf about the size of Earth — the same mass we discussed here, squeezed into a body one-hundredth its present diameter. The Sun's bigness is a phase, not a permanent property.

Misconceptions: 'surface,' 'yellow,' and 'the biggest thing'

The Sun's size is surrounded by tidy errors worth correcting.

It has no solid surface. The "surface" is the photosphere, the depth at which the plasma finally becomes transparent enough for light to escape — a layer only a few hundred kilometers thick against a body 1.4 million km across, like the skin on a soap bubble. Where the visible edge sits is a matter of optical depth, not a wall. Above it lie the chromosphere and the tenuous corona, which extends millions of kilometers and reaches over a million kelvin — far hotter than the surface below it, a puzzle ("coronal heating") still not fully solved.

It isn't yellow. The Sun emits across the spectrum and peaks in green-blue; integrated, its true color is essentially white. It looks yellow, orange, or red from Earth's ground because our atmosphere scatters away the shorter blue wavelengths (the same Rayleigh scattering that makes the sky blue and sunsets red). Astronauts above the atmosphere see a white Sun.

It is not the largest thing around — only the closest big thing. The Sun's radius of 695,700 km is dwarfed by giant stars (Betelgeuse ~700 R☉; UY Scuti and similar hypergiants ~1,000+ R☉). It merely dominates our sky because it is 250,000+ times closer than the next-nearest star, Proxima Centauri (about 4.24 light-years, or ~270,000 AU, away). Distance, not intrinsic size, is why nothing else in the sky comes close.

How we actually measured a ball of fire 150 million km away

Knowing the Sun's size means knowing its distance first, and that history is a triumph of patient geometry. In the 3rd century BCE, Aristarchus of Samos tried to measure the Sun–Earth distance from the geometry of the half-Moon; his method was sound but his angles too crude, and he underestimated badly — yet he correctly deduced the Sun was far larger than Earth and argued for a Sun-centered cosmos nearly 1,800 years before Copernicus.

The Astronomical Unit was finally pinned down using the transits of Venus. Edmond Halley proposed in 1716 that timing a Venus transit from widely separated points on Earth would yield the solar parallax and hence the AU; the 1761 and 1769 transits, observed by expeditions worldwide (Captain Cook's voyage to Tahiti among them), gave a distance within a few percent of the modern value. Today the AU is fixed by radar ranging to planets and spacecraft telemetry at 149,597,870.7 km — an exact defined value since 2012. Multiply the Sun's measured angular radius by that distance and you get R☉ directly.

Modern tools refine the picture inside and out. Helioseismology reads sound waves resonating through the Sun's interior — recorded by missions like SOHO (launched 1995) and NASA's SDO (2010) — to map its internal density and rotation. NASA's Parker Solar Probe, launched in 2018, has flown through the corona to within a few million kilometers of the photosphere, and the ESA/NASA Solar Orbiter (2020) images its poles. We have gone from Aristarchus's shadow geometry to spacecraft dipping into the Sun's outer atmosphere — and every measurement keeps returning the same staggering verdict: one ordinary star holds virtually all the mass and volume of the entire solar system.

The Sun versus the planets and a familiar yardstick — real values that make the scale concrete

Body	Radius (km)	Volume in Earths	Share of solar-system mass
Sun	695,700	≈1,300,000	99.86%
Jupiter	69,911	≈1,321	0.095%
Saturn	58,232	≈764	0.029%
Earth	6,371	1	0.0003%
All 8 planets combined	—	≈2,200	0.135%
Sun ÷ all 8 planets (volume)	—	≈590×	≈746× (by mass)