Comparative sizes of planets and stars

Planets and stars: dimensions that defy imagination — Comparative sizes of the 4 terrestrial planets, Mercury, Venus, Earth and Mars. These 4 terrestrial planets represent ≈ 11% of the total mass of the planets in the solar system.
Image source: astronoo.com (new window)

What are the differences in scale between the planets of the solar system and the stars of the Universe?

Sizes in the Universe vary over staggering orders of magnitude. In the solar system, Jupiter is the largest planet with a diameter about 11 times that of Earth. The Sun, with a diameter 109 times that of Earth, could contain 1,305,620 Earths within its volume. But compared to giant stars, our Sun is a dwarf: Betelgeuse (red giant) has a diameter about 1300 times that of the Sun, or 141,863 times that of Earth. The Stefan-Boltzmann law (L = 4πσR²T⁴) allows calculating the radius of stars from their luminosity and temperature.

Comparative sizes of terrestrial planets

Our Universe is truly vast and empty, however a few grains of matter dot this cosmic void, from small dust grains to the largest stars. Between the small planets of the solar system and the largest stars, the size difference is enormous; for example, the diameter of the star Betelgeuse is 141,863 times larger than the diameter of the Earth. This page shows, in images, some size comparisons between planets and between stars. In the solar system, the Sun captured 99.86% of the total mass of dust and gas from the original nebula.

Jupiter, the largest planet in the system, captured 71% of the remainder. The other planets shared the residue of this gravitational evolution, that is, 0.038% of the total mass. The 4 terrestrial planets represent only ≈ 11% of the total mass of the planets in the solar system.

Comparative sizes of some objects compared to Earth

Size comparison between dwarf planets and Earth — Sizes of trans-Neptunian objects, the asteroid Ceres, and the Moon compared to Earth.
Image source: astronoo.com (new window)

A dwarf planet, since the new definition of August 2006, is a celestial body orbiting the Sun that has sufficient mass for its gravity to overcome the cohesive forces of the solid body and maintain it in hydrostatic equilibrium (in a nearly spherical shape), and that is not a satellite, but that has not cleared its orbital neighborhood.

Comparison of the diameters of the planets of the solar system

This graph illustrates the relative diameters of the eight planets of the solar system, using Earth as a reference (diameter = 1).

Diameter of stars compared to the Sun

This graph illustrates the relative diameters of the stars Sun, Sirius, Pollux, Arcturus, Aldebaran, Rigel, Antares and Betelgeuse, using the Sun as a reference (diameter = 1). It is thanks to the Stefan‑Boltzmann law that astronomers can easily calculate the radii of stars. Betelgeuse has a diameter ≈ 1300 times that of the Sun.

N.B.:
In 1879, the Austrian physicist Josef Stefan (1835-1893) discovered that the total energy emitted by an object is proportional to the 4th power of its absolute temperature. Thanks to the Stefan-Boltzmann law, astronomers can calculate the radii of stars. The luminosity of a star is written: \( L = 4\pi \sigma R^2 T^4 \). (L = luminosity, σ = Stefan-Boltzmann constant, R = radius of the star, and T = temperature).

Relative sizes of planets and stars

Our Sun is truly very small compared to some stars, and our planets are only dust compared to the Blue and Red Giants of our Universe.

Size of the Earth compared to the Sun

The Earth is quite small compared to the Sun. We could fit 1,305,620 Earths within the Sun's volume. Its mean diameter is ≈ 12,742 km and that of the Sun ≈ 1,392,684 km (≈109 times larger). The image shows this Earth/Sun size ratio, if the Earth were on the same plane, very close to the Sun.

Size of Pluto and Charon with Earth and the Moon

The new measurements and images of Pluto and Charon obtained by the New Horizons probe (July 2015) allow us to make this comparative montage.

Size comparison between Earth and Pluto and Charon

FAQ: Everything you need to know about the comparative sizes of planets and stars

What is the largest planet in the solar system and how does it compare to others?

Jupiter is the largest planet in the solar system. Its diameter is about 11 times that of Earth and its mass represents 71% of the total mass of the planets (excluding the Sun). Saturn comes next with a diameter about 9.5 times that of Earth, then Uranus (~4 times) and Neptune (~3.9 times). The terrestrial planets (Earth, Venus, Mars, Mercury) are much smaller: Venus has a diameter similar to Earth (about 95%), Mars is about half the size of Earth, and Mercury is the smallest of the planets (about 38% of Earth's diameter).

How do astronomers measure the size of stars?

The size of stars is generally not measured directly (except for the closest ones by interferometry). It is calculated using the Stefan-Boltzmann law: L = 4πσR²T⁴, where L is the luminosity of the star, σ the Stefan-Boltzmann constant, R the radius of the star, and T its surface temperature. By measuring the luminosity (amount of energy emitted) and the surface temperature (via its spectrum), one can deduce the radius of the star. This law shows that luminosity is proportional to the surface area (R²) and to the 4th power of the temperature.

Why can stars be so enormous compared to planets?

Stars are enormous balls of gas held together by the balance between the pressure of nuclear fusion (outward) and gravity (inward). Their size depends on their mass, their evolutionary stage, and radiation pressure. Red giant stars (like Betelgeuse) are at an advanced stage of their life where they have expanded tremendously after exhausting the hydrogen in their core. Brown dwarfs and gas giant planets (Jupiter) occupy an intermediate zone, but the boundary between planet and star is defined by nuclear fusion: a star is an object that fuses hydrogen into helium (or has done so). Below about 13 Jupiter masses, fusion is not possible.