The Solar System

In a dark room, adults and children lean over a brass machine made of rings and small balls, their faces lit by a light at the centre. — A public lecture in front of a model of the Solar System: a lamp, in place of the Sun, lights up the faces of the onlookers (painting by Joseph Wright of Derby, c. 1766). Joseph Wright of Derby, Public domain, source

On 24 August 2006, Pluto ceased to be a planet. It had not moved.¹

What it is made of

At the centre, a star: the Sun. Around it revolve eight planets, in order of distance from the Sun: Mercury, Venus, Earth, Mars, then, much further out, Jupiter, Saturn, Uranus and Neptune. The first four are small and rocky; the other four are immense giants of gas and ice. Between Mars and Jupiter lies the asteroid belt; beyond Neptune lies another, far vaster belt of icy bodies. Add moons, comets, dust — and a word, “planet”, which has a history of its own.²

On 24 August 2006, Pluto ceased to be a planet.

It did not move. It did not shrink. It did nothing at all.

That day, in a hall in Prague, a sentence was written.

Why that sentence had to be written

Astronomers began to discover icy bodies beyond Neptune: a few at first, then a great many. This region is called the Kuiper belt.

Then one of them turned out to be able to rival Pluto. For a time there was talk of a tenth planet.

The problem then became obvious: if Pluto was a planet, so was that one. And others were waiting behind it.

There were only two ways out, not three: either the Solar System would see a queue of new planets arrive, or the word would be redefined.

What was decided

The International Astronomical Union adopted a definition with three conditions. A planet is a celestial body that:

a. is in orbit around the Sun;
b. has enough mass for its own gravity to give it a nearly round shape;
c. has cleared the neighbourhood around its orbit.³

A body that meets the first two conditions but not the third, and is not a satellite, becomes a dwarf planet. This is the case with Pluto: it shares its neighbourhood with a host of other icy bodies.⁴

The result: the Solar System now has eight planets — Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus and Neptune.⁵

How it was decided

This is the part of the story that is almost never told, and yet it is the most instructive.

The vote took place on the last day, at the closing ceremony. Yellow ballots were handed out to members of the Union at the entrance; to vote, members raised their ballot in the air, and the raised ballots were counted.⁶

The assembly, in Prague, had brought together more than two thousand five hundred astronomers. On the resolution concerning Pluto, one of the few whose count was published, there were 237 votes in favour, 157 against and 17 abstentions — about four hundred voters.⁷

A petition launched immediately afterwards by planetary scientists and astronomers opposed to the text does the arithmetic: about 428 members took part in this vote, out of the Union’s nearly ten thousand.⁸

It criticises something else as well: neither the initial definition nor the one that was adopted had been submitted for review by the specialist community as a whole before the meeting.⁹

The gap in the text

The first condition deserves a second reading: “is in orbit around the Sun”.

Since the 1990s, thousands of planets have been discovered around other stars. These are exoplanets, and new ones are discovered every month.

The 2006 text is titled “Definition of a Planet *in the Solar System*”. Taken literally, it says nothing about the others: it does not cover them.¹⁰

This is not a rhetorical trap: it is a limit that the adopted sentence set for itself. A proposal to extend it to exoplanets was made in 2015; the 2006 text itself has not been reopened.¹¹

Twenty years on, the disagreement is not settled.

What this story teaches

This mechanism appears elsewhere on this site, in two other forms.

A box invented to sell. On 29 June 1987, in London, about twenty record-industry professionals created the “World Music” category because shops did not know which section to put certain records in. The minutes of their meeting have been preserved: they were looking for a name under which to group their catalogue, and they voted by a show of hands. Among the other suggestions: “World Beat” and “Tropical”.
A definition deliberately left unwritten. The United Nations refuses to define the word “indigenous”, so that each people decides for itself who it is.
And here, a definition written by a vote. A few hundred yellow ballots raised in the air, one day in August, at the end of a congress.¹²

Three very different situations, one and the same question: who makes categories, and how?

It is almost never nature that makes them, but people, at a given moment, to solve a problem they faced. Knowing which problem helps one understand the category much better.

The asteroid belt is not what people think

In films, a spaceship crosses the asteroid belt by weaving between rocks that graze its hull.

One fact settles the question: to get near an asteroid, you have to aim for it.

There are more than a hundred thousand asteroids larger than one kilometre, but they are scattered through an immense volume: their average spacing is measured in millions of kilometres. The probes that have crossed the belt — the Pioneers, the Voyagers, Galileo, Cassini, New Horizons — all did so unscathed. They pass through, says an astronomy magazine, as though it were empty space.¹³

More striking still: in the early 1990s, NASA wanted a probe to encounter an asteroid while crossing the belt, on its way to Jupiter. It took a search to find one roughly along its path, and then it had to be specially targeted.¹⁴

Collisions between asteroids themselves are just as rare: a one-kilometre asteroid suffers one every few billion years.¹⁵

A figure that depends on the question

The average spacings quoted vary a great deal: less than a million kilometres here, several million elsewhere.

None is wrong. The figure depends on what one decides to count. If only the large ones are included, they are very far apart. If the pebbles are counted too, they are closer together.

There is no single average spacing: there is a spacing for each threshold. Faced with a figure of this kind, the real question is not “is it right?” but “what does it count?”

Why every drawing of the Solar System lies

This question is more important than it seems, and it explains where the previous error comes from.

An illustrator faces an insoluble problem.

If the Solar System is drawn to scale, the planets become invisible dots — and the asteroids disappear completely. If they are enlarged enough to be seen, the result is a tight ring of gravel.

The dots have to be visible. This visibility creates the false impression that they are close to one another.

The error is not in the drawing. It is in the need to draw.

Interactive

Drawing the Solar System

An illustrator faces an insoluble problem: the Solar System does not fit on a page. Try it yourself.

The question

Can you draw the Solar System to scale, and still see anything?

Without JavaScript, here are the animation’s data.

The Sun and the eight planets
Body	Diameter (km)	Mean distance from the Sun (million km)	Source
Sun	1,391,400	—	1
Mercury	4,879	57.9	2
Venus	12,104	108.2	2
Earth	12,756	149.6	2
Mars	6,792	228.0	2
Jupiter	142,984	778.5	2
Saturn	120,536	1,432.0	2
Uranus	51,118	2,867.0	2
Neptune	49,528	4,515.0	2

At that scale, if Neptune were 1 metre from the Sun, the Earth would measure 0.003 mm, Jupiter 0.03 mm and the Sun 0.3 mm: less than a grain of sand.

These scaled sizes are calculated from the table. The order of the planets is the one given in the article.

The takeaway To scale, the planets are invisible. To see them, you have to lie about the sizes, or about the distances. Every picture of the Solar System lies about at least one of the two.

Photo credits 9

The Sun in visible light, photographed by NASA’s SDO satellite on 10 August 2024. NASA/SDO, AIA, EVE and HMI science teams, Public domain, source
Mercury, a mosaic of images from NASA’s MESSENGER probe (2011). NASA/JHUAPL, Public domain, source
Venus, wrapped in clouds, photographed by NASA’s Mariner 10 probe in February 1974. NASA/JPL-Caltech, Public domain, source
The Earth photographed on 7 December 1972 by the Apollo 17 crew on their way to the Moon: Africa, Antarctica and the Arabian Peninsula are visible. NASA, Apollo 17 crew, Public domain, source
Mars, a mosaic of 102 images from the Viking 1 orbiter (1980); in the centre, the Valles Marineris canyon. NASA / USGS (PIA04304), Public domain, source
Jupiter photographed in full by the Hubble Space Telescope on 21 April 2014. NASA, ESA and A. Simon (Goddard), Public domain, source
Saturn in natural colour, from images taken by the Cassini probe (2008). NASA / JPL / Space Science Institute, Public domain, source
Uranus photographed by the Voyager 2 probe in 1986. NASA/JPL-Caltech, Public domain, source
Neptune, from the last whole-planet images taken by Voyager 2 through green and orange filters (1989); the Great Dark Spot is visible. NASA, Public domain, source

Sources 3

Williams, David R., “Sun Fact Sheet”, NASA Goddard Space Flight Center (NSSDCA), updated 9 May 2024, mean radius (the diameter in the table is twice this).
Volumetric mean radius (km) 695,700.
Open access open the source · checked on 27 October 2026
Williams, David R., “Planetary Fact Sheet – Metric”, NASA Goddard Space Flight Center (NSSDCA), updated 18 March 2025, rows “Diameter (km)” and “Distance from Sun (10^6 km)”, columns Mercury to Neptune.
Diameter (km) 4879 12,104 12,756 6792 142,984 120,536 51,118 49,528 […] Distance from Sun (10^6 km) 57.9 108.2 149.6 228.0 778.5 1432.0 2867.0 4515.0
Open access open the source · checked on 27 October 2026
Union astronomique internationale, “IAU 2006 General Assembly: Result of the IAU Resolution votes”, communiqué iau0603 du 24 August 2006, results-of-votes press release.
the Solar System consists of eight "planets" Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus and Neptune
Open access open the source · checked on 9 September 2026

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The question to ask of an image

From this follows a useful habit, one that serves in all circumstances.

Faced with a figure, the question is not: is it right?

But rather: what did it have to lie about?

Every figure of the Solar System lies about at least one thing — the sizes, or the distances, or both. This is not dishonesty: it is the price of putting on a page what does not fit on a page.

This exact problem recurs elsewhere on this site. In the entry on the classification of living things, a famous illustration became the most misleading representation of evolution — simply because it had been folded to fit in a book.

Note 1

24 August 2006, Prague […] Pluto is a "dwarf planet" by the above definition and is recognized as the prototype of a new category of trans-Neptunian objects.

Union astronomique internationale, “IAU 2006 General Assembly: Result of the IAU Resolution votes”, communiqué iau0603 du 24 August 2006