A tour of the solar system
Most of this blog is about taking binaries apart: no source code, no documentation, only the output, and the job is to work out how the machine behind it is built. Astronomy is the same job on a larger scale. Nobody has ever stood outside the Solar System and looked at it. We reconstruct it from light, from timing, and from the way one object tugs on another. This post is a guided tour of the result: what is out there, in order, and how big the gaps really are.
The Sun, where nearly everything is
The Sun holds 99.86 percent of the mass of the whole system. It is about 1.39 million km across, 109 Earths side by side, and roughly 1.3 million Earths would fit inside it. In its core hydrogen fuses into helium, turning about 4 million tonnes of matter into energy every second. It is 4.6 billion years old, about halfway through its stable life, and its light needs 499 seconds, a little over 8 minutes, to reach us.
Here are the eight planets in order, with their sizes to scale:
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1 2 3 4 5 6 7 8
1 Mercury 2 Venus 3 Earth 4 Mars 5 Jupiter 6 Saturn 7 Uranus
8 Neptune. Scale: 2 columns = 1 Earth diameter (12,756 km).
On this scale the Sun would be 218 columns wide.The rocky planets
- Mercury is the smallest (4,879 km) and closest (0.39 AU) planet, with an 88-day year. It turns so slowly that one sunrise to the next takes 176 Earth days, two Mercury years, and with almost no atmosphere its surface swings from about -180 to +430 C.
- Venus is almost Earth's twin in size (12,104 km), but its carbon dioxide atmosphere presses down with about 92 times our surface pressure and keeps the ground near 465 C, hotter than Mercury. It spins backwards, so slowly that its day (243 Earth days) outlasts its year (225).
- Earth is the yardstick: one astronomical unit (AU) is defined as roughly its average distance from the Sun, 149.6 million km.
- Mars is about half Earth's diameter (6,779 km), with a thin carbon dioxide atmosphere under 1 percent of Earth's pressure. Its volcano Olympus Mons rises about 22 km, roughly two and a half times the height of Everest. A Martian day is 24 h 37 min.
Their orbits, drawn to scale together with the asteroid belt:
. . .. .... .
. asteroid belt, 2.2 - 3.2 AU .
.... ::: ... . ::.:. ..:..::....::.
.. ....: ..:.: .:. . .::.....: ... ..
.:.. : :::. ..... . ......: :. . ..:..:.. ..:..
. : .:.:: ..:. : ::.. . .:........ .:... . .
: : ..:... ::. .. ::.. .
:.. :.:.: :: .: .::.... .:
: : .:.: .. ::. .: .
. ....:.:.:.: --------------- :: : . . . ..:
.. . .:: :. ///4- -\\\\ . ..:..... ..
: .. .. . /// - \\\ .. .:.. . .
. . :::. . // //---------\\ \\ .. :. . :
..:.. ... // /// /-------\ \\\ \\ : :... :...
:.:.. :. . || /// /2/ \\\ \3\ || .. ::.:. :
:.:.. | || // //--1\\ \\ || | :.:. . . .:
. . : || | || |/ \| || | || : : . . ....
|--1 AU---| | | | | (*) | | | |
1 Mercury 0.39 AU 2 Venus 0.72 3 Earth 1.00 4 Mars 1.52
Top view of the inner Solar System, upper half only, radii to scale.
Jupiter (5.2 AU) would sit 57 columns from the Sun: off this page.The asteroid belt
Between Mars and Jupiter lies the asteroid belt, from about 2.2 to 3.2 AU. Films show a field of tumbling boulders, but the real belt is mostly empty space: all of it together has only about 3 percent of the Moon's mass, and typical asteroids are hundreds of thousands of kilometres apart. The largest member, Ceres, is 940 km across and holds roughly a third of the belt's mass; NASA's Dawn probe orbited it from 2015 to 2018. The belt is leftover building material that Jupiter's gravity never let assemble into a planet.
Gas and ice giants
Jupiter is 11 Earth diameters wide and has 318 Earth masses, two and a half times all the other planets combined, yet only about a thousandth of the Sun. Its Great Red Spot is a storm wider than Earth that astronomers have watched for well over a century. Saturn is a bit smaller and so light for its size that its average density (0.69 g/cm3) is below that of water. Both are mostly hydrogen and helium.
Uranus and Neptune are the ice giants. "Ice" here is planetary-science slang for water, ammonia and methane under huge pressure, not something you could skate on. Uranus rotates tipped on its side, its axis inclined 98 degrees. Neptune has a good origin story: in 1846 Urbain Le Verrier calculated from irregularities in the orbit of Uranus where an unseen planet had to be, and Johann Galle found it on the first night he looked, within a degree of the prediction. A planet found with a pencil before a telescope is reverse engineering at its best. Jupiter and Saturn each have dozens of known moons, and the counts keep rising, so I will not quote totals.
The table
| Body | Diameter (km) | Mean distance (AU) | Year | Day (one rotation) |
|---|---|---|---|---|
| Sun | 1,391,400 | 0 | - | about 25 days (equator) |
| Mercury | 4,879 | 0.39 | 88 days | 58.6 days |
| Venus | 12,104 | 0.72 | 225 days | 243 days, backwards |
| Earth | 12,756 | 1.00 | 365.25 days | 23 h 56 min |
| Mars | 6,779 | 1.52 | 687 days | 24 h 37 min |
| Ceres | 940 | 2.77 | 4.6 years | 9.1 h |
| Jupiter | 142,984 | 5.20 | 11.9 years | 9 h 56 min |
| Saturn | 120,536 | 9.54 | 29.5 years | about 10.6 h |
| Uranus | 51,118 | 19.2 | 84 years | 17.2 h, backwards |
| Neptune | 49,528 | 30.1 | 165 years | 16.1 h |
| Pluto | 2,377 | 39.5 (29.7 to 49.3) | 248 years | 6.4 days, backwards |
Diameters are equatorial (for gas giants, at the 1 bar level). "Day" means one turn against the stars; Saturn has no surface to time, so its value is only good to a few minutes.
Pluto, and the vote of 2006
Clyde Tombaugh found Pluto in 1930 at Lowell Observatory in Arizona, and for 76 years it was the ninth planet. Its estimated size kept shrinking. Today we know it is 2,377 km across, smaller than our Moon (3,475 km). Then came other objects in the same region. In 2005 a team led by Mike Brown announced Eris, about Pluto's size and roughly 27 percent more massive.
So was Eris the tenth planet, with hundreds more to follow? On 24 August 2006 the International Astronomical Union, meeting in Prague, voted on a definition. A planet:
- orbits the Sun,
- is massive enough for gravity to pull it into a round shape,
- has cleared the neighbourhood around its orbit.
Pluto fails the third test and became a dwarf planet, together with Ceres and Eris, joined in 2008 by Haumea and Makemake. Not everyone is happy. "Cleared the neighbourhood" is fuzzy (Earth shares its orbit with thousands of asteroids), and some planetary scientists, among them Alan Stern, who led the New Horizons mission, would define planets by their own properties instead. New Horizons flew past Pluto on 14 July 2015 and found ice mountains, nitrogen glaciers and a heart-shaped plain, hardly a dead rock. Pluto did not change in 2006, only our label did. A category is a decision about where to draw a line on a continuum, much like deciding where one function ends and the next begins in a stripped binary.
The Kuiper belt and the Oort cloud
Beyond Neptune lies the Kuiper belt, from about 30 to 50 AU: icy leftovers of planet formation, with Pluto among its residents. The first member after Pluto was found only in 1992, and in 2019 New Horizons flew by a snowman-shaped one called Arrokoth. Farther out is the Oort cloud, a hypothetical shell of comets proposed by Jan Oort in 1950 to explain why long-period comets arrive from every direction. It has never been observed directly, and its extent is uncertain: roughly 2,000 to 100,000 AU.
The ruler below is logarithmic, because a linear one would not fit on any screen:
AU 1 10 100 1k 10k 100k 1M
------+---------+-----===--+----------+--========+==========+---------+-
| | | | | +- Oort cloud (?) |
| | | | +- Voyager 1 at heliopause, 2012 |
| | | +- Neptune 30; Kuiper belt 30-50 |
| | +- Jupiter 5.2 |
| +- Earth 1 |
+- Mercury 0.39 |
Proxima Centauri 268,500 -+
Log scale: each tick is 10 times farther. '=' marks a belt or cloud;
the Oort cloud has never been observed directly.Voyager 1, launched in 1977, crossed the edge of the Sun's magnetic bubble, the heliopause, at about 122 AU in 2012. It is now near 160 AU and signals take about 22 hours one way. At roughly 3.6 AU per year it needs about 500 years to reach the inner edge of the Oort cloud and some 27,000 more to cross it. The nearest star, Proxima Centauri, is 268,500 AU away.
A model you can walk
Numbers like 149.6 million km mean little to a body that walks at 1.4 m/s. So scale everything with one rule, 1 AU = 10 metres, and let a few lines of Python do the bookkeeping:
AU_KM = 149_597_870.7
M_PER_AU = 10 # the whole model: 1 AU = 10 metres
bodies = [ # name, diameter in km, distance in AU
("Sun", 1_391_400, 0.0),
("Mercury", 4_879, 0.387),
("Venus", 12_104, 0.723),
("Earth", 12_756, 1.000),
("Mars", 6_779, 1.524),
("Jupiter", 142_984, 5.203),
("Saturn", 120_536, 9.537),
("Uranus", 51_118, 19.19),
("Neptune", 49_528, 30.07),
("Pluto", 2_377, 39.48),
]
print(f"{'body':8} {'size':>9} {'distance':>9} {'walk @1.4 m/s':>14}")
for name, d_km, au in bodies:
size_mm = d_km / AU_KM * M_PER_AU * 1000
dist_m = au * M_PER_AU
print(f"{name:8} {size_mm:7.2f} mm {dist_m:7.1f} m {dist_m/1.4:11.0f} s")Real output:
body size distance walk @1.4 m/s
Sun 93.01 mm 0.0 m 0 s
Mercury 0.33 mm 3.9 m 3 s
Venus 0.81 mm 7.2 m 5 s
Earth 0.85 mm 10.0 m 7 s
Mars 0.45 mm 15.2 m 11 s
Jupiter 9.56 mm 52.0 m 37 s
Saturn 8.06 mm 95.4 m 68 s
Uranus 3.42 mm 191.9 m 137 s
Neptune 3.31 mm 300.7 m 215 s
Pluto 0.16 mm 394.8 m 282 sThe Sun becomes a ball 9.3 cm across, the size of a small grapefruit. Earth is a poppy-seed speck 10 m away, and the Moon would sit 2.6 cm from it. Jupiter is a pea 52 m down the path. The whole planetary system, out to Neptune, fits in 300 m, a stroll of 3.6 minutes. Here it is as a path:
0 m 100 m 200 m 300 m 400 m 500 m
+-------------+--------------+-------------+=============+=============+
| | | | | | | +- Pluto 395 m
| | | | | | +- Neptune 301 m
| | | | | +- Uranus 192 m
| | | | +- Saturn 95 m
| | | +- Jupiter 52 m
| | +- Ceres 28 m
| +- Mercury 4, Venus 7, Earth 10, Mars 15 m
+- Sun: a ball 9.3 cm across
1 column = 7 m. '=' is the Kuiper belt (30-50 AU = 300-500 m).On this scale light itself crawls at 2 cm per second, so you would outrun it by a factor of 70. And Proxima Centauri would stand about 2,700 km away, a bit farther than London is from Moscow. Almost everything is empty space.
Try it
Pick a straight 100 m path in a park, put a small grapefruit at one end and a pea at the 52 m mark, and count your steps. Mars sits at 15 m and Jupiter at 52 m, so the 37 m of bare path between them is where the asteroid belt lives. Next in this series is our nearest neighbour, in The Moon, tides and eclipses; whether other stars have systems like this one is the subject of Exoplanets: how we find other worlds.