Appearance
11.2 — The Solar System
The Sun contains 99.86 % of the solar system's mass. Jupiter has two thirds of what is left. Everything else — the other seven planets, all the moons, all the asteroids and comets — is 0.05 % of the total.
But almost all the angular momentum is in the planets. The Sun holds under 1 % of it despite holding 99.86 % of the mass, and explaining that split is one of the main tests of any formation model.
The numbers
| Planet | Distance (AU) | Radius (Earth=1) | Mass (Earth=1) | Day | Year | Moons |
|---|---|---|---|---|---|---|
| Mercury | 0.387 | 0.383 | 0.055 | 58.6 d | 88 d | 0 |
| Venus | 0.723 | 0.949 | 0.815 | -243 d | 225 d | 0 |
| Earth | 1.000 | 1.000 | 1.000 | 23.93 h | 365.25 d | 1 |
| Mars | 1.524 | 0.532 | 0.107 | 24.62 h | 687 d | 2 |
| Jupiter | 5.203 | 11.21 | 317.8 | 9.93 h | 11.86 y | 95 |
| Saturn | 9.537 | 9.45 | 95.2 | 10.7 h | 29.4 y | 146 |
| Uranus | 19.19 | 4.01 | 14.5 | -17.2 h | 84 y | 28 |
| Neptune | 30.07 | 3.88 | 17.1 | 16.1 h | 165 y | 16 |
The negative rotation periods mean retrograde — Venus and Uranus spin backwards relative to their orbits.
The obvious split is four small rocky planets inside 2 AU and four giants outside 5 AU, with the asteroid belt between. That split is the single most important fact about the system's formation, and the reason is below.
The rocky planets
Mercury
The extremes. Closest to the Sun, smallest planet, most eccentric orbit (e = 0.206), and the largest day–night temperature range in the solar system: from 430 °C to -180 °C, a swing of 610 degrees, because there is no atmosphere to move heat around.
A 3:2 spin–orbit resonance. Mercury rotates exactly three times for every two orbits, which is unusual — most tidally influenced bodies end up 1:1 like the Moon. The eccentricity is what allows it, because the tidal torque is strongest at perihelion and 3:2 is stable there.
The consequence is bizarre. A solar day on Mercury — noon to noon — lasts 176 Earth days, which is two Mercury years. The Sun rises, and near perihelion its apparent motion briefly reverses, so from some longitudes the Sun rises, stops, sets again, and rises a second time.
An enormous iron core, about 85 % of the radius, giving a density of 5.43 g/cm³ — nearly Earth's, in a body a third the size. The leading explanation is a giant impact that stripped away most of the mantle.
And its perihelion precession was the first confirmed test of general relativity (Chapter 6.9).
Venus
Earth's twin by size and nothing else.
Surface temperature 464 °C — hotter than Mercury, despite being twice as far from the Sun. A runaway greenhouse effect (Chapter 3.7): an atmosphere 96.5 % CO₂ at 92 bar, equivalent to being 900 m underwater on Earth.
Compute the equilibrium temperature with Venus's albedo of 0.77 and solar intensity 2601 W/m²:
T = \left(\frac{S(1-\alpha)}{4\sigma}\right)^{1/4} = \left(\frac{2601\times0.23}{4\times5.67\times10^{-8}}\right)^{1/4} = \left(2.637\times10^{9}\right)^{1/4} = 227\ \text{K}
-46 °C predicted; +464 °C measured. A greenhouse effect of 510 degrees, against Earth's 33 (Chapter 3.7).
Why it ran away. Venus started with oceans. Being closer to the Sun, more water evaporated; water vapour is a greenhouse gas, so it warmed further, evaporating more. Positive feedback with no stopping point. Eventually all the water was in the atmosphere, ultraviolet split it, and the hydrogen escaped to space — which is confirmed by Venus's deuterium-to-hydrogen ratio being 150 times Earth's, exactly what you expect when the lighter isotope preferentially escapes.
With the water gone, the carbon that on Earth is locked into carbonate rocks stayed in the air. Earth has about as much CO₂ as Venus; ours is in limestone.
And it rotates backwards, once every 243 days — longer than its 225-day year, so a Venusian solar day is 117 Earth days. The most likely cause is atmospheric tides in that massive atmosphere acting over billions of years.
Mars
Half Earth's diameter, a tenth its mass, and it lost its atmosphere.
The evidence for past water is overwhelming: dried river valleys, deltas, lake sediments, clay minerals that only form in water, and rounded pebbles photographed by rovers.
Why it dried out is a chain of three consequences.
Mars is small, so its interior cooled faster and its core dynamo shut down about 4 billion years ago.
No magnetic field means no protection from the solar wind (Chapter 4.5). MAVEN has measured the current stripping rate directly.
Low gravity, 3.71 m/s², makes escape easier. Escape velocity is 5.03 km/s against Earth's 11.2.
Present atmosphere: 6 millibars, 0.6 % of Earth's, 95 % CO₂. Water cannot be liquid at that pressure — the triple point of water is 6.1 mbar (Chapter 3.6), so surface water sublimes directly.
Olympus Mons is 22 km high, three times Everest, and 600 km across. Two reasons it grew so large: Mars has no plate tectonics, so a hotspot builds one volcano indefinitely rather than a chain of them, and lower gravity allows a taller structure before it collapses under its own weight.
Its two moons are almost certainly captured asteroids. Phobos orbits below synchronous altitude and is spiralling in, and will break up into a ring in about 50 million years.
The giants
Jupiter
318 Earth masses, more than twice all the other planets combined.
No surface. Hydrogen and helium becoming denser with depth, through liquid molecular hydrogen, into metallic hydrogen at about 100 GPa where the molecules dissociate and the hydrogen conducts like a metal. That layer is what generates Jupiter's magnetic field, which is 20,000 times Earth's and would appear larger than the full Moon in our sky if it were visible.
The Great Red Spot is a storm at least 190 years old and probably 350, currently about 1.3 Earth diameters across and shrinking. It persists because there is no surface to provide friction and no land to disrupt it.
Jupiter radiates about 1.6 times as much energy as it receives from the Sun. The excess is gravitational — the planet is still slowly contracting, at about 2 cm per year, converting potential energy to heat (the Kelvin–Helmholtz mechanism).
Four Galilean moons, discovered in January 1610 and each a world:
Io — the most volcanically active body in the solar system, with over 400 active volcanoes, heated by tidal flexing from the resonance with Europa and Ganymede.
Europa — an ice shell over a liquid water ocean containing roughly twice the water of all Earth's oceans. One of the two best candidates for life beyond Earth, and the target of the Europa Clipper mission.
Ganymede — the largest moon in the solar system, bigger than Mercury, and the only moon with its own magnetic field.
Callisto — the most heavily cratered object known, essentially unchanged for four billion years.
Galileo's observation that these moons orbit Jupiter was the decisive blow against the idea that everything orbits the Earth. You can see them yourself with binoculars, and their positions change visibly in a few hours.
Saturn
Density 0.687 g/cm³ — less than water. It is the only planet that would float, if you had a large enough bath.
The rings are 99.9 % water ice, span 280,000 km, and are on average 10 metres thick. Scaled to a sheet of paper, they would be 100 km across.
They are young. Cassini's measurement of the ring mass, combined with the rate at which they are darkened by micrometeorites and drained into the planet, gives an age of 10 to 100 million years. Saturn did not have rings when the dinosaurs lived, and they will be gone in another 100 million years.
Why rings exist at all: the Roche limit. Inside a certain distance, tidal forces exceed a body's self-gravity and it cannot hold together:
d = 2.44R_p\left(\frac{\rho_p}{\rho_m}\right)^{1/3}
For Saturn with an icy moon, d \approx 140{,}000 km, and the rings lie inside that. Nothing can accrete there.
Titan is the only moon with a substantial atmosphere — 1.45 bar, mostly nitrogen — and the only body besides Earth with stable surface liquid. The liquid is methane and ethane, in lakes and seas, with a full hydrological cycle of methane rain, rivers and evaporation. Huygens landed there in 2005 and photographed rounded pebbles in what is clearly a dry riverbed.
Enceladus is 500 km across and shoots plumes of water from its south pole. Cassini flew through them and found salts, silica and organic molecules, indicating a subsurface ocean in contact with a rocky seafloor — which is the setting for hydrothermal chemistry.
Uranus and Neptune
Ice giants, not gas giants. About 80 % water, ammonia and methane ices around a rocky core, with only a modest hydrogen–helium envelope.
Uranus is tipped 98° — it rolls along its orbit. Each pole gets 42 years of continuous sunlight then 42 years of darkness. The likely cause is one or more giant impacts.
Neptune has the fastest winds in the solar system, up to 2100 km/h, despite receiving 1/900 of Earth's sunlight. How that is powered is not fully understood.
Neptune was found by mathematics. Uranus's orbit did not match prediction, Le Verrier and Adams independently computed where a perturbing planet must be, and Galle found it within one degree of Le Verrier's position on the first night of looking, 23 September 1846. The most striking prediction in the history of celestial mechanics — and the same method applied to Mercury gave Vulcan, which does not exist (Chapter 6.9).
Both have offset, tilted magnetic fields — Uranus's is 59° from its rotation axis and displaced a third of a radius from centre — suggesting the dynamo runs in a thin conducting shell rather than a core.
Small bodies
Asteroids occupy the belt between Mars and Jupiter, with total mass about 4 % of the Moon's. Ceres alone is a third of it.
They are not a destroyed planet. Jupiter's gravity prevented one from forming, and the Kirkwood gaps — depleted zones at orbital periods in simple ratios with Jupiter's — show the mechanism directly. Resonant perturbations pump up the eccentricity until the object is ejected or collides.
Comets come from two reservoirs:
The Kuiper belt, 30–50 AU, source of short-period comets.
The Oort cloud, out to 100,000 AU — a light year and a half, a quarter of the way to Proxima Centauri — source of long-period comets. It has never been directly observed and is inferred entirely from the orbits of the comets that arrive.
A comet's tail always points away from the Sun (Chapter 4.7), driven by radiation pressure and the solar wind, so on the outbound leg the tail leads.
Pluto was reclassified in 2006 because it fails the third criterion for a planet: it has not cleared its orbital neighbourhood. Eris, found in 2005, is nearly the same size, and it was clear that either Pluto was demoted or the count of planets would grow indefinitely. New Horizons found it far more interesting than expected in 2015 — nitrogen ice glaciers flowing, mountains of water ice 3 km high, and a surface with regions less than 10 million years old, meaning it is geologically active.
Formation
The nebular hypothesis, in outline.
A cloud of gas and dust collapses. Conservation of angular momentum (Chapter 1.8) means any initial rotation speeds up enormously as it shrinks, flattening the cloud into a disc.
The centre becomes the Sun. The disc becomes the planets.
The snow line is the key structural feature. Inside about 2.7 AU it was warm enough that water stayed vapour; outside, it froze.
Beyond the snow line there was far more solid material to build with, so cores grew large enough — about 10 Earth masses — to capture hydrogen and helium directly from the disc before it dispersed. Inside the line, only rock and metal were available, so the planets stayed small.
\boxed{\text{The snow line is why the inner planets are rocky and the outer ones are giants.}}
Evidence the model gets right: all planets orbit the same way in nearly the same plane, the composition gradient, the age of meteorites at 4.567 billion years, and the observation of protoplanetary discs around young stars — including ALMA images showing gaps carved by forming planets.
And two things it does not.
The angular momentum problem. The Sun should be spinning far faster than it is. Magnetic braking — the young Sun's field coupling to the ionised disc and transferring angular momentum outward — is the leading explanation.
Hot Jupiters. The first exoplanet found around a Sun-like star, 51 Pegasi b in 1995, is a Jupiter-mass planet orbiting in four days, far inside where Mercury is. The nebular model says giant planets cannot form there.
The resolution is migration. Planets do not stay where they form; they exchange angular momentum with the disc and spiral inwards or outwards.
And the Grand Tack model applies this to our own system: Jupiter migrated in to about 1.5 AU, then Saturn caught up into a resonance and both migrated back out. This would explain why Mars is so small — Jupiter's passage cleared the material Mars would otherwise have grown from — and why the asteroid belt is depleted and mixed.
The Nice model proposes a later instability that scattered the outer planets and drove the Late Heavy Bombardment, whose scars are the lunar maria.
The honest summary: the broad picture is solid and the details are actively contested, and exoplanet discoveries since 1995 have overturned more of it than anything else.
Exoplanets
Over 5,800 confirmed, and the statistics are the interesting part.
Detection methods:
Transit — the star dims as a planet crosses. Gives radius, and requires the orbit to be edge-on, so only about 1 in 200 systems is detectable. Kepler and TESS use this.
Radial velocity — the star wobbles, shifting its spectral lines (Chapter 2.5). Gives minimum mass.
Together they give density and hence composition, which is why systems with both measurements are so valuable.
What has been found:
Planets are common. Statistically, most stars have at least one, and there are more planets than stars in the galaxy.
The most common size has no analogue here. Super-Earths and mini-Neptunes, between 1 and 4 Earth radii, are the commonest type found — and our system has none.
Systems look nothing like ours. Hot Jupiters, planets in resonant chains, planets orbiting binary stars, planets on wildly eccentric orbits.
TRAPPIST-1 has seven Earth-sized planets around a red dwarf 40 light years away, three in the habitable zone, all in a resonant chain. The whole system would fit inside Mercury's orbit.
Whether our system is typical is genuinely unknown, because current methods are strongly biased towards large planets on short orbits. A survey with our instruments looking at the Sun from 40 light years would probably find nothing.
Where this shows up in your life
GPS satellites must account for the Earth's oblateness, the Moon's and Sun's gravity, and solar radiation pressure.
Space weather. Solar flares and coronal mass ejections disrupt satellites, power grids and aviation. The Carrington Event of 1859 set telegraph offices on fire, and an equivalent event today would be a multi-trillion-dollar disaster.
Asteroid impact risk is now systematically surveyed, and in 2022 the DART mission deliberately hit the asteroid Dimorphos and changed its orbital period by 32 minutes — the first demonstration that deflection works.
Planetary science drives Earth science. Venus is what a runaway greenhouse looks like and Mars is what atmospheric loss looks like, and both inform climate models here.
And the search for life is now a mainstream research programme, with Europa, Enceladus and Titan as targets in this system and atmospheric spectroscopy of exoplanets as the method beyond it.
What the next chapter fixes
The orbits have been described and never derived. Chapter 11.3 does the derivation both ways: Kepler's three laws are obtained from Newton's inverse-square law, and then Newton's law is recovered from Kepler's third — closing the loop, and showing exactly what each derivation assumes and what it proves.