Ch. 11 — Jupiter
- Position — identifies Jupiter as the 5th planet and innermost gas giant, ~5.2 AU from the Sun with an ~11.86-year orbit
- Rotation — describes its fast differential rotation (~9 h 55 min, the fastest-spinning planet) and that it spins faster at the equator than at the poles
- Consequence — connects the rapid spin to the planet's oblateness (equatorial radius 71,500 km vs polar 66,900 km)
- Composition — states it is almost entirely hydrogen and helium (H₂ ≈86%, He ≈14%) with no solid surface
- Scale — gives the comparative figures (~320× Earth's mass, ~11× Earth's radius) rather than a raw kg value, and notes the low density (~1300 kg/m³)
- Why H/He — connects its massive gravity to its ability to retain light gases, keeping the composition H/He-dominated
- Bands — distinguishes zones (bright, rising, high-pressure) from belts (dark, sinking, low-pressure) and describes the stable alternating zonal wind flow
- Great Red Spot — describes it as a long-lived (≥300 yr) anticyclonic storm ~2× Earth's size
- Energy & color — notes Jupiter emits ~2× the energy it receives (still cooling from formation) and that colors come from trace-molecule photochemistry, not H/He
- Layers — names the outward sequence — compact core → liquid metallic hydrogen → liquid molecular hydrogen → gaseous atmosphere — with gradual, not sharp, boundaries
- Core — states the core is ~5–10 Earth masses (possibly “dilute”/dissolved per Juno)
- Conditions — gives the central pressure (~100 million bars) and temperature (~20,000 K)
- Source — attributes the field to electric currents in the conductive liquid metallic hydrogen layer, amplified by rapid rotation
- Strength/size — notes it is ~19,500× Earth's and that the magnetosphere extends beyond Saturn's orbit
- Io torus — describes the Io plasma torus as a ring of charged particles (Na, S) fed by Io's volcanoes
- Galilean set — names the four Galilean moons (Io, Europa, Ganymede, Callisto) and notes the ~1:2:4 resonance
- Io & Europa — identifies Io as the most volcanically active body in the Solar System and Europa as an ice-covered world with a probable subsurface salt-water ocean
- Ganymede & Callisto — notes Ganymede is the largest moon in the Solar System (bigger than Mercury) and Callisto is old, heavily cratered, and geologically dead
- Mechanism — explains that a varying tidal bulge flexes a moon's interior and the internal friction generates heat
- Two requirements — states that BOTH proximity to a massive planet AND a slightly eccentric orbit are needed
- Circular-orbit case — notes a perfectly circular orbit produces tidal bulges but NO tidal heating, and ranks Io (heated most) → Europa → Callisto (least)
Ch. 12 — Saturn
- Basics — identifies Saturn as the 2nd gas giant, ~2× Jupiter's distance, with an ~29.4-yr orbit and ~10 h rotation
- Interior contrast — notes a larger solid core but a much thinner liquid metallic hydrogen zone than Jupiter, giving a weaker magnetic field
- Mass comparison — gives the comparative mass (~95 Earth masses vs Jupiter's ~320) rather than a raw kg value
- Density — states its mean density is ~700 kg/m³ — less than water — so Saturn would float
- Shape — connects fast rotation + low density to it being the flattest planet
- Excess heat — explains it radiates ~3× the energy it receives via helium precipitation (He condenses and sinks, heating the interior)
- Ring nature — describes the rings as ~200,000 km wide but only tens of meters thick, made of countless mostly water-ice particles
- Roche limit — defines the Roche limit as the distance inside which tidal forces tear a moon apart, and notes the rings lie inside it
- Origin — connects this to a disrupted ~250 km body that could not re-form (Cassini data suggest the rings may be young, ~100 Myr)
- Atmosphere — notes Titan is the only moon with a thick, dense atmosphere (~N₂ 98% + CH₄ 2%), surface pressure ~1.6× Earth's
- Surface — describes liquid methane/ethane lakes and a surface temperature ~94 K
- Size/probe — notes it is the 2nd-largest moon and that the Huygens probe landed on solid icy ground
- Key factor — explains that it is mass PLUS low temperature, not mass alone
- Mechanism — states that Titan's great distance and cold make gas molecules too slow to escape
- Contrast — notes that if moved to Ganymede's warmer distance Titan would lose its atmosphere, explaining why bigger, warmer moons lack one
- Plumes — describes cryo-volcanic plumes (mostly water vapor) from the south polar region, found by Cassini
- Heat source — attributes the activity to tidal heating sustained by a 2:1 resonance with Dione
- E-ring — notes Enceladus orbits in, and is the likely source of, the E-ring
Ch. 13 — Uranus & Neptune
- Definition — states Uranus & Neptune are ice giants — ~4× Earth's radius but < 20 Earth masses (Uranus ~14, Neptune ~17)
- Composition — notes most of the mass (~80%) is an icy mantle of water/ammonia/methane over a rocky core, with only a modest H/He envelope
- Contrast — explains they are too dense to be pure H/He gas giants at that size
- Layers — names rocky core (~10 M⊕) → massive icy mantle → H/He gas atmosphere
- Mass fraction — states core + mantle ≈ 80% of total mass, central pressure ~100 million bars
- No metallic H — notes the absence of a thick metallic-hydrogen zone (unlike Jupiter/Saturn), which changes how the field is generated
- Tilt — states Uranus is tilted ~98°, essentially on its side
- Seasons — describes the resulting extreme seasons (a pole can face continuous sunlight for years, then continuous dark)
- Cause — notes a giant collision is the likely cause, but the moons' matching tilt complicates a single-impact explanation
- Methane — attributes the color to methane in the upper atmosphere absorbing red light and reflecting blue/green
- Uranus vs Neptune — contrasts featureless, heat-poor Uranus with the deeper-blue, dynamic Neptune
- Neptune's heat — notes Neptune radiates ~2.7× the energy it receives (a strong internal heat source)
- Odd geometry — states both fields are strongly tilted (Uranus ~60°, Neptune ~46°) and offset ~⅓ of the radius from center
- Source — attributes them to electric currents in a conducting ionic solution within the icy mantle (e.g., dissolved ammonia)
- Contrast — notes this differs from Earth's molten core or Jupiter's metallic hydrogen, explaining the misalignment and offset
- Retrograde/capture — states Triton has a retrograde, inclined orbit → almost certainly a captured Kuiper-belt object
- Surface — notes it is the coldest object yet visited (~37 K), with a thin N₂ atmosphere and nitrogen geysers
- Fate — describes it spiraling inward, to be tidally torn into a ring in ~100 Myr
Ch. 14 — Solar System Debris
- Motivation — explains astronomers searched for a “Planet X” to explain apparent irregularities in Uranus/Neptune's orbits
- Discovery — states Clyde Tombaugh discovered Pluto in 1930
- Twist — notes the irregularities were not real (Voyager 2 corrected Neptune's mass) and Pluto is far too small to have caused them — a lucky accident
- Orbit — gives key values: e=0.25, perihelion 29.7 AU / aphelion 49.3 AU, inclination 17.2°, period 248 yr
- Resonance — describes the 3:2 resonance with Neptune (2 Pluto orbits per 3 Neptune orbits)
- Protection — explains the resonance keeps their minimum separation > 17 AU despite crossing orbits
- Atmosphere — describes a tenuous N₂/CO atmosphere (traces of CH₄, ~44 K) that freezes out when far from the Sun
- New Horizons — notes the 2015 flyby revealed nitrogen-ice plains (Sputnik Planum) convecting like a lava lamp, plus water-ice hills
- Surface detail — mentions a brighter methane-frost south pole and haze layers
- Charon — describes the large moon Charon, tidally locked with Pluto, orbiting a barycenter outside Pluto
- Small moons — names the smaller moons (Nix, Hydra, Kerberos, Styx)
- Origin — connects Charon's formation to a giant impact, analogous to the Earth–Moon system
- Definition — states the IAU 2006 definition's three conditions (orbits the Sun; ~spherical; has cleared its orbital neighborhood)
- Which it fails — notes Pluto meets conditions 1 and 2 but not 3
- Reason — explains Pluto is embedded in the Kuiper belt among many similar objects, so it hasn't cleared its neighborhood
- Location — describes a disk of icy planetesimals just beyond Neptune (>100,000 members)
- Members — notes Pluto is one of the largest and names Eris and Sedna
- Origin — explains it is the residual planetesimal disk that never formed a major planet, later sculpted by ice-giant migration
- Definition — describes rocky minor planets in eccentric prograde orbits, mostly between Mars and Jupiter
- Not a broken planet — states asteroids are NOT fragments of a destroyed planet
- Origin — identifies them as primordial planetesimals that never accreted into a planet
- Location — states the belt lies between Mars and Jupiter
- Mass — notes the original mass was ~1 Earth mass but is now < 0.1% of Earth's
- Clearing — connects the depleted mass to gravitational clearing over time
- What they are — describes depleted zones at specific orbital radii within the belt
- Cause — attributes them to orbital resonances with Jupiter
- Mechanism — explains asteroids at those periods get repeatedly tugged and cleared out
- Three types — names C-type (carbonaceous, ~75%, dark, most primitive), S-type (silicate/rocky, ~15%), and M-type (metallic, ~10%)
- Trend — notes composition trends from metallic/rocky in the inner belt to carbonaceous in the outer belt
- Primitive — connects C-types to material largely unchanged since ~4.6 Gyr
- Leftovers — states asteroids are leftover planetesimals (rockier and less icy than Kuiper-belt objects)
- Jupiter's role — explains Jupiter's gravity prevented them from forming a planet
- Dawn — cites Dawn results — Ceres primitive and wet (~25% water), Vesta evolved, dry, and differentiated
- NEAs — defines NEAs as asteroids coming within ~1 AU of Earth's orbit; ~1000 are potentially hazardous (≥150 m)
- K/T event — describes the ~10–15 km impactor 66 Myr ago → Chicxulub, with extinction driven by the following “nuclear winter,” not the blast
- Deflection — notes DART (2022) shortened Dimorphos's orbit by 32 min, showing deflection is feasible
- Comet — describes a comet as a small (few-km) icy “dirty snowball,” among the oldest and most primitive material
- Meteoroid/meteor/meteorite — distinguishes meteoroid (the debris), meteor (the streak of light), and meteorite (what reaches the ground)
- Light — notes comets emit only reflected sunlight
- Orbits — states comet orbits are far more elliptical and occur at all inclinations and orientations (prograde and retrograde)
- Contrast — contrasts this with planets/asteroids/KBOs, which stay near the ecliptic and prograde
- Two classes — distinguishes short-period (P < 200 yr, e.g. Halley) from long-period (Oort cloud) comets
- Sublimation — explains that near the Sun subsurface ice sublimates (solid → gas) forming a coma
- Push — attributes the tail to solar radiation pressure and the solar wind
- Direction — states the tail always points away from the Sun, not backward along the path
- Nucleus — describes a few-km “dirty snowball” of ices + dust holding most of the comet's mass, with a very dark surface
- Mass — gives the rough mass range (~10¹²–10¹⁶ kg)
- Erosion — notes close solar passes erode comets over time (Halley/Hale-Bopp destroyed in a few thousand orbits)
- Mission — states ESA's Rosetta orbited comet 67P/Churyumov–Gerasimenko
- Landing — notes the Philae lander touched down 11/12/2014 — the first-ever comet landing
- Science — mentions it imaged gas jets near perihelion
- Structure — describes a vast spherical shell of trillions of icy comets out to ~100,000 AU (~5 Earth masses)
- Origin — explains it formed from icy planetesimals ejected outward by gravitational interactions with the young giant planets
- Long-period comets — connects it to the source of long-period comets
- Shooting star — defines it as a small dust particle burning up (~75–120 km altitude) from ram-pressure heating
- Meteor showers — explains they occur when Earth crosses a comet's debris stream, with meteors radiating from a “radiant”
- Examples — cites the Leonids (~Nov 16) and Geminids (~Dec 13)
Ch. 15 — Exoplanets
- Indirect vs direct — defines indirect detection as measuring the planet's effect on the star's light rather than imaging the planet
- Why indirect — explains direct imaging is extremely hard because the planet sits beside a vastly brighter star at huge distance
- The two methods — names radial velocity and transit as the two workhorse indirect methods
- Mechanism — explains the star wobbles around the common center of mass and the Doppler effect measures its velocity along the line of sight
- What it yields — states it gives minimum mass (M·sin i), period, and eccentricity — minimum mass because inclination is usually unknown
- 51 Pegasi b — cites 51 Peg b (Mayor & Queloz 1995) as the first, a surprising “hot Jupiter” (P=4.3 d, a=0.05 AU)
- Mechanism — explains an edge-on planet crossing its star causes a small periodic dip in brightness
- What it yields — states it gives radius (from ΔL/L ≈ (R_p/R_star)²), period, and duration — but not mass or eccentricity
- Probability/density — notes transit probability is set mainly by orbital distance (closer = higher; Mercury only ~1%) and that transit + RV together give mean density
- Method — describes a space telescope using the transit method on ~160,000 stars over ~4 years
- Why space — explains an Earth-size transit signal is smaller than the light-curve noise from Earth's atmosphere
- Goal — states its primary goal was the frequency of Earth-size planets in the habitable zone of Sun-like stars
- RV finds — notes RV surveys found many close-in giant “hot Jupiters” with surprisingly high eccentricities
- Kepler finds — states Kepler's most common result was super-Earths and mini-Neptunes
- Frequency — cites the estimate of ~300 million potentially habitable worlds in the galaxy (examples: Proxima b, TRAPPIST-1)
Orbit & spin
Jupiter is the fifth planet from the Sun and the innermost jovian (gas giant) world. It is the third-brightest object in the night sky (after the Moon and Venus), brightest near opposition. It orbits at about 5.2 AU with a period of ~11.86 Earth years, and spins extremely fast — a differential rotation of about 9 h 55 min, faster at the equator than at the poles (the fastest-spinning planet). That rapid spin flattens the planet noticeably: the equatorial radius (71,500 km) exceeds the polar radius (66,900 km).
What a gas giant is
Jupiter is composed almost entirely of hydrogen and helium (H₂ ≈ 86.1%, He ≈ 13.8%, with trace CH₄, NH₃, C₂H₂, C₂H₆, H₂O). There is no solid surface — the atmosphere transitions smoothly into a fluid interior. Its mass is ~318–320 Earth masses (more than twice all the other planets combined), its radius ~11× Earth's, and its density is low (~1300 kg/m³). Its enormous gravity lets it hold onto light gases, which is why it stays H/He-dominated. It is the prototype for the other jovian worlds and a model for exoplanetary gas giants.
Prof. note: don't memorize the mass in kilograms — know the comparative figures (~320× Earth's mass, ~11× Earth's radius). The four-giant-planets comparison slide (gas giants Jupiter/Saturn vs ice giants Uranus/Neptune) is flagged as maybe the most important slide of the chapter.
Atmosphere & storms
Two features dominate: the banded structure and the Great Red Spot. The bands are alternating zones (bright, upward-moving convective currents, high pressure) and belts (dark, sinking material, low pressure). This zonal flow forms stable alternating east/west wind bands, exaggerated by the rapid rotation; there are no seasons. Colors come from photochemistry of trace molecules, not from H/He themselves. Jupiter emits about twice the energy it receives from the Sun — it is still cooling from its formation. The Great Red Spot is a giant anticyclonic storm about twice Earth's size, observed for ≥ 300 years, rotating counter-clockwise about once every 6 days over rising cool air; why it is red and so long-lived remain open puzzles.
Interior structure
| Layer (outward) | Description |
|---|---|
| Core | Dense/compact, ~5–10 M⊕ (Juno data suggest it may be "dilute"/dissolved) |
| Liquid metallic hydrogen | Highly conductive; source of the magnetic field |
| Liquid molecular hydrogen | Thick fluid layer |
| Gaseous atmosphere | Visible cloud decks |
Boundaries are gradual, not sharp. Central pressure reaches ~100 million bars at ~20,000 K.
Magnetic field & magnetosphere
The field is generated by electric currents in the large, highly conductive liquid metallic hydrogen zone, amplified by the fast rotation — about 19,500× Earth's field strength. The magnetosphere is enormous, extending beyond Saturn's orbit, with a flat current sheet in the magnetic equatorial plane. The Io plasma torus is a ring of charged particles (sodium, sulfur) fed by Io's volcanoes.
The Galilean moons
| Moon | a (km) | Period | Signature |
|---|---|---|---|
| Io | 422,000 | 1.8 d | Most volcanically active body in the Solar System; sulfur lava; youngest surface |
| Europa | 671,000 | 3.6 d | Ice-covered; probable subsurface salt-water ocean; life candidate |
| Ganymede | 1,070,000 | 7.2 d | Largest moon in the Solar System (bigger than Mercury); own magnetic field |
| Callisto | 1,883,000 | 16.7 d | Heavily cratered, old, geologically dead; effectively no tidal heating |
The inner three (Io:Europa:Ganymede) sit near a 1:2:4 orbital resonance. All four were discovered by Galileo and are visible in binoculars.
Tidal heating
A moon on a slightly eccentric orbit close to a massive planet feels a varying tidal bulge; the flexing creates internal friction and therefore heat. Two conditions are required: proximity to a massive planet (large tides) and a slightly eccentric orbit (so the bulge changes size). On a perfectly circular orbit there would be tidal bulges but no tidal heating. Heating falls off with distance/period: Io (1.8 d) is heated most → continuous volcanism; Europa is intermediate; Callisto (16.7 d) is heated least.
Rings, Shoemaker–Levy 9, and extra moons
Jupiter has a thin, dusty ring produced by micrometeorite erosion of the small moons Metis and Adrastea, plus 60+ additional small/irregular moons. In 1994, comet Shoemaker–Levy 9 — torn into fragments spread over ~1 million km, largest piece ~1–2 km — struck Jupiter, demonstrating the planet's strong gravitational influence on small bodies; such impacts are relatively common there.
Saturn as the 2nd gas giant
Saturn is the second gas giant and the outermost planet visible to the naked eye, roughly twice Jupiter's distance from the Sun. Its orbital period is 29.4 Earth years, it has a rapid differential rotation (~10 h), and an axial tilt of 27°. Its interior mirrors Jupiter's but with a larger solid core and a much thinner liquid metallic hydrogen zone beneath a thick liquid molecular hydrogen layer; the thinner metallic-H layer gives it a weaker magnetic field (~1/20 of Jupiter's). Its mass is ~95 Earth masses — only slightly smaller than Jupiter in size but far less massive.
Prof. note: again, don't memorize kilograms — remember Jupiter ~320 M⊕ vs Saturn ~95 M⊕. Saturn is only slightly smaller than Jupiter but much less massive, hence its very low density.
Least dense & flattest planet
Saturn's mean density is ~700 kg/m³ — less than water — so it would float. Its fast rotation combined with low density make it the flattest planet (equatorial 60,000 km vs polar 54,000 km). It radiates about three times the energy it receives; formation heat alone is insufficient, so the extra heat comes from helium precipitation — helium condenses and sinks through the liquid hydrogen, compressing and heating the interior (leaving the upper atmosphere ~50% poorer in helium than Jupiter's).
The rings & the Roche limit
The rings span ~200,000 km but are only tens of meters thick, made of countless particles (fractions of a mm to tens of m) that are mostly water ice (reflecting ~80% of sunlight), lying in the equatorial plane. The Roche limit is the critical distance inside which a body's tidal forces would tear a moon apart (~2.4 Earth radii for the Earth–Moon case). Saturn's rings lie inside its Roche limit, so they most likely came from a ~250 km moon/asteroid/comet that was tidally disrupted and could not re-form. Cassini data suggest the rings may be only ~100 Myr old (still debated).
| Year | Discovery |
|---|---|
| 1610 | Galileo sees the rings (as "ears") |
| 1655 | Huygens identifies them as a ring |
| 1675 | Cassini finds the Cassini Division |
| 1875 | Maxwell: rings are many small particles |
| 1895 | Doppler measurements confirm Keplerian motion |
Titan
Titan is the second-largest moon in the Solar System and the only moon with a thick, dense atmosphere: ~98% N₂ + ~2% CH₄ (traces of argon), with a surface pressure about 60% greater than Earth's (~1.6×) and a surface temperature ~94 K. Its surface has lakes and oceans of liquid methane and ethane (mostly methane), "titanquakes," and likely a water layer tens of km down. The Huygens probe landed on solid icy ground.
Why Titan keeps its atmosphere
It is not mass alone — it is mass + low temperature. Titan's great distance from the Sun makes it cold enough that gas molecules move too slowly to escape. If Titan were moved to Ganymede's (warmer) distance, it would lose its atmosphere. This explains why larger but warmer moons (Ganymede, our Moon) lack thick atmospheres.
Enceladus
Cassini found jet-like cryo-volcanic plumes from the south polar region of the small moon Enceladus — mostly water vapor plus N₂, CH₄, and CO₂ — driven by tidal heating. Enceladus orbits within the densest part of the E-ring and is likely its source. It is in a 2:1 resonance with Dione, which pumps its orbital eccentricity (0.0047) and sustains the internal heating; a mean density ~1.6 g/cm³ implies silicates/iron and possibly molten pockets.
Ice giants defined
Uranus and Neptune are ice giants, not gas giants. Each is about 4× Earth's radius but under 20 Earth masses (Uranus ~14 M⊕, Neptune ~17 M⊕) — too dense to be pure hydrogen/helium at that size. Most of their mass (~80%) is a mantle of "ices" — water, ammonia, and methane compounds of C, N, and O — over a rocky core, topped by only a modest H/He gas envelope.
Prof. note: the interiors slide is flagged as "very important." At their size and mass these two cannot be pure H/He gas giants — hence the massive ice mantle, with mantle + core ≈ 80% of the mass. Expect a gas-giant-vs-ice-giant short answer.
Interior
Layers run rocky core (~10 M⊕, ~10× Earth's core) → a massive icy mantle (water/ammonia/methane) → an H/He molecular gas atmosphere. Core plus mantle make up ≈ 80% of the total mass, and central pressure reaches ~100 million bars. There is no thick metallic-hydrogen zone as in Jupiter/Saturn, which is why their magnetic fields are generated differently.
Uranus's extreme tilt
Uranus's rotation axis is tilted about 98° — essentially on its side. Its poles alternately point near the Sun, producing extreme seasons (a pole can face continuous sunlight for years, then continuous darkness). The likely cause is a giant collision, though the fact that the moons' orbits are tilted the same way is hard to explain with a single impact (there is no direct evidence).
Blue/green color
Methane (CH₄) in the upper atmosphere absorbs red wavelengths and reflects blue/green. Uranus is a fairly featureless blue-green with little internal heat; Neptune is a deeper blue and surprisingly dynamic, with a strong internal heat source (radiating ~2.7× the energy it receives) and winds up to ~1500 km/h. Neptune's Great Dark Spot appeared in 1989 and later vanished.
Strange magnetic fields
Both fields are highly tilted relative to the rotation axis (Uranus ~60°, Neptune ~46°) and offset from the planet's center by about a third of the radius. They are generated not in metallic hydrogen or a molten core but by electric currents in a conducting ionic solution within the icy mantle (for example, dissolved ammonia), which explains the misalignment and offset.
Triton
Neptune's large moon Triton has a retrograde orbit inclined about -23° (opposite Neptune's spin), so it is almost certainly a captured Kuiper-belt object rather than one formed in place. It is the coldest object yet visited (~37 K), with a very thin N₂ atmosphere (~1/100,000 of Earth's), a water+nitrogen-ice surface, few craters (young, active), and nitrogen "geysers" several km high. It is spiraling inward and may be tidally torn into a ring in ~100 Myr.
Uranus vs Neptune — quick compare
| Uranus | Neptune | |
|---|---|---|
| Distance | 19.2 AU | 30.1 AU |
| Period | 84 yr | 164 yr |
| Axis tilt | 98° | 29.6° |
| Field tilt | ~60° | ~46° |
| Internal heat | Little | ~2.7× received |
| Mass | ~14 M⊕ | ~17 M⊕ |
Voyager 2 (launched 1977) is the only spacecraft to visit both, on its "Grand Tour." Miranda (a moon of Uranus) hosts Verona Rupes, the tallest cliff in the Solar System (~20 km).
Chapter 14 spans three lecture parts — Pluto/Kuiper belt, asteroids, and comets/meteorites — and carries the largest share of study-guide bullets. All three feed this one section.
Pluto's discovery
Astronomers thought unexplained irregularities in the orbits of Uranus and Neptune implied a "Planet X." Clyde Tombaugh discovered Pluto in 1930 (named by 11-year-old Venetia Burney; "PL" also honors Percival Lowell). The irregularities turned out not to be real — Voyager 2 gave Neptune's correct mass and they vanished, and Pluto is far too small to have caused them anyway. Its discovery was essentially a lucky accident.
Pluto's orbit
Eccentricity 0.25; perihelion 29.7 AU, aphelion 49.3 AU; inclination 17.2°; orbital period 248 yr; rotation 6.5 days. Pluto is in a 3:2 resonance with Neptune (two Pluto orbits for every three of Neptune's) — a protective resonance: despite crossing Neptune's orbit, their minimum separation stays above 17 AU (Pluto actually comes closer to Uranus, ~11 AU, than to Neptune).
Pluto's surface, interior & atmosphere
A tenuous atmosphere of N₂ and CO with traces of CH₄ is retained at ~44 K and freezes out when Pluto is far from the Sun; a brighter south pole marks a frozen methane cap. The New Horizons flyby (07/14/2015) revealed water-ice "floating hills," nitrogen-ice plains (Sputnik Planum) convecting like a lava lamp, and haze layers.
Pluto's moons
The large moon Charon is tidally locked with Pluto, and the pair orbit a barycenter that lies outside Pluto. Smaller moons include Nix, Hydra, Kerberos, and Styx. Charon likely formed in a giant impact, analogous to the Earth–Moon system.
Why Pluto was demoted
The IAU planet definition (2006) has three conditions: (1) orbits the Sun; (2) is massive enough to be roughly spherical; (3) has cleared its orbital neighborhood. Pluto meets 1 and 2 but not 3 — it is embedded in the Kuiper belt among many similar objects — so it is classified as a dwarf planet.
Prof. note: the phrase "cleared the neighborhood" is hammered — the key is no similar-size object nearby. Jupiter's orbit has debris but nothing comparable in mass, so it qualifies; Pluto fails because it sits among similar Kuiper-belt bodies. A prime short-answer.
The Kuiper belt
A disk of icy planetesimals just beyond Neptune (predicted in the 1950s, first object found in 1992), with over 100,000 members and ~1100 Pluto-like objects; Pluto is one of the largest, near the inner edge. Notable members: Eris (~Triton-sized) and Sedna (76–928 AU). The belt is the residual planetesimal disk that never formed a major planet, later sculpted by the outward migration of the ice giants.
Asteroids: what, where, why gapped, composition
Asteroids are rocky "minor planets" in eccentric, prograde orbits, mostly between Mars and Jupiter; sizes range from ~1/10 km up to Ceres (~940 km), with ~500,000 known. They are not fragments of a broken-up planet — they are primordial planetesimals that never accreted into one (Jupiter's gravity prevented it). The belt's original mass was ~1 Earth mass but is now under 0.1% of Earth's. The Kirkwood gaps are depleted zones caused by orbital resonances with Jupiter, which repeatedly tug and clear asteroids at those periods.
| Type | Share | Composition / location |
|---|---|---|
| C-type | ~75% | Carbonaceous, dark, most primitive; outer belt |
| S-type | ~15% | Silicate/rocky; inner belt |
| M-type | ~10% | Metallic (Ni/Fe) |
The Dawn mission found Ceres primitive and wet (~25% water) and Vesta evolved, dry, and differentiated (core/mantle/crust).
Near-Earth Asteroids & impacts
NEAs come within ~1 AU of Earth's orbit; Apollo asteroids actually cross it. There are ~6500 Earth-crossers and ~1000 "potentially hazardous" (≥150 m). The K/T impactor (~10–15 km, 66 Myr ago) formed the ~180 km Chicxulub crater; the extinction was driven by the following "nuclear winter," not the blast itself, and K/T-scale events happen roughly every 100 Myr. DART (2022) shortened the moonlet Dimorphos's orbit by 32 minutes, showing deflection is feasible — blowing an asteroid up Hollywood-style is not the plan.
| Event | Size / date |
|---|---|
| Tunguska | ~40–80 m, 1908 |
| Chelyabinsk | ~17–20 m, ~440 kT, 02/15/2013 |
| Meteor Crater (AZ) | ~50 m Ni/Fe, ~50,000 yr ago, ~10 MT |
Comets & meteorites
A comet is a small (few-km) icy "dirty snowball" nucleus — the oldest, most primitive Solar System material — that emits only reflected sunlight. A meteoroid is small rocky/metal debris; a meteor is the streak of light as it burns up ("shooting star"); a meteorite is the piece that survives to the ground. Comet orbits are far more elliptical than planets' and occur at all inclinations and orientations (prograde and retrograde), unlike planets/asteroids/KBOs, which stay near the ecliptic and prograde. Short-period comets (P < 200 yr, e.g. Halley) come from the Kuiper belt; long-period comets come from the Oort cloud.
Why comets have tails
Near the Sun (~1 AU), subsurface ice sublimates (solid → gas directly, because of the low pressure), forming a coma. Solar radiation pressure and the solar wind push gas and dust outward, so the tail always points away from the Sun — not backward along the path. The ion tail is straight and glowing (CO, N₂), the dust tail broad and curved.
Prof. note: expect the "why a tail / why away from the Sun" short-answer, and remember comets are more ice-rich than asteroids (closer to KBOs) yet only a few km across.
Cometary nuclei & Rosetta
A nucleus is a few km across — a "dirty snowball" of ices (water, methane, ammonia, CO₂) plus dust — holding most of the comet's mass, with a very dark surface and a mass of ~10¹²–10¹⁶ kg. Close solar passes erode comets (Halley and Hale-Bopp would be destroyed in a few thousand orbits). ESA's Rosetta orbited comet 67P/Churyumov–Gerasimenko, and its Philae lander touched down on 11/12/2014 — the first-ever comet landing — imaging gas jets near perihelion.
The Oort cloud
A vast spherical shell of trillions of icy comets extending up to ~100,000 AU, ~5 Earth masses in total, still gravitationally bound to the Sun. It formed from icy planetesimals ejected from the inner Solar System by gravitational interactions with the young giant planets (especially Jupiter and Saturn), onto long, eccentric orbits where they spend most of their time near aphelion. It is the source of long-period comets.
Shooting stars & meteor showers
A shooting star is a small dust particle burning up in the atmosphere (~75–120 km altitude) from ram-pressure heating. Meteor showers occur when Earth crosses a stream of debris left along a comet's orbit; the meteors appear to radiate from one point (the radiant). Examples: the Leonids (~Nov 16, ~12/hr) and the Geminids (~Dec 13, ~50/hr).
Indirect vs direct detection
Indirect detection measures a planet's effect on its star's light rather than imaging the planet directly. Direct imaging is extremely hard because the planet sits right next to a vastly brighter star at huge distance. The two workhorse indirect methods are radial velocity and transit.
Radial velocity (RV)
A star and its planet both orbit their common center of mass, so the star wobbles; the Doppler effect measures the star's velocity along the line of sight (only the star is observed). RV yields the planet's minimum mass (M·sin i), the orbital period (→ distance), and the eccentricity — minimum mass only, because the orbital inclination is usually unknown. The signal is largest for a massive planet close to the star. 51 Pegasi b (Mayor & Queloz, 1995; half of the 2019 Nobel) was found this way — a "hot Jupiter" with P = 4.3 d and a = 0.05 AU, the big surprise being its very short period and close orbit.
Transit
When the orbit is edge-on, the planet crosses in front of the star and causes a small periodic dip in brightness. Transit yields the planet's radius (from ΔL/L ≈ (R_planet/R_star)²), the period, and the transit duration — but not mass or eccentricity. Depths: Jupiter ~1–2%, Earth ~0.0084%. Transit probability is set mainly by orbital distance (closer = higher probability); even Mercury's is only ~1%. Combining transit (radius) with RV (mass) gives the planet's mean density. HD 209458 b was the first transiting exoplanet, with a lower mean density than Saturn.
| Method | Yields | Does not give |
|---|---|---|
| Radial velocity | Minimum mass (M·sin i), period, eccentricity | Radius |
| Transit | Radius, period, duration | Mass, eccentricity |
| RV + transit | Mean density | — |
Prof. note: transit probability depending on orbital distance is called out as "an important point" — even the best-case (closest) planet gives a low probability, which is why Kepler needed so many stars. The RV/transit contrast (mass vs radius, both → density) is framed as the spine of the chapter.
NASA's Kepler mission
Kepler was a space telescope (continuous view, no atmosphere) that used the transit method on ~160,000 stars over ~4 years, designed for ~20 ppm precision (an Earth transit is ~85 ppm); its PI was Bill Borucki at NASA Ames. It had to go to space because an Earth-size transit signal is smaller than the light-curve noise from Earth's atmosphere. Its primary goal was the frequency of Earth-size planets in the habitable zone of Sun-like stars — an answer that is a statistical range, not a single number.
What has been found
RV surveys found many close-in giant planets ("hot Jupiters") and surprisingly high eccentricities — unexpected, since giants were assumed to form far out — plus the first multi-planet systems, often in orbital resonances. Kepler's most common finds were super-Earths and mini-Neptunes (sizes between Earth and Neptune). Estimates suggest ~300 million potentially habitable worlds in the galaxy (≥7% of Sun-like stars conservatively, ~50% on average). Examples: Proxima b (~1.3 M⊕, around the nearest star), Kepler-62 (a habitable-zone system), and TRAPPIST-1 (a tiny star giving large transit signals for Earth-size planets).
Short-answer / discussion prep — self-grade using the checklist after you write your response. ~50% of the last exam was short-answer, so use these to build fluency.
🎥 Video Lectures (Dr. Endl) — study on the go
Tap a link to open the lecture on YouTube. Great for reviewing while walking or commuting.
Ch. 12 — Saturn
Ch. 13 — Uranus & Neptune
Ch. 14 — Solar System Debris
Ch. 15 — Exoplanets
Ch. 11 — Jupiter
Orbit & spin (5.2 AU, ~11.86 yr, 9h55m): ____________________________
Gas giant: composition & scale (~320×, ~11×): ____________________________
Atmosphere: zones/belts & Great Red Spot: ____________________________
Interior layers & metallic hydrogen: ____________________________
Magnetic field & Io plasma torus: ____________________________
Galilean moons (Io/Europa/Ganymede/Callisto): ____________________________
Tidal heating (2 requirements): ____________________________
Ch. 12 — Saturn
Interior vs Jupiter (thin metallic-H): ____________________________
Least dense / flattest / He precipitation: ____________________________
Rings & the Roche limit: ____________________________
Titan (atmosphere, methane lakes): ____________________________
Why Titan keeps its atmosphere: ____________________________
Enceladus (plumes, Dione 2:1): ____________________________
Ch. 13 — Uranus & Neptune
Ice giants defined (~4× R, <20 M⊕): ____________________________
Interior (icy mantle ~80% mass): ____________________________
Uranus's 98° tilt: ____________________________
Blue/green color (methane): ____________________________
Tilted, offset magnetic fields: ____________________________
Triton (retrograde / captured): ____________________________
Ch. 14 — Solar System Debris
Pluto discovery (1930, Planet X): ____________________________
Pluto orbit & 3:2 resonance: ____________________________
Pluto surface/atmosphere/New Horizons: ____________________________
Pluto moons (Charon): ____________________________
Dwarf-planet definition (cleared neighborhood): ____________________________
Kuiper belt & origin: ____________________________
Asteroids: what / belt / Kirkwood gaps: ____________________________
Asteroid types (C/S/M) & Dawn: ____________________________
NEAs & K/T impact / DART: ____________________________
Comets vs meteoroid/meteor/meteorite: ____________________________
Comet orbits & why tails point away from Sun: ____________________________
Cometary nuclei & Rosetta/Philae: ____________________________
Oort cloud (origin, ~100,000 AU): ____________________________
Meteor showers (Leonids/Geminids): ____________________________
Ch. 15 — Exoplanets
Indirect vs direct detection: ____________________________
Radial velocity (min mass, 51 Peg b): ____________________________
Transit (radius, probability ← distance): ____________________________
RV + transit → density: ____________________________
Kepler (why space, primary goal): ____________________________
What's been found (hot Jupiters, super-Earths): ____________________________