The Solar System

MODIFIED: 3 October 2025 terms 'giant planets, gas giants, ice giants' clarified

POSTED: 10 June 2025

• Origin of the Solar System

• age of universe: about 14 billion years (a more precise value is 13.799 +/- 0.021 billion years)
(NB for the interested: we know this from estimates of the lifetime of protons which are considered "stable" particles.)
• age of solar system⁽¹⁾ (and Earth): 4.5 billion years (Moon about 100,000 years younger)
⁽¹⁾estimated from the study of meteorites

Currently, no hypothesis can explain all observations adequately but the modified nebular hypothesis, initially by Immanuel Kant (1755), then updated, is the most widely used/accepted. The nebular hypothesis supports the idea that sun and planets formed at the same time.

• initially huge rotating spherical cloud (nebula) of ice, gas, debris
• spinning cloud contracts
• to conserve angular momentum, spinning speeds up (like figure skater pulling arms in)
• flattening to disk
  (NB for the interested: the exact reason for this is poorly understood but one hypothesis is that particles from above and below the disk collided near the disk and propelled each other further out - this is needed to explain why the planets carry most of the angular momentum in the solar system)
• immense density at center leads to gravitational collapse and formation of sun
(NB for the interested: immense pressure and temperature during gravitational collapse started thermo-nuclear processes in the sun)
• in rotating disk, masses collide and stick together (accretion)
(NB: exactly how this was done is still under debate; two possible models are called gravitational instability theory and core-accretion theory)
• accretion first leads to planetesimals (a few km radius), then planets (> 1000km radius)
A publication in "American Scientist" likens the core-accretion process to what happens to dust particles under your bed when you don't vacuum for a while. If you are logged in from a UCSD campus machine, you can access the article.

• Earth's History

• ~ 4.56 Ga ago: initially aggregating mass of particles and gas
(NB: Ga comes from Giga for billion and "annum", latin for year)
• random collision of pieces -> planetesimal -> planet
• collision and compaction led to heating
• differentiation: heavier components (e.g. iron-rich) pulled toward center; lighter components toward surface
• formation of Earth's iron core while Earth's outer shells were still soft
• Mars-size impactor hit earth, ejecting parts of mantle into space to form the Moon
• Earth cools and forms lithosphere
• ~3.9 billion years ago: (small) continents, oceans, dense atmosphere (CO2 dominant)
• ~3.5 billion years ago: first life (bacteria)
• ~2.5 billion years ago: large continents
• plate tectonics for at least last 1.5 billion years

• Planets and Major Bodies in the Solar System

• sun has 99.9% of all the mass in the solar system
• planets have 99% of all the angular momentum in the solar system


• terrestrial planets, inner planets: Mercury, Venus, Earth, Mars
(rocky planets)


• giant planets, outer planets: Jupiter, Saturn, Uranus, Neptune
(Jupiter and Saturn are gas planets with relatively small cores; Uranus and Neptune are icy planets; all four consist of mainly H₂ and He)
• giant planets are typically much larger than terrestrial planets
• Jupiter has 60% of all the mass of the planets and 70% of all the angular momentum
• all planets orbit the sun in the same direction (counterclockwise when viewed from above)
• all planets have nearly circular orbits
NB: a revolution about the sun can take from 80 days (Mercury) to 165 years (Neptune)
• all planets also revolve about a spin axis
a spin can take from 9h (Jupiter) to 243 days (Venus)
• all but Mercury and Venus have Moons/satellites


• definition of a planet
• has own orbit around Sun
• has sufficient mass to assume nearly round shape
• is not a satellite of a planet or other nonstellar body
• has "cleared the neighborhood" around its orbit of planetesimals
• Moon
• a satellite, not a planet
• Earth's satellite, not a planet
• formed during early formation when Mars size impactor hit Earth (~4.4 billion years ago)
• Moon made mostly of Earth mantle rock



DID YOU KNOW? Scientists have recently determined that a spot on the Moon that is never exposed to sunlight may be the coldest spot in the solar system. see newsclip.

DID YOU KNOW? As of December 2021, scientists have discovered five quasi-satellites of Earth. According to a Science News article(1), at least Kamo'oalewa (discovered in 2016) may be a junk of the Moon and ejected when a giant impact on Earth formed the Moon. Kamo'oalewa in only the size of a Ferris wheel and strays between 40 and 100 times as far from Earth and the moon, weaving in and out of Earth's orbit around the sun. Keep reading here. Update: as of September 2023, Earth has seven quasi-satellites.





Body	Type	Radius (ER)	Mass (EM)	Distance from Sun (AU)	# Satellites
Sun	star	109.25	332946.8	0	-
Mercury	planet	0.383	0.055	0.4	0
Venus	planet	0.950	0.815	0.7	0
Earth	planet	1.0	1.0	1.0	1
Moon	Earth satellite	0.27	1/81.3	*	0
Mars	planet	0.53	0.107	1.5	2
Cereso	dwarf planet in asteroid belt	0.074	1.5x10-4	2.77	0
Jupiter	planet	10.7	317.8**	5.2	63***
Saturn	planet	9.0	95.15	9.5	60+3?****
Uranus	planet	4.0	14.54	19.6	27
Neptune	planet	3.882	17.15	30	13
Pluto	dwarf planet	0.19	0.002	39	5
Charonoo	Pluto satellite	0.095	2.5x10-4	39	0
Haumea	dwarf planet	0.12	6.77x10-4	43	2
Makemake	dwarf planet	0.11	4.92x10-4	46	2
Eris--	dwarf planet	0.183	0.0028	68	1
Sedna	dwarf planet	0.078	0.0002	4845*	0


NB: ER, Earth radius = 6371 km; EM, Earth mass = 5.974x10²⁴ kg; AU, astronomical unit = 1.4960x10⁸ km
* distance to Earth: 3.844x10⁵ km
** this is 2.5 times the mass of all the other planets taken together
*** Ganymede, Callisto, Io and Europa are similar to terrestrial planets; Ganymede is larger than Mercury
**** Titan (largely made of ice) is the only satellite with an atmosphere and is larger than Mercury
^oCeres was discovered on 1 January, 1801, 45 years before Netpune. It was considered a planet for half a century before reclassification as an asteroid.
^oo Charon is currently considered Pluto's satellite but Pluto/Charon are really a binary dwarf planet system
-- Eris is larger slightly smaller and but more massive than Pluto
^5*Sedna's orbit is highly elliptical, with the perihelion (closest to the Sun) at 76 AU and the aphelion (farthest away from the SUN) at 937 AU




DID YOU KNOW? The atmosphere of Jupiter is really hot. At mid-latitudes, you would find a temperature of 530°C, hotter than the surface of Venus which is "only" 460°C. At such temps, the rocks on the surface start to glow reddish. If the Sun were the only source of heat, Jupiter's temps would be a chilling -70°. Scientists now think that strong storms could provide that extra heat (through sound wave generated by turbulences in the flowing air). An article in Science News on 20 Aug 2016 mentions that temps in Jupiter's Great Red Spot are a whopping 1,300°C. At such temperatures, all rocks on Earth would be molten.




• The Sun-Earth-Moon system and Earth Tides

• gravitational forces: Moon and Earth attract each other, so does Sun and Earth
• Moon and Sun pulling on Earth cause Earth tides
• Moon's position relative to Earth and Sun determine the phases of the Moon:
  • New Moon: Sun-Moon-Earth as viewed from above
  • Quarter or Waxing Quarter Moon: Sun-Earth-Moon make right angle
  • Full Moon: Sun-Earth-Moon as viewed from above Earth's orbit around the Sun
  • Three-Quarter or Waning Quarter Moon: Sun-Earth-Moon make right angle, but is on other side
• eclipses as special cases: lunar and solar eclipses occur when Sun, Earth, Moon are perfectly aligned. During a lunar eclipse, Earth is between Sun and Moon casting its full shadow on the Moon. During a solar eclipse, Moon is between Sun and Earth obstructing the view to the sun from Earth. Eclipses do not happen every month because Moon's orbit around Earth is tilted with respect to Earth's orbit around the Sun. I.e., Moon's orbit around the Earth is no fully located within Earth's ecliptic (Earth's orbital plane around the Sun).
• high and low tides: time scale: diurnal, typically twice daily; because Earth is spinning around it's own axis; twice daily high and low tides in most places on Earth, once daily in some places
• tidal range: difference in sea level between high and low tide
• spring and neap tides: time scale: monthly, usually (and at least) twice a month; when tidal range is largest; 2 spring tides (Full and New Moon); 2 neap tides (Waxing and Waning Quarter Moons)
• Blue Moon: another Full Moon in addition to the regular Full Moon; e.g. a second Full Moon in a month, or the 4th in a season, or the 13th in a year; in these cases, there is an additional spring tide
• king tides: not a scientific term but often used for particularly large tidal range, e.g. twice a year (June, December), when Earth is particularly close to (perihelion) (December/January) or far from the Sun (aphelion) (June/July). Most sources discuss only the larger of the two King tides which occurs during perihelion.
  NB, as of 9/29/2017, the Wikipedia entry on this, claiming that the king tide is reduced in July is wrong! The point that is wrong is that the gravitational force is reduced not the tidal range as claimed! The claim is based on a NOAA education page which is also wrong! Our video 3b is also confusing in this respect and needs re-editing. It says there that particularly large tidal ranges occur twice a year and implies relation to perigee with concurrent perihelion but the latter occurs only once a year.


trouble remembering from high school? download a printable crash course on tides


2016

2017

2018

2019

Three examples of a monthly tide calendar for the Scripps Pier

• Dwarf Planets

• definition of a dwarf planet
• has own orbit around Sun
• has sufficient mass to assume nearly round shape
• is not a satellite of a planet or other nonstellar body
• has not "cleared the neighborhood" around its orbit of planetesimals
The only difference between the definition of a dwarf planet and a real planet is that real planets have "cleared the neighborhood" around its orbit, meaning that they have "scooped" up all the debris along its orbit as a result of its large mass and gravitational force.


• Pluto
• a dwarf planet, not a planet
• discovered relatively late: on 13 February 1930 by Clyde W. Tombaugh
• Pluto is one of the trans-Neptunian objects (TNOs), meaning they lie beyond Neptune
• Pluto is relatively small (less than half the diameter of Mercury; only 2/3 diameter of Moon); smaller than other celestial objects found in the solar system
• Pluto's orbit is highly elliptical and inclined with respect to all the other planets' orbits
• Pluto has a very large moon, Charon, whose diameter is about half that of Pluto
• Pluto was found to be too unusual to fit the typical 'planet of the solar system'
• used to be one of the 9 planets but was downgraded to a dwarf planet in August 2006 during a conference of the IAU (International Astronomical Union) in Prague, Czech Republic
• since then, some astronomers continue to make the case to 'up-grade' Pluto back to a planet

Pluto is now a member of the dwarf planets. Other examples are Ceres in the asteroid belt and Eris (formerly known as Xena), a TNO out farther than Pluto but larger and whose discovery was ultimately responsible to downgrade Pluto to a dwarf planet. Read about Pluto on wikipedia



DID YOU KNOW? Pluto actually has 5 moons, but the 4 smaller ones, Hydra, Nix, Styx and Kerberos, are tiny compared to Charon. Hydra, the largest of the four, is only 55 km across. Styx and Kerberos, the two smallest ones are only 12 km across.



• Current list of dwarf planets as of 2022, the IAU recognizes nine dwarf planets in the Solar System: Ceres (discovered in 1801), Pluto (1930), Haumea (2004), Makemake (2005), Eris (2005), Gonggong (2007), Quaoar (2002), Orcus (2004), Sedna (2003). The first five were declared dwarf planet in 2011. Only Ceres and Pluto have been observed in enough detail to demonstrate that they meet the definition of a dwarf planet. Eris has been accepted because it is more massive (by 27%) than Pluto. The two trans-Neptunian objects Haumea (rather egg-shaped, having 2 moons) and Makemake currently qualify because their brightness of +1 or larger suggests that their diameter is 838 km or larger). However, hundreds more objects are proposed for consideration.
  Ceres is the only dwarf planet in the Asteroid Belt.


• Pluto's Fall Continues: Pluto's struggle for status isn't over. Charon is currently considered Pluto's satellite but it does not really orbit it. Due to the size of Charon relative to Pluto, the center of mass for the Pluto-Charon system (which orbits the Sun) lies outside of Pluto. One could argue that Pluto does not have its own orbit around the Sun (one of the criteria to be a dwarf planet). There is therefore the suggestion that Pluto and Charon should be re-classified together to be a dual, or binary dwarf planet system.



NOW READ THIS? Science News, Jun 23, 2018(3): Images from the spacecraft New Horizon's flyby in 2015 shows structures along the edges of Pluto's heart-shaped region that resemble sand dunes. On Earth, sand dunes form when winds pick up, transport and subsequently drop loose grains. Pluto's atmosphere is thought to be too thin for winds to pick up particles. But they can transport methane ice particles that sublimate from puffs of air that rise from the sun-lit ground. Ultimately, this could lead to ice dune formation. Keep reading here.

DID YOU KNOW? As of August 2021, scientists are still debating the fate of Pluto. A Science News article(2) features Pluto's discovery, its unique place in the solar system, and its demise as a planet and continuing debate. Keep reading here.




• Asteroids and the Asteroid Belt

• located between Mars and Jupiter
• belt where there should be another planet, according to Bode's Law
• scientists believe that either an impactor destroyed a planet initially formed there or that, due to the tug of Jupiter, a planet could never form
• most objects too small to be rounded
• 3 objects larger than 500 km in diameter (Ceres, Pallas, Vesta)
• some of the larger objects are planetesimals or dwarf planets, with Ceres being the largest at about 1000 km in diameter
• some have Earth-crossing orbit (Apollos) -> past and future impactors on Earth
• some have Mars-crossing orbit (Amors)
• asteroids are low-density, rocky objects
• Ceres: largest object in the Asteroid Belt; one of the five currently accepted dwarf planets; composed of rocky core and icy mantle; thought to carry 1/3 of mass of entire Asteroid Belt; organic compounds (tholins) detected in Feb 2017; was actually considered a planet from 1801 until the 1860s

NB: Neptune was discovered in 1846 and both Neptune and Pluto are not located where Bode's Law would place them. check out wikipedia.



• Trojans and Greeks

• small bodies located along the orbit of a planet
• gravitational force of the sun and planet on the body are in balance (Lagrangian point)
• two Lagrangian points at 60°, as viewed from the Sun
• one 'in front of' planet (L4) and one 'behind' planet (L5) along its orbit
• L4 bodies are Greeks; L5 bodies are Trojans
• since gravitational forces are in balance, small bodies a stable along the orbit (e.g., do not crash into the planet)
• trojans can be found along orbits of all planets in Solar System but Jupiter has most
• probably >1 M Jupiter trojans larger than 1 km across; 7000 cataloged
• Mars has only 9 trojans, Neptune 28, Uranus 2, Earth 2, Venus 1
• moons can also have trojans though they are much smaller; all of the currently known ones belong to moons of Saturn
• even asteroids Ceres and Vesta have trojans



• Comets

• have rocky core (a few km across) embedded in a shell of ice and dust ("dirty snowball")
• volatile ices consist of water, carbon dioxide, ammonia
• upon approach to sun, solar energy causes glow and ejections of icy material
• left as debris to make for bright meteor showers when Earth approaches debris on its orbit (e.g. Perseid Meteor Showers in August)
• the core of some comets are large enough to pose a serious impactor risk (e.g. Shoemaker-Levy 9 comet that hit Jupiter in 1994) view wikipedia page
• In July 2009, another comet impacted on Jupiter see newsclip
• as of Nov 2021, there are 4584 known comets
• actual number is estimated to be one trillion

NOW READ THIS? TIME Magazine, Nov 9, 2015: A comet that passed through our solar system this year was shown to contain alcohol and sugar, making it the first with those organic molecules. The so-called happy-hour comet won't be a regular, however; researchers say it won't return for 8000 years.




• Two tails of a comet:
  tails form when volatile particles (light elements or gases) of the comet's core are hit by solar radiation and vaporize.
  • whitish, more visible, dust tail
  • blueish, gas tail
• Kuiper belt:
  • on the plane of the solar system (ecliptic) beyond Neptune
  • extends out to 55 AU (1AU=Earth-Sun distance)
  • source of short-period comets (recurrence < 200 yrs)
  An example:
  • e.g. Halley's comet comes about every 75 yrs though Halley's comet originally may have come from the Oort cloud, last appearance/apparition: 1986)


• Oort cloud:
  • vast, diffuse spherical cloud around the solar system
  • extends out to 50,000 AU (1 light-year)
  • source of long-period comets (recurrence > 200 yrs)
  • period of comets change after encounter with gravitational pull of giant planets
  
  
• Recent Great Comets:
  • 1973: Kohoutek (not actually listed as one of the great comets) last came 150,000 yrs ago, will come again in 75,000 yrs; its rocky composition suggests that it was originally from the Kuiper belt; hailed as the 'comet of the century', its showing was disappointing, perhaps due partial disintegration of the core
  • 1976: West; discovered about 6 months before reacing perihelion; largely unreported in press even though it was one of the brightest objects in 1976 to pass through the inner solar system (perhaps because Kohoutek was disappointing); chaotic orbit, so its recurrence is unknown
  • 1986: Halley; short-period comet returning every 75.3 yrs; recorded by astronomers since at least 240 BC; will reappear in 2061;
  • 1996: Hyakutake used to come every 17,000 yrs but now comes every 70,000 yrs
  • 1997: Hale-Bopp last came 4,200 yrs ago, will come again in 2,533 yrs; orbit almost perpendicular to the ecliptic, so collision unlikely
  • 2007: McNaught; visible only in southern hemisphere; brightest comet in over 40 years; discovered 5 months before reaching perihelion; will not come back in a very long time (> 6 Mio yrs)
  • 2011: C/2011 or Lovejoy: discovered a few weeks before reaching perihelion; visible mostly in southern hemisphere; entered Sun's corona 12/16; core disintegrated 4 days later; its orbital period is 622 years so it may come back in 2633 if some piece of the core survived
  • 2013: C/2013 R1 (also called Lovejoy): visible to the naked eye; long-period comet;
  • 2020: NEOWISE: visible to the naked eye throughout July; brightest comet in the northern hemisphere since Hale-Bopp; long-period comet; discovered March 2020 by the Wide-field Infrared Survey Explorer (WISE) space telescope that is supposed to find near-Earth objects (NEOs), objects that could potentially collide with Earth;



• Meteoroids, Meteorites and Meteors

• meteoroid: fragment of asteroid or comet entering Earth's atmosphere
• meteor: visual phenomenon when meteoroid burns up in Earth's atmosphere
• meteorite: remaining piece(s) of meteoroid after it impacts on Earth's surface;
  there are two types
  iron/metallic meteorites are thought to resemble Earth's core
  stony/rocky meteorites are thought to resemble Earth's mantle; rarer than iron meteorites



DID YOU KNOW? 'Shooting stars' are not actual stars. Shooting stars come in different colors and sizes. They occur when dust from the tail of a comet that passed Earth's orbit long ago burns up in the atmosphere as Earth passes the dust. So in a way shooting stars are the little cousins of meteors. 'Meteor showers' occur when Earth passes through particularly dense clusters of left-over comet dust. The Perseids from mid-July through end of August are associated with dust from comet Swift-Tuttle. The Leonids occur in November and are associated with dust from comet Tempel-Tuttle.




• References

• ⁽¹⁾ Temming, M., "A space rock called Kamo'oalewa may be a piece of the moon" 2021, Science News, 18 December issue; published online 22 November; link here
• ⁽²⁾ Grossman, L., "Pluto's Place - The definition of planet is still a sore point" 2021, Science News, 28 August issue; published online 24 August; link here
• ⁽³⁾ Grossman, L., "Never-before-seen dunes on Pluto spotted in New Horizons images" 2018, Science News, 23 June issue; published online 31 May; link here

• Additional Resources

study guide to book chapter 3
lecture slides #1 (available AFTER the lecture!)
not-for-credit online quiz for practice testing (do not request edit access!)

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• Seven Random Review Questions (not a complete review!)

1) What is the approximate age of the universe?
1. 1,400 years
2. 1.4 million years
3. 1.4 billion years
4. 14 billion years
2) Which of these is NOT a terrestrial planet?
1. Mercury
2. Mars
3. Pluto
4. Earth
3) Which of these are typically larger?
1. gas giants
2. terrestrial planets
3. both about the same size
4) The Sun has more than 90% of the mass of the solar system.
1. true
2. false
5) What is the tidal range?
1. the distance between Moon and Earth
2. the difference in sea level between high and low tide
3. the number of days between high and low tide
4. the attraction of Moon and Sun
6) When do spring tides occur?
1. during quarter moons
2. during full moons
3. during new moons
4. all of the above
5. b) and c)
7) Which of these is the source of short-period comets?
1. Andromeda Galaxy
2. Asteroid Belt
3. Kuiper Belt
4. Oort Cloud
8) Which of these are most closely related to shooting stars?
1. meteorite
2. meteor
3. meteoroid
4. all of the above
9) What is so special about comets?
1. glowing tails when they approach the Sun
2. could impact on Earth
3. may have brought water to Earth
4. all of the above
5. none of the above


Answers: 1d; 2c; 3a; 4a; 5b; 6e; 7c; 8b; 9d