The Solar System is currently recognized to contain eight major planets. By physical structure and chemical composition they fall into two broad classes: the inner-orbiting, rock- and metal-dominated, small but high-density terrestrial planets—Mercury, Venus, Earth, Mars; and the outer-orbiting, huge but low-density giant planets composed mainly of hydrogen and helium gas or of water, ammonia, and methane ices—among which Jupiter and Saturn are gas giants and Uranus and Neptune are ice giants. This page focuses on the classification, geometric configurations, and observing highlights of each planet as they relate to observation and astrophotography; for coordinate and orientation concepts, see Celestial Coordinate Systems and Apparent Motion.
The two classes differ markedly in composition, density, moons, and ring systems. The table below gives a qualitative comparison; specific values appear in the physical-parameter table further on.
| Characteristic | Terrestrial planets | Giant planets |
|---|---|---|
| Examples | Mercury, Venus, Earth, Mars | Jupiter, Saturn, Uranus, Neptune |
| Main composition | Silicate rock and iron-nickel metal | Hydrogen-helium gas (Jupiter, Saturn); water, ammonia, and methane ices (Uranus, Neptune) |
| Mean density | About 3900–5500 kg/m³ | About 690–1640 kg/m³ |
| Surface | A solid rocky surface, possibly with mountains, canyons, and impact craters | No solid surface; a small dense core wrapped in a thick gas layer |
| Moons | Few: Earth 1, Mars 2; Mercury and Venus none | Numerous; each has dozens of known moons |
| Planetary rings | None | All four have rings, but only Saturn’s are easily seen from Earth |
| Rotation period | Slower (about 24 hours to tens of days) | Very fast (about 10–17 hours) |
Saturn’s mean density is about 687 kg/m³, lower than that of water (1000 kg/m³), making it the least dense planet; Jupiter’s mean density is about 1326 kg/m³. The terrestrial planets generally have densities more than 4–5 times that of water, owing to their large content of iron and rock.
The data in the table below are compiled from the NASA Planetary Fact Sheet (values rounded by convention). Diameter is the equatorial diameter; distance from the Sun is the orbital semi-major axis; a negative sign in the sidereal rotation period indicates that the spin direction is opposite to the orbital direction (retrograde rotation).
| Planet | Equatorial diameter (km) | Mass (Earth = 1) | Mean density (kg/m³) | Distance from Sun (AU) | Orbital period | Rotation period | Known moons |
|---|---|---|---|---|---|---|---|
| Mercury | 4879 | 0.055 | 5429 | 0.39 | 88.0 days | 58.6 days | 0 |
| Venus | 12104 | 0.815 | 5243 | 0.72 | 224.7 days | −243 days | 0 |
| Earth | 12756 | 1.000 | 5514 | 1.00 | 365.2 days | 23.93 h | 1 |
| Mars | 6792 | 0.107 | 3934 | 1.52 | 687 days | 24.62 h | 2 |
| Jupiter | 142984 | 317.8 | 1326 | 5.20 | 11.86 years | 9.93 h | 90+ |
| Saturn | 120536 | 95.2 | 687 | 9.58 | 29.46 years | 10.66 h | 140+ |
| Uranus | 51118 | 14.5 | 1270 | 19.19 | 84.0 years | −17.24 h | 27+ |
| Neptune | 49528 | 17.1 | 1638 | 30.07 | 164.8 years | 16.11 h | 14+ |
Mercury and Venus orbit inside Earth’s orbit and are called inferior planets. As seen from Earth, they never stray far from the Sun and can appear only in the eastern sky before sunrise (morning star) or in the western sky after sunset (evening star); they cannot culminate at midnight.
Greatest elongation: the moment when a planet’s angular separation from the Sun (its elongation) reaches a maximum—the best time to observe an inferior planet.
The elongation of an inferior planet is limited by its orbital radius:
| Planet | Maximum elongation (range) | Visibility window |
|---|---|---|
| Mercury | About 18°–28° | Briefly visible at dawn/dusk around greatest elongation; its large orbital eccentricity causes large variations in elongation |
| Venus | About 45°–47° | Visible for several hours around greatest elongation; one of the easiest planets to observe |
Inferior planets also have two conjunction configurations: inferior conjunction, when the planet passes between Earth and the Sun; and superior conjunction, when the planet passes behind the Sun. In both conjunctions the planet is lost in sunlight and is difficult to observe.
Like the Moon, inferior planets display waxing and waning phases. This was key evidence used by Galileo in 1610 when he observed the phases of Venus to support the heliocentric model.
The apparent diameter of Venus varies between about 10″ (near superior conjunction) and about 66″ (near inferior conjunction), a difference of more than sixfold. Because the “illuminated fraction” and the “apparent size” trade off against each other, the peak brightness of Venus occurs in the slender-crescent stage at about 25% phase, not at full phase.
When an inferior planet reaches inferior conjunction exactly over the Sun’s apparent disk, it crosses the solar disk as a small black dot, an event called a transit.
Mars and the planets farther out orbit beyond Earth’s orbit and are called superior planets. Their observing rhythm is governed by opposition and conjunction, and their elongation can take any value between 0° and 180°.
Superior planets also show phases, but because of their great distance and small phase angle the deficiency is very slight (it is most noticeable for Mars, reaching about 12%).
A planet usually moves from west to east relative to the background stars (prograde motion). When Earth, on its inner orbit, “catches up to and overtakes” a superior planet, that planet temporarily appears to move from east to west in retrograde motion. Retrograde motion is not the planet actually moving backward but a visual effect produced by the relative motion of Earth and the planet.
Before and after retrograde, the point at which the planet’s direction of motion reverses appears stationary and is called a stationary point (station). The full sequence is: prograde → station (prograde to retrograde) → retrograde → station (retrograde to prograde) → prograde. The midpoint of a superior planet’s retrograde occurs exactly at the moment of opposition; the retrograde of an inferior planet occurs around inferior conjunction. The farther a planet is, the shorter its retrograde lasts and the smaller its retrograde arc.
The synodic period is the time a planet takes to return to the same configuration relative to the Sun (e.g., between two successive oppositions, or two successive conjunctions of the same kind). Its relationship to the planet’s sidereal period and Earth’s orbital period E (about 365.26 days) is:
Superior planet: 1/S = 1/E − 1/PInferior planet: 1/S = 1/P − 1/Ewhere S is the synodic period and P is the planet’s sidereal orbital period. The table below gives measured synodic periods for each planet:
| Planet | Synodic period | Notes |
|---|---|---|
| Mercury | About 116 days | Multiple greatest elongations per year |
| Venus | About 584 days | About one cycle every 1.6 years; completes 5 synodic periods in 8 years |
| Mars | About 780 days | An opposition roughly every 26 months; the most dramatic variation |
| Jupiter | About 399 days | An opposition roughly every 13 months |
| Saturn | About 378 days | An opposition roughly every 12.5 months |
| Uranus | About 370 days | Nearly one opposition per year |
| Neptune | About 367 days | Nearly one opposition per year, owing to its extremely slow orbital motion |
The farther a superior planet is from the Sun, the slower it orbits and the closer its synodic period is to Earth’s one year, so planets beyond Jupiter have a favorable observing window near opposition almost every year.
Mercury
The hardest planet to observe, visible only briefly low in the dawn or dusk sky around greatest elongation. A telescope shows Moon-like phase changes, but surface detail is nearly impossible to resolve. The rare transits of Mercury require observation with a safe solar filter.
Venus
The brightest planet, showing a complete cycle of waxing and waning phases. Near inferior conjunction its disk is large and crescent-thin; near superior conjunction it is small and round. Its surface is covered by thick clouds, so no surface detail is visible through a telescope.
Opposition of Mars
Around opposition it is red and bright, and one can make out the white polar caps (composed of water ice and dry ice), dark surface markings, and occasional dust storms. An opposition that also falls near perihelion is called a perihelic (great) opposition, when the apparent disk is largest—occurring about once every 15–17 years.
Jupiter
A telescope shows alternating light and dark cloud bands (belts and zones) and the Great Red Spot, a giant anticyclonic storm that has persisted for centuries; the four Galilean moons change their arrangement night to night and are visible even in binoculars.
Saturn
The most popular eyepiece target. The rings of Saturn and the Cassini Division within them are clearly visible in a medium-aperture telescope; the ring tilt changes from year to year, presenting an edge-on view about once every 15 years.
Uranus / Neptune
Uranus is about magnitude 5.7 and barely visible to the naked eye under excellent conditions; Neptune is about magnitude 7.8 and requires a telescope. In a telescope both appear as tiny blue-green/blue disks and must be located by chart coordinates.
Mars shows the largest variation in apparent diameter and brightness of all the planets: its apparent diameter ranges from about 3.5″ (near conjunction, far from Earth) to about 25″ (near perihelic opposition, close to Earth), and its brightness ranges from about magnitude +1.8 to about −2.9 (a brightness difference of about 75 times). The period around opposition is the only good opportunity to make out the polar caps and dark surface patches, usually requiring a magnification above 50×.
Jupiter is the second-brightest planet in the night sky (about magnitude −2.9 at opposition), with an apparent diameter of about 30″–50″, and it is the planet in which detail is easiest to see through a telescope. The Great Red Spot is about 1.3 Earth diameters wide and has been shrinking steadily over the past century. The Galilean moons are Io, Europa, Ganymede, and Callisto; at opposition they are about magnitude 4.6–5.6, arranged from inner to outer, and change their relative positions night to night. They can be resolved with binoculars, and one can often watch their transits across Jupiter, their shadow transits, and their occultations by Jupiter.
Saturn’s rings consist mainly of the A, B, and C rings. The Cassini Division, about 4500 km wide, between the A and B rings was cleared by orbital resonance with the moon Mimas and was discovered by Cassini in 1675. Saturn’s brightness is about magnitude −0.5 to +1, with an apparent diameter (of the body) of about 16″–20″, reaching about 40″ overall including the rings. The tilt of the ring plane relative to Earth varies cyclically over Saturn’s 29.5-year orbit: at maximum opening the rings spread wide like a disk, while about once every 15 years they appear nearly edge-on, when they thin to a bright line or even briefly vanish.
Saturn’s rings and Jupiter’s cloud bands and Great Red Spot are the two most classic targets in planetary photography:
Unlike the stars, planets do not have a constant brightness; their apparent diameter and brightness change markedly with the relative positions of the Sun, Earth, and planet. The table below gives approximate brightness ranges and maximum apparent diameters (for observing-expectation reference).
| Planet | Brightness range (apparent magnitude) | Maximum apparent diameter | When brightest/largest |
|---|---|---|---|
| Mercury | About −2.5 to +5 | About 13″ | Brightest near superior conjunction but hard to see; largest disk near inferior conjunction |
| Venus | About −3.9 to −4.9 | About 66″ | A slender crescent around inferior conjunction, peaking at about 25% phase |
| Mars | About −2.9 to +1.8 | About 25″ | Near perihelic opposition |
| Jupiter | About −2.9 to −1.6 | About 50″ | Near opposition |
| Saturn | About −0.5 to +1.5 | About 20″ (body) | At opposition with the rings widely open |
| Uranus | About +5.4 to +5.9 | About 4″ | Near opposition |
| Neptune | About +7.8 to +8.0 | About 2.4″ | Near opposition |
Observing conditions (atmospheric seeing, transparency, and altitude above the horizon) are equally critical to whether these details can be resolved; see Observing Conditions. To judge how high a given planet rises at your latitude and whether it is visible all night, see Hemisphere Visibility.
All the planets travel near the ecliptic, so they frequently “meet” the Moon or one another in the sky.
Planetary positions change with the date; the specific dates of opposition, greatest elongation, and conjunction with the Moon should be looked up in the current year’s astronomical almanac or in ephemeris software. For a quick reference of catalogs and terms, see the Object Catalog and the Glossary.
To capture planetary detail with a telescope, planetary photography uses the “short exposures, vast numbers of frames, stack and sharpen” approach (lucky imaging), which is entirely different from the long-exposure approach used for deep-sky objects.
To learn more about the other members of the Solar System, head to the Sun, the Moon, and Small Bodies.