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Planets
to appear in Encyclopedia of Time, Sage Publishing, in preparation, H.J. Birx (Ed.)
Astronomy as one of the oldest sciences of humankind always provided orientation in space and time: Cardinal directions (east, north, west, south) are defined and obtained by basic astronomical measurements. Time and calender issues are also definable and measurable by astronomical observations: One "year" is the period the Earth needs for one full revolution around the Sun (originally, before the Copernican resolution, it was seen the other way around), and one "month" is roughly the time our Moon needs to orbit the Earth. The currently most widely used calender system, the Christian calender in use in Europe, Northern and Southern Americas and many other parts of the Earth, is mainly based on the Sun. Other cultures have developed slightly different calenders based either on the moon (e.g. moslem calender) or a combination of Sun and moon (e.g. jewish).
We count "seven" days per week, because long time ago, people considered "seven" objects as "planets" or "planet­like objects", namely the real planets, which can be observed by naked eye (before the invention of the astronomical telescopes), i.e. Mercury, Venus, Mars, Jupiter, and Saturn, as well as the other two large visible bodies in the solar system, the Sun and our Moon, together "seven" objects, hence also the names of the seven days of the week:
"Sunday", as original first day of the week, is the day of the Sun, the brightest object in the sky, often even worshiped as god in several ancient cultures.
The word "monday" obviously refers to the "Moon".
"Tuesday" is named after Mars, the God of War (notice in french, italian, and spanish ­ the word for tuesday is still closed to the Roman God Martius (for Mars), namely mardi, martedi, and martes, respectively) and it originally comes from "Tiwes dag" or "Tyr dag", i.e. from the old teutonic word "Tyr" for "Mars".
"Wednesday" is named after the Roman God "Mercury" (in romanian, the day is still known as "miercuri"), and the word wednesday itself comes from "Wodan dag" for the teutonic god "Wodan".
In roman times, the fifth day of the week ("thursday") was known as "dies jovis", after their God of Thunder and chief of the Gods, "Jupiter", where "thursday" itself comes from "Thunor dag", i.e. the day of the teutonic god "Thor".
The Romans named another day after their goddess of beauty, "Venus", and called it "dies veneris" (still similar in french), when german tribes invaded England more than 500 years ago, they imposed their goddess upon that day and called it "frigedaeg", now "Friday".
And finally, "saturday" is obviously called after "Saturn".
Nowadays, time and its unit second are defined by the speed of light. Previously, a second was defined by the atomic clocks and earlier also as one certain small part of a day, i.e. a revolution of the Earth. Still, astronomical observations are important for fixing "time": Due to tidal interaction between Sun, Earth, and Moon, the rotation period of the Earth is very slowly slowing down.
The definition of "planet" has changed over the centuries, always following new astronomical observations and new understanding. The word "planet" comes from the greek word for "wanderer" meaning a wandering or fast moving star­like object, e.g. the old arabic name for the Egyptian captital Cairo is "Al Qahira" for "the backwards wandering" meaning Mars. As mentioned above, few hundred to three thousand years ago, people could see by naked eye seven objects apparently moving fast on the sky (compared to the "fixed stars"), incorrectly thought to orbit around the Earth in the center, namely Sun, Mercury, Venus, Moon, Mars, Jupiter, and Saturn; the next planet known today behind Saturn, called Uranus, is also visible to the naked eye during clear and dark nights, when Uranus is close to the Sun and the Earth roughly in between Sun and Uranus (a constellation called "opposition", when an outer planet like Uranus is brightest as seen from the inner planet like Earth), but no such reports are known so far, possibly because Uranus only moves slowly and is quite faint.
During the period of Reconnaissance in general and the co­called Copernican revolution specifically, it became clear through a number of new observations, that the old theory placing Earth into the center of the Universe, is not perfect. Those observations became possible with the invention of the astronomical telescope. In 1609, Galileo Galilei observed the phases on Venus, craters on our Moon, and discovered moons around Jupiter (first called "Mediceiscue planets" or "Mediceiscue stars" after his supporters, the italian Medice family, now known as "Galileian Moons"). All this together favored an alternative explanation putting the Sun into the center of our Solar System and having the planets orbiting around the Sun. At this moment, it became clear that also the Earth is orbiting the Sun and, hence, was now seen as "planet". Soon later, two more planets were discovered beyond Saturn, namely Uranus and Neptune.
While it was always possible to estimate the orbital periods of planets around the Sun by their periodic appearance and disappearance on the sky, it was originally difficult to measure distances between the planets, or from Earth to either its moon or the Sun. The distance between Earth and Sun is now called the Astronomical Unit being about 150 million kilometers. First good estimates of such distances were obtained a few centuries ago by observing eclipses of the Sun by the inner planets Venus and Mercury, which happen only very rarely (usually one or few per century): One has to measure exactly either the angular distance between the apparent path of the planet across the solar disk (as seen from two different locations on Earth), or the exact times if ingress and egress of the planet moving in front of the Sun, i.e. the four so­called contacts, to be observed and measured from different locations on Earth with as large as possible distance in between them, e.g. from the South Africa and Europe. Few centuries ago, it was still difficult to coordinate such efforts and also to run precise clocks. A first observation was done in 1639, a Venus transit. After several attempts, first good values for the distance between Earth and Sun were obtained in 1761 and 1769 ­ also giving evidence about the size of the Sun. Together with the laws of gravity just found by Newton and its application to the Solar System by Kepler, this immediately yielded all distances between each planets and the Sun.
Towards the end of the 18th century, the so­called Titius­Bode law was found and discussed: According to this "law", the distance from planet to planet roughly doubles, e.g. Saturn is roughly twice as distant from the Sun as Jupiter, Uranus is roughly twice as distant as Saturn, etc. However, from Mars to Jupiter, the distance increases roughly by a factor of 4, so that there would be space for one more planet. Even the famous philosopher Hegel wrote his dissertation about this problem at the University of Jena in Germany. Many astronomers were already hunting for this new planet. Then, in January 1801, an object was found at the expected distance from the Sun, called Ceres, and celebrated as new planet. However, soon later, more similar objects were found, all at a similar distance, few decades later, the Solar System had more than 20 "planets". It was also found that those new objects are smaller than all other previous planets, so that it was decided to call them "minor planets" as a new class of objects. Hence, objects celebrated and counted as planets were removed from the list of planets by a new definition.
Early in the 20th century, another new object was discovered and celebrated as new, ninth planet, called "Pluto", located most of the time beyond Neptune, but sometimes crossing its orbit.
In August 2006, the general assembly of the International Astronomical Union (recognized by the UN as international body to define and name celestial objects), among many other issues, discussed the definition of "planet" again. The definition was prompted by two new discoveries in the 1990ies: Minor bodies like Pluto were discovered near and beyond Pluto; planets around other stars, so­called "extra­
solar planets" or "exo­planets" were discovered, which were apparently different in many respect to the Solar System planets, so that a new definition seemed necessary. After lengthy heated debates, a new definition was confirmed by a majority vote. This definition reads as follows:
"The IAU resolves that planets and other bodies (except moons) in our Solar System be defined into three distinct categories in the following way:
A "planet" (note 1) is a celestial body that (a) is in orbit around the Sun,
(b) has sufficient mass for its self­gravity to overcome rigid body forces, so that it assumes a hydrostatic equilibrium (nearly round) shape (note 2), and
(c) has cleared the neighborhood around its orbit.
A "dwarf planet" is a celestial body that
(a) is in orbit around the Sun,
(b) has sufficient mass for its self­gravity to overcome rigid body forces, so that it assumes a hydrostatic equilibrium (nearly round) shape (note 2),
(c) has not cleared the neighborhood around its orbit, and
(d) is not a moon.
All other objects (note 3) ­ except moons ­ orbiting the Sun shall be
referred to collectively as "Small Solar System Bodies".
Note 1: The eight planets are Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus, and Neptune.
Note 2: An IAU process will be established to assign borderline objects into either dwarf planets or other categories.
Note 3: including asteroids, trans­neptunian objects, comets, and other small bodies.”
This mainly says, that objects which are (a) in orbit around the Sun (i.e. not being moons of planets), (b) have at least a certain mass (to be round by gravitational effects), and (c) have cleared their orbits by their own gravitational forces, are the "planets" of our Solar System. According to this definition, Pluto is not a planet (anymore). However, it was not the first time that an object was deleted from the list of planets (see above regarding Ceres and the other "minor planets"). This effect, however, is highly controversial and it is very well possible that the definition will be changed again soon.
There are now eight planets in the Solar System:
(1) Mercury, as innermost known planet, is the smallest known planet in our Solar System with less than 5000 km diameter (Pluto is smaller, but not a planet anymore according to the new definition). It has a rotation period of 59 days, which is quite exactly two thirds of its orbital period around the Sun (88 days), hence one "Mercury­day" is equal to two "Mercury­years". Mercury does not have an atmosphere which would be comparable to the Earth, and its surface is similar to the moon. Two thirds of its material and mass is made of iron. According to Einstein's general theory of relativity, the orbit of Mercury should change slowly: The location of the perihelion, the point of smallest distance between Sun and planet on the orbit, moves by a small angle of 43 seconds or arc per century, which has been confirmed observationally.
(2) Venus needs 225 days for orbiting the Sun (compared to 365 days for one Earth orbit around the Sun). The rotation of Venus around its own axis is retrograde, i.e. in the rotation direction opposite to the direction in which it revolves around the Sun, and one such "Venus­day" lasts 243 days, i.e. is longer than one "Venus­year". Venus has a dense atmosphere consisting mostly of CO2 and N2 with strong pressure on the surface, from where one would never be able to see the stars in the night sky through the dense clouds.
Mercury and Venus as planets inside the Earth orbit, i.e. orbit the Sun faster than the Earth and are often close to the Sun, as seen from earth. Hence, they are observable either in the evening sky just after sunset or in the morning sky just before sunrise; e.g. Venus is also called "morning star" or "evening stars". The Greek called Mercury as evening star "Hermes" and as morning star "Apollo" and Venus either "Hesperus" or "Phosphorus", respectively.
(3) The Earth is the third planet from the Sun, it needs 365 days for a complete orbit around the central star and 24 hours for one rotation. Its atmosphere consists mainly of O2 and N2. This planet is the only one known so far to harbor living beings like plants, animals, and intelligent life.
(4) The fourth planet is called Mars. It has an orbital period of 687 days and a rotation period of 24,6 hours, so that a "Mars­day" is only slightly longer than a day on Earth. Its thin atmosphere consists mostly of CO2 and N2, but not identical to Earth. There is frozen CO2­ and water­ice on the poles, but no fluid water is detected yet. However, some surface structures look like dry river beds and may indicate that fluid water was present some billions of years ago. It is not impossible that life has formed on Mars, too, but there is no clear evidence for life on Mars found, yet. Mars is orbited by two small moons, called Phobos and Deimos, with 8 and 30 hours orbital periods, respectively. Like the moon of the Earth, their rotation is bound: Their orbital period equals their rotational period; they are rotating around themselves only by orbiting their planet and always show the same side to their planet.
The innermost four planets are also called "terrestrial planets" as they are all made mostly of solid material like Earth ("terra"). Between Mars, the fourth planet, and Jupiter, the fifth planet, there is a larger gap, where many small bodies are orbiting the Sun, called minor planets ­ sometimes also called planetoids, because they are physically like terrestrial planets, namely rocky objects, or also asteroids, because ­ on the sky – they looked like stars, when they were discovered, namely point like (while the planets of our Solar System appear to be extended on the sky even in naked eye observations, because of their larger size and small distance).
The four outermost known planets Jupiter, Saturn, Uranus, and Neptune all are larger in size than the terrestrial planets, mostly because of their large atmospheres and only small (or in case if Jupiter maybe even no) solid or fluid core. Hence, they are called the "gaseous giant planets".
(5) Jupiter is the largest planet in our Solar System with a diameter of 143000 km and a mass of 318 times the mass of Earth. It needs almost 12 years for one orbit around the Sun, but only 10 hours for a rotation around itself, as can be observed with even a small telescope with the moving large red spot in the outer atmosphere. Given its diameter, mass, and composition (mostly molecular hydrogen), it is not absolutely clear whether it has solid or fluid core or possibly even no core, i.e. no solid surface. If it has a core, it could have a mass of a few or maybe 10 Earth masses. Due to contraction, Jupiter is still re­
radiating more energy to outer space than receiving from the Sun. This giant planet also has a small ring system and a large number of moons, probably a few dozen, new small moons are still being discovered. The four largest moons were original discovered by Galileo, when he observed Jupiter for the first time with a telescope. These four moons (Io, Europa, Ganymed, and Callisto) are called the "Galileian Satellites".
(6) Saturn has twice the distance to the Sun as compared to Jupiter. Saturn is mostly known for its large ring system. It also has a large number of moons. Saturn needs 29.5 years to orbit the Sun and rotation period of 10 hours and 40 minutes. It has a solid core of a few Earth masses and a large atmosphere made mostly of molecular hydrogen gas. Saturn is the second largest (120000 km diameter) and second most massive planet (95 times the mass of Earth) in our system.
All planets from Mercury to Saturn (including the Earth) were known since several thousand years to most cultures on Earth, since they can be observed by naked eye. The outermost planets Uranus and Neptune (as well as Pluto) were discovered after the invention of the telescope. While Jupiter and Saturn are called "gas giants", Uranus and Neptune can be seen as "ice giants".
(7) Uranus was discovered (and recognized as planet) in 1781 by Herschel. Other had observed it before, but did not recognize that it is a planet. Uranus is also a gaseous planet with central solid core, but in total only 15 times as massive as Earth. Uranus needs 84 years to circle around the Sun and one "day" on Uranus lasts around 17 hours. Uranus' atmosphere consists of 83% hydrogen, 15% helium, and 2% methane. So far, 21 moon are discovered (and counting). Uranus also has a small ring system as discovered by the Voyager satellites.
(8) Neptune is the outermost known (and accepted) planet. It was observed by Galileo in 1612, but not recognized as planet. Because of apparent deviations in the orbit of Uranus, both Adams and Leverrier predicted the existence of another planet theoretically and tried to forecast its rough location in the sky. Soon later, in 1846, the observer Galle in Berlin, Germany, searched that area on the sky for a small moving object and discovered Neptune within a few hours. Neptune needs 165 years for one full circle around the Sun. One "day" on Neptune lasts 16 hours. Neptune has a small solid core, a large gaseous atmosphere, mostly molecular hydrogen, and a total mass of 17 times the mass of Earth. Neptune like all gaseous planets in our Solar System has moons and rings.
The object Pluto was discovered in 1930, celebrated as new planet, but deleted from the list of planets in the 2006 definition of "planet" by the International Astronomical Union.
The new definition of "planet" is formulated for the Solar System, but can and should be applied analogously to other planetary systems around other stars. However, there is yet no consensus or definition for the upper mass limit of planets. Such an upper mass limit, however, would be very important for extra­solar planets, to be able to decide whether they are planets or so­called brown dwarfs.
In history, the last few definitions have always worked for about 200 years, one from the Copernican revolution to the discovery of Ceres and other minor planets (which now form the asteroid belt between Mars and Jupiter); the new definition excluding the minor planets was in effect again for about 200 years until 2006.
Both, the problem regarding Pluto and the missing upper mass limit for planets, may very well lead the a new definition in one of the next meeting of the International Astronomical Society, holding very three years a general assembly.
Prof. Dr. Ralph Neuhäuser
University of Jena
Schillergässchen 2
D­07745 Jena
Germany
for further reading:
Karttunen H., Kroeger P., Oja H. (2007).
Fundamental Astronomy, Springer
Corfield R. (2007).
Lives of the Planets: A Natural History of the Solar System, Basic Books
Encrenaz T., Bibring K.­P., Blanc M. (2002).
The Solar System (Astronomy and Astrophysics Library), Springer
Maunder M., Moore P. (1999):
Transit: When Planets Cross the Sun (Practical Astronomy), Springer