Classical planet

Lore & Background

The Babylonians recognized seven planets, recorded in a bilingual list in the British Museum in the order: Moon (Sin), Sun (Shamash), Jupiter (Merodach), Venus (Ishtar), Saturn (Ninip), Mercury (Nebo), and Mars (Nergal). In Mandaeism, the seven classical planets are called 'šuba šibiahia' and are generally not viewed favorably, as they form part of the entourage of Ruha, Queen of the World of Darkness, though some planets can be associated with positive qualities. Their names in Mandaic are borrowed from Akkadian, ultimately derived from Sumerian.

The astrological symbols for the classical planets appear in medieval Byzantine codices. In original Greek horoscope papyri, the Sun is represented by a circle with one ray and the Moon by a crescent. Symbols for Mercury, Venus, Jupiter, and Saturn trace to late Greek papyri, with Jupiter and Saturn as monograms of Greek names and Mercury as a stylized caduceus. Bianchini's planisphere from the 2nd century shows Greek personifications of planetary gods with early versions of the symbols. The modern Sun symbol (circle with a dot) first appeared in the Renaissance.

The Ptolemaic system placed the planets in geocentric order from closest to furthest: Moon, Mercury, Venus, Sun, Mars, Jupiter, Saturn. The day was divided into seven-hour intervals, each ruled by a planet, and the first hour of each day named after the ruling planet gave rise to the Roman seven-day week. Modern English days of the week mostly inherited from Germanic Norse culture, with Wednesday as Wōden's-day (equivalent to Mercury), Thursday as Thor's-day (Jupiter), and Friday as Frige-day (Venus).

Reader's Guide

The classical planets have left a lasting legacy across multiple domains. In alchemy, each classical planet was associated with one of the seven known metals: the Sun ruled gold, the Moon silver, Mercury quicksilver, Venus copper, Mars iron, Jupiter tin, and Saturn lead. Alchemical glyphs for these metals coincide with the planetary symbols. Alchemy in the Western world was intertwined with Babylonian-Greek style astrology, both seeking hidden knowledge. Most modern astrologers still use the four classical elements extensively.

In contemporary astrology, Western and Indian traditions recognize the seven visible planets (including the Sun and Moon), with Indian astrology adding two invisible planets. Chinese astrology also recognizes seven visible planets, flourishing during the Han dynasty. The naked-eye visibility of these planets varies: Mercury and Venus are visible only in twilight due to their interior orbits; Venus is the third-brightest object in the sky. Mars is brightest at opposition every twenty-five months. Jupiter and Saturn are the largest but receive less sunlight; Jupiter is often the next brightest after Venus, and Saturn's luminosity is enhanced by its rings, though the rings are not visible to the naked eye.

Did You Know?

• The term 'planet' comes from the Greek word for 'wanderer'.
• The Babylonians recorded seven planets in a bilingual list now in the British Museum.
• The modern English name 'Saturday' retains a vestige of the Roman convention for naming days after planets.

A Slow-Motion Detection

Eris's discovery was nearly a casualty of its own glacial motion. In October 2003, Mike Brown, Chad Trujillo, and David Rabinowitz were conducting routine scans with the 1.2-meter Samuel Oschin Schmidt telescope at Palomar Observatory, California. Their automated image-searching pipeline was tuned to reject any object drifting slower than 1.5 arcseconds per hour, a threshold set to suppress false positives. Eris, moving even more slowly, simply vanished from their software's output. The breakthrough came indirectly: after Sedna was identified in 2003 at 1.75 arcsec/h, the team lowered their angular-motion cutoff and manually re-sorted the previously discarded frames. By January 2005, Eris's faint crawl against the background stars was finally visible. The public announcement landed on July 29, 2005, just two days after a Spanish team controversially announced Haumea, prompting the discovery group to release Eris and Makemake ahead of their preferred schedule. Precovery photographs now trace Eris's path back to September 3, 1954.

From Xena to the Goddess of Strife

For a stretch of time, the object most people recognized by a name lifted straight from a television series. "Xena" was an informal moniker the discovery team used internally, drawn from the warrior-princess character of the same name. Mike Brown later explained the logic: it began with X (for planet "X"), carried a mythological ring, and aligned with the team's desire to feature more female deities among their finds. The show was still on air at the time, which he joked was proof of how long they had been searching. The nickname escaped when a New York Times reporter who happened to be a college friend of Brown's pressed him in an interview, asking what the group called the object among themselves. Brown shared it once, and it spread across the press. Official naming had to wait until the IAU's August 24, 2006 ruling settled the object's classification. On September 6, 2006, the Caltech team proposed Eris, the Greek goddess of strife and discord, and it was formally assigned on September 13, ending a long stretch under the provisional label 2003 UB313.

The Crisis That Rewrote the Planetary Roster

Eris's emergence shattered the informal, uncodified notion of what counted as a planet. Because it initially appeared larger than Pluto, NASA publicly described it as the Solar System's tenth planet. The realistic possibility that additional bodies of similar scale might yet surface in the outer reaches made the existing framework untenable. The International Astronomical Union was forced to define the word "planet" for the very first time. The resulting definition, approved on August 24, 2006, reclassified Eris, Pluto, and Ceres as "dwarf planets," shrinking the recognized planetary count back to eight—the same number that had stood before Pluto's 1930 discovery. A 2010 stellar-occultation observation confirmed Eris was in fact slightly smaller than Pluto, with a diameter of 2,326 ± 12 kilometres versus Pluto's 2,377 ± 2 kilometres measured by New Horizons in July 2015. Nevertheless, Eris's mass exceeds Pluto's by roughly 27 percent, making it the most massive known dwarf planet despite its marginally smaller volume.

The Farthest Unvisited World

Eris carves a highly eccentric path through the scattered disk, a population of trans-Neptunian objects whose orbits stretch far beyond the classical planetary zone. In February 2016 it sat at 96.3 AU from the Sun—about 14.41 billion kilometres—more than triple the distance of either Neptune or Pluto. Paired with its confirmed satellite Dysnomia, Eris held the distinction of being the most distant known natural objects in the Solar System, excluding long-period comets, until 2018 AG37 and 2018 VG18 were identified in 2018. Despite ranking as the ninth-most massive body orbiting the Sun and the sixteenth-most massive object in the entire Solar System once moons are included, Eris remains the largest known body never touched by a spacecraft. Dysnomia's orbital observations let scientists pin down Eris's mass at (1.66 ± 0.02) × 10²² kg by June 2007, and there is ongoing speculation about a second, substantially larger moon. Both Eris and Pluto boast surface areas in the ballpark of Russia or South America.