Eris

Eris

I

INTRODUCTION

Eris, dwarf planet orbiting the Sun in the outer solar system beyond Neptune. Like Pluto, Eris is a spherical Kuiper Belt Object (KBO) made of ice and rock. Its estimated diameter of 2,400 km (1,490 mi) makes Eris slightly bigger than Pluto and the largest known KBO. Eris completes one orbit in 560 years and is currently the most distant known body in the solar system at 14.5 billion km (9 billion mi) from the Sun. Its orbit is much more eccentric and tilted than that of Pluto. Eris has a small moon named Dysnomia.

All the information scientists have about Eris comes from observations made with telescopes, and some facts are not firmly established yet, including its rotation period and its axis tilt. The dwarf planet was discovered in 2005 after astronomers restudied data recorded in 2003. Eris is named for the Greek goddess of discord.

II

ORBIT AND COMPOSITION

Eris orbits at an average distance of 67 AU or 10 billion km (6.3 billion mi) from the Sun. (AU stands for astronomical unit, the average distance from the Earth to the Sun). However, its orbit is so elliptical that Eris’s distance from the Sun can range from its present far position (aphelion) of about 97 AU—14.5 billion km (9 billion mi)—to a near point (perihelion) of 38 AU—5.7 billion km (3.5 billion mi)—an event that will next happen in 2257. At its nearest point, Eris passes inside the orbit of Pluto, but beyond the orbit of Neptune.

The orbit of Eris is also tilted much more steeply than Pluto’s relative to the plane of the ecliptic (the plane in which Earth orbits the Sun, roughly the main plane of the solar system). Eris’s orbit is inclined 44° to the ecliptic, while Pluto's orbit is inclined 17.2°. Like Pluto, Eris likely formed in the disk of the Kuiper Belt, which lies in the plane of the ecliptic. The gravitational influence of the planet Neptune on the Kuiper Belt is thought to have affected the orbit of Pluto. Neptune may also have disturbed the original path of Eris early in the history of the solar system, sending the dwarf planet into its steep orbit.

In addition to being a dwarf planet and a KBO, Eris is considered a scattered disk object (SDO) because of its highly inclined orbit. Scattered disk objects are bodies that orbit out of the main plane of the solar system and form part of the population of trans-Neptunian objects (TNOs) that exist at the outer edge of the solar system beyond Neptune.

Observations made with the Hubble Space Telescope indicate that Eris is similar to Pluto in composition. Both bodies likely have a rocky core surrounded by a mantle of ice and a surface covered by frozen methane. However, Eris reflects about 86 percent of the sunlight that falls on its surface compared to 60 percent for Pluto. Scientists originally overestimated the size of Eris because they assumed it reflected about the same amount of light as Pluto does.

Why Eris is so bright is currently not understood. One theory is that Eris has an atmosphere of nitrogen and methane, as does Pluto, but unlike Pluto the atmosphere of Eris is currently frozen in a uniform whitish layer that covers its surface. At its aphelion, the temperature on Eris is about –243°C (–406°F), colder than Pluto’s aphelion. When Eris reaches its perihelion, it will warm to around –218°C (–360°F), possibly enough for frozen nitrogen and methane to change into gases and create a thin atmosphere.

All data about Eris's small moon Dysnomia are preliminary. Dysnomia is thought to be about 250 km (155 mi) in diameter. It apparently takes 14 days to orbit Eris at an estimated distance of 36,000 km (22,320 mi). Its surface is much darker than that of Eris and its shape may be irregular.

III

DISCOVERY AND CONTROVERSY

Eris was first detected in October 2003 as part of a search for distant solar system objects funded by the National Aeronautics and Space Administration (NASA). The search was done using a telescope at the Palomar Observatory in California. The telescope operated robotically and scanned large regions of the sky. Electronic images gathered by a camera linked to the telescope were then analyzed by computers that compared images of the same part of the sky at different times. If a point of light shifted against the background of stars, it could indicate a distant object in orbit around the Sun.

Scientists did not notice one particular slow-moving new object in their data until they did a reanalysis in January 2005. The astronomers who made the discovery—Michael Brown of the California Institute of Technology, Chad Trujillo of Gemini Observatory, and David Rabinowitz of Yale University—gave the distant body the temporary name 2003 UB313. After more study, they were able to find photographic images of the previously unnoticed object taken as far back as the 1950s, allowing them to calculate its orbit more accurately.

The astronomers publicly announced their find in July 2005 as a new “planet” and gave it the nickname Xena, the name of a fictional female warrior on a popular television program. Since Xena began with the letter “x,” the name was also a play on the term “planet X,” historically used in astronomy for a long-sought planet beyond Pluto. In September 2005 astronomers at the Keck Observatory in Hawaii discovered a small moon orbiting 2003 UB313. Now called Dysnomia, the moon was informally nicknamed Gabrielle (after the companion of Xena).

The discovery of 2003 UB313 posed a problem for astronomers. Because 2003 UB313 was bigger than Pluto, the ninth planet, the new object logically ranked as the tenth planet in the solar system. Other planet-like bodies smaller than Pluto had been detected in the region of the Kuiper Belt in recent years. Such KBOs might be considered planets as well, potentially adding dozens of such small icy bodies to the planet count in the solar system as more finds are made.

The definition of a planet became a topic of heated debate among astronomers and planetary scientists. In August 2006 the International Astronomical Union (IAU) adopted a new definition of a planet that excluded Pluto and 2003 UB313. The eight larger planets (Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus, Neptune) qualified as 'classical planets.” Pluto and 2003 UB313 were classified instead as 'dwarf planets.” The new IAU definitions of classical planet and dwarf planet were controversial and some scientists have refused to accept them.

According to the IAU, dwarf planets and classical planets share two properties—they both orbit the Sun and have a rounded shape from effects of their own gravity. Dwarf planets differ from classical planets by their formation and environment. Unlike classical planets, dwarf planets are not massive enough to have cleared the neighborhoods of their orbits of other objects by their presence. Both Pluto and Eris are associated with the Kuiper Belt, a region containing thousands of small icy bodies ranging from objects similar to Pluto in size to comets and chunks of ice. Ceres, once ranked as the largest asteroid, also qualifies as a dwarf planet because it is spherical in shape and orbits in the asteroid belt, a zone between Mars and Jupiter scattered with thousands of small rocky bodies.

The IAU gave 2003 UB313 the official name Eris in September 2006 and put it on the catalog of minor planets as number 136199 Eris, along with Pluto, other KBOs, Ceres, and asteroids. The IAU also gave its moon the official name Dysnomia. In Greek mythology, Eris was the goddess of discord and strife. Dysnomia was her daughter and the demon goddess of lawlessness. The names Eris and Dysnomia were suggested by astronomer Michael Brown and allude in part to the scientific controversy over Eris's status as a planet. The name Dysnomia is also a sly play on the name Lucy Lawless, the actress who played Xena on television.

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