Eris (dwarf planet)

• 1. Introduction

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  Eris, minor-planet number 136199, upended the Solar System's address book in a single announcement on the 29th of July 2005. Here was an object beyond Neptune, more massive than Pluto, initially described by NASA as a tenth planet. Its existence forced astronomers to ask a question they had somehow avoided for decades: what, exactly, is a planet? The answer they arrived at in August 2006 shrank the count from nine to eight, reclassified Pluto, and left Eris as something entirely its own category. How a faint smear of light photographed in October 2003 set all of that in motion, what Eris is made of, and where it sits in the cold depths of the outer Solar System are the threads this documentary follows.

• 2. Finding Eris

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  Mike Brown, Chad Trujillo, and David Rabinowitz captured the image that would change planetary science on the 21st of October 2003, using the 1.2 m Samuel Oschin Schmidt telescope at Palomar Observatory in California. They did not know they had found Eris that night. Their automated image-searching software discarded any object moving slower than 1.5 arcseconds per hour to keep false positives manageable, and Eris moved too slowly to pass that filter. The discovery of Sedna in 2003, moving at 1.75 arcseconds per hour, nudged the team to reconsider their threshold. They lowered the cutoff and sorted through previously rejected images by hand, and in January 2005, Eris's sluggish drift against the background stars finally revealed itself.

  Precovery work later turned up images of Eris dating as far back as the 3rd of September 1954, meaning the object had been photographed for decades without anyone recognizing what it was. The team planned to hold their announcement until more orbital data was in hand, but events forced their schedule. On the 27th of July 2005, a Spanish team controversially announced the discovery of Haumea, a large TNO that Brown's group had also been tracking. Two days later, on July 29, Brown's team announced both Eris and Makemake at the same time, compressing what would have been a carefully staged series of disclosures into a single crowded day.

  Observations published in October 2005 revealed that Eris had a moon, later named Dysnomia. By June 2007, measurements of Dysnomia's orbit allowed scientists to calculate Eris's mass with precision, finding it to be 27% greater than Pluto's.

• 3. The Name and Its Mythology

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  Before Eris had a formal name, the discovery team called it Xena, after the title character of the television series Xena: Warrior Princess. Brown later explained the reasoning: the name started with an X, recalling the old idea of Planet X; it sounded mythological; and the team had been saving it for the first body they found that was larger than Pluto. Xena's fictional companion Gabrielle lent her name informally to the moon, before official designations replaced both.

  Brown had initially wanted to call the object Lila, after a concept in Hindu mythology describing the cosmos as the result of a game played by Brahman. The name happened to match the name of his newborn daughter. He quietly registered the discovery page at the web address /~mbrown/planetlila, but in the chaos following the Haumea controversy, forgot to change the URL. Rather than invite more conflict with colleagues, he quietly set Lila aside, saying the webpage had simply been named for his daughter.

  Persephone, wife of the god Pluto, was another candidate and had won a public poll run by New Scientist magazine. It was ruled out because 399 Persephone, a minor planet, already held the name, and the IAU could not assign it twice. According to science writer Govert Schilling, Brown then settled on Eris, whom he described as his favorite goddess. The name was proposed on the 6th of September 2006, and accepted by the IAU on the 13th of September 2006. Brown summed up the choice in a 2006 statement: Eris caused strife and discord by causing quarrels among people, and that is what this one has done too.

• 4. Redefining a Planet

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  NASA's initial description of Eris as the Solar System's tenth planet created immediate pressure on the International Astronomical Union. The prospect of more Pluto-sized objects turning up as telescope surveys improved made the situation urgent. If Eris was a planet, then several other recently found bodies might qualify as well, and the count could keep climbing.

  The IAU convened a group of astronomers to draft a precise definition, something the organization had never formally done. The resulting resolution, adopted on the 24th of August 2006, drew three criteria a body must meet to be called a planet. Eris and Pluto both failed the third criterion, clearing the neighborhood around their orbits, and were placed in the newly defined category of dwarf planets. Ceres, the largest object in the asteroid belt between Mars and Jupiter, joined them. The total number of recognized planets fell to eight, exactly the count that had stood before Pluto's discovery in 1930.

  Brown publicly endorsed the reclassification. The IAU added Eris to its Minor Planet Catalogue as (136199) Eris. The episode produced a new astronomical vocabulary and settled a debate that had been quietly building for years about whether Pluto fit comfortably among the eight large planets or belonged to a distinct population of outer Solar System bodies.

• 5. An Extreme Orbit

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  Eris is a scattered-disk object, meaning Neptune's gravity flung it from the Kuiper belt into an unusual path during the Solar System's formation. Its orbit is tilted roughly 44 degrees relative to the ecliptic, the plane in which Earth and the other planets travel. That steep inclination is why Eris went unnoticed for so long: sky surveys hunting for large outer Solar System bodies naturally concentrated on the ecliptic plane.

  The orbit itself takes 558 years to complete. At its farthest reach, aphelion, Eris sits 97.7 astronomical units from the Sun; at its closest, perihelion, it comes to within 38.4 AU. Numerical integration by JPL Horizons places its last perihelion around 1699 and its last aphelion around 1977. It will next reach perihelion on the 6th of December 2257. In about 800 years, Eris will briefly draw closer to the Sun than Pluto for a period.

  In February 2016, Eris was 96.3 AU from the Sun, more than three times Neptune's distance. With the exception of long-period comets, Eris and its moon Dysnomia were the most distant known natural objects in the Solar System until two newly discovered bodies surpassed them in 2018. As of 2007, Eris had an apparent magnitude of 18.7, faint but within reach of a 200 mm telescope with a CCD under favorable conditions.

• 6. Ice, Rock, and Hidden Water

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  Spectroscopic observations made at the 8 m Gemini North Telescope in Hawaii on the 25th of January 2005, revealed methane ice on Eris's surface. At the time, Pluto was the only other trans-Neptunian object known to carry surface methane; Neptune's moon Triton also had it. In 2022, the James Webb Space Telescope found deuterated methane ice on Eris, at lower abundances than those seen in Jupiter-family comets such as 67P/Churyumov-Gerasimenko. The low deuterium level suggests Eris's methane did not survive from the Solar System's earliest era but was instead produced by subsurface geochemical processes. The JWST also detected substantial quantities of nitrogen ice, presumed to share a similar non-primordial origin, present at roughly one-third the volume of the methane.

  A stellar occultation observed in November 2010 allowed researchers to measure Eris with precision unavailable from direct imaging. The final results, released in October 2011, set its diameter at 2326 km, making it slightly smaller than Pluto, which the New Horizons mission measured at a mean diameter of 2376.6 km in July 2015. Eris remains the more massive of the two, and its density of 2.52 g/cm3 is substantially higher than Pluto's, indicating a composition dominated by rocky materials rather than ice.

  Models of internal heating through radioactive decay suggest Eris could maintain a subsurface ocean of liquid water at its mantle-core boundary. Tidal heating from Dysnomia may help preserve that possible ocean. Research grouping Eris with Pluto and Makemake found all three could harbor active subsurface oceans and show geothermal activity.

• 7. A Pale, Locked World

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  Eris's surface is nearly white and unusually uniform, in contrast to the reddish and varied terrain of Pluto and Neptune's moon Triton. Pluto's color comes from tholins, organic compounds that darken the surface and trigger local temperature increases that cause methane to evaporate. Eris is far enough from the Sun that methane can condense even on low-albedo patches, smoothing out any such contrasts and burying red tholin deposits under a fresh layer of ice. The measured geometric albedo of 0.96 makes Eris one of the most reflective bodies in the Solar System.

  The JWST found no ethane on Eris's surface; ethane is a byproduct of methane broken down by radiation, so its absence suggests the surface is being continuously refreshed. Researchers have proposed that this refreshing could stem from a methane and nitrogen ice glacier cycling through radiogenic convection, similar to Pluto's Sputnik Planitia region, or from a sublimation-condensation cycle linked to Eris's eccentric orbit. Surface temperatures range from roughly 30 to 56 K as Eris moves between perihelion and aphelion.

  The planet's rotation period proved difficult to pin down because the uniform surface produces very little brightness variation as it spins. Long-term monitoring eventually showed that Eris is tidally locked to Dysnomia, rotating once every 15.78 Earth days, exactly matching Dysnomia's orbital period. Dysnomia is also tidally locked to Eris, making the pair the second known case of double-synchronous rotation in the Solar System, after Pluto and its moon Charon.

• 8. Dysnomia and Future Exploration

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  The moon now called Dysnomia was spotted on the 10th of September 2005, by the adaptive optics team at the Keck telescopes in Hawaii. They were surveying the four brightest trans-Neptunian objects using a newly commissioned laser guide star system. Brown's team gave it the informal name Gabrielle, after the warrior princess's sidekick, to match the Xena nickname already attached to Eris. When the IAU assigned Eris its official name, the moon followed as Dysnomia, after the Greek goddess of lawlessness and daughter of Eris. Brown has said he chose the name partly for its similarity to his wife's name, Diane; the connection to Lucy Lawless, the actress who played Xena on television, was an unintentional bonus.

  Dysnomia's diameter is approximately 615 km and its semi-major axis is roughly 37,273 km. Together the two objects form a gravitationally coupled system with an orbital period of 15.774 days.

  Human-built spacecraft have never visited Eris. The New Horizons probe observed it from a distance in May 2020, during its extended mission after the Pluto flyby. At that point, New Horizons was actually farther from Eris than Earth was, but its position deep inside the Kuiper belt allowed it to observe Eris at high phase angles impossible from Earth, yielding data on how the surface scatters light. Mission planners have since calculated that a dedicated flyby of Eris using a Jupiter gravity-assist, launched on the 3rd of April 2032, or the 7th of April 2044, would take 24.66 years, with Eris sitting between 90.19 and 92.03 AU from the Sun on arrival.