Why Did Eris Change Our Definition of Planets?

Eris: dwarf planet — Image of the dwarf planet Eris (2003 UB313 or Xena) taken with the Samuel Oschin Schmidt Telescope at Palomar Observatory. Bottom left: Simulated view of Eris.
Image source: NASA - Eris (new window)

Scientific Summary

Discovered in 2005 by Mike Brown's team, Eris is the most massive known trans-Neptunian dwarf planet (diameter: 2,326 km), orbiting in the scattered disc beyond the Kuiper Belt. It has a crust of water ice and methane covering a differentiated silicate core. Its discovery directly led the IAU to redefine the term "planet" in 2006, and thus the status of Pluto. Eris is slightly smaller than Pluto but 27% more massive. Its satellite Dysnomia allowed its mass (1.66 × 10²² kg) to be determined using Kepler's laws.

Why Did Eris Disrupt the Definition of a Planet and What Is Its Status Today?

Discovered in 2005, Eris caused a stir in the astronomical community. With a size comparable to Pluto but 27% more massive, it raised a simple question: should it be called the tenth planet of the Solar System? If so, Pluto should retain its status. But then, how many other Kuiper Belt objects would also deserve this title? Faced with this deadlock, the International Astronomical Union had to decide. In 2006, it established a new definition of a planet, requiring three criteria: orbiting the Sun, having sufficient mass to be in hydrostatic equilibrium (spherical shape), and having cleared its orbital neighborhood. Eris fails the third criterion, as does Pluto. Both are therefore classified as dwarf planets. Today, Eris remains a fascinating object: it takes 559 years to orbit the Sun, and its icy surface, mainly composed of frozen methane and nitrogen, makes it one of the brightest bodies in the Solar System, despite its extreme distance.

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Physical Characteristics: A Frozen and Mysterious World

Eris has a diameter of 2,326 km (slightly smaller than Pluto) but a mass of 1.66 × 10²² kg, or 0.28% of Earth's mass. Its high density (≈ 2.52 g/cm³) suggests an internal composition dominated by silicate rocks (70%) and a crust of water ice and methane (30%). The temperature on its surface is around -230 °C, making Eris one of the coldest objects in the solar system.

Eris's surface is covered with a thin layer of methane ice, which partially sublimates as it approaches perihelion, creating a tenuous and temporary atmosphere. This dynamic is similar to that observed on Pluto, but on a much smaller scale.

Internal Structure and Differentiation

Geophysical models indicate that Eris is differentiated: a dense rocky core (radius ≈ 1,200 km) would be surrounded by a water ice mantle (≈ 100 km thick), itself covered by a crust of volatile ice (methane, nitrogen). This structure suggests a geologically active past, possibly marked by episodes of cryovolcanism, although no direct evidence has been observed to date.

Classification Criteria: Why Eris Is a Dwarf Planet

Eris meets the IAU's three criteria for a dwarf planet:

Orbits the Sun: Orbital period of 557 years.
Hydrostatic shape: Its mass gives it a spherical shape (minimal flattening).
Uncleared orbit: It shares its environment with other trans-Neptunian objects.

Unlike classical planets, Eris has not "cleared" its orbit, a key criterion that distinguishes it from the eight planets of the solar system.

Dysnomia: Eris's Solitary Moon

Eris has a single natural satellite, Dysnomia (diameter ≈ 700 km), discovered in 2005. Its nearly circular orbit (period of 15.8 days) and its inclination suggest formation by a giant impact, similar to the Earth-Moon system. The study of Dysnomia made it possible to determine Eris's mass using Kepler's laws, confirming its status as the most massive dwarf planet.

Kepler's third law states that the square of the orbital period (T) is proportional to the cube of the semi-major axis (a) of the orbit. Newton later reformulated it to include the mass of the central body: $ T^2 = \frac{4\pi^2}{G(M + m)} \, a^3 $

Scientific Importance: A Fossil from the Early Solar System

Eris is a primordial object, preserved since the formation of the solar system 4.5 billion years ago. Its study provides valuable clues about:

The migration of giant planets (Nice model).
The composition of trans-Neptunian objects and their role in delivering water to Earth.
The differentiation mechanisms in intermediate-sized icy bodies.

Comparison: Eris, Pluto, Sedna, and Haumea

Comparison of the main characteristics of Eris with Pluto, Sedna, and Haumea
Characteristic	Eris	Pluto	Sedna	Haumea
Diameter (km)	2,326	2,377	≈ 1,000	≈ 1,632 (ellipsoidal shape: 2,100 × 1,680 × 1,074)
Mass (×10²¹ kg)	16.6	13.0	≈ 0.1	≈ 4.2
Density (g/cm³)	2.52	1.85	≈ 2.0	≈ 2.0
Orbital period (years)	557	248	≈ 11,400	≈ 284
Surface temperature (°C)	-230	-229	-240	≈ -241
Atmosphere	Tenue (sublimated methane)	Nitrogen/methane (seasonal)	None detected	None detected

N.B.: Haumea is distinguished by its highly ellipsoidal shape, a consequence of its extremely rapid rotation (≈ 3.9 hours, the fastest known for a body in hydrostatic equilibrium). The diameter indicated corresponds to an equivalent average diameter in volume, not a spherical body like Eris or Pluto.

References

NASA - Eris in Depth (new window), Brown et al. (2005), Science (new window), Wikipedia - Haumea (new window).

FAQ: Everything You Need to Know About the Dwarf Planet Eris

Why Did Eris Cause Pluto's Demotion?

When discovered in 2005, Eris appeared slightly more massive than Pluto, raising the question of whether it deserved to be the ninth or tenth planet. Rather than adding a new member to the list of planets, the IAU formalized three planetary definition criteria in 2006, including "clearing its orbit." Since neither Eris nor Pluto meet this criterion—both evolve among other objects in the scattered disc or Kuiper Belt—the two bodies were classified as dwarf planets.

What Is Eris Made Of and Why Is It So Dense?

With a density of 2.52 g/cm³, higher than Pluto's (1.85 g/cm³), Eris has an interior dominated by silicate rocks accounting for about 70% of its mass. Geophysical models suggest a differentiated structure: a rocky core about 1,200 km in radius, surrounded by a water ice mantle about a hundred kilometers thick, itself covered by a thin crust of volatile ices (methane, nitrogen). This differentiation implies a geologically active past, with possible cryovolcanism, although this has not been confirmed by direct observation.

What Is Dysnomia and What Role Does It Play in the Study of Eris?

Dysnomia is Eris's only known natural satellite, discovered in 2005, with an estimated diameter of about 700 km. Its nearly circular orbit, with a period of 15.8 days, was essential for precisely determining Eris's mass using Kepler's laws: by measuring the period and semi-major axis of Dysnomia's orbit, astronomers were able to calculate the mass of the system, confirming that Eris is indeed the most massive known dwarf planet to date.

What Is Eris's Orbit and Why Is It So Unusual?

Eris's orbit is one of the most extreme in the Solar System. Its eccentricity of 0.44 takes it as far as 97 AU from the Sun (14.5 billion km) at aphelion, and as close as 38 AU at perihelion. Its inclination of 44° relative to the ecliptic plane places it outside the plane in which most planets orbit. A year on Eris lasts 559 Earth years, and its current perihelion is on the opposite side from Pluto's, permanently separating the two.

Does Eris Have an Atmosphere?

Eris has a tenuous and seasonal atmosphere. As it approaches the Sun at perihelion, the temperature reaches about −230 °C, sufficient to sublimate the methane and nitrogen ices on its surface, creating a thin exosphere. As it moves away toward aphelion, these gases refreeze and fall back to the surface. This phenomenon is analogous to that observed on Pluto, but Eris's extreme distance makes its atmosphere even more tenuous and difficult to observe from Earth.