Object profile · ERIS
Eris
Dwarf planet · Scattered disc
A massive, distant ice world whose discovery forced astronomers to define the word planet.
- Atlas data reviewed
- Editorial stage
- Published
- Visibility
- Public
Artist visualizationArtist visualization, not a spacecraft image. Surface texture, lighting and moon position are illustrative.
Vastward original reconstruction; scientific basis: NASA Science, Hubble and occultation measurements ↗What this world is
Eris is slightly smaller than Pluto by current size estimates but more massive. Its discovery exposed a classification problem: if Eris were a planet, many similar worlds might need the same label.
Why it looks this way now
No spacecraft has visited. A moon orbit supplies mass, stellar occultation supplies size, and spectra constrain a frozen methane-rich surface.
Where it sits
Eris follows a highly inclined and elongated orbit in the scattered disc, far beyond the classical Kuiper Belt.
Swipe sideways to inspect the full map
Eris orbits the Sun directly at a mean distance of about 67.9 AU; the marker is not a live position.
Neptune provides the outer-planet reference. The shaded zone shows the broad relationship to the Scattered disc, while muted dots are other profiled outer-system dwarf planets; horizontal distance is heavily compressed.
Read the numbers
Reference values retain their units, context, evidence state and published uncertainty.
Physical measurements
- Mean radiusglobal mean
- 1,163.0kmScientific modelClose to Pluto in size.
A global reference radius reconstructed from imaging, thermal data or occultation.
No uncertainty published in this source table
The selected summary source does not publish a compact uncertainty for this reference value.
- Massglobal mean
- 1.6466E22kgCalculated measurementFar below Earth's mass, yet enough gravity to become rounded.
Calculated from the motion of a spacecraft, moon or dynamical system.
No uncertainty published in this source table
The selected summary source does not publish a compact uncertainty for this reference value.
- Mean densityglobal mean
- 2.520g/cm³Calculated measurementA clue to the relative mixture of rock, water ice and voids.
Density is calculated from mass and volume; it does not reveal a unique interior by itself.
No uncertainty published in this source table
The selected summary source does not publish a compact uncertainty for this reference value.
- Reference gravitysurface
- 0.82m/s²Calculated measurementOnly a few percent of Earth's surface gravity.
A reference value derived from mass and radius.
No uncertainty published in this source table
The selected summary source does not publish a compact uncertainty for this reference value.
- Temperaturesurface
- -230°CScientific modelCold enough for volatile compounds to remain as surface ice.
A model-supported representative temperature, not a thermometer reading on the ground.
Reported range: -243 to -217 °C
Orbit and rotation
- Average distancerelative to the Sun
- 67.900AUCalculated measurementDozens of times farther from the Sun than Earth.
The semi-major-axis reference summarizes an orbit whose actual distance changes.
No uncertainty published in this source table
The selected summary source does not publish a compact uncertainty for this reference value.
- Rotation periodrelative to distant stars
- 25.9000hCalculated measurementMeasured from repeating brightness or surface patterns.
A sidereal rotation period relative to distant stars.
Approximate value
JPL flags the adopted light-curve rotation period as uncertain and potentially subject to revision.
- Orbital periodrelative to the Sun
- 561.30Earth yearsCalculated measurementOne full season cycle lasts many human lifetimes.
Calculated from long-baseline astrometry and a fitted heliocentric orbit.
No uncertainty published in this source table
The selected summary source does not publish a compact uncertainty for this reference value.
Atmosphere and inside
Atmosphere
At its present distance, any atmosphere is expected to be frozen onto the surface. It may reappear as Eris approaches the Sun.
Surface and interior
Its high bulk density points to a substantial rocky fraction mixed with ice; internal layers are not directly observed.
- 1Dense rock-and-ice interior
Moon-orbit mass and occultation size imply a denser mixture than Pluto, but exact layering is unknown.
Scientific model - 2Bright volatile-ice surface
A highly reflective frozen surface likely includes methane ice deposited from a collapsed atmosphere.
Scientific model - 3Dysnomia satellite system
Tracking Dysnomia's orbit provides Eris's mass—the critical quantity that a single image cannot supply.
Scientific model
How we know
Size comes from imaging, thermal data or occultation; orbit and rotation from repeated positions and brightness; mass only when gravity leaves a measurable trace.
- 01Calculated measurement
Stellar-occultation shape reconstruction
Multiple observatories time the object's shadow crossing a background star to reconstruct chords across its silhouette.
Where this method stops
A single event samples one orientation and requires a shape model.
- 02Calculated measurement
Gravity from orbital motion
The motion of a moon or spacecraft reveals the central body's gravitational parameter.
Where this method stops
The solution depends on orbit coverage and a dynamical model.
- 03Calculated measurement
Thermal radiometry and spectroscopy
Brightness at visible, infrared and thermal wavelengths constrains temperature, reflectivity and surface ices.
Where this method stops
Temperature and size can trade off against albedo, roughness and thermal assumptions.
- 04Calculated measurement
Long-baseline astrometry
Repeated sky positions over years are fitted to a heliocentric orbit.
Where this method stops
Long-period distant objects need many years of observations and future values remain predictions.
- 05Calculated measurement
Rotational light curve
Repeating brightness changes reveal a candidate rotation period.
Where this method stops
Shape, surface markings and viewing angle can create aliases or double-peaked solutions.
Evidence key
- Direct observation
- An instrument or sample recorded the phenomenon, with processing still disclosed.
- Calculated measurement
- Observed motion or signal is converted into a physical quantity using equations and reference constants.
- Scientific model
- A tested interpretation that fits observations but is not directly imaged or sampled.
- Vastward explanation
- Original beginner-facing synthesis, traceable to the source records but not itself a measurement.
- Artist visualization
- An interpretive image, not observational evidence.
Missions and instruments
Hubble Space Telescope
Hubble resolved Dysnomia and helped establish the orbit used to calculate Eris's mass.
Official mission ↗- Hubble Advanced Camera for SurveysResolved astrometry of Eris and Dysnomia.
Official source trail
Links below are the exact records used for this profile. Access dates are retained with the content.
- 01
NASA Science · official-page
Eris Facts
- Source updated
- No source update date published
- Access checked
- 2026-07-31
- 02
NASA Jet Propulsion Laboratory, Solar System Dynamics · dataset
JPL Small-Body Database Lookup: 136199 Eris
- Source updated
- No source update date published
- Access checked
- 2026-07-31
- 03
NASA Hubble Space Telescope · official-page
Astronomers Measure Mass of Largest Dwarf Planet
- Source updated
- No source update date published
- Access checked
- 2026-07-31
- 04
Nature · research-paper
A Pluto-like radius and a high albedo for the dwarf planet Eris from an occultation
- Source updated
- 2011-10-26
- Access checked
- 2026-07-31
Continue exploring
Atlas is a starting point. Use these relationships to move into explanation, experiment and mission thinking.
Why is it a dwarf planet?
Use Eris to separate roundness, orbit and orbital clearing—the three ideas behind the classification.
academyCompare the small worlds
Place radius, density and sunlight side by side without pretending every value is equally certain.
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