Explainer · Outer Solar System
Hunting dwarf planets at the edge of the Solar System
See how astronomers use sky-survey images to spot slow-moving worlds beyond Neptune, and how far the known dwarf planets are from the Sun.
Educational explainer. Not connected to live NASA data.
- Ceres — asteroid belt1801
- Pluto — Kuiper belt1930
- Haumea — Kuiper belt2004
- Makemake — Kuiper belt2005
- Eris — scattered disc2005
How a dwarf-planet search works
New dwarf-planet candidates come from repeated images of the same patch of sky, followed by careful orbit and size measurements.
Survey the sky again and again
Wide-field telescopes photograph the same region over several nights. Background stars stay fixed, while Solar System objects drift slightly between exposures.
- Multi-night image sets
- Deep exposures to catch faint, distant objects
- Searches focus near the ecliptic plane
Find the slow movers
Software subtracts the static stars and looks for faint points that move along a consistent path. Very distant objects move only a few arcseconds per hour.
- Difference imaging removes fixed stars
- Shift-and-stack reveals faint movers
- Humans vet every candidate
Pin down the orbit and distance
Follow-up observations over months and years refine the orbit. The orbit gives the object's distance from the Sun in AU.
- Follow-up over multiple oppositions
- Semi-major axis gives average distance
- Orbital period follows from Kepler's laws
Is it big and round enough?
Brightness, reflectivity and sometimes a moon's orbit are used to estimate size and mass. The IAU decides whether an object is officially a dwarf planet.
- Size from brightness and albedo
- Mass from any orbiting moon
- Official status set by the IAU
The known dwarf planets
The five dwarf planets currently recognised by the IAU, with approximate figures.
| Ceres | 1801 | Asteroid belt | 939 | 2.8 | 0.38 | 4.6 |
|---|---|---|---|---|---|---|
| Pluto | 1930 | Kuiper belt | 2,377 | 39.5 | 5.47 | 247.9 |
| Haumea | 2004 | Kuiper belt | 1,632 | 43.1 | 5.97 | 284.0 |
| Makemake | 2005 | Kuiper belt | 1,430 | 45.4 | 6.29 | 306.0 |
| Eris | 2005 | Scattered disc | 2,326 | 67.9 | 9.41 | 559.0 |
Source: NASA / JPL Solar System Dynamics (approximate values)
Questions
Does this app analyse NASA images itself?
No. This app explains how dwarf-planet searches work and shows reference data. It does not download or process telescope images.
What makes something a dwarf planet?
Under the IAU's 2006 definition, a dwarf planet orbits the Sun and has enough mass to pull itself into a nearly round shape. It has not cleared other objects from its orbital path, and it is not a moon.
Why not give distances in light-years?
Dwarf planets are inside our Solar System, so light-years are far too large a unit. Even Eris is only about 9.4 light-hours from the Sun on average.
Are there more dwarf planets waiting to be found?
Very likely. Objects such as Gonggong, Quaoar and Sedna are strong candidates, but they are not yet officially recognised. Ongoing sky surveys are expected to find more in the outer Solar System.