This is the classification diagram astronomers actually use: every trans-Neptunian object (TNO) is plotted by its semi-major axis a (horizontal) against its eccentricity e (vertical). Perihelion, aphelion and period follow directly:
q = a(1-e) Perihelion — closest approach to the Sun
Q = a(1+e) Aphelion — farthest point
T = a^1.5 yr Kepler's third law (a in AU)
The dashed curves are constant-perihelion lines e = 1 - q/a; the vertical dashed lines mark Neptune mean-motion resonances a_res = a_Neptune·(p/q)^(2/3) (simplified from Gladman et al. 2008):
- Resonant — a sits within 0.4 AU of a resonance line, e.g. 3:2 "Plutinos" at 39.4 AU or 2:1 "Twotinos" at 47.8 AU. Its period is a simple integer ratio of Neptune's, so it never has a close encounter even at high eccentricity.
- Classical / Cubewano — inside the shaded box (42–48 AU, e < 0.24) and not resonant: a dynamically cold, roughly circular belt just beyond Neptune.
- Scattered Disk — below the q = 37 AU curve: close enough to Neptune's 30.1 AU orbit for repeated gravitational scattering to keep pumping eccentricity.
- Detached — above the q = 40 AU curve outside the classical box: decoupled from Neptune's gravity entirely (Sedna, q ≈ 76 AU, is the archetype).
The small dial in the corner is an orbital phase clock: it solves Kepler's equation for the test object's true anomaly and sweeps at the rate set by its own period, exactly as the object's real position along its ellipse would advance — without drawing the ellipse itself. Population dots pulse at their own Kepler rate the same way, so faster-orbiting inner objects visibly flicker quicker than distant detached ones.
Drag the sliders to move the white marker and watch its class, resonance lock and stats update instantly against the live population of 1,200 classified TNOs.