Say what is known about a world, and how it is known

Four changes to the readout panel and the body cards, which between them were showing less than
the catalogues hold and not always distinguishing a measurement from an inference.

Picking a planet used to navigate straight to /body/:id. That tore down the system scene and the
camera with it, so comparing two planets meant flying back into the system between each. Hovering
a body now raises a card over the live view and clicking pins it; Full view still opens the route
for the full 3D inspection. Clicking empty space unpins, and leaving the system clears it.

The card and the detail page were assembling "what do we know about this world" independently,
which is the shape of bug where a planet reads 255 K in one panel and 254 K in the other. Both now
build from one shared view model.

Orbital period was absent everywhere. For a heliocentric orbit it follows exactly from the
semi-major axis, because in these units the Sun's mass is the unit of mass — Mars comes back
687.0 d against a published 686.98. It is deliberately not computed for moons, whose elements are
relative to a parent planet the catalogue has no mass for, nor for exoplanets: periodDays is
populated for none of the 6319 shipped records and hostStarMassSolar for none either, so any
figure would assume a solar-mass host and mis-state every planet around an M dwarf. Where a period
does exist it is filed under Measured or Derived according to which it is, not by its field name.

The system readout showed a flat 0.00 pc for the Sun's distance, which is arithmetically right and
reads as a bug — the distance from here to here is not a measurement, so it is suppressed. It
gains the host star's luminosity, marked as derived, and counts moons separately from planets. The
neighbourhood readout gains the one thing the star field cannot show: how many of those points can
actually be entered. Derived readouts carry a marker and a footnote saying so.

Every quantity now formats through one module whose precision follows magnitude, rather than a
fixed decimal count per call site that read as false precision at one end and lost real
information at the other: 0.0026 AU stays legible instead of rounding to 0.00, and Pluto's period
reads 248 yr rather than 90560 d.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01WaySiNst4HhDXBHnMy8p5G

src/app/features/body-detail/body-detail-scene.component.ts

@@ -6,9 +6,6 @@ import { OrbitControls } from 'three/addons/controls/OrbitControls.js';
 
 import { DataLoaderService } from '../../core/data/data-loader.service';
 import { EngineService } from '../../core/engine/engine.service';
-import { appearanceForBody, appearanceForExoplanet } from '../../shared/astro/body-appearance';
-import { EARTH_RADIUS_KM } from '../../shared/astro/planet-appearance';
-import { luminositySolar } from '../../shared/astro/stellar';
 import { planetTexture } from '../../shared/rendering/procedural-planet-texture';
 import { applyMilkyWaySkybox, createGlowSprite } from '../../shared/rendering/skybox';
 import { atmosphereColorFor, bodyTexturePath, loadCachedTexture, MILKY_WAY_SKYBOX_PATH, SATURN_RING_TEXTURE_PATH } from '../../shared/rendering/texture-catalog';
@@ -17,6 +14,7 @@ import { ExoplanetRecord } from '../../shared/models/exoplanet.model';
 import { StarRecord } from '../../shared/models/star.model';
 import { NavigationStore } from '../../shared/state/navigation.store';
 import { BodyDetailViewModel } from './body-detail.model';
+import { buildBodyViewModel } from './body-view-model';
 import { InfoPanelComponent } from './info-panel.component';
 
 /** Gas giants read as smoother/less rocky than terrestrial bodies under the same lighting rig. */
@@ -118,44 +116,24 @@ export class BodyDetailSceneComponent implements AfterViewInit, OnDestroy {
       return;
     }
 
-    const body = this.bodies.find((candidate) => candidate.id === id);
-    const exoplanet = this.exoplanets.find((candidate) => candidate.id === id);
-
-    if (body) {
-      const hostStar = this.stars.find((star) => star.id === body.systemStarId);
-      this.viewModel.set({
-        id: body.id,
-        name: body.name,
-        kind: body.kind,
-        hostStarName: hostStar?.name ?? 'Unknown star',
-        radiusKm: body.radiusKm,
-        orbit: body.orbit,
-        appearance: appearanceForBody(body, this.bodies, this.luminosityOf(hostStar)),
-        hasPhotography: bodyTexturePath(body.id) !== undefined
-      });
-      this.navigationStore.selectStar(body.systemStarId);
-    } else if (exoplanet) {
-      const hostStar = this.stars.find((star) => star.id === exoplanet.hostStarId);
-      this.viewModel.set({
-        id: exoplanet.id,
-        name: exoplanet.name,
-        kind: 'exoplanet',
-        hostStarName: exoplanet.hostStarName,
-        radiusKm: exoplanet.radiusEarth ? exoplanet.radiusEarth * EARTH_RADIUS_KM : undefined,
-        massEarth: exoplanet.massEarth,
-        discoveryYear: exoplanet.discoveryYear,
-        orbit: exoplanet.orbit,
-        appearance: appearanceForExoplanet(exoplanet, this.luminosityOf(hostStar)),
-        hasPhotography: bodyTexturePath(exoplanet.id) !== undefined
-      });
-      if (exoplanet.hostStarId !== null) {
-        this.navigationStore.selectStar(exoplanet.hostStarId);
-      }
-    } else {
+    // Shared with the system view's object card, so the same body cannot read differently there.
+    const viewModel = buildBodyViewModel(id, { bodies: this.bodies, exoplanets: this.exoplanets, stars: this.stars });
+    if (!viewModel) {
       this.viewModel.set(undefined);
       this.notFound.set(true);
       return;
     }
+    this.viewModel.set(viewModel);
+
+    const body = this.bodies.find((candidate) => candidate.id === id);
+    if (body) {
+      this.navigationStore.selectStar(body.systemStarId);
+    } else {
+      const exoplanet = this.exoplanets.find((candidate) => candidate.id === id);
+      if (exoplanet?.hostStarId != null) {
+        this.navigationStore.selectStar(exoplanet.hostStarId);
+      }
+    }
 
     this.notFound.set(false);
     this.navigationStore.selectBody(id);
@@ -164,18 +142,6 @@ export class BodyDetailSceneComponent implements AfterViewInit, OnDestroy {
     }
   }
 
-  /**
-   * The host star's luminosity in solar units, from its own catalogued magnitude and distance.
-   * `null` for an exoplanet whose host never cross-referenced to the star catalogue, which
-   * leaves its planets with no derived temperature rather than a guessed one.
-   */
-  private luminosityOf(star: StarRecord | undefined): number | null {
-    if (!star) {
-      return null;
-    }
-    return luminositySolar({ magnitude: star.magnitude, distancePc: Math.hypot(star.x, star.y, star.z), spectralType: star.spectralType });
-  }
-
   private applyViewModelToScene(): void {
     const viewModel = this.viewModel();
     if (!viewModel || !this.planetMaterial) {

src/app/features/body-detail/body-detail.model.ts

@@ -26,4 +26,16 @@ export interface BodyDetailViewModel {
   appearance: PlanetAppearance;
   /** True when a real photograph is being shown rather than the derived surface. */
   hasPhotography: boolean;
+  /**
+   * Sidereal orbital period. Measured where the archive published one; otherwise derived from the
+   * semi-major axis for heliocentric orbits, where the central mass is known exactly. Undefined
+   * when neither applies — see `heliocentricPeriodDays`.
+   */
+  orbitalPeriodDays?: number;
+  /**
+   * Which of those two the period is, so the surfaces can file it under the right heading. A
+   * derived period presented as an observation is the same category of error as presenting a
+   * derived surface as a photograph.
+   */
+  orbitalPeriodSource?: 'measured' | 'derived';
 }

src/app/features/body-detail/body-view-model.spec.ts

@@ -0,0 +1,125 @@
+import { describe, expect, it } from 'vitest';
+
+import { BodyRecord, OrbitalElements } from '../../shared/models/body.model';
+import { ExoplanetRecord } from '../../shared/models/exoplanet.model';
+import { StarRecord, SUN_STAR_ID } from '../../shared/models/star.model';
+import { buildBodyViewModel, heliocentricPeriodDays } from './body-view-model';
+
+const orbit = (overrides: Partial<OrbitalElements> = {}): OrbitalElements => ({
+  semiMajorAxisAu: 1,
+  eccentricity: 0.0167,
+  inclinationDeg: 0,
+  longitudeOfAscendingNodeDeg: 0,
+  argumentOfPeriapsisDeg: 0,
+  meanAnomalyAtEpochDeg: 0,
+  epochJd: 2451545,
+  ...overrides,
+});
+
+const sun: StarRecord = {
+  id: SUN_STAR_ID,
+  name: 'Sol',
+  x: 0,
+  y: 0,
+  z: 0,
+  magnitude: -26.7,
+  spectralType: 'G2V',
+  colorIndex: 0.65,
+};
+
+const earth: BodyRecord = {
+  id: 'earth',
+  systemStarId: SUN_STAR_ID,
+  name: 'Earth',
+  kind: 'planet',
+  radiusKm: 6371,
+  orbit: orbit(),
+};
+const luna: BodyRecord = {
+  id: 'luna',
+  systemStarId: SUN_STAR_ID,
+  name: 'Moon',
+  kind: 'moon',
+  radiusKm: 1737,
+  parentBodyId: 'earth',
+  orbit: orbit({ semiMajorAxisAu: 0.00257 }),
+};
+
+describe('heliocentricPeriodDays', () => {
+  it('recovers a known period from the semi-major axis alone', () => {
+    // P² = a³ in these units, so Earth must come back a year.
+    expect(heliocentricPeriodDays(earth)).toBeCloseTo(365.25, 1);
+  });
+
+  it('scales as the three-halves power', () => {
+    const jupiter: BodyRecord = {
+      ...earth,
+      id: 'jupiter',
+      name: 'Jupiter',
+      orbit: orbit({ semiMajorAxisAu: 5.2044 }),
+    };
+    // Jupiter's real sidereal period is 4332.6 days.
+    expect(heliocentricPeriodDays(jupiter)).toBeCloseTo(4335, -1);
+  });
+
+  it('refuses to compute a period for a moon', () => {
+    // A moon's elements are relative to its planet, whose mass is not in the catalogue — the
+    // same arithmetic would be wrong by the ratio of that planet's mass to the Sun's.
+    expect(heliocentricPeriodDays(luna)).toBeUndefined();
+  });
+});
+
+describe('buildBodyViewModel', () => {
+  const catalogues = { bodies: [earth, luna], exoplanets: [] as ExoplanetRecord[], stars: [sun] };
+
+  it('marks a period computed from the semi-major axis as derived', () => {
+    const model = buildBodyViewModel('earth', catalogues);
+    expect(model?.orbitalPeriodSource).toBe('derived');
+    expect(model?.orbitalPeriodDays).toBeCloseTo(365.25, 1);
+  });
+
+  it('leaves a moon without a period rather than inventing one', () => {
+    const model = buildBodyViewModel('luna', catalogues);
+    expect(model?.orbitalPeriodDays).toBeUndefined();
+    expect(model?.orbitalPeriodSource).toBeUndefined();
+  });
+
+  it('marks a published exoplanet period as measured, not derived', () => {
+    const exoplanet: ExoplanetRecord = {
+      id: 'kepler-22-b',
+      hostStarId: null,
+      hostStarName: 'Kepler-22',
+      name: 'Kepler-22 b',
+      periodDays: 289.9,
+      orbit: { semiMajorAxisAu: 0.849 },
+    };
+    const model = buildBodyViewModel('kepler-22-b', {
+      bodies: [],
+      exoplanets: [exoplanet],
+      stars: [],
+    });
+    expect(model?.orbitalPeriodSource).toBe('measured');
+    expect(model?.orbitalPeriodDays).toBe(289.9);
+  });
+
+  it('leaves an exoplanet with no published period undefined rather than assuming a solar-mass host', () => {
+    const exoplanet: ExoplanetRecord = {
+      id: 'x',
+      hostStarId: null,
+      hostStarName: 'X',
+      name: 'X b',
+      orbit: { semiMajorAxisAu: 0.05 },
+    };
+    const model = buildBodyViewModel('x', { bodies: [], exoplanets: [exoplanet], stars: [] });
+    expect(model?.orbitalPeriodDays).toBeUndefined();
+  });
+
+  it('returns undefined for an id in neither catalogue', () => {
+    expect(buildBodyViewModel('nowhere', catalogues)).toBeUndefined();
+  });
+
+  it('reads the same body identically however it is reached', () => {
+    // The whole point of the shared builder: the card and the detail route must not drift.
+    expect(buildBodyViewModel('earth', catalogues)).toEqual(buildBodyViewModel('earth', catalogues));
+  });
+});

src/app/features/body-detail/body-view-model.ts

@@ -0,0 +1,96 @@
+import { appearanceForBody, appearanceForExoplanet } from '../../shared/astro/body-appearance';
+import { EARTH_RADIUS_KM } from '../../shared/astro/planet-appearance';
+import { luminositySolar } from '../../shared/astro/stellar';
+import { bodyTexturePath } from '../../shared/rendering/texture-catalog';
+import { BodyRecord } from '../../shared/models/body.model';
+import { ExoplanetRecord } from '../../shared/models/exoplanet.model';
+import { StarRecord, SUN_STAR_ID } from '../../shared/models/star.model';
+import { BodyDetailViewModel } from './body-detail.model';
+
+/** Everything the view model is assembled from — the three catalogues, already loaded. */
+export interface BodyCatalogues {
+  readonly bodies: readonly BodyRecord[];
+  readonly exoplanets: readonly ExoplanetRecord[];
+  readonly stars: readonly StarRecord[];
+}
+
+/**
+ * Bolometric luminosity of a star in solar units, from what the catalogue measured: apparent
+ * magnitude, parallax distance, and a bolometric correction read off the spectral type.
+ */
+export function luminosityOf(star: StarRecord | undefined): number | null {
+  if (!star) {
+    return null;
+  }
+  return luminositySolar({
+    magnitude: star.magnitude,
+    distancePc: Math.hypot(star.x, star.y, star.z),
+    spectralType: star.spectralType,
+  });
+}
+
+/**
+ * Builds the flattened view model for one body or exoplanet.
+ *
+ * Shared rather than duplicated per surface: the in-map card and the full detail page show
+ * overlapping subsets of the same quantities, and two independent assemblies of "what do we
+ * know about this world" is exactly the shape of bug where a planet reads 255 K in one panel
+ * and 254 K in the other.
+ */
+export function buildBodyViewModel(id: string, catalogues: BodyCatalogues): BodyDetailViewModel | undefined {
+  const body = catalogues.bodies.find((candidate) => candidate.id === id);
+  if (body) {
+    const hostStar = catalogues.stars.find((star) => star.id === body.systemStarId);
+    return {
+      id: body.id,
+      name: body.name,
+      kind: body.kind,
+      hostStarName: hostStar?.name ?? 'Unknown star',
+      radiusKm: body.radiusKm,
+      orbit: body.orbit,
+      appearance: appearanceForBody(body, catalogues.bodies, luminosityOf(hostStar)),
+      hasPhotography: bodyTexturePath(body.id) !== undefined,
+      orbitalPeriodDays: heliocentricPeriodDays(body),
+      orbitalPeriodSource: heliocentricPeriodDays(body) === undefined ? undefined : 'derived',
+    };
+  }
+
+  const exoplanet = catalogues.exoplanets.find((candidate) => candidate.id === id);
+  if (!exoplanet) {
+    return undefined;
+  }
+  const hostStar = catalogues.stars.find((star) => star.id === exoplanet.hostStarId);
+  return {
+    id: exoplanet.id,
+    name: exoplanet.name,
+    kind: 'exoplanet',
+    hostStarName: exoplanet.hostStarName,
+    radiusKm: exoplanet.radiusEarth ? exoplanet.radiusEarth * EARTH_RADIUS_KM : undefined,
+    massEarth: exoplanet.massEarth,
+    discoveryYear: exoplanet.discoveryYear,
+    orbit: exoplanet.orbit,
+    appearance: appearanceForExoplanet(exoplanet, luminosityOf(hostStar)),
+    hasPhotography: bodyTexturePath(exoplanet.id) !== undefined,
+    // `periodDays` is populated for none of the shipped records, and deriving one would need the
+    // host star's mass, which is equally absent. Left undefined rather than assuming a solar-mass
+    // host, which would silently mis-state the period of every planet around an M dwarf.
+    orbitalPeriodDays: exoplanet.periodDays,
+    orbitalPeriodSource: exoplanet.periodDays === undefined ? undefined : 'measured',
+  };
+}
+
+/**
+ * Kepler's third law for a body orbiting the Sun: P² = a³ with P in years and a in AU, which
+ * holds exactly in these units because the Sun's mass is the unit of mass.
+ *
+ * Only for heliocentric orbits. A moon's elements are relative to its parent planet, whose mass
+ * the catalogue does not carry, so the same arithmetic there would be wrong by the ratio of the
+ * planet's mass to the Sun's — a factor of a thousand for Jupiter.
+ */
+export function heliocentricPeriodDays(body: BodyRecord): number | undefined {
+  if (body.parentBodyId !== undefined || body.systemStarId !== SUN_STAR_ID) {
+    return undefined;
+  }
+  const a = body.orbit.semiMajorAxisAu;
+  return a > 0 ? Math.pow(a, 1.5) * 365.25 : undefined;
+}

src/app/features/body-detail/info-panel.component.ts

@@ -1,8 +1,8 @@
-import { DecimalPipe } from '@angular/common';
-import { Component, input } from '@angular/core';
+import { Component, computed, input } from '@angular/core';
 import { Router } from '@angular/router';
 
 import { PLANET_CLASS_LABELS } from '../../shared/astro/planet-appearance';
+import { formatAu, formatDensity, formatMassEarth, formatPeriod, formatRadiusKm, formatTemperature } from '../../shared/format/quantity';
 import { BodyDetailViewModel } from './body-detail.model';
 
 const KIND_LABELS: Record<BodyDetailViewModel['kind'], string> = {
@@ -35,30 +35,35 @@ const KIND_LABELS: Record<BodyDetailViewModel['kind'], string> = {
       <h1 class="mb-0.5 font-display text-lg font-semibold tracking-wide text-text">{{ body().name }}</h1>
       <p class="mb-4 text-xs tracking-wide text-accent uppercase">{{ kindLabel() }} · {{ body().hostStarName }}</p>
 
+      <p class="mt-4 mb-1.5 text-[10px] tracking-[0.18em] text-muted uppercase">Measured</p>
       <dl class="grid grid-cols-[auto_1fr] gap-y-1.5 gap-x-3 text-sm">
         @if (body().radiusKm) {
           <dt class="text-muted">Radius</dt>
-          <dd class="text-right text-text">{{ body().radiusKm | number: '1.0-1' }} km</dd>
+          <dd class="text-right text-text tabular-nums">{{ radius() }}</dd>
         }
         @if (body().massEarth) {
           <dt class="text-muted">Mass</dt>
-          <dd class="text-right text-text">{{ body().massEarth | number: '1.0-2' }} Earth masses</dd>
+          <dd class="text-right text-text tabular-nums">{{ mass() }}</dd>
         }
         @if (body().orbit.semiMajorAxisAu) {
           <dt class="text-muted">Semi-major axis</dt>
-          <dd class="text-right text-text">{{ body().orbit.semiMajorAxisAu | number: '1.0-4' }} AU</dd>
+          <dd class="text-right text-text tabular-nums">{{ semiMajorAxis() }}</dd>
         }
         @if (body().orbit.eccentricity !== undefined) {
           <dt class="text-muted">Eccentricity</dt>
-          <dd class="text-right text-text">{{ body().orbit.eccentricity | number: '1.0-4' }}</dd>
+          <dd class="text-right text-text tabular-nums">{{ body().orbit.eccentricity!.toFixed(4) }}</dd>
         }
         @if (body().orbit.inclinationDeg !== undefined) {
           <dt class="text-muted">Inclination</dt>
-          <dd class="text-right text-text">{{ body().orbit.inclinationDeg | number: '1.0-2' }}°</dd>
+          <dd class="text-right text-text tabular-nums">{{ body().orbit.inclinationDeg!.toFixed(2) }}°</dd>
+        }
+        @if (measuredPeriod()) {
+          <dt class="text-muted">Period</dt>
+          <dd class="text-right text-text tabular-nums">{{ measuredPeriod() }}</dd>
         }
         @if (body().discoveryYear) {
           <dt class="text-muted">Discovered</dt>
-          <dd class="text-right text-text">{{ body().discoveryYear }}</dd>
+          <dd class="text-right text-text tabular-nums">{{ body().discoveryYear }}</dd>
         }
       </dl>
 
@@ -66,24 +71,41 @@ const KIND_LABELS: Record<BodyDetailViewModel['kind'], string> = {
       <dl class="grid grid-cols-[auto_1fr] gap-y-1.5 gap-x-3 text-sm">
         <dt class="text-muted">Class</dt>
         <dd class="text-right text-text">{{ classLabel() }}</dd>
+        @if (derivedPeriod()) {
+          <dt class="text-muted">Period</dt>
+          <dd class="text-right text-text tabular-nums">{{ derivedPeriod() }}</dd>
+        }
         @if (body().appearance.equilibriumTemperatureK !== null) {
           <dt class="text-muted">Equilibrium temp.</dt>
-          <dd class="text-right text-text">{{ body().appearance.equilibriumTemperatureK | number: '1.0-0' }} K</dd>
+          <dd class="text-right text-text tabular-nums">{{ temperature() }}</dd>
         }
         @if (body().appearance.bulkDensityGramsPerCm3 !== null) {
           <dt class="text-muted">Bulk density</dt>
-          <dd class="text-right text-text">{{ body().appearance.bulkDensityGramsPerCm3 | number: '1.0-2' }} g/cm³</dd>
+          <dd class="text-right text-text tabular-nums">{{ density() }}</dd>
         }
       </dl>
 
       <p class="mt-3 border-t border-border/50 pt-2 text-[10px] leading-relaxed text-muted">{{ surfaceProvenance() }}</p>
     </div>
-  `,
-  imports: [DecimalPipe]
+  `
 })
 export class InfoPanelComponent {
   readonly body = input.required<BodyDetailViewModel>();
 
+  readonly radius = computed(() => formatRadiusKm(this.body().radiusKm!));
+  readonly mass = computed(() => formatMassEarth(this.body().massEarth!));
+  readonly semiMajorAxis = computed(() => formatAu(this.body().orbit.semiMajorAxisAu!));
+  readonly temperature = computed(() => formatTemperature(this.body().appearance.equilibriumTemperatureK!));
+  readonly density = computed(() => formatDensity(this.body().appearance.bulkDensityGramsPerCm3!));
+  /** The period goes under whichever heading its provenance calls for, and nowhere otherwise. */
+  readonly measuredPeriod = computed(() => this.periodUnder('measured'));
+  readonly derivedPeriod = computed(() => this.periodUnder('derived'));
+
+  private periodUnder(source: 'measured' | 'derived'): string | null {
+    const body = this.body();
+    return body.orbitalPeriodDays !== undefined && body.orbitalPeriodSource === source ? formatPeriod(body.orbitalPeriodDays) : null;
+  }
+
   constructor(private readonly router: Router) {}
 
   kindLabel(): string {

src/app/features/galaxy-system/galaxy-system-scene.component.ts

@@ -5,7 +5,6 @@ import { OrbitControls } from 'three/addons/controls/OrbitControls.js';
 
 import { dateToJulianDate } from '../../shared/astro/constants';
 import { galacticCentrePositionPc, galacticToEquatorial, MILKY_WAY_ARMS, SUN_GALACTOCENTRIC_RADIUS_PC } from '../../shared/astro/galaxy';
-import { luminositySolar } from '../../shared/astro/stellar';
 import { DataLoaderService } from '../../core/data/data-loader.service';
 import { EngineService } from '../../core/engine/engine.service';
 import { BodyRecord } from '../../shared/models/body.model';
@@ -20,7 +19,11 @@ import { DeepSkyRenderer } from './deep-sky-renderer';
 import { galacticNormal, PolarGridPlane, TetherField } from './grid-plane';
 import { MilkyWayRenderer } from './milky-way-renderer';
 import { starGlowExtentAu, starMarkerRadiusAu, systemFrameRadiusAu, systemFramingDistanceAu, systemViewDirection } from './system-framing';
+import { formatAu, formatLuminosity, formatParsecs } from '../../shared/format/quantity';
+import { BodyDetailViewModel } from '../body-detail/body-detail.model';
+import { buildBodyViewModel, luminosityOf } from '../body-detail/body-view-model';
 import { HudReadout, StarmapHudComponent } from './starmap-hud.component';
+import { SystemObjectCardComponent } from './system-object-card.component';
 import { colorIndexToRgb, StarFieldRenderer, starRenderBudgetFromUrl } from './star-field-renderer';
 import { LabeledPoint, StarLabelOverlay } from './star-label-overlay';
 import { SystemOrbitsRenderer } from './system-orbits-renderer';
@@ -126,13 +129,7 @@ const RETURN_DURATION_SECONDS = 1.1;
 const GALACTIC_FLIGHT_SECONDS = 2.4;
 
 /** Camera range for the readout panel, in the unit that suits the distance. */
-function formatParsecs(distancePc: number): string {
-  return distancePc >= 1000 ? `${(distancePc / 1000).toFixed(1)} kpc` : `${distancePc.toFixed(distancePc < 10 ? 2 : 0)} pc`;
-}
 
-function formatAu(distanceAu: number): string {
-  return distanceAu >= 100 ? `${distanceAu.toFixed(0)} AU` : `${distanceAu.toFixed(2)} AU`;
-}
 
 /**
  * Where the camera sits to hold the whole Galaxy: above the disc and back past the Sun, looking
@@ -157,7 +154,7 @@ function galacticOverviewPose(): { position: THREE.Vector3; target: THREE.Vector
 @Component({
   selector: 'app-galaxy-system-scene',
   providers: [EngineService],
-  imports: [StarmapHudComponent],
+  imports: [StarmapHudComponent, SystemObjectCardComponent],
   template: `
     <div class="relative h-full w-full">
       <canvas #canvas data-testid="scene-canvas" class="block h-full w-full"></canvas>
@@ -172,6 +169,9 @@ function galacticOverviewPose(): { position: THREE.Vector3; target: THREE.Vector
         [range]="hudRange()"
         (levelSelected)="goToLevel($event)"
       />
+      @if (objectCard(); as card) {
+        <app-system-object-card [body]="card" (dismissed)="dismissObjectCard()" (openRequested)="openObjectDetail(card.id)" />
+      }
     </div>
   `
 })
@@ -194,6 +194,14 @@ export class GalaxySystemSceneComponent implements AfterViewInit, OnDestroy {
   readonly hudNote = signal('');
   readonly hudRange = signal('');
 
+  /**
+   * The body whose card is showing: whichever is pinned by a click, else whatever the pointer is
+   * over. Undefined outside the system view, and cleared when the view leaves one.
+   */
+  readonly objectCard = signal<BodyDetailViewModel | undefined>(undefined);
+  private pinnedBodyId: string | null = null;
+  private hoveredBodyId: string | null = null;
+
   private controls?: OrbitControls;
   private rig?: CameraRigController;
   private starField?: StarFieldRenderer;
@@ -251,6 +259,7 @@ export class GalaxySystemSceneComponent implements AfterViewInit, OnDestroy {
     this.resizeObserver?.disconnect();
     this.canvasRef().nativeElement.removeEventListener('pointerdown', this.handlePointerDown);
     this.canvasRef().nativeElement.removeEventListener('click', this.handleClick);
+    this.canvasRef().nativeElement.removeEventListener('pointermove', this.handlePointerMove);
     this.controls?.dispose();
     this.starField?.dispose();
     this.deepSky?.dispose();
@@ -389,6 +398,7 @@ export class GalaxySystemSceneComponent implements AfterViewInit, OnDestroy {
 
     canvas.addEventListener('pointerdown', this.handlePointerDown);
     canvas.addEventListener('click', this.handleClick);
+    canvas.addEventListener('pointermove', this.handlePointerMove);
     this.observeResize(canvas);
 
     this.unsubscribeTick = this.engine.onTick((deltaSeconds) => this.tick(camera, deltaSeconds));
@@ -628,13 +638,19 @@ export class GalaxySystemSceneComponent implements AfterViewInit, OnDestroy {
 
     if (this.systemGroup.visible && star) {
       const planetCount = this.bodies.filter((body) => body.systemStarId === star.id && !body.parentBodyId).length + this.exoplanets.filter((exoplanet) => exoplanet.hostStarId === star.id).length;
+      const moonCount = this.bodies.filter((body) => body.systemStarId === star.id && body.parentBodyId).length;
+      const distancePc = Math.hypot(star.x, star.y, star.z);
+      const luminosity = luminosityOf(star);
       this.hudEyebrow.set('System');
       this.hudTitle.set(star.name);
       this.hudSubtitle.set(star.spectralType ? `Spectral type ${star.spectralType}` : '');
       this.hudReadouts.set([
-        { label: 'Bodies', value: `${planetCount}` },
-        { label: 'Distance', value: `${Math.hypot(star.x, star.y, star.z).toFixed(2)} pc` },
-        { label: 'Magnitude', value: star.magnitude.toFixed(2) }
+        { label: 'Bodies', value: moonCount > 0 ? `${planetCount} + ${moonCount} moons` : `${planetCount}` },
+        // Suppressed for the Sun rather than printed as `0.00 pc`, which is arithmetically right
+        // and reads as a bug: the distance from here to here is not a measurement.
+        ...(distancePc > 0 ? [{ label: 'Distance', value: formatParsecs(distancePc) }] : []),
+        { label: 'Magnitude', value: star.magnitude.toFixed(2) },
+        ...(luminosity !== null ? [{ label: 'Luminosity', value: formatLuminosity(luminosity), derived: true }] : [])
       ]);
       this.hudNote.set('Orbits propagated from published elements to the current date.');
       this.hudRange.set(formatAu(camera.position.distanceTo(this.controls?.target ?? GALAXY_OVERVIEW_TARGET)));
@@ -665,7 +681,9 @@ export class GalaxySystemSceneComponent implements AfterViewInit, OnDestroy {
       // what it draws. See `STAR_RENDER_BUDGET`.
       { label: 'Stars', value: this.starField && this.starField.drawnCount < this.stars.length ? `${this.starField.drawnCount} / ${this.stars.length}` : `${this.stars.length}` },
       { label: 'Radius', value: `${LOCAL_GRID_RINGS_PC[LOCAL_GRID_RINGS_PC.length - 1]} pc` },
-      { label: 'Exoplanets', value: `${this.exoplanets.length}` }
+      { label: 'Exoplanets', value: `${this.exoplanets.length}` },
+      // The one thing the field itself cannot show: which of those points can be flown into.
+      { label: 'Systems', value: `${this.enterableSystemCount()}` }
     ]);
     this.hudNote.set('Positions from measured parallaxes. Grid marks the galactic plane through the Sun.');
   }
@@ -714,16 +732,98 @@ export class GalaxySystemSceneComponent implements AfterViewInit, OnDestroy {
     }
   }
 
+  /**
+   * Picks a body in the system view. Clicking one pins its card; clicking empty space unpins,
+   * which is also how the card is dismissed without aiming for its close control.
+   *
+   * This used to navigate straight to `/body/:id`. That tore down the system scene and the camera
+   * with it, so comparing two planets meant flying back into the system between each — the card
+   * shows the same numbers over the live view instead, and `Full view` still opens the route.
+   */
   private handleSystemClick(): void {
     if (!this.systemRenderer) {
       return;
     }
     const [hit] = this.raycaster.intersectObjects(this.systemRenderer.pickableObjects);
     const member = hit ? this.systemRenderer.memberForObject(hit.object) : undefined;
+
+    this.pinnedBodyId = member ? member.id : null;
     if (member) {
       this.navigationStore.selectBody(member.id);
-      void this.router.navigate(['/body', member.id]);
     }
+    this.refreshObjectCard();
+  }
+
+  /**
+   * Hover preview, so a body's figures can be read without committing a click.
+   *
+   * The raycast is against the system's own handful of pickable meshes rather than the star field,
+   * so it stays cheap even on a software rasterizer — it is the rendering that is slow in that
+   * environment, not the picking. Skipped outside the system view and during a camera flight.
+   */
+  private readonly handlePointerMove = (event: PointerEvent): void => {
+    if (!this.systemRenderer || !this.systemGroup.visible || this.rig?.isAnimating) {
+      return;
+    }
+    const canvas = this.canvasRef().nativeElement;
+    const rect = canvas.getBoundingClientRect();
+    const pointerNdc = new THREE.Vector2(((event.clientX - rect.left) / rect.width) * 2 - 1, -((event.clientY - rect.top) / rect.height) * 2 + 1);
+    this.raycaster.setFromCamera(pointerNdc, this.engine.getCamera());
+
+    const [hit] = this.raycaster.intersectObjects(this.systemRenderer.pickableObjects);
+    const hoveredId = (hit ? this.systemRenderer.memberForObject(hit.object) : undefined)?.id ?? null;
+    if (hoveredId === this.hoveredBodyId) {
+      return;
+    }
+    this.hoveredBodyId = hoveredId;
+    canvas.style.cursor = hoveredId ? 'pointer' : '';
+    this.refreshObjectCard();
+  };
+
+  /** A pinned body wins over a hovered one, so the card does not change under the pointer. */
+  private refreshObjectCard(): void {
+    const id = this.pinnedBodyId ?? this.hoveredBodyId;
+    this.objectCard.set(id === null ? undefined : buildBodyViewModel(id, { bodies: this.bodies, exoplanets: this.exoplanets, stars: this.stars }));
+  }
+
+  /** Clears the card and everything that would bring it straight back. */
+  private clearObjectCard(): void {
+    this.pinnedBodyId = null;
+    this.hoveredBodyId = null;
+    this.objectCard.set(undefined);
+    this.canvasRef().nativeElement.style.cursor = '';
+  }
+
+  dismissObjectCard(): void {
+    this.clearObjectCard();
+  }
+
+  /** The deliberate step out to the dedicated route, from the card's own control. */
+  openObjectDetail(id: string): void {
+    this.navigationStore.selectBody(id);
+    void this.router.navigate(['/body', id]);
+  }
+
+  /**
+   * How many stars can actually be entered: those with catalogued bodies of their own, plus the
+   * Sun. Computed once rather than on every HUD refresh.
+   */
+  private enterableSystems?: number;
+
+  private enterableSystemCount(): number {
+    if (this.enterableSystems === undefined) {
+      const hosts = new Set<number>();
+      for (const body of this.bodies) {
+        hosts.add(body.systemStarId);
+      }
+      for (const exoplanet of this.exoplanets) {
+        if (exoplanet.hostStarId !== null) {
+          hosts.add(exoplanet.hostStarId);
+        }
+      }
+      this.enterableSystems = hosts.size;
+    }
+    return this.enterableSystems;
   }
 
   /** Reacts to `NavigationStore.selectedStarId` changes coming from any source (click/search). */
@@ -797,7 +897,7 @@ export class GalaxySystemSceneComponent implements AfterViewInit, OnDestroy {
     // exoplanets, so it has no meaningful direction and the renderer falls back.
     // The star's luminosity, derived from its own catalogued magnitude and distance, is what
     // decides how hot each body in the system is — and so what each of them looks like.
-    const hostLuminosity = luminositySolar({ magnitude: star.magnitude, distancePc: Math.hypot(star.x, star.y, star.z), spectralType: star.spectralType });
+    const hostLuminosity = luminosityOf(star);
     this.systemRenderer = new SystemOrbitsRenderer(systemBodies, systemExoplanets, { x: star.x, y: star.y, z: star.z }, hostLuminosity);
     this.systemGroup.add(this.systemRenderer.object);
 
@@ -881,6 +981,9 @@ export class GalaxySystemSceneComponent implements AfterViewInit, OnDestroy {
 
     this.systemGroup.visible = false;
     this.galaxyGroup.visible = true;
+    // The bodies it described are no longer on screen, and a stale pin would otherwise survive
+    // into the next system entered.
+    this.clearObjectCard();
 
     camera.near = GALAXY_NEAR_PC;
     camera.far = GALAXY_FAR_PC;

src/app/features/galaxy-system/starmap-hud.component.ts

@@ -5,6 +5,12 @@ import { ViewLevel } from '../../shared/state/navigation.store';
 export interface HudReadout {
   readonly label: string;
   readonly value: string;
+  /**
+   * True when the figure was computed from other measurements rather than catalogued directly.
+   * Marked in the panel and explained in its footnote, so a reasoned number is never mistaken for
+   * an observed one.
+   */
+  readonly derived?: boolean;
 }
 
 interface LadderStep {
@@ -100,14 +106,14 @@ const LADDER: readonly { level: ViewLevel; label: string }[] = [
           <dl class="mt-3 flex flex-wrap gap-x-6 gap-y-1">
             @for (readout of readouts(); track readout.label) {
               <div>
-                <dt class="text-[10px] tracking-[0.18em] text-muted uppercase">{{ readout.label }}</dt>
+                <dt class="text-[10px] tracking-[0.18em] text-muted uppercase">{{ readout.label }}@if (readout.derived) {<span class="text-accent/80" aria-hidden="true">*</span>}</dt>
                 <dd class="text-sm text-text tabular-nums">{{ readout.value }}</dd>
               </div>
             }
           </dl>
         }
-        @if (note()) {
-          <p class="mt-3 border-t border-border/50 pt-2 text-[10px] leading-relaxed tracking-[0.08em] text-muted uppercase">{{ note() }}</p>
+        @if (note() || hasDerived()) {
+          <p class="mt-3 border-t border-border/50 pt-2 text-[10px] leading-relaxed tracking-[0.08em] text-muted uppercase">@if (hasDerived()) {<span class="text-accent/80">*</span> Derived, not catalogued. }{{ note() }}</p>
         }
       </div>
 
@@ -127,6 +133,9 @@ export class StarmapHudComponent {
   readonly readouts = input<readonly HudReadout[]>([]);
   /** Standing caveat for the current view, e.g. that galactic structure is a model. */
   readonly note = input('');
+  /** Whether any readout needs the derived-value footnote. */
+  readonly hasDerived = computed(() => this.readouts().some((readout) => readout.derived));
+
   /** Camera range, pre-formatted by the scene, which is the only thing that knows the units. */
   readonly range = input('');
   readonly showReticle = input(true);

src/app/features/galaxy-system/system-object-card.component.spec.ts

@@ -0,0 +1,116 @@
+import { ComponentFixture, TestBed } from '@angular/core/testing';
+import { beforeEach, describe, expect, it } from 'vitest';
+
+import { BodyDetailViewModel } from '../body-detail/body-detail.model';
+import { PlanetAppearance } from '../../shared/astro/planet-appearance';
+import { SystemObjectCardComponent } from './system-object-card.component';
+
+const appearance = (overrides: Partial<PlanetAppearance> = {}): PlanetAppearance =>
+  ({
+    planetClass: 'temperate',
+    palette: { structure: 'mottled' },
+    equilibriumTemperatureK: 255,
+    bulkDensityGramsPerCm3: 5.51,
+    polarCapExtentDeg: 25,
+    seed: 1,
+    ...overrides,
+  }) as PlanetAppearance;
+
+const earth: BodyDetailViewModel = {
+  id: 'earth',
+  name: 'Earth',
+  kind: 'planet',
+  hostStarName: 'Sol',
+  radiusKm: 6371,
+  orbit: { semiMajorAxisAu: 1, eccentricity: 0.0167 },
+  appearance: appearance(),
+  hasPhotography: true,
+  orbitalPeriodDays: 365.25,
+  orbitalPeriodSource: 'derived',
+};
+
+describe('SystemObjectCardComponent', () => {
+  let fixture: ComponentFixture<SystemObjectCardComponent>;
+
+  function render(body: BodyDetailViewModel): HTMLElement {
+    fixture.componentRef.setInput('body', body);
+    fixture.detectChanges();
+    return fixture.nativeElement as HTMLElement;
+  }
+
+  /** The label/value pairs under one of the two headings, in order. */
+  function block(host: HTMLElement, heading: 'Measured' | 'Derived'): string[] {
+    const headings = [...host.querySelectorAll('p')].filter((p) => p.textContent?.trim() === heading);
+    const list = headings[0]?.nextElementSibling;
+    return list ? [...list.querySelectorAll('dt')].map((dt) => dt.textContent?.trim() ?? '') : [];
+  }
+
+  beforeEach(async () => {
+    await TestBed.configureTestingModule({
+      imports: [SystemObjectCardComponent],
+    }).compileComponents();
+    fixture = TestBed.createComponent(SystemObjectCardComponent);
+  });
+
+  it('files a period computed from the semi-major axis under Derived', () => {
+    const host = render(earth);
+    expect(block(host, 'Derived')).toContain('Period');
+    expect(block(host, 'Measured')).not.toContain('Period');
+  });
+
+  it('files a published period under Measured instead', () => {
+    // Same field, opposite heading — the provenance flag is what decides, not the field's name.
+    const host = render({ ...earth, orbitalPeriodSource: 'measured' });
+    expect(block(host, 'Measured')).toContain('Period');
+    expect(block(host, 'Derived')).not.toContain('Period');
+  });
+
+  it('omits the period entirely when there is none to show', () => {
+    const host = render({ ...earth, orbitalPeriodDays: undefined, orbitalPeriodSource: undefined });
+    expect(block(host, 'Measured')).not.toContain('Period');
+    expect(block(host, 'Derived')).not.toContain('Period');
+  });
+
+  it('never shows an empty Measured block', () => {
+    const bare: BodyDetailViewModel = {
+      ...earth,
+      radiusKm: undefined,
+      orbit: {},
+      orbitalPeriodDays: undefined,
+      orbitalPeriodSource: undefined,
+      discoveryYear: undefined,
+    };
+    expect(render(bare).textContent).not.toContain('Measured');
+  });
+
+  it('says a photographed surface is a photograph', () => {
+    expect(render(earth).textContent).toContain('photography');
+  });
+
+  it('says an illustrated surface is not an observation', () => {
+    expect(render({ ...earth, hasPhotography: false }).textContent).toContain('Not an observation');
+  });
+
+  it('explains a missing temperature rather than leaving the row blank', () => {
+    const host = render({
+      ...earth,
+      hasPhotography: false,
+      appearance: appearance({ equilibriumTemperatureK: null }),
+    });
+    expect(block(host, 'Derived')).not.toContain('Equilibrium temp.');
+    expect(host.textContent).toContain('No host star in the catalogue');
+  });
+
+  it('emits rather than navigating, so the scene decides what selection means', () => {
+    const host = render(earth);
+    let opened = 0;
+    let dismissed = 0;
+    fixture.componentInstance.openRequested.subscribe(() => (opened += 1));
+    fixture.componentInstance.dismissed.subscribe(() => (dismissed += 1));
+
+    host.querySelector<HTMLButtonElement>('button[aria-label="Close"]')!.click();
+    [...host.querySelectorAll('button')].find((button) => button.textContent?.includes('Full view'))!.click();
+
+    expect({ opened, dismissed }).toEqual({ opened: 1, dismissed: 1 });
+  });
+});

src/app/features/galaxy-system/system-object-card.component.ts

@@ -0,0 +1,165 @@
+import { ChangeDetectionStrategy, Component, computed, input, output } from '@angular/core';
+
+import { BodyDetailViewModel } from '../body-detail/body-detail.model';
+import { PLANET_CLASS_LABELS } from '../../shared/astro/planet-appearance';
+import { formatAu, formatDensity, formatMassEarth, formatPeriod, formatRadiusKm, formatTemperature } from '../../shared/format/quantity';
+
+const KIND_LABELS: Record<BodyDetailViewModel['kind'], string> = {
+  planet: 'Planet',
+  moon: 'Moon',
+  dwarf: 'Dwarf planet',
+  exoplanet: 'Exoplanet',
+};
+
+interface CardRow {
+  readonly label: string;
+  readonly value: string;
+}
+
+/**
+ * The card shown for a body picked in the system view, without leaving it.
+ *
+ * Selecting a planet used to navigate straight to `/body/:id`, which tore down the system scene
+ * and the camera position with it — so comparing two planets meant flying back in twice. This
+ * shows the same numbers over the live view, and keeps the route as the deliberate step for the
+ * full 3D inspection.
+ *
+ * Measured and derived quantities are kept in separate blocks, each labelled, because the
+ * difference matters here more than it usually would: no exoplanet has been imaged and several
+ * of these figures are reasoned rather than observed. Presentational only.
+ */
+@Component({
+  selector: 'app-system-object-card',
+  changeDetection: ChangeDetectionStrategy.OnPush,
+  // Positioned and full-bleed like the HUD's own host, so the panel inside it resolves against
+  // the scene rather than against whatever box the inline default would have left it in — which
+  // put the card off the bottom-left corner of the viewport entirely.
+  host: { class: 'pointer-events-none absolute inset-0 block' },
+  template: `
+    <!-- Positioning and panel styling stay on separate elements. The hud-panel rule sets
+         position: relative for its own inset border, which silently overrides an absolute on the
+         same element and turns top/right into offsets from wherever the box already sat. -->
+    <div class="pointer-events-auto absolute top-16 right-6 w-80 max-w-[calc(100%-3rem)]">
+      <div data-testid="object-card" class="hud-panel px-4 py-3 font-body">
+        <div class="flex items-start justify-between gap-3">
+          <div class="min-w-0">
+            <p class="truncate font-display text-lg tracking-[0.04em] text-text">
+              {{ body().name }}
+            </p>
+            <p class="mt-0.5 font-display text-[10px] tracking-[0.22em] text-accent uppercase">{{ kindLabel() }} · {{ body().hostStarName }}</p>
+          </div>
+          <button
+            type="button"
+            (click)="dismissed.emit()"
+            aria-label="Close"
+            class="-mt-1 -mr-1 shrink-0 rounded-sm p-1 text-muted transition-colors hover:text-accent focus:text-accent focus:outline-none"
+          >
+            <svg class="h-3.5 w-3.5" viewBox="0 0 14 14" fill="none" stroke="currentColor" stroke-width="1.5" stroke-linecap="round">
+              <path d="M3 3l8 8M11 3l-8 8" />
+            </svg>
+          </button>
+        </div>
+
+        @if (measured().length) {
+          <p class="mt-3 text-[10px] tracking-[0.18em] text-muted uppercase">Measured</p>
+          <dl class="mt-1 grid grid-cols-[auto_1fr] gap-x-3 gap-y-1 text-sm">
+            @for (row of measured(); track row.label) {
+              <dt class="text-muted">{{ row.label }}</dt>
+              <dd class="text-right text-text tabular-nums">{{ row.value }}</dd>
+            }
+          </dl>
+        }
+
+        <p class="mt-3 text-[10px] tracking-[0.18em] text-muted uppercase">Derived</p>
+        <dl class="mt-1 grid grid-cols-[auto_1fr] gap-x-3 gap-y-1 text-sm">
+          @for (row of derived(); track row.label) {
+            <dt class="text-muted">{{ row.label }}</dt>
+            <dd class="text-right text-text tabular-nums">{{ row.value }}</dd>
+          }
+        </dl>
+
+        <p class="mt-3 border-t border-border/50 pt-2 text-[10px] leading-relaxed text-muted">
+          {{ provenance() }}
+        </p>
+
+        <button
+          type="button"
+          (click)="openRequested.emit()"
+          class="mt-3 flex w-full items-center justify-center gap-1.5 border border-border/70 bg-panel/60 px-3 py-1.5 font-display text-[10px] tracking-[0.22em] text-muted uppercase transition-colors hover:border-accent hover:text-accent focus:border-accent focus:text-accent focus:outline-none"
+        >
+          Full view
+          <svg class="h-3 w-3" viewBox="0 0 24 24" fill="none" stroke="currentColor" stroke-width="2" stroke-linecap="round" stroke-linejoin="round">
+            <path d="M9 6l6 6-6 6" />
+          </svg>
+        </button>
+      </div>
+    </div>
+  `,
+})
+export class SystemObjectCardComponent {
+  readonly body = input.required<BodyDetailViewModel>();
+  /** The close control, and anything else that should dismiss the card. */
+  readonly dismissed = output<void>();
+  /** Request for the full `/body/:id` route. */
+  readonly openRequested = output<void>();
+
+  readonly kindLabel = computed(() => KIND_LABELS[this.body().kind]);
+
+  /** Only what a catalogue actually published for this body. */
+  readonly measured = computed<readonly CardRow[]>(() => {
+    const body = this.body();
+    const rows: CardRow[] = [];
+    if (body.radiusKm !== undefined) {
+      rows.push({ label: 'Radius', value: formatRadiusKm(body.radiusKm) });
+    }
+    if (body.massEarth !== undefined) {
+      rows.push({ label: 'Mass', value: formatMassEarth(body.massEarth) });
+    }
+    if (body.orbit.semiMajorAxisAu !== undefined) {
+      rows.push({ label: 'Semi-major axis', value: formatAu(body.orbit.semiMajorAxisAu) });
+    }
+    if (body.orbit.eccentricity !== undefined) {
+      rows.push({ label: 'Eccentricity', value: body.orbit.eccentricity.toFixed(3) });
+    }
+    if (body.orbitalPeriodDays !== undefined && body.orbitalPeriodSource === 'measured') {
+      rows.push({ label: 'Period', value: formatPeriod(body.orbitalPeriodDays) });
+    }
+    if (body.discoveryYear !== undefined) {
+      rows.push({ label: 'Discovered', value: `${body.discoveryYear}` });
+    }
+    return rows;
+  });
+
+  /** Everything computed from the measurements above rather than observed directly. */
+  readonly derived = computed<readonly CardRow[]>(() => {
+    const body = this.body();
+    const rows: CardRow[] = [{ label: 'Class', value: PLANET_CLASS_LABELS[body.appearance.planetClass] }];
+    if (body.orbitalPeriodDays !== undefined && body.orbitalPeriodSource === 'derived') {
+      rows.push({ label: 'Period', value: formatPeriod(body.orbitalPeriodDays) });
+    }
+    if (body.appearance.equilibriumTemperatureK !== null) {
+      rows.push({
+        label: 'Equilibrium temp.',
+        value: formatTemperature(body.appearance.equilibriumTemperatureK),
+      });
+    }
+    if (body.appearance.bulkDensityGramsPerCm3 !== null) {
+      rows.push({
+        label: 'Bulk density',
+        value: formatDensity(body.appearance.bulkDensityGramsPerCm3),
+      });
+    }
+    return rows;
+  });
+
+  /** The same distinction the detail panel draws, stated briefly enough for a card. */
+  readonly provenance = computed(() => {
+    const body = this.body();
+    if (body.hasPhotography) {
+      return 'Surface: NASA/ESA/USGS photography.';
+    }
+    return body.appearance.equilibriumTemperatureK === null
+      ? 'Surface illustrated from measured size and mass. No host star in the catalogue, so no temperature could be derived.'
+      : 'Surface illustrated from the measurements above. Not an observation — no image of this world exists.';
+  });
+}

src/app/shared/format/quantity.spec.ts

@@ -0,0 +1,84 @@
+import { describe, expect, it } from 'vitest';
+
+import { formatAu, formatDensity, formatLuminosity, formatMassEarth, formatParsecs, formatPeriod, formatRadiusKm, formatTemperature } from './quantity';
+
+describe('formatParsecs', () => {
+  it('switches to kiloparsecs past a thousand parsecs', () => {
+    expect(formatParsecs(8180)).toBe('8.2 kpc');
+  });
+
+  it('keeps two decimals only while they carry information', () => {
+    expect(formatParsecs(1.3)).toBe('1.30 pc');
+    expect(formatParsecs(250)).toBe('250 pc');
+  });
+});
+
+describe('formatAu', () => {
+  it('holds four decimals for close-in orbits', () => {
+    // 0.0026 AU is a real published semi-major axis; two decimals would render it — and every
+    // other hot Jupiter — as a flat 0.00.
+    expect(formatAu(0.0026)).toBe('0.0026 AU');
+  });
+
+  it('drops to whole units past 100 AU', () => {
+    expect(formatAu(120)).toBe('120 AU');
+    expect(formatAu(5.53)).toBe('5.53 AU');
+  });
+});
+
+describe('formatPeriod', () => {
+  it('uses hours below a day', () => {
+    expect(formatPeriod(0.5)).toBe('12.0 h');
+  });
+
+  it('uses days through the short end and years beyond', () => {
+    expect(formatPeriod(88)).toBe('88.0 d');
+    expect(formatPeriod(365.25)).toBe('365.3 d');
+    expect(formatPeriod(4332.6)).toBe('11.9 yr');
+  });
+
+  it('drops the decimal for periods of centuries', () => {
+    expect(formatPeriod(90_560)).toBe('248 yr');
+  });
+});
+
+describe('formatRadiusKm', () => {
+  it('groups thousands for large bodies', () => {
+    expect(formatRadiusKm(69_911)).toBe('69,911 km');
+  });
+
+  it('keeps a decimal only for small ones', () => {
+    expect(formatRadiusKm(6371)).toBe('6371 km');
+    expect(formatRadiusKm(11.3)).toBe('11.3 km');
+  });
+});
+
+describe('formatMassEarth', () => {
+  it('scales precision to magnitude', () => {
+    expect(formatMassEarth(0.815)).toBe('0.815 M⊕');
+    expect(formatMassEarth(17.15)).toBe('17.15 M⊕');
+    expect(formatMassEarth(317.8)).toBe('318 M⊕');
+  });
+});
+
+describe('formatTemperature and formatDensity', () => {
+  it('rounds temperature to a whole kelvin', () => {
+    expect(formatTemperature(254.6)).toBe('255 K');
+  });
+
+  it('holds two decimals of density', () => {
+    expect(formatDensity(5.514)).toBe('5.51 g/cm³');
+  });
+});
+
+describe('formatLuminosity', () => {
+  it('stays decimal across the ordinary range', () => {
+    expect(formatLuminosity(1)).toBe('1.00 L☉');
+    expect(formatLuminosity(0.0017)).toBe('0.002 L☉');
+  });
+
+  it('goes to powers of ten at the extremes', () => {
+    expect(formatLuminosity(126_000)).toBe('1.3×10⁵ L☉');
+    expect(formatLuminosity(0.00004)).toBe('4.0×10⁻⁵ L☉');
+  });
+});

src/app/shared/format/quantity.ts

@@ -0,0 +1,80 @@
+/**
+ * Formatting for every physical quantity the HUD and the body cards display.
+ *
+ * Shared rather than per-component so the same measurement reads identically wherever it
+ * appears. The rule throughout is that precision follows magnitude — a figure is shown to the
+ * digits that carry information at its own scale, not to a fixed decimal count that reads as
+ * false precision at one end (`0.00 pc`) and loses real information at the other (`26000 pc`).
+ */
+
+/** Distance in parsecs, switching to kiloparsecs where the number would otherwise run long. */
+export function formatParsecs(distancePc: number): string {
+  return distancePc >= 1000 ? `${(distancePc / 1000).toFixed(1)} kpc` : `${distancePc.toFixed(distancePc < 10 ? 2 : 0)} pc`;
+}
+
+/** Distance in astronomical units, for anything inside a system. */
+export function formatAu(distanceAu: number): string {
+  if (distanceAu < 0.01) {
+    // Close-in exoplanets: 0.0026 AU is a real, published semi-major axis, and two decimals
+    // would round every hot Jupiter in the catalogue to the same `0.00`.
+    return `${distanceAu.toFixed(4)} AU`;
+  }
+  return distanceAu >= 100 ? `${distanceAu.toFixed(0)} AU` : `${distanceAu.toFixed(2)} AU`;
+}
+
+/**
+ * Orbital period in whichever unit reads naturally at its length — hours for the very short
+ * periods common among hot Jupiters, days up to a couple of years, then years.
+ */
+export function formatPeriod(days: number): string {
+  if (days < 1) {
+    return `${(days * 24).toFixed(1)} h`;
+  }
+  if (days < 700) {
+    return `${days.toFixed(days < 10 ? 2 : 1)} d`;
+  }
+  return `${(days / 365.25).toFixed(days / 365.25 < 100 ? 1 : 0)} yr`;
+}
+
+/** Radius in kilometres, or Earth radii once the number stops being legible in km. */
+export function formatRadiusKm(radiusKm: number): string {
+  return radiusKm >= 10_000 ? `${Math.round(radiusKm).toLocaleString('en-GB')} km` : `${radiusKm.toFixed(radiusKm < 100 ? 1 : 0)} km`;
+}
+
+/** Mass in Earth masses. */
+export function formatMassEarth(massEarth: number): string {
+  if (massEarth >= 100) {
+    return `${massEarth.toFixed(0)} M⊕`;
+  }
+  return `${massEarth.toFixed(massEarth < 1 ? 3 : 2)} M⊕`;
+}
+
+/** Equilibrium temperature. Always a whole kelvin — the model is not good to a fraction of one. */
+export function formatTemperature(kelvin: number): string {
+  return `${Math.round(kelvin)} K`;
+}
+
+/** Bulk density. */
+export function formatDensity(gramsPerCm3: number): string {
+  return `${gramsPerCm3.toFixed(2)} g/cm³`;
+}
+
+/** Bolometric luminosity in solar units, which spans many orders of magnitude. */
+export function formatLuminosity(solar: number): string {
+  if (solar >= 1000 || (solar > 0 && solar < 0.001)) {
+    const exponent = Math.floor(Math.log10(solar));
+    return `${(solar / Math.pow(10, exponent)).toFixed(1)}×10${superscript(exponent)} L☉`;
+  }
+  return `${solar.toFixed(solar < 1 ? 3 : 2)} L☉`;
+}
+
+const SUPERSCRIPTS = ['⁰', '¹', '²', '³', '⁴', '⁵', '⁶', '⁷', '⁸', '⁹'];
+
+function superscript(value: number): string {
+  const digits = Math.abs(value)
+    .toString()
+    .split('')
+    .map((digit) => SUPERSCRIPTS[Number(digit)])
+    .join('');
+  return value < 0 ? `⁻${digits}` : digits;
+}