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1 known planet · 904.4 light years

HAT-P-21

HAT-P-21 is a yellow star (G3) 904.4 light years away, with 1 confirmed planet. Distances and orbital speeds are to scale; the star and planets are enlarged so you can see them.

Day 0
habitable zone (estimate)planet orbits

Green band: where liquid water could exist on a planet's surface. Drag to turn the system, pinch or scroll to zoom.

The star

The star

HAT-P-21

Type
G3
Surface
≈ 5,588 K
Size
1.21 × the Sun
Mass
1.24 × the Sun
Brightness
1.52 × the Sun
Habitable zone
0.936–2.22 AU

The planets

HAT-P-21

HAT-P-21 b

Year
4.12 days
Distance from star
0.0504 AU
Mass
4.9 Jupiters
Size
12 × Earth
Temperature
≈ 1367 K (+1094 °C)
Found
2010, transit (HATNet)
Too hot

Masses found by the wobble method are minimums, because the orbit's tilt is unknown. Temperatures are equilibrium estimates for a bare planet, before any atmosphere.

How we know

The shadow

The planet passes in front of its star and dims it by a fraction of a percent, every orbit. The dip gives the planet's size and year.

Sources

  1. Planet and star parameters: NASA Exoplanet Archive, Planetary Systems Composite Parameters, retrieved 1 Nov 2026. This research has made use of the NASA Exoplanet Archive, which is operated by the California Institute of Technology, under contract with NASA under the Exoplanet Exploration Program.
  2. Position: HYG Database v4.1 (CC BY-SA 4.0).
  3. Habitable zone: a simple estimate from the star's brightness (√L × 0.95 to 1.67 AU), shown as a guide.
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