Imaginary Picture of HAT-P-21 b
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Exoplanet Name : HAT-P-21 b
Alternate Name :
Radius (Rjup / Re): 1.0240 (Rjup) 11.4780 (Rearth)
Mass (Mjup / Me): 4.0630 (Mjup) 1291.7821 (Mearth)
Semi Major Axis / Orbital Distance Calculated 0.0494 (AU) 0.0494 (AU Est.)
Eccentricity : 0.2280
Orbital Period(Yrs) 0.0113
Orbital Period(Observed/Estimated) 4.12 (Days Obs.) 4.12 (Days Est.)
Star Radiation at Atmospheric Boundary: 598275.3 (W/m2)
Measured Temperature (K) 0.0 (K)
Blackbody T 0.1(K) 1241.3 (K)
Blackbody T 0.3(K) 1165.7 (K)
Blackbody T 0.7(K) 943.2 (K)
Tidally Locked Blackbody T 0.1(K) 1476.1 (K)
Tidally Locked Blackbody T 0.3(K) 1386.2 (K)
Size Class: Jupiter-size
Mass Class:
Detection Type : Primary Transit
Mass Detection :
Radius Detection :
Habitability by Solar Eq. Astronomical Unit: Not habitable
Habitability by Kopparapu: Not habitable
Star Name : HAT-P-21
Alternate Name :
Star Distance(LY) : 874.4
Stellar Radius(Rsun) : 1.1 (Rsun Measured)
Stellar Mass(Msun) : 0.9 (Msun Measured) 0.9 (Msun Estimated)
Star Age : 10.2 (GY)
Temperature : 5588 (K)
Spectral type : G3
Metalicity : 0.0
Absolute Magnitude: 4.3
Apparent Magnitude: 11.5
Right Ascension(RA) 171.2749
Declination (DEC) 41.0281
Hubble Space Telescope Guide Star Catalog (GSC) ID 03013-01229
Discovered by
Discovery Paper - Authors HOG E.; FABRICIUS C.; MAKAROV V.V.; URBAN S.; CORBIN T.; WYCOFF G.; BASTIAN U.; SCHWEKENDIEK P.; WICENEC A.
Discovery Paper - Title The Tycho-2 catalogue of the 2.5 million brightest stars.
Discovery Paper - Journal 0
Discovery Paper - URL 0
SIMBAD LINK http://simbad.u-strasbg.fr/simbad/sim-basic?Ident=HAT-P-21+&submit=SIMBAD+search
Open Exoplanet Database Link http://www.openexoplanetcatalogue.com/planet/HAT-P-21%20b/
HAT-P-21 b
HAT-P-21 b

  • HAT-P-21 b is an exoplanet orbiting the star HAT-P-21, located about 874.4 light-years (268.1 pc) away from Solar System. Its discovery was publicly announced on 2010.
  • The host star HAT-P-21 has apparent magnitude of 11.5, with absolute magnitude of 4.3.
  • It is 0.9 times more massive and 1.1 times bigger compared with our Sun. The surface temperature is 5588 with its spectral types of G3
  • In this planetary system, the extrasolar planet HAT-P-21 b orbits around the star HAT-P-21 every 4.1 days with its orbital distance of 0.05 AU ( 7390134.8 km)
  • (Projected orbit of HAT-P-21 b around its host star HAT-P-21)


  • The Star HAT-P-21 's habitable zone is located at the following distance
  • Inner Boundary (the orbital distance at Venus's Equivalent Radiation ) : 0.747 AU ( 111823526.8 km)
  • Earth Boundary (the orbital distance at Earth's Equivalent Radiation) : 1.033 AU ( 154569337.0 km)
  • Outer Boundary (the orbital distance at Mars's Equivalent Radiation) : 1.574 AU ( 235529463.5 km)
  • Snow Line (the orbital distance at Snow Line Equivalent Radiation) : 2.317 AU ( 346591923.4 km)
  • Radiation at Planetary Boundary of HAT-P-21 b : 598275.28 W/m2

    (Habitable zone calculated based on SEAU(Solar Equivalent Astronomical Unit) around the star HAT-P-21)



  • Kopparapu Recent Venus for Star HAT-P-21 distance : 0.784 AU
  • Kopparapu Runaway Greenhouse for Earth-sized planet for Star HAT-P-21 distance : 0.992 AU
  • Kopparapu Runaway Greenhouse for Super-Earth-sized planet for Star HAT-P-21 distance : 0.958 AU
  • Kopparapu Runaway Greenhouse for Mars-sized planet for Star HAT-P-21 distance : 1.051 AU
  • Kopparapu Outer Boundary for Maximum Greenhouse for Star HAT-P-21 distance : 1.762 AU
  • Kopparapu Outer Boundary for Early Mars for Star HAT-P-21 distance : 1.858 AU

    (Habitable zone calculated based on Kopparapu et al.(2013) around the star HAT-P-21)



  • Original Kopparapu Recent Venus for Star HAT-P-21 distance : 0.782 AU
  • Original Kopparapu Runaway Greenhouse for Star HAT-P-21 distance : 1.020 AU
  • Original Kopparapu Moist Greenhouse for Star HAT-P-21 distance : 1.034 AU
  • Original Kopparapu Outer Boundary for Maximum Greenhouse for Star HAT-P-21 distance : 1.792 AU
  • Original Kopparapu Outer Boundary for Early Mars for Star HAT-P-21 distance : 1.864 AU

    (Habitable zone calculated based on Kopparapu et al.(Original) around the star HAT-P-21)



    (Position in Stellar Map of star HAT-P-21 and its Exoplanet HAT-P-21 b)



    (Zoomed position in Stellar Map of star HAT-P-21 and its Exoplanet HAT-P-21 b (zoom level 3))