Key parameters at a glance
- A radius of 12.44 Earth radii
- A mass of 1,547.83 Earth masses
- Surface gravity around 10.00 g
- An orbital period of 4.124 days
- Semi-major axis 0.0504 AU
- Equilibrium temperature 1,367 K (1094 °C)
- Distance from Earth 904.36 light-years
- Earth Similarity Index 0.120
- Travel time at Voyager 1 speed 15,948,367 years
HAT-P-21 b Planet Profile
Physical Specs
Classification
Planet Type
Gas Giant
Discovery
Size rank in cohort
Rank by radius
#1287of 1771
top 72.6%
This planet
12.44R⊕
Gas Giant median
13.52R⊕
Nearest-size peers
| Metric | Earth | HAT-P-21 b | Jupiter |
|---|---|---|---|
| Radius (R⊕) | 1.00 | 12.44 | 11.21 |
| Mass (M⊕) | 1.00 | 1,547.83 | 317.83 |
| Density (g/cm³) | 5.51 | 3.63 | 1.33 |
| Surface Gravity (g) | 1.00 | 10.00 | 2.53 |
| Insolation (S⊕) | 1.00 | 459.54 | 0.037 |
Alternative Mass Estimates
| Method | Value | Interpretation |
|---|---|---|
| Best measured mass | 1,547.832 M⊕ | Direct mass measurement (not derived). |
Star Catalogue Identifiers
TIC
TIC 17993892
Gaia DR2
Gaia DR2 770622651659107712
Gaia DR3
Gaia DR3 770622651659107712
System
HAT-P-21
Percentile among Gas Giant cohort
Orbit & Habitability
Orbital Dynamics
Year Length
A year here lasts approximately 4.12 Earth days (1.1% of a terrestrial year) on a mildly elliptical path at a mean orbital distance of 0.0504 AU.
Transit Fingerprint
Depth
0.900 %
Duration
3.672 h
Impact parameter b
0.615
Rp / R★
0.093100
Transit midpoint (BJD)
2,458,159.8964
Photometric dip
Transit produces a flux drop of 9,000 ppm lasting ≈ 3.67 h.
Extended Orbital Architecture
Planet / star radius ratio
0.093100
Semi-major axis / stellar radius
9.650
Impact parameter (b)
0.615
RV semi-amplitude (K)
548.300 m/s
Transit mid-time (BJD)
2,458,159.8964
Long. of periastron (ω)
305.80°
Sky-projected obliquity (λ)
-0.70°
True obliquity (ψ)
25.00°
Angular separation (arcsec)
0.18200
Eq. Temperature
1,367K
(1094 °C)
Insolation (S⊕)
459.54
Earth = 1.00
Earth Similarity (ESI)
0.120
0 = alien, 1 = Earth-identical
Inside the inner HZ boundary (too hot for liquid water) — the dayside is scorched.
Detection Signatures
Transit photometry
A transit dip has been observed from at least one telescope.
Radial velocity
Stellar wobble has been measured spectroscopically.
Discovery Paper
Primary reference
Bakos et al. 2010Instrument
2K CCD Sensor
Publication
2011-12
Observation locale
Ground
Discovery cohort
Planets confirmed in 2010 at HATNet (12 shown).
Host System: HAT-P-21
Spectral Class
G-type yellow (Sun-like)
Effective Temperature
5,588 K — cooler than the Sun (5,778 K)
Estimated Age
10.20 Gyr — older than the Sun (4.6 Gyr)
Stellar Radius
1.210 R☉
Stellar Mass
1.240 M☉
Metallicity [Fe/H]
0.01
Extended Stellar Properties
Surface gravity (log g)
4.330 dex
Stellar density
1.000 g/cm³
Systemic radial velocity
-53.02 km/s
Rotational v·sin i
3.50 km/s
Rotation period
15.88 days
Metallicity ratio
[Fe/H]
Multi-band Host Photometry — 16 bands
Astrometric Data
Parallax
3.578 mas
Total Proper Motion
13.287 mas/yr
PM Right Ascension
-1.09 mas/yr
PM Declination
13.24 mas/yr
Galactic Cartesian (pc)
x = -0.746 · y = 0.114 · z = 0.656
Equatorial (J2000)
RA 171.27493° · Dec 41.02802°
Galactic ℓ, b
169.325° · 67.464°
Ecliptic λ, β
153.846° · 33.831°
HTM-20 index
1119497618
Similar Worlds
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