Key parameters at a glance
- A radius of 17.78 Earth radii
- A mass of 1,242.72 Earth masses
- Surface gravity around 3.93 g
- An orbital period of 3.613 days
- Semi-major axis 0.0522 AU
- Equilibrium temperature 2,051 K (1778 °C)
- Distance from Earth 1,535.80 light-years
- Earth Similarity Index 0.060
- Travel time at Voyager 1 speed 27,083,776 years
KELT-21 b Planet Profile
Physical Specs
Classification
Planet Type
Gas Giant
Discovery
Size rank in cohort
Rank by radius
#84of 1771
top 4.7%
This planet
17.78R⊕
Gas Giant median
13.52R⊕
Nearest-size peers
| Metric | Earth | KELT-21 b | Jupiter |
|---|---|---|---|
| Radius (R⊕) | 1.00 | 17.78 | 11.21 |
| Mass (M⊕) | 1.00 | 1,242.72 | 317.83 |
| Density (g/cm³) | 5.51 | 1.24 | 1.33 |
| Surface Gravity (g) | 1.00 | 3.93 | 2.53 |
| Insolation (S⊕) | 1.00 | 2,950.00 | 0.037 |
Alternative Mass Estimates
| Method | Value | Interpretation |
|---|---|---|
| Best measured mass | 1,242.715 M⊕ | Direct mass measurement (not derived). |
| Minimum mass (M sin i) | 1,242.715 M⊕ | Lower bound from radial-velocity fitting. |
Star Catalogue Identifiers
HD
HD 332124
TIC
TIC 203189770
Gaia DR2
Gaia DR2 2054523105274216704
Gaia DR3
Gaia DR3 2054523105274216704
System
KELT-21
Percentile among Gas Giant cohort
Orbit & Habitability
Orbital Dynamics
Year Length
A year here lasts approximately 3.61 Earth days (1.0% of a terrestrial year) on a nearly circular orbit at a mean orbital distance of 0.0522 AU.
Transit Fingerprint
Depth
0.990 %
Duration
4.105 h
Impact parameter b
0.423
Rp / R★
0.099520
Transit midpoint (BJD)
2,457,295.9343
Photometric dip
Transit produces a flux drop of 9,900 ppm lasting ≈ 4.11 h.
Extended Orbital Architecture
Planet / star radius ratio
0.099520
Semi-major axis / stellar radius
6.860
Impact parameter (b)
0.423
RV semi-amplitude (K)
399.600 m/s
Transit mid-time (BJD)
2,457,295.9343
Sky-projected obliquity (λ)
-5.60°
Angular separation (arcsec)
0.11100
Eq. Temperature
2,051K
(1778 °C)
Insolation (S⊕)
2,950.00
Earth = 1.00
Earth Similarity (ESI)
0.060
0 = alien, 1 = Earth-identical
Inside the inner HZ boundary (too hot for liquid water) — the dayside is scorched.
Discovery Paper
Primary reference
Johnson et al. 2018Instrument
Apogee 4K CCD Sensor
Publication
2018-02
Observation locale
Ground
Host System: KELT-21
Spectral Class
A-type white
Effective Temperature
7,598 K — hotter than the Sun (5,778 K)
Estimated Age
1.60 Gyr — younger than the Sun (4.6 Gyr)
Stellar Radius
1.638 R☉
Stellar Mass
1.458 M☉
Metallicity [Fe/H]
-0.41
Extended Stellar Properties
Surface gravity (log g)
4.173 dex
Stellar density
0.468 g/cm³
Systemic radial velocity
-13.00 km/s
Rotational v·sin i
146.03 km/s
Metallicity ratio
[Fe/H]
Multi-band Host Photometry — 11 bands
Astrometric Data
Parallax
2.095 mas
Total Proper Motion
2.242 mas/yr
PM Right Ascension
2.24 mas/yr
PM Declination
-0.11 mas/yr
Galactic Cartesian (pc)
x = 0.481 · y = -0.692 · z = 0.538
Equatorial (J2000)
RA 304.80003° · Dec 32.58104°
Galactic ℓ, b
71.481° · -1.987°
Ecliptic λ, β
318.821° · 50.290°
HTM-20 index
-1070449284
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