Key parameters at a glance
- A radius of 2.70 Earth radii
- A mass of 17.70 Earth masses
- Surface gravity around 2.43 g
- An orbital period of 25.129 days
- Semi-major axis 0.1726 AU
- Equilibrium temperature 637 K (364 °C)
- Distance from Earth 258.34 light-years
- Earth Similarity Index 0.333
- Travel time at Voyager 1 speed 4,555,855 years
HD 21520 b Planet Profile
Physical Specs
Classification
Planet Type
Sub-Neptune
Discovery
Size rank in cohort
Rank by radius
#877of 1978
top 44.3%
This planet
2.70R⊕
Sub-Neptune median
2.71R⊕
Nearest-size peers
| Metric | Earth | HD 21520 b | Jupiter |
|---|---|---|---|
| Radius (R⊕) | 1.00 | 2.70 | 11.21 |
| Mass (M⊕) | 1.00 | 17.70 | 317.83 |
| Density (g/cm³) | 5.51 | 1.35 | 1.33 |
| Surface Gravity (g) | 1.00 | 2.43 | 2.53 |
| Insolation (S⊕) | 1.00 | 25.74 | 0.037 |
Alternative Mass Estimates
| Method | Value | Interpretation |
|---|---|---|
| Best measured mass | 17.700 M⊕ | Direct mass measurement (not derived). |
Star Catalogue Identifiers
HD
HD 21520
HIP
HIP 16038
TIC
TIC 404518509
Gaia DR2
Gaia DR2 4847732649308331648
Gaia DR3
Gaia DR3 4847732649308331648
System
HD 21520
Percentile among Sub-Neptune cohort
Orbit & Habitability
Orbital Dynamics
Year Length
A year here lasts approximately 25.13 Earth days (6.9% of a terrestrial year) on a nearly circular orbit at a mean orbital distance of 0.1726 AU.
Transit Fingerprint
Depth
0.047 %
Duration
4.885 h
Impact parameter b
0.478
Rp / R★
0.023690
Transit midpoint (BJD)
2,459,160.0157
Photometric dip
Transit produces a flux drop of 465 ppm lasting ≈ 4.89 h.
Extended Orbital Architecture
Planet / star radius ratio
0.023690
Semi-major axis / stellar radius
35.600
Impact parameter (b)
0.478
RV semi-amplitude (K)
1.640 m/s
Transit mid-time (BJD)
2,459,160.0157
Angular separation (arcsec)
2.18000
Eq. Temperature
637K
(364 °C)
Insolation (S⊕)
25.74
Earth = 1.00
Earth Similarity (ESI)
0.333
0 = alien, 1 = Earth-identical
Inside the inner HZ boundary (too hot for liquid water) — the surface is too hot to retain surface water.
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
Nies et al. 2024Instrument
TESS CCD Array
Publication
2024-09
Observation locale
Space
Discovery cohort
Planets confirmed in 2024 at Transiting Exoplanet Survey Satellite (TESS) (12 shown).
Host System: HD 21520
Spectral Class
G-type yellow (Sun-like)
Effective Temperature
5,871 K — hotter than the Sun (5,778 K)
Estimated Age
5.30 Gyr — older than the Sun (4.6 Gyr)
Stellar Radius
1.044 R☉
Stellar Mass
1.090 M☉
Metallicity [Fe/H]
0.05
Extended Stellar Properties
Surface gravity (log g)
4.420 dex
Stellar density
1.350 g/cm³
Rotational v·sin i
2.50 km/s
Activity index (log R'HK)
-4.930
Metallicity ratio
[Fe/H]
Multi-band Host Photometry — 11 bands
Astrometric Data
Parallax
12.596 mas
Total Proper Motion
22.934 mas/yr
PM Right Ascension
-2.36 mas/yr
PM Declination
-22.81 mas/yr
Galactic Cartesian (pc)
x = 0.449 · y = 0.568 · z = -0.690
Equatorial (J2000)
RA 51.64034° · Dec -43.61366°
Galactic ℓ, b
251.108° · -54.770°
Ecliptic λ, β
28.744° · -59.171°
HTM-20 index
-665214928
Similar Worlds
Kepler-1545 b
Sub-Neptune · G-type yellow (Sun-like)
Radius 2.70 R⊕ · 2,380.8 ly
Kepler-259 c
Sub-Neptune · G-type yellow (Sun-like)
Radius 2.70 R⊕ · 3,268.5 ly
Kepler-811 b
Sub-Neptune · G-type yellow (Sun-like)
Radius 2.70 R⊕ · 3,212.8 ly
TOI-2000 b
Sub-Neptune · G-type yellow (Sun-like)
Radius 2.70 R⊕ · 573.1 ly
Kepler-668 b
Sub-Neptune · G-type yellow (Sun-like)
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Kepler-995 b
Sub-Neptune · G-type yellow (Sun-like)
Radius 2.70 R⊕ · 1,988.4 ly