Key parameters at a glance
- A radius of 7.16 Earth radii
- A mass of 51.00 Earth masses
- Surface gravity around 0.99 g
- An orbital period of 5.186 days
- Semi-major axis 0.0525 AU
- Equilibrium temperature 716 K (443 °C)
- Distance from Earth 552.50 light-years
- Earth Similarity Index 0.228
- Travel time at Voyager 1 speed 9,743,368 years
- Almost certainly tidally locked to its host star given the close orbit
K2-121 b Planet Profile
Physical Specs
Classification
Planet Type
Neptune-like
Discovery
Size rank in cohort
Rank by radius
#192of 574
top 33.3%
This planet
7.16R⊕
Neptune-like median
6.39R⊕
Nearest-size peers
| Metric | Earth | K2-121 b | Jupiter |
|---|---|---|---|
| Radius (R⊕) | 1.00 | 7.16 | 11.21 |
| Mass (M⊕) | 1.00 | 51.00 | 317.83 |
| Density (g/cm³) | 5.51 | 0.70 | 1.33 |
| Surface Gravity (g) | 1.00 | 0.99 | 2.53 |
| Insolation (S⊕) | 1.00 | 62.50 | 0.037 |
Alternative Mass Estimates
| Method | Value | Interpretation |
|---|---|---|
| Best measured mass | 51.000 M⊕ | Direct mass measurement (not derived). |
Star Catalogue Identifiers
TIC
TIC 7059054
Gaia DR2
Gaia DR2 662078902422542336
Gaia DR3
Gaia DR3 662078902422542336
System
K2-121
Percentile among Neptune-like cohort
Orbit & Habitability
Orbital Dynamics
Year Length
A year here lasts approximately 5.19 Earth days (1.4% of a terrestrial year) on a mildly elliptical path at a mean orbital distance of 0.0525 AU.
Transit Fingerprint
Depth
1.311 %
Duration
2.030 h
Impact parameter b
0.140
Rp / R★
0.100500
Transit midpoint (BJD)
2,457,143.5607
Photometric dip
Transit produces a flux drop of 13,112 ppm lasting ≈ 2.03 h.
Extended Orbital Architecture
Planet / star radius ratio
0.100500
Semi-major axis / stellar radius
14.460
Impact parameter (b)
0.140
Transit mid-time (BJD)
2,457,143.5607
Long. of periastron (ω)
72.97°
Angular separation (arcsec)
0.31000
Eq. Temperature
716K
(443 °C)
Insolation (S⊕)
62.50
Earth = 1.00
Earth Similarity (ESI)
0.228
0 = alien, 1 = Earth-identical
Inside the inner HZ boundary (too hot for liquid water) — the surface is too hot to retain surface water. Close-orbiting tidally-locked around a cool host.
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
Dressing et al. 2017Instrument
Kepler CCD Array
Publication
2017-11
Observation locale
Space
Discovery cohort
Planets confirmed in 2017 at K2 (12 shown).
Host System: K2-121
Spectral Class
K-type orange
Effective Temperature
4,526 K — cooler than the Sun (5,778 K)
Estimated Age
—
Stellar Radius
0.675 R☉
Stellar Mass
0.710 M☉
Metallicity [Fe/H]
0.04
Extended Stellar Properties
Surface gravity (log g)
4.630 dex
Stellar density
6.532 g/cm³
Systemic radial velocity
30.15 km/s
Activity index (log R'HK)
-4.534
Metallicity ratio
[Fe/H]
Multi-band Host Photometry — 17 bands
Astrometric Data
Parallax
5.875 mas
Total Proper Motion
20.220 mas/yr
PM Right Ascension
-13.87 mas/yr
PM Declination
-14.71 mas/yr
Galactic Cartesian (pc)
x = -0.573 · y = 0.762 · z = 0.302
Equatorial (J2000)
RA 126.93666° · Dec 17.57933°
Galactic ℓ, b
206.933° · 28.937°
Ecliptic λ, β
124.960° · -1.489°
HTM-20 index
1448951889
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