Key parameters at a glance

A radius of 2.30 Earth radii
A mass of 6.31 Earth masses
Surface gravity around 1.19 g
An orbital period of 3.560 days
Semi-major axis 0.0446 AU
Equilibrium temperature 1,012 K (739 °C)
Distance from Earth 660.74 light-years
Earth Similarity Index 0.256
Travel time at Voyager 1 speed 11,652,204 years

5 siblings around K2-138

K2-138 c shares its host star with 5 other confirmed planets. Side-by-side measurements below.

Planet	Type	Radius (R⊕)	Mass (M⊕)	Period (d)	Eq. T (K)	Year
K2-138 b	Super-Earth	1.51	3.10	2.353	1,157	2017
K2-138 c this	Sub-Neptune	2.30	6.31	3.560	1,012	2017
K2-138 d	Sub-Neptune	2.39	7.92	5.405	888	2017
K2-138 e	Sub-Neptune	3.39	12.97	8.261	757	2017
K2-138 f	Sub-Neptune	2.90	1.63	12.758	653	2017
K2-138 g	Sub-Neptune	3.01	4.32	41.968	445	2021

K2-138 c Planet Profile

Physical Specs

Radius

2.30 R⊕

Radius (Jupiter)

0.205 R♃

Mass

6.31 M⊕

Mass (Jupiter)

0.020 M♃

Density

2.80 g/cm³

Surface Gravity

1.19 g (Earth)

Classification

Planet Type

Sub-Neptune

ESI Score 0.256

HZ Position Inner

Controversial No

Discovery

Year 2017

Method Transit

Facility K2

Telescope 0.95 m Kepler Telescope

Size rank in cohort

Rank by radius

#1514of 1978

top 76.5%

This planet

2.30R⊕

Sub-Neptune median

2.71R⊕

Nearest-size peers

GJ 536 c 2.30 R⊕ Kepler-328 b 2.30 R⊕ Kepler-156 b 2.30 R⊕

Metric	Earth	K2-138 c	Jupiter
Radius (R⊕)	1.00	2.30	11.21
Mass (M⊕)	1.00	6.31	317.83
Density (g/cm³)	5.51	2.80	1.33
Surface Gravity (g)	1.00	1.19	2.53
Insolation (S⊕)	1.00	279.00	0.037

Alternative Mass Estimates

Method	Value	Interpretation
Best measured mass	6.310 M⊕	Direct mass measurement (not derived).

Star Catalogue Identifiers

TIC

TIC 4610830

Gaia DR2

Gaia DR2 2413596935442139520

Gaia DR3

Gaia DR3 2413596935442139520

System

K2-138

Percentile among Sub-Neptune cohort

Radius 2.299 R⊕ · percentile 23 / cohort 1978

Mass 6.310 M⊕ · percentile 31 / cohort 1978

Orbital period 3.56 d · percentile 7 / cohort 1946

Distance 202.59 pc · percentile 29 / cohort 1975

ESI 0.256 · percentile 18 / cohort 1978

Orbit & Habitability

Orbital Dynamics

Period

3.560 days

Semi-major axis

0.0446 AU

Eccentricity

0.045

Inclination

88.10 °

Year Length

A year here lasts approximately 3.56 Earth days (1.0% of a terrestrial year) on a nearly circular orbit at a mean orbital distance of 0.0446 AU.

Transit Fingerprint

Duration

2.380 h

Impact parameter b

0.369

Rp / R★

0.024390

Transit midpoint (BJD)

2,457,740.3218

Extended Orbital Architecture

Planet / star radius ratio

0.024390

Semi-major axis / stellar radius

11.100

Impact parameter (b)

0.369

RV semi-amplitude (K)

2.780 m/s

Transit mid-time (BJD)

2,457,740.3218

Long. of periastron (ω)

200.00°

Angular separation (arcsec)

0.22000

Eq. Temperature

1,012K

(739 °C)

Insolation (S⊕)

279.00

Earth = 1.00

Earth Similarity (ESI)

0.256

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

Christiansen et al. 2018

Instrument

Kepler CCD Array

Publication

2018-02

Observation locale

Space

Discovery cohort

Planets confirmed in 2017 at K2 (12 shown).

K2-129 b · Rocky Terrestrial K2-130 b · Super-Earth K2-133 b · Super-Earth K2-128 b · Super-Earth K2-133 c · Super-Earth K2-116 b · Rocky Terrestrial K2-125 b · Sub-Neptune K2-118 b · Sub-Neptune K2-136 d · Super-Earth K2-123 b · Sub-Neptune K2-136 c · Sub-Neptune K2-124 b · Sub-Neptune

Host System: K2-138

Spectral Class

G-type yellow (Sun-like)

Effective Temperature

5,356 K — cooler than the Sun (5,778 K)

Estimated Age

2.80 Gyr — younger than the Sun (4.6 Gyr)

Stellar Radius

0.863 R☉

Stellar Mass

0.935 M☉

Metallicity [Fe/H]

0.14

Extended Stellar Properties

Surface gravity (log g)

4.540 dex

Stellar density

2.064 g/cm³

Systemic radial velocity

0.64 km/s

Rotational v·sin i

2.70 km/s

Metallicity ratio

[M/H]

Distance

202.59 parsec

Light-years 660.74 ly

V-band magnitude

12.25 mag

Voyager-speed travel 11,652,204 yr

Multi-band Host Photometry — 17 bands

Astrometric Data

Parallax

4.908 mas

Total Proper Motion

10.571 mas/yr

PM Right Ascension

-1.02 mas/yr

PM Declination

-10.52 mas/yr

Galactic Cartesian (pc)

x = 0.964 · y = -0.188 · z = -0.188

Equatorial (J2000)

RA 348.94903° · Dec -10.84974°

Galactic ℓ, b

64.216° · -62.239°

Ecliptic λ, β

345.590° · -5.614°

HTM-20 index

217304828