Key parameters at a glance

A radius of 2.39 Earth radii
A mass of 7.92 Earth masses
Surface gravity around 1.39 g
An orbital period of 5.405 days
Semi-major axis 0.0589 AU
Equilibrium temperature 888 K (615 °C)
Distance from Earth 660.74 light-years
Earth Similarity Index 0.296
Travel time at Voyager 1 speed 11,652,204 years

5 siblings around K2-138

K2-138 d 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	Sub-Neptune	2.30	6.31	3.560	1,012	2017
K2-138 d this	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 d Planet Profile

Physical Specs

Radius

2.39 R⊕

Radius (Jupiter)

0.213 R♃

Mass

7.92 M⊕

Mass (Jupiter)

0.025 M♃

Density

3.10 g/cm³

Surface Gravity

1.39 g (Earth)

Classification

Planet Type

Sub-Neptune

ESI Score 0.296

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

#1343of 1978

top 67.8%

This planet

2.39R⊕

Sub-Neptune median

2.71R⊕

Nearest-size peers

Kepler-193 b 2.39 R⊕ Kepler-152 c 2.39 R⊕ Kepler-238 c 2.39 R⊕

Metric	Earth	K2-138 d	Jupiter
Radius (R⊕)	1.00	2.39	11.21
Mass (M⊕)	1.00	7.92	317.83
Density (g/cm³)	5.51	3.10	1.33
Surface Gravity (g)	1.00	1.39	2.53
Insolation (S⊕)	1.00	160.00	0.037

Alternative Mass Estimates

Method	Value	Interpretation
Best measured mass	7.920 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 2.390 R⊕ · percentile 31 / cohort 1978

Mass 7.920 M⊕ · percentile 54 / cohort 1978

Orbital period 5.40 d · percentile 14 / cohort 1946

Distance 202.59 pc · percentile 29 / cohort 1975

ESI 0.296 · percentile 27 / cohort 1978

Orbit & Habitability

Orbital Dynamics

Period

5.405 days

Semi-major axis

0.0589 AU

Eccentricity

0.043

Inclination

89.00 °

Year Length

A year here lasts approximately 5.40 Earth days (1.5% of a terrestrial year) on a nearly circular orbit at a mean orbital distance of 0.0589 AU.

Transit Fingerprint

Depth

0.084 %

Duration

2.700 h

Impact parameter b

0.241

Rp / R★

0.025320

Transit midpoint (BJD)

2,457,743.1599

Photometric dip

Transit produces a flux drop of 838 ppm lasting ≈ 2.70 h.

Extended Orbital Architecture

Planet / star radius ratio

0.025320

Semi-major axis / stellar radius

14.700

Impact parameter (b)

0.241

RV semi-amplitude (K)

3.030 m/s

Transit mid-time (BJD)

2,457,743.1599

Long. of periastron (ω)

83.00°

Angular separation (arcsec)

0.29100

Eq. Temperature

888K

(615 °C)

Insolation (S⊕)

160.00

Earth = 1.00

Earth Similarity (ESI)

0.296

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

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