Key parameters at a glance

A radius of 1.51 Earth radii
A mass of 3.10 Earth masses
Surface gravity around 1.36 g
An orbital period of 2.353 days
Semi-major axis 0.0339 AU
Equilibrium temperature 1,157 K (884 °C)
Distance from Earth 660.74 light-years
Earth Similarity Index 0.255
Travel time at Voyager 1 speed 11,652,204 years

5 siblings around K2-138

K2-138 b 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 this	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	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 b Planet Profile

Physical Specs

Radius

1.51 R⊕

Radius (Jupiter)

0.135 R♃

Mass

3.10 M⊕

Mass (Jupiter)

0.010 M♃

Density

4.90 g/cm³

Surface Gravity

1.36 g (Earth)

Classification

Planet Type

Super-Earth

ESI Score 0.255

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

#730of 1176

top 62.0%

This planet

1.51R⊕

Super-Earth median

1.60R⊕

Nearest-size peers

Kepler-249 c 1.51 R⊕ Kepler-205 b 1.51 R⊕ Kepler-389 b 1.51 R⊕

Metric	Earth	K2-138 b	Jupiter
Radius (R⊕)	1.00	1.51	11.21
Mass (M⊕)	1.00	3.10	317.83
Density (g/cm³)	5.51	4.90	1.33
Surface Gravity (g)	1.00	1.36	2.53
Insolation (S⊕)	1.00	486.00	0.037

Alternative Mass Estimates

Method	Value	Interpretation
Best measured mass	3.100 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 Super-Earth cohort

Radius 1.510 R⊕ · percentile 37 / cohort 1176

Mass 3.100 M⊕ · percentile 43 / cohort 1176

Orbital period 2.35 d · percentile 14 / cohort 1164

Distance 202.59 pc · percentile 25 / cohort 1171

ESI 0.255 · percentile 24 / cohort 1176

Orbit & Habitability

Orbital Dynamics

Period

2.353 days

Semi-major axis

0.0339 AU

Eccentricity

0.048

Inclination

87.20 °

Year Length

A year here lasts approximately 2.35 Earth days (0.6% of a terrestrial year) on a nearly circular orbit at a mean orbital distance of 0.0338 AU.

Transit Fingerprint

Duration

2.010 h

Impact parameter b

0.419

Rp / R★

0.016000

Transit midpoint (BJD)

2,457,773.3168

Extended Orbital Architecture

Planet / star radius ratio

0.016000

Semi-major axis / stellar radius

8.500

Impact parameter (b)

0.419

RV semi-amplitude (K)

1.560 m/s

Transit mid-time (BJD)

2,457,773.3168

Long. of periastron (ω)

244.00°

Angular separation (arcsec)

0.16700

Eq. Temperature

1,157K

(884 °C)

Insolation (S⊕)

486.00

Earth = 1.00

Earth Similarity (ESI)

0.255

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