Key parameters at a glance

A radius of 3.01 Earth radii
A mass of 4.32 Earth masses
Surface gravity around 0.48 g
An orbital period of 41.968 days
Semi-major axis 0.2311 AU
Equilibrium temperature 445 K (172 °C)
Distance from Earth 660.74 light-years
Earth Similarity Index 0.453
Travel time at Voyager 1 speed 11,652,204 years

Context from the literature

K2-138, also designated EPIC 245950175 or EE-1, is a large early K-type main sequence star with a system of at least 6 planets discovered by citizen scientists. Four were found in the first two days of the Exoplanet Explorers project on Zooniverse in early April 2017, while two more were revealed in further analysis. The system is about 660 light-years away in the constellation Aquarius, within K2 Campaign 12.

Excerpted from Wikipedia · full article

5 siblings around K2-138

K2-138 g 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	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 this	Sub-Neptune	3.01	4.32	41.968	445	2021

K2-138 g Planet Profile

Physical Specs

Radius

3.01 R⊕

Radius (Jupiter)

0.269 R♃

Mass

4.32 M⊕

Mass (Jupiter)

0.014 M♃

Density

0.90 g/cm³

Surface Gravity

0.48 g (Earth)

Classification

Planet Type

Sub-Neptune

ESI Score 0.453

HZ Position Inner

Controversial No

Discovery

Year 2021

Method Transit

Facility K2

Telescope 0.95 m Kepler Telescope

Size rank in cohort

Rank by radius

#507of 1978

top 25.6%

This planet

3.01R⊕

Sub-Neptune median

2.71R⊕

Nearest-size peers

Kepler-1767 b 3.01 R⊕ Kepler-1036 b 3.01 R⊕ Kepler-414 c 3.01 R⊕

Metric	Earth	K2-138 g	Jupiter
Radius (R⊕)	1.00	3.01	11.21
Mass (M⊕)	1.00	4.32	317.83
Density (g/cm³)	5.51	0.90	1.33
Surface Gravity (g)	1.00	0.48	2.53
Insolation (S⊕)	1.00	9.29	0.037

Alternative Mass Estimates

Method	Value	Interpretation
Best measured mass	4.320 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 3.013 R⊕ · percentile 74 / cohort 1978

Mass 4.320 M⊕ · percentile 2 / cohort 1978

Orbital period 41.97 d · percentile 82 / cohort 1946

Distance 202.59 pc · percentile 29 / cohort 1975

ESI 0.453 · percentile 63 / cohort 1978

Orbit & Habitability

Orbital Dynamics

Period

41.968 days

Semi-major axis

0.2311 AU

Eccentricity

0.059

Inclination

89.40 °

Year Length

A year here lasts approximately 41.97 Earth days (11.5% of a terrestrial year) on a nearly circular orbit at a mean orbital distance of 0.2311 AU.

Transit Fingerprint

Depth

0.084 %

Duration

4.750 h

Impact parameter b

0.558

Rp / R★

0.031870

Transit midpoint (BJD)

2,457,773.8616

Photometric dip

Transit produces a flux drop of 842 ppm lasting ≈ 4.75 h.

Extended Orbital Architecture

Planet / star radius ratio

0.031870

Semi-major axis / stellar radius

57.700

Impact parameter (b)

0.558

RV semi-amplitude (K)

0.830 m/s

Transit mid-time (BJD)

2,457,773.8616

Long. of periastron (ω)

164.00°

Angular separation (arcsec)

1.14000

Eq. Temperature

445K

(172 °C)

Insolation (S⊕)

9.29

Earth = 1.00

Earth Similarity (ESI)

0.453

0 = alien, 1 = Earth-identical

Inside the inner HZ boundary (too hot for liquid water) — the surface is too hot to retain surface water.

Discovery Paper

Primary reference

Hardegree Ullman et al. 2021

Instrument

Kepler CCD Array

Publication

2021-05

Observation locale

Space

Discovery cohort

Planets confirmed in 2021 at K2 (12 shown).

EPIC 220492298 b · Rocky Terrestrial EPIC 206042996 c · Rocky Terrestrial EPIC 212624936 b · Rocky Terrestrial EPIC 206024342 c · Super-Earth EPIC 201427007 b · Super-Earth K2-332 b · Sub-Neptune EPIC 212624936 c · Sub-Neptune EPIC 206024342 d · Sub-Neptune K2-345 b · Sub-Neptune K2-349 b · Super-Earth K2-343 c · Sub-Neptune K2-346 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

Wikipedia → Image: Wikimedia Commons · NASA/JPL-Caltech/R. Hurt (IPAC) · Public domain