Confirmed Exoplanet Discovered 2017

K2-138 f

A sub-neptune orbiting the g-type yellow (sun-like) K2-138, located approximately 660.7 light-years from Earth.

Key parameters at a glance

  • A radius of 2.90 Earth radii
  • A mass of 1.63 Earth masses
  • Surface gravity around 0.19 g
  • An orbital period of 12.758 days
  • Semi-major axis 0.1045 AU
  • Equilibrium temperature 653 K (380 °C)
  • Distance from Earth 660.74 light-years
  • Earth Similarity Index 0.261
  • Travel time at Voyager 1 speed 11,652,204 years

5 siblings around K2-138

K2-138 f 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 this 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 f Planet Profile

Physical Specs

Radius
2.90 R⊕
Radius (Jupiter)
0.259 R♃
Mass
1.63 M⊕
Mass (Jupiter)
0.005 M♃
Density
0.40 g/cm³
Surface Gravity
0.19 g (Earth)

Classification

Planet Type

Sub-Neptune

ESI Score 0.261
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

#618of 1978

top 31.2%

This planet

2.90R⊕

Sub-Neptune median

2.71R⊕

Nearest-size peers

Kepler-107 e 2.90 R⊕ TOI-1772.01 2.91 R⊕ K2-365 b 2.91 R⊕
Metric Earth K2-138 f Jupiter
Radius (R⊕)1.002.9011.21
Mass (M⊕)1.001.63317.83
Density (g/cm³)5.510.401.33
Surface Gravity (g)1.000.192.53
Insolation (S⊕)1.0051.000.037

Alternative Mass Estimates

Method Value Interpretation
Best measured mass 1.630 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 2.904 R⊕ · percentile 69 / cohort 1978
Mass 1.630 M⊕ · percentile 0 / cohort 1978
Orbital period 12.76 d · percentile 43 / cohort 1946
Distance 202.59 pc · percentile 29 / cohort 1975
ESI 0.261 · percentile 19 / cohort 1978

Orbit & Habitability

Orbital Dynamics

Period
12.758 days
Semi-major axis
0.1045 AU
Eccentricity
0.062
Inclination
88.80 °

Year Length

A year here lasts approximately 12.76 Earth days (3.5% of a terrestrial year) on a nearly circular orbit at a mean orbital distance of 0.1045 AU.

Transit Fingerprint

Depth

0.085 %

Duration

3.200 h

Impact parameter b

0.560

Rp / R★

0.030850

Transit midpoint (BJD)

2,457,738.7024

Photometric dip

Transit produces a flux drop of 851 ppm lasting ≈ 3.20 h.

Extended Orbital Architecture

Planet / star radius ratio

0.030850

Semi-major axis / stellar radius

26.100

Impact parameter (b)

0.560

RV semi-amplitude (K)

0.470 m/s

Transit mid-time (BJD)

2,457,738.7024

Long. of periastron (ω)

144.00°

Angular separation (arcsec)

0.51600

Eq. Temperature

653K

(380 °C)

Insolation (S⊕)

51.00

Earth = 1.00

Earth Similarity (ESI)

0.261

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

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

8.815.115.10U13.06B12.25V12.03Gaia12.07Kepler11.50TESS14.89Sloan g12.07Sloan r11.86Sloan i13.05Sloan z10.76J10.38H10.31K10.27W110.33W210.28W38.84W4

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

Similar Worlds

Kepler-107 e

Sub-Neptune · G-type yellow (Sun-like)

Radius 2.90 R⊕ · 1,715.6 ly

TOI-1772.01

Sub-Neptune · G-type yellow (Sun-like)

Radius 2.91 R⊕ · 318.8 ly

K2-365 b

Sub-Neptune · G-type yellow (Sun-like)

Radius 2.91 R⊕ · 610.6 ly

Kepler-314 c

Sub-Neptune · G-type yellow (Sun-like)

Radius 2.90 R⊕ · 877.0 ly

Kepler-130 c

Sub-Neptune · G-type yellow (Sun-like)

Radius 2.90 R⊕ · 1,032.8 ly

Kepler-178 b

Sub-Neptune · G-type yellow (Sun-like)

Radius 2.90 R⊕ · 2,335.4 ly

Explore Related Categories

Sub-Neptune Planets Transit Discoveries K2 Mission G-type yellow (Sun-like) Hosts