Key parameters at a glance

A radius of 2.37 Earth radii
A mass of 8.92 Earth masses
Surface gravity around 1.59 g
An orbital period of 32.940 days
Semi-major axis 0.1429 AU
Equilibrium temperature 284 K (11 °C)
Distance from Earth 124.03 light-years
Earth Similarity Index 0.824
Travel time at Voyager 1 speed 2,187,199 years
Almost certainly tidally locked to its host star given the close orbit

Context from the literature

K2-18b, also known as EPIC 201912552 b, is an exoplanet orbiting the red dwarf K2-18, located 124 light-years (38 pc) away from Earth. The planet is a sub-Neptune about 2.6 times the radius of Earth, with a 33-day orbit within the star's habitable zone; it receives approximately a similar amount of light as the Earth receives from the Sun. Initially discovered with the Kepler space telescope, it was later observed by the James Webb Space Telescope (JWST) in order to study the planet's atmosphere.

Excerpted from Wikipedia · full article

K2-18 b Planet Profile

Physical Specs

Radius

2.37 R⊕

Radius (Jupiter)

0.211 R♃

Mass

8.92 M⊕

Mass (Jupiter)

0.028 M♃

Density

4.11 g/cm³

Surface Gravity

1.59 g (Earth)

Classification

Planet Type

Sub-Neptune

ESI Score 0.824

HZ Position Habitable

Controversial No

Discovery

Year 2015

Method Transit

Facility K2

Telescope 0.95 m Kepler Telescope

Size rank in cohort

Rank by radius

#1375of 1978

top 69.5%

This planet

2.37R⊕

Sub-Neptune median

2.71R⊕

Nearest-size peers

Kepler-81 c 2.37 R⊕ Kepler-597 b 2.37 R⊕ Kepler-125 b 2.37 R⊕

Metric	Earth	K2-18 b	Jupiter
Radius (R⊕)	1.00	2.37	11.21
Mass (M⊕)	1.00	8.92	317.83
Density (g/cm³)	5.51	4.11	1.33
Surface Gravity (g)	1.00	1.59	2.53
Insolation (S⊕)	1.00	1.01	0.037

Alternative Mass Estimates

Method	Value	Interpretation
Best measured mass	8.920 M⊕	Direct mass measurement (not derived).

Star Catalogue Identifiers

TIC

TIC 388804061

Gaia DR2

Gaia DR2 3910747531814692736

Gaia DR3

Gaia DR3 3910747531814692736

System

K2-18

Percentile among Sub-Neptune cohort

Radius 2.370 R⊕ · percentile 30 / cohort 1978

Mass 8.920 M⊕ · percentile 64 / cohort 1978

Orbital period 32.94 d · percentile 75 / cohort 1946

Distance 38.03 pc · percentile 9 / cohort 1975

ESI 0.824 · percentile 100 / cohort 1978

Orbit & Habitability

Orbital Dynamics

Period

32.940 days

Semi-major axis

0.1429 AU

Eccentricity

0.200

Inclination

89.58 °

Year Length

A year here lasts approximately 32.94 Earth days (9.0% of a terrestrial year) on a mildly elliptical path at a mean orbital distance of 0.1429 AU.

Transit Fingerprint

Depth

0.310 %

Duration

2.682 h

Impact parameter b

0.601

Rp / R★

0.053750

Transit midpoint (BJD)

2,457,264.3914

Photometric dip

Transit produces a flux drop of 3,098 ppm lasting ≈ 2.68 h.

Extended Orbital Architecture

Planet / star radius ratio

0.053750

Semi-major axis / stellar radius

83.830

Impact parameter (b)

0.601

RV semi-amplitude (K)

3.550 m/s

Transit mid-time (BJD)

2,457,264.3914

Long. of periastron (ω)

-5.70°

Angular separation (arcsec)

3.76000

Eq. Temperature

284K

(11 °C)

Insolation (S⊕)

1.01

Earth = 1.00

Earth Similarity (ESI)

0.824

0 = alien, 1 = Earth-identical

Within the conservative habitable zone — liquid water is physically possible with adequate atmospheric pressure. Close-orbiting tidally-locked around a cool host.

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

Montet et al. 2015

Instrument

Kepler CCD Array

Publication

2015-08

Observation locale

Space

Discovery cohort

Planets confirmed in 2015 at K2 (12 shown).

K2-9 b · Sub-Neptune K2-3 d · Super-Earth K2-3 c · Super-Earth K2-21 c · Sub-Neptune K2-5 c · Sub-Neptune K2-13 b · Super-Earth K2-3 b · Sub-Neptune K2-17 b · Sub-Neptune K2-21 b · Super-Earth K2-16 c · Sub-Neptune K2-5 b · Super-Earth K2-25 b · Sub-Neptune

Host System: K2-18

Spectral Class

M-type red dwarf

Effective Temperature

3,457 K — cooler than the Sun (5,778 K)

Estimated Age

—

Stellar Radius

0.411 R☉

Stellar Mass

0.359 M☉

Metallicity [Fe/H]

0.12

Extended Stellar Properties

Surface gravity (log g)

4.786 dex

Stellar density

7.073 g/cm³

Systemic radial velocity

0.65 km/s

Rotation period

39.63 days

Activity index (log R'HK)

-5.183

Metallicity ratio

[Fe/H]

Distance

38.03 parsec

Light-years 124.03 ly

V-band magnitude

13.48 mag

Voyager-speed travel 2,187,199 yr

Multi-band Host Photometry — 17 bands

Astrometric Data

Parallax

26.269 mas

Total Proper Motion

155.522 mas/yr

PM Right Ascension

-80.38 mas/yr

PM Declination

-133.14 mas/yr

Galactic Cartesian (pc)

x = -0.983 · y = 0.128 · z = 0.132

Equatorial (J2000)

RA 172.56014° · Dec 7.58783°

Galactic ℓ, b

254.633° · 62.575°

Ecliptic λ, β

170.167° · 4.019°

HTM-20 index

-244884223

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