Key parameters at a glance

A radius of 2.37 Earth radii
A mass of 4.30 Earth masses
Surface gravity around 0.77 g
An orbital period of 12.040 days
Semi-major axis 0.0835 AU
Equilibrium temperature 502 K (229 °C)
Distance from Earth 1,135.97 light-years
Earth Similarity Index 0.479
Travel time at Voyager 1 speed 20,032,865 years
Almost certainly tidally locked to its host star given the close orbit

2 siblings around Kepler-81

Kepler-81 c shares its host star with 2 other confirmed planets. Side-by-side measurements below.

Planet	Type	Radius (R⊕)	Mass (M⊕)	Period (d)	Eq. T (K)	Year
Kepler-81 b	Sub-Neptune	2.42	17.20	5.955	635	2012
Kepler-81 c this	Sub-Neptune	2.37	4.30	12.040	502	2012
Kepler-81 d	Rocky Terrestrial	1.21	1.92	20.838	418	2014

Kepler-81 c Planet Profile

Physical Specs

Radius

2.37 R⊕

Radius (Jupiter)

0.211 R♃

Mass

4.30 M⊕

Mass (Jupiter)

0.014 M♃

Density

1.77 g/cm³

Surface Gravity

0.77 g (Earth)

Classification

Planet Type

Sub-Neptune

ESI Score 0.479

HZ Position Inner

Controversial No

Discovery

Year 2012

Method Transit

Facility Kepler

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-125 b 2.37 R⊕ Kepler-597 b 2.37 R⊕ K2-18 b 2.37 R⊕

Metric	Earth	Kepler-81 c	Jupiter
Radius (R⊕)	1.00	2.37	11.21
Mass (M⊕)	1.00	4.30	317.83
Density (g/cm³)	5.51	1.77	1.33
Surface Gravity (g)	1.00	0.77	2.53
Insolation (S⊕)	1.00	15.62	0.037

Alternative Mass Estimates

Method	Value	Interpretation
Best measured mass	4.300 M⊕	Direct mass measurement (not derived).

Star Catalogue Identifiers

TIC

TIC 270611154

Gaia DR2

Gaia DR2 2077816087307845248

Gaia DR3

Gaia DR3 2077816087307845248

System

Kepler-81

Percentile among Sub-Neptune cohort

Radius 2.370 R⊕ · percentile 30 / cohort 1978

Mass 4.300 M⊕ · percentile 2 / cohort 1978

Orbital period 12.04 d · percentile 41 / cohort 1946

Distance 348.29 pc · percentile 38 / cohort 1975

ESI 0.479 · percentile 68 / cohort 1978

Orbit & Habitability

Orbital Dynamics

Period

12.040 days

Semi-major axis

0.0835 AU

Eccentricity

0.000

Inclination

89.51 °

Year Length

A year here lasts approximately 12.04 Earth days (3.3% of a terrestrial year) on a nearly circular orbit at a mean orbital distance of 0.0835 AU.

Transit Fingerprint

Depth

0.132 %

Duration

2.736 h

Impact parameter b

0.850

Rp / R★

0.032857

Transit midpoint (BJD)

2,455,014.2227

Photometric dip

Transit produces a flux drop of 1,317 ppm lasting ≈ 2.74 h.

Extended Orbital Architecture

Planet / star radius ratio

0.032857

Semi-major axis / stellar radius

34.890

Impact parameter (b)

0.850

Transit mid-time (BJD)

2,455,014.2227

Angular separation (arcsec)

0.24000

Eq. Temperature

502K

(229 °C)

Insolation (S⊕)

15.62

Earth = 1.00

Earth Similarity (ESI)

0.479

0 = alien, 1 = Earth-identical

Inside the inner HZ boundary (too hot for liquid water) — the surface is too hot to retain surface water. Close-orbiting tidally-locked around a cool host.

Detection Signatures

Transit photometry

A transit dip has been observed from at least one telescope.

Transit timing variations

Gravitational interactions from neighbours have been detected.

Discovery Paper

Primary reference

Xie 2013

Instrument

Kepler CCD Array

Publication

2013-10

Observation locale

Space

Discovery cohort

Planets confirmed in 2012 at Kepler (12 shown).

Kepler-54 c · Rocky Terrestrial Kepler-49 c · Sub-Neptune Kepler-55 b · Sub-Neptune Kepler-49 b · Sub-Neptune Kepler-55 c · Sub-Neptune Kepler-52 c · Super-Earth Kepler-83 c · Sub-Neptune Kepler-32 d · Sub-Neptune Kepler-83 b · Sub-Neptune Kepler-54 b · Sub-Neptune Kepler-80 c · Sub-Neptune Kepler-81 b · Sub-Neptune

Host System: Kepler-81

Spectral Class

M-type red dwarf

Effective Temperature

4,385 K — cooler than the Sun (5,778 K)

Estimated Age

3.63 Gyr — younger than the Sun (4.6 Gyr)

Stellar Radius

0.590 R☉

Stellar Mass

0.640 M☉

Metallicity [Fe/H]

-0.18

Extended Stellar Properties

Surface gravity (log g)

4.700 dex

Stellar density

4.193 g/cm³

Metallicity ratio

[M/H]

Distance

348.29 parsec

Light-years 1,135.97 ly

V-band magnitude

15.26 mag

Voyager-speed travel 20,032,865 yr

Multi-band Host Photometry — 16 bands

Astrometric Data

Parallax

2.843 mas

Total Proper Motion

24.701 mas/yr

PM Right Ascension

-1.37 mas/yr

PM Declination

-24.66 mas/yr

Galactic Cartesian (pc)

x = 0.294 · y = -0.672 · z = 0.680

Equatorial (J2000)

RA 293.63694° · Dec 42.82483°

Galactic ℓ, b

75.746° · 10.830°

Ecliptic λ, β

310.352° · 62.991°

HTM-20 index

-84634903