Key parameters at a glance

A radius of 8.08 Earth radii
A mass of 29.90 Earth masses
Surface gravity around 0.46 g
An orbital period of 38.985 days
Semi-major axis 0.2270 AU
Equilibrium temperature 523 K (250 °C)
Distance from Earth 2,049.10 light-years
Earth Similarity Index 0.257
Travel time at Voyager 1 speed 36,135,839 years

2 siblings around Kepler-9

Kepler-9 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-9 d	Super-Earth	1.64	8.41	1.593	2,026	2010
Kepler-9 b	Neptune-like	8.29	43.40	19.239	661	2010
Kepler-9 c this	Neptune-like	8.08	29.90	38.985	523	2010

Kepler-9 c Planet Profile

Physical Specs

Radius

8.08 R⊕

Radius (Jupiter)

0.721 R♃

Mass

29.90 M⊕

Mass (Jupiter)

0.094 M♃

Density

0.31 g/cm³

Surface Gravity

0.46 g (Earth)

Classification

Planet Type

Neptune-like

ESI Score 0.257

HZ Position Inner

Controversial No

Discovery

Year 2010

Method Transit

Facility Kepler

Telescope 0.95 m Kepler Telescope

Size rank in cohort

Rank by radius

#152of 574

top 26.3%

This planet

8.08R⊕

Neptune-like median

6.39R⊕

Nearest-size peers

Kepler-849 b 8.08 R⊕ HATS-72 b 8.10 R⊕ Kepler-117 b 8.06 R⊕

Metric	Earth	Kepler-9 c	Jupiter
Radius (R⊕)	1.00	8.08	11.21
Mass (M⊕)	1.00	29.90	317.83
Density (g/cm³)	5.51	0.31	1.33
Surface Gravity (g)	1.00	0.46	2.53
Insolation (S⊕)	1.00	18.90	0.037

Alternative Mass Estimates

Method	Value	Interpretation
Best measured mass	29.900 M⊕	Direct mass measurement (not derived).
Minimum mass (M sin i)	27.287 M⊕	Lower bound from radial-velocity fitting.

Star Catalogue Identifiers

TIC

TIC 120571842

Gaia DR2

Gaia DR2 2099925719951103872

Gaia DR3

Gaia DR3 2099925719951103872

System

Kepler-9

Percentile among Neptune-like cohort

Radius 8.080 R⊕ · percentile 74 / cohort 574

Mass 29.900 M⊕ · percentile 49 / cohort 574

Orbital period 38.99 d · percentile 65 / cohort 524

Distance 628.26 pc · percentile 61 / cohort 572

ESI 0.257 · percentile 41 / cohort 574

Orbit & Habitability

Orbital Dynamics

Period

38.985 days

Semi-major axis

0.2270 AU

Eccentricity

0.067

Inclination

89.19 °

Year Length

A year here lasts approximately 38.99 Earth days (10.7% of a terrestrial year) on a nearly circular orbit at a mean orbital distance of 0.2270 AU.

Transit Fingerprint

Depth

0.616 %

Duration

4.565 h

Impact parameter b

0.740

Rp / R★

0.075600

Transit midpoint (BJD)

2,454,968.8400

Photometric dip

Transit produces a flux drop of 6,159 ppm lasting ≈ 4.57 h.

Extended Orbital Architecture

Planet / star radius ratio

0.075600

Semi-major axis / stellar radius

49.800

Impact parameter (b)

0.740

RV semi-amplitude (K)

5.068 m/s

Transit mid-time (BJD)

2,454,968.8400

Long. of periastron (ω)

167.50°

Angular separation (arcsec)

0.36100

Eq. Temperature

523K

(250 °C)

Insolation (S⊕)

18.90

Earth = 1.00

Earth Similarity (ESI)

0.257

0 = alien, 1 = Earth-identical

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

Detection Signatures

Transit photometry

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

Radial velocity

Stellar wobble has been measured spectroscopically.

Transit timing variations

Gravitational interactions from neighbours have been detected.

Discovery Paper

Primary reference

Holman et al. 2010

Instrument

Kepler CCD Array

Publication

2010-10

Observation locale

Space

Discovery cohort

Planets confirmed in 2010 at Kepler (11 shown).

Kepler-11 g · Sub-Neptune Kepler-11 f · Sub-Neptune Kepler-11 d · Sub-Neptune Kepler-11 e · Neptune-like Kepler-11 b · Super-Earth Kepler-11 c · Sub-Neptune Kepler-9 b · Neptune-like Kepler-9 d · Super-Earth Kepler-40 b · Gas Giant Kepler-8 b · Gas Giant KOI-217 b · Gas Giant

Host System: Kepler-9

Spectral Class

G-type yellow (Sun-like)

Effective Temperature

5,774 K — cooler than the Sun (5,778 K)

Estimated Age

2.00 Gyr — younger than the Sun (4.6 Gyr)

Stellar Radius

0.958 R☉

Stellar Mass

1.022 M☉

Metallicity [Fe/H]

0.05

Extended Stellar Properties

Surface gravity (log g)

4.490 dex

Stellar density

1.640 g/cm³

Rotational v·sin i

2.74 km/s

Rotation period

16.49 days

Metallicity ratio

[Fe/H]

Distance

628.26 parsec

Light-years 2,049.10 ly

V-band magnitude

13.92 mag

Voyager-speed travel 36,135,839 yr

Multi-band Host Photometry — 17 bands

Astrometric Data

Parallax

1.563 mas

Total Proper Motion

14.922 mas/yr

PM Right Ascension

2.49 mas/yr

PM Declination

-14.71 mas/yr

Galactic Cartesian (pc)

x = 0.210 · y = -0.755 · z = 0.621

Equatorial (J2000)

RA 285.57399° · Dec 38.40082°

Galactic ℓ, b

68.980° · 14.490°

Ecliptic λ, β

295.275° · 60.481°

HTM-20 index

489015344