Key parameters at a glance

A radius of 2.49 Earth radii
A mass of 6.76 Earth masses
Surface gravity around 1.09 g
An orbital period of 15.502 days
Semi-major axis 0.1070 AU
Equilibrium temperature 420 K (147 °C)
Distance from Earth 272.81 light-years
Earth Similarity Index 0.658
Travel time at Voyager 1 speed 4,811,049 years
Almost certainly tidally locked to its host star given the close orbit

Context from the literature

K2-21, also known as EPIC 206011691, is a red dwarf star located 273 light-years away in the constellation Aquarius. It hosts two known exoplanets, discovered in 2015 by the transit method as part of Kepler's K2 mission. Both planets have significantly lower densities than Earth, indicating that they are not rocky planets and are better described as mini-Neptunes. The inner planet, K2-21b, is less dense than the outer planet, K2-21c.

Excerpted from Wikipedia · full article

1 sibling around K2-21

K2-21 c shares its host star with 1 other confirmed planet. Side-by-side measurements below.

Planet	Type	Radius (R⊕)	Mass (M⊕)	Period (d)	Eq. T (K)	Year
K2-21 b	Super-Earth	1.84	4.04	9.325	500	2015
K2-21 c this	Sub-Neptune	2.49	6.76	15.502	420	2015

K2-21 c Planet Profile

Physical Specs

Radius

2.49 R⊕

Radius (Jupiter)

0.222 R♃

Mass

6.76 M⊕

Mass (Jupiter)

0.021 M♃

Density

5.40 g/cm³

Surface Gravity

1.09 g (Earth)

Classification

Planet Type

Sub-Neptune

ESI Score 0.658

HZ Position Inner

Controversial No

Discovery

Year 2015

Method Transit

Facility K2

Telescope 0.95 m Kepler Telescope

Size rank in cohort

Rank by radius

#1191of 1978

top 60.2%

This planet

2.49R⊕

Sub-Neptune median

2.71R⊕

Nearest-size peers

Kepler-184 d 2.49 R⊕ HD 176986 d 2.49 R⊕ Kepler-11 f 2.49 R⊕

Metric	Earth	K2-21 c	Jupiter
Radius (R⊕)	1.00	2.49	11.21
Mass (M⊕)	1.00	6.76	317.83
Density (g/cm³)	5.51	5.40	1.33
Surface Gravity (g)	1.00	1.09	2.53
Insolation (S⊕)	1.00	5.10	0.037

Star Catalogue Identifiers

TIC

TIC 240766850

Gaia DR2

Gaia DR2 2596888100773300224

Gaia DR3

Gaia DR3 2596888100773300224

System

K2-21

Percentile among Sub-Neptune cohort

Radius 2.490 R⊕ · percentile 39 / cohort 1978

Mass 6.760 M⊕ · percentile 37 / cohort 1978

Orbital period 15.50 d · percentile 51 / cohort 1946

Distance 83.64 pc · percentile 18 / cohort 1975

ESI 0.658 · percentile 89 / cohort 1978

Orbit & Habitability

Orbital Dynamics

Period

15.502 days

Semi-major axis

0.1070 AU

Eccentricity

0.210

Inclination

88.85 °

Year Length

A year here lasts approximately 15.50 Earth days (4.2% of a terrestrial year) on a mildly elliptical path at a mean orbital distance of 0.1070 AU.

Transit Fingerprint

Depth

0.129 %

Duration

2.333 h

Impact parameter b

0.660

Rp / R★

0.035332

Transit midpoint (BJD)

2,456,988.4710

Photometric dip

Transit produces a flux drop of 1,294 ppm lasting ≈ 2.33 h.

Extended Orbital Architecture

Planet / star radius ratio

0.035332

Semi-major axis / stellar radius

35.500

Impact parameter (b)

0.660

Transit mid-time (BJD)

2,456,988.4710

Long. of periastron (ω)

59.96°

Angular separation (arcsec)

1.28000

Eq. Temperature

420K

(147 °C)

Insolation (S⊕)

5.10

Earth = 1.00

Earth Similarity (ESI)

0.658

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.

Discovery Paper

Primary reference

Petigura et al. 2015

Instrument

Kepler CCD Array

Publication

2015-10

Observation locale

Space

Discovery cohort

Planets confirmed in 2015 at K2 (12 shown).

K2-18 b · Sub-Neptune K2-9 b · Sub-Neptune K2-3 d · Super-Earth K2-3 c · Super-Earth 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: K2-21

Spectral Class

M-type red dwarf

Effective Temperature

4,222 K — cooler than the Sun (5,778 K)

Estimated Age

1.00 Gyr — younger than the Sun (4.6 Gyr)

Stellar Radius

0.647 R☉

Stellar Mass

0.676 M☉

Metallicity [Fe/H]

-0.11

Extended Stellar Properties

Surface gravity (log g)

4.828 dex

Stellar density

3.168 g/cm³

Systemic radial velocity

4.80 km/s

Metallicity ratio

[Fe/H]

Distance

83.64 parsec

Light-years 272.81 ly

V-band magnitude

12.85 mag

Voyager-speed travel 4,811,049 yr

Multi-band Host Photometry — 15 bands

Astrometric Data

Parallax

11.927 mas

Total Proper Motion

81.584 mas/yr

PM Right Ascension

20.74 mas/yr

PM Declination

-78.91 mas/yr

Galactic Cartesian (pc)

x = 0.912 · y = -0.326 · z = -0.250

Equatorial (J2000)

RA 340.30378° · Dec -14.48933°

Galactic ℓ, b

48.936° · -57.179°

Ecliptic λ, β

336.361° · -5.725°

HTM-20 index

-864288784

Wikipedia →