Key parameters at a glance

A radius of 1.84 Earth radii
A mass of 4.04 Earth masses
Surface gravity around 1.19 g
An orbital period of 9.325 days
Semi-major axis 0.0760 AU
Equilibrium temperature 500 K (227 °C)
Distance from Earth 272.81 light-years
Earth Similarity Index 0.529
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 b 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 this	Super-Earth	1.84	4.04	9.325	500	2015
K2-21 c	Sub-Neptune	2.49	6.76	15.502	420	2015

K2-21 b Planet Profile

Physical Specs

Radius

1.84 R⊕

Radius (Jupiter)

0.164 R♃

Mass

4.04 M⊕

Mass (Jupiter)

0.013 M♃

Density

2.70 g/cm³

Surface Gravity

1.19 g (Earth)

Classification

Planet Type

Super-Earth

ESI Score 0.529

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

#201of 1176

top 17.0%

This planet

1.84R⊕

Super-Earth median

1.60R⊕

Nearest-size peers

Kepler-52 c 1.84 R⊕ Kepler-354 b 1.84 R⊕ Kepler-1651 b 1.84 R⊕

Metric	Earth	K2-21 b	Jupiter
Radius (R⊕)	1.00	1.84	11.21
Mass (M⊕)	1.00	4.04	317.83
Density (g/cm³)	5.51	2.70	1.33
Surface Gravity (g)	1.00	1.19	2.53
Insolation (S⊕)	1.00	10.00	0.037

Star Catalogue Identifiers

TIC

TIC 240766850

Gaia DR2

Gaia DR2 2596888100773300224

Gaia DR3

Gaia DR3 2596888100773300224

System

K2-21

Percentile among Super-Earth cohort

Radius 1.840 R⊕ · percentile 82 / cohort 1176

Mass 4.040 M⊕ · percentile 75 / cohort 1176

Orbital period 9.33 d · percentile 63 / cohort 1164

Distance 83.64 pc · percentile 16 / cohort 1171

ESI 0.529 · percentile 75 / cohort 1176

Orbit & Habitability

Orbital Dynamics

Period

9.325 days

Semi-major axis

0.0760 AU

Eccentricity

0.100

Inclination

88.98 °

Year Length

A year here lasts approximately 9.33 Earth days (2.6% of a terrestrial year) on a mildly elliptical path at a mean orbital distance of 0.0760 AU.

Transit Fingerprint

Depth

0.081 %

Duration

2.372 h

Impact parameter b

0.700

Rp / R★

0.026158

Transit midpoint (BJD)

2,456,989.4220

Photometric dip

Transit produces a flux drop of 809 ppm lasting ≈ 2.37 h.

Extended Orbital Architecture

Planet / star radius ratio

0.026158

Semi-major axis / stellar radius

25.370

Impact parameter (b)

0.700

Transit mid-time (BJD)

2,456,989.4220

Long. of periastron (ω)

34.48°

Angular separation (arcsec)

0.90900

Eq. Temperature

500K

(227 °C)

Insolation (S⊕)

10.00

Earth = 1.00

Earth Similarity (ESI)

0.529

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

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