Key parameters at a glance

A radius of 0.83 Earth radii
A mass of 0.50 Earth masses
Surface gravity around 0.72 g
An orbital period of 1.606 days
Semi-major axis 0.0216 AU
Equilibrium temperature 789 K (516 °C)
Distance from Earth 208.76 light-years
Earth Similarity Index 0.395
Travel time at Voyager 1 speed 3,681,501 years
Almost certainly tidally locked to its host star given the close orbit

K2-257 b Planet Profile

Physical Specs

Radius

0.83 R⊕

Radius (Jupiter)

0.074 R♃

Mass

0.50 M⊕

Mass (Jupiter)

0.002 M♃

Density

4.79 g/cm³

Surface Gravity

0.72 g (Earth)

Classification

Planet Type

Rocky Terrestrial

ESI Score 0.395

HZ Position Unknown

Controversial No

Discovery

Year 2018

Method Transit

Facility K2

Telescope 0.95 m Kepler Telescope

Size rank in cohort

Rank by radius

#461of 570

top 80.7%

This planet

0.83R⊕

Rocky Terrestrial median

1.01R⊕

Nearest-size peers

Kepler-1412 b 0.83 R⊕ Kepler-314 b 0.83 R⊕ Kepler-450 d 0.84 R⊕

Metric	Earth	K2-257 b	Jupiter
Radius (R⊕)	1.00	0.83	11.21
Mass (M⊕)	1.00	0.50	317.83
Density (g/cm³)	5.51	4.79	1.33
Surface Gravity (g)	1.00	0.72	2.53
Insolation (S⊕)	1.00	—	0.037

Star Catalogue Identifiers

TIC

TIC 130640162

Gaia DR2

Gaia DR2 3695235654973028736

Gaia DR3

Gaia DR3 3695235654973028736

System

K2-257

Percentile among Rocky Terrestrial cohort

Radius 0.830 R⊕ · percentile 19 / cohort 570

Mass 0.498 M⊕ · percentile 16 / cohort 570

Orbital period 1.61 d · percentile 18 / cohort 567

Distance 64.01 pc · percentile 20 / cohort 566

ESI 0.395 · percentile 59 / cohort 570

Orbit & Habitability

Orbital Dynamics

Period

1.606 days

Semi-major axis

0.0216 AU

Eccentricity

—

Inclination

— °

Year Length

A year here lasts approximately 1.61 Earth days (0.4% of a terrestrial year) at a mean orbital distance of 0.0216 AU.

Transit Fingerprint

Depth

0.034 %

Duration

1.300 h

Impact parameter b

0.400

Rp / R★

0.015300

Transit midpoint (BJD)

2,457,583.2875

Photometric dip

Transit produces a flux drop of 340 ppm lasting ≈ 1.30 h.

Extended Orbital Architecture

Planet / star radius ratio

0.015300

Semi-major axis / stellar radius

8.200

Impact parameter (b)

0.400

Transit mid-time (BJD)

2,457,583.2875

Angular separation (arcsec)

0.33700

Eq. Temperature

789K

(516 °C)

Insolation (S⊕)

—

Earth = 1.00

Earth Similarity (ESI)

0.395

0 = alien, 1 = Earth-identical

Insufficient insolation data — the surface is too hot to retain surface water. Close-orbiting tidally-locked around a cool host.

Discovery Paper

Primary reference

Livingston et al. 2018

Instrument

Kepler CCD Array

Publication

2018-08

Observation locale

Space

Discovery cohort

Planets confirmed in 2018 at K2 (12 shown).

K2-190 c · Rocky Terrestrial K2-183 b · Rocky Terrestrial K2-209 b · Rocky Terrestrial K2-210 b · Rocky Terrestrial K2-165 b · Super-Earth K2-169 b · Super-Earth K2-148 b · Super-Earth K2-219 b · Super-Earth K2-190 b · Super-Earth K2-200 b · Super-Earth K2-201 b · Super-Earth K2-170 b · Super-Earth

Host System: K2-257

Spectral Class

K-type orange

Effective Temperature

3,725 K — cooler than the Sun (5,778 K)

Estimated Age

—

Stellar Radius

0.500 R☉

Stellar Mass

0.520 M☉

Metallicity [Fe/H]

-0.03

Extended Stellar Properties

Surface gravity (log g)

4.760 dex

Stellar density

5.567 g/cm³

Systemic radial velocity

12.59 km/s

Metallicity ratio

[Fe/H]

Distance

64.01 parsec

Light-years 208.76 ly

V-band magnitude

13.72 mag

Voyager-speed travel 3,681,501 yr

Multi-band Host Photometry — 17 bands

Astrometric Data

Parallax

15.595 mas

Total Proper Motion

55.284 mas/yr

PM Right Ascension

-47.15 mas/yr

PM Declination

-28.86 mas/yr

Galactic Cartesian (pc)

x = -0.989 · y = -0.144 · z = -0.034

Equatorial (J2000)

RA 188.27828° · Dec -1.95329°

Galactic ℓ, b

293.578° · 60.585°

Ecliptic λ, β

188.370° · 1.488°

HTM-20 index

-694150515