Key parameters at a glance

A radius of 0.91 Earth radii
A mass of 0.70 Earth masses
Surface gravity around 0.84 g
An orbital period of 2.021 days
Semi-major axis 0.0163 AU
Equilibrium temperature 471 K (198 °C)
Distance from Earth 12.11 light-years
Earth Similarity Index 0.660
Travel time at Voyager 1 speed 213,509 years
Almost certainly tidally locked to its host star given the close orbit

2 siblings around YZ Cet

YZ Cet b 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
YZ Cet b this	Rocky Terrestrial	0.91	0.70	2.021	471	2017
YZ Cet c	Rocky Terrestrial	1.05	1.14	3.060	410	2017
YZ Cet d	Rocky Terrestrial	1.03	1.09	4.656	357	2017

YZ Cet b Planet Profile

Physical Specs

Radius

0.91 R⊕

Radius (Jupiter)

0.081 R♃

Mass

0.70 M⊕

Mass (Jupiter)

0.002 M♃

Density

5.05 g/cm³

Surface Gravity

0.84 g (Earth)

Classification

Planet Type

Rocky Terrestrial

ESI Score 0.660

HZ Position Inner

Controversial No

Discovery

Year 2017

Method Radial Velocity

Facility La Silla Observatory

Telescope 3.6 m ESO Telescope

Size rank in cohort

Rank by radius

#405of 570

top 70.9%

This planet

0.91R⊕

Rocky Terrestrial median

1.01R⊕

Nearest-size peers

Kepler-1934 b 0.91 R⊕ TOI-700 b 0.91 R⊕ Kepler-1525 b 0.91 R⊕

Metric	Earth	YZ Cet b	Jupiter
Radius (R⊕)	1.00	0.91	11.21
Mass (M⊕)	1.00	0.70	317.83
Density (g/cm³)	5.51	5.05	1.33
Surface Gravity (g)	1.00	0.84	2.53
Insolation (S⊕)	1.00	8.21	0.037

Alternative Mass Estimates

Method	Value	Interpretation
Minimum mass (M sin i)	0.700 M⊕	Lower bound from radial-velocity fitting.

Star Catalogue Identifiers

HIP

HIP 5643

TIC

TIC 439403362

Gaia DR2

Gaia DR2 2358524597030794112

Gaia DR3

Gaia DR3 2358524597030794112

System

YZ Cet

Percentile among Rocky Terrestrial cohort

Radius 0.913 R⊕ · percentile 29 / cohort 570

Mass 0.700 M⊕ · percentile 25 / cohort 570

Orbital period 2.02 d · percentile 22 / cohort 567

Distance 3.71 pc · percentile 2 / cohort 566

ESI 0.660 · percentile 81 / cohort 570

Orbit & Habitability

Orbital Dynamics

Period

2.021 days

Semi-major axis

0.0163 AU

Eccentricity

0.060

Inclination

— °

Year Length

A year here lasts approximately 2.02 Earth days (0.6% of a terrestrial year) on a nearly circular orbit at a mean orbital distance of 0.0163 AU.

Eq. Temperature

471K

(198 °C)

Insolation (S⊕)

8.21

Earth = 1.00

Earth Similarity (ESI)

0.660

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

Astudillo Defru et al. 2017

Instrument

HARPS Spectrograph

Publication

2017-09

Observation locale

Ground

Discovery cohort

Planets confirmed in 2017 at La Silla Observatory (12 shown).

GJ 273 c · Rocky Terrestrial Ross 128 b · Rocky Terrestrial GJ 3138 b · Rocky Terrestrial GJ 3323 c · Super-Earth GJ 3323 b · Rocky Terrestrial GJ 273 b · Super-Earth GJ 3293 e · Super-Earth YZ Cet d · Rocky Terrestrial GJ 3138 c · Super-Earth YZ Cet c · Rocky Terrestrial GJ 3293 d · Sub-Neptune GJ 3138 d · Sub-Neptune

Host System: YZ Cet

Spectral Class

M-type red dwarf

Effective Temperature

3,151 K — cooler than the Sun (5,778 K)

Estimated Age

3.80 Gyr — younger than the Sun (4.6 Gyr)

Stellar Radius

0.157 R☉

Stellar Mass

0.142 M☉

Metallicity [Fe/H]

-0.18

Extended Stellar Properties

Surface gravity (log g)

5.170 dex

Systemic radial velocity

28.30 km/s

Rotational v·sin i

2.00 km/s

Rotation period

68.28 days

Metallicity ratio

[Fe/H]

Distance

3.71 parsec

Light-years 12.11 ly

V-band magnitude

12.07 mag

Voyager-speed travel 213,509 yr

Multi-band Host Photometry — 6 bands

Astrometric Data

Parallax

269.363 mas

Total Proper Motion

1,363.523 mas/yr

PM Right Ascension

1,205.18 mas/yr

PM Declination

637.76 mas/yr

Galactic Cartesian (pc)

x = 0.909 · y = 0.298 · z = -0.292

Equatorial (J2000)

RA 18.13308° · Dec -16.99624°

Galactic ℓ, b

149.724° · -78.755°

Ecliptic λ, β

9.785° · -22.742°

HTM-20 index

987040254