Key parameters at a glance

A radius of 2.50 Earth radii
A mass of 6.82 Earth masses
Surface gravity around 1.09 g
An orbital period of 13.727 days
Semi-major axis 0.0900 AU
Distance from Earth 59.18 light-years
Earth Similarity Index 0.704
Travel time at Voyager 1 speed 1,043,709 years
Almost certainly tidally locked to its host star given the close orbit

2 siblings around GJ 3998

GJ 3998 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
GJ 3998 b	Super-Earth	1.39	2.50	2.650	—	2016
GJ 3998 c this	Sub-Neptune	2.50	6.82	13.727	—	2016
GJ 3998 d	Sub-Neptune	2.34	6.07	41.780	—	2025

GJ 3998 c Planet Profile

Physical Specs

Radius

2.50 R⊕

Radius (Jupiter)

0.223 R♃

Mass

6.82 M⊕

Mass (Jupiter)

0.021 M♃

Density

2.40 g/cm³

Surface Gravity

1.09 g (Earth)

Classification

Planet Type

Sub-Neptune

ESI Score 0.704

HZ Position Inner

Controversial No

Discovery

Year 2016

Method Radial Velocity

Facility Roque de los Muchachos Observatory

Telescope 3.58 m Telescopio Nazionale Galileo

Size rank in cohort

Rank by radius

#1168of 1978

top 59.0%

This planet

2.50R⊕

Sub-Neptune median

2.71R⊕

Nearest-size peers

Kepler-126 d 2.50 R⊕ Kepler-254 d 2.50 R⊕ GJ 163 c 2.50 R⊕

Metric	Earth	GJ 3998 c	Jupiter
Radius (R⊕)	1.00	2.50	11.21
Mass (M⊕)	1.00	6.82	317.83
Density (g/cm³)	5.51	2.40	1.33
Surface Gravity (g)	1.00	1.09	2.53
Insolation (S⊕)	1.00	5.40	0.037

Alternative Mass Estimates

Method	Value	Interpretation
Minimum mass (M sin i)	6.820 M⊕	Lower bound from radial-velocity fitting.

Star Catalogue Identifiers

HIP

HIP 84460

TIC

TIC 459558974

Gaia DR2

Gaia DR2 4540061704988054400

Gaia DR3

Gaia DR3 4540061704989418240

System

GJ 3998

Percentile among Sub-Neptune cohort

Radius 2.500 R⊕ · percentile 40 / cohort 1978

Mass 6.820 M⊕ · percentile 38 / cohort 1978

Orbital period 13.73 d · percentile 47 / cohort 1946

Distance 18.15 pc · percentile 4 / cohort 1975

ESI 0.704 · percentile 93 / cohort 1978

Orbit & Habitability

Orbital Dynamics

Period

13.727 days

Semi-major axis

0.0900 AU

Eccentricity

0.000

Inclination

— °

Year Length

A year here lasts approximately 13.73 Earth days (3.8% of a terrestrial year) on a nearly circular orbit at a mean orbital distance of 0.0900 AU.

Eq. Temperature

—

Insolation (S⊕)

5.40

Earth = 1.00

Earth Similarity (ESI)

0.704

0 = alien, 1 = Earth-identical

Inside the inner HZ boundary (too hot for liquid water). Close-orbiting tidally-locked around a cool host.

Discovery Paper

Primary reference

Affer et al. 2016

Instrument

HARPS-N Spectrograph

Publication

2016-10

Observation locale

Ground

Discovery cohort

Planets confirmed in 2016 at Roque de los Muchachos Observatory (4 shown).

GJ 3998 b · Super-Earth HD 5583 b · Gas Giant Pr0211 c · Gas Giant BD+15 2375 b · Gas Giant

Host System: GJ 3998

Spectral Class

M-type red dwarf

Effective Temperature

3,726 K — cooler than the Sun (5,778 K)

Estimated Age

8.38 Gyr — older than the Sun (4.6 Gyr)

Stellar Radius

0.500 R☉

Stellar Mass

0.520 M☉

Metallicity [Fe/H]

-0.13

Extended Stellar Properties

Surface gravity (log g)

4.750 dex

Stellar density

5.188 g/cm³

Systemic radial velocity

-44.81 km/s

Rotational v·sin i

0.93 km/s

Metallicity ratio

[Fe/H]

Distance

18.15 parsec

Light-years 59.18 ly

V-band magnitude

10.84 mag

Voyager-speed travel 1,043,709 yr

Multi-band Host Photometry — 17 bands

Astrometric Data

Parallax

55.080 mas

Total Proper Motion

373.473 mas/yr

PM Right Ascension

-137.31 mas/yr

PM Declination

-347.32 mas/yr

Galactic Cartesian (pc)

x = -0.187 · y = -0.963 · z = 0.192

Equatorial (J2000)

RA 259.00205° · Dec 11.05618°

Galactic ℓ, b

32.272° · 26.150°

Ecliptic λ, β

256.944° · 33.998°

HTM-20 index

206453640