Key parameters at a glance

A radius of 2.34 Earth radii
A mass of 6.07 Earth masses
Surface gravity around 1.11 g
An orbital period of 41.780 days
Semi-major axis 0.1890 AU
Distance from Earth 59.18 light-years
Earth Similarity Index 0.728
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 d 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	Sub-Neptune	2.50	6.82	13.727	—	2016
GJ 3998 d this	Sub-Neptune	2.34	6.07	41.780	—	2025

GJ 3998 d Planet Profile

Physical Specs

Radius

2.34 R⊕

Radius (Jupiter)

0.209 R♃

Mass

6.07 M⊕

Mass (Jupiter)

0.019 M♃

Density

2.60 g/cm³

Surface Gravity

1.11 g (Earth)

Classification

Planet Type

Sub-Neptune

ESI Score 0.728

HZ Position Habitable

Controversial No

Discovery

Year 2025

Method Radial Velocity

Facility Multiple Observatories

Telescope Multiple Telescopes

Size rank in cohort

Rank by radius

#1429of 1978

top 72.2%

This planet

2.34R⊕

Sub-Neptune median

2.71R⊕

Nearest-size peers

Kepler-246 b 2.34 R⊕ Kepler-275 b 2.34 R⊕ Kepler-338 c 2.34 R⊕

Metric	Earth	GJ 3998 d	Jupiter
Radius (R⊕)	1.00	2.34	11.21
Mass (M⊕)	1.00	6.07	317.83
Density (g/cm³)	5.51	2.60	1.33
Surface Gravity (g)	1.00	1.11	2.53
Insolation (S⊕)	1.00	1.20	0.037

Alternative Mass Estimates

Method	Value	Interpretation
Minimum mass (M sin i)	6.070 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 2.340 R⊕ · percentile 27 / cohort 1978

Mass 6.070 M⊕ · percentile 26 / cohort 1978

Orbital period 41.78 d · percentile 82 / cohort 1946

Distance 18.15 pc · percentile 4 / cohort 1975

ESI 0.728 · percentile 95 / cohort 1978

Orbit & Habitability

Orbital Dynamics

Period

41.780 days

Semi-major axis

0.1890 AU

Eccentricity

0.000

Inclination

— °

Year Length

A year here lasts approximately 41.78 Earth days (11.4% of a terrestrial year) on a nearly circular orbit at a mean orbital distance of 0.1890 AU.

Eq. Temperature

—

Insolation (S⊕)

1.20

Earth = 1.00

Earth Similarity (ESI)

0.728

0 = alien, 1 = Earth-identical

Within the conservative habitable zone. Close-orbiting tidally-locked around a cool host.

Discovery Paper

Primary reference

Stefanov et al. 2025

Instrument

Multiple Instruments

Publication

2025-03

Observation locale

Ground

Discovery cohort

Planets confirmed in 2025 at Multiple Observatories (12 shown).

Gl 725 A b · Super-Earth HD 20794 f · Sub-Neptune GJ 251 c · Super-Earth HD 86728 b · Sub-Neptune TOI-2015 c · Sub-Neptune Kepler-139 f · Neptune-like HD 73344 d · Gas Giant TOI-1472 c · Sub-Neptune TOI-6695 c · Gas Giant AT2021ueyL b · Gas Giant HIP 111909 b · Gas Giant TOI-6041 c · 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