Key parameters at a glance

A radius of 0.79 Earth radii
A mass of 0.39 Earth masses
Surface gravity around 0.62 g
An orbital period of 4.049 days
Semi-major axis 0.0223 AU
Equilibrium temperature 286 K (13 °C)
Distance from Earth 40.54 light-years
Earth Similarity Index 0.915
Travel time at Voyager 1 speed 714,938 years
Almost certainly tidally locked to its host star given the close orbit

Context from the literature

TRAPPIST-1d is a small exoplanet, which orbits on the inner edge of the habitable zone of the ultracool dwarf star TRAPPIST-1, located 40.7 light-years away from Earth in the constellation of Aquarius. The exoplanet was found by using the transit method. The first signs of the planet were announced in 2016, but it was not until the following years that more information concerning the probable nature of the planet was obtained. TRAPPIST-1d is the second-least massive planet of the system. It receives just 4.3% more sunlight than Earth, placing it on the inner edge of the habitable zone. A 2025 study, based on a James Webb Space Telescope observation, found that the data was "satisfactorily" consistent with TRAPPIST-1d having no atmosphere at all. Nevertheless, there is still a "marginal possibility" the planet has an atmosphere, but determining this with greater confidence would require a different detection technique.

Excerpted from Wikipedia · full article

6 siblings around TRAPPIST-1

TRAPPIST-1 d shares its host star with 6 other confirmed planets. Side-by-side measurements below.

Planet	Type	Radius (R⊕)	Mass (M⊕)	Period (d)	Eq. T (K)	Year
TRAPPIST-1 b	Rocky Terrestrial	1.12	1.37	1.511	398	2016
TRAPPIST-1 c	Rocky Terrestrial	1.10	1.31	2.422	340	2016
TRAPPIST-1 d this	Rocky Terrestrial	0.79	0.39	4.049	286	2016
TRAPPIST-1 e	Rocky Terrestrial	0.92	0.69	6.101	250	2017
TRAPPIST-1 f	Rocky Terrestrial	1.05	1.04	9.208	218	2017
TRAPPIST-1 g	Rocky Terrestrial	1.13	1.32	12.352	197	2017
TRAPPIST-1 h	Rocky Terrestrial	0.76	0.33	18.773	172	2017

TRAPPIST-1 d Planet Profile

Physical Specs

Radius

0.79 R⊕

Radius (Jupiter)

0.070 R♃

Mass

0.39 M⊕

Mass (Jupiter)

0.001 M♃

Density

4.37 g/cm³

Surface Gravity

0.62 g (Earth)

Classification

Planet Type

Rocky Terrestrial

ESI Score 0.915

HZ Position Habitable

Controversial No

Discovery

Year 2016

Method Transit

Facility La Silla Observatory

Telescope 0.60 m Transiting Planets and Planetesimals Small Telescope

Size rank in cohort

Rank by radius

#492of 570

top 86.1%

This planet

0.79R⊕

Rocky Terrestrial median

1.01R⊕

Nearest-size peers

Kepler-390 c 0.79 R⊕ Kepler-1566 b 0.79 R⊕ Kepler-1222 b 0.79 R⊕

Metric	Earth	TRAPPIST-1 d	Jupiter
Radius (R⊕)	1.00	0.79	11.21
Mass (M⊕)	1.00	0.39	317.83
Density (g/cm³)	5.51	4.37	1.33
Surface Gravity (g)	1.00	0.62	2.53
Insolation (S⊕)	1.00	1.12	0.037

Alternative Mass Estimates

Method	Value	Interpretation
Best measured mass	0.388 M⊕	Direct mass measurement (not derived).

Star Catalogue Identifiers

TIC

TIC 278892590

Gaia DR2

Gaia DR2 2635476908753563008

Gaia DR3

Gaia DR3 2635476908753563008

System

TRAPPIST-1

Percentile among Rocky Terrestrial cohort

Radius 0.788 R⊕ · percentile 14 / cohort 570

Mass 0.388 M⊕ · percentile 11 / cohort 570

Orbital period 4.05 d · percentile 45 / cohort 567

Distance 12.43 pc · percentile 6 / cohort 566

ESI 0.915 · percentile 91 / cohort 570

Orbit & Habitability

Orbital Dynamics

Period

4.049 days

Semi-major axis

0.0223 AU

Eccentricity

0.008

Inclination

89.90 °

Year Length

A year here lasts approximately 4.05 Earth days (1.1% of a terrestrial year) on a nearly circular orbit at a mean orbital distance of 0.0223 AU.

Transit Fingerprint

Depth

0.368 %

Duration

0.815 h

Impact parameter b

0.063

Rp / R★

0.060630

Transit midpoint (BJD)

2,457,670.1463

Photometric dip

Transit produces a flux drop of 3,676 ppm lasting ≈ 0.81 h.

Extended Orbital Architecture

Planet / star radius ratio

0.060630

Semi-major axis / stellar radius

40.216

Impact parameter (b)

0.063

Transit mid-time (BJD)

2,457,670.1463

Long. of periastron (ω)

-8.73°

Angular separation (arcsec)

1.79000

Eq. Temperature

286K

(13 °C)

Insolation (S⊕)

1.12

Earth = 1.00

Earth Similarity (ESI)

0.915

0 = alien, 1 = Earth-identical

Within the conservative habitable zone — liquid water is physically possible with adequate atmospheric pressure. Close-orbiting tidally-locked around a cool host.

Detection Signatures

Transit photometry

A transit dip has been observed from at least one telescope.

Transit timing variations

Gravitational interactions from neighbours have been detected.

Discovery Paper

Primary reference

Gillon et al. 2016

Instrument

TRAPPISTCAM

Publication

2016-05

Observation locale

Ground

Discovery cohort

Planets confirmed in 2016 at La Silla Observatory (5 shown).

TRAPPIST-1 c · Rocky Terrestrial TRAPPIST-1 b · Rocky Terrestrial GJ 536 b · Sub-Neptune HD 111998 b · Gas Giant NGC 2682 YBP 401 b · Gas Giant

Host System: TRAPPIST-1

Spectral Class

M-type red dwarf

Effective Temperature

2,566 K — cooler than the Sun (5,778 K)

Estimated Age

7.60 Gyr — older than the Sun (4.6 Gyr)

Stellar Radius

0.119 R☉

Stellar Mass

0.090 M☉

Metallicity [Fe/H]

0.05

Extended Stellar Properties

Surface gravity (log g)

5.240 dex

Stellar density

75.050 g/cm³

Rotational v·sin i

2.04 km/s

Rotation period

1.40 days

Metallicity ratio

[Fe/H]

Distance

12.43 parsec

Light-years 40.54 ly

V-band magnitude

17.02 mag

Voyager-speed travel 714,938 yr

Multi-band Host Photometry — 13 bands

Astrometric Data

Parallax

80.451 mas

Total Proper Motion

1,026.579 mas/yr

PM Right Ascension

901.00 mas/yr

PM Declination

-492.00 mas/yr

Galactic Cartesian (pc)

x = 0.969 · y = -0.230 · z = -0.088

Equatorial (J2000)

RA 346.62639° · Dec -5.04346°

Galactic ℓ, b

69.715° · -56.649°

Ecliptic λ, β

345.733° · 0.630°

HTM-20 index

915425164

Wikipedia → Image: Wikimedia Commons · NASA, ESA, CSA, J. Olmsted (STScI) · CC BY 4.0