Host star — HD 40307
- Spectral type
- K2.5 V
- Temperature
- 4,956 K
- Radius
- 0.72 R☉
- Mass
- 0.77 M☉
- Luminosity
- 0.230 L☉
- Distance
- 12.9 pc (42.2 ly)
Orange dwarf, cooler and longer-lived than the Sun.
Cooler than the Sun. Orange or red dwarf.
HD 40307 g
Orbits HD 40307 · 42.2 light-years from Earth
Neptune-likeRadial Velocity2013ESI 85 · Very Earth-like
Radius
2.56×
Earth
Mass
7.1×
Earth
Year
198d
Temp
255 K
-18°C
Gravity
1.1×
Earth
Distance
42.2
ly
Could life exist here?
AI analysisHD 40307 g orbits an orange dwarf star 42.2 light-years away and ranks among the most promising candidates for habitability discovered to date, with a habitability score of 85 out of 100. The planet is 2.56 times Earth's radius and 7.1 times Earth's mass, giving it a density of 2.33 grams per cubic centimeter—suggesting a rocky core wrapped in a thicker atmosphere than Earth's, placing it in the "super-Earth" category. Its equilibrium temperature of 255 Kelvin is tantalizingly close to Earth's 288 Kelvin, cool enough that liquid water could exist on its surface, though the exact amount of atmospheric insulation remains unknown. The planet orbits every 198 days at 0.6 AU from its parent star, well within the habitable zone. The main uncertainty is composition: we know only its mass and radius, not whether a thick hydrogen-helium envelope or a thin atmosphere caps this world. HD 40307 g's radial-velocity discovery and relative proximity make it a natural target for future atmospheric characterization with next-generation telescopes.
What it would be like
HD 40307 g is a Neptune-like world — probably wrapped in thick layers of hydrogen, helium, and ices. There may be no solid surface at all, just clouds all the way down.
Surface gravity is about 1.1g — noticeably heavier what you're used to on Earth.
With an equilibrium temperature around -18°C, this planet sits in the temperature range where liquid water could potentially exist on the surface — a key ingredient for life as we know it.
An orbital period of 198 days makes the year 1.8× shorter than Earth's. You'd celebrate your birthday more often here.
Earth comparison
Logarithmic bars so Jupiter-class planets fit the same scale as Earth-size worlds.
Radius2.56R⊕
1/25×Earth = 125×
Mass7.10M⊕
1/10000×Earth = 110000×
Surface gravity1.08g
1/100×Earth = 1100×
Equilibrium temp255 K(-18°C)
0 KEarth 255 K2500 K
Side-by-side with Earth
Temperature in context
Liquid N₂Mars avgEarth eq.Earth sfc.Boiling H₂OVenus
Discovery & orbit
- Method
- Radial Velocity
- Year
- 2013
- Facility
- La Silla Observatory
- Semi-major axis
- 0.6000 AU
- Period
- 197.80 days
- Eccentricity
- 0.2900
- Density
- 2.33 g/cm³
Detected by the star's wobble — gravitational tug from the orbiting planet shifts spectral lines.
Noticeably elliptical. Seasons (if any) would vary in intensity.
Low density — probably icy or gas-rich.
Discovered via · Radial velocity
The star's wobble — gravitational tug from the planet shifts its spectrum
A planet orbiting a star pulls it slightly back and forth. That motion compresses the star's light when moving toward us (blueshift) and stretches it away (redshift). Precision spectrographs detect the wobble at metres-per-second — enough to infer a planet's mass and orbit.
- Overall share
- ~19% of discoveries
- Best for
- Massive, close-in planets around nearby bright stars
Orbital Animation
HD 40307HD 40307 gOrbitEarth orbitHabitable zone
Drag to rotate · scroll to zoomSemi-major axis: 0.600 AUEccentricity: 0.2900Period: 197.8 days
Hertzsprung–Russell Diagram
Where this host star sits among … exoplanet host stars. The main sequence band runs diagonally — giants and supergiants sit above, white dwarfs below.
OBAFGKMCurrent star
How far is 42.2 light-years?
- A light beam leaving Earth right now would arrive in 42.2 years.
- At Voyager 1's speed (17 km/s), the trip would take approximately 744,080 years.
- A radio signal sent today would arrive in 42.2 years — and the reply wouldn't come back for twice that.
Earth Similarity Index
85/1000 — Nothing like Earth100 — Identical to Earth
ESI combines radius similarity and equilibrium temperature similarity. Earth = 100. Mars ≈ 73. Venus ≈ 44. This score reflects two physical parameters only — not atmosphere, water, or magnetic field.