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
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.