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Entering orbit…
Habitability
0/ 100
Uninhabitable

Reading HD 128717 b’s atmosphere

Dominant elements: H, He, O, C, NThis is what a telescope would measure while HD 128717 b crosses in front of its star: starlight passes through the atmosphere and spreads into a rainbow band. Each dark line marks a wavelength the atmosphere absorbed.Every element absorbs its own signature wavelengths — hydrogen always darkens the same two spots, calcium another pair, and so on. Astronomers match the lines in a real spectrum against a library of these signatures to say what an atmosphere is made of, light-years away, without ever visiting. A deeper line generally means more of that element is present.HD 128717 b sits about 240 light-years away, found by the astrometry method. Most catalogued distances trace back to parallax: as Earth orbits the Sun, a nearby star appears to shift very slightly against the distant background stars, and the size of that shift gives its distance directly.

HD 128717 b

Brown DwarfUninhabitable240 light years awayFound 2025

HD 128717 b, from the NASA Exoplanet Archive (found 2025). Measured: 6290 Earth masses, 4.85 AU, a 6210 K F-type star. Composition inferred from a mass of 6290 Earths, which is giant territory whatever its 3.55x density suggests, with an assumed 24-hour day — rotation cannot be measured for exoplanets. Note: TerraForge comes out at 938 K against the archive's 152 K, because it models stars by spectral class, and HD 128717 is not an average F-type.

Host star

HD 128717

Type
F-type (White)
Temperature
6,210 K
Radius
1.25 ☉
Mass
1.21 ☉
Known planets
1 (this one)
Luminosity
2.10 ☉

Published by the NASA Exoplanet Archive. This world’s temperature and habitability score are computed from this star’s measured luminosity, not from a stand-in for its class.

Brown Dwarf

Magnetic FieldNone Activity
Why is this a Brown Dwarf?Your planet has an unusual composition that doesn't fit standard classifications, or lacks the elements needed for complex geological processes.
Barren worlds can form from uncommon element combinations or may be remnants of larger bodies that lost their atmospheres and volatile materials.
Classification Criteria:
Composition doesn't match standard planet typesMass: 6290.0x EarthTemperature: 938K
Further reading
The Little Book of ExoplanetsJoshua WinnHow these worlds are found, and what a measurement can honestly tell us.Exoplanets: Diamond Worlds, Super Earths, Pulsar PlanetsMichael Summers & James TrefilThe strange end of the catalogue — carbon worlds, molten rock, planets orbiting dead stars.Envisioning ExoplanetsMichael CarrollWhat these worlds might look like from the ground, painted from the same measurements this page renders.Imagined LifeJames Trefil & Michael SummersWhat could live on a world like this one: ice shells, supergravity, tidally locked twilight.
As an Amazon Associate I earn from qualifying purchases — if you buy through these links, TerraForge may receive a small commission at no extra cost to you.
Habitability ScoreUninhabitable
0
Contributing Factors
temperature5
Scorching - hot enough to melt lead, though rock stays solid; no chance for life
atmosphere30
Hydrogen-rich atmosphere typical of gas giants
water15
No solid surface — any water is locked in a supercritical mantle and hot ice, not oceans
magnetic Field20
A strong field, but no surface beneath it — there is nothing here for it to shelter
geology30
Geologically dead - no nutrient recycling
Organic Chemistry80
Core organic elements (CHNO) present
rotation100
Optimal rotation period - balanced day/night cycle, moderate weather, and good heat distribution

Physical Properties

Mass
6290.00x Earth (3.76e+28 kg)
Radius
18.46x Earth (117,605 km)
This planet: 18.46× Earth radiusEarth: 1× (baseline)
Surface Gravity
18.46x Earth (181.1 m/s²)
Surface Temperature
938 K (665 °C / 1,228 °F)
Rotation Period
24.0 hours/day
This planet: 24 h/dayEarth: 24 h/day

Orbital Environment

Star Type
F-type (White)
Distance from Star
4.85 AU (726 million km)
Orbital Period
9.37 years

Atmosphere

Hydrogen and helium dominated by methane absorption, with water clouds below

Elemental Composition

H - Hydrogen
53.8%
He - Helium
32.3%
O - Oxygen
5.4%

Surface Characteristics

No surface: a cooling failed star, its methane atmosphere absorbing so much red light it would look magenta to the eye
A binary star system's orrery: two suns close together, a red dashed dead-zone boundary, and a debris trail left by a destroyed world

New: Binary Star Systems 🌀

A system can now orbit two suns. Add a companion star when you build one — close, medium, or wide — and the physics decides what survives: a shared center of mass carves out a dead zone where gravity from both stars tears a world apart, leaving it as debris, and every surviving planet's orbit goes elliptical, moving fastest at closest approach and slowest out at aphelion. Push the separation to wide and the dead zone opens tens of AU across, wide enough to gut a system that would have been stable around one star alone. Stand on a world that made it through and you'll watch two suns cross the sky together.

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