Astronomical evidence
KELT-9 b is an ultra-hot giant on a roughly 1.48-day orbit. Intense irradiation makes its atmosphere a useful test of physics that is much less prominent in cooler planets. The distinction between the planet’s incoming stellar energy and its emitted heat is central to interpreting observations. On the extremely hot dayside, molecular hydrogen can break apart into atoms. Dissociation absorbs energy. When those atoms move to cooler regions and recombine into molecules, energy is released again. Atmospheric circulation can therefore transport energy in chemical form as well as through changes in gas temperature. Thermal observations taken over an orbit constrain how the system’s brightness varies with viewing phase. Interpreting that variation involves temperature structure, heat transport and the spectrum emitted by the atmosphere; it is not equivalent to directly photographing a local weather front.
The illustrated viewpoint
The pale glowing limb is viewed through an invented protective window. Its filtering and exposure mean that the palette is not a naked-eye color prediction. No ordinary surface, habitable atmosphere or feasible shelter is established by the illustration.
The pale glowing limb is viewed through an invented protective window. Its filtering and exposure mean that the palette is not a naked-eye color prediction. No ordinary surface, habitable atmosphere or feasible shelter is established by the illustration. All depicted life and technology are fictional.
Catalog measurements & sources
Host: KELT-9. Snapshot: 25 September 2026. Errors, limits and source provenance are retained. Calculated values and model estimates are not direct measurements. Unknown is not zero; equilibrium temperature is not surface temperature.
| Quantity / unit | Value / reported errors | Source |
|---|---|---|
| Radius · Earth radii | 21.7006 (+0.526822 / -0.526822) | Borsa et al. 2019 |
| Mass · Earth masses | 915.346 (+111.24 / -111.24) | Borsa et al. 2019 |
| Density · g/cm³ | 0.491 (+0.072 / -0.066) | Borsa et al. 2019 |
| Orbital period · days | 1.48112 (+1.1e-06 / -1.1e-06) | Borsa et al. 2019 |
| Orbital semimajor axis · AU | 0.03368 (+0.00078 / -0.00078) | Borsa et al. 2019 |
| Eccentricity | 0 | Borsa et al. 2019 |
| Inclination · degrees | 86.79 (+0.25 / -0.25) | Borsa et al. 2019 |
| Irradiation · Earth flux | 44900 (+8100 / -7200) | Gaudi et al. 2017 |
| Equilibrium temperature · K | 3921 (+182 / -174) | Borsa et al. 2019 |
| Stellar effective temperature · K | 9270 (+240 / -180) | Borsa et al. 2019 |
| Stellar radius · Solar | 2.418 (+0.058 / -0.058) | Borsa et al. 2019 |
| Stellar mass · Solar | 2.32 (+0.16 / -0.16) | Borsa et al. 2019 |
| Stellar luminosity · log Solar | 1.58995 (+0.04553 / -0.03728) | Borsa et al. 2019 |
| Stellar age · Gyr | 0.45 (+0.14 / -0.13) | Borsa et al. 2019 |
| Stellar metallicity · dex | 0.07 (+0.2 / -0.23) | Borsa et al. 2019 |
| Distance · pc | 204.455 (+1.582 / -1.558) | TICv8 |