GJ 367 b, as measured

A world of 0.70meas. Earth radii that rounds its red dwarf every 7.7meas. hours, one face always to it. Turn it, and where you look from is where the telescope stood in the orbit.

Starlit rock of Bond albedo 0.11meas., exposed as for white ground under the star, so it shows as dark as it is. Craters on the solid rock and the smooth melt inside the blue 1,500 K lineassumed are imaginedimagined.

GJ 367 is behind you, 35° across from the ground; turn toward transit to see it. Seen from phase 0.40, 3.1 h after transit.
Drag to turn; B the band, E the eye, C the slice.

The phase curve this model makes in MIRI’s 5–12 µm band, round one orbit (orange), against what Zhang et al. measured (blue): the eclipse depth, 79 ± 4 ppmmeas.; the night side, 4 ± 8 ppmmeas.; the peak 10.6 ± 4.8° eastmeas., so a little before the eclipse. The ring is the planet as you see it now.

Where the model and the measurement part

The map is fitted to the measured dayside temperature, so it peaks where MIRI saw the peak, by construction, and its night side gives 0.1 ppm against the measured 4 ± 8: the same within the error. Its eclipse is not: 66.3 ppmderived against 79 ± 4, 12.7 ppm shallower, 3.2σ of the measured error.

The gap is the model’s, in two parts. Its planet-to-star size comes from two radii measured apart, 0.0142, where the MIRI fit found 0.01364 ± 0.00023meas.; at the fit’s size the model would be 4.9 ppm shallower still, 61.4 ppm. The remaining 17.6 ppm is the star, taken here as a blackbody at 3,522 Kmeas. when turning the dayside’s temperature back into a depth. Zhang et al. went the other way, from the depth to the temperature through the star’s own spectrum, measured by MIRI in the same data; an M dwarf is no blackbody in this band.

The line where the rock melts

With the hottest point at 1,935 Kderived, the 1,500 K lineassumed falls 69° of arc from it. Inside it lies 87% of the dayside disc as this model computes it; the paper’s own figure is 88%meas..

“One major complication which we ignore is that GJ 367b is hot enough to melt most plausible crusts over at least some regions of the day side. In the case of zero Bond albedo and no heat recirculation, the temperature at the substellar point would be 1930 K; by comparison, iron melts at 1811 K, while silicate rocks start melting at 850 K, and all silicate rocks are molten by ~1500 K [50]. 88% of the dayside planetary disk has a temperature higher than 1500 K. Molten lava has different spectral properties from its solid counterpart. [17] found that lava worlds should have low albedos (A_g ≲ 0.1), in agreement with our observations.”

Michael Zhang et al., GJ 367b is a dark, hot, airless sub-Earth (2024)

Its own sky

From the ground, GJ 367 spans 35° of sky, from its radiusmeas. and the orbit’s sizederived. Turn the planet to its night side and it crosses that disc. The disc is darkened to its limb as an M dwarf’s is, by the quadratic law with u₁ = 0.50assumed and u₂ = 0.25assumed, coefficients assumed, not fitted to this star; nothing else is drawn on it.

An eye would see dark rock. Its own heat is real but dim beside the starlight it returns; the glow is drawn as the kit computes it, and barely shows. All surface compositions Zhang et al. tried fit the spectrum:

“As shown in Table 2, all surface compositions we tried are consistent with the data, largely because all have low albedos (high emissivities) in the MIRI bandpass. Airless worlds with a wavelength-independent albedo of 0 or 0.1 and a corresponding blackbody emission spectrum are also consistent with the data. Our observations therefore do not constrain the surface composition. Many complications make our observations even less constraining on the surface composition than the χ² numbers imply [55]. The airless rocky solar system bodies are generally dark, with Bond albedos of 0.07 for Mercury, 0.11 for the Moon, and 0.03 for Ceres. Graphite contaminants, space weathering, and surface roughness could all decrease the albedo below the already low values measured in the lab (e.g. Brunetto et al. 7, Dumusque et al. 16, Mansfield et al. 55). On the other hand, if the surface were smoother and had fewer non-metallic elements, it could have a higher albedo than the compositions we assumed.”

Michael Zhang et al., GJ 367b is a dark, hot, airless sub-Earth (2024)

Inside

An iron core of 91 −23/+7%meas. of the mass, with iron at 7.87assumed and rock at 3.3 g/cm³assumed, reaches 93% of the radius: the rock is a skin about 300 km thick. Across the core fraction’s range the core spans 78–98% of the radius (the lighter band). The densities are uncompressed and assumed, so this is a proportion, not a structure model.

“Very little is known about hot sub-Earths. It is currently unclear whether or not sub-Earth planets form their cores and acquire atmospheres via the same channels as their more massive super-Earth and sub-Neptune cousins, or whether these small planets represent a distinct formation channel with correspondingly different outcomes for their atmospheric and surface properties. There is some empirical evidence for the latter hypothesis from exoplanet demographics [60]. The suggestion by [63] (inspired by Cameron 9, Fegley & Cameron 18, Valencia et al. 65) that the hottest sub-Earths may be the remnants of more massive planets whose silicate mantles evaporated away is particularly intriguing because GJ 367b’s exceptional density (10.2 ± 1.3 g cm-3) is consistent with an iron core comprising 91_-23⁺⁷% of the mass [22]. Another possibility, not exclusive with the first, is that GJ 367b formed like Mercury [5]: a giant impact stripped the mantle and left the core behind. This process does not necessarily strip all volatiles, as evidenced by pyroclastic volcanism on Mercury [38]. Finally, [51] suggested that iron-rich planets can simply be formed from iron-rich materials, as outwardly drifting iron vapour condenses and increases the pebble iron mass fraction.”

Michael Zhang et al., GJ 367b is a dark, hot, airless sub-Earth (2024)

Measured: the band, the dayside, the night side, the offset, the eclipse, the albedo, the sizes. Derived by the kit: the map’s shape (the simplest the numbers ask for, cos(z)^¼ about the hottest point), the curve, the star’s size. Imagined: every crater, the relief, the mottling and where exactly the melt’s shore wanders. No telescope has resolved any of it.