HD 3167 b’s measured brightness temperature ratio in the context of other Earth-sized (<1.9 R_⊕) planets observed in thermal emission. Dashed lines represent different assumptions about the planet’s heat redistribution efficiency ε and effective albedo A_eff, and point size represents planet surface area. HD 3167 b better constrains a potential dichotomy between lava worlds with atmospheres and warm rocky M star planets without. Future emission observations of lava worlds with JWST, shown as purple stars—GJ 9827 b, TOI-1442 b, TOI-1075 b, TOI-1416 b, TOI-500 b, TOI-1807 b, HD 20329 b (JWST GO 4818; PI: Weiner Mansfield); TOI-6255 b, TOI-2431 b, TOI-2260 b (JWST GO 8864; PI: Dang); WASP-47 e (JWST GO 3615; PI: Zieba)—will better constrain this potential transition temperature. See Appendix E for a discussion of the data used in these plots.