Editing Openai/6946e998-79f0-8007-9a65-19828450838d (section)

==== Short answer: η⊕ by itself does not distinguish “Earth-like climate” from Venus-like or Mars-like. It is defined in terms of: ====
* Radius (e.g. 0.5–1.5 R⊕)
* Incident stellar flux / orbital period (within some HZ bounding flux range)

You can choose different inner and outer HZ boundaries (e.g. “recent Venus” inner edge, “early Mars” outer edge ADS<ref>{{cite web|title=ADS|url=https://ui.adsabs.harvard.edu/abs/2014ApJ...787L..29K/abstract|publisher=ui.adsabs.harvard.edu|access-date=2025-12-21}}</ref>), but that only says “in the range where an Earth-like atmosphere could in principle support surface liquid water,” not what the planet actually did with that flux.

Hence:
* A Venus analog (similar size, runaway greenhouse) at the inner HZ edge can easily be counted as an “HZ Earth-size” planet if you use an optimistic HZ boundary.
* A Mars analog near the outer edge can also be counted, even if it lost most of its atmosphere and surface water.

Because of this, η⊕ is better read as “Earth-radius planets in the right sunlight zone”, not “actual Earth-like habitability.”