From rod, on old Venus notions: "This dense atmosphere strongly reflects the sun’s rays and thus prevents the surface of the planet from attaining a temperature too elevated for highly organized life. The planet would be regarded as habitable.” —Scientific American, March 1911"
They clearly did not have all the details of Venus! But it could be habitable, if you read enough of the posts by all the dreaming space explorers. They just have to replace its atmosphere, bring in (or dig up) a lot of water for golf courses and suburban sprawl. Otherwise, live a subterranean life. Nobody says you have to live "on" the planet to live on the planet! Imaginations run wild.....
Let us hope the ELTs will find the oxygen. That would be a unique observation, apparently.
Wait, looks like I may have found some O2 after all*:
"The first observed extrasolar planetary atmosphere was made in 2001. Sodium in the atmosphere of the planet HD 209458 b was detected during a set of four transits of the planet across its star. Later observations with the Hubble Space Telescope showed an enormous ellipsoidal envelope of hydrogen, carbon and oxygen around the planet. This envelope reaches temperatures of 10,000 K. The planet is estimated to be losing (1-5)×108 kg of hydrogen per second. This type of atmosphere loss may be common to all planets orbiting Sun-like stars closer than around 0.1 AU. In addition to hydrogen, carbon, and oxygen, HD 209458 b is thought to have water vapor in its atmosphere."
Sounds like a newly formed planet and/or its atmosphere. It won't be losing that quantity for a very long time, one imagines. You must have data on this one (which would be nice confirmation, or not, of this Wiki data at any rate).
But they also note later that :
"The presence of molecular oxygen (O2) may be detectable by ground-based telescopes, and it can be produced by geophysical processes, as well as a byproduct of photosynthesis by life forms, so although encouraging, O2 is not a reliable biosignature. >> In fact, planets with high concentration of O2 in their atmosphere may be uninhabitable.<< "
end quotes
That last observation (my >> <<) would tend to rule out earth at 20% oxygen (surely that qualifies as a "high concentration of O2"). And then they go on about abiogensis (one of my specialties, as you know!). Whoever wrote this might want to stick to the space stuff and forget about life and its formation, activity and longevity. They have a few gaps on that one......
While O2 can clearly be produced by geophysical processes, it is not at all clear that those processes would last for billions of years, like the source of O2 on earth, which is not a geophysical process. Earth is believed to have fossilized photosynthetic algae dating back over 3 bya. No doubt a young planet like HD 209458 b most likely formed O2 by a nonbiological process since it is at high temperature and not exactly what even the wildest imagination would suggest is a habitable planet.
Apparently the opinions of a habitable planet is ever-changing, even as we exchange these messages!
To repeat: "Let us hope the ELTs will find the oxygen."
*
https://en.wikipedia.org/wiki/Extraterrestrial_atmosphere