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How confident are they in detecting planets of earth size? I have a distinct memory that they weren’t.

Depends how close to the parent star and how far that star is from us. But for earth sized planetes about 1 AU from the star we are rather blind.

For those who don't know (I don't think it's a common unit): 1 AU is the distance between the earth and the sun (Astronomical Unit). So apparently [citation needed] we're blind for earth-sized planets at an earth's distance from its star.

I am not a planetary scientist (my office mate is) and didn't spend much time looking at the literature. "A Revised Exoplanet Yield from the Transiting Exoplanet Survey Satellite (TESS)" by Barclay et al. (https://iopscience.iop.org/article/10.3847/1538-4365/aae3e9) might be the citation you are looking for. Figure 14 shows that anything below 1.5 Earth radii is basically invisible to TESS. I can not find a good chart for the detection limit in the planet size vs star distance plane.

The question that popped into my head in a disturbing manner was whether beings on those planets could see ours.

I'm not sure that's relevant - the fact that we can detect enough earth-ish sized planets to reveal a consistent bimodal size distribution is compelling on its own.

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