The domain-specific notation in this paper is inscrutable to me. I assume that this means a possible "Minshara class" planet (as Gene would say) around a star much different than our own, in that the distance from its sun and insolation received don't rule it out?
EDIT: R⊕ appears to be something related to Earth. Perhaps a planetary radius?
⊕ is the symbol for earth. R⊕ is earth-radius, as you guessed.
It is domain-specific, so it should probably be changed in the title.
I will say that it is a very nice symbol - easy to write out by hand, makes for quick note-taking. Same for the symbol used for the sun (which isn't rendering for me, but is a circle with a dot in the middle).
It's a planet that has 1.55 Earth's radius orbiting a much cooler star. Planets in the habitable zone are those that are at a distance from their star, such that they have the reasonable potential for liquid water to exist on the planet.
EDIT: R⊕ appears to be something related to Earth. Perhaps a planetary radius?