Hmmmm. (ETA: following claim is incorrect) They’re judging that the planets are rocky by measuring their mass, not by noticing that they’re actually rocky.
If you don’t have a Jupiter-sized core out there sucking up all the gas, why would gas planets need to end up as giants? They naturally could do that—that happened with the star, after all, but it doesn’t seem inevitable to me, and it might not even be common.
In that case, the earth-mass planets would be gas planets after all. If you think this is a stretch, keep in mind that these are specifically in systems noted to be low metallicity. Suggesting that they might not be high in metals after all is not much of a stretch.
Actually, Kepler is able to determine both size and mass of planet candidates, using the method of transit photometry.
For further info, I found a non-paywalled copy of Bucchave et al’s Nature paper. Figure 3 plots planet radius against star metallicity, and some of the planets are clearly of Earth-radius or smaller. I very much doubt that it is possible to form gas “giants” of Earth size, and in any case they would have a mass much lower than Earth mass, so would stand out immediately.
Hmmmm. (ETA: following claim is incorrect) They’re judging that the planets are rocky by measuring their mass, not by noticing that they’re actually rocky.
If you don’t have a Jupiter-sized core out there sucking up all the gas, why would gas planets need to end up as giants? They naturally could do that—that happened with the star, after all, but it doesn’t seem inevitable to me, and it might not even be common.
In that case, the earth-mass planets would be gas planets after all. If you think this is a stretch, keep in mind that these are specifically in systems noted to be low metallicity. Suggesting that they might not be high in metals after all is not much of a stretch.
Actually, Kepler is able to determine both size and mass of planet candidates, using the method of transit photometry.
For further info, I found a non-paywalled copy of Bucchave et al’s Nature paper. Figure 3 plots planet radius against star metallicity, and some of the planets are clearly of Earth-radius or smaller. I very much doubt that it is possible to form gas “giants” of Earth size, and in any case they would have a mass much lower than Earth mass, so would stand out immediately.
I forgot about photometry.