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Orbiting less than two stellar radii above the visible surface of a Sun-like star, the Earth-sized exoplanet Kepler-78b is a hellish world. But its existence bodes well for the discovery and characterization of habitable planets. See Letters p.377 & p.381 A few exoplanets of about the size or mass of Earth have been discovered. Now, for the first time, both size and mass have been determined for one of them. Kepler-78b, first described in August this year, is close-in to its host star, which it orbits every 8.5 hours. Two groups have been able to exploit the closeness of planet and star to make Doppler spectroscopic measurements of the mass of Kepler-78b. The teams, led by Andrew Howard and Francesco Pepe, used different telescopes to arrive at mass estimates of 1.69 ± 0.41 and 1.86 +0.38/−0.245 Earth masses, respectively. They calculate the planet's mean density at 5.3 and 5.57 g cm−3, very similar to Earth's and consistent with an Earth-like composition of rock and iron.

OWING in the main part to the swift axial rotation of the planet Jupiter, it is usual to find the dark belts, which constitute the principal configuration of his visible surface, lying both parallel to one another and to the planet's equator. An instance of obliquity of one of the bands relatively to the others is rare, and a most definite and striking example of the kind was recorded in the northern hemisphere in 1860. A recent phenomenon akin to this was observed in the spring months of the present year. Although in this case the band was a faint one, yet the marked trend which it exhibited called: for special notice, and the more important facts relating to it might he briefly recorded here.

I was much interested in the account which your last number (vol. xviii. p. 197) contained of the presumably new lunar crater discovered by Dr. Klein in the Mare Vaporum. Is it really necessary to ascribe the formation of such a crater to present volcanic action? It seems to me that this singular phenomenon of the birth of a new crater may be more likely owing to such action having, in long-past ages, left (as in all probability it would leave) extensive caverns beneath the visible surface of our satellite. Such caverns might, in consequence of the gradual changes which the action of the sun's rays, alternating with intense cold, must produce on the lunar rocks, occasionally give way. A crater-like cavity would then be caused on the moon's surface by this subsidence, such as are not unfrequently seen in mining districts where old workings have fallen in. The fact that the new crater is elliptical, and not round, seems to add to the probability of its having been caused by some such “settling” process. If the crater were produced by active volcanic agency, it would surely be circular, or nearly so. I do not find this mode of quasi-crater formation suggested in Nasmyth's book, nor, so far as I can remember, in any other. Is it not, however, a possible cause of change on the surface of our satellite?