UPDATE: I have a story on a slightly different aspect of the Kepler results — asteroseismology — up on the main Nature News site.
So the Kepler mission announced its first exoplanetary discoveries today: four huge planets bigger than Jupiter, and one about the size of Neptune, all hotly hugging their parent stars in tight orbits of a few days. There were a few neat tidbits that Kepler PI Bill Borucki offered up about the finds in talks today. One of the hot Jupiters has a density as fluffy as styrofoam. Some of them are hotter than molten lead. Looking at them is like looking into a “blast furnace”, says Borucki. Overall, the discoveries serve notice: Kepler is in action. The finds themselves? Not so special really. Of the 415 known exoplanets, the vast majority are hot Jupiters.
One interesting angle in the discoveries, however, is the gap between the four Jupiters and the one Neptune. Neptune is just 17 times the mass of Earth; Jupiter is more than 300 times the Earth’s mass. Why didn’t Kepler find anything in between? Dimitar Sasselov, a Kepler co-investigator from Harvard, says it is because the gap is real. There is a cutoff mass, bigger than Neptune, where a runaway accumulation process occurs during planetary formation. Anything bigger than, say, 30 Earth masses, will eat up any gas it can find in a solar disk and become a big hulking Jupiter. Smaller than the cut off, and it will stay Neptune-like. UC Santa Cruz’s Greg Laughlin was pointing this out a year and a half ago on his terrific blog — that this cutoff mass was expected. But Kepler — one of the first instruments to be capable of reaching low mass planets without selection effects — is starting to show that the “Neptune Gap” is real.