In a logarithmic sense, the largest gap in mass among the planets in our solar system lies between the Earth (which has, unsurprisingly, one Earth mass) and Uranus, which is 14.536 times more massive than Earth. One of the most interesting facets of the ongoing detection of extrasolar planets is that we’re now getting real information on planets that fall into this previously unobserved planetary regime.
Indeed, the most startling exoplanet-related revelation of the past few years has been the announcement by the Geneva Planet Search Team that planets in the Earth-Uranus gap are extraordinarily common. Their take-away message has consistently been that 30%-50% of the quiet solar-type stars in the Sun’s neighborhood harbor at least planet with Msin(i)<17 Earth masses, and an orbital period of less than 50 days. Tens of billions of worlds! The Milky Way Galaxy is essentially a Costco full of HD 40307 b’s, c’s, and d’s.
With super-Earths and sub-Neptunes out there in such quantities, it’s not surprising that the Kepler mission has returned a large number of candidates. Rather alarmingly, however, it appears at first glance that Kepler may be seeing significantly fewer planets than the Geneva Team’s predictions might imply. Given a 40% overall occurrence rate of planets with P<50d and mass between Earth and Neptune, and assuming one planet per star, there should be ~60,000 potentially detectable planets orbiting the 150,000 target stars in Kepler’s field of view. For planets in orbits of 50 days or less, the geometric probabilities of transit lie in the 1-15% range. Taking a 5% transit probability (a 10-day orbit) as a benchmark, one ball parks that the number of sub-Neptune-mass planets that Kepler would have been able to detect is ~3,000. If we use a simple mass-radius scaling law, we find that a bit less than 1,000 of Kepler’s planet candidates fall in the sub-Neptune mass range. Naively, it thus seems that the Geneva team’s occurrence rate appears to overestimate the number of planets that Kepler would have detected by a factor of around three.
So what’s up? It seems a-priori highly unlikely that either Kepler or the HARPS analysis pipeline have made a significant error. In collaboration with UCSC Grad student Angie Wolfgang, we’ve made a very detailed attempt to compare the two observational programs, with the goal of seeing whether there’s a sensible way to bring the two surveys into agreement. This task is tricky because Kepler employs transit photometry, where as the Geneva Team’s results are based on radial velocity measurements from HARPS.
To see the details of our work, have a look at the paper that we’ve recently posted to arXiv. The bottom line is that concordance can be obtained, provided that there exist two very different planetary populations in the sub-Neptune mass regime. One population, which is numerically dominant, consists of dense scaled-up terrestrial planets, super-Mercuries if you will. The other population (to which Kepler is selectively sensitive) consists of planets with much lower densities, akin to scaled-down versions of Uranus and Neptune.