Kepler-93b: A Terrestrial World Measured to within 120 km

http://arxiv.org/abs/1405.3659

Kepler-93b: A Terrestrial World Measured to within 120 km, and a Test Case for a New Spitzer Observing Mode

We present the characterization of the Kepler-93 exoplanetary system, based on three years of photometry gathered by the Kepler spacecraft. The duration and cadence of the Kepler observations, in tandem with the brightness of the star, enable unusually precise constraints on both the planet and its host. We conduct an asteroseismic analysis of the Kepler photometry and conclude that the star has an average density of 1.652+/-0.006 g/cm^3. Its mass of 0.911+/-0.033 M_Sun renders it one of the lowest-mass subjects of asteroseismic study. An analysis of the transit signature produced by the planet Kepler-93b, which appears with a period of 4.72673978+/-9.7x10^-7 days, returns a consistent but less precise measurement of the stellar density, 1.72+0.02-0.28 g/cm^3. The agreement of these two values lends credence to the planetary interpretation of the transit signal. The achromatic transit depth, as compared between Kepler and the Spitzer Space Telescope, supports the same conclusion. We observed seven transits of Kepler-93b with Spitzer, three of which we conducted in a new observing mode. The pointing strategy we employed to gather this subset of observations halved our uncertainty on the transit radius ratio R_p/R_star. We find, after folding together the stellar radius measurement of 0.919+/-0.011 R_Sun with the transit depth, a best-fit value for the planetary radius of 1.481+/-0.019 R_Earth. The uncertainty of 120 km on our measurement of the planet's size currently renders it one of the most precisely measured planetary radii outside of the Solar System. Together with the radius, the planetary mass of 3.8+/-1.5 M_Earth corresponds to a rocky density of 6.3+/-2.6 g/cm^3. After applying a prior on the plausible maximum densities of similarly-sized worlds between 1--1.5 R_Earth, we find that Kepler-93b possesses an average density within this group

Regarding that new observing mode, they better use it fast.

See Steinn Sigurðsson's post at Dynamics of Cats, and the links therein.

http://arxiv.org/abs/1412.8687

The Mass of Kepler-93b and The Composition of Terrestrial Planets

Kepler-93b is a 1.478 +/- 0.019 Earth radius planet with a 4.7 day period around a bright (V=10.2), astroseismically-characterized host star with a mass of 0.911+/-0.033 solar masses and a radius of 0.919+/-0.011 solar radii. Based on 86 radial velocity observations obtained with the HARPS-N spectrograph on the Telescopio Nazionale Galileo and 32 archival Keck/HIRES observations, we present a precise mass estimate of 4.02+/-0.68 Earth masses. The corresponding high density of 6.88+/-1.18 g/cc is consistent with a rocky composition of primarily iron and magnesium silicate. We compare Kepler-93b to other dense planets with well-constrained parameters and find that between 1-6 Earth masses, all dense planets including the Earth and Venus are well-described by the same fixed ratio of iron to magnesium silicate. There are as of yet no examples of such planets with masses > 6 Earth masses: All known planets in this mass regime have lower densities requiring significant fractions of volatiles or H/He gas. We also constrain the mass and period of the outer companion in the Kepler-93 system from the long-term radial velocity trend and archival adaptive optics images. As the sample of dense planets with well-constrained masses and radii continues to grow, we will be able to test whether the fixed compositional model found for the seven dense planets considered in this paper extends to the full population of 1-6 Earth mass planets.

Surprised that didn't get any follow-up here... the apparent uniformity of iron-to-silicate ratio for terrestrial planets is not something I would have necessarily expected. Would have thought that different systems would have different amounts of iron available that would produce more variations. Apparently not.

Though it is an interesting proposal, you must remember that the compositional model is approximate. Figure 4 shows that the errors are also not particularly good in their current state, which compounds the issue of giving all of these planets the same composition - it could very well be a coincidence.

In the near future, when that diagram has a lot more datapoints, a meaningful study of terrestrial planet composition will be possible. Perhaps there will be a correlation between the host star's Fe/SiO ratio and planetary density, perhaps not.

That was more or less my thoughts on the issue. It will be interesting to have more RV measurements for such planets.

One thing worth noting is that when systems like Kepler-11 and other 'fluffy' low-density planets whose masses have been determined with transit timing variations are added to that diagram, the correlation becomes a lot more blurred. It may be that systems likely to show TTVs are also likely to have experienced a formation history that leads to significant planetary H/He envelopes. In any event, I would like to see more RV-derived masses for low-mass planets, especially for planets which also have TTV-derived mass measurements.

Though the variation does seem to be in line with the variation in element abundances observed in solar photospheres. So perhaps there is a suggestion that for these planets, processes like collisions are not particularly important for setting planetary abundances.

I think you have forgotten about C/O ratio as it can influence possibility planet is rocky with density as low as 4.0 g/cm3 as well habitability