Atmosphere Loss by Aerial Bursts

We present a simple analytic description of atmospheric mass loss by aerial bursts and demonstrate that mass loss from aerial bursts becomes significant when the maximum impactor size that leads to an aerial burst rather than a ground explosion, $r_o$, is larger than the minimum impactor size needed to achieve atmospheric loss, $r_{min}$. For vertical trajectories, which give the most stringent limit, this condition is approximately satisfied when $\rho_o/\rho_i \gtrsim 0.4 v_e/v_\infty$, which implies atmospheric densities need to be comparable to impactor densities for impactor velocities that are a few times the escape velocity of the planet. The range of impactor radii resulting in aerial burst-induced mass loss, $r_o-r_{min}$, increases with the ratio of the atmosphere to the impactor density and with the trajectory angle of the impactor. The range of impactor radii that result in aerial burst-induced mass loss and the atmospheric mass lost is larger in adiabatic atmospheres than isothermal atmospheres of equivalent total mass, scale height, and atmospheric surface density. Our results imply that aerial bursts are not expected to significantly contribute to the atmospheric mass-loss history of Earth, but are expected to play an important role for planets and exoplanets similar to Neptune with significant atmospheres. For Neptune-like atmospheres, the atmospheric mass ejected per impactor mass by aerial bursts is comparable to that lost by ground explosions, which implies that, for impactors following a Dohnanyi size distribution, overall loss by aerial busts is expected to exceed that by ground explosions by a factor of $(r_{ground}/r_{aerial})^{0.5}$.Comment: 10 pages, 9 figure

A Chondritic Solar Neighborhood

A persistent question in exoplanet demographics is whether exoplanetary systems form from similar compositional building blocks to our own. Polluted white dwarf stars offer a unique way to address this question as they provide measurements of the bulk compositions of exoplanetary material. We present a statistical analysis of the rocks polluting oxygen-bearing white dwarfs and compare their compositions to rocks in the Solar System. We find that the majority of the extrasolar rocks are consistent with the composition of typical chondrites. Measurement uncertainties prevent distinguishing between chondrites and bulk Earth, but do permit detecting the differences between chondritic compositions and basaltic or continental crust. We find no evidence of crust amongst the polluted white dwarfs. We show that the chondritic nature of extrasolar rocks is also supported by the compositions of local stars. While galactic chemical evolution results in variations in the relative abundances of rock-forming elements spatially and temporally on galaxy-wide scales, the current sample of polluted white dwarfs are sufficiently young and close to Earth that they are not affected by this process. We conclude that exotic compositions are not required to explain the majority of observed rock types around polluted white dwarfs, and that variations between exoplanetary compositions in the stellar neighborhood are generally not due to significant differences in the initial composition of protoplanetary disks. Nonetheless, there is evidence from stellar observations that planets formed in the first several billion years in the Galaxy have lower metal core fractions compared with Earth on average.Comment: Accepted to PS

New chondritic bodies identified in eight oxygen-bearing white dwarfs

We present observations and analyses of eight white dwarf stars that have accreted rocky material from their surrounding planetary systems. The spectra of these helium-atmosphere white dwarfs contain detectable optical lines of all four major rock-forming elements (O, Mg, Si, Fe). This work increases the sample of oxygen-bearing white dwarfs with parent body composition analyses by roughly thirty-three percent. To first order, the parent bodies that have been accreted by the eight white dwarfs are similar to those of chondritic meteorites in relative elemental abundances and oxidation states. Seventy-five percent of the white dwarfs in this study have observed oxygen excesses implying volatiles in the parent bodies with abundances similar to those of chondritic meteorites. Three white dwarfs have oxidation states that imply more reduced material than found in CI chondrites, indicating the possible detection of Mercury-like parent bodies, but are less constrained. These results contribute to the recurring conclusion that extrasolar rocky bodies closely resemble those in our solar system, and do not, as a whole, yield unusual or unique compositions.Comment: Accepted for publication in ApJ. 7 Figures, 7 Table

A Chondritic Solar Neighborhood

A persistent question in exoplanet demographics is whether exoplanetary systems form from similar compositional building blocks to our own. Polluted white dwarf stars offer a unique way to address this question, as they provide measurements of the bulk compositions of exoplanetary material. We present a statistical analysis of the rocks polluting oxygen-bearing white dwarfs and compare their compositions to rocks in the solar system. We find that the majority of the extrasolar rocks are consistent with the composition of typical chondrites. Measurement uncertainties prevent distinguishing between chondrites and bulk Earth but do permit detecting the differences between chondritic compositions and basaltic or continental crust. We find no evidence of crust among the polluted white dwarfs. We show that the chondritic nature of extrasolar rocks is also supported by the compositions of local stars. While galactic chemical evolution results in variations in the relative abundances of rock-forming elements spatially and temporally on galaxy-wide scales, the current sample of polluted white dwarfs are sufficiently young and close to Earth that they are not affected by this process. We conclude that exotic compositions are not required to explain the majority of observed rock types around polluted white dwarfs and that variations between exoplanetary compositions in the stellar neighborhood are generally not due to significant differences in the initial composition of protoplanetary disks. Nonetheless, there is evidence from stellar observations that planets formed in the first several billion years in the Galaxy have lower metal core fractions compared with Earth on average