Superhydrophobic surfaces reduce drag

Rare and common The hot grains of tektite and obsidian that show up in HD 172555’s IR spectrum are small enough that the star’s radiation pressure would drive them away from their current 6-AU orbit within 0.1 million years. The SiO molecules would likely condense and reform minerals on the same time scale. Given that rocky planets take 100 My to form, catching a giant impact in HD172555, despite its being an ideal setting, seems fortunate, though not wildly improbable. The odds of catching other giant impacts would be higher if one could detect not just the ground-up, kicked-out debris but also the hot glowing surface of the impacted planet. As Michael Meyer of ETH Zürich and his collaborators pointed out in a recent paper, a single stellar system could experience several impacts as its rocky planets form.2 If a system has two Earth-sized planets, each might have suffered two collisions with Mars-sized objects to reach its final mass. The impacted surfaces could glow for 2 My, so the chance of spotting one glowing surface during the 100 My of rocky planet formation would be roughly 1 in 10. Not bad. Surprisingly, thanks to its bigger combined area, the ground-up dust from a Pluto-sized impactor is far brighter than the hot glowing surface of an Earth-sized impactee. The next generation of giant ground-based telescopes will be needed to spot those surfaces. Still, the evidence of one impact outside our solar system suggests that other Earth-like planets do lie within a detectable range. Charles Day References