WHEN young exoplanets crash into each other, heavy, salty skies could give them away. According to theory, rocky planets like Earth grow when smaller bodies smash together and merge. But most traces of such events in our solar system have long since faded. Roxana Lupu at the SETI Institute in Mountain View, California, and her team think we could test the idea if we can find exoplanets that have just collided (Astrophysical Journal, doi.org/rw2). If the planets resemble Earth, post-impact they should have thick, steamy atmospheres full of hydrogen chloride gas and vaporised minerals such as salt. Next-generation telescopes should be able to spot them.
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