Astronomers have begun scrutinising a new class of planet that might support alien life: the hycean. This is a portmanteau coinage combining “hydrogen” and “ocean”, since the planets are hot waterworlds with hydrogen-rich atmospheres. In ancient Greek, Oceanus was the great river encompassing the disc of the Earth, personified as the son of Uranus and Gaia. Hydrogen, meanwhile, is Greek for “water-generating”, as H indeed is when combined with O2. That reaction also occurs in hydrogen fuel cells that produce electricity, but where do we get the hydrogen from in the first place? Mainly, as it turns out, from burning fossil fuels. One type of this CO2-emitting process is called, in a truly marvellous act of Unspeak, “blue hydrogen”, even though hydrogen is colourless – presumably in the hope that the association with the colour of clear skies will act as rhetorical greenwashing. (“Blue hydrogen” captures and stores some but not all of the CO2 created, while your actual “green hydrogen” is made with renewable energy.) But let’s not be too hard on human ingenuity: with enough sea-level rise and global heating, we could even turn Earth itself into a hycean for aliens to study.