There are, Loeb admits, a couple of catches. First, the Webb can only pull this off if a planet in question orbits a white dwarf star—the tiny, white-hot ember left behind when a star like the Sun dies. That’s because the CFCs’ signature would appear as a distortion of starlight passing through the planet’s atmosphere. If the star is much bigger than the planet, most of the light hitting the telescope would bypass the planet entirely, and the distortion would be lost in the glare.

Another issue is that high concentrations of CFC’s might signal a civilization that has already managed to destroy itself. Some of these gases attack and destroy a planet’s protective ozone layer, which is why, here on Earth, the worst offenders were banned in 1987.

But it’s also possible that an alien civilization would have pumped its atmosphere full of CFC’s on purpose, says Loeb. “They act as greenhouse gases,” he says, “so if a planet were at the colder edge of its habitable zone, you could imagine using CFC’s to warm things up.” As for the ozone problem, he says, “they could in principle produce extra ozone as well, or design CFC’s that were less harmful.”