The image at left shows observations from the Hubble Space Telescope. These data were collected as the planet GJ 1214 b passed in front of (transited) its host star. When that happens, light emitted by the host star passes through the planet’s atmosphere, and the atmosphere can imprint a spectral signature on that light, telling us what it’s made of.
But for GJ 1214 b, as shown by the bottom at left, there were NO spectral signatures — the spectrum is completely flat. The most likely explanation is that the planet has clouds high in its atmosphere that block the star light from passing through the part of the atmosphere where spectral signatures would be imprinted.
To make such a flat spectrum, GJ 1214 b’s clouds have to be very high in its atmosphere. Kreidberg and colleagues estimate the cloud deck can’t be lower than about 1 millibar in pressure. On the Earth, cirrus clouds, some of the highest clouds, live at pressures of about 300 millibars or 10 km in altitude. Earth’s atmospheric pressure doesn’t drop to 1 millibar until an altitude of about 70 km, above a height where meteors typically burn up. So GJ 1214 b’s clouds are very unusual.