TRAPPIST 1-b and 1c are the two innermost planets and are likely to have rocky cores and be surrounded by atmospheres much thicker than Earth's.

TRAPPIST-1d is the lightest of the planets at about 30 percent the mass of Earth. We're uncertain whether it has a large atmosphere, an ocean or an ice layer.

TRAPPIST-1e is a bit of a surprise. It's the only planet in the system slightly denser than Earth. It may have a denser iron core, and it does not necessarily have a thick atmosphere, ocean or ice layer. TRAPPIST-1e is a mystery because it appears to be so much rockier than the rest of the planets. It's the most similar to Earth, in size, density and the amount of radiation it receives from its star.

TRAPPIST-1f, g and h might have frozen surfaces. If they have thin atmospheres, they would be unlikely to contain the heavy molecules that we find on Earth, such as carbon dioxide.