The fourth trait is the presence of liquids, ideally water, on the planet's surface or below it. Temperature and atmospheric pressure play important roles in retaining surface water.
Some information on whether there are liquids on a planet can be gleaned indirectly. The team points out that the presence of lakes filled with liquid hydrocarbons on Saturn's frosty moon Titan was first proposed in 1983. NASA's Cassini mission confirmed their existence 22 years later. The potential for hosting lakes was inferred from Titan's temperature, estimates of atmospheric pressure, and chemistry.
The research team devising the rating system suggests that current or proposed space-based observatories – NASA's James Webb Space Telescope and the European Space Agency's Gaia mission – should be able to pin down conditions affecting each of these four key categories for some exo-planets, especially those closest to Earth. Answers for others may remain well out of reach.
