This graph presents the measured properties of the seven TRAPPIST-1 exoplanets (labeled bah), showing how they stack with each other, as well as with Earth and the other inner rocky worlds of our own solar system. The relative sizes of the planets are indicated by the circles. All known TRAPPIST-1 planets are larger than Mars, with five of them within 15% of Earth’s diameter.
The vertical axis shows the uncompressed densities of the planets. Density, calculated from the mass and volume of a planet, is the first important step in understanding its composition. Uncompressed density takes into account that the larger a planet, the more its own gravity will pack the planet’s material and increase its density. Therefore, uncompressed density usually provides a better means to compare the composition of planets.
The plot shows that the uncompressed densities of the TRAPPIST-1 planets are similar to each other, suggesting that they may all have a similar composition. The four rocky planets in our own solar system show more variations in density compared to the seven planets TRAPPIST-1. Mercury, for example, contains a much higher percentage of iron than the other three rocky planets and therefore has a much higher uncompressed density.
The horizontal axis shows the level of illumination that each planet receives from its host star. The star TRAPPIST-1 is only 9% of the mass of our Sun and its temperature is much cooler. But because the TRAPPIST-1 planets orbit so close to their star, they receive comparable levels of light and heat to Earth and its neighboring planets.
At the top of the plot are indicated the corresponding “living areas” (regions where an Earth-like planet could support liquid water on its surface) of the two planetary systems. The two zones do not align exactly because the cooler star TRAPPIST-1 emits more light in the form of infrared radiation that is absorbed more efficiently by an Earth-like atmosphere. Because less lighting is needed to reach the same temperatures, the living area is further away from the star.
The masses and densities of the planets TRAPPIST-1 were determined by measurements of slight variations in the times of their orbits by extensive observations made by NASA’s Spitzer and Kepler space telescopes, in combination with Hubble data and various ground-based telescopes. The latest analysis, which includes Spitzer’s complete record of more than 1,000 hours of TRAPPIST-1 observations, has reduced the uncertainties of mass measurements to just 3-6%. These are one of the most accurate measurements of planetary masses outside our solar system.