TRAPPIST-1 is a planetary system, located 12 parsecs away from the Solar system, near the ecliptic, within the constellation of Aquarius. Around a star which is 12 times less massive than the Sun and only slightly larger than Jupiter, there are at least seven planets in orbit. The initial discovery was made by TRAPPIST, the Transiting Planets and PlanetesImals Small Telescope.
All the planets in the TRAPPIST-1 system transit their star, meaning that they pass in front of it. The planets were discovered from the regular and repeated shadows that are cast during transit. Thanks to the transit signals we could measure the orbital periods deep space exploration of the planets and could calculate the sizes of the planets. The exact time at which the planets transit also provide us with a means to measure their masses, which leads to knowing their densities and therefore their bulk properties. The planets are consistent with a rocky composition.
We found that the planets have sizes and masses comparable to the Earth and Venus. Because we know the distance of the planets to their star, and the temperature of the star, we can deduce that they receive an amount of jet propulsion laboratory light that is similar to many of the planets in the Solar system, from Mercury to beyond Mars.
During transit, some of the starlight goes through the atmosphere of the planets, getting transformed by the chemical composition of the atmosphere and by its vertical structure. This means that we can remotely study the climates of terrestrial worlds beyond our Solar system! The TRAPPIST-1 worlds are the most optimal currently at our disposal. They are providing humanity with it first opportunities at discovering evidence of biology beyond the Solar system.