An international team of astronomers led by Dr. Michaël Gillon from the Institute of Astrophysics and Geophysics of the University of Liege has discovered a planetary system around TRAPPIST-1. It is only 40 light years away and has at least seven earth-sized planets. This discovery was supported by an Andor CCD-camera iKon-L 936 which is used at the "TRAnsiting Planets and PlanetesImals Small Telescope (TRAPPIST)"-North in Morocco, a high-sensitivity Back Illuminated Deep Depletion CCD sensor. TRAPPIST-1 is a small and dim red dwarf, having only about 8% of our Sun's mass and it is only slightly larger than Jupiter, the largest planet in our solar system.
Fig. 1: Size comparison of the TRAPPIST-1 planets with bodies from the solar system. All planets found around TRAPPIST-1 are of similar size as Earth. Source: ESO/O. Furtak.
In 2016, astronomers already reported the discovery of three terrestrial planets in the TRAPPIST-1 system using the TRAPPIST-South telescope on the premises of the European Southern Observatory (ESO) in La Silla, Chile. Follow-up observations with different telescopes, such as the NASA Spitzer space telescope, ESO's Very large telescope (VLT), the Belgian TRAPPIST-North and -South telescopes and observatories on Hawaii, La Palma and in South Africa, then revealed that the TRAPPIST-1 system actually hosts seven small planets, labelled TRAPPIST-1b, -c, -d, -e, -f, -g, -h. Three of them orbit their host star at a distance which suggests the possibility of liquid water on the surface. This means the three exoplanets TRAPPIST-1e, f and g are potential candidates for extraterrestrial life. At the time of the discovery, TRAPPIST-1 is the exoplanetary system with the largest number of rocky, and potentially habitable planets.
Because the TRAPPIST-1 planets orbit their host directly in our line of sight (as viewed from Earth), astronomers can see that the planets pass in front of their host star in periodic intervals and thus, dim its observed brightness (transit method). By using this data, it is possible to calculate the orbits, diameters, and estimate masses of the planets (transit and transit timing variation method).
Fig. 2: Comparison of the TRAPPIST-1 system, with its planets orbiting very close to the host star, with the inner solar system and the Galilean moons of Jupiter. For example, the outermost planet TRAPPIST-1h has an orbital period of only approx. 20 days. All planets around TRAPPIST-1 are subject to similar radiation levels as Venus, Earth and Mars.
All seven orbits are very close to their host star. The innermost planet TRAPPIST-1b only needs 1.51 days for one full revolution and even the outermost planet TRAPPIST-1h orbits the host star in approx. 20 days. Their orbital radii lie between 0.011 and 0.06 astronomical units. One astronomical unit (1 AU) is the medium distance between Earth and Sun (approx. 149.6 million km). The seven TRAPPIST-1 planets’ masses range from 0.62 to 1.38 Earth masses and their diameters vary between 0.76 and 1.13 Earth radii.
In addition to the TRAPPIST observatories, the astronomers of the University of Liège are also involved in the "Search for habitable planets eclipsing ultra-cool stars (SPECULOOS)" project. The aim of SPECULOOS is to detect terrestrial planets around cool stars in our solar neighborhood, supported by five iKon-L CCD cameras (1x SPECULOOS Northern Observatory, 4x SPECULOOS Southern Observatory).
Fig. 3 (left): Light curves of the seven TRAPPIST-1 planets during their transits. The graph shows how the light of the very small, ultra-cool red dwarf TRAPPIST-1 gets weaker while each of the seven known planets passes in front of the star and blocks some of the light. The larger planets cause larger dips in the light output, the planets that are further away have longer transit times, as they revolve at a slower pace. All this data comes from observations with the NASA Spitzer telescope. Fig. 4 (right): VLT observations of the light curve of TRAPPIST-1 during the triple transit of 11 December 2015. The star was observed with the HAWK-I instrument on the VLT of ESO, as three planets passed in front of the star and block the observed light. This light curve shows for the first time three temperate Earth-sized planets and temperate Earth-sized planes, with two of them being in the habitable zone, transiting their host star. Source: ESO/M. Gillon et al.
Find further information on the TRAPPIST-1 system and the search of the astronomers for habitable exoplanets:
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