About a year ago, the discovery of 3 Earth-sized planets orbiting the very cold star TRAPPIST-1 stirred excitement in the astronomical community. At a distance of forty light-years, this star is among our nearest celestial neighbours. These planets have been uncovered by the so-called transit method. By monitoring very accurately the flux from a star, one can detect the minute flux decrease due to the passage of planets in front of the star. The transit technique requires a very large number of observations, and observations of a large number of stars. Only a small percentage of systems have the right orientation such that planets are seen to transit in front of their stars, and this happens only once per orbital period. An international team of astronomers, collaborating within the “TRAPPIST” consortium, observed a large number of cool stars in the solar vicinity with a dedicated telescope. Thanks to these observations, they were able to identify the sought-after signature of transits around the star named “TRAPPIST-1”, a minute red dwarf weighing only 8% of our Sun’s mass. Follow-up measurements confirmed the presence of a least 3 planets around this star with radiuses comparable to that of the Earth. This spectacular discovery led to a publication in Nature in early 2016.
This discovery prompted further observations, and the TRAPPIST team obtained observing time on the Spitzer Space Telescope, observing continuously the TRAPPIST-1 star for 20 days. While these measurements improved upon existing measurements of the known planets, they also allowed for the search of further transiting worlds. The results were beyond all expectations: 34 individual transits were detected, arising from no fewer than 7 planets! All planets orbiting TRAPPIST-1 have radii comparable to that of the Earth, from slightly smaller than Venus (75% of Earth) to slightly larger than Earth (110% of Earth). Considering the brightness of the host star and the orbital separation of TRAPPIST-1 planets, atmosphere models suggest that 3 or 4 planets are potentially located within the “habitable zone” where liquid water could exist on their surfaces. The properties of the atmosphere of these planets are unknown, and whether they are habitable, let alone inhabited, remains a matter of speculation. While the discovery of a single habitable world among our nearest stellar neighbours is significant news in itself, the discovery of seven earths around a single star is unprecedented and will enter astronomy textbooks.
The 7 earth-sized planets around TRAPPIST-1 allow for detailed study of the planets properties. As they orbit tightly around their star, their mutual gravitational interactions lead to perturbations of the planetary orbits that allows one to determine their masses. These masses combined with the radii derived from the transit measurements constrains their densities and bulk compositions. Most planets have a nominal density comparable to that of the Earth, and we can rule-out the possibility that these are largely gaseous worlds. Future observations of the planetary transits with specialized instruments will probe the outer layers of their atmospheres. An instrument that will fly aboard the James Webb Space Telescope, NIRISS, was conceived especially to do this kind of analysis. This Canadian Space Agency instrument, for which René Doyon, the director of iREx, is the principal investigator, will be able to study these planets as early as 2019. The discovery of 7 similarly-sized planets around the same star is perfect for comparative studies, and the search for eventual traces of life in the atmosphere of these planets will be a priority.
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