ULg astronomers discovered seven telluric planets around the star TRAPPIST-1. “The TRAPPIST-1 system is the largest treasure of terrestrial planets ever detected around a single star.” This discovery, published this week in the journal Nature, revives the quest for life in the Universe.
This artist’s impression shows the view from the surface of one of the planets in the TRAPPIST-1 system. At least seven planets orbit this ultra cool dwarf star 40 light-years from Earth and they are all roughly the same size as the Earth. They are at the right distances from their star for liquid water to exist on the surfaces of several of them. ©ESO
Discovered in May 2016 by an international team led by par Michaël Gillon, an astronomer at the University of Liege, the exoplanetary system TRAPPIST-1 has revealed more secrets.
The intensive surveillance of the system using several telescopes, including NASA’s Spitzer space telescope, has not only reinforced the results received previously by the TRAPPIST-Sud telescope in Liege, but has also revealed the presence of four planets in the system bringing their number up to seven. According to the results published in the journal Nature (1), these seven planets are similar in size to Earth and could have liquid water on their surface, particularly in the case of three of them which orbit in the so-called “habitable” zone of their star. TRAPPIST-1 is the system with the biggest number of rocky planets and the greatest number of potentially habitable worlds ever discovered up to the present day. It has reignited the quest for extra-terrestrial life in the Universe.
This discovery was made in the context of the SPECULOOS project, a new more ambitious project for the detection of potentially inhabitable exoplanets. Directed by Michael Gillon, this project, supported by a Starting grant from the European Research Council (ERC), is in the final preparation phase on the site of the European Southern Observatory at Paranal (ESO) in Chile.
A fascinating exoplanetary system
The story began at the end of 2015 when Michael Gillon and the members of his team, including Emmanuël Jehin, an astronomer at ULg, decoded the data acquired by the Liege telescope TRAPPIST-Sud, located in Chile. They had just observed a new exoplanetary system (2). Given the name TRAPPIST-1, and detected by means of the transiting method, the system already presented some very interesting characteristics, notably, the presence of three planets with a size and temperature similar to Earth and suitable for detailed study of their atmospheric conditions using the current technology available. The new observations made in 2016 by the team only served to increase this interest by gradually revealing other planets. Their exact number and their orbital characteristics remained unclear until autumn 2016, when the team used NASA’s Spitzer space telescope to continuously observe the system during a three-week period. These space observations revealed that there were in fact seven planets orbiting the star. These planets, called TRAPPIST-1b, c, d, e, f, g and h in order of increasing distance from their star, are all of a similar size to Earth. “The TRAPPIST-1 system is the greatest treasure trove of Earth-sized planets ever detected around a star”, says Michaël Gillon. “It is a fascinating planetary system, not only for the number of planets it has, but also because these planets present characteristics which are very similar to those on Earth”!
According to the results published in the journal Nature, three of the planets are in the habitable zone of the system (3). By “habitable zone” we mean the range of distances from a star at which a rocky planet like Earth can harbor large quantities of liquid water on most of its surface. And the existence of water means the existence of life, at least this is the case on Earth! “By way of comparison”, continues Michael Gillon, “our solar system contains two Earth-sized planets only one of which is in the habitable zone. With its seven Earth-sized planets, three of which are in its habitable zone, TRAPPIST-1 seems to be an incredibly rich planetary system. Studying it will be even more fascinating”!
The TRAPPIST-1 star is an ultracool dwarf, that is to say, a much smaller and colder star then the Sun. This explains why, even though they are much closer to their star than the Earth is to the Sun, the planets of the TRAPPIST-1 system could contain liquid water on at least part of their surface. In this respect, the three planets e, f and g, which orbit in the habitable zone, are the most promising. “While the other planets of the system can only contain water on a small part of their surface, these three planets e, f, and g could have oceans similar to those on Earth”!, explains Julien de Wit, a graduate of ULg who is currently doing a PhD at MIT
Next step : determinate the atmospheres’ composition
Thanks to their current data, the researchers have been able to very precisely measure the radius of the planets, but also to obtain a first estimate of the mass of six of them. “We are continuing to intensely observe the system from the ground and from space”, explains Emmanuël Jehin, an astronomer at the University of Liege and co-head of the TRAPPIST project, “And soon we should have sufficiently precise masses to be able to establish the composition of the planets. In addition, we hope to determine the existence and extent of their atmospheres very soon”. These observations will be possible by means of the Hubble space telescope, but we will have to wait for the launch of the James Webb Space Telescope, expected in 2018 by NASA and the ESA, to carry the research further in the hope of determining the composition of these atmospheres or even to detect chemical signs of biological activity on the surface of these planets. “We will be able to compare the planets to each other”, continues Michaël Gillon, “and conduct what is called comparative planetology. In the next five to ten years, we should know a lot more about these planets: what they are composed of, how they were formed, what their surface conditions are, etc. And who knows? Perhaps we will have detected signs of life on one or several of them? If so, we will never look at the stars in the same way again…”
This discovery marks an important turning point in space exploration and makes TRAPPIST-1 an important target in the search for extra-terrestrial life in the Universe, these worlds represent our best chance of detecting life elsewhere at the current time.