An exceptional system of exoplanets

A planetary solar system composed of a star, TRAPPIST-1, and 7 Earth-sized exoplanets has been discovered by a team of international astrophysicists, including researchers from the Astrophysics Laboratory of Bordeaux.

  • 07/03/2017

Trappist-1 et les 7 exoplanètes © NASA/R. Hurt/T. Pyle Trappist-1 et les 7 exoplanètes © NASA/R. Hurt/T. Pyle

These findings, published in the international journal Nature at the end of February 2017, are considered by the European Southern Observatory (ESO) to be one of the ten most important discoveries since its creation. This is the first time a star with almost as many telluric planets as our solar system has been found. In addition, at least three of the planets have conditions (distance from star…) compatible with the presence of liquid water on their surfaces.

For the researchers at the Bordeaux Astrophysics Laboratory, (LAB, CNRS and the University of Bordeaux), Franck Selsis, Sean Raymond and Jérémy Leconte, the presence of these 7 planets (all around +/-15% the size of our planet) is not only astonishing but also very promising, especially with regard to scientific potential: as well as determining the orbits and masses of the planets, it will be possible in the near future to detect the potential presence of atmospheres.

A solar system 40 light-years away

At the beginning of 2016, an international team of researchers, lead by astrophysicists from the University of Liège, completed high-precision brightness (“photometric”) monitoring with ESO's TRAPPIST telescope (La Silla Observatory in Chile) and discovered three Earth-sized planets (TRAPPIST-1 b, c and d) orbiting around the star TRAPPIST-1.

A dwarf-star, TRAPPIST-1's very small size (only 12 times Earth's radius, roughly the size of Jupiter) and its low luminosity (0.05% of the Sun's) provide exceptional conditions for observing its planets. Telescopes are less blinded by the low luminosity and can observe the planets as they pass in front of the star during each orbit, causing a fall in brightness known as a transit.

The researchers then received observation time with NASA's Spitzer Space Telescope pointed towards Trappist-1. The star was systematically monitored to find out whether it contained any other planets. At 40 light-years distance, it’s impossible to actually “see” the planets. Therefore decryptions, data modelling and hypothesis tests were conducted and discussed. Jérémy Lecont, specialist in weather and tides, describes how the researchers initially thought the 3rd planet was a double system (two planets very close together). However, the hypothesis wasn’t compatible with the results found by the Bordeaux researchers. This information enabled the researchers to continue their investigations and to find out that the 3rd planet wasn’t one, or two, but three different planets.

7 resounding exoplanets

One of the interesting elements of this system is the resounding orbits of these planets, explains Sean Raymond, who works on the creation and orbital dynamics of planets. When the 6th planet g finishes a revolution around the TRAPPIST-1 star, planets b, c, d, e and f have respectfully made 8, 5, 3, 2 and 4/3 times their revolutions. A link between these planets is not random, especially in their creation. Their interactions depend on their mass, thus studying these interactions can indicate their mass.  

According to Franck Selsis, astrophysicist specializing in the observability of these exoplanets, these latest discoveries will facilitate further research in this domain. It will be possible to observe the planet Proxima b, located only 4,2 light-years from Earth  thanks to future ground-based telescopes and this TRAPPIST-1 seven planet system proposes many interesting targets for the upcoming James Webb infra-red telescope. It will enable comparative planetology around one star and the discovery of the presence of atmospheres. It’s only just the beginning!

More information

Seven temperate Earth-sized planets revolve around the star TRAPPIST-1. The discovery was made by an international team led by a Belgian scientist and including researchers from the CNRS, the French Alternative Energies and Atomic Energy Commission (CEA) and the Université Pierre-et-Marie-Curie (UPMC), the Laboratoire d'Astrophysique de Bordeaux (CNRS/University of Bordeaux), Laboratoire de Météorologie Dynamique (CNRS/UPMC/École polytechnique/ENS Paris) and Laboratoire Astrophysique, Instrumentation et Modélisation (CNRS/CEA/Université Paris Diderot).

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