A trio of Earths 40 light years away?

In the end, fortune favored the brave and the TRAPPIST telescope found its Holy Grail even before observation of all fifty candidates was finished. A first exceptional discovery which demonstrated the potential of the project. At the beginning of 2017, four bigger and more powerful telescopes which will be operational from the famous European observatory of Cerro Paranal, in the Acatama desert in Chile. One of the best astronomy sites in the world says a jubilant Emmanuël Jehin. A twin telescope of TRAPPIST financed by the University of Liege will also very soon be installed in Morocco giving access to the brightest ultracool red dwarf stars in the Northern Hemisphere. “In the coming years, we are going to probe 20 times more ultracool stars than those observed to date. We can expect to find a lot more planetary systems, unless we were incredibly lucky with this one, which is hard to believe. This discovery is the result of a first five years of work, and that seems to indicate that small planets are very frequent around this type of star!” Say the two delighted researchers.

Planets similar in size to Earth

TRAPPIST-1 is a star that is barely bigger than Jupiter (1.2 times its radius). Its light emission was only detected for the first time in 2000, during the course of a systematic observation programme of the heavens in the infrared. Its mass is 80 times greater than that of the gas giant but it remains 12 times dimmer than the Sun. Its surface temperature of only 2550 Kelvin, is barely half the temperature of our star.

Around the TRAPPIST-1 star, three planets comparable in size to ours have been discovered. Two of them have a radius equivalent to 110% that of the Earth, the third 100%. What remains unknown is their mass. “It is a big challenge but we will succeed”, say the astrophysicists. “There are two possible methods. For the technique known as “radial velocity” which is the most common and involves observing the small movement of the star around the center of mass of the system due to the attraction of the planets that orbit it. But TRAPPIST-1 is so weakly luminous in the visible spectrum that a high-precision IR spectrograph would be required. A first instrument of this type is available in Spain, but it has not yet proved its worth. Our best chance lies rather in the presence of several planets in this system. These planets disturb one another in fact. Therefore, the gravitational interactions between them will influence their orbital periods according to their respective masses. By observing dozens of transits and the small variations in the moments when they are produced, we will be able to deduce the mass of each of these planets. We will then have a much better idea of their composition: rocky planets like the Earth, quite rich in metals like Mercury, or even rich in ice like the moons of Jupiter”.  

Similarities with the Jovian system

The size of the star and the distances separating the planets remain two of the biggest differences with our solar system. “The first planet orbits at 0.011 astronomical units (AU) from its star, the second at 0.15 and the third at probably between 0.02 and 0.06”, explains Michaël Gillon.  In less academic terms, this would correspond to orbital periods of a day and a half, two and a half days, and a third orbit of between 4 and 20 days. “We have only been able to observe two transits of the furthest planet which is not sufficient to determine its period”. The fact remains that this system is very different to ours. By way of comparison, Mercury, the planet closest to the Sun needs 88 days to complete an orbit while Earth, the only viable planet in our system, takes 365.25.

By way of contrast, there is a quite astonishing resemblance with the Jovian system. The satellites of the giant planet are certainly closer to each other, but they orbit with very similar periods. “This similarity is not surprising”, comments Michaël Gillon. “When they are born, these small stars are surrounded by protoplanetary discs that are much more compact than those of sun-like stars. The planets therefore form closer to their star. According to theoretical predictions, it is what we expected. What is most surprising is the fact that there are so many Earth-sized planets, even though we know that protoplanetary discs do not necessarily have a mass that is in proportion to their star. Everything depends on the conditions under which they were formed, the composition of the initial gas cloud and dust, the speed of rotation of the system, etc. Above all, we know that objects can form further away before migrating towards their star. If I had to bet, I would say that these objects formed beyond the ice line of this star, which is only at 0.06 astronomical units. Emmanuël Jehin continues, “At that distance, temperatures do not exceed 100°C. It is cold enough so that the volatile elements remain solid in the form of ice. They can then mix with rocks to form planets. There is therefore more material and we can more easily imagine how Earth-sized objects are formed, even within such small discs. Planets then only need to migrate closer to their star to create a system like TRAPPIST-1”.

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