A system with seven earths of terrestrial size

A team of astronomers has revealed the first system known to harbor seven Earth-sized planets around an individual star (called TRAPPIST-1b, c, d, e, f, g and h). Three of these planets lies within the habitable zone, the area around the star where a rocky planet could hold liquid water on its surface, a key to life as we know it.

With only 8% of the mass of the Sun, the star TRAPPIST-1 is very small in stellar terms (only 162,793 km in diameter, compared to 139,822 on the planet Jupiter) and although it is relatively close to us (40 light-years), Is very dim. Astronomers had hoped that this type of dwarf stars could house many Earth-sized planets in tight orbits, making them promising targets for the search for extraterrestrial life, but TRAPPIST-1 is the first such system discovered.

"The emission of energy from dwarf stars like TRAPPIST-1 is much weaker than that of our Sun. So that there would be water on their surfaces the planets would have to be in orbits much closer than we can see in the Solar System. Fortunately, it seems that this type of compact configuration is what we are seeing around TRAPPIST-1! "Says Amaury Triaud, co-author of the study.

However, a planet containing liquid water on its surface does not depend only on the distance to which it is from its star, but also influences factors such as the atmospheric composition of the planet and the eccentricity of its orbit among others. Because of this, in reality the seven planets of the system could possess liquid water depending on the conditions of each one, but it is much more probable to find it in the three planets of the habitable zone.

The orbits of the TRAPPIST-1 planets are much smaller than Mercury's orbit in the Solar System. The planets are so close to one another that if a person stands on the surface of one of them, it is possible that he saw geological features or clouds on the nearest planets, which could sometimes look larger than the Moon in the sky from the earth. However, despite the astonishing closeness between the planets, the small size of the star and its low temperature mean that the energy it provides to its worlds is similar to that received by the inner planets of our Solar System; TRAPPIST-1c, dyf receives similar amounts of energy as Venus, Earth and Mars, respectively.

Astronomers have confirmed the existence of the seven planets of the system using the TRAPPIST-South telescope, installed at La Silla Observatory, the Very Large Telescope (VLT) in Paranal, and the NASA Spitzer Space Telescope, as well as other telescopes around the world.

Planets of TRAPPIST-1

Artistic illustration of how the TRAPPIST-1 planets could be viewed, based on the data available so far. Credit: NASA / R. Hurt / T. Pyle.

Based on their densities, it is estimated that all TRAPPIST-1 planets are rocky. Further observations of the system will not only help determine if they are rich in water, but also possible to reveal if any could have liquid water on their surface. The mass of TRAPPIST-1h, the most distant planet, has yet to be estimated, but scientists believe it could be similar to a frozen world, which could be determined with more observations.

"The seven wonders of TRAPPIST-1 are the first Earth-sized planets have been found orbiting this kind of star," said Michael Gillon, lead author of the study and lead investigator of the exoplanet TRPPIST survey at the University of Liege in Belgium. "It is also the best goal yet to study the potentially habitable earth-sized worlds."