Only 25 years ago we did not know of any planets outside the Solar System. Today, though, we have a list of more than 3,500 exoplanets around other stars. There are a number of techniques for discovering them, and one of the most used is the radial velocity technique. This involves measuring the changes in the position and velocity of a star when the star and a planet in orbit around it rotate around their common centre of gravity. Depending on the relative masses of the two objects gravity will determine the magnitude of the change in velocity of the star, which can be measured using its observed spectrum.
Thanks to this technique, a group of astronomers headed by Alejandro Suárez Mascareño, a PhD student of the Instituto de Astrofísica de Canarias (IAC) and the University of La Laguna (ULL), and his thesis directors Rafael Rebolo, Director of the IAC and Scientific Director of the Severo Ochoa Program of this institute, and Jonay Isaí González Hernández, IAC Ramón y Cajal researcher, have discovered two ‘super-Earth’ type planets, GJ 625 b and GJ 536 b, whose masses are around 3 and 5 Earth masses, respectively, in orbit around nearby red dwarf stars.
The observations that have led to these discoveries have been obtained with the HARPS (High Accuracy Radial velocity Planet Seeker) spectrograph on the 3.6m ESO Telescope at La Silla (Chile) and HARPS North, on the Telescopio Nazionale Galileo (TNG) at the Roque de los Muchachos Observatory, Garafia (La Palma).
GJ 625 b
At just 21 light-years away from us and with a mass almost three times that of the Earth, this is one of the least massive Super-earth-like planets known to date, and it is located at the limit of the habitable zone of the red dwarf star GJ 625.
“Since GJ625 is a relatively cool star”, explained Alejandro Suárez Mascareño, “the planet would be at the border of the habitable zone, where liquid water could exist. Also, depending on the composition of its atmosphere and its rotation, it could be potentially habitable”.
Despite red dwarfs are the most common stars in the Universe, only a few hundred of earth-like planets are known orbiting around them to date. The majority of exoplanets have been discovered orbiting much more distant stars, using the transit method, in which the planet causes a minor ‘eclipse’ when it passes in front of the star. In contrast, only a few tens of rocky planets have been discovered around nearby stars with the radial velocity technique, and very few have been found in their habitability zones.
GJ 536 b
This exoplanet –the planet orbiting the star GJ 536- is not within the star’s habitable zone, but its short orbital period of 8.7 days and the luminosity of its star, a red dwarf which is quite cool and near to our Sun, make it an attractive candidate for investigating its atmospheric composition.
“This rocky exoplanet is observable from both the northern and southern hemispheres”, comments Jonay Isaí González, “so that it is very interesting for future high stability spectrographs and in particular for the possible detection of another rocky planet in the habitability zone of the star”.
“To detect the planet”, states Rafael Rebolo “we had to measure the velocity of the star with an accuracy of the order of a metre per second. With the construction of the new instrument ESPRESSO, co-directed by the IAC, we will improve this accuracy by a factor of ten, and will be able to extend our search to planets with conditions very similar to Earth, around this and many other nearby stars”.
Front and in-text image "Artistic design of the super-Earth GJ 625 b and its star, GJ625 (Gliese 625)", by Gabriel Pérez, SMM (IAC).