An article published in “The Astronomical Journal” shows the confirmation of 15 exoplanets that orbit red dwarfs. A team of researchers led by Teruyuki Hirano from the Tokyo Institute of Technology used data collected by NASA’s Kepler space telescope and follow-up observations. Another article in the same journal focuses on 3 confirmed super-Earths including K2-155d, which could be in its system’s habitable zone.
In just over twenty years since the discovery of the first exoplanet, research has focused above all on Sun-like stars but in recent years it’s expanding. The small red dwarfs are the most numerous stars and have a very long life because they consume their hydrogen very slowly. However, they can hit hard with flares any planets close enough to receive enough energy to keep water liquid on their surface.
The chances of a planet orbiting a red dwarf maintaining a stable atmosphere are still a controversial subject because we know too little about it. From this point of view, the system of the tiny ultra-cool dwarf TRAPPIST-1 with its 7 rocky planets represents the frontier of research on those exoplanets.
NASA’s Kepler space telescope continues to be crucial in the search for exoplanets in its K2 mission and during the campaigns between 5 and 10 it observed 12 red dwarfs finding a number of potential exoplanets. Teruyuki Hirano’s team confirmed the existence of 15 of them thanks to follow-up observations with the Subaru telescope, the Nordic Optical Telescope (NOT) and others.
The most interesting situation was found in the K2-155 system, about 200 light years from Earth, where the researchers confirmed the presence of 3 super-Earths a little larger than the Earth. The outermost, K2-155d, with a year that lasts just over 40 Earth’s days and a diameter 1.6 times the Earth’s, is the most promising one concerning habitability potential but there are still many unknowns about its characteristics.
Because of these doubts it was possible to create only simulations based on hypotheses that could be completely wrong. The parameters used in the simulations are summarized in the left part of the image (courtesy Tokyo Institute of Technology), where the temperature on the surface of the planet K2-155d is a function of the amount of radiation coming from its star. When that amount exceeds 1.5 a runaway greenhouse effect is triggered.
The researchers simulated the possible conditions on the planet K2-155d if it were really a larger version of the Earth but with a composition and atmosphere similar to the Earth’s. Teruyuki Hirano is the first to be cautious stating that there’s no guarantee that this is the case.
This research on the planet K2-155d after the confirmation of its existence and in general the discovery of various exoplanets orbiting red dwarfs indicates that this type of research must be continued. Further research could also solve some mysteries such as the scarcity of exoplanets between 1.5 and 2 Earth masses compared to those a little smaller and a little larger.
In the right part of the image a histogram is shown that indicates the number of exoplanets divided by Earth masses. In pink-purple there are the ones that orbit red dwarfs with surface temperatures around 3,500-4,000 Kelvin, in shades of blue the ones that orbit around red dwarfs with surface temperatures of less than 3,500 Kelvin.
A help in the investigation of red dwarf systems will come with the launch of the new NASA planet hunter, the TESS (Transiting Exoplanet Survey Satellite) space telescope, scheduled in a few weeks. This new mission should constitute a step forward in this type of research, also improving the possibilities for follow-up observations.