An article published in the journal “Monthly Notices of the Royal Astronomical Society” reports the identification of the exoplanet TOI-715 b in an orbit around its star in a position that meets the most conservative definition of a habitable zone. A team of researchers used observations conducted with NASA’s TESS space telescope confirmed by other instruments to identify what appears to be a super-Earth with a radius about 1.5 times the Earth’s. A second exoplanet candidate was discovered that would have a size very similar to the Earth’s a little further away from its star but follow-up observations are needed to verify that it’s not a false positive.
About 137 light-years away from Earth, the star TOI-715 is a red dwarf with a mass and size about a quarter of the Sun’s. This means that its system’s habitable zone, the one in which water can be in a liquid state on the surface of a planet with Earth-like conditions, is much closer to TOI-715 than Mercury is to the Sun.
Discussions regarding calculating the size of a system’s habitable zone continue because there are various factors that can influence whether a planet can have liquid water on its surface. According to a conservative definition, it’s a region around a star where a rocky planet receives between 42% and 84.2% of the stellar energy that Earth receives from the Sun. The exoplanet TOI-715 b orbits in this region with its year lasting about 19.2 Earth days at an average distance from its star of about 13 million kilometers.
The exoplanet TOI-715 b is also interesting because planets with a radius between 1.5 and 2 times the Earth’s are rare. The most likely explanation is that there are sub-Neptune planets that form with a certain mass but some of it is stripped away by their star. TOI-715’s system is older than the solar system, at an estimated age of 6.2 billion years, so the star had plenty of time to strip away a thick early atmosphere from its planet.
The problem of stellar activity is important due to the consequences that powerful flares can have on planets. Red dwarfs are small but can be very active with very negative consequences on their planets. TOI-715 is a quiet red dwarf, which is another reason for optimism.
More follow-up studies will be necessary to better understand the characteristics of the exoplanet TOI-715 b and assess its real habitability potential. First of all, its mass is not known, so it’s not entirely certain that it’s a rocky planet. Hypotheses range from a sub-Neptune, although unlikely, to a water world completely covered in water. The possible atmosphere makes an extreme difference in its nature but at the moment, we know nothing about it.
The James Webb Space Telescope was also designed to examine the atmospheres of exoplanets. The ones that are very close to their stars and therefore often pass in front of them from Earth’s perspective are easier targets to study. The relative proximity of the TOI-715 system represents another advantage.
Other traces were found by the TESS space telescope. The candidate exoplanet was classified as TIC 271971130.02 because it’s potentially the second planet of a star that in one of the astronomical catalogs is classified as TIC 271971130. It received some confirmations but not enough to rule out that it’s a false positive.
Information suggested by observations of candidate TIC 271971130.02 is of a planet just larger than Earth with a year of approximately 25.6 Earth days. Its presumed location puts it on the outer edge of its system’s conservative habitable zone. If confirmed, it would be the smallest exoplanet discovered so far in its system’s habitable zone by the TESS space telescope.
Establishing if the TIC candidate 271971130.02 actually exists is also important in the new follow-up studies of TOI-715 b due to the effects it would have on its star. The masses of planets are estimated by exploiting their gravitational influence on their stars, even if it’s small, so the presence of another nearby planet must absolutely be considered.
Given the reasons for interest in the TOI-715 system, its study will continue. The hope is always to find a planet habitable by life forms similar to the Earth’s. At worst, astronomers will gather more information that will help understand how rocky planets may evolve in their system’s habitable zone.