There might be at least one habitable world orbiting a red dwarf star only 105 light-years from Earth. Two rocky exoplanets have been discovered and verified in tight orbit around the chilly, faint star; one of them is comfortably within the star's habitable zone.
The discovery is intriguing and suggests that there may be more similar worlds waiting to be found in the solar neighborhood, but further investigations must be done to ascertain its nature. Worlds in the habitable zone are relatively uncommon, especially rocky ones, even if the exoplanet isn't habitable.
In either case, the discovery is a crucial piece of information for describing the composition of this group of exoplanets. NASA's TESS exoplanet-hunting telescope made observations that led to the discovery of the two exoplanets. It detected the faint, regular dips in illumination indicative of an exoplanet orbiting between us and the star, known as transits, on a 2.7-day cycle when it observed the tiny red dwarf star LP 890-9 (also known as TOI-4306).
We can learn quite a bit about an exoplanet from transit data. One is the simple fact that it exists. The orbital period is another factor. The diameter of the exoplanet can also be estimated based on how much the starlight dims, but more observations with other instruments are needed to confirm the discovery and gather more information on the exoplanets.
This follow-up is crucial for very cold stars like TOI-4306, which emit the majority of their light in the near-infrared and for which TESS has a rather low sensitivity, according to astronomer Laetitia Delrez of the University of Liège in Belgium.
The LP 890-9b exoplanet was discovered by her team using telescopes from the SPECULOOS (Search for livable Planets EClipsing ULtra-cOOl Stars) consortium, which are sensitive to the near-infrared wavelengths radiated by TOI-4306.
LP 890-9c, which has an orbital period of 8.4 days, was discovered after the team searched for exoplanets that TESS may have missed. Other data, including radial velocity measurements that determine the gravitational pull an exoplanet exerts on a star and thus derive their mass, allowed the team to characterize the two exoplanets in detail.
The radial velocity data provided the researchers with an upper mass limit for both exoplanets even though the two masses have not been measured. LP 890-9b is about 1.32 times the diameter of Earth and up to 13 times its mass, while LP 890-9c is about 1.37 times the diameter of Earth and up to 25 times its mass.
These results are compatible with the density of rocky worlds, like Earth, Mars, and Venus, rather than gaseous or ice worlds, like Jupiter or Neptune, which means the exoplanets can be categorized as super-Earths - rocky worlds that are larger than Earth, and smaller than Neptune.
An exoplanet that is too close to its star or that is too far away from it will be too hot or too cold, respectively, for life as we know it. However, there is a temperate, or habitable, zone in every star's orbital neighborhood in which liquid water may comfortably sit on a planetary surface.
At its orbital position, the exoplanet is within its star's conservative habitable zone, receiving similar stellar radiation levels to Earth, and is the second-most promising possibly habitable world discovered so far after the TRAPPIST system, according to the researchers.
"However, we must not jump to conclusions. "Our neighbor planet Venus, which is, in a sense, a carbon dioxide-rich, close to 500 degrees Celsius pressure cooker, is also close to this so-called habitable zone orbiting the Sun," explains astronomer Robert Wells of the University of Bern in Germany.
A second team of scientists, including some of the authors on the LP 890-9 paper, has already submitted a preprint interrogating this mystery, but we likely won't know until we obtain observations of the exoplanet's atmosphere, if it has one. LP 890-9c is very close to the point at which a young planet might be caught in a runaway greenhouse effect, like Venus.
We may have to wait a while for observations, as the James Webb Space Telescope is in high demand, but the telescope is already proven adept at this. On the other hand, a target as promising as LP 890-9c may merit a hustle, as whatever the outcome, we have something to learn from it.
"It is crucial to identify as many temperate terrestrial worlds as possible to study the diversity of exoplanet climates, and eventually to be in a position to quantify how frequently biology has emerged in the Cosmos," says astronomer Amaury Triaud of the University of Birmingham in the UK.
Reference: Astronomy & Astrophysics.
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