An article published in “The Astrophysical Journal Letters” reports a study on the exoplanet GJ 367 b, or Gliese 367 b, formally known by the name Tahay, which concludes that its core is composed almost entirely of iron for a density that is almost twice the Earth’s. The researchers also announced the discovery of two more planets in the system of the star GJ 367, formally known as Añañuca, which may be super-Earths.
Just over 30 light-years from Earth, Añañuca is a red dwarf with a mass and size just under half of the Sun’s. Observations began in 2019 with NASA’s TESS space telescope and in 2021 the data collected led to the exploration of the possibility that a planet orbited very close to this star, subsequently confirmed with other instruments. Tahay’s roughly 7.7-hour year is the shortest in the cosmic neighborhood, one reason for interest and worthy of its own name.
Another peculiarity noted when the exoplanet Tahay was detected and studied is that it’s made up of a dense core and very little else. It’s not a usual type of planet among the over 5,000 now known and was compared to Mercury, hence the proposal of the super-Mercury class.
The interest in the exoplanet Tahay led to this new follow-up study based on detections conducted with the High-Accuracy Radial Velocity Planet Searcher (HARPS) spectrograph at ESO’s La Silla Observatory in Chile. The result is greater precision in estimates of its characteristics with the determination that its radius is 70% of the Earth’s while its mass is 63% of the Earth’s. The consequence is that its density is almost twice the Earth’s with a core composed of about 91% iron.
Tahay’s composition suggests that it’s the core of a bigger planet whose mantle was stripped in a cosmic catastrophe. Collisions between protoplanets could be common in compact planetary systems such as those around red dwarfs. What perplexes researchers is the fact that the possible collision stripped the planet of almost all materials other than iron.
A collision that stripped the mantle of the primitive Tahay is decidedly more likely than a planet that formed already like this, in an area of the protoplanetary disk made up almost exclusively of iron. A further hypothesis is that Tahay was originally a gas planet that approached its star and lost its atmosphere but the fact that its core is composed almost exclusively of iron would remain truly anomalous.
Finding other planets in the Añañuca system was important to study possible collisions but also the hypothesis that Tahay formed further away from its star and then moved closer as a result of various gravitational influences. This new study discovered two more planets that at least for the moment haven’t been seen directly and only have catalog designations. GJ 367 c has a year that lasts about 11.5 Earth days and a mass that is probably just over four times the Earth’s. GJ 367 d has a year that lasts just over 34 Earth days and a mass that is probably about six times the Earth’s.
The formation of Tahay still remains uncertain and there will be further studies for sure, even more so now that traces of other planets in the Añañuca system were detected. Finding an explanation can offer important insights into the formation and evolution of planetary systems.
