Iron oddball planet challenges theories of planetary formation

To truly grasp the intricacies of nature, it’s imperative to embrace its diverse range. This axiom holds true in the realm of exoplanet science and our overarching theories on planetary formation. Nature’s outliers and peculiarities serve as a driving force, challenging the limits of our existing models and spurring scientists to embark on deeper explorations.

One such cosmic oddity that has piqued the curiosity of astronomers is Gliese 367 b, also affectionately known as Tahay. This exoplanet stands out not only for its designation as an Ultrashort Period (USP) planet, circling its host star in an astonishingly brisk 7.7 hours, but also for its extraordinary density – nearly double that of Earth.

This exceptional density strongly suggests that Gliese 367 b is predominantly composed of iron, making it a celestial anomaly. While there are nearly 200 other USP planets in our ever-expanding catalog of over 5,000 exoplanets, Gliese 367 b sets itself apart with its remarkable attributes.

The tale of Tahay began with its discovery in 2021, thanks to data from NASA’s Transiting Exoplanet Survey Satellite (TESS). Its detection was a challenging endeavor, as the transit signal it emitted from the red dwarf star Gliese 367 was exceedingly faint, teetering on the edge of TESS’s detection capabilities. However, this feeble signal hinted at the planet’s diminutive size, akin to our own Earth.

In the initial 2021 investigation, researchers utilized the High-Accuracy Radial Velocity Planet Searcher (HARPS) spectrograph at the European Southern Observatory to discern key properties of Gliese 367 b. Their findings unveiled that the planet had a radius equivalent to 72% of Earth’s and a mass roughly 55% that of Earth. These figures pointed to the likelihood that Gliese 367 b was once part of a more substantial celestial body, now reduced to its iron-rich core.

Fast forward to the present, and new research led by Elisa Goffo, a Ph.D. student at the Physics Department of the University of Turin, has provided further refinement to our understanding of this enigmatic planet. Utilizing HARPS once again, they amassed a comprehensive dataset comprising 371 observations of Gliese 367 b. These fresh findings unveiled an even greater density than previously estimated. Instead of accounting for 55% of Earth’s mass, this updated research indicates that Gliese 367 b now encompasses 63% of Earth’s mass, and its radius has constricted to 70% of Earth’s dimensions.

The crux of the matter is that Gliese 367 b stands as a celestial enigma, being twice as dense as our own planet. The natural question arises: How did it attain such a unique composition? The prevailing consensus is that Gliese 367 b did not form in its present state. Rather, it likely emerged as an Earth-like planet, sporting a dense iron core enveloped by a silicate-rich mantle.

The transformation into its current iron-rich state is believed to be the result of a cataclysmic event. This event could have stripped away its rocky mantle, leaving behind the dense planetary core we observe today. This notion aligns with the theory of early collisions with other protoplanets, which could have resulted in the removal of Gliese 367 b’s outer layers.

Another conjecture, initially pondered upon the planet’s 2021 discovery, is that Gliese 367 b might have been the remnant of a colossal gas giant akin to Neptune. In this scenario, the planet would have formed farther from its host star before migrating inward. Its current proximity to the red dwarf star suggests that the planet’s gaseous envelope would have been obliterated due to the intense irradiation.

Gliese 367 b belongs to an exclusive category of exoplanets known as super-Mercuries. While they share a similar composition with Mercury, these planets are larger and denser. Even within this exclusive club, Gliese 367 b holds a unique position as the densest USP planet known to us, boasting an iron core with a mass fraction of 0.91.

The questions surrounding this celestial oddity do not end with its composition. Recent research has unveiled two companion planets in the Gliese 367 system, denoted as G 367 c and d. This aligns with the prevailing notion that USP planets are often found in multi-planet systems. These two newly discovered siblings do not transit their host star, rendering them undetectable by TESS. Their presence, however, adds a layer of complexity to the formation scenarios.

The existence of these companion planets raises further inquiries about how such a system came into existence. While an iron-rich environment is a conceivable origin, the possibility of collisional events shaping these planets cannot be dismissed. This underscores the complex interplay of factors governing the formation of celestial bodies.

In the concluding remarks of their paper, the research team delves deeper into the potential formation scenarios. The protoplanetary disk surrounding Gliese 367 may have harbored an iron-enriched region, though it remains uncertain whether such iron-rich zones exist in protoplanetary disks, as a 2020 study cast doubts on this possibility. Alternatively, Gliese 367 b might have evolved over time, shedding its outer layers through repeated collisions, thereby increasing its bulk density.

In essence, three possibilities are considered: formation in an iron-rich environment, size reduction through collisions, or the transformation from a gas giant to its current state. It’s even conceivable that a combination of these processes contributed to shaping the enigmatic Gliese 367 b.

For now, the Gliese 367 system stands as a cosmic enigma, a puzzle waiting to be solved. Its exceptional properties serve as a stimulus for astronomers to push the boundaries of their understanding. In the quest to unravel the mysteries of outliers like Gliese 367 b, scientists refine their models and theories, advancing our comprehension of the cosmos.

In conclusion, the Gliese 367 system, with its exceptional properties, offers a captivating puzzle for astronomers to decipher, pushing the boundaries of our knowledge in exoplanet science and planetary formation theories. It reminds us that nature’s outliers are catalysts for scientific exploration, encouraging us to delve deeper into the mysteries of the universe.

Source: Universe Today

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