A superheated planetary body, characterized by a molten surface and enveloped by a dense vaporized rock shell, may represent the most compelling evidence unearthed to date for a rocky celestial body with an atmospheric envelope situated beyond our native Solar System.
Designated TOI-561 b, this celestial object is an execessively hot super-Earth, which, according to a recent investigation spearheaded by researchers from Carnegie Science, appears to possess a global molten magma layer situated beneath a substantial atmosphere composed of volatile chemical compounds.
Furthermore, TOI-561 b presents an enduring cosmic puzzle, substantially challenging prevailing understandings of exoplanets subjected to extreme thermal conditions and locked in rapid orbital trajectories around their parent stars.
This exoplanet traverses its stellar orbit at an almost negligible proximity, less than 1.6 million kilometers (0.99 million miles), a distance that equates to merely one-fortieth of the separation between our Sun and Mercury. This proximity creates a tidally locked environment, a veritable inferno where one hemisphere is perpetually illuminated by its star, while the opposing side endures eternal night.
Remarkably, it has managed to retain its atmospheric mantle for eons, defying the potent stellar radiation that is typically understood to erode the gaseous envelopes of comparable celestial bodies, leaving them as barren, incandescent rocks or even entirely molten spheres.
“Based on our current comprehension of planetary systems, astronomical projections would have suggested that a celestial body of this magnitude and thermal intensity would be incapable of sustaining its own atmospheric layer for an extended period post-formation,” states Nicole Wallack, an astronomer at Carnegie Science.
TOI-561 b is classified as an ultra-short period (USP) planet, a designation stemming from its extremely condensed orbit, which necessitates less than an 11-hour duration for a complete revolution. In terms of physical dimensions, it possesses a mass approximately twice that of Earth and a radius 1.4 times Earth’s radial extent.
It circulates an exceptionally ancient star, marginally less massive and cooler than our Sun. This stellar entity exhibits a low abundance of iron and a rich composition of alpha elements such as oxygen and magnesium, elements that were synthesized within massive stars during the primordial epochs of the Universe.
Its location within the Milky Way’s thick disk, a galactic region often likened to a stellar sanctuary for aging stars, further substantiates its antiquity. These collective indicators point to the star being approximately 10 billion years old, more than double the age of the Sun.
The research collective also observed that TOI-561 b exhibits a notably reduced density, being only about four times denser than water. This characteristic might be attributable to TOI-561 b possessing a comparatively diminutive iron core, and potentially being composed of rocky materials with a lower density than those found in Earth’s crust. Such a composition would be consistent with TOI-561 b’s formation in the nascent stages of the Universe, when iron was a scarcer element.
Alternatively, this reduced density could also be a consequence of TOI-561 b housing an atmosphere that artificially inflates its apparent size.
To definitively determine whether the lower-than-anticipated density of TOI-561 b was indeed due to an atmospheric phenomenon, the investigators leveraged data acquired from the James Webb Space Telescope (JWST). The telescope meticulously observed the planet’s system for a duration of 37 hours, capturing nearly four full orbital passes around its parent star.
By meticulously quantifying the dayside radiance of TOI-561 b in the near-infrared spectrum using Webb’s NIRSpec (Near-Infrared Spectrograph) instrument, the researchers were able to ascertain its surface temperature. This temperature data subsequently enabled them to infer the probability of an atmospheric presence.
Without an atmosphere, TOI-561 b would be expected to register a surface temperature of approximately 2,700 degrees Celsius (4,900 degrees Fahrenheit). However, the empirical measurements indicated a more temperate surface temperature, closer to 1,800 degrees Celsius.
The research team postulates that an atmosphere could be contributing to the apparent “cooling” of TOI-561 b’s stellar-facing hemisphere through several potential mechanisms. Atmospheric winds might facilitate the redistribution of thermal energy from the sunlit side to the nocturnal hemisphere. Concurrently, water vapor within the atmosphere could absorb near-infrared radiation emanating from the planet’s surface, thereby creating an optical illusion of lower temperatures.
However, the pertinent question remains: how has TOI-561 b succeeded in preserving this substantial atmospheric shell over billions of years, while maintaining such an intimate proximity to its host star?
The investigative team proposes that the exoplanet might have achieved a state of equilibrium between its atmospheric envelope and its surface-spanning magma ocean. Without an atmosphere, this magma ocean would have ostensibly solidified into a frozen crust on the nightside.
Instead, the researchers hypothesize that gaseous substances may have been continuously released from the exoplanet’s crust, replenishing the atmosphere. While some of these gases would inevitably escape into the void of space, the vast magma ocean could concurrently function as a geothermal reservoir, reabsorbing gases back into the planet’s interior.
The iron content of this exoplanet may also play a crucial role in this phenomenon. The very element that facilitates oxygen binding within the hemoglobin of our red blood cells might be instrumental in TOI-561’s atmospheric retention by sequestering volatile compounds within its molten mantle or core.
“Analysis of rocky planets for which we have constraints on dayside brightness temperatures suggests that celestial bodies experiencing irradiation temperatures exceeding approximately 2000 Kelvin possess the capacity to replenish their volatile envelopes at a rate exceeding their loss,” the researchers articulate in their published work.
Nevertheless, “precisely elucidating the underlying reasons for TOI-561 b’s atmospheric retention will necessitate further dedicated theoretical analysis and comprehensive observational investigation.”
