The celestial body known as TOI-561 b, with a mass approximately twice that of Earth, presents a strikingly different profile. Positioned extremely close to its star--at a distance merely one-fortieth of Mercury's distance from the Sun--this planet completes a full orbit in just 10.56 hours. Despite being around a slightly smaller and cooler star, TOI-561 b remains in perpetual daylight, as one side is always facing its sun.
Nicole Wallack, a postdoctoral fellow at Carnegie Science and co-author of the study, remarked, "Astronomers would typically assume that a planet of this size and heat would struggle to maintain an atmosphere long after its formation. However, our findings indicate that it is enveloped by a surprisingly thick layer of gas, challenging the traditional views on ultra-short-period planets."
In our Solar System, smaller planets subjected to intense heat tend to lose their gas envelopes early in their evolution. Yet, TOI-561 b orbits a much older star than our Sun, and intriguingly, it appears to have retained its atmosphere despite the extreme conditions.
Unusual Composition Suggested by Low Density
The potential existence of an atmosphere may also clarify the planet's unexpectedly low density. Johanna Teske, the lead author of the study, stated, "While it's not classified as a super-puff or 'cotton candy' planet, its density is lower than what would be anticipated if it had an Earth-like structure."
Initially, the research team considered whether the planet's internal structure could explain its density. One hypothesis suggested that TOI-561 b might possess a smaller iron core and a mantle of lighter rock compared to Earth. Teske elaborated that the planet's unique characteristics align with its formation history: "TOI-561 b orbits a very old, iron-poor star, indicating it formed in a different chemical environment than planets in our Solar System."
This hints that TOI-561 b may resemble planets that formed in the early universe. However, the composition alone does not fully account for the observations.
JWST Data Unveils Hidden Atmospheric Features
The research team utilized the JWST's Near-Infrared Spectrograph (NIRSpec) to measure the temperature of the planet's daylight side, observing its brightness in near-infrared light. This technique, which also applies to planets in the TRAPPIST-1 system, revealed that the dayside temperature is around 3,200 degrees Fahrenheit (1,800 degrees Celsius), significantly lower than the expected nearly 4,900 degrees Fahrenheit (2,700 degrees Celsius). This temperature discrepancy suggests that heat is being distributed across the planet.
Atmospheric Dynamics and Cooling Mechanisms
Scientists proposed that a substantial atmosphere could account for the cooler temperature. Anjali Piette, co-author from the University of Birmingham, noted, "We need a thick, volatile-rich atmosphere to explain our findings. Strong winds could transport heat from the dayside to the nightside, while gases like water vapor would absorb specific wavelengths of near-infrared light, making the planet appear cooler."
While the evidence strongly supports the existence of an atmosphere, it raises intriguing questions about how such a planet can retain gas in the face of intense radiation. Some material may escape into space, but perhaps not as quickly as anticipated.
A Balance of Atmosphere and Magma
One hypothesis is that a dynamic equilibrium exists between the planet's molten interior and its atmosphere. Tim Lichtenberg from the University of Groningen explained, "There seems to be a balance where gases escape to form the atmosphere while the magma ocean pulls them back in. This planet is likely far more volatile-rich than Earth, resembling a 'wet lava ball.'
The discovery opens up numerous questions, as highlighted by Teske: "What's fascinating is that this new data set is generating more questions than it resolves."
Future Implications of JWST Discoveries
These findings stem from JWST's General Observers Program 3860, which monitored TOI-561 b for over 37 hours. Researchers are currently analyzing the complete dataset to map temperature patterns and further understand its atmospheric composition. The legacy of Carnegie Science with JWST continues to thrive, as researchers anticipate more groundbreaking discoveries in the upcoming year.
