A new study reveals that the exoplanet TOI-561 b presents significant evidence of a rocky world with an atmosphere outside our Solar System. This ultra-hot super-Earth, which features a global ocean of molten rock beneath a dense layer of volatile gases, challenges existing theories about the nature of searingly hot exoplanets. The research, led by scientists from Carnegie Science, showcases findings that could reshape our understanding of planetary atmospheres.
Located less than 1.6 million kilometers (approximately 0.99 million miles) from its star, TOI-561 b orbits at a distance that is merely one-fortieth of that between the Sun and Mercury. This proximity results in a tidally locked environment where one hemisphere is perpetually illuminated while the other remains shrouded in darkness. Remarkably, despite the intense radiation from its host star, the planet has retained its atmosphere for billions of years, a feat that defies expectations.
Nicole Wallack, an astronomer at Carnegie Science, expressed surprise at TOI-561 b’s ability to maintain its atmospheric conditions. “Based on what we know about other systems, astronomers would have predicted that a planet like this is too small and hot to retain its own atmosphere for long after formation,” Wallack noted. The planet is classified as an ultra-short period (USP) planet, completing an orbit in less than 11 hours.
The characteristics of TOI-561 b are further intriguing given that it orbits an ancient star, estimated to be around 10 billion years old, more than twice the age of our Sun. This star is slightly less massive and cooler than the Sun, with a peculiar composition that includes low iron and high levels of alpha elements, such as oxygen, magnesium, and silicon. Such elements were generated by massive stars in the early universe, providing a unique context for TOI-561 b’s formation.
Researchers discovered that TOI-561 b has an unusually low density, calculated to be only four times denser than water. This could indicate a smaller iron core or a composition of less dense rocks than those found on Earth. The findings may also suggest that an atmosphere could contribute to the planet’s apparent size.
To investigate TOI-561 b’s density and atmosphere, the team utilized data from the James Webb Space Telescope (JWST), which observed the planet’s system for over 37 hours and nearly four full orbits. By analyzing the dayside brightness in near-infrared light via the telescope’s NIRSpec (Near-Infrared Spectrograph), researchers were able to estimate the planet’s temperature.
The analysis indicated that TOI-561 b’s temperature is approximately 1,800 degrees Celsius (about 3,272 degrees Fahrenheit), significantly cooler than the expected 2,700 degrees Celsius (approximately 4,892 degrees Fahrenheit) for a planet of its type without an atmosphere. This discrepancy raises the possibility that the atmosphere plays a role in regulating temperature. Researchers suggest that winds might distribute heat across the planet, while water vapor could absorb infrared light, making the surface appear cooler.
The longevity of TOI-561 b’s atmosphere remains an open question. The research team proposes that the planet may have reached a balance between its atmosphere and the magma ocean, which would solidify on the nightside without such a protective layer. Gases possibly leaking from the crust could replenish the atmosphere, although some would inevitably escape into space. The presence of iron in the planet’s composition may facilitate the retention of these volatile chemicals.
The findings highlight the complexity of TOI-561 b’s atmosphere and its potential implications for understanding rocky exoplanets in extreme environments. As the researchers conclude, “pinpointing exactly why TOI-561 b has a thick atmosphere will require further theoretical and observational investigation.” The study is detailed in the latest issue of The Astrophysical Journal Letters, paving the way for future exploration of similar worlds.
