TOI-3261 b, a Neptune-sized exoplanet, has become a focal point for scientists studying planets that orbit extremely close to their stars, making it one of the rarest types of exoplanet ever discovered. Located in the “hot Neptune desert”—a category of planets that is notable for its lack of members—this scorching world provides valuable insights into planetary formation, atmospheric evolution, and the physical processes that shape such extreme environments. Discovered using NASA’s Transiting Exoplanet Survey Satellite (TESS) and further observed by ground-based telescopes in Australia, Chile, and South Africa, TOI-3261 b’s extreme characteristics and its place in the hot Neptune desert could help unravel mysteries about how planets like it are formed and how they evolve.
Hot Neptunes, so named for their size and proximity to their stars, are planets similar in size and composition to Neptune, but they orbit their stars in ultra-short periods, often taking only hours or days to complete one orbit. The most extreme of these, such as TOI-3261 b, are part of an exclusive group of ultra-short-period hot Neptunes whose masses have been precisely measured. TOI-3261 b is located approximately 350 light years from Earth and completes a full orbit in just 21 hours, making it one of the fastest orbiting exoplanets ever discovered.
The discovery of TOI-3261 b, published in The Astronomical Journal in August 2024, is significant because hot Neptunes are rare—so rare, in fact, that their scarcity has earned them a spot in the “desert” of exoplanet classification. The few that have been discovered tend to have properties that make them valuable for understanding planetary formation processes. What makes TOI-3261 b particularly interesting is its size and its ability to provide clues about how planets of this kind form and survive close to their stars.
One of the challenges in studying hot Neptunes is understanding how they can retain their thick gaseous atmospheres despite being so close to their stars. Stars emit enormous amounts of energy, which can strip away the atmospheres of nearby planets. This phenomenon, called photoevaporation, occurs when the star’s radiation heats the planet’s atmosphere to the point where gas particles escape into space. Additionally, gravitational forces from the nearby star can cause tidal stripping, where the star’s gravity pulls the planet’s atmosphere away over time. These processes can cause hot Neptunes to lose a significant portion of their mass, leaving them with a much thinner atmosphere than they might have originally had.
The team of astronomers, led by Emma Nabbie from the University of Southern Queensland, used their observations to build models of TOI-3261 b’s formation and evolution. Their findings suggest that TOI-3261 b is about 6.5 billion years old and likely began as a much larger gas giant, similar to Jupiter. Over time, it lost much of its mass due to both photoevaporation and tidal stripping. According to their models, TOI-3261 b might have formed farther from its star, where these effects would have been less intense, allowing the planet to retain more of its atmosphere before migrating inward to its current position.
The remaining atmosphere of TOI-3261 b is one of the most intriguing aspects of the planet. The planet is about twice as dense as Neptune, which suggests that lighter components of its atmosphere—such as hydrogen and helium—have been stripped away, leaving behind heavier elements. This composition provides hints that TOI-3261 b might have once had a more complex atmosphere, rich in different molecules, but its current atmosphere is likely dominated by heavier elements.
To uncover more details about the composition of TOI-3261 b’s atmosphere, astronomers will likely turn to NASA’s James Webb Space Telescope (JWST). JWST, with its ability to observe exoplanets in infrared light, is well-suited for identifying the molecular “fingerprints” of the gases in the planet’s atmosphere. Infrared observations could reveal what elements and compounds remain in TOI-3261 b’s atmosphere and how these elements might have evolved over billions of years. These observations are expected to provide a deeper understanding not only of TOI-3261 b’s past but also of the general processes that govern the formation and evolution of hot giant planets.
TOI-3261 b is just the latest addition to a small group of ultra-short-period hot Neptunes. The first-ever discovery of this type of planet, LTT-9779 b, was made in 2020, followed by TOI-849 b and TOI-332 b. These planets, with their short orbits and extreme temperatures, present unique challenges to planetary science, especially when it comes to understanding how such planets can form and survive in such hostile environments.
As more of these planets are discovered and studied, especially with advanced instruments like JWST, scientists hope to learn more about their atmospheres, how they evolve, and what processes allow some of them to retain thick atmospheres despite the intense radiation they experience from their stars. The study of these extreme exoplanets could reveal much about the broader processes of planetary formation in other star systems as well, providing insights into the potential diversity of planets in the universe.
Source: NASA