TOI-1231 b: A Temperate World with Secrets of Water and Atmosphere Beyond Our Solar System

The search for exoplanets—planets that orbit stars outside our solar system—has yielded many fascinating discoveries. One of the most recent is TOI-1231 b, a unique exoplanet that offers exciting opportunities for scientists to explore atmospheres beyond our solar neighborhood. Discovered in 2021 by NASA’s Transiting Exoplanet Survey Satellite (TESS) and confirmed by additional ground-based observations, TOI-1231 b has quickly become a topic of interest due to its favorable characteristics and potential to offer insights into water-rich planetary systems.

Discovery of TOI-1231 b

TOI-1231 b was first identified by TESS, a NASA mission designed to survey the brightest stars near the Earth for transiting exoplanets. The star system, TOI-1231, lies about 90 light-years away from Earth in the constellation Vela. Astronomers utilized the transit method to detect TOI-1231 b, observing the regular dimming of the host star as the planet passed in front of it, which allowed them to estimate its size and orbit.

The planet orbits a relatively cool M-dwarf star, also known as a red dwarf. These stars are smaller and cooler than the Sun, which means that even planets that orbit close to them can have moderate temperatures. Using follow-up data from observatories, including the Planet Finder Spectrograph (PFS) on the Magellan Clay telescope in Chile, astronomers confirmed the existence of the planet and began analyzing its characteristics.

Characteristics of TOI-1231 b

TOI-1231 b is classified as a sub-Neptune exoplanet, meaning it is slightly smaller than Neptune but larger than Earth. Its radius is about 3.65 times that of Earth, and it has a mass approximately 15.4 times Earth’s mass. The planet’s density suggests it has a significant atmosphere, likely composed of hydrogen and helium, but potentially containing water vapor as well.

The planet's orbit is remarkably short, taking just 24.2 days to complete one revolution around its star. Despite this proximity to its host star, TOI-1231 b has a relatively mild temperature of around 330 Kelvin (57°C or 134°F) due to the coolness of the M-dwarf star it orbits. This temperature places it in the category of temperate exoplanets, meaning it's neither too hot nor too cold—making it an intriguing target for atmospheric studies.

The Uniqueness of TOI-1231 b

TOI-1231 b stands out among other exoplanets for several reasons, making it a subject of interest for astronomers:

1. Potential Water-Rich Atmosphere: One of the most exciting aspects of TOI-1231 b is the potential for a water-rich atmosphere. Due to its relatively low temperature and large size, it is likely to have a thick atmosphere capable of supporting water vapor. This makes TOI-1231 b a prime candidate for follow-up atmospheric studies. Researchers are particularly eager to determine whether the planet’s atmosphere contains hydrogen, helium, or possibly even water, as this could provide clues about planetary formation and migration processes.
2. Slow Escape Velocity: The exoplanet’s atmosphere has a relatively low escape velocity compared to other known sub-Neptunes. This means that gases like hydrogen and helium, which are lighter and typically escape quickly from exoplanetary atmospheres, may remain longer in TOI-1231 b’s atmosphere. Studying these gases will help astronomers understand how atmospheres evolve over time.
3. Temperate Climate: The moderate climate of TOI-1231 b makes it a prime candidate for studying planetary atmospheres in a range that is not extremely hot or cold. This exoplanet resides in a habitable zone-like temperature range, although its size and composition likely prevent it from being directly habitable in the same way as Earth. Still, its temperature range is favorable for detecting water vapor, and such discoveries would be groundbreaking in the study of exoplanetary atmospheres.
4. Potential for Follow-Up Observations: TOI-1231 b’s host star, an M-dwarf, is particularly bright in the infrared spectrum, making it easier for astronomers to study the planet's atmosphere using space-based telescopes like the Hubble Space Telescope and the upcoming James Webb Space Telescope (JWST). Infrared observations will be critical for detecting and characterizing molecules in the planet’s atmosphere, such as water vapor, methane, and carbon dioxide, which are key to understanding its composition and potential for supporting life.
5. Comparative Planetology: TOI-1231 b falls within a size category between Earth and Neptune, which astronomers call “sub-Neptunes.” These planets are common in our galaxy, but the mechanisms that govern their formation, atmospheric retention, and evolution remain unclear. Studying TOI-1231 b could help scientists understand the diversity of planetary systems and how they differ from our own.

What’s Next for TOI-1231 b?

With TOI-1231 b’s discovery, astronomers have identified an exoplanet that presents an excellent opportunity for future research. Upcoming observations with the James Webb Space Telescope, which has the ability to capture high-resolution infrared data, will be essential for probing the planet's atmosphere in unprecedented detail. If water vapor or other molecules are detected, it could help scientists gain insight into the planet's formation history and the potential for other sub-Neptunes to host similar environments.

TOI-1231 b is not likely to be directly habitable, but its study offers valuable knowledge in understanding the types of planets that could host life or share similar traits with habitable worlds. This discovery underscores the growing diversity of exoplanets and the potential for finding more Earth-like planets in the future.