TOI-2257 b
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| Discovery[1] | |
|---|---|
| Discovered by | Schanche et al. |
| Discovery date | November 2021 |
| Transit | |
| Orbital characteristics[1] | |
| 0.145±0.003 AU | |
| Eccentricity | 0.496+0.216 −0.133 |
| 35.189346(90) d | |
| Inclination | 89.786°+0.078° −0.062° |
| Star | TOI-2257 |
| Physical characteristics[1] | |
| 2.194+0.113 −0.111 R🜨 | |
| Mass | 5.712+4.288 −2.311 M🜨 (predicted) |
| Temperature | 256+61 −17 K (−17 °C; 1 °F, equilibrium) |
TOI-2257 b is an extremely eccentric (0.496)[2] exoplanet in or near the circumstellar habitable zone of the star TOI-2257, 188 light-years away. It is likely a sub-Neptune exoplanet, with a mass of 5.71 M🜨 and a radius of 2.19 R🜨.[3] As a small planet in the habitable zone, it is included in the Planetary Habitability Laboratory's list of potentially habitable exoplanets.[4]
The planet was discovered using the transit method, by TESS in November 2021. It has one of the longest orbital periods of any TESS planet. Further observations intend to find possible water vapor in the atmosphere of the planet, as well as any other planets in the system if they exist. The planet is most likely not a false positive, and its existence is supported by photometry and the high-resolution observations of ground-based telescopes.[3]
Properties
Mass, radius, and temperature
The planet has a radius 2.19 times that of Earth. Its mass and density are unknown, although it is predicted to have a mass roughly 3.4-10 times that of Earth based on mass-radius relationships.[1] Based on its size, it is likely a Neptune-like world. Its average equilibrium temperature is 256 K (−17 °C; 1 °F), similar to the average temperature of Alert, Canada on Earth, and varies from approximately 193 K (−80 °C; −112 °F) during aphelion to 373 K (100 °C; 212 °F) at perihelion.[3][5] However, the actual temperature could differ and would also vary throughout the planet's eccentric orbit.[2]
Orbit
The planet has an orbital period of 35.19 days, with an extremely high eccentricity of almost 0.5.[3] It has a semimajor axis of 0.145 AU, approximately half of Mercury's at the point in its orbit nearest to the Sun.[2] According to NASA Exoplanet Exploration, the planet's eccentric orbit takes it through the "too hot" zone (albeit for a very short amount of time), then out to the outer fringes of the habitable zone, near the border with the "too cold" zone.[6]
The planet has the highest eccentricity ever recorded around an M-type star, and the third highest of any known mini-Neptune as of 2021.[7]
Star
The planet's star is M3V, with a temperature of 3,430 K. It has a metallicity of -0.27 and is about 8 billion years old, with ~0.3 times the mass and 0.33 times the radius of the Sun.[3][2] For comparison, the Sun has a temperature of 5,778 K and is 4,572,000,000 years old, with a spectral class of G2V. The metallicity is 0.00.
