Planet name: TOI 2257 b
Planet mass (J): 0.018 (compares to Earth at 0.00315), 5.7x the mass of Earth
Planet semi-major axis (AU): 0.145 (compares to Earth at 1 AU; compares to Mercury at 0.4 AU)
Orbital Period (sidereal days): 35.19 (compares to Earth at 365.26)
Star name: TOI 2257
Star distance: 57.8 parsecs (188.5 light years)
Spectral type: M3 (a coolish red star, compared to the sun which is a hotter, yellow G class star)
Star mass (solar masses): 0.33 (compares to the sun at 1 solar mass)
Star luminosity (solar luminosity): 0.014 (compares to the sun at 1.0 solar luminosity)
Theoretical max. time star in main sequence (billion years): 137 (compares to sun at 10)
Habitable zone inner radius (AU): 0.111 (compares to the sun’s at 0.95 AU)
Habitable zone outer radius (AU): 0.160 (compares to the sun’s at 1.37 AU)
Planet orbits in star's circumstellar habitable zone: Yes, near the outer rim of the habitable zone
Comments
This exoplanet is about six times the mass of Earth, orbiting a cool red star about 0.1 the distance that Earth orbits the sun. For reference, Mercury orbits the sun at about 0.4 AU. So, this planet orbits its host star 4x closer than Mercury. And even though this reddish star is only about a third as massive as the sun, it would appear very large in the sky, when viewed by the planet – because the planet orbits so closely.
Approximating the star’s history based on the star’s mass (and not its observed apparent magnitude), it will be in its main sequence for 137 billion years.
Given the planet’s close orbit, it is a prime candidate to become tidally locked – like the moon is tidally locked to Earth. That is, one side of the planet will always face the star, and the opposite side of the planet will always face away from the star.
Class M stars have very long main sequence stages. Theoretically, this star could be in main sequence for 639 billion years, if the Universe lasts that long.
Given the prospect of tidal locking, this planet won't experience a day-night cycle. There won't be warming during daylight, and cooling at night. So, the "day" side will be constantly very hot -- maybe too hot for liquid water. And the night side will be constantly very cold -- maybe too cold for liquid water . There is a possible habitable ring at the day-night terminator.
Let’s think more about atmosphere. This is a very large planet. 5.7x the mass of Earth. Probably more like a rocky world than a gas giant world. Its powerful gravity could accumulate and hold on to gases, sustaining a thick atmosphere. But wait. If the planet is tidally locked to its host star, then atmospheric gases on the perpetually “night” side of the planet could freeze and fall to the planetary surface as solids or liquids. Those gases would later be back-filled by circulation of atmospheric gases from the “day” side of the planet – which would also freeze out. The dark side would act as a gas sink, leeching atmospheric gases from the "day" side and depositing them as liquids or solids on the "night" side. This could result in a large rocky body with little or no atmosphere, but an interesting accumulation of ice and cold liquids on the dark side of the planet. But wait a moment. If there is no atmosphere, liquid or solid depositions from chilled atmospheric gases would not last for long. Without any atmospheric pressure, they would evaporate into space and be blown away by the solar wind. The planet would have no fluids whatsoever.
So if a star's circumstellar habitable zone is too close to the star, tidal locking will spoil the chances for the persistence of an atmosphere or surface liquids.
Getting there. As this planet is 188.5 light years away, rockets using gravitational slings can attain speeds of over 394,736 miles per hour (NASA Parker solar probe). At that speed, it would take about 321,627 years to get there.
My imagined view of exoplanet TOI-2257 b (using Blender and the Custom Planet add-on). Image created by Tom Morris / PlanetaryBiology.com
Imagined view of exoplanet TOI-2257 b in exoExplorer. Notice the moon in the frame with the sun. When the planet is tidally locked with the host star, the sun position will remain fixed in this viewpoint. But the moon will travel across the sky in its own orbit. Very weird. See it for yourself with exoExplorer.