Using a three-dimensional global climate model typically employed for Earth studies, the team found that a white dwarf exoplanet exhibited significantly higher temperatures than a comparable exoplanet orbiting Kepler-62, despite similar stellar energy distributions.
"While white dwarf stars may still give off some heat from residual nuclear activity in their outer layers, they no longer exhibit nuclear fusion at their cores. For this reason, not much consideration has been given to these stars' ability to host habitable exoplanets," Shields explained. "Our computer simulations suggest that if rocky planets exist in their orbits, these planets could have more habitable real estate on their surfaces than previously thought."
A major factor influencing habitability is the rotational characteristics of the exoplanets. The study found that the habitable zone of a white dwarf is much closer to the star than that of a main sequence star like Kepler-62. As a result, an exoplanet orbiting a white dwarf would have a much shorter rotation period-approximately 10 hours-compared to the 155-day period of an exoplanet orbiting Kepler-62.
Both exoplanets are expected to be tidally locked, meaning one side always faces the star while the other remains in perpetual darkness. However, the white dwarf planet's rapid rotation enhances atmospheric circulation, preventing excessive cloud cover on the dayside. Conversely, the Kepler-62 exoplanet's slower rotation fosters substantial dayside cloud formation, which can reflect stellar radiation and lead to surface cooling.
"We expect synchronous rotation of an exoplanet in the habitable zone of a normal star like Kepler-62 to create more cloud cover on the planet's dayside, reflecting incoming radiation away from the planet's surface," Shields said. "That's usually a good thing for planets orbiting close to the inner edge of their stars' habitable zones, where they could stand to cool off a bit rather than lose their oceans to space in a runaway greenhouse. But for a planet orbiting squarely in the middle of the habitable zone, it's not such a good idea."
She elaborated that while the Kepler-62 exoplanet's extensive cloud cover results in significant cooling and reduced habitable surface area, the white dwarf planet's minimal cloud cover allows it to retain more heat, thereby enhancing its potential habitability.
The study indicates that the nightside of the white dwarf planet experiences a stronger greenhouse effect due to atmospheric dynamics, leading to a warmer overall climate compared to the Kepler-62 planet.
"These results suggest that the white dwarf stellar environment, once thought of as inhospitable to life, may present new avenues for exoplanet and astrobiology researchers to pursue," Shields noted. "As powerful observational capabilities to assess exoplanet atmospheres and astrobiology have come on line, such as those associated with the James Webb Space Telescope, we could be entering a new phase in which we're studying an entirely new class of worlds around previously unconsidered stars."
Research Report:Increased Surface Temperatures of Habitable White Dwarf Worlds Relative to Main-sequence Exoplanets
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