White dwarf planets are more habitable than we thought

For years, astronomers have searched for habitable exoplanets around main-sequence stars similar to the Sun. These stars provide stable energy for billions of years, making them prime candidates for life-supporting planets.

However, recent research suggests that another type of star – known as a white dwarf – may also offer suitable conditions for habitability.

White dwarfs are the remnants of stars that have exhausted their nuclear fuel. They no longer sustain fusion at their cores, yet they continue to emit residual heat. This lingering warmth raises an intriguing question: Could planets orbiting these stars maintain the right conditions for life?

A new study led by astronomers at the University of California, Irvine, suggests that the answer might be yes. Their findings indicate that white dwarf systems may be more hospitable to life than previously thought.

Comparing white dwarfs to Sun-like stars

The research team, led by UC Irvine professor Aomawa Shields, explored the climates of exoplanets orbiting two very different stars.

One was a theoretical white dwarf nearing the final stages of its existence. The other was Kepler-62, a main-sequence star undergoing a phase of stellar evolution similar to that of the Sun.

To understand how these exoplanets might behave, the team used a 3D global climate model originally designed to study Earth’s climate.

The analysis revealed that the exoplanet orbiting the white dwarf was significantly warmer than the one around Kepler-62. This finding was unexpected, as white dwarfs are often assumed to be too dim to support habitable worlds.

Unique characteristics of white dwarf planets

“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,” explained Professor Shields.

“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.”

One of the most critical differences between the two planetary systems studied was the way their planets rotate. A planet’s rotation determines how heat is distributed across its surface, which in turn affects its climate and potential habitability.

The role of planetary rotation

The white dwarf’s habitable zone – the region where an exoplanet could maintain liquid water – sits much closer to the star than that of Kepler-62.

As a result, the planet orbiting the white dwarf has a much shorter rotation period of just ten hours. Meanwhile, the Kepler-62 planet takes 155 days to complete one rotation.

Both exoplanets likely experience synchronous rotation, meaning one side always faces the star while the other remains in permanent darkness.

This phenomenon, known as tidal locking, creates extreme temperature differences between the day and night sides of a planet. However, the speed of rotation affects how atmospheric circulation distributes heat.

How cloud cover affects temperature

“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,” noted Professor Shields.

“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.”

The slow-rotating Kepler-62 exoplanet develops a thick layer of clouds on its day side. These clouds reflect a significant portion of incoming radiation, causing the planet to cool down more than expected.

In contrast, the white dwarf exoplanet’s rapid rotation prevents the accumulation of extensive cloud cover. Without this reflective shield, the planet retains more heat, making it warmer and potentially more habitable.

Heat retention and cloud formation

“The planet orbiting Kepler-62 has so much cloud cover that it cools off too much, sacrificing precious habitable surface area in the process. On the other hand, the planet orbiting the white dwarf is rotating so fast that it never has time to build up nearly as much cloud cover on its dayside, so it retains more heat, and that works in its favor,” explained Professor Shields.

These insights suggest that white dwarf planets may have a natural advantage in maintaining surface temperatures that support liquid water.

While cloud cover on the Kepler-62 exoplanet reduces its habitable surface, the minimal clouds on the white dwarf planet create a more balanced climate.

New perspective on white dwarf habitability

Scientists have long assumed that planets orbiting white dwarfs would be too cold to sustain life. However, this study challenges that belief. Fewer clouds on the day side and an enhanced greenhouse effect on the night side keep white dwarf planets warmer than expected.

“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,” said Professor Shields.

“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.”

Future of exoplanet research

The discovery that white dwarf exoplanets could maintain habitable conditions opens new doors for future research.

With advanced telescopes like the James Webb Space Telescope, scientists can now analyze the atmospheres of exoplanets in ways that were previously impossible. This could lead to the identification of rocky planets orbiting white dwarfs with atmospheres that support life.

This research, funded by the National Science Foundation (NSF) and the National Center for Atmospheric Research, involved collaboration with experts from multiple institutions, including the University of Colorado Boulder, the University of Washington, and the University of Warwick.

The findings highlight the importance of considering a wider range of stellar environments in the search for life beyond Earth.

As astronomers continue to refine their understanding of exoplanet climates, white dwarf systems may become key targets in the search for potentially habitable worlds. The universe could be home to many more life-supporting planets than previously imagined.

The study is published in The Astrophysical Journal.

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