10 billion Milky Way stars might have habitable exoplanets after all todayheadline

Astronomers have identified a surprising feature of planets orbiting white dwarf stars – they could potentially maintain warmer surface conditions than previously thought, opening a new frontier in the search for habitable worlds.

A team led by Aomawa Shields at the University of California, Irvine compared the potential climates of planets orbiting two different types of stars using sophisticated 3D climate models. Their findings, published in The Astrophysical Journal, challenge conventional wisdom about the habitability of planets around white dwarfs – the dense remnant cores of stars that have exhausted their nuclear fuel.

“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,” said Shields.

The research team’s computer models revealed that a hypothetical Earth-like planet orbiting a white dwarf would have a global mean surface temperature approximately 25 degrees Kelvin higher than an equivalent planet orbiting Kepler-62, a main-sequence star with similar effective temperature.

This temperature difference stems primarily from the planets’ rotation rates. White dwarf stars’ habitable zones – regions where planets could support liquid water – lie extremely close to their stars. This proximity results in a much faster rotation period of about 10 hours for white dwarf planets, compared to 155 days for planets orbiting stars like Kepler-62.

Both types of planets would likely be “tidally locked” to their stars – with one side permanently facing the star and one side in perpetual darkness. However, the physical effects of their different rotation speeds create dramatically different climatic conditions.

“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. This reflection might benefit planets close to the inner edge of habitable zones, preventing overheating, but it can be problematic for planets in the middle of those zones.

“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.”

The researchers discovered that white dwarf planets generate strong zonal winds and unique atmospheric circulation patterns that distribute heat more effectively. These planets exhibit what scientists call a “bat rotator” pattern, with banded temperature distributions rather than the simpler hot-side/cold-side division seen on slower-rotating planets.

This study comes at a significant time for exoplanet research. Recent discoveries have identified giant planet candidates orbiting white dwarfs, and evidence of circumstellar debris disks around these stars suggests rocky planets might also exist there, either having survived their star’s red giant phase or formed from remaining stellar material afterward.

As white dwarfs cool extremely slowly, planets in their habitable zones could remain stable for up to 8 billion years – nearly twice Earth’s current age. Their warmer surface conditions might provide additional compensation for their stars’ ever-diminishing light output.

“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 said.

The research has particularly exciting implications for observations using NASA’s James Webb Space Telescope (JWST). Because white dwarfs are much smaller than main-sequence stars, they create less glare, potentially making their planets easier to detect and characterize through direct imaging techniques.

“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,” Shields concluded.

The study, supported by the National Science Foundation and the National Center for Atmospheric Research, was conducted in collaboration with Eric Wolf from the University of Colorado Boulder, Eric Agol from the University of Washington, and Pier-Emmanuel Tremblay from the University of Warwick.