Over 10 billion stars in our Milky Way galaxy can support planets with life

Stellar corpses are supposed to cool, fade, and mind their own business. Yet a new climate‑modeling study hints that some of these faint objects – white dwarfs – could keep nearby planets toasty enough for liquid water to exist.

That possibility expands the hunting ground for habitable worlds by billions of targets and challenges the old idea that life can flourish only around stars that are very much alive.

Today’s tools make that hunt more than wishful thinking. The James Webb Space Telescope (JWST) has already glimpsed gas‑giant candidates circling white dwarfs, proving that planets can survive their star’s violent endgame.

With Webb’s infrared eyes, astronomers can now probe rocky worlds the size of Earth. That capability sets the stage for fresh computer experiments comparing two very different star systems.

White dwarf corpses don’t chill out

After a Sun‑like star sheds its outer layers, its core contracts into a white dwarf that is roughly as big as Earth but packs about half the Sun’s mass.

Current models suggest the Milky Way hosts around 10 billion of these embers, and more than 97 percent of all stars will finish their lives the same way.

Aomawa Shields of the University of California, Irvine, and colleagues wondered whether such dim stars could light the way for life.

“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.”

Two‑sun water world

The team used a three‑dimensional climate model that is normally used for Earth’s environment to compare the climates of two a hypothetical ocean‑covered planets with Earth‑like atmospheres. The two exoplanets were modeled to be in orbit around two different stars.

One exoplanet was placed in the so‑called habitable zone of a hypothetical 5,000 K white dwarf that has passed through most of its lifecycle and is on the slow path to stellar death.

For comparison, they placed the other exoplanet in the habitable zone around the main-sequence star known as Kepler‑62. This is a K-dwarf star that has an equivalent temperature to the white dwarf, but is still actively fusing hydrogen in its core. Kepler-62 is in a similar life stage as our Sun.

Both modeled planets were assumed to be tidally locked, with one hemisphere fixed in daylight and the other in eternal night. That configuration is common for worlds that hug close to small stars.

Despite receiving the same amount of starlight overall, the climates of the two test exoplanets diverged in a surprising way.

Speed keeps white dwarfs warm

The white‑dwarf planet zips around its star every 10 hours. That tight orbit forces a 10‑hour day – much faster than Earth’s orbit, and far quicker than the 155‑day slog of its Kepler‑62 counterpart.

Rapid rotation whips up powerful winds that whisk heat from the day side to the night side and back again. Those winds also shear away the thick, reflective clouds that tend to gather over a slow‑spinning planet’s hot hemisphere.

As a result, the white‑dwarf world ran about 25 °C warmer at its surface than the planet orbiting Kepler‑62.

“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 noted.

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.

Clouds, winds, extra shoreline

The model showed that fewer dayside liquid‑water clouds let more sunlight stream down, while stronger greenhouse gases on the nightside trapped outgoing heat.

“The planet orbiting Kepler‑62 has so much cloud cover that it cools off too much, sacrificing precious habitable surface area in the process,” Shields continued.

“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.”

Taken together, the temperature bump and the efficient heat circulation spread balmy conditions over a larger fraction of the white‑dwarf planet’s globe.

That result matters because real estate near the terminator – the ring of perpetual sunset between day and night – often offers the best prospects for life on tidally locked worlds.

Wider playground for planet hunters

Planets in orbit around dead stars are no longer hypothetical. In 2020, astronomers spotted WD 1856+534 b, a Jupiter‑sized object that circles its white dwarf host every 34 hours without being torn apart.

In 2024, the Webb telescope directly detected two more gas‑giant candidates orbiting separate white dwarfs, demonstrating Webb’s knack for sniffing out faint planets against dim stellar backgrounds.

“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.

As powerful observational capabilities to assess exoplanet atmospheres and astrobiology have come on line, such as those associated with the Webb telescope, we could be entering a new phase in which we’re studying an entirely new class of worlds around previously unconsidered stars.

What happens next?

Every white dwarf cools for billions of years, granting any close‑in planet a stable light source for far longer than the Sun’s remaining lifespan.

With ten billion of these stars scattered through our Milky Way galaxy, even a tiny fraction hosting temperate rocky worlds would translate into a staggering number of potential abodes.

The next step is clear: point Webb and its successors at the brightest white dwarfs, catch a transit, and read the atmospheric fingerprints. If the models are right, the ghostly glow of a dying star could someday soon reveal a living planet.

The full study was published in The Astrophysical Journal.

—–

Like what you read? Subscribe to our newsletter for engaging articles, exclusive content, and the latest updates.

Check us out on EarthSnap, a free app brought to you by Eric Ralls and Earth.com.

—–