An astronomer explains the case for all times round white dwarfs

The solar will sometime die. This may occur when it runs out of hydrogen gas in its core and may now not produce power through nuclear fusion because it does now. The demise of the solar is commonly regarded as the top of the photo voltaic system. However in actuality, it could be the start of a brand new part of life for all of the objects residing within the photo voltaic system.

When stars just like the solar die, they undergo a part of speedy enlargement referred to as the Red Giant part: The radius of the star will get greater, and its coloration will get redder. As soon as the gravity on the star’s floor is now not sturdy sufficient for it to carry on to its outer layers, a big fraction—as much as about half—of its mass escapes into area, abandoning a remnant referred to as a white dwarf.

I’m a professor of astronomy on the College of Wisconsin-Madison. In 2020, my colleagues and I discovered the first intact planet orbiting round a white dwarf. Since then, I have been fascinated by the prospect of life on planets round these, tiny, dense white dwarfs.

Researchers seek for indicators of life within the universe by ready till a planet passes between a star and their telescope’s line of sight. With mild from the star illuminating the planet from behind, they will use some easy physics rules to determine the forms of molecules current within the planet’s environment.

In 2020, researchers realized they might use this system for planets orbiting white dwarfs. If such a planet had molecules created by residing organisms in its environment, the James Webb House Telescope would in all probability be capable to spot them when the planet handed in entrance of its star.

In June 2025, I published a paper answering a query that first began bothering me in 2021: Might an ocean—possible wanted to maintain life—even survive on a planet orbiting near a lifeless star?

A universe stuffed with white dwarfs

A white dwarf has about half the mass of the solar, however that mass is compressed right into a quantity roughly the scale of Earth, with its electrons pressed as shut collectively because the legal guidelines of physics will enable. The solar has a radius 109 times the scale of Earth’s—this dimension distinction signifies that an Earth-like planet orbiting a white dwarf could possibly be about the identical dimension because the star itself.

White dwarfs are extraordinarily widespread: An estimated 10 billion of them exist in our galaxy. And since each low-mass star is destined to finally develop into a white dwarf, numerous extra have but to kind. If it seems that life can exist on planets orbiting white dwarfs, these stellar remnants may develop into promising and plentiful targets within the seek for life past Earth.

However can life even exist on a planet orbiting a white dwarf? Astronomers have known since 2011 that the habitable zone is extraordinarily near the white dwarf. This zone is the situation in a planetary system the place liquid water may exist on a planet’s floor. It will possibly’t be too near the star that the water would boil, nor so far-off that it could freeze.

The liveable zone round a white dwarf can be 10 to 100 times closer to the white dwarf than our personal liveable zone is to our solar, since white dwarfs are a lot fainter.

The problem of tidal heating

Being so near the floor of the white dwarf would deliver new challenges to rising life that extra distant planets, like Earth, don’t face. One in every of these is tidal heating.

Tidal forces—the variations in gravitational forces that objects in area exert on totally different elements of a close-by second object—deform a planet, and the friction causes the fabric being deformed to warmth up. An instance of this may be seen on Jupiter’s moon Io.

The forces of gravity exerted by Jupiter’s different moons tug on Io’s orbit, deforming its inside and heating it up, leading to a whole bunch of volcanoes erupting always throughout its floor. Consequently, no floor water can exist on Io as a result of its floor is simply too scorching.

In distinction, the adjoining moon Europa can be topic to tidal heating, however to a lesser diploma, because it’s farther from Jupiter. The warmth generated from tidal forces has induced Europa’s ice shell to partially soften, leading to a subsurface ocean.

Planets within the liveable zone of a white dwarf would have orbits shut sufficient to the star to expertise tidal heating, just like how Io and Europa are heated from their proximity to Jupiter.

This proximity itself can pose a problem to habitability. If a system has a couple of planet, tidal forces from close by planets may trigger the planet’s environment to entice warmth till it turns into hotter and warmer, making the planet too hot to have liquid water.

Enduring the crimson big part

Even when there is just one planet within the system, it could not retain its water.

Within the strategy of changing into a white dwarf, a star will broaden to 10 to 100 occasions its unique radius throughout the crimson big part. Throughout that point, something inside that expanded radius can be engulfed and destroyed. In our personal photo voltaic system, Mercury, Venus and Earth can be destroyed when the solar finally turns into a crimson big earlier than transitioning into a white dwarf.

For a planet to outlive this course of, it must begin out a lot farther from the star—maybe on the distance of Jupiter and even past.

If a planet begins out that far-off, it could have to migrate inward after the white dwarf has fashioned with a purpose to develop into liveable. Computer simulations show that this kind of migration is possible, however the course of may trigger extreme tidal heating that will boil off floor water—just like how tidal heating causes Io’s volcanism. If the migration generates sufficient warmth, then the planet may lose all its floor water by the point it lastly reaches a liveable orbit.

Nevertheless, if the migration occurs late enough within the white dwarf’s lifetime—after it has cooled and is now not a scorching, shiny, newly fashioned white dwarf—then floor water could not evaporate away.

Below the precise circumstances, planets orbiting white dwarfs may maintain liquid water and doubtlessly assist life.

Seek for life on planets orbiting white dwarfs

Astronomers have not but discovered any Earth-like, liveable exoplanets round white dwarfs. However these planets are troublesome to detect.

Conventional detection strategies just like the transit approach are much less efficient as a result of white dwarfs are a lot smaller than typical planet-hosting stars. Within the transit approach, astronomers look ahead to the dips in mild that happen when a planet passes in entrance of its host star from our line of sight. As a result of white dwarfs are so small, you would need to be very fortunate to see a planet passing in entrance of 1.

However, researchers are exploring new strategies to detect and characterize these elusive worlds utilizing superior telescopes such because the Webb telescope.

If liveable planets are discovered to exist round white dwarfs, it could considerably broaden the vary of environments the place life would possibly persist, demonstrating that planetary methods could stay viable hosts for all times even lengthy after the demise of their host star.

This text is republished from The Conversation beneath a Artistic Commons license. Learn the original article.