My curiosity about white dwarfs continues to be piqued by the occasional journal article, like a recent study from Caldon Whyte and colleagues reviewing the possibilities for living worlds around such stars. Aimed at Astrophysical Journal Letters, the paper takes note of the expansion of the search space from stars like the Sun (i.e., G-class) in early thinking about astrobiology to red dwarfs and even the smaller and cooler brown dwarf categories. Taking us into white dwarf territory is exciting indeed.
How lucky to live in a time when our technologies are evolving fast enough to start producing answers. I sometimes imagine what it would have been like to have been around in the great age of ocean discovery, when a European port might witness the arrival of a crew with wondrous tales of places that as yet were on no maps. Today, with Earth-based instruments like the Extremely Large Telescope, the Giant Magellan Telescope and the Vera Rubin Observatory, we can expect data from near-ultraviolet to mid-infrared to complement what space telescopes like the Nancy Grace Roman instrument, not to mention the Habitable Worlds Telescope, will eventually tell us.
These instruments aren’t that far from completion, so the process of prioritizing stellar systems is crucial, given that we have thousands of exoplanets to deal with. Whyte and team (Florida Institute of Technology), working with Paola Pinilla (University College, London) point out that while we’ve neglected white dwarfs in terms of astrobiology, there are reasons for reassessing the category. Sure, a white dwarf is a stellar remnant, having blown off mass in its red giant phase and disrupted any existing planetary system. But around many white dwarfs circumstellar material may allow the formation of new planets, as is illustrated in the image below. And if such planets form, the possibilities for life are intriguing.
Image: In this illustration, an asteroid (bottom left) breaks apart under the powerful gravity of LSPM J0207+3331, the oldest, coldest white dwarf known to be surrounded by a ring of dusty debris. Scientists think the system’s infrared signal is best explained by two distinct rings composed of dust supplied by crumbling asteroids. Credit: NASA’s Goddard Space Flight Center/Scott Wiessinger.
After all, a white dwarf offers a long lifetime for biological life to emerge. Based on studies using luminosity to estimate star lifetimes, it appears that almost all the white dwarfs in the 0.6 solar mass range (typical for the category, although lower-mass dwarfs have been observed) are less than 10 billion years old. As long as accretion keeps their mass below the Chandrasekhar limit (around ~1.4 solar masses), they’re stabilized by electron degeneracy pressure and continue cooling for billions of years. Interestingly, 97 percent of all stars in the Milky Way will become white dwarfs. I’m drawing that figure, which surprised me, from a 2001 paper called “The Potential of White Dwarf Cosmochronology” (citation below), but I see Whyte and team use the same number. The 2001 paper is by Fontaine, Brassard and Bergeron, from which this:
…white dwarf stars represent the most common endpoint of stellar evolution. It is believed that over 97% of the stars in the Galaxy will eventually end up as white dwarfs. The defining characteristic of these objects is the fact that their mass is typically of the order of half that of the Sun, while their size is more akin to that of a planet. Their compact nature gives rise to large average densities, large surface gravities, and low luminosities.
In the near future, I want to dig into this further. Because red dwarfs, 80 percent of the galaxy, live a long, long time. How much do we really know about what their ultimate fate will be? Looking around in this epoch, all we see are red dwarfs in their comparative infancy.
But back to dwarfs of the white variety. Remember how small these stars are. 0.6 solar masses corresponds to a radius of 1.36 times that of Earth. We get terrific transit depth, but as always the odds favor detecting larger planets. The low luminosity of a white dwarf means the habitable zone is going to be close to the star, but we know little about how an existing stellar system fares as its star undergoes the red giant phase. Would planets be cleared out of this region?
Observations by our latest instruments will better inform us about the stellar dynamics at work here. But let me quote the paper for the good news about habitable zones:
…a major challenge to planetary habitability for such a system is the inward migration of the habitable zone. After the white dwarf is formed, it rapidly cools until it is between 2 and 4 Gyr old, where the cooling slows down…, remaining stable until it attains an age of at least 8 Gyr. This creates a range of orbital distances where there are billions of years between the time the planet would enter the habitable zone and when it would leave it, which we refer to as the habitable lifetime. Another promising result from this habitable zone is that its width seems to remain constant as the star ages until the end of its habitable lifetime, which is understood to manifest when it is cut-off by the Roche limit. This period of stability creates a window of time for life to begin and evolve that is very similar in duration to that of Earth’s own habitability interval.
The authors consider the maximum habitable lifetime for a white dwarf to be about 7 billion years. Assume that it takes a billion years for life to emerge, then the paper’s numbers show that an Earth-class planet with a constant orbital distance between ∼ 0.006 AU and ∼ 0.06 AU would remain in the habitable zone and offer conditions that would support liquid water on the surface. The former figure is the Roche limit, which sets the limit on how close a planet can orbit a star without being torn apart by tidal forces. Get too close and a planet breaks into what we will see as a debris disk. Contrast this 7 billion years with the emergence of life on Earth at 0.8 billion years, and the the appearance of a tool-using technological species some 3.7 billion years after that.
Plenty of time exists, in other words, for interesting things to happen. We also have to factor in the presence of an atmosphere, whose constitution will vary the inner and outer limits of the habitable zone. Remember that we are dealing with a star that is gradually cooling, so factors that lead to more warming will be beneficial. The problem at this point is that the specifics of a white dwarf’s effects upon a planetary atmosphere are not well understood, and I’m not sure I’ve even seen them modeled.
The James Webb Space Telescope should help in expanding our knowledge of these factors, and may also explain why so few planets have been found around white dwarfs thus far. Earlier work has indicated that the transit probabilities for an Earth-class exoplanet around a white dwarf are a slim 0.006, further compromised by the size and luminosity of white dwarfs. Planets in the tight habitable zone of such a star would surely have been destroyed during the red giant phase, but studies of white dwarf atmospheric pollution are encouraging. They indicate that the formation of new close-in planets out of a circumstellar debris disk is a distinct possibility.
This is an exciting paper that paints a serious upside for habitability around a kind of star that has seldom been examined in this context. Thus the conclusion:
The effective temperature of a typical white dwarf during its habitable lifetime provides conditions for abiogenesis and photosynthesis similar to those seen on Earth. Moreover, the habitable lifetime could be slightly longer than the Sun’s at the ideal orbital distance of 0.012 AU. With a long habitable lifetime and overlap in PAR [photosynthetically active radiation] and abiogenesis zones white dwarfs are an ideal host for the origin and advanced evolution of life.
The paper is Whyte et al., “Potential for life to exist and be detected on Earth-like planets orbiting white dwarfs,” accepted at Astrophysical Journal Letters (preprint). The 2001 paper cited within the text is Fontaine, Brassard & Bergeron, “The Potential of White Dwarf Cosmochronology,” Publications of the Astronomical Society of the Pacific Vol. 113, No. 782 (2001), 409 (full text). See also Saumon et al. for a more recent look at the question: “Current challenges in the physics of white dwarf stars,” Physics Reports Volume 988 (19 November 2022), 1-63 (abstract).
Yes the tidal forces would be a problem with the Roche limit and Hill Sphere. The planet might break apart in the life belt around a white dwarf star or crash into the white dwarf. The planet has to be too close to the white dwarf to be in the life belt.
Actually, the authors calculate that the habitable zone extends well beyond the Roche limit. Quoting myself:
“the paper’s numbers show that an Earth-class planet with a constant orbital distance between ∼ 0.006 AU and ∼ 0.06 AU would remain in the habitable zone and offer conditions that would support liquid water on the surface. The former figure is the Roche limit, which sets the limit on how close a planet can orbit a star without being torn apart by tidal forces. Get too close and a planet breaks into what we will see as a debris disk.”
In other words, the Roche limit is at about 0.006 AU. The habitable zone extends out to 0.06 AU. There is definitely room there for a planet if one can form, and it could remain habitable.
Still, there might be too much tidal stress even beyond Roche limit. The planet will end up like Io which orbits about 2x the Roche distance IIRC.
I can see a planet surviving just outside the Roche limit. It would be tidally locked, but it might have to migrate there since the gases that are “burped’ off or expelled from the red giant are mostly hydrogen and helium which can’t form rocky planets. The main problem with white dwarfs is the migrating habitable zone since they cool off pretty quickly and therefore dim fast over one billion years. The life belt around a white dwarf won’t remain stable long enough for life to form their or get restarted on a migrated planet. Some life might have survived on a migrated planet if it migrated far enough from the star.
The authors of the paper disagree. They calculate a 7 billion year lifetime for a planet in the habitable zone in some situations. Have a look at the paper — it’s on arXiv.
I think they are right about that the life belt might be stable based on the light from a white dwarf taking several billion years to dim, but the atmospheric physics or planetology is against it considering that it is not just about temperature, but also atmosphere. Without an atmosphere life can’t evolve. If the world migrated from far enough away and was large enough and kept it’s atmosphere, so life might evolve around a white dwarf star. It would be limited to microbial life.
These tidal forces are so remarkable that I wonder if even a small libration of the planet would significantly affect its ability to develop life, either positively by powering geothermal processes or negatively by overly disrupting the shorelines.
The paper doesn’t appear to address the composition of those post-nova planets. Existing planets out to at least Mars orbit would not survive. First, the atmosphere and other surface volatiles would be driven off by heat and the stellar wind, then they will spiral inward and be consumed as the red giant’s radius expands outward. Any volatiles that survive will be in a gaseous form and probably blown out of the system.
The remaining material from which debris might coalesce into planets will be dry: no water and no atmosphere, most likely. Regardless of the stability of a habitable zone there isn’t likely to be a habitable planet available to occupy it. The volatiles might become part of a nebula elsewhere and become material for an entirely new stellar system birth, such as ours.
If we look at our solar system as an analogy the Kuiper belt and Orit cloud would not be greatly affected by the luminosity or material outflow in the red giant phase as its so cold out there. This outflow of material could perhaps destabilise objects out there bringing them inwards to form new planets but cant see them being very big.
The “spiral inward” process only works for planets close to the star before it becomes a white dwarf (e.g., Mercury, Venus, maybe Earth), because the expansion of the star in the red giant phase brings those planets within the surface of the star.
The subsequent mass loss (“outflow”) in the red-giant-to-white-dwarf transition would cause Kuiper Belt and Oort Cloud objects to move to *larger* radii, because the mass of the star they are orbiting becomes smaller.
“The “spiral inward” process only works for planets close to the star before it becomes a white dwarf (e.g., Mercury, Venus, maybe Earth), because the expansion of the star in the red giant phase brings those planets within the surface of the star.”
Yes, that’s what I said.
“…cause Kuiper Belt and Oort Cloud objects to move to *larger* radii, because the mass of the star they are orbiting becomes smaller.”
Maybe (and I know this has been simulated), however the interaction of light and fluffy Oort and perhaps also KBO (or their equivalent in a different stellar system!) with the large mass outflows from the star would appear to be less than certain. Also, their orbital velocities are such that their trajectories will becomes hyperbolic and will no longer be in orbit around the stellar remnant.
White dwarfs emit a lot in UV which is very destructive to organic molecules including life. However that UV breaks down water to form hydrogen and oxygen so atmospheres of oxygen are possible which could accelerate life’s development.
Interestingly if a planet did form near the white dwarf hab zone it would be very close to the gravity focal line enabling any smart alien species that evolved on it to use it as a very powerful telescope indeed.
Read “The Integral Trees” and “Smoke Ring” by Larry Niven. Enjoy.
Don’t know about white dwarves, but sure can on a neutron star – in the mind of Robert Forward.
Do white dwarfs retain the original star’s magnetic field? If not, would cosmic rays not be problematic for any potential life there?
Jupiter has a powerful magnetic field and the environment is dangerous around it, white dwarf magnetic fields can be on another level making them extremely dangerous !
Jupiter is dangerous because of the radiation belts in its magnetic field. There’s no guarantee white dwarfs would have the same, they’re not even planets.
Hi Paul
White dwarf stars are potential fusion reactors if trickle fed hydrogen, courtesy of their intense surface gravity. The trick is keeping them from imploding as gigantic thermonuclear bombs – the fusion of carbon is very temperature sensitive and can runaway rapidly. The risk is greatest within a few percent of the Chandrasekhar Mass of 1.44 solar masses.
With that in mind, say we start with a 0.5 solar mass white-dwarf. How long can it radiate providing steady illumination to a 0.01 AU habitable zone before it reaches a dangerous mass of 1.4 solar masses? Around 1 quadrillion years.
I suppose a powerful laser could be used to ignite a small amount of the surface, the density of carbon/oxygen on the surface are in the range for the fusion of these elements and only needs a boost to around 50 kev to go bang. Perhaps a ring around the white dwarf that fires lasers to keep it cooking.
I deleted the article about life around white dwarf stars too soon. At least I jotted down r = 0.012 AU and M = 0.6 solar mass. Even if r = 0.12 AU, you’ll see the problem.
I’ll take the reciprocals and invert at the end. 83.333 times the radius to the 3/2 power = 760.27 times the period. The square root of 1.6666 = 1.291, or a year that is 981.5 time longer. Going 83.333 times the orbital distance in 760.27 the time means the velocity is reduced to 0.1096.
Invert: a planet zipping around a white dwarf in less than 12 hours moves 9.123 times faster than earth. The kinetic energy per kilo or megaton of impactor will be 83.229 times greater.
An asteroid with 1.2% of the mass which killed the dinosaurs could kill the exo-dinosaurs. Does the number of asteroids diminish in linear proportion to their mass, or to the 2/3 or 3/2 power? I forget the relationship.
The point: there will be an extinction level impact not once in every 200 million Earth years of so, but once every 5 million years, or even more often. Mass extinction after mass extinction!
When I started this calculation, I did not want to sound like a fan of the Rare Earth Hypothesis. Like it or not, did I reason correctly?
Orbital velocities of a planet in the hab zone around a white dwarf would be in the hundreds of kilometres per second range, small rocks become big atomic bombs !
The potential energy GM/r will indeed be 83.333 times greater with a radius of 0.012 AU, or 50 times greater if we count the 0.6 solar masses. So I expect the kinetic energy of the orbiting planet will be correspondingly greater, as will the kinetic energy of an infalling body. The relationship I found at https://geology.com/articles/near-earth-asteroids.shtml is that a 10^8 difference in frequency corresponds to a 10^10 difference in energy (and asteroid mass), so a 50-fold increase in energy would equate to a 23-fold increase in frequency. But I don’t know if that power law relationship has any relevance at all to a white dwarf system. It seems like it could be impossible to find out if it does without a spaceship handy… but astronomers seem to wring a magical amount of data out of their pixel of interest, and I suppose the collisions will, at least, be bright. :)
Careful with some wording! The author states the following as fact: “Interestingly, 97 percent of all stars in the Milky Way __will__ become white dwarfs.” (Emphasis mine). However, the paper he cites only says: “It is __believed__ that over 97% of the stars in the Galaxy will eventually end up as white dwarfs.” (Emphasis mine again). The author states as fact something that sounds much more like conjecture. Very well studied, and possibly backed up by actual statistics, but nonetheless this author essentially creates a fact where none was intended.
My reading of that is that this wording reflects the discussion of the long lives of red dwarf stars, whose main sequence lifetime exceeds the current age of the universe. So this evolution from red dwarf to white dwarf has not been, and could not have been, observed. The paper’s authors write a little more cautiously than this article does.
In principle no one can say with absolute certainty anything about the future… A speed of light phase change in the universe may reach us before the next sunrise after all! Anyone who says “X will happen” in physics is implicitly prefixing “According to out best current understanding”.
A follow-up: I double checked. Yes, for Zero Age Main Sequence stars, the luminosity increase as the mass to the 3.5 power.
Without crunching any numbers, that means an Earth 2.0 around an F9 star orbits slightly slower than our Earth travels. Earth 1.0 orbits faster than it would around a G9 or K8 star. As for M stars, any planet in their inhabitable zone might as well orbit a white dwarf when it come to frequent killer asteroids. More bad news :(
Sorry!
Mike
Did you mean slower?
It is also quite easy to calculate orbital velocity if given the orbital radius and period. I did this for the Trappist 1 planets in the HZ as a quick check to confirm your general statement that orbital velocity must increase in the HZ as the star mass decreases – as long as the mass: luminosity relationship holds.