https://www.centauri-dreams.org/2024/12/06/can-life-emerge-around-a-white-dwarf/ Centauri Dreams * Home * About * Administrative * Booklist * Contact Select Page [ ] Can Life Emerge around a White Dwarf? by Paul Gilster | Dec 6, 2024 | Astrobiology and SETI | 11 comments 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. [white-dwarf] 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). [tzf_img_post] 11 Comments 1. Geoffrey Hillend on December 6, 2024 at 16:18 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. Reply + Paul Gilster on December 6, 2024 at 20:50 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. Reply o Juraj on December 7, 2024 at 6:04 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. Reply + Mike Serfas on December 7, 2024 at 7:51 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. Reply 2. Ron S. on December 6, 2024 at 17:42 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. Reply + Michael on December 7, 2024 at 0:53 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. Reply 3. Michael on December 6, 2024 at 20:39 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. Reply 4. Michael on December 7, 2024 at 0:57 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. Reply 5. Oren on December 7, 2024 at 6:32 Read "The Integral Trees" and "Smoke Ring" by Larry Niven. Enjoy. Reply 6. Robin Datta on December 7, 2024 at 6:50 Don't know about white dwarves, but sure can on a neutron star - in the mind of Robert Forward. Reply 7. Arun Prasaad on December 7, 2024 at 7:05 Do white dwarfs retain the original star's magnetic field? If not, would cosmic rays not be problematic for any potential life there? Reply Submit a Comment Cancel reply Your email address will not be published. Required fields are marked * [ ] [ ] [ ] [ ] [ ] [ ] [ ] Comment * [ ] Name * [ ] Email * [ ] Website [ ] [ ] Save my name, email, and website in this browser for the next time I comment. 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