If we ever find life on a planet orbiting a white dwarf star, it will be life that has emerged only after the red giant phase has passed and the white dwarf has emerged as a stellar relic. That’s the conclusion of a study being discussed today at the National Astronomy Meeting of Britain’s Royal Astronomical Society, which convened online due to COVID concerns. The work is also recently published in Monthly Notices of the Royal Astronomical Society.
At issue is the damage caused by powerful stellar winds that occur as a star makes the transition from red giant to white dwarf stage. This is the scenario that awaits our own Sun, which should swell to red giant status in roughly five billion years, eventually becoming a dense white dwarf about the size of the Earth. We’ve speculated in these pages about life surviving this phase of stellar evolution, but the study, in the hands of Dimitri Veras (Warwick University) concludes that this is all but impossible.
We know that the Earth is protected by a magnetosphere that thwarts atmospheric depletion by channeling harmful particles along magnetic field lines. You would think that a magnetosphere would ease this kind of erosion in the far future for those planets that have one (Mars, for example, does not) but the stellar winds of the evolving star will be far stronger than the Sun’s today. The authors modeled the winds from eleven different kinds of stars in a range of masses. They find this:
The plot shows that an exo-Jovian analogue would just reach the threshold for hosting a magnetopause at some point during giant branch evolution. However, much higher fields would be required to maintain any magnetopause throughout these giant branch phases. For terrestrial and potentially habitable planets, any protection previously afforded by the magnetosphere would effectively disappear. This lack of protection, compounded with orbital expansion and varying stellar luminosities, suggest that life would be challenged to survive throughout the giant branch phases of stellar evolution.
Could scenarios emerge in which moons around the gas giants maintain life under an ice crust? It’s hard to see how. Veras, working with Aline Vidotto (Trinity College, Dublin) points out that a habitable zone supporting liquid water would move from some 150 million kilometers from the Sun to up to 6 billion kilometers, pushing it beyond the orbit of Neptune. Planets can migrate during this phase, but the paper argues that the habitable zone moves outward faster than the planet, a likely fatal threat. Thus life around a white dwarf will need to start over.
Image: An illustration of material being ejected from the Sun (left) interacting with the magnetosphere of the Earth (right). When the Sun evolves to become a red giant star, the Earth may be swallowed by our star’s atmosphere, and with a much more unstable solar wind, even the resilient and protective magnetospheres of the giant outer planets may be stripped away. MSFC / NASA. Licence type Attribution (CC BY 4.0).
Thus the movement of the habitable zone outward and the difficulty in maintaining a magnetosphere throughout this phase of stellar evolution make preserving habitability extremely unlikely. The authors’ model shows that the strong stellar wind combines with the expanding orbits of surviving planets to first shrink and then expand the magnetosphere of a planet over time. It would take a magnetic field 100 times stronger than Jupiter’s to maintain a stable magnetosphere all the way through the transition of red giant to white dwarf:
“We find that a planetary magnetosphere will always be quashed at some point during the giant branch phases, unless the planet’s magnetic field strength is at least two orders of magnitude higher than Jupiter’s current value.”
And afterwards? White dwarfs do not emit stellar winds, so that threat disappears. Any life we find around a white dwarf will doubtless have developed during the white dwarf phase. If such exists, we may be able to detect its biomarkers through future space missions — recall that white dwarfs are roughly the size of the Earth, and a transiting planet would produce profound transit depth and would seemingly be an ideal target for transmission spectroscopy, in which we analyze the components of a planetary atmosphere as starlight passes through it.
Most of the exoplanets we know about orbit main sequence stars, but about 100 are known to orbit red giants, and at least four have been found orbiting white dwarf stars. These worlds are survivors of stellar evolution and thus useful as benchmarks in tracing the lifetime of their systems. Two of the white dwarf planets, says Veras, are close to their star’s habitable zone, an indication of planet migration showing that an Earth-sized planet could exist in such an orbit. And he adds:
“These examples show that giant planets can approach very close to the habitable zone. The habitable zone for a white dwarf is very close to the star because they emit much less light than a Sun-like star. However, white dwarfs are also very steady stars as they have no winds. A planet that’s parked in the white dwarf habitable zone could remain there for billions of years, allowing time for life to develop provided that the conditions are suitable.”
The paper is Veras & Vidotto, “Planetary magnetosphere evolution around post-main-sequence stars,” Monthly Notices of the Royal Astronomical Society Vol. 506, Issue 2 (September 2021), pp. 1697-1703. Abstract / Preprint.
Mmmm UV channeling along magnetic fields….Paul only charged particles can do that.
I was about to say, but you beat me to it. =)
The Ozone layer is our best friend to stop the worst part of the UV, the magnetic field divert charged particles and send some spinning into the VanAllen belts while some others loose energy when they hit the highest atmosphere and is seen as borealis light.
Yes, a gaffe now fixed. Thanks for catching it.
UV and magnetic fields…
I wonder if this concerns Cherenkov radiation?
Charged particles accelerated in a magnetic field give off
radiation – and UV is the frequent example.
I think it is more correct to say the charged particles undergo photo emission while in the magnetic field and slow down due to energy emission.
Two observations on surviving the red giant era:
On the plus side for life, this paper does not address (nor was it meant to) lithospheric life, much of which is already highly heat resistant and protected by kilometers of rock. Yes, not helpful for life in rocks on Earth, but on Triton or Pluto?
On the downside also it doesn’t address cooking during the helium flash. A brief cooking, but I wonder if it may melt a rocky world – or half-rocky world – to magma, which makes the issue of solar radiation rather minor.
Would the habitability outcome be any different if no particle winds were emitted by a star? The red giant phase would swallow up and fry the planets within the current HZ, and these planets would in turn freeze at the white dwarf stage. The result would be the same – no life unless it reappeared with a new genesis.
In turn, how many astrobiologists will be turning their instruments towards white dwarf stars to look for biosignatures of planets in the current HZ?
While the red giant stage of our star is long in our future, we also think that the slow increase in luminosity will make Earth uninhabitable on the surface billions of years earlier, condemning almost a 100% extinction rate for complex life.
Assuming humanity in some form is still around in 10 millennia, and that our civilization[s] have managed to retain a high degree of technological capability, we will know if life is common or not in the galaxy. If it proves rare or even absent, then we should prepare to preserve life as best we can, with arks and seed ships to do as much as we can to preserve and spread terrestrial life across the stars. Even maintaining life in artificial worlds within the solar system would be worthwhile.
In all likelihood, I suspect we will have started this process well within the next millennium even if our vessels remain incapable of travelling more than a tiny fraction of light speed. Maybe like the 2 remaining robot servitors in Silent Running maintaining the last living dome around Saturn, our robot descendants will tirelessly manage the process for a post human future.
I suspect that if life does develop around a main sequence star, that after several billion years, when the star enters its red giant phase, the star may have more to fear from that evolved life than vice versa. That is, when it becomes overly problematic it’ll be dismantled or adjusted by any local advanced civilization that happened to develop in its vicinity in a bid to protect more primitive ecologies or the local infrastructure they’ve built. Or they’ll take a vacation and monitor the process from afar.
Would stars that have been “adjusted” be distinguishable from natural ones? If yes (however unlikely) it would we something we could look for…
There is also the possibility that life on a WD planet could indicate that some civilization took an active role in preserving that life somehow.
If it’s life you’re talking about, then the so-called “habitable zone” is an outdated concept, as John Gertz makes clear in his latest JBIS paper (July 2021; see p.261). A fair fraction of the life of a planet like our own is microbial and subterranean, and I do not see any arguments above as to why the stellar winds during the red giant phase should inconvenience such life. (Plus, I’m not so sure about the claim that the magnetosphere thwarts ultraviolet radiation!)
Re: the last quote – it’s obvious that Earth is a “Goldilocks” world, its environment almost literally threading between extremes that would prevent life from arising. This will be true of any habitable world, but assuming that any such world must follow the same path as Earth to reach that state goes against the Copernican principle.
In theory, an advanced civilization might choose to use technology to live completely underground in such a scenario. Of course if they are that intelligent they would probably just leave. But maybe they are xenophobic.
I also agree the concept of a habitual zone should include technologically assisted living as a separate consideration different from the question of life being able to originate on some world. Any star with an Oort Cloud should be techno-habitable given O’Neill space colonies and that’s just with our current level of technology.
The Earth might also be tidally locked by the time it reaches the red giant phase in 5 billion years, so there will be no rotation and therefore no magnetic field. Will there be any atmosphere left after that time? Probably most of it will have completely escaped due to the high surface temperature, but maybe there we be some traces left enough to appear in spectra, a thin atmosphere.
A look up in Google says the Moon won’t be tidally locked to the Earth for 50 billion years, so Earth’s magnetic field will be compressed by the solar wind.
“Can Life Survive a Star’s Red Giant Phase?”
Depends on what we call “life”. A rather extreme speculative example is in Robert L. Forward’s “Dragon’s Egg”.
The novel “Titan” by Stephen Baxter describes the development of life including intelligent life on the Saturnian moon Titan during the red gian phase of solar evolution. I don’t recall if that life was from panspermia resulting from a human expedition billions of years earlier. In an event, two humans from that expedition were resurrected. The novel was well-written, full of flawed characters and depressing. The Titanians were, on the other hand, less crazy.
Obviously the planetary engineers will have wrapped Earth in an AirShell (TM) to avoid atmosphere loss and reflect most of the heat away during the RGB phase. They might also cause enough solar mass loss to “turn off” the RGB phase solar swelling even if they can’t avoid the solar core’s collapse into the Helium Main Sequence. During the enhanced mass loss, they might maneuver Earth into Jupiter’s orbit to enjoy the Equitable climate during the Helium Main Sequence, assuming Jupiter is still in residence…
Perhaps as the Sun heats up any low mass moons will be blown out into the Oort cloud of comets carrying microbes and then later they come in to seed life on potentially habitable worlds.
A smaller gas giant might lose a moon that might orbit the dwarf…ice melting and freeing things up?
So far the discussion centers on single red giants, but a significant number of white dwarfs exist in the solar neighborhood residing in binary systems, the Dog Stars being two examples. Their presence makes the habitable zones for the remaining main sequence stars dynamically unstable would likely have fried them much as described. Possibly other systems have
greater mean separation. If so, then the survival issue could be resolved by transferring to another system on order of 100 AUs away.
But in addition to the traditional HR diagram issues of mass, luminosity and HZ standoff distance, over time there is considerable mass loss or transfer. Procyon and Sirius could do some explaining on why we have F and A stars remaining after a red giant and white dwarf phase for their neighbor. Mass must have been shed to get to WD phases and some of it might have influenced the development of the neighbor stars, perhaps near contact binaries of lower mass. If for example, sentient life existed in those two systems aeons ago, the events since must have really rocked their boats and I suspect that they abandoned the systems
entirely for better camping grounds – if they could.
Up to around 40% of the Suns mass will be ejected into space as it turns into a White Dwarf, that’s over a 100 000 Earth masses of material, H, He, He3, N and Carbon. An intelligent species could use magnetic fields and grab a lot of that to form habitats, worlds or a few red dwarfs and ancient aliens could be doing this in space somewhere, Now imagine the view of the Nebula from these colonies !
The Helix Nebula looks to have kicked off over 5 solar masses of material from its original 6.5 solar start leaving a 100L / 120 000K white dwarf behind.
My main cripe with WD habitat zones is the very high orbital velocity of hundreds of km/s. A head on impact with an object coming the the other way, however rare, would do vastly more damage than our orbit velocity contribution.
Unless the whole planet is engulfed and ablated away, or turned into a lava ball from ultra-intense penetrating radiation (neutrinos?) at some point, lithospheric life would do just fine. Just a few dozen km down, the temperature and other conditions are determined only by the internal geology of the planet and wouldn’t be affected by solar winds or UV radiation at all. Others here have noted this, I am not sure if the authors did. Lithospheric life does exist, and it would reseed surface life as soon as conditions become favorable again. So the conclusion that life has to start from scratch is not supported, really.
White dwarfs usually have very strong magnetic field. Wouldn’t it lead to similar radiation as around Jupiter, making surface life impossible?
The four observed planets orbiting WD stars could have been
captured or be artifcial rather than survivors of their star’s RG phases ..