Encouraging News re Red Dwarf Planets

by Paul Gilster on June 28, 2010

Knowing of my fascination with small red stars, a friend recently asked why they seemed such problematic places for life. M-dwarfs are all over the galaxy, apparently accounting for 75 percent or more of all stars (I’m purposely leaving the brown dwarfs out of this, because we’re still learning about how prolific they may be). Anyway, asked my friend, is it just that a habitable planet would have to be so close to the star that it would always present the same side to it? That’s tidal lock, and it looks as if it would play havoc with any chances for a stable environment.

But maybe not. In the absence of observational evidence, we have to apply models to M-dwarf planets to see what might or might not work, and some very solid modeling out of NASA Ames back in the 1990s showed that there were ways an atmosphere could circulate so as to keep the dark side of the planet from freezing out its atmosphere. This work, by Robert Haberle and Manoj Joshi, was followed by Martin Heath (Greenwich Community College, London), who showed a viable mechanism for getting liquid water circulating between night and day sides. Tidal lock may not be a showstopper after all.

Image: The M-dwarf AD Leonis, a flare star that may offer clues to habitability. Credit: ESO Online Digitized Sky Survey.

No, in recent times the rap against M-dwarf planets has been that their stars are prone to violent convulsions that launch potentially lethal flares into their planetary systems. Many M-dwarfs produce high energy charged particles and short-wavelength radiation from X-rays to ultraviolet. All of this activity can also affect a planet’s atmosphere, so that a key question becomes whether a planet in an M-dwarf’s habitable zone can retain its atmosphere, or whether terrestrial worlds would lose hydrogen and helium and gas giants would erode into Neptune-mass cores.

My friend and I kicked this around before parting company, he returning to studies unrelated to astronomy, while I returned to my office to find a message from Adam Crowl on red dwarfs and flare activity. A new study demonstrates that red dwarf planets may be shielded from these flares after all. As is standard practice in these matters, Antigona Sugura (Universidad Nacional Autónoma de México) and team went to work with computer models, simulating how a 1985 flare from the star AD Leonis would have affected an Earth-like planet orbiting it at 0.16 AU. AD Leonis is an M-dwarf about 16 light years from Earth, and 0.16 AU, about half Mercury’s distance from the Sun, is in the zone where liquid water could exist at the surface.

The results are promising. It turns out that in the simulation, bursts of UV radiation hitting an Earth-like atmosphere produced a thicker ozone layer, protecting the surface. From the preprint:

For an oxygen-rich, Earth-like planet in the habitable zone of an active M dwarf, stellar flares do not necessarily present a problem for habitability. Much of the potentially life-damaging UV radiation goes into photolyzing ozone in the stratosphere, preventing it from reaching the planetary surface. Ozone variations cause temperature fluctuations in the upper atmosphere, but these fluctuations are small, and the climate at the surface is unaffected.

In fact, in a feature on this work in Science, Lucianne Walkowicz (UC-Berkeley), a co-author of the paper, is quoted as saying “Throughout most of the flare, the surface of our model Earth-like planet experienced no more radiation than is typical on a sunny day here on Earth.” That’s good news indeed, for AD Leonis wasn’t chosen at random. At less than 300 million years old, it’s young and energetic, one of the most active M-dwarfs known, and the same article notes that the 1985 flare studied was 1000 times as energetic as a similar flare on our Sun.

Are we out of the woods? Not exactly. The paper goes on to note further problems:

Ionizing particles emitted during a flare may be more dangerous depending on how much of the particle flux strikes the planet. The additive effects of repeated flares over the duration of the planet’s lifetime are not well understood– as M dwarfs can be active on timescales of days to weeks, the atmosphere may not return to equilibrium before another flare occurs.

So there’s still plenty to think about regarding M-dwarf planets as abodes for life. But the coronal mass ejections and dangerous flares that characterize younger M-dwarfs don’t necessarily rule out life in the system, based on this work, and it’s also true that as these stars age, they offer lengthy lifetimes of up to 100 billion years (compared to a G-class star like the Sun, whose life will be around 10 billion years) during which life processes can emerge. That long, slow maturation is often accompanied by a decrease in problematic stellar activity.

The paper is Segura et al., “The Effect of a Strong Stellar Flare on the Atmospheric Chemistry of an Earth-like Planet Orbiting an M Dwarf,” accepted by Astrobiology and available online as a preprint. And in the ‘work remaining to be done’ category, note this sentence: “…there has not yet been a detailed, dynamic model exploring the
evolution of an Earth-like atmosphere over the course of a flare.”

tzf_img_post

Zen Blade June 28, 2010 at 10:41

Regarding lifetime of a red dwarf, you say that their lives are “100 billion years”, but to my knowledge this is many times older than the estimated age of the universe.

So, two things. 1. How certain are people of this estimate
and 2. This may suggest that the most likely time for complex life to form in such a system may be some time… 20 or 40 billion years from now.

-Zen Blade

Phil June 28, 2010 at 12:41

So what about a moon??

Isn’t it possible to envision a scenario where a moon around a gas giant (ie further out from the Red Dwarf) harbors life… something like a Europa w life. Doesn’t this do away with the danger from solar Flares and the need to be tidally locked, etc, etc??

I honestly believe that the life is out there and that it’s mostly humans suffering from a lack of imagination or some kind of self importance issues who can’t envision it. I can’t wait till we find life on Titan, Europa, Triton, Enceladus, Io, etc, etc (hopefully I will be here to talk about it). To me that would prove that life can exist almost anywhere in the universe. Even around a brown dwarf or a naked planet out there by itself!!

Administrator June 28, 2010 at 13:19

Re the age of red dwarfs, Zen Blade asks how certain people are of this estimate. Good question, and one I have to leave to our resident astronomers re degree of certainty. My assumption is that Zen is right in suggesting that complex life might arise later in such systems, and they do have a lot of time to play with.

Also, the age estimates are all over the map. I cited a conservative one, but check this out from the Wikipedia:

The lower the mass of a red dwarf, the longer the lifespan. It is believed that the lifespan of these stars exceeds the expected 10 billion year lifespan of the sun by the third or fourth power of the ratio of their masses to the solar mass; thus a red dwarf with 0.1 solar mass may continue burning for 10 trillion years.

The article cites several recent articles in the literature.

Ron S June 28, 2010 at 14:43

Even if flares do kill immediately or over a somewhat extended period, I can see where this is not a negative indication for an M-dwarf planet to harbor life. This can be true whether the effect is moderate, such as from a forest fire on Earth, or a global catastrophe such as an asteroid impact.

In the case of forest fires, many ecosystems rely on them for their long term health by clearing the area of stagnant growth and even entire species, creating opportunities for new growth and species. Some plants, including some conifers, depend on fire to free their seeds from the cones lying on the forest floor. That is, they’ve adapted to periodic fires to propagate the species.

Large asteroid impacts are more extreme in that they may have eliminated 90% of species and an even larger fraction of all life more than once in Earth’s history. Again, this created opportunities for new species to evolve and lay claim to territory they had previously been excluded from. For example, mammals.

The same may be true of periodic flaring, creating opportunities for, and even promoting, evolution and ecosystem renewal. Who knows, some species may even evolve to depend on flares much as those conifers that depend on fire.

Zen Blade June 28, 2010 at 14:53

Until I hear otherwise,

I am going to be under the opinion that complex life on a planet orbiting a red dwarf is “HIGHLY UNLIKELY”… at least for the next several (to 10 or 20) billion years. I’ll check back again in a couple billion years to see if prospects have improved. :)

Michel S. June 28, 2010 at 15:01

An earth-like planet did not start out with an oxygen-rich atmosphere, though — before photosynthesizing plants drastically alter the balance, most of the oxygen is locked-up as CO2.

How would solar flares affect the atmosphere during the period in which there is insufficient O2 to be turned into O3? Would solar flare activity break down CO2 and release the oxygen?

James Jason Wentworth June 28, 2010 at 15:01

Paul, I was about to post about the multi-trillion year lifetime estimates for red dwarf stars, but I’m glad to see that you beat me to it!

Regarding the rotation of worlds in such stars’ habitable zones, Phil’s suggestion about moons of close-in Jovian-type planets (whose magnetospheres could protect their moons) is a good one. Also, what might be the rotation status of Earth-type planets whose rotation axes are parallel to–or nearly so–to their orbit planes (like Uranus’ axis)? Could they possibly avoid being tidally locked for longer periods (if not indefinitely)? Also, could a planet with retrograde rotation like Venus “resist” becoming tidally locked longer than a planet with direct (prograde) rotation?

Antonio June 28, 2010 at 15:38

Salve, a voi tutti.

Leggendo la traduzione di questo interessante articolo, mi sono posto questo quesito che porto all’attenzione dei lettori di questo interessante “blog” scientifico.

Un pianeta di tipo “terrestre”, in orbita nella zona abitabile di una “nana rossa”(o anche “nana bruna”)sarebbe in grado di avere un campo magnetico adeguato, che potesse fare da “scudo” protettivo alle tempeste solari, nonostante fosse “bloccato” dall’effetto mareale della piccola stella?

Scusate la mia ignoranza in materia, ma non so che rapporto ci sia tra l’esistenza(o meno)di un campo magnetico planetario, e il fatto di avere un pianeta che volge sempre una sola parte, rispetto alla stella presso cui orbita.

In ogni caso, l’esistenza di un campo magnetico adeguato, dovrebbe proteggere l’esistenza di una ecosfera sui pianeti(o anche grossi satelliti)che orbitano attorno a una “nana rossa”.

Voi, che ne pensate, di questa mia considerazione?

E scusate, se scrivo in lingua italiana, ma non conosco la lingua inglese…

Saluti a voi tutti, da Antonio.

Administrator June 28, 2010 at 15:42

Google Translate renders Antonio’s message this way:

——-

Hello to you all.

Reading the translation of this interesting article, I asked myself this question I bring to the attention of readers of this interesting “blog” scientific.

A planet-type land, into orbit in the habitable zone of a “red dwarf” (or “brown dwarf”) would be able to have a magnetic field properly, which could act as a “shield” protection to solar storms, despite was “locked” tidal effect of the small star?

Excuse my ignorance on the subject, but I do not know what relationship there is between the existence (or not) of a planetary magnetic field, and having a planet that always turns one party, with respect to star in orbit.

In any case, the existence of a suitable magnetic field, should secure the existence of an ecosphere on planets (or even large satellites) that orbit around a “red dwarf”.

You, who do you think of my account?

And sorry, if I write in Italian, but I do not know the English language …

Greetings to you all, from Anthony.

kurt9 June 28, 2010 at 15:59

Maybe M-star flares wipe out land life on orbiting Earth-like worlds but that ocean life would remain unaffected.

andy June 28, 2010 at 17:00

Talking of red dwarf planets, there’s recently been the discovery of a fourth planet in the Gliese 876 system, which is in a 1:2:4 multiple resonance with the two previously-known gas giant planets. The only other such configuration currently known is Io-Europa-Ganymede. Probably Gliese 876 does not contain habitable worlds, but it does seem to represent a fairly extreme case of planet formation around such stars.

Colin Weaver June 28, 2010 at 19:35

@Ron S – your post regarding life on an M-dwarf planet brings to mind the Larry Niven story “Flare Time”, which paints a vivid picture of such an ecology. In the story, the occurrence of a flare brings about a rapid response in the local lifeforms, and indeed their breeding (and some hunting behaviour) depends upon the flares. The story also supposes that the habitable world “Medea” is orbiting a red hot superjovian (we’d probably call it a Brown dwarf today), both to avoid tidal lock with the red dwarf sun and as a source of additional heat..

philw1776 June 28, 2010 at 21:03

Regarding the favorite Earthlike moon of a gas giant around an M dwarf. Think of this, the Earth IS tidally locked to its primary planet. As it orbits the primary, yes different sides get sunlight, but they also get solar tides as well as the orbit sweeps one quarter of the planet after another through the star’s tidal vector. And I don’t mean tides like here on Earth. Much closer to the primary with the resultant tidal force rising with the inverse square of solar distance. Huge tides, such that it’s unlikeky that any continents would simply be tidal mud flats with huge influx and egress of water. Not a place for a civilization to arise or colonize.

Tobias Holbrook June 29, 2010 at 15:05

“Huge tides, such that it’s unlikeky that any continents would simply be tidal mud flats with huge influx and egress of water. Not a place for a civilization to arise or colonize.”
Reminds me of one of the worlds visited in Permanence…

philw1776 June 29, 2010 at 15:59

Another way around the one face to the star tidal locking is orbital resonance. We see it here with Mercury which until the 60s astronomers “knew” it kept one face permanently towards the sun.

Imagine a system with one or more Neptune class planets next out where the HZ planet could have a number of resonances producing reasonable day/night cycles. The problem: the damn stellar tides again draging the oceans and lakes with them. Worse yet, I see internal heating as a result of the Neptune(s) influence and a wicked volcanic planet emerging. Yet another lousy place for surface dwelling animal life.

andy June 29, 2010 at 17:39

Much closer to the primary with the resultant tidal force rising with the inverse square of solar distance

Actually tidal forces go as the inverse cube of the distance, as they arise from the gradient of the gravitational field.

Tobias Holbrook June 30, 2010 at 7:59

“Worse yet, I see internal heating as a result of the Neptune(s) influence and a wicked volcanic planet emerging.”
On the other hand, it might extend the habitable zone.

Ronald July 1, 2010 at 5:17

Zen Blade: “Regarding lifetime of a red dwarf, you say that their lives are “100 billion years”, but to my knowledge this is many times older than the estimated age of the universe.”

I think you are confusing potential lifespan with age here. Of course, the first is meant here. The actual age could never be more than some 14 billion years.
I read that red dwarfs will become blue when they approach their later stages (moving off main sequence). Obviously then the universe contains no blue dwarfs yet and won’t for at least the next 100 gy (now, that would be really something, the discovery of an old red/blue dwarf).

andy July 1, 2010 at 18:34

Yes red dwarfs are predicted not to go through a giant stage, but increase in temperature and luminosity as they age. As far as I can make out, the “blue dwarf” stage might look something similar to the sdB stars, which are produced in today’s universe by mass transfer from red giant stars.

Rick in Canada July 16, 2010 at 14:20

Antonio (June 28, 2010 at 15:38) asked about magnetic fields. Actually, a gravitationally locked-planet would likely have a weaker magnetic field. Earth’s magnetic field is generated by a hot liquid core rotating due to Coriolis effect. If the Earth was permanently facing the Sun, the field would weaken considerably.

Cheers

————————————————————————————————

Caro Antonio,
Sulla tua domanda sul campo magnetico: un pianeta bloccato gravitazionalmente avrebbe probabilmente un campo magnetico più debole. Il campo magnetico terrestre è generato da un nucleo fato de liquido caldo qui é rotante a causa dell’effetto di Coriolis. Se la Terra è stata definitivamente verso il Sole, il campo indebolirebbe considerevolmente.

Saluti

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