Learning that there is flowing water on Mars encourages the belief that human missions there will have useful resources, perhaps in the form of underground aquifers that can be drawn upon not just as a survival essential but also to produce interplanetary necessities like rocket fuel. What yesterday’s NASA announcement cannot tell us, of course, is whether there is life on Mars today, though if the detected water is indeed flowing up from beneath the surface, it seems a plausible conjecture that some form of bacterial life may exist below ground, a life perhaps dating back billions of years.
I’ve speculated in these pages that we may in fact identify life around other stars — through studies of exoplanet atmospheres — before we find it elsewhere in our Solar System, given the length of time we have to wait before return missions to places like Enceladus and Europa can be mounted. Perhaps the Mars news can help us accelerate that schedule, at least where the Red Planet is concerned.
Meanwhile, we continue to construct models of habitability not just for Martian organisms, but for more advanced creatures on planets around other suns. As witness today’s topic, recent work out of the University of Washington that is showing us that what seemed to be a major problem for life on planets around red dwarfs may in some cases actually be a blessing.
Of Tides and Magnetic Fields
Our understanding about planets around red dwarf stars is that potentially habitable worlds are close enough to their star to be tidally locked, with one side always facing the star. We’re seeing interesting depictions of such worlds in recent science fiction, such as Stephen Baxter’s Proxima (Roc, 2014), where a habitable planet around Proxima Centauri undergoes ‘winters’ due not to axial tilt but varying levels of activity on the star itself. But tidal locking is problematic, as is the process of getting into it. Circularizing an orbit creates tidally generated heat that can affect surface conditions as well as any magnetic field.
Will planets like these have magnetic fields in the first place?According to Rory Barnes (University of Washington), the general belief among astronomers is that they’re unlikely, a conclusion the new work rejects. It’s an important issue because magnetic fields are believed to protect planetary atmospheres from the charged particles of the stellar wind, thus preventing them from being dissipated into space. Such fields can also protect surface life from dangerous radiation, as from flare-spitting M-dwarfs.
Image: An artist’s impression of the Gliese 667 system from one of the super-Earths that orbit Gliese 667C. Image credit: ESO/M. Kornmesser.
The new paper from Barnes and former UW postdoc Peter Driscoll (now at the Carnegie Institution for Science) takes a look at magnetic fields on planets around red dwarfs. Driscoll began with an examination of tidal effects. In our Solar System, think of Io, its surface punctuated by volcanic activity, to see tidal heating in action. Says Driscoll: “The question I wanted to ask is, around these small stars, where people are going to look for planets, are these planets going to be roasted by gravitational tides?”
And what would be the effect of tidal heating on magnetic fields over the aeons? To find out, Driscoll and Barnes used simulations of planets around stars ranging from 0.1 to 0.6 of a solar mass. Their finding is that tidal heating can help by making a planetary mantle more able to dissipate interior heat, a process that cools the core and thus helps in the creation of a magnetic field.
Thus we have a way to protect the surface of a red dwarf’s planet in an environment that can show a good deal of flare activity in the early part of the star’s lifespan. “I was excited to see that tidal heating can actually save a planet in the sense that it allows cooling of the core,” says Barnes. “That’s the dominant way to form magnetic fields.” A planet in the habitable zone of a red dwarf in its early flare phase may have just the protection it needs to allow life.
But note the mass threshold described in the paper:
…tides are more influential around low mass stars. For example, planets around 0.2 Msun stars with eccentricity of 0.4 experience a tidal runaway greenhouse for 1 Gyr and would be tidally dominated for 10 Gyr. These time scales would increase if the orbits were fixed, for example by perturbations by a secondary planetary companion. We find a threshold at a stellar mass of 0.45Msun, above which the habitable zone is not tidally dominated. These stars would be favorable targets in the search for geologically habitable Earth-like planets as they are not overwhelmed by strong tides.
With stellar mass as the key, as explored in the paper over a range of masses and orbital eccentricities, various outcomes emerge. Planets with low initial eccentricity experience only weak tides, while planets on highly eccentric orbits experience much stronger effects – high initial eccentricity and tight orbits around low mass stars produce extreme tides that help to circularize planetary orbits. As the mass of the star increases, the habitable zone moves to larger orbital distances and tidal dissipation decreases.
Given all these scenarios, helpful magnetic fields are only one possible outcome, and even when they form, they may not be sufficient to protect life. The Driscoll/Barnes model includes planetary cores that undergo super-cooling, thus solidifying and killing the magnetic dynamo. Also, hotter mantle temperatures and lower core cooling rates can weaken the magnetic field below the point at which it can protect the planet’s surface.
Other possibilities: Planets orbiting close to their star in highly eccentric orbits will experience enough tidal heating to produce a molten surface. Tidal heating can also produce high rates of volcanic eruption, producing a toxic environment for life (the atmospheres of such planets may well be detectable with future generations of space- and ground-based telescopes). Tidal heating effects are most extreme for planets in the habitable zone around very small stars, those less than half the mass of the Sun.
So we don’t exactly have a panacea that makes all red dwarf star planets in the habitable zone likely to support life. What we do have is a model showing that for worlds orbiting a star of above 0.45 solar masses, the tidal effects do not overwhelm the possibilities for surface life while they do allow the formation of a protective magnetic field. Some of the magnetic fields generated last for the lifetime of the planets.
Needed improvements in the model, the paper suggests, include factors like variable internal composition and dissipation in oceans or internal liquid layers. “With growing interest in the habitability of Earth-like exoplanets, the development of geophysical evolution models will be necessary to predict whether these planets have all the components that are conducive to life.”
The paper is Driscoll and Barnes, “Tidal Heating of Earth-like Exoplanets around M Stars: Thermal, Magnetic, and Orbital Evolutions,” Astrobiology Vol. 15, Issue 9 (22 September 2015). Abstract / preprint.
Comments on this entry are closed.
“Other possibilities: Planets orbiting close to their star in highly eccentric orbits will experience enough tidal heating to produce a molten surface.”
This might be the case for GJ667Cc (whose primary has an estimated mass of 0.33 times that of the Sun). Because of the proximity of this exoplanet to GJ667C (~0.13 AU) and the fairly high eccentricity of its orbit (~0.3) , work by Valeri V. Makarov and Ciprian Berghea of the US Naval Observatory (“Dynamical evolution and spin-orbit resonances of potentially habitable exoplanet. The Case of GJ 667C”, The Astrophysical Journal, Vol. 780, No. 2, article id. 124, January 2014) strongly suggests that GJ667Cc is experiencing extreme tidal heating – enough for this HZ world to be rendered molten. Of course this assumes that GJ667Cc is a rocky planet. But with a minimum mass of ~4 times that of the Earth, odds are that it is most likely a mini-Neptune.
I’m glad to hear that, although I don’t think tides are the biggest problem with looking for habitable planets in M-dwarf systems. The biggest problem is the extended pre-main sequence period for them, which would cook any planets in what would later be their habitable zones.
There is an interesting historical analogy with early attempts to measure the rotational period of the planet Mercury. Giovanni Schiaparelli (famous for being the first to see “canals” on Mars) suggested in the 1880s that, due to tidal locking, Mercury’s rotational period is 88 days, the same as its orbital period. This suggestion was widely accepted (Antoniadi, for example, accepts it in his 1934 book on Mercury) until 1965 when radio signals from Earth were bounced off the planet showing that its rotational period is 59 days. In “The Extraterrestrial Life Debate 1750-1900,” Michael Crowe argues that Schiaparelli was led to his incorrect measurement partly due to his belief that there is life on other planets in the Solar System. On page 484 Crowe writes:
“Schiaparelli […] must have realized that his 88-day rotation period was the only period that would save part of Mercury from repeated fluctuations between the frigid darkness of prolonged night and a solar radiation seven times more intense than that reaching earth. In fact, his rotation period, when combined with his 47 degree libration, provides, as he notes, one-fourth of Mercury’s surface (the libration zone) with a sun that rises and sets every 88 days on the same horizon, but never inflicts the intensity of vertical or nearly vertical rays. Having implied that Mercury has a region of moderate seasons, he proceeds to provide seas for the planet, arranged in a manner that earthlings ‘might envy.'”
Schiaparelli described the presence of an atmosphere on Mercury as “almost a certainty” although he had no good reason for doing so. Schiaparelli also claimed the rotational period of Venus is equal to its period of revolution.
Has there been any work done on what the actual consequences of periodic flares are on the beginning of life, or are we just displaying our land-dwelling prejudice? Unless abiogenesis is 100% required to occur in shallow water open to the sky(I believe the currently prevailing theory is that the structures that form above “black smokers” are the best candidate.) surely life that starts under the sea would have significant protection? That wouldn’t help with the atmospheric erosion issue, of course.
Christian G is correct, I think. Flare activity is a red herring, it is easily counteracted by an ocean or a thick atmosphere. Similarly, the “protective magnetic field”, in my opinion, is simply not necessary. A somewhat heavier planet does much more to retain atmosphere than any magnetic field. Lastly, the “cooking” of planets in the extended pre-main sequence period, mentioned by Brett: Why would this cooking be detrimental? If this is about the water, it seems to me there must be planets with too much water that could benefit from a thorough cooking. Or, bigger planets that would retain a steam atmosphere during the cooking.
I suppose the general point here is that, too often, we are tempted to see our own planet as the pinnacle of habitability, with any deviation from it automatically considered detrimental. I do not think this perception is accurate.
It seems extraordinary that almost 40 years on from the Viking landers we don’t have any mission, even the pipeline ( as far as I’m aware) to seriously test for the presence of life elsewhere in the solar system.
The negative conclusions to the Viking results seems unfortunate in hindsight, given the rather ambiguous nature of the actual data and what we have learned since.
Given the increading likelihood that panspermia mechanisms are actually operating life may well be quite widespread in the solar system. Serious efforts to definitively test panspermia and to actually test if life is present elsewhere is now at least a decade overdue.
There are other problems with red dwarf life origination as well. The low energy photons a red dwarf exists may not allow the evolution of chlorophyll or a surrogate, meaning only chemotrophs would evolve there. This means life could be there, without the most discussed biosignature, oxygen in the atmosphere.
@Anthony Mugan September 30, 2015 at 8:24
The one thing that the, at best, ambiguous results from the Viking biological experiments taught us was that detecting extraterrestrial life (or even defining it, for that matter) is not anywhere near as easy as was imagined 40 or more years ago. That is why there has been no further attempts at flying life detection instruments on subsequent spacecraft which have limited scientific instrument payload capacity. Given the choice between flying another experiment that will produce more ambiguous results and an instrument that has clearly defined outcomes, the latter will always be chosen to fly.
As for your comment about panspermia, the connection between ambiguous life detection results and the claim that panspermia mechanisms are increasingly likely escape me. While there is certainly evidence to suggest that it is possible, without the detection of a single extraterrestrial life form (so far), it is impossible to make any statement that its origins are related to panspermia.
We may get CONFIRMATION(and perhaps even CHARACTERIZATION) of these theorized magnetic fields in the not to distant future. Abel Mendez and collegues have initiated a study to moniter stellar interaction with every earth-siced and super-earth sized “habitable zone” planet disovered to date with the Arecebo Radio Telescope. Each star will be monitered for an EXTENDED PERIOD OF TIME(unlike the previous SHORT DURATION SETI searches). I am currently unclear whether an EXTENDED SETI search can be PIGGYBACKED on this experiment or not, but I sincerely hope that it can. Check out the PHL UPRA website dor details.
The artist’s depiction of Gliese 667 system is beautiful indeed!
Fascinating entry, and thank you very much. I wonder if people think about tidal locking as only happening the way it does with the Moon — which could be limiting. There are other possibilities, such as 3:2 spin-orbit resonances in which the planet rotates three times in every two orbits (I think Mercury has this.) Perhaps such planets would not bake the way planets in a 1:1 resonance would. This paper is of interest: Brown et al. (2014). Photosynthetic Potential of Planets in 3:2 Spin Orbit Resonances. International Journal of Astrobiology 13:4 (279-289). It has fascinating planet maps that show how much solar exposure each part of a 3:2-locked planet would get, varied by orbital eccentricity. Basically, you could get a planet that looks like a sphere with hot ends on either side, cold everywhere else, and a temperate zone in between the hot and cold zones.
Red dwarfs flares can be very powerful indeed, DG 20CV went hot big time!
‘The initial blast from this record-setting series of explosions was as much as 10,000 times more powerful than the largest solar flare ever recorded. At its peak, the flare reached temperatures of 360 million degrees Fahrenheit (200 million Celsius), more than 12 times hotter than the center of the sun.’
Even if intelligent life evolved on a planet orbiting these stars travelling into space could be a real challenge. Might be great for sending sail probes out of their systems though.
@James M Essig September 30, 2015 at 10:44
‘The artist’s depiction of Gliese 667 system is beautiful indeed!’
At least they have it about right this time with the colour, red dwarfs are more orange than red.
I’m pretty interested in potential habitability of extrasolar planets orbiting around red dwarfs being conscious that they’re most likely the most common type of star in the Universe.
I’ve a doubt on that field. Astrobiologists agree that extrasolar planets orbiting red dwarfs inside their habitability range must be tidally locked due to closeness to the star, making half of the planet a burning hell and the other half completely frozen, in permanent darkness. Their logic is that red dwarfs are cooler and less brighter than G (like our Sun) and K stars so that’s why they should keep close to the star.
My question about it is… Why must they be tidally locked? I mean, okay, they’re cooler and brighter, but they’re also supposed to be much less massive, so the star shouldn’t be able to apply enough gravity to stop the planet rotating itself.
Thank you for your interesting comment. I don’t know if your views are typical within those working in the field or not. I sincerely hope they are not but if so that would go a long way to explain the lack of any real focus on answering such an utterly fundamental question.
Just to clarify what may be a misunderstanding. There is no connection between ambiguous Viking data and increasing credibility of panspermia…a lot has happened in the last 40 years, but there seems little point in going into all that here.
Whilst I do hope a rather more ‘can do’ approach will emerge I sadly suspect we will continue to avoid the question as long as possible, perhaps in part for the reason you outline.
@Anthony Mugan October 1, 2015 at 3:56
” I don’t know if your views are typical within those working in the field or not. I sincerely hope they are not but if so that would go a long way to explain the lack of any real focus on answering such an utterly fundamental question.”
First a little background: I have been involved with a variety of space projects over the course of the past three decades of my professional career designing experiments and instruments for astronomy and remote sensing applications as well as analyzing their data. While I have not been involved professionally in any planetary missions or in any serious astrobiological investigations, I have been a serious observer of these fields decades and have written extensively about them for the past quarter of a century. As a result, I have a pretty good understanding of these programs, their history, the science and how they are managed.
It is a matter of public record that the reason no life detection experiments have been flown on any US planetary mission for the last 40 years is because of the ambiguous results of the Viking missions. That and the realization in the planetary science/astrobiology community that life detection is not as easy as it was originally thought. Space missions in general and planetary missions in particular have very tight mass budgets for instruments and there is keen competition among various teams to get a piece of that budget for an instrument. When review boards are making decisions about what instruments will fly on any particular mission and which won’t, only those with a high likelihood of producing unambiguous, usable results that address stated mission objectives have any chance of being selected. Life detection experiments typically do not meet this critical criterion.
Instead of risking precious payload space on another high risk life detection experiment (and the back lash of embarrassingly ambiguous results, e.g. no Mars missions for a decade and a half after Viking), NASA has concentrated on addressing more basic questions about astrobiology that can be answered. These include “following the water”, studies of soil chemistry as well as a range of geological investigations to determine the climate and conditions on Mars at various points in its past. As these investigations continue, there has also been progress in designing better formulated and more generic life detection schemes:
With some luck, these might be eventually flown. However, I think a better approach would be to return samples for examination in Earth-bound laboratories. A much wider range of tests can be performed here on Earth (with the prospect of performing new types of tests designed to address new issues) than could ever be performed with an automated instrument on a spacecraft.
@Raul M. September 30, 2015 at 18:30
‘I’ve a doubt on that field. Astrobiologists agree that extrasolar planets orbiting red dwarfs inside their habitability range must be tidally locked due to closeness to the star, making half of the planet a burning hell and the other half completely frozen, in permanent darkness.’
It might be hot and bright on the dayside but the nightside could still be lit up at least in the polar regions through an aura borealis if the planet had a magnetic field.
‘My question about it is… Why must they be tidally locked? I mean, okay, they’re cooler and brighter, but they’re also supposed to be much less massive, so the star shouldn’t be able to apply enough gravity to stop the planet rotating itself.’
There are possible resonances and atmospheric tidal effects that could cause non synchronised rotation or at least substantial liberation. As for tidal locking the planet must hug the star for heat and light but the luminosity is not linear to the mass of the star. Tidal effects become more prominent the closer you get to the mass.
Its interesting that the threshold identified by the paper is around the same size as the transition from radiative to fully convective cores occurs. I wonder if that’s significant?
Alas if its true it makes the habitable world of Baxter’s ‘Proxima’ less likely.
Just had a thought on the amount of light that could land on the dark side of inner planets when other outer planets are in orbit as they should get plenty of planet shine, at least enough to see by.