It’s easy to see why interest in planets around red dwarfs is growing. The low mass of such a star makes finding smaller planets feasible. It also produces orbits closer to the star, another aid to their detection. We know that planets can form near the habitable zone of such stars because we have the example of Gliese 581, where two planets orbit close to if not just within that region. But is a habitable planet always habitable? If not, what could make these conditions change?
I’m looking at a paper that examines tidal effects, an important factor when dealing with M dwarfs. Planets in the habitable zone around these stars experience effects that can cause both their orbital distance and orbital eccentricity to decrease [see comments below re my original misstatement of the eccentricity change, now corrected]. The paper, by Rory Barnes (University of Arizona, Tucson), Sean Raymond (University of Colorado, Boulder) and team, examines an interesting parameter: The habitable lifetime. The authors define this as the time needed for a planet to move from a habitable region to one that is not. More massive stars have habitable zones far enough away that this tidal evolution does not occur, but M dwarfs below about 0.35 solar masses can be affected.
The result is striking and potentially devastating to life: A planet with a large enough orbital eccentricity (larger than about 0.5) around a low-mass star can, because of this tidal effect, be pulled out of the habitable zone in less than a billion years. Given the fact that M dwarfs account for over 75 percent of the stars in our galaxy, and given the fraction of known exoplanets with high eccentricity, the authors suggest that tidal effects may be a noteworthy constraint on the total number of habitable planets. The definition of habitability used in this study is the classic one, based upon the presence of liquid water upon the surface.
The results for Gliese 581 c are intriguing. This is the planet that caused such a stir when researchers announced that it was within the habitable zone of its star, a finding that has since been sharply questioned. Looking at the interactions between Gliese 581 c and inner planet 581 b, the authors state:
Tidal theory suggests that planet c orbited with larger values of semi-major axis and eccentricity in the past. Therefore, it may have been habitable in the past, but tides subsequently moved the planet into an uninhabitable orbit. If planet c was the only planet in the system, plausible physical properties indicate that it was habitable. However, when constraints from the mutual interactions of the additional planets are considered, planet c has likely never been habitable.
The details of these interactions are complicated, but what I want to focus on is the deeper implication here:
The detection of a terrestrial planet around a low-mass star is insufficient to determine that planet’s past and future habitability. The tidal forces between planet and star can significantly change the orbits and hence limit the habitable lifetime. Planets detected in the HZ with large eccentricities may be bound for hotter temperatures and, ultimately, a global extinction. On the other hand, planets detected interior to the HZ may have been habitable in the past. Gliese 581 c was most likely not habitable in the past, but if its companion planets were on different orbits, past habitability would have been possible.
As if we didn’t have reason enough for caution about these matters, we’re now reminded that just finding a rocky world at a particular distance from its parent star is no guarantee of its long-term habitability, particularly when we’re dealing with M dwarfs. Because we’re talking about major orbital evolution over a span comparable to the age of the Earth, it’s clear that sustained complex life demands planets that form with low eccentricities. The good news is that most exoplanets are thought to have been formed with relatively low eccentricities.
So now we’re looking at a true science fiction scenario, a planet that once supported life but moved ultimately inside the habitable zone of its star. What might future exobiologists find among the wreckage? The authors of this paper note that we should extend our work on habitable atmospheres and their evolution to include the possibility of detecting the signatures of extinct life on planets like this around M dwarfs. In many ways, the thought is poignant, and it’s easy to agree with this statement:
Perhaps the most distressing aspect of this work (from a SETI perspective) is the prediction that planets can be habitable long enough for complex life to develop, but then that life is extinguished by tides. Yet this work suggests that such a “tidal extinction” may occur on some planets around low-mass stars.
The paper is Barnes et al., “Tides and the Evolution of Planetary Habitability,” accepted by Astrobiology and available online. Be aware as well of Jackson, Greenberg and Barnes, “Tidal Heating of Extra-Solar Planets,” accepted by the Astrophysical Journal (abstract). An earlier Centauri Dreams story on that paper is here. Thanks to Adam Crowl for his assistance on this story.
I’m not sure how much more “distressing” this fate is for a planet than to have the habitable zone be pushed outwards by stellar evolution, both of which can happen after there’s been enough time for complex life to evolve. All habitable planets die eventually, whether by stellar evolution, migration, cessation of geological activity, etc.
Administrator:
“Planets in the habitable zone around these stars experience effects that can cause their orbital distance to decrease while the eccentricity of their orbit *increases*”.
?
What I understand from the abstract and from your 3rd and 6th paragraph, is that this orbital decay is a problem for planets (in close orbit around low-mass stars) that have a great eccentricity in the first place, not that the eccentricity itself increases over time, on the contrary: both semi-major axis and eccentricity appear to decrease with time.
So this should not be too much of a problem for these planets if they were formed with low eccentricity to begin with.
@andy: so what we are looking for then, in view of a very long life-span, are not too low mass, non-flare M dwarfs (about M0-M2), with an earth to super-earth mass planet (about 1-5 earth masses) in a circular orbit in the habitable zone.
Or this confirms that there may be a ‘Goldilocks zone’, not just for a star and a galaxy, but indeed also with respect to spectral class: in between the big hot short-lived (O, B, A, early F and all giants) on the one side and the small cool (M, late K, and all subdwarfs etc.) on the other side. In other words, the solar type stars.
This would not bode very well for the (distant) future of our universe, since it seems to consist mainly and increasingly of dwarfs stars.
Such a cheery thought andy, though inexorably certain for all worlds.
It’s a toss-up whether Earth will die from the sun’s evolution, water-loss to the mantle, or arteriosclerosis when the geophysical cycles grind to a halt. Tides are presently the least of our worries. Paul’s point is that tidal constraints are important for estimates of existing, mature biospheres – something that’s a natural interest for the TZF.
Absolutely right, Ronald, and thanks for pointing out my mistake. The eccentricity does indeed diminish with time due to tidal effect, as the paper notes. Sorry. Now fixed in the main entry, with reference to this correction.
The question to ask here is just how long can a planet’s habitability be prolonged in the absence of stellar evolution? Cessation of geological activity might be postponed by tidal heating e.g. if the planet’s eccentricity is pumped by an additional planet in the system, however this process would then tend to cause the planet’s orbit to move inwards. The companion planet would also experience orbital evolution, which means various resonances might become important… this problem is very complex indeed!
Hi Folks;
I can imagine what a beautiful sight of the star or an M-class sun at high noon on an Earth like planet in orbit around such a star would provide. Assuming an orbital distance of say 0.1 AU to 0.033 AU, such a star would appear as a disk with an angular diameter several times that of our Sun or Earth’s moon.
The star might have an spherical angle brightness that might almost permit a human person to view it with the unaided eye, sort of like looking at a roaring fire in a fireplace, due to the T EXP 4 dependence of integrated spectral radiance of a blackbody of which stars are a good approximation. In fact, some low end massed M class stars have a surface temperature close to 2,000 K. Thus the spherical area irradiance of the star on a planet or within the human eye gazing at it would be about (2,000/5,800) EXP 4 times that of the Sun or roughly 80 times less than that of the Sun. Well, if not with the unaided eye, perhaps good sun glasses would permit such. The star sets and star rises on any orbiting Earth like planets should be beautiful also.
Thanks;
Jim
Hi Folks;
I revisit an idea I posted at another Tau Zero thread several months ago.
The idea essentially involves prolonging the life of a very low end mass range M class star. Basically, one can imagine that because the luminosity of a such a red dwarf is as much as 4 orders of magnitude less than the Sun, commensurate with the fusion of about 40,000 metric tons of hydrogen per second, one can imagine developing a mechanism whereby say 120 trillion metric tons of hydrogen are collected every 1,000 years and deposited into such a star and 120 trillion metric tons of stellar material are removed every millennium.
The deposition and removal of material in such a batch mode seems more feasible to me than the constant continuous influx and removal of 40,000 metric tons of matter per second. 120 trillion metric tons of hydrogen in metallic form with a density of roughly H20 or water would only have the volume equivalent of a 50 kilometer cube or a 30 mile cube. It would seem to me that such a volume of material could be collected and deposited into such a red dwarf every 1,000 years by a civilization capable of interstellar travel and planetary scale engineering projects
My hope is that such stars could be islands of energy and hope for advanced humans and ETI beings indefinitely into the future. As the stores of easily recoverable interstellar and intergalactic hydrogen were depleted, perhaps cold dark matter reserves could be converted into hydrogen, perhaps by intermediary reaction pathways wherein the cold dark matter would be converted into energy which would then run particle accelerators or other machines to convert the electromagnetic or electro-dynamic energy into protons and electrons. When the stores of easily recoverable CDM where depleted, perhaps the Cosmic Background Radiation could be captured by huge porous membranous structures which would then power proton and electron generators.
Thus, M class stars might be our best hope of perpetuating our species in recognizable form indefinitely into the future.
Thanks;
Jim