When it comes to exoplanet speculations, we’re still in the era when data are few and dominated by selection effect, which is why we began by finding so many ‘hot Jupiters’ — such planets seem made to order for relatively short-term radial velocity detections. It’s a golden age for speculation, with the promise of new instrumentation and a boatload of information from missions like Kepler and CoRoT to be delivered within a few years. What an extraordinary time to be doing exoplanetary science.
The big questions can’t be answered yet, but it shouldn’t be long before we have an inkling about what kind of stars are most likely to produce terrestrial planets. And maybe a qualification is in order. M-dwarfs are so common in our galaxy — some estimates run to seventy percent of all stars and up — that finding habitable worlds around them would hugely increase the possible venues for life. But is there any way we could call planets around M-dwarfs ‘Earth-like?’ Maybe in terms of temperature in a specific habitable zone on the surface, but little else applies.
M-dwarfs vs. the Early Sun
An M-dwarf planet in the habitable zone is, around many such stars, going to be subject to the kind of solar flare activity that could either prevent life from gaining a foothold altogether, or else serve as an evolutionary stimulus. Either way, conditions like this don’t seem Earth-like, even if the early Earth was subjected to its own barrage of harsh ultraviolet radiation before life forms could produce enough oxygen to yield an ozone layer. Rocco Mancinelli (SETI Institute) talked about this at the recent IAU Symposium on Solar and Stellar Variability — Impact on Earth and Planets. Here he discusses the importance of UV:
“We also see ultraviolet radiation as a kind of selection mechanism. All three domains of life that exist today have common ultraviolet protection strategies such as a DNA repair mechanism and sheltering in water or in rocks. Those that did not were likely wiped out early on.”
Clearly, intense ultraviolet can’t be considered prohibitive to life. But again, that’s from Earth’s history around a G-class star. In addition to their flare activity, M-dwarf planets are likely to be tidally locked, producing weather patterns that will keep meteorologists up nights and probably reducing habitable zones to specific areas on the Sun side. Life may well be possible, but even if Kepler turns up M-dwarf rocky worlds in large numbers, we’ll be talking about conditions that are only marginally like Earth, though obviously with an astrobiological fascination all their own. What we may one day find living on such worlds should be exotic creatures indeed.
The Case for K-class Stars
Recently we’ve been kicking around the subject of K-class stars in the comments to various posts here, and with K stars we really can start talking about planets much more like our own. Here I fall back to the IAU meeting, where Jean-Mathias Grießmeier (ASTRON, The Netherlands) looked at the role of magnetic fields in determining how likely life is to develop. Such fields provide a shield against incoming charged particles from stellar mass ejections as well as pervasive solar winds. They also offer protection against high-energy cosmic rays.
And here’s the quote from Grießmeier that resonates with me. He’s looking at the kind of stars we might expect to find life around, and concludes that our Sun probably wouldn’t top a list of such stars as compiled by the average extraterrestrial astronomer:
“The Sun does not seem like the perfect star for a system where life might arise. Although it is hard to argue with the Sun’s ‘success’ as it so far is the only star known to host a planet with life, our studies indicate that the ideal stars to support planets suitable for life for tens of billions of years may be a smaller slower burning ‘orange dwarf’ with a longer lifetime than the Sun ― about 20-40 billion years. These stars, also called K stars, are stable stars with a habitable zone that remains in the same place for tens of billions of years. They are 10 times more numerous than the Sun, and may provide the best potential habitat for life in the long run.”
K stars — now we’re talking! A stable habitable zone that offers a long period for life’s development, and a population that far outnumbers G-class stars like the Sun. It’s nice to speculate about the closest such star, the K-dwarf Alpha Centauri B, but of course we still have to resolve the question of planetary formation in binary systems like this one. We should have some answers fairly quickly, what with two ongoing attempts to find planets in the Alpha Centauri system, and may well know about Centauri planets before we start getting hard returns from Kepler.
Describing a Life-Bearing Planet
What does Grießmeier lean to when it comes to planets that would make good astrobiological candidates? Planets more massive than the Earth by two or three times, where higher gravity can make it easier to retain the atmosphere, and a larger liquid iron core offers robust magnetic field protection. The clincher here is the slower cooling of such a planet, allowing it to keep that magnetic protection for longer periods.
Meanwhile, Manfred Cuntz (University of Texas, Arlington) told the IAU meeting about his own team’s work on ultraviolet radiation and its effect on DNA. This is also quite interesting:
“The most significant damage associated with ultraviolet light occurs from UV-C, which is produced in enormous quantities in the photosphere of hotter F-type stars and further out, in the chromospheres, of cooler orange K-type and red M-type stars. Our Sun is an intermediate, yellow G-type star. The ultraviolet and cosmic ray environment around a star may very well have ‘chosen’ what type of life could arise around it.”
So many life factors, and so many stars to study! More in this IAU news release, which also looks at Edward Guinan’s work at Villanova, where he’s been studying stars that are analogues to the Sun at various stages of their life cycles. Among the findings: The Sun rotated ten times faster four billion years ago than it does now, thus producing a far stronger magnetic field. Our young Sol emitted X-rays and ultraviolet radiation several hundred times stronger than the Sun does today.