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A Yale researcher has published a study that suggests that because the presumed "self-regulating mechanism" for planetary internal temperature does not exist, the distance at which an exoplanet orbits its star might not matter as much as random factors such as giant impacts during the formation of the planet:
A new study, appearing in the journal Science Advances on Aug. 19, suggests that simply being in the habitable zone isn't sufficient to support life. A planet also must start with an internal temperature that is just right.
"If you assemble all kinds of scientific data on how Earth has evolved in the past few billion years and try to make sense out of them, you eventually realize that mantle convection is rather indifferent to the internal temperature," said Jun Korenaga, author of the study and professor of geology and geophysics at Yale. Korenaga presents a general theoretical framework that explains the degree of self-regulation expected for mantle convection and suggests that self-regulation is unlikely for Earth-like planets.
"The lack of the self-regulating mechanism has enormous implications for planetary habitability," Korenaga said. "Studies on planetary formation suggest that planets like Earth form by multiple giant impacts, and the outcome of this highly random process is known to be very diverse." Such diversity of size and internal temperature would not hamper planetary evolution if there was self-regulating mantle convection, Korenaga said. "What we take for granted on this planet, such as oceans and continents, would not exist if the internal temperature of Earth had not been in a certain range, and this means that the beginning of Earth's history cannot be too hot or too cold."
Can mantle convection be self-regulated? (open, DOI: 10.1126/sciadv.1601168)
[More..]
From the paper:
However, the overall effect of increasing planetary mass remains to reduce the magnitude of the Tozer number [ratio of the thermal adjustment rate over the decay constant], and if plate tectonics on Earth cannot achieve thermal equilibrium, then it would be more unlikely for super-Earths. Deviation from thermal equilibrium would be even more pronounced for the case of stagnant lid convection. A lower Tozer number also means a longer e-folding time scale (Fig. 3C), indicating that how a planet forms in the first few tens of million years could have a profound impact on its subsequent evolution over a few billion years. Parameterized convection models with the effect of mantle melting suggest that the influence of initial conditions on present-day observables is significant even for planets smaller than Earth (48).
(Score: 2) by aristarchus on Monday August 22 2016, @07:02PM
If we find life on a body, we know that the conditions for life arising (or being introduced?) exist, or did once exist. If the life continues to live, we know the conditions to sustain life exist, and are sufficient for life. Now what exactly those conditions are is a bit more difficult to ascertain.
We can speculate that certain conditions, like a surface temperature that permits liquid water, are necessary, but not sufficient for life. We might also relate surface temperature to stellar distance. Simplistic, but for large variations, more or less reasonable. Of course, we may be wrong about what the necessary condition for life are in the first place.
(Score: 2) by takyon on Monday August 22 2016, @07:43PM
Bringing this in:
https://en.wikipedia.org/wiki/Circumstellar_habitable_zone [wikipedia.org]
It would be great if microbes or RNA could form in a subsurface ocean on Europa, Enceladus, or wherever, but I wouldn't consider it habitable without an atmosphere. Titan is an interesting case, but appears to be too cold.
That's not to say that exomoons aren't important. If you have a gas giant in the habitable zone, the exomoons become interesting (and are probably larger and more likely to have a dense atmosphere than moons orbiting Mars/Venus/Earth mass planets). The exomoons may be able to extend the size of the habitable zone, since an exomoon at a Venus distance would presumably be cooler than the planet. If Jupiter was around where Mars is, maybe the combination of tidal heating and energy from the Sun would make the moons more likely to be habitable (not sure about that one).
Maybe "habitable" should be redefined as "life-supporting object with surface liquid water and an atmosphere that humans can breathe". Because that's what our criteria are looking for in the absence of any crazy counterexamples, and subsurface oceans seem to be excluded.
[SIG] 10/28/2017: Soylent Upgrade v14 [soylentnews.org]
(Score: 2) by PartTimeZombie on Tuesday August 23 2016, @12:32AM
Of course, we may be wrong about what the necessary condition for life are in the first place.
I suspect this might be correct.
We are making assumptions about how life can arise with almost no data. It's like trying to determine what Yankee Stadium looks like from inside the broom closet, peering through the keyhole.
My view is that there are lots of variables needed for life (Earth type life) to arise on a planet, which will make it relatively rare.
There are however many stars, so the odds of life elsewhere seem pretty good to me.