In order to support life, an exoplanet should simply hang out where heat from its star is just right for liquid water. Right?
Not necessarily. New research is suggesting that in order to support life, such a planet might also need plate tectonics, and those are triggered in a narrower band of distance from the parent star.
Rory Barnes, a University of Washington astronomer, is lead author of a paper to be published by The Astrophysical Journal Letters that uses new calculations from computer modeling to define a “tidal habitable zone.”
Besides liquid water, scientists think plate tectonics are needed to pull excess carbon from its atmosphere and confine it in rocks, to prevent runaway greenhouse warming. Tectonics, or the movement of the plates that make up a planet’s surface, typically is driven by radioactive decay in the planet’s core, but a star’s gravity can cause tides in the planet, which creates more energy to drive plate tectonics.
“If you have plate tectonics, then you can have long-term climate stability, which we think is a prerequisite for life,” Barnes said.
The tectonic forces cannot be so severe that geologic events quickly repave a planet’s surface and destroy life that might have gotten a foothold, he said. The planet must be at a distance where tugging from the star’s gravitational field generates tectonics without setting off extreme volcanic activity that resurfaces the planet in too short a time for life to prosper.
“Overall, the effect of this work is to reduce the number of habitable environments in the universe, or at least what we have thought of as habitable environments,” Barnes said. “The best places to look for habitability are where this new definition and the old definition overlap.”
The new calculations have implications for planets previously considered too small for habitability. An example is Mars, which used to experience tectonics but that activity ceased as heat from the planet’s decaying inner core dissipated.
But as planets get closer to their suns, the gravitational pull gets stronger, tidal forces increase and more energy is released. If Mars were to move closer to the sun, the sun’s tidal tugs could possibly restart the tectonics, releasing gases from the core to provide more atmosphere. If Mars harbors liquid water, at that point it could be habitable for life as we know it.
Various moons of Jupiter have long been considered as potentially harboring life. But one of them, Io, has so much volcanic activity, the result of tidal forces from Jupiter, that it is not regarded as a good candidate. Tectonic activity remakes Io’s surface in less than 1 million years.
“If that were to happen on Earth, it would be hard to imagine how life would develop,” Barnes said.
A potential Earth-like planet, but eight times more massive, called Gliese 581d was discovered in 2007 about 20 light years away in the constellation Libra. At first it was thought the planet was too far from its sun, Gliese 581, to have liquid water, but recent observations have determined the orbit is within the habitable zone for liquid water. However, the planet is outside the habitable zone for its sun’s tidal forces, which the authors believe drastically limits the possibility of life.
“Our model predicts that tides may contribute only one-quarter of the heating required to make the planet habitable, so a lot of heat from decay of radioactive isotopes may be required to make up the difference,” Jackson said.
Barnes added, “The bottom line is that tidal forcing is an important factor that we are going to have to consider when looking for habitable planets.”
Source: The University of Washington via Eurekalert. The paper is available here.
Interesting idea, but:
– what does it make of the absence of tectonics on Venus (or at least of the kind of tectonics that might ‘pull carbon’)?
– on the other hand, planets around Mars distance could use more carbon in their atmosphere to make them ‘habitable’, so tectonics would be unhelpful there.
I really dont think plate tectonics are that vital to the long term climate as this article indicates.
I know its just a theory, but a planet that has some form of ocean current (not necessarily water) would be fine for moderating the long term climate of a planet without plate tectonics.
‘“If that were to happen on Earth, it would be hard to imagine how life would develop,” Barnes said.’
I think that’s the problem with a lot of these sorts of proposals… a lack of imagination. 😉
I suppose it does help to narrow the search, but we’ve hardly even started exploring our own system, and we’ve found very few serious extrasolar candidates, so it seems rather premature.
Anyhow, might also a runaway greenhouse effect expand the habitability zone, since more distant planets that might otherwise be too cool would be able to hold their heat?
Bah, never close doors to the possibilities of life on different worlds.
There could be some really weird stuff out there that we would miss out on.
cipater’s got it right. I also once heard an opinion that life could not have developed without a large high-inclanation moon.
Give me a break. Somewhere out there is a planet orbiting a black hole, sustaining its biosphere with a food chain that’s based on gamm-ray-induced reactions, and one of their less-imaginative researcher is writing a paper about how he can’t imagine life around anything less compact than a Neutron Star.
Well, maybe not quite, but just maybe.
# Dark Gnat Says:
June 11th, 2009 at 2:42 pm
“There could be some really weird stuff out there that we would miss out on.”
Absolutely. I never ceases to amaze me that people seem to be so opinionated on what is and isn’t necessary for life to form. We understand almost nothing about it. I have several gripes with this study – most of all, plate tectonics is so poorly understood that I fail to see how they can have any significant degree of confidence in their models. But the other thing is – who says plate tectonics is necessary, or even desirable for life to form?
Consider this – we feel like we have a reasonable (if incomplete) grasp of the basic physics governing the universe these days. And yet still we are surprised more often than not by the way the universe works when we actually go out and observe it – effects combine in surprising and unpredictable ways, even if we do have a good understanding of the underlying physics.
Why should it be any different with life? How could we possibly have ANY idea what life may be capable of, or even if our definition of life is a sensible one? We have but one example of what is possible, under a very limited set of conditions. ONE!
Granted, it is important to try to work out where life may be more likely to be found so that future observations can be targeted most effectively, but this is a prime example of speculation overshooting observational/modeling capability by a huge amount. It may mean something, it may not. It is barely rates above suggestive (in the absolute sense) from my point of view…
Rare Earth hypothesis.
I’ll pitch in with the above comments. It’s interesting to work it out, and I’m happy with that there is a simple model, but what about the Venus/water ideas? The authors takes tidal heating as a limiting constraint when in fact it could open up the radiogenic heating “zone” on systems and planet sizes. (Also, ironically they point out that Earth can do tectonics without tidal heating. I see what they say about this maybe being an exception, but that then needs extraordinary evidence.)
I note that the upper limit posed is actually a constraint on abiogenesis times.
IIRC ocean circulation through heat vents sets another constraint on Earth of ~ 10 ky for floating organics systems to replicate, thus circumventing heat dissociation. This was argued as a realistic constraint from some researcher or other, i.e. he thought abiogenesis was possible on those time scales. Viewed in that light the considered 10^6 y constraint seems much too tight.
Also, the paper that looked at how bacterias could survive late heavy bombardment resurfacing was IIRC using rather weak system dispersal and still got good survivability. Admittedly, by systems already replicating fully, but then again what is the difference? Cellular mobility and sea recirculation isn’t too different AFAIU, i.e. the later dominates.
I would argue that resurfacing could imply two different types of HZs, one for mono-cellular life type biospheres, another for multi-cellular life which may not survive this so well. (But if it does, again there will be only one type of “resurfacing constrained” HZ.)
On this topic, there is also a more subtle difference, regarding biosphere lifetime.
As noted in the comments tectonics may or may not help biospheres vs HZ. But there was a recent paper on stellar heating effects, which IIRC got Earth biosphere lifetime possibly extended to 2.5 Gy hence on account of thinning it out. (After 2.5 Gy again mono-cellular life would be the only biosphere left.)
The thinning was a consequence of how tectonics and biology interrelated. (I.e. tectonics and biology have not only sequestered carbon but here more importantly lots of nitrogen.) So you probably want to have tectonics to extend the galactic habitable zone into the future.
“thinning it out” – thinning the atmosphere out.