Habitable worlds around red giants

[timb]

von Bloh, ****z, Schroeder, Bounama & Franck claim that initially cold 10 M_E "super-Earths" orbiting Sun-like stars going through their RGB phase may experience periods of habitability. The further the planet orbits from the star, the shorter its period of habitability. For example, at 2 AU the period is 3.7 Ga, but at 5 AU it is only 0.1 Ga. More massive and watery planets are more suited to this transition to habitability than smaller, dryer worlds because they retain their internal heat and atmospheric CO₂ longer. No planet or satellite in the solar system fits the bill.

I am rather dubious about this paper because it makes some assumptions that smack of a will to believe.

[Vultur]

How long would the period of habitability be? I could see simple bacteria popping up very fast, but probably not anything more complex.

I think we will find life all sorts of places. (I'm still betting Earth isn't the only living world in this solar system.)

[Fiery Phoenix]

How long would the period of habitability be? I could see simple bacteria popping up very fast, but probably not anything more complex.

I think we will find life all sorts of places. (I'm still betting Earth isn't the only living world in this solar system.)

Same here; there's Venus, Mars, Europa, fascinating Titan, Enceladus, and possibly even the frozen moon Triton. A single living critter walking on or under the surface of any of those worlds would immediately make two living worlds in our Solar System.

To stay on-topic, I think this isn't a bad theory at all. But I guess planets around red giants would always be frigid (and maybe barren) worlds, as red giants are naturally relatively cold stars. Unless, of course, the planet orbits very close to its parent red giant, that could do it - as mentioned in the article. I've personally thought of the same thing before.

[timb]

How long would the period of habitability be? I could see simple bacteria popping up very fast, but probably not anything more complex.

You should read more closely. I stated 3.7 Ga (billion years) at 2AU, etc! Although life started quite early on Earth, not much very exciting developed for the first 3.8 Ga or so. Just bacteria, algae, colonies of algae, colonies of bacteria, that sort of thing. This may be because the Earth's climate was not stable enough for that period.

I think we will find life all sorts of places. (I'm still betting Earth isn't the only living world in this solar system.)

I'd take you on at 2/1 your way, if I could be sure of settlement in my lifetime.

[timb]

Same here; there's Venus, Mars, Europa, fascinating Titan, Enceladus, and possibly even the frozen moon Triton. A single living critter walking on or under the surface of any of those worlds would immediately make two living worlds in our Solar System.

To stay on-topic, I think this isn't a bad theory at all. But I guess planets around red giants would always be frigid (and maybe barren) worlds, as red giants are naturally relatively cold stars.

Mmm actually red giants are bright stars. When a star enters its red giant phase it brightens by a factor of 1000 or more. The surface is cooler, and so emits less power per unit area, but this is much more than made up for by the immense increase in the star's size.

[eburacum45]

The type of planet they are talking about is a super-terrestrial, ten times the mass of Earth and with a gravity of 2.8 gees. This planet would be about 23000 km in diameter, and the continents, if any, would be almost flat.

I think that most worlds of this type in the habitable zone would be waterworlds with no land at all, but if that is not the case then land-based lifeforms might evolve on some of them. The habitable zone itself is a moving location because Sun-like stars gradually get hotter, as von Bloh et al. point out; and a superterrestrial world would remain temperate for a longer period of time because it retains more atmosphere and moisture.

I wonder if that means that high gravity species are more common than species of our kind. The Earth has the highest gravity of any terrestrial world in our system, but larger worlds are not only possible, but probably commonplace. Humans might be weedy 57-kilogram weaklings compared to the majority of intelligent species out there.

Note that when von Bloh et al. talk about 'habitability', they are not talking about a planet suitable for humans- with a surface gravity of 2.8 and presumably a greater air pressure than Earth's, a superterrestial would be an uncomfortable place. We could not live on such a world, without radical genetic engineering.

[timb]

The type of planet they are talking about is a super-terrestrial, ten times the mass of Earth and with a gravity of 2.8 gees. This planet would be about 23000 km in diameter, and the continents, if any, would be almost flat.

I think that most worlds of this type in the habitable zone would be waterworlds with no land at all, but if that is not the case then land-based lifeforms might evolve on some of them. The habitable zone itself is a moving location because Sun-like stars gradually get hotter, as von Bloh et al. point out; and a superterrestrial world would remain temperate for a longer period of time because it retains more atmosphere and moisture.

I wonder if that means that high gravity species are more common than species of our kind. The Earth has the highest gravity of any terrestrial world in our system, but larger worlds are not only possible, but probably commonplace. Humans might be weedy 57-kilogram weaklings compared to the majority of intelligent species out there.

The greater the gravity the more likely complex life is confined to the Oceans. I doubt we'll be meeting any such creatures unless we visit them. If you thought humans had trouble carrying hundreds of kg of air with them on short space flights, imagine getting a spacecraft filled with hundreds of tons of water off the ground, against much higher gravity.

Note that when von Bloh et al. talk about 'habitability', they are not talking about a planet suitable for humans- with a surface gravity of 2.8 and presumably a greater air pressure than Earth's, a superterrestial would be an uncomfortable place. We could not live on such a world, without radical genetic engineering.

I think von Bloh et al. are displaying a will to believe when they say that so-called super Earths resemble the Earth. In fact the expression super Earth refers simply to the exoplanet's mass being in the range 1-10 M_E. That's the only physical data we have at the moment on these "super Earths". They consider only worlds where the surface is between 10% and 90% water, and note that the wetter the wider their habitable zone. However, surfaces that are part water, part rock require that the amount of water in the planet fall into a very narrow range. It's likely that the most common scenario for a planet of 10 M_E, particularly when one considers that it is forming in a colder part of the protoplanetary disk, is that it will accrete relatively more volatiles than did the Earth in the course of its formation, and any rock it may possess will end up buried under hundreds or thousands of kilometers of water/ice, probably with a heavy atmosphere of primordial gases above that. Something like Uranus with a bigger core and less H. This is a poor scenario for life because, while water is necessary for life, life does not live on water alone. Any surface water will be isolated from minerals by the thick layer of various solid allotropes of water.

[PraedSt]

von Bloh, ****z, Schroeder, Bounama & Franck claim that initially cold 10 M_E "super-Earths" orbiting Sun-like stars going through their RGB phase may experience periods of habitability. The further the planet orbits from the star, the shorter its period of habitability. For example, at 2 AU the period is 3.7 Ga, but at 5 AU it is only 0.1 Ga. More massive and watery planets are more suited to this transition to habitability than smaller, dryer worlds because they retain their internal heat and atmospheric CO₂ longer. No planet or satellite in the solar system fits the bill.

Thanks timb. That was a good read. The relationship between continental fraction and habitability was something I hadn't known before.

[timb]

Thanks timb. That was a good read. The relationship between continental fraction and habitability was something I hadn't known before.

Not only the area but the arrangement of the continents is also relevant to how hospitable a planet's climate will be. Earlier in Earth's history the land was arranged in a belt around the equator, and the result was a vicious cycling between global ice ages and greenhouse conditions. So a planet with a given insolation, mass, continental fraction etc may be a tropical paradise or a snowball. It's not easy to predict climate.

[Rhaedas]

Continental drift and carbon recycling might also be an important factor, so far as life as we know it. Wouldn't a super earth be too big for this?

The biggest factor though is still the habitable zone time limit. Even with a bit of padding allotted for heat retention, it doesn't leave a lot of time for a jump from cellular size to larger forms of life (again assuming our one sample of existing life holds as a good example of what's typical universally) .

But, how about this scenario...an Europa type world around a gas giant, that is jumpstarted by the red giant phase, and then sealed back under the ice as the red giant contracts, with the life forms then living off geothermal heat generated from gravitational stress.

Of course this is a problem if my first comment is necessary for long term biosphere stability.

[timb]

Continental drift and carbon recycling might also be an important factor, so far as life as we know it. Wouldn't a super earth be too big for this?

Not according to von Bloh et al. They consider carbon recycling and plate tectonics. In fact they conclude more massive Earths would have stronger plate tectonics. You should read the paper. It's a good habit to get into.

The biggest factor though is still the habitable zone time limit. Even with a bit of padding allotted for heat retention, it doesn't leave a lot of time for a jump from cellular size to larger forms of life (again assuming our one sample of existing life holds as a good example of what's typical universally) .

But, how about this scenario...an Europa type world around a gas giant, that is jumpstarted by the red giant phase, and then sealed back under the ice as the red giant contracts, with the life forms then living off geothermal heat generated from gravitational stress.

Of course this is a problem if my first comment is necessary for long term biosphere stability.

As the star heats up the habitable zone sweeps outwards. This happens quite slowly at first as the star goes becomes a pre-giant (eg Procyon) and planets can have residences of over a billion years. Later as the star swells into a true red giant the HZ moves outwards much more quickly. According to von Bloh, the Sun's projected peak luminosity is 2730, which will put every planet in the oven.

[eburacum45]

Valencia, O'Connell and Sasselov say that tectonics will be 'inevitable' on a superEarth, even if dry.
http://arxiv.org/abs/0710.0699v1
What plate tectonics on a superterrestrial would look like is another matter. Perhaps, since the plates are expected to be thinner on a superterrestrial, smaller plates will predominate. I imagine a superterrestrial covered in dozens of tiny continents. Whether the ocean would overlay these microplates or not is another matter.

[PraedSt]

I think von Bloh et al. are displaying a will to believe when they say that so-called super Earths resemble the Earth. In fact the expression super Earth refers simply to the exoplanet's mass being in the range 1-10 M_E. That's the only physical data we have at the moment on these "super Earths". They consider only worlds where the surface is between 10% and 90% water, and note that the wetter the wider their habitable zone. However, surfaces that are part water, part rock require that the amount of water in the planet fall into a very narrow range. It's likely that the most common scenario for a planet of 10 M_E, particularly when one considers that it is forming in a colder part of the protoplanetary disk, is that it will accrete relatively more volatiles than did the Earth in the course of its formation, and any rock it may possess will end up buried under hundreds or thousands of kilometers of water/ice, probably with a heavy atmosphere of primordial gases above that. Something like Uranus with a bigger core and less H. This is a poor scenario for life because, while water is necessary for life, life does not live on water alone. Any surface water will be isolated from minerals by the thick layer of various solid allotropes of water.

I see your point about the assumption, and I agree with it. But, having made that assumption, I don't see why ice may be a problem. A Habitable Zone has surface water by definition, doesn't it?

[timb]

I see your point about the assumption, and I agree with it. But, having made that assumption, I don't see why ice may be a problem. A Habitable Zone has surface water by definition, doesn't it?

Not really, no. There are many definitions of "habitable zone", some of them are more to do with planetary conditions than a zone of potentially habitable orbits. If you are talking about orbits then the Moon is in the Sun's HZ yet it has no surface water. If a scaled 10 M_E Uranus were in Earth's orbit its surface water would be difficult to distinguish and probably continuous with its supercritical fluid atmosphere. A scaled up Ganymede with an atmosphere like Earth's would have a global ocean resting on warm ice.

[PraedSt]

Not really, no. There are many definitions of "habitable zone", some of them are more to do with planetary conditions than a zone of potentially habitable orbits. If you are talking about orbits then the Moon is in the Sun's HZ yet it has no surface water. If a scaled 10 M_E Uranus were in Earth's orbit its surface water would be difficult to distinguish and probably continuous with its supercritical fluid atmosphere. A scaled up Ganymede with an atmosphere like Earth's would have a global ocean resting on warm ice.

Yes, I've just been reading up on 'criticisms of the Habitable Zone approach'. With habitability depending on so many variables, I'm wondering how useful the concept is. And I mean that as a query, not a criticism.

[PraedSt]

As the star heats up the habitable zone sweeps outwards. This happens quite slowly at first as the star goes becomes a pre-giant (eg Procyon) and planets can have residences of over a billion years. Later as the star swells into a true red giant the HZ moves outwards much more quickly. According to von Bloh, the Sun's projected peak luminosity is 2730, which will put every planet in the oven.

Yes, I was surprised how fast the HZ moves outwards after a certain point. That was something I didn't know.

The width of the pHZ during the main-sequence evolution is found to be approximately constant, but for higher ages, it increases over time and moves outward, a phenomenon most noticeable beyond 11.5 Gyr. For example, for ages of 11.0, 11.5, 12.0, and 12.1 Gyr, the pHZ is found to extend from 1.41 to 2.60, 1.58 to 2.60, 4.03 to 6.03, and 6.35 to 9.35 AU, respectively.

And regarding our Sun, I found the fact that we may be limited by geology interesting..

In case of Earth-mass planets (1 M⊕), a detailed investigation of geodynamic habitability was presented by Franck et al. (2000b) with respect to the Sun as well as stars of somewhat lower and higher mass as central stars. Franck et al. found that Earth is rendered uninhabitable after 6.5 Gyr as a result of plate tectonics, notably the growth of the continental area (enhanced loss of atmospheric CO2 by the increased weathering surface) and the dwindling spreading rate (diminishing CO2 output from the solid Earth).

This implies that there is no merit in investigating the future habitability of Earth during the post–main sequence evolution of the Sun, as in the framework of pHZ models, the lifetime of habitability is limited by terrestrial geodynamic processes...

[transreality]

Why is there any requirement for geology for early life. Early life requires protection from UV so almost certainly arises in the ocean. Early metazoan life on earth was purely marine, before settling some terrestrial areas with algal matts, and a subsequent colonisation by ecologically connected animals. Every terrestrial and benthic niche is a subsequent colonisation from a free-living pelagic marine habitat.

[timb]

You might find the answer if you read the paper. Hint: it's not about protection from UV.