What would a habitable planet look like twice the size of Earth

Astronomers have already found several thousand exoplanets - some of them are stony and are located within the habitable zone of their star. A rather large part of them is larger than the Earth, in connection with which the question arises: how would a habitable planet look more than twice the size of the Earth?

Structure

The first difficulty: twice as much is not the same thing as twice as heavy. Earth-like planet with a double mass is quite simple to analyze, but if we double the radius, then everything will depend on what it consists of.
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Please note that if the ratio of stone and water corresponds to that of the earth, then the planet is 15 times heavier, but with a surface area of ​​only 4 times more, there will be a hydrosphere with a depth of 3.75 times more, other things being equal. And these are oceans 16 km deep.

Much depends on whether we assume that the Double Earth appeared on the outskirts of the Solar System, in the ice zone, and then moved inward (then it will be very wet), or appeared close to the sun. In the first case of the Wet Double Earth, its mass will be 3 times the Earth's, and the density will be 37% of the Earth's, the force of gravity on the surface will be 0.73 g, and the speed of runaway - 13.6 km / s. There will be oceans hundreds of kilometers deep, surrounding a rocky core covered with warm ice at high temperatures. In the second case of the Dry Double Earth, its mass will be 15 times greater than the Earth’s, density 167%, gravity 3.4 g, escape speed - 30 km / s. For modeling, I used the model Sotin et al. in Sotin, C., Grasset, O., Mocquet, A. 2007. Mass-radius curve for extrasolar planets and ocean planets. Icarus191, 337-351.



How big is the Moist core? If its density is like that of the Earth (5520 kg / m ³ ), and it will be surrounded by water (1000 kg / m ³ ), then the core radius will be 1.22 times greater than that of the Earth (7772 km), and water cover - 0.78 from the earth (4969 km). This is the first approximation, since it will have a crust of ice under high pressure, which appears when pressures approach 1 GPa. Additional calculations give me an estimate of the core with a radius of 6060 km (0.95 terrestrial) with an ice crust of 12,600 km (1.97 terrestrial), which leaves the oceans "only" at a depth of 160 km. If the ocean is colder in the depths, then its depth may be only 104 km.

Atmosphere

We need to make guesses about the atmosphere and temperatures. The temperature of a gray body with an albedo equal to that of the earth in an orbit with a radius of 1 AU. around a sun-like star will be 250 K, and if we add a correction for a greenhouse gas of 36 K, we will get an average temperature of 13 ° C.



There is another equilibrium state, similar to the “Earth-Snow”, where the entire surface is cold and covered with a glacier (there may be such oceanic worlds completely covered with ice) and effectively reflects energy. With an albedo of 0.8, we get a temperature of -15 ° C. Of course, the vast oceans will remain liquid anyway, especially since the freezing point of water decreases with pressure.

In the case of a Wet scale of heights will be 11.3 km - the pressure at this altitude will be 36% less. In order for the molecules to escape, the temperature must be 1.49 times higher than the Earth's one: in this case, hydrogen will definitely escape, and, as it seems to me, helium too (depends on the temperature of the exosphere , which is difficult to calculate). Methane and ammonia can remain, but in the presence of life and oxygen they will turn into carbon dioxide and nitrogen.

In the case of a dry altitude scale it will be 2.4 km: the clouds will be flat and lie close to the ground. Retention temperature is 7.5 times higher than Earth's - Dry, in principle, can retain hydrogen. This means that a more dense atmosphere can accumulate on it from the very beginning, which will turn it into a gas giant. However, this planet should have been formed in the dry zone next to the star, so that it could not collect so many gases. But still its atmosphere should be denser than that of the Wet.

If we assume that the surface pressure is proportional to surface gravity, then on the surface the wet pressure will be 0.73 atm and dry at 3.4 atm.

Then the wet density of the atmosphere will be 0.9 Earth. For people is quite acceptable.

Let's also assume that the wind speed will be equal to 10 m / s terrestrial - it is very difficult to estimate without launching the full circulation model. And, finally, which is unlikely, suppose that the orbital period of the planet is 24 hours. The radiation timeline is 18 days and advection is 14 days - that is, the weather is as complex as on Earth and reacts quite quickly to the seasons (yes, I implicitly assumed that the tilt of the axis of rotation is also equal to that of the earth - in the case of a larger incline will become very strange). Wet will have 9-10 high-altitude jet streams (about 7 of them are on the Earth). The dry air density at the surface is 4.3 times greater than ours, so the time scales will be shortened, and the jet streams will be about 10. Nothing too unusual.

The weather, in particular, is buoyant. On Humid, it is smaller — the clouds will be higher and will move slowly, and on Dry, high gravity will cause small changes in density to have a big impact — there will be flatter and more intense convection.

The Coriolis force will also be twice as strong, so there will be more zonal, rather than meridional winds - the predominance of flows from east to west over flows from north to south will be greater than on Earth.

The amount of water in a cloud will be approximately proportional to its height and density of the atmosphere: on Wet and Dry precipitation in a typical cloud will be greater than on Earth (30-40%, if I'm not mistaken), and Wet will be slightly wetter - low air density is compensated large scale heights. In practice, it all depends on the more complex aspects of the atmosphere - the adiabatic temperature gradient and so on.

Hail on the Wet will be terrible, for its formation will be enough distance. The radius of hailstones is probably proportional to the scale of altitudes, so large hailstones will be 3.5 times larger - but due to low gravity they will weigh only 2.6 times more. The maximum speed of fall is proportional to the square root of gravity divided by the density, so the speed of hailstones will be only 90% of terrestrial ones of the same size. On Dry it will be 89% due to denser air. But still their kinetic energy will be three times greater.

The strength of hurricanes depends on the temperature difference between the ocean and the stratosphere - I do not know how easy it is to calculate. In the absence of land, they can last longer before crawling towards the poles, where they will scatter. With sufficiently strong zonal winds, hurricanes can become almost constant - like a big red spot on Jupiter , but I think that there will be enough meridian winds to prevent this from happening.

Also, I'm not quite sure about whether the latitudinal mixing will be strong enough so that the poles will be heated and will not form polar caps. I suspect that the absence of land and the presence of a huge ocean with a large heat capacity will reduce the formation of ice.

If we assume the presence of 20% oxygen, then the dry partial pressure of oxygen will be about 537 mm Hg, which will be toxic to humans. Worse, the partial pressure of CO ₂ will be 10.4 mm Hg, which is fraught with hypercapnia . However, local life is likely to evolve to cope with this problem. The atmosphere on Wet looks suitable for people.

The optical depth of the atmospheres on the Double Lands will be comparable to that of the Earth (since, as I assumed, pressure = gravity on the surface), therefore, it will be possible to see at the same distance. The vertical optical depth will be 1.37 times the Earth's on Wet: the sky is milder, but does not look too alien. On Dry only 1.1 times - almost as usual. When flying by plane, the situation will not turn dark blue at some extremely low altitude.

Geosphere

Radiogenic heat (if the composition of the earth corresponds) on the Dry will be 3.34 times greater than on Earth, 0.29 W / m ² . It is not enough to melt the crust and turn it into the volcanic orgy of Io, but the situation is much more active - the thickness of the crust is only 3 km. Often there are underwater volcanoes, many hydrothermal sources. Increased buoyancy enhances volcanic convection - the depths of the ocean will be mixed much more actively than on Earth.

If the dry orbit is eccentric enough to warm it up a little more, volcanism may be enough for a state like Io with semi-molten crust. Mixed oceans will accumulate large quantities of minerals, including sulfur. It may turn out to be a world with sulfuric acid oceans. The development of life is possible, but on the basis of more expensive biochemistry. On such a planet will be observed abundant emission of carbon dioxide, which will enhance the greenhouse effect. Adjusting the parameters so that it remains in the habitable state will be quite difficult.

Radiogenic heat on Humid is less than on Earth (95%). This is enough to ensure the continental drift and mixing of the deep ice crust. However, on the ice surface such mixing is hardly possible. The energy flow on the surface of the ice will be 0.02 W / m ² - not enough to carry out continental drift on a rocky planet, but perhaps enough to move the ice.

Stony mountains on Wet will be 5% more than on Earth, but they will all be at the bottom of the ultra-deep ocean under a crust of ice. On Dry, their height will be only 29% of the earth's - local Everest will reach only 2.4 km in height. Given my hunch about the depths of the oceans, this is likely to be a water world.

Hydrosphere

Ocean waves will move in different ways. On the Wet they will move at a speed of 85% of the earth, on the Dry - 184%. The height will be inversely proportional to gravity - 136% on Wet, 29% on Dry. Therefore, the seas will be more wavy, but slower in the case of Wet (but the waves will carry more energy per square meter), and on the Dry they will be fast and low.

On both worlds, the light will penetrate through the water in the same way as on Earth, and the illumination zone will be about 200 m deep - where photosynthesis can work.

Large hydrospheres will operate with temperature buffers and resist temperature changes in daily and seasonal cycles.

Ocean currents occur due to the trade winds: the air, moving at the equator, and deviating due to the Coriolis force , forms the trade winds; at the same time part of the wind energy is transferred to water. In this way currents similar to those in the central part of the Pacific Ocean are obtained: the northern and southern equatorial current going to the west, and between them the opposite current directed to the east. Towards the north, swirling eddies may appear, or perhaps other currents directed east or west. If the currents mostly go east or west, the temperature differences between the equator and the poles will be greater, which will lead to strong convection - colder water will sink into the polar regions and float up at the equator. At a distance from the equator, Ekman flows will occur at depths of up to 100 m, which will complicate the entire circulation system.

The oceans will be layered, since less dense warm waters will lie on more dense cold ones (and even volcanic activity on Dry will provide much less warming from below than from above). Wind and salinity differences due to evaporation will lead to surface convection, but the deep layers will remain in place. Polar waters can extend to the very bottom, at least on the dry. But there will be no seamounts mixing the layers, or the deep currents generated by the continents. In the intratropical convergence zone (Intertropical Convergence Zone, ITCZ), upward flows will be observed along the equator, which, at least on the Dry, will be the main source of nutrient-rich water. The wet ocean will be so deep that the currents generated by the winds will not be too deep, and the ascending currents will not be so useful.

Volcanism can be the main factor leading to the emergence of rising currents or deep water rich in minerals: even a small temperature difference is enough for an upward flow to appear. Rising streams from great depths will be subject to the Coriolis force. This happens on Earth, too, but on Wet the effects will be much stronger, since the streams will have to go through a large proportion of the planetary radius. When moving up, they will deviate to the west, as well as acquire a rotational moment if they are not at the equator.

In general, the oceans will not be as salty as on Earth, for there will be no continents leached out by pure rain — all dissolved salt will be the result of volcanism and slow alignment with open crust. The wet will be especially fresh - the water there will not be in contact with the bark, and the total volume of water is much more than on the dry one.

Biosphere

The dry and wet surface biospheres can work in the same way as the open sea biosphere on Earth. Algae photosynthesis will be the basis of the food chain, in which various forms of plankton and large organisms will feed on them and on each other. As on Earth, most of the biomass will be located in the illuminated surface layer, and more rare detritophages and predators will be hidden in the depths.



As in terrestrial oceans, gravitational restrictions on the size of organisms will not be there - only environmental restrictions (larger animals need more food and more time to grow up, so at some point they will have a reduced ability to collect food and the likelihood of survival with subsequent reproduction ) [there is also the issue of heat removal , although they live in a denser environment than air - approx. trans.]. Vertical surface plants on Dry will be one third less than on Earth due to gravity.

At the bottom there may be ecological niches based on hydrothermal sources. On Wet hot water, it will be necessary to overcome the thick ice crust, and their structure (and generally the likelihood of availability) will depend on how the ice behaves under high pressure — I have no idea about that. An interesting opportunity would be lithoautotrophs , similar to those of the earth, living in icy crevices. On dry biology will be similar to the earth.

Note that for such ecosystems, oxygen is practically not needed: on Earth, they use available oxygen, but with fairly strong chemical flows from volcanoes, life can be sustained in other ways. For example, terrestrial anammox bacteria convert ammonia into nitrogen with nitrites instead of oxygen. Thiobacillus denitrificans convert sulfur to sulfates using nitrates, hydrogen bacteria convert hydrogen to water using sulfates, phosphitic bacteria convert phosphites to phosphates using sulfates, methanogens convert hydrogen to water using carbon dioxide, and carboxylic bacteria turn carbon dioxide into carbon dioxide, turning while water is hydrogen.

The big problem of living on Wet is a lack of salt. Most of life on earth is based on CHON, but it needs to absorb other elements for specific enzymes and molecules. Most likely, life will develop structures that catch rarer heavy atoms like terrestrial siderophores. Cells will have problems with osmosis - if the concentration of solutions is higher than that of sea water, the water molecules will be sucked inwards and threaten the cells with a gap. They need to constantly pump out water to maintain stability - like organisms living in fresh water. Osmoconformers that maintain a concentration that coincides with the ambient concentration will have larger cells with a lower reaction rate.

And although the surface area on both planets will be four times that of the earth, there will be fewer animal species there. Dry can support at least two almost independent layer ecosystems and something in between. Whether the mind will develop there - who knows.

Total

Both Twin Lands are water worlds, but one of them is very deep. There is no sushi on them. They may have interesting ecological niches near hydrothermal sources in high-pressure ice, moreover, the dry sources will look more like terrestrial ones. The weather on the surfaces of the oceans will resemble the earth, although the clouds there will be either unusually high or very flat. Life can flourish on both worlds, but it will be limited in minerals - there is no land, no leaching, less minerals in the oceans. From Wet to fly into space is as hard as from Earth, and from Dry to fly much harder.

Anders Sandberg is a researcher, scientific debater, futurologist, transhumanist, and writer. He received his Ph.D. in computational neuroscience at the University of Stockholm, and is currently a research fellow at the James Martin Research Society at the Institute for the Future of Humanity at Oxford University.