Number of habitable planets could be limited by stifling atmospheres

New research has revealed that fewer than predicted planets may be capable of harbouring life because their atmospheres keep them too hot.

When looking for planets that could harbour life, scientists look for planets in the 'habitable zones' around their stars - at the right distance from the stars to allow water to exist in liquid form. Traditionally, this search has focused on looking for planets orbiting stars like our Sun, in a similar way to Earth.

However, recent research has turned to small planets orbiting very close to stars called M dwarfs, or red dwarfs, which are much smaller and dimmer than the Sun. M dwarfs make up around 75 per cent of all the stars in our galaxy, and recent discoveries have suggested that many of them host planets, pushing the number of potentially habitable planets into the billions.

This month, both the TRAPPIST and Kepler planet-hunting telescopes have announced the discovery of multiple near-Earth-sized planets orbiting M dwarf stars, some within the .

New research from Imperial College London and the Institute for Advanced Studies in Princeton, published in the Monthly Notices of the Royal Astronomical Society, has revealed that although they orbit smaller and dimmer stars, many of these planets might still be too hot to be habitable.

The scientists suggest that some of the planets might still be habitable, but only those with a smaller mass than Earth, comparable to Venus or Mars.

Dr James Owen, Hubble Fellow and lead author of the study from the Institute for Advanced Studies in Princeton, said: "It was previously assumed that planets with masses similar to Earth would be habitable simply because they were in the 'habitable zone'. However, when you consider how these planets evolve over billions of years this assumption turns out not to be true."

It was known previously that many of these planets are born with thick atmospheres of hydrogen and helium, making up roughly one percent of the total planetary mass. In comparison, the Earth's atmosphere makes up only a millionth of its mass. The greenhouse effect of such a thick atmosphere would make the surface far too hot for liquid water, rendering the planets initially uninhabitable.

However, it was thought that over time, the strong X-ray and ultraviolet radiation from the parent M dwarf star would evaporate away most of this atmosphere, eventually making the planets potentially habitable.

The new analysis reveals that this is not the case. Instead, detailed computer simulations show that these thick hydrogen and helium envelopes cannot escape the gravity of planets that are similar to or larger in mass than the Earth, meaning that many of them are likely to retain their stifling atmospheres.

However, all is not lost, according to the researchers. While most of the M that are Earth-mass or heavier would retain thick atmospheres, smaller planets, comparable to Venus or Mars, could still lose them to evaporation.

Dr Subhanjoy Mohanty, the other study author from the Department of Physics at Imperial College London, said: "There are hints from recent exoplanet discoveries that relatively puny planets may be even more common around than Earth mass or larger ones, in which case there may indeed be a bonanza of potentially habitable planets whirling around these cool red ."

Ongoing ground- and space-based searches, and new space missions to be launched in the near future, should provide a definitive answer to this question as well as other questions about the potential suitability of these for life.

Explore further

Hunting for hidden life on worlds orbiting old, red stars

More information: James E. Owen et al. Habitability of terrestrial-mass planets in the HZ of M Dwarfs – I. H/He-dominated atmospheres, Monthly Notices of the Royal Astronomical Society (2016). DOI: 10.1093/mnras/stw959
Citation: Number of habitable planets could be limited by stifling atmospheres (2016, May 26) retrieved 18 July 2019 from
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May 26, 2016
Earth-sized planets in Earth-like orbits may not be Earth-like unless they were formed in an Earth-like manner, and the most widely accepted theory for that is Giant Impact. It is clear Earth's gravity and global magnetic field could hold onto a much larger atmosphere, just compare with Venus, but it is possible that much of our original atmosphere may have been lost in the Giant Impact.

So there might be two (or more) classes of Earth like planets: 1. True analogs that formed in the same way with similar materials; and 2. Smaller planets roughly 1/3 to 2/3 the mass of Earth that end up with similar conditions. I suggested this possibility last year:

May 26, 2016
All these possibilities and unknowns suggest that we won't properly assess nearby solar systems like Alpha Centauri unless we can detect all exoplanets there down to a size significantly smaller than Earth, at least Mars-sized and preferably moon-sized. This makes the job a great deal more difficult, but it keeps the door open to there being habitable planets there even if none of the habitable zone planets are as massive as Earth.

May 26, 2016
Uh, what about Earth-sized moons of giant planets ? Sorry, that silly SciFi movie has rather muddied the waters but, if the host sub-Jovian or Neptunian lacks our Jupiter's killer 'VanAllen' zones, there's more of a chance...

One tangential thought; given majority of exo-planets are being found by optical Doppler measurements, that means sensitivity goes by the sine (?) of their ecliptic's angle to our line of sight. What proportion are we missing ?

May 26, 2016
Soon there will be a scientific consensus it's all due to the civilizations' on those currently uninhabitable planets discovery of the combustion engine and their failure to give all their liberties and treasure to the followers of the alien equivalent of Karl Marx.

May 27, 2016
Uh, geo', it's more likely that any life there has yet to get beyond the single-cell stage. IIRC, took a while here, aka 'The Boring Billion'...

May 28, 2016
@Mark Thomas:

Terrestrials are believed to be formed by accretion like Earth, and Mercury, Earth, Moon and Mars all shows large last impacts.

The Moon-forming impact leaves Earth as either rare, or like Venus and perhaps unlike Mars the last accretion impacts didn't eject a debris ring that formed moons. It looks like the process typically makes 3 giant impacts on an Earth analog, [Range is 1-8, see fig 10 in http://iopscience.../126/pdf ] The upshot is that an atmosphere stripping impact is rare, ~ 10 % can strip an Earth atmosphere, none looks able to strip an Earth ocean, and if there were no ocean in the first place atmosphere stripping is harder. [Ibid.]


May 28, 2016

@Nik: " Earth-sized moons"

Good point! Those are not well studied. But at first order the moon-forming disks should behave much as planet-forming protoplanetary disks. E.g. expect Earth massed moons to look like Earth. So if they live in the rHZ, radiative HZ, of the star, no change. But if they live in the tHZ, tidal HZ, of the giant, maybe that HZ stays put.

"percentage ... missing".

I think you mean that most planet discoveries are by transit methods like Kepler. (Used to be that confirmation of candidates needed radial, doppler shift, methods. But the latest Kepler release used a different statistical method to reject implausible candidates.)

Transit methods sees only those where the planet aligns with the star sufficiently when viewed from Earth. That means we can see typically 1-2 % of the total planet inventory out to the distances we have looked. (Now out to Earth, even Mars, orbit in some cases.)

May 28, 2016
Nik, I think geokstr was trolling. But yes, unicellular life is most likely.

However, it is as far as I know an open question what the boring billions meant.

It looks indeed to be stasis on the geological (so biological) level, but on the other hand it is likely eukaryotes evolved at the time, or just before. The BB is 1.7 to 0.7 Ga, consensus is that eukaryotes evolved 2 to 1 Ga, small but complex multicellular red algae has been found at 1.2 Ga and possibly as far back as 1.6-1.7 Ga. [ https://en.wikipe...#Fossils ] "Complex" (differentiated multicellular with nitrification specialist cell) cyanobacteria is older.

If it was stasis you can argue that it means it is hard to evolve tissue forming animals,. They are known from after the BB, but there may have been a "slow fuse" tail from much earlier as in so many similar cases. (C.f. mammals, birds, et cetera.)

Details, details, ... We have many models, few good tests showing which are the valid ones.

May 28, 2016
@Mark: Oh, shoot, I forgot to describe Venus re great impacts.

It is a planet that is hard to tell early history on because its run-away greenhouse. It is believed the crust thickened due to the loss of temperature differential over it, little difference between upper mantle and lower atmosphere temp compared to Earth. (Perhaps 200 K vs 700 K as a simple estimate.) Any impact scars should be lava covered after billions of years.

And its thick atmosphere should mostly be from after the run-away greenhouse cooked the crust and liberated most of its carbon and some of its oxygen. The question is open I think, but again it is hard to tell the early history.

May 28, 2016
Other research has revealed that the Electric Universe is incapable of harbouring any life - sentient or otherwise - since the stifling, toxic atmosphere pervading it prevents such life-forms from appearing, let alone surviving. Whether the colossal electrical storms ravaging that particular cosmos also play a rôle in suppressing any sort of life at all is a subject for idle speculation.

Meanwhile, here on the good Earth, the EU is attempting to model its' own pitiful existence on such a cosmos, with a stifling atmosphere created - in their underground "labs", of course - from the farts emanating from such EU luminaries as Thornhill, Perratt and Scott, heated by the bitchy in-fighting and torrid, incestuous relationships exhibited by members of the group. Coming up for air occasionally, they are treated to having their noses rubbed(...)and gruesome sustenance.

- FineStructureConstant
You seem bitter. Had a bad week, did you? When EU is proven right, then what?

May 28, 2016
Or.. maybe just maybe move the thick atmosphere planets further out from their sun?

May 31, 2016
TBGL, the IOP paper you cited concluded that ~3 giant impacts (as they defined them) are most typical for a Earth-like planet in an Earth-like orbit, but atmosphere stripping giant impacts are rare. According to their simulations (to be taken with a grain of salt), moon-forming impacts are just 5% of all giant impacts while less than 0.2% completely strip an atmosphere (Page 8, cited IOP paper).

While the precise extent Earth's early atmosphere was reduced is unclear, given the large moon we have, it seems likely there was a significant reduction in atmosphere. Obviously, none of the other terrestrial planets (Mercury, Venus and Mars) have anything like our moon. So my original point remains, while all these exo-Earths were probably battered as well, it seems likely they lost less atmosphere on average than Earth as evidenced by the existence of our large impact-created moon.

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