A recent issue of Nature Astronomy reported on a detection of a watery super-earth in the habitable zone of a nearby M-Class star, in a paper entitled “Water vapour in the atmosphere of the habitable-zone eight-Earth-mass planet K2-18b”. This is a very interesting discovery, the first of its kind, but likely we will hear more and more about finds of this type in the future, as the observational technology steadily improves.
The interesting features of the detection are included in the title of the paper – water, habitable zone, eight-Earth mass. These are all thought to be necessary (though not sufficient) for the development of life, as we know it. So, let’s break down the title of the paper further, to explore the significance of each phrase in the title:
in the atmosphere of the habitable-zone eight-Earth-mass planet K2-18b
Water is now known to be extremely common, throughout the universe. It has been found on many planets of our solar system (ice, liquid or vapour), in galactic molecular clouds, and in stellar atmospheres. Water is essential for life as we know it, due to its peculiar chemical properties, which render it an excellent solvent, that supports the cellular activities of living things. In the search for extra-terrestrial life, NASA and other agencies of this type have emphasized the strategy “follow the water”.
In the atmosphere
of the habitable-zone eight-Earth-mass planet K2-18b
In the case of this detection, the water was discovered in vapour form, during a series of transits of the star that it orbits. These observations were made by the Hubble Space Telescope, which orbits Earth. As the planet passes in front of the star (transits) from the Earth’s perspective, the light of the star dims and the spectrum of light changes. Some light will actually pass through the planet’s atmosphere, altering the spectrum because of certain quantum effects, which leaves behind a fingerprint of the types of gases found in that atmosphere.
The modelling detected a statistically significant signal indicating that there was an atmosphere around K2-18b i.e. the spectrum differed from a flat spectrum (a model with no active gases) at better than 3 standard deviations. This in itself was important, as it was the first time an atmosphere had been detected around a “super-Earth” with a high level of statistical significance.
The observed spectrum was then compared to a synthetic spectrum of how light would be expected to be altered, as it passed through an atmosphere with a typical range of gases in the observed wavelengths, or potential absorbers, in the language of spectral analysis. These gases included water vapour (H2O), carbon monoxide (CO), carbon dioxide (CO2), methane (CH4), and ammonia (NH3). Hydrogen (H2), Helium (He) and Nitrogen (N2) were also modelled as possible constituents.
The strength of the match between the observed spectrum and the various synthetic spectrums was computed, via statistical “best fit” modelling. The candidates that best matched the observed spectra were:
· a cloud-free atmosphere of H2O and H2-He.
· a cloud-free atmosphere of H2O and H2-He, as well as molecular nitrogen.
· a cloudy atmosphere of H2O and H2-He.
All of these models indicated that water vapour was in the atmosphere, with high statistical confidence. However, none of the models was clearly superior to the others, though the cloud-free H2O and H2-He model was somewhat favoured. If that is the case, it would yield an atmosphere with some 30% to 50% water vapour. There may be other trace gases as well.
Water vapour In the atmosphere
of the Habitable Zone
eight-Earth-mass planet K2-18b
The habitable zone of a star is defined as that region that is far enough away from the star to allow liquid water to exist on the surface, but not so far that all water would become frozen as ice. That’s because liquid water is needed to carry on the life processes of cells.
The exact range of orbital distances that are within in the habitable zone depend on the type of star (mainly size and temperature) as well as certain features of the planet (mainly albedo or reflectivity and greenhouse effect). Bigger stars are hotter, so their habitable zones are farther out than is the case for smaller stars. The temperature of a star is measurable via its spectrum (where the spectrum peaks).
Planets with high albedos will reflect a lot of incoming light, while those with low albedos will absorb a lot of light. So, a planet that has a has a high albedo will have to be closer to its star, to sustain liquid water. As to the greenhouse effect, a planet with an atmosphere with a lot of greenhouse gases can be farther from its star, and still sustain liquid water. Features of a distant planet, such as albedo and greenhouse gas composition are much more difficult to measure.
Obviously, these factors (and many more, such as the existence of plate tectonics) will interact over a short and long term basis, making the precise distance of the habitable zone difficult to estimate. That’s also why modelling the Earth’s climate is difficult.
Water vapour In the atmosphere of the Habitable Zone
eight-Earth-mass planet K2-18b
Planets can be grouped into various classes, such as:
· Terrestrial (rocky, smaller and similar in size to Earth).
· Super-Earth (rocky, up to ten Earth masses).
· Mini-Neptunes (over ten Earth masses, up to Neptune size, rocky core, thick gas atmosphere.
· Gas giants (really big planets, such as the out planets of our solar system, such as Jupiter and larger)
This particular planet, K2-18b is thought to be about 8 Earth masses, and around 2.3 Earth radii. That gives it a density 3.3 g/cm3, which compares to Earth’s density of about 5.5. The lower density would imply a rocky planet, but with a substantial atmosphere, or perhaps a very substantial water ocean (a water world).
Given its orbital distance from its star (about 1.4 astronomical units or about 200,000 km) its temperature is probably between 200 and 320 degrees Kelvin. The freezing point of water is about 273 Kelvin and the boiling point is about 373, so there is a good chance that the planet’s temperature might be suitable for life as we know it. The finding of substantial water vapour in the atmosphere might imply a rather steamy water world.
The star that it is circling is much smaller than the sun, of the class M2.5, also known as a red dwarf. It is substantially cooler than our sun at about 3500 K, whereas our sun is at about 5800 K. Since it is cooler, the habitable zone is much closer to the star, so the planet in question in the habitable zone for that star.
There has been a lot of conjecture about whether M-class stars are suitable for life, as these stars give off frequent bursts of energetic radiation, via flares. These flares might effectively sterilize the surface of any planet in orbit around such a star, or strip it of its atmosphere.
Also, there is a greater tendency for planets orbiting such a star in the habitable zone to become tidally locked over time, similar to how the moon is tidally locked to the Earth. That would result in one side of the star being intolerably hot, with other side being freezing cold.
The very fact that these are “red” dwarfs indicate that the radiation peaks at a less energetic level than does the sun’s radiation. That means less energy for photosynthesis, which would inhibit life.
However, there has been some re-thinking of these issues. For example, red dwarf flares may not be as dangerous as once thought, especially as the star ages. As for tidal locking, it might result in crazy weather, but an atmosphere and ocean might well redistribute heat well enough for life to take hold.
Finally, it is quite possible that plants would evolve to make use of less energetic sunlight. These issues are explored in the blog and paper cited below.
And, let’s not forget that life has been found on a Red Dwarf, namely the British comedy SF series, Red Dwarf.
It should also be noted that this star is only about 124 or so light years from the Earth (about 38 parsecs). That’s a long way off in human terms, but in galactic terms (about 10,000 light years across the galaxy), that’s just about next door. The distance still makes visiting a dream for some far-flung future, but it will make close observation possible, especially as larger and more accurate space telescopes are launched. So, I think it is fair to say that this is “in the neighborhood”.
This ties into some of the recent objections to calling this planet a Super-Earth, rather than a mini-Neptune. If it is the latter, then it would seem to be a much less interesting discovery. But, at present, nobody can be sure, as the observational science is far from clear-cut. Time, and more observations with increasingly better instruments, will tell.
1 – Nature Astronomy paper
Water vapour in the atmosphere of the habitable-zone eight-Earth-mass planet K2-18 b, Angelos Tsiaras, Ingo P. Waldmann, Giovanna Tinetti, Jonathan Tennyson & Sergey N. Yurchenko, Nature Astronomy (2019).
2 – Life and Red Dwarf Stars
3 – Red Dwarfs and Photosynthesis
The Potential of planets orbiting red dwarf stars to support oxygenic photosynthesis and complex life. Joseph Gale and Amri Wandel, International Journal of Astrobiology, 16(1): 1-9 (2017)
Now that you have read some real science (astronomy and astrophysics), you should read some science fiction. The Witch’s Stones series would be an excellent choice. Alternatively, you could try the short story “The Magnetic Anomaly”, a SF story which includes plenty of interesting geophysics, including magnetic fields.
The Witches’ Stones
Or, you might prefer, the trilogy of the Witches’ Stones (they’re psychic aliens, not actual witches), which follows the interactions of a future Earth confederation, an opposing galactic power, and the Witches of Kordea. It features Sarah Mackenzie, another feisty young Earth woman (they’re the most interesting type – the novelist who wrote the books is pretty feisty, too).
The Magnetic Anomaly: A Science Fiction Story
“A geophysical crew went into the Canadian north. There were some regrettable accidents among a few ex-military who had become geophysical contractors after their service in the forces. A young man and young woman went temporarily mad from the stress of seeing that. They imagined things, terrible things. But both are known to have vivid imaginations; we have childhood records to verify that. It was all very sad. That’s the official story.”