Is Exoplanet K2-18b Habitable?
In recent years, astronomers have begun to discover and characterize planets from outside of our solar system, and these discoveries feature prominently in popular science reporting. The exoplanets that receive the most sensational coverage are those that seem likely to support life as we know it on Earth. Among these was K2-18b, an exoplanet discovered in 2015 using data from the KEPLER 2 mission. In 2019, it grabbed headlines based on the discovery of liquid water in its atmosphere and the associated likelihood that this planet might support life.
This paper will review the discovery and characterization of K2-18b, including the discovery of water in its atmosphere and explain how and why we know that this world may be able to support life. First, this paper covers the properties of planets that are required to support life, and how those properties are used to decide which planets are good candidates for supporting life. Next is a closer look at how astronomers measured these properties for K2-18b. Lastly, this paper discusses how habitable K2-18b might be based on what we know and what conditions might be like there.
How do we classify planets for habitability?
Understanding what properties an exoplanet might require to support life starts by understanding the reasons Earth supports life. Life as we understand it requires liquid water, a particular combination of elements, protection from radiation, and an atmosphere.
Water – The Habitable Zone
Liquid water is critical for Earth-like life. In order for liquid water to exist on the surface of a word, some part of the surface of that world must be warmer than freezing and cooler than a temperature that could trigger a runaway greenhouse effect1.
A star’s habitable zone is the range of distances from the star where it may be possible for liquid water to exist1. If a planet is too close to the star a runaway greenhouse effect would boil any water that might be on the planet away2. If a planet is too far from the star insufficient energy reaches the world to keep water above freezing, even under maximum greenhouse conditions1. Astronomers use climate modelling along with the energy output of the star to estimate the range where it is possible for a planet to support liquid water1.
In addition to liquid water the chemical composition of a planet has to be right in order to support life. The minimum set of elements required for life as we understand it on earth are carbon, hydrogen, oxygen, potassium and sulfur along with an assortment of essential metals3. The sort of metals required depends on the kind of organism that could live there. As an example, methanogens (a kind of simple bacteria that breathes carbon-dioxide and produces methane) require nickel as part of that process4.
The best chance for these basic materials to exist is on Telluric (or Terrestrial) worlds5. In our solar system, Mercury, Venus, Earth and Mars are all Telluric6. These planets have solid rocky surfaces composed of silicates, metals and elements less common in the rest of the universe6. Telluric worlds may also have metallic cores, magma and plate tectonics, all of which are crucial to maintaining life on earth7.
Atmosphere and Magnetosphere
On Earth, the atmosphere plays a complex role in the ecosystem. Living creatures breathe the air, and the greenhouse effect warms the earth to above the freezing point of water, among many other crucial effects. Astronomers use the basic physical properties of an exoplanet to determine if it is capable of keeping an atmosphere. Escape velocity, which is the speed something needs to be travelling to escape the gravity of the planet8, is an important consideration in how a planet maintains an atmosphere. Escape velocity is higher when gravity is stronger, so planets with a large mass have a higher escape velocity, and planets with a large radius have a lower escape velocity9. If the planet’s mass is too low, or it’s radius too large relative to its mass, then the escape velocity for airborne molecules would be low enough that the atmosphere would drift away over time. The Earth’s magnetosphere deflects harmful radiation which can disrupt organic molecules or remove water from the atmosphere as it did on Venus10.
As this is an active area of astronomy, a number of habitability classification systems have been proposed. The purpose of these classification systems is to sort exoplanets by their likelihood to support life, to help prioritize future study by astrobiologists. This paper will discuss three of these to illustrate how they work.
The first is the Earth Similarity Index, which makes the assumption that planets that are measurable closest to Earth are the most likely to support life11. It is a weighted index from 0 to 1 that makes it easy to sort planets based on radius, density, surface temperature and escape velocity11. All of these characteristics can be estimated directly from astronomical observations11.
The second is the Planet Habitability Index11 which makes fewer assumptions about what life requires. It instead ranks worlds on having a stable substrate, some sort of liquid solvent (not necessarily water), available chemical basis, and if there is sufficient energy available to support life11.
A third is the Biological Complexity Index12 which attempts to assess the likelihood of complex biological life based on properties that can be observed today12. It builds on the ideas of the Planet Habitability Index by considering geology, temperature and age, and computes a result using index values for substrate, energy, geophysics, temperature and age12. One of the interesting results of this index is that one exoplanet scored higher than Earth, meaning that some exoplanets may be better suitable to complex life than Earth13. The planet that scored higher than Earth is larger and warmer than Earth, and orbits a star much older than ours12. The implication is that if life arose there, there’s a chance that the planet has supported life for longer than Earth has, which would mean that it would have had more time to evolve, and therefore possibly be more complex than our own ecosystem13.
How did astronomers measure habitability characteristics for k12-18b?
Discovery and the Circumstellar Habitable Zone
K2-18b was discovered during Campaign 1 of the Kepler 2 (K2) mission using the Kepler Space telescope14. This mission had to contend with the failure of two reaction wheels used to keep the telescope steadily oriented14. This meant that between observations, the telescope would sometimes drift a few pixels, making it hard to track exactly which pixel in the images the telescope sent home. Statistical and analytical methods were applied to the observations to separate evidence of transits15 from noise caused by this drift.
Once the data was cleaned up, astronomers were able to plot light curves for the star K2-18 that covered the 80 days the telescope was looking in its direction15. A light curve is simply a graph that tracks the brightness of a star over time16. When looking at these light curves, observers noted that light coming from the star K2-18, was slightly dimmer once every 32.9 days14. The dimmer light indicates that something passed between the Kepler photometer and the star. The regularity and predictability of this observation17 suggested that it was a planet orbiting the star14. This technique for detecting exoplanets, which the Kepler mission was designed for, is called the transit method17. This observation was confirmed using the Spitzer telescope18.
Determining if K2-18b was in the habitable zone required understanding properties of both the star and the planet. In order to draw the boundaries of the habitable zone, astronomers need to know how much energy would reach a planet at a given distance1. Then, they needed to know how far K2-18b was from the star in order to determine if it was in that zone. All of these questions were answered by studying the star itself.
Temperature, Mass and Luminosity of K2-18
The hotter a star is the further away the habitable zone will be, so one thing astronomers needed to measure was the surface temperature of K2-18. Astronomers used a technique that analyzed the wavelengths of light, or spectra, coming from the star to determine the temperature14. Two different spectra were collected from the uSpeX spectrograph at the NASA Infrared Telescope Facility, and the Supernova Integral Field Spectrograph at the University of Hawaii 2.2m telescope14. They compared the real spectra with spectra for theoretical stars generated with computer models of stellar atmospheres (modelled using PHOENIX atmosphere modelling software). This gave astronomers a temperature for K2-18 because its spectra matched modelled stars and the computer model included a temperature14. Empirical studies have shown that these models are likely to match a temperature determined using more lengthy and costly observations19.
By studying stars closer to earth, astronomers know that the mass of some stars are related to the amount of energy they radiate, or luminosity. Fortunately, K2-18b is a red dwarf20 whose mass can be determined this way. Luminosity is calculated from the absolute magnitude (brightness) and radius of a star21. The absolute magnitude of a star is how bright a star is at a distance of 3.26 light years21. That can’t be measured directly, but astronomers can measure the apparent magnitude that we can see from Earth, and then use the distance to the star to compute the absolute magnitude21. The apparent magnitude of K2-18 was measured using spectroscopy14. The distance to K2-18 and the radius of the star were measured using parallax data from the GAIA space telescope20. Parallax is the change in angle a distant object appears to have if you view it from two different positions22, and the GAIA spacecraft is able to produce observations from different points along its orbit around the sun23. While we can measure the apparent magnitude using a spectroscopy, knowing both the absolute magnitude and the radius of a star requires parallax data to give us the distance to the star and its radius. This filled in the missing pieces, giving K2-18b a Luminosity of 2.34% of the sun20.
With both the distance to the planet, and the amount of energy coming from the star in hand, the Habitable Zone could be calculated as 0.12 – 0.25 Au24, or roughly 12-25% of the distance from the Earth to the Sun.
Thanks to Kepler’s Laws of planetary motion, astronomers know that the distance a planet orbits its star at related to the mass of the star, the mass of the planet and the orbital period of the planet25. For an initial estimate, the mass of the planet is so much smaller than the mass of the star that the orbital distance can be determined using the mass of the star alone26. This allowed astronomers to place the orbital distance of K2-18b at 0.1429 AU, which is within the Habitable Zone20.
Planetary temperature can be estimated by knowing the amount of radiation given off by the star and the distance from the star to the planet, and by making some assumptions about how much energy is reflected from the star back into space (albedo)27. By assuming that this property is similar to earth, astronomers estimated that the equilibrium temperature is 284K28 without considering a greenhouse effect. The likely surface temperature depends on a number of things that have yet to be determined including the true albedo and greenhouse effect29. If more solar energy is reflected back into space, the planet will be cooler. If the greenhouse effect is strong, the planet will be hotter.
The distance K2-18b orbits at means it is likely tidally locked30. This means that one side of the planet is likely facing the star at all times, meaning on one half of the world, it is always day, and on the other, it is perpetual night, with a strip of twilight dividing the two30.
The radius of the planet was determined from the light curves produced by its transit. These are the same light curves from the Kepler Space Telescope and the Spitzer Telescope that were used to detect it28. The more the star dims during the transit (transit depth) the bigger the planet is relative to the star it transits in front of31. Astronomers were able to combine the transit measurements with the radius of the star to determine the radius of the planet28.
Mass was measured using Radial Velocity (RV) measurements in the visible spectrum using two separate spectrometers28. The first was the High Accuracy Radial velocity Planet Searcher Spectrograph (HARPS)28. The second was the Calar Alto high-Resolution search for M dwarfs with Exoearths with Near-infrared and optical Échelle Spectrographs (CARMENES)28. Estimating mass using radial velocity relies on the fact that the star moves slightly when a planet orbits it32. Even though the planet is much less massive than the star, Newton’s Universal Law of Gravitation tells us that its gravitational influence still moves the star slightly33. This movement is visible as a slight shift in the spectrum of light observed coming from the star in a period that corresponds with the orbital period of the planet32. This information was used along with the orbital period of the planet to determine its mass to be 8.8x as massive as Earth28.
With mass and radius understood, the escape velocity was calculated as 21.0 km/s34, which is almost twice the escape velocity of Earth8. K2-18b is capable of keeping its atmosphere.
Two separate teams have detected the presence of water vapour in the atmosphere of K2-18b24,35. K2-18b was considered an ideal candidate for attempting to detect water due to its position in the Habitable Zone, the stability of its star, and its tentative classification as a super-earth 24,35. The presence of water acts as a confirmation that at least some parts of the world are likely to have a temperature above the freezing point of water35.
To detect water, repeated observations of transits of K2-18b were made by the Wide Field Camera on the Hubble Space Telescope, focusing on wavelengths that are characteristic of light passing through possible atmospheric compounds24. What they were looking for was differences in the transit depths (how much the light emitted by the star was dimmed when blocked by the planet) at different wavelengths24. Because some of this light passed through the atmosphere of the planet, the dips would be due to photons being absorbed by matter in the atmosphere24. These repeated observations were combined to create a stronger signal, and the resulting spectra were compared with spectra expected for a wide range of atmospheric compositions24. The result was a fit for three different possible atmospheres containing water: a cloudless atmosphere with water and helium, a cloudless atmosphere with water, helium and nitrogen or a cloudy atmosphere with water and helium24. Analyzing similar data, another team noted a water absorption band in the infrared spectrum35. It is therefore likely that water vapour exists in the atmosphere of K2-18b24,35.
Life on K2-18b
Of the exoplanets discovered and characterized to date, K2-18b is among the most likely to support life36. The Planetary Habitability Laboratory maintains a list of exoplanets sorted by the Earth Similarity Index. On this list K2-18b has an ESI of 0.71 and is currently twentieth on optimistic habitability list36 which expands the consideration to less Earth-like worlds. Considering ESI alone it is the 37th most habitable exoplanet discovered to date36. The optimistic habitability list does not exclude super-earths, ocean worlds, and possible miniature Neptunes36. Based on its density and radius, it is more likely that K2-18b is either an ocean world, or a miniature warm Neptune with a thick atmosphere and a small rocky core34 than it is a super-earth, which is why it is on this second list. The ESI does not consider the presence of water vapour in the atmosphere11, but based on this additional evidence, it is somewhat more likely to harbour life than planets with similar properties35.
TODO: ESI Table
Consider the most optimistic possible outcome for K2-18b, given what we know today. The most optimistic composition for K2-18b is an ocean world28 with a hydrogen/helium atmosphere with some nitrogen24. In this case, the planet would have a rocky core of silicates and iron, with a deep layer of water or ice, and a deep gaseous atmosphere over that28. While the gravity is estimated at 1.37 times Earth’s gravity28, if the atmospheric layer is deep, the pressure and temperature could be crushing near the surface of the ocean37. As the planet is likely tidally locked, K2-18b has regions of perpetual day, night and twilight30. Temperature is likely to be tolerable in some areas of the planet, and boiling and freezing in others38. Depending on how well the atmosphere is able to move heat around the world, the twilight band might be the most stable and temperate region where life could safely arise39. Planets like K2-18b are unlikely to have seasons40 due to the influence of tidal forces removing any axial tilt the planet may have had upon formation. This tilt erosion is caused by the gravity of the nearby star which deforms the planet to create bulges pointing toward and away from the star40. This change in shape eventually leads to a loss of tilt40. It is not known whether the planet has a magnetosphere to protect the atmosphere and biological life from radiation as magnetospheres have only been detected on hot Jupiter exoplanets41. It is possible that some form of life could arise under these conditions, though it might be very unlike life on Earth38. However, even on Earth, life is able to thrive in extreme conditions42, including extreme heat, radiation, pressure and acidity42. While these extremophiles tend to be less complex42, they might be the most abundant form of life on Earth42.
Due to its large radius (2.3x earth)28, however, and current understanding of planetary formation43, it is more likely that K2-18b is a Neptune-like world37, with a deep atmosphere and crushing pressure and high temperatures near the surface of the rocky core37. If enough of the atmosphere was lost, scientists believe that such planets could support life44. In our solar system, Neptune does have a magnetosphere45, so it’s not impossible that in this case K2-18b could have this protection.
This paper has shown how astronomers know whether K2-18b may be able to support life, and what sort of conditions that life might exist under. K2-18b orbits its star every 32.9 days14 at a distance of 0.1429 AU28, which is within the habitable zone1. It has an equilibrium temperature between 268 and 300 Kelvin (-5 to 27 degrees Celsius)28, is 2.3x28 the size of Earth and is 8.8x as massive28. It ranks 20th on the optimistic list of habitable exoplanets36. Current research indicates that K2-18b is probably not habitable in the way that we understand life on Earth, but the presence of liquid water24 means that it does meet at least one of the key characteristics for supporting life. Classifying worlds for habitability is a challenging field within planetary astronomy and astrobiology that helps prioritize future research. Those worlds mostly likely to support life can be studied first when improved techniques emerge for measuring the properties of extrasolar planets. As this process continues, so will our understanding of the distribution of potentially habitable worlds in the observable universe.
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