Habitable Planet Reality Check: Kepler’s K2-155d

Without a doubt, NASA’s Kepler mission has been the most prolific discoverer of extrasolar planets to date. It has done this by looking for periodic dips in the brightness of hundreds of thousands of stars caused by the transits of orbiting exoplanets. Even after its primary mission ended in May 2013 when the failure of a second reaction wheel after four years in space prevented Kepler from pointing at its target area straddling the border of the constellations Lyra and Cygnus, project engineers and scientists were able to formulate an alternate extended mission to continue hunting for exoplanets. Since the official start of this “K2” extended mission in May 2014, the spacecraft has been observing a sequence of star fields along the ecliptic for stretches of about 80 days at a time before moving on to the next star field. This observation strategy was possible using the remaining pair of reaction wheels by balancing the slight pressure of sunlight reflecting off of the spacecraft to maintain an accurate fix during the observation runs (see “The First Year of Kepler’s K2 Mission”).

During Kepler’s four-year primary mission, it nearly continuously observed a single patch of sky on the edge of the constellation Cygnus. The ongoing K2 extended mission is observing a succession of fields for 80 days at a time along the ecliptic. (NASA/Ames Research Center/Wendy Stenzel)

Although the short observation times inherent in the K2 mission means that only exoplanets with orbital periods shorter than about 40 days can be detected, potentially habitable exoplanets orbiting red dwarfs are still within reach of this mission. The first such discovery, K2-3d, was announced in January 2015 (see “Habitable Planet Reality Check: Kepler’s New K2 Finds”). The latest such discovery was announced on March 12, 2018 in a press release from the Tokyo Institute of Technology among 15 new exoplanets which had been found in the K2 data set by the KESPRINT collaboration – a group formed in 2016 by the merger of KEST (Kepler Exoplanet Science Team) and ESPRINT (Equipo de Seguimiento de Planetas Rocosos Intepretando sus Transitos). Out of this group, three super-Earth size exoplanets were found orbiting a star designated K2-155 which are described in a paper published in the Astronomical Journal with Teruyuki Hirano (Tokyo Institute of Technology) as the lead author. One of these new finds, K2-155d, appears to orbit inside the star’s habitable zone (HZ) and could have surface conditions which allow for the existence of liquid water. So what are the prospects that K2-155d is potentially habitable?

Background

The star K2-155 (also known as EPIC 210895787 and 2MASS J04215245+2121131) is a red dwarf star with a V magnitude of 12.8 located in the constellation of Taurus roughly 200 light years away. Kepler observed this star as part of its K2 Campaign 13 whose high-cadence observations ran from March 8 to May 27, 2017. Analysis of the Kepler photometry of K2-155 revealed the presence of three sets of regularly occurring dips in the star’s brightness with periods of about 6, 14 and 40 days. While the lack of secondary eclipses helped to eliminate the possibility that these were false positives caused by K2-155 being an eclipsing binary, additional observations were needed to confirm the planetary nature of these signals. In addition, since Kepler observations provide information on the orbital period (from the time between successive transits) and the size of the candidate exoplanet relative to the star it orbits (by the depth of the transit), detailed knowledge of the properties of the star were needed to calculate other parameters such as the exoplanet’s absolute radius, the size of its orbit and the amount of energy it receives from its sun (i.e. its effective stellar flux or S_eff). Once again, more detailed observations of K2-155 were required.

Shown here is the star field for K2 Campaign 13 which ran from March 8 to May 27, 2017. Click on image to enlarge. (NASA/Ames Research Center)

Among the first follow up observations made by the KESPRINT collaboration involved obtaining high resolution imagery of K2-155 to look for close stellar companions whose presence could affect the interpretation of K2 results. On the night of September 5, 2017, NESSI (NASA Exoplanet Star and Speckle Imager) on the 3.5-meter WIYN Consortium’s telescope at the Kitt Peak Observatory in Arizona was used to obtain high resolution images of K2-155. No additional stellar companions were seen at projected distances greater than several AU. An examination of archival imagery from the Palomar Observatory Sky Survey obtained in 1950, when K2-155 was 14 arc seconds away from its current position in the sky, showed no background stars present down to a V magnitude of about 18.

Sample images reconstructed using data from NESSI/WIYN showing K2-155. The plot shows the detectable difference in magnitude as a function of distance from these images. (Hirano et al.)

In order to eliminate the possibility that a closer orbiting companion star might be present, the radial velocity (RV) of K2-155 was measured by members of KESPRINT using spectra obtained from the FIES (Fibre-fed Echelle Spectrograph) on the 2.56-meter NOT (Nordic Optical Telescope) at the Observatorio del Roque de los Muchachos at La Palma in the Canary Islands. Analysis of spectra obtained on December 24, 25 and 27, 2017 and January 10, 2018 showed no RV variations down to the tens of meters per second level. These RV measurements, plus an archived RV measurement made a decade earlier, along with the lack of any “contamination” in the spectra of K2-155 effectively eliminates the possibility that the three signals found in the K2 photometry are the result of a stellar companion bolstering the case that these are bona fide exoplanets.

The Nordic Optical Telescope (NOT) at the Observatorio del Roque de los Muchachos in the Canary Islands was used to obtain spectra to measure the radial velocity (RV) of K2-155 as part of follow up observations of this system. (Magnus Gålfalk)

In order to pin down the properties of these exoplanets, the properties of the star itself needed to be refined. Members of the KESPRINT collaboration used the Tull Coude Spectrograph on the 2.7-meter Harlan J. Smith Telescope at the McDonald Observatory in Texas to obtain high dispersion spectra on the nights of September 14 and October 14, 2017. By comparing these spectra to a library of spectra from hundreds of well-characterized stars, it was possible for Hirano et al. to estimate key stellar parameters of K2-155. The best fit for their data was for a star with an effective temperature of 3919±70 K, a radius 0.526±0.053 times that of the Sun, a mass of 0.540±0.056 times and a luminosity of 0.059±0.013 times. Based on these estimated properties, the existing photometry for K2-155 suggests a distance of 203±30 light years. The metallicity of K2-155 (i.e. the concentration of elements heavier than helium) was found to be only about 38% that of the Sun. All of these properties are consistent with an early-type M dwarf star that is somewhat larger and brighter than the average red dwarf.

The upper panel shows the K2 photometry of K2-155 obtained during Campaign 13. The bottom panels show closeups of the transits of the three super-Earth size exoplanets that were found. Click on image to enlarge. (Hirano et al.)

Using these newly derived stellar parameters, it was possible to calculate the properties of the three exoplanets detected orbiting K2-155. These properties, taken from Hirano et al., are summarized below in Table 1 along with the uncertainties where they are significant. It should be noted that the fairly large uncertainties in the star’s parameters translate into large uncertainties in the exoplanets radii, their orbital semimajor axis and especially the effective stellar flux, S_eff. Despite the uncertainties, these parameters are good enough to begin to assess the types of worlds which orbit K2-155 and the potential habitability of K2-155d. Hirano et al. also noted how similar this system is to that of K2-3 whose outer planet is also considered to be potentially habitable (see “Habitable Planet Reality Check: Kepler’s New K2 Finds”).

Table 1: Properties of Planets Orbiting K2-155

Potential Habitability

A thorough assessment of the habitability of any extrasolar planet would require a lot of detailed data on the properties of that planet, its atmosphere, its spin state, the evolution of its volatile content and so on. Unfortunately, at this very early stage, the only information typically available to scientists about extrasolar planets are basic orbit parameters, a rough measure of its size and/or mass and some important properties of its sun. Combined with theoretical extrapolations of the factors that have kept the Earth habitable over billions of years (not to mention why our neighbors are not habitable today), the best we can hope to do at this time is to compare the known properties of extrasolar planets to our current understanding of planetary habitability to determine if an extrasolar planet is “potentially habitable”. And by “habitable”, I mean in an Earth-like sense where the surface conditions allow for the existence of liquid water – one of the presumed prerequisites for the development of life as we know it. While there may be other worlds that might possess environments that could support life, these would not be Earth-like habitable worlds of the sort being considered here.

The first step in assessing the potential habitability of K2-155d is to determine what sort of world it is: is it a rocky planet like the Earth or is it volatile-rich mini-Neptune with little prospect of being habitable in an Earth-like sense. If we know the radius and mass of an exoplanet, its mean density can be readily calculated which in turn can be used to constrain its bulk composition. While the radii of the exoplanets orbiting K2-155 have been derived from Kepler measurements, unfortunately there are no mass measurements currently available.

One method used today which could provide the mass of an exoplanet is the analysis of precision radial velocity (RV) measurements. The initial RV measurements derived by Hirano et al. using FIES/NOT spectra set a rather high upper limit of 75 times that of the Earth or M_E for the mass of K2-155d. While sufficient to confirm the planetary nature of this object, it is not enough to help determine what kind of world this is. Hirano et al. estimate that the three exoplanets orbiting K2-155 should produce RV variations with amplitudes in the 1 to 2 meter per second range. A dedicated observation program with an 8 to 10-meter telescope like the 10-meter Keck Telescope with HIRES (High Resolution Echelle Spectrometer) should be able to provide the needed data in the future. Future data on the transit timing variations (TTV) caused by the interactions of these exoplanets could also provide the needed mass estimates especially for the inner two exoplanets of this system whose orbital periods are close to a 2:1 resonance (increasing the magnitude of their gravitational interactions).

A view of the two ten-meter telescopes of the Keck Observatory atop of Mauna Kea. Keck I is fitted with the HIRES spectrometer which could be used to measure the mass of K2-155d. (NASA)

Without any information on the masses of the transiting planets of K2-72 immediately forthcoming, we are forced to rely on statistical arguments based on the observed mass-radius relationship of other exoplanets whose radii and masses have been measured. A series of analyses of Kepler data and follow-up observations published over the last several years has shown that there are limits on how large a rocky planet can become before it starts to possess increasingly large amounts of water, hydrogen and helium as well as other volatiles making the planet more of a Neptune-like world. Rogers has shown that planets have even chances of being mini-Neptunes at a radius of no greater than 1.6 times that of the Earth (or R_E) although 1.5 R_E seems more probable (see “Habitable Planet Reality Check: Terrestrial Planet Size Limits”). The probability that an exoplanet has a more Earth-like rocky composition would decrease with increasing radius.

A more recent analysis of the mass-radius relationship with a much larger collection of exoplanetary data by Chen and Kipping suggests that that the gradual transition from rocky to volatile-rich exoplanets starts at about 1.2 R_E again with the probability that a planet is rocky decreasing with increasing radius. Hints of this transition in exoplanet populations from primarily rocky to volatile-rich worlds is evident in the statistical analysis of Kepler finds which shows comparatively fewer exoplanets in the 1.5 to 2.0 R_E size range. This result suggests that once rocky planets reach a radius of about 1.5 R_E, they become massive enough to retain more volatiles including a puffy envelope of hydrogen which allows them to jump the size gap to radii of 2.0 R_E and greater. Only the largest rocky planets and the smallest mini-Neptunes fill this gap.

The distribution of the number of planets as a function of radius shows a gap in the 1.5 to 2.0 RE range where rocky planets transition to being volatile-rich mini-Neptunes. Click on image to enlarge. (Tokyo Institute of Technology)

With a derived radius of 1.64 R_E, K2-155d seems to be just above the threshold where it is more likely to be a volatile-rich mini-Neptune – a possibility which Hirano et al. readily acknowledge. However, the low metallicity of its host star might make it a bit more likely that K2-155d is a rocky planet. Some studies suggest that low metallicity stars are less likely to form mini-Neptunes because it takes longer for their cores to form. If it takes too long, the circumstellar disk out of which these planets form may become depleted of gas and other volatiles leaving the forming protoplanet with a rocky composition. But before we get too far into this argument, it should be remembered that the current uncertainty in the radius of K2-155d is rather large (dominated by the uncertainty in the stellar radius of K2-155) and has a one-sigma range of 1.29 to 2.05 R_E. A more accurate radius measurement will be needed to help resolve the issue. All that can be stated given what little we know is that it is possible that K2-155d is a rocky planet but the odds might favor it being a mini-Neptune instead.

Another important criterion which can be used to determine if a planet is potentially habitable is the amount of energy it receives from its parent star known as the effective stellar flux or S_eff. According to the work by Kopparapu et al. (2013, 2014) on the limits of the habitable zone (HZ) based on detailed climate and geophysical modeling, the inner limit of the HZ is conservatively defined by the runaway greenhouse limit where a planet’s temperature would soar even with no CO₂ present in its atmosphere resulting in the loss of all of its water in a geologically brief time in the process. For an Earth-mass planet orbiting K2-155, this happens at an S_eff value of 0.94 which corresponds to a mean orbital distance of 0.25 AU. With a S_eff calculated to be 1.67±0.38, K2-155d orbits too close to its sun to be considered habitable by this definition. However, there are other definitions for the HZ worth considering for this case.

Because of the tight orbit of K2-144d, it would be expected to be a synchronous rotator which keeps the same side pointing towards its sun during the course of its orbit. Detailed climate modeling over the last two decades has shown that synchronous rotation is not the impediment to global habitability as it was once thought. In fact, it has been predicted that slow or synchronous rotation can actually result in an increase of the S_eff corresponding to the inner edge of the HZ owing to feedback mechanisms which result in the formation of a reflective cloud layer on the daylight side. According to the recent work by Yang et al., the inner edge of the HZ for a slow rotator orbiting a star like K2-155 would have an S_eff of 1.75 corresponding to an orbital distance of just 0.18 AU. With the large uncertainty in its S_eff, there is about a 58% probability that K2-155d orbits inside this definition for the HZ for synchronous rotators.

An even more recent paper by Kopparapu et al. (2017) which incorporates the latest data of how key greenhouse gases transmit and absorb infrared radiation suggests that changes in the atmospheric structure for synchronous rotators can lead to rapid and permanent water loss for an Earth-size exoplanet at an S_eff of around 1.38 even before a runaway greenhouse effect sets in. Various forms of elevated activity red dwarfs typically experience would be yet another loss mechanism that would exacerbate the situation (although the large orbit of K2-155d compared to other HZ exoplanets orbiting smaller red dwarfs would help limit these loss processes). With the permanent loss of water, the carbonate-silicate cycle which helps act as a global thermostat breaks down allowing CO₂ to build up in the atmosphere resulting in a dry runaway greenhouse much as Venus experiences today in our own solar system. Given the current uncertainties in the S_eff of K2-155d, there is about a 22% chance that this exoplanet orbits inside this more conservative “water loss” definition of the HZ.

Climate models by Hirano et al. show that liquid water can be supported on the surface of K2-155d if the incident flux is less than ~1.5. Click on image to enlarge. (Hirano et al.)

Hirano et al. used a 3D general circulation model to assess the potential habitability of K2-155d for themselves assuming that it has an Earth-like composition. They assumed that K2-155d was a synchronous rotator with a radius of 1.6 R_E and a mass of 4.2 M_E (a value that is consistent with a roughly Earth-like mean density). They further assumed that the planet possesses a 900-meter deep ocean (whose circulation can help transport heat) and has an atmosphere similar to that of the Earth – a one-bar N₂ atmosphere with CO₂ present at the one part per million level. By varying the S_eff of their model, they found that moderate surface temperatures were possible for S_eff values less than about 1.5 essentially splitting the difference between the models of Yang et al. and Kopparapu et al.. At higher values, the surface temperatures begin to rise quickly resulting in a runaway greenhouse effect. I estimate that there is a 33% chance that K2-155d has an S_eff value lower than the limit found by Hirano et al. who clearly concede in their paper that their new find would only be habitable if the S_eff value is towards the low end of the range they found.

Given the uncertainties in the various red dwarf HZ models currently available (compounded by the uncertainties in the actual S_eff of K2-155d), the possibility that this exoplanet orbits inside the HZ does not appear especially promising. Folding in the real likelihood that K2-155d may be a mini-Neptune with no prospect of being habitable in the Earth-like sense and the argument that this new discovery is potentially habitable is diminished even further. This assessment is not too different than that of K2-3d which Hirano et al. cite as being similar. Although with a radius currently pegged at 1.35±0.16 R_E and an S_eff of 1.20±0.27, K2-3d would seem to have better prospects for being potentially habitable than K2-155d.

We should not be too disheartened by this initial assessment, however. It must be remembered that the uncertainties in the stellar properties of K2-155 are still rather large. Updated values could result in better prospects for K2-155d being habitable. The situation with another red dwarf, K2-72, provides a perfect example. When the discovery of K2-72e was announced in July 2016, it was calculated that it had a radius of 0.82±0.22 R_E and an S_eff of 0.76±0.46. Based on its Earth-like size and stellar flux, it seemed that K2-72e had excellent prospects of being potentially habitable (see “Habitable Planet Reality Check: Kepler’s New Finds At K2-72”). But at the time it was known that the estimated properties of the host star found in Kepler’s Ecliptic Plane Input Catalog (EPIC) were not as accurate as they should be. Subsequent follow up observations by two separate groups cooperating with the Kepler science team were able to refine our knowledge of K2-72 finding that it was larger and brighter than originally estimated. In March 2017 these groups published their findings which resulted in updated radius and S_eff values for K2-72e of 1.29±0.13 R_E and 1.2±0.2, respectively. Although both were larger, K2-72e still appears to have good prospects for being potentially habitable (see “Habitable Planet Reality Check: Update on Kepler’s K2-72”). A change in values of similar magnitude in the other direction could result in a better assessment for K2-155d.

In order to make a better assessment of the potential habitability of K2-155d, a more detailed characterization of the stellar properties of K2-155 will be essential. New ground-based observations and a more accurate distance measurement from ESA’s ongoing Gaia astrometry mission will be of great help. Hirano et al. suggest that K2-155 would be a good target for ESA’s upcoming CHEOPS (CHaracterizing ExOPlanets Satellite) mission slated for launch in late 2018. Its observations should help refine the size estimates for these exoplanets and could provide data to estimate the masses of these exoplanets using the TTV technique. As mentioned earlier, a dedicated precision RV measurement campaign on an 8 to 10 meter telescope could provide mass measurements. A mass measurement combined with better knowledge of its radius would allow the bulk composition of K2-155d to be constrained. Better knowledge of the orbit and the properties of its host star would help to refine its effective stellar flux value.

CHEOPS, shown here in an artist’s impression, could observe K2-155 and help refine the properties of this system’s planets. (ESA/C. Carreau)

Regardless of how the question of the potential habitability of K2-155d is resolved, this is still an important system for future study. With a V magnitude of 12.8, K2-155 is one of the brightest red dwarfs observed by Kepler and found to possess transiting planets – in this case, planets which straddle the transition from rocky worlds to volatile-rich ones. Follow up observations by future instruments like NASA’s James Webb Space Telescope (JWST) of such a comparatively bright red dwarf could begin to probe the atmospheres of its planets providing fresh insights into the properties and evolution of such worlds. The potential habitability of K2-155d could be then more thoroughly assessed giving scientists the data they need to validate and further refine their models of planetary habitability. With this in mind, K2-155 is an excellent target for future scientific investigation.

Summary

Given what is currently known about K2-155d, it does not seem that it has especially promising prospects for being potentially habitable in the Earth-like sense. Its radius of 1.64 R_E seems to favor it being a volatile-rich mini-Neptune and its high effective stellar flux, S_eff, of 1.67 seems to be too high to avoid experiencing a runaway greenhouse – possible outcomes which Hirano et al. acknowledge in their paper. However, the uncertainties in the derived properties of K2-155d combined with those of the mass-radius relationship for exoplanets as well as the uncertainties in exoplanet climate models means that this new find could be potentially habitable after all. What is needed is more data to help refine and expand our knowledge of the bulk properties of K2-155d. Fortunately, K2-155 is a relatively bright red dwarf making it an excellent target for future ground and space-based studies. And the data scientists gather about this and similar worlds will allow them to validate and refine their climate models bringing us one step closer to understanding how common habitable exoplanets are.

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Related Reading

In addition to the articles cited above, there is an ever-growing list of articles on Drew Ex Machina related to the results from NASA’s Kepler mission. A complete list of these articles can be found on this web site’s Kepler mission page.

General References

Jingjing Chen and David Kipping, “Probabilistic Forecasting of the Masses and Radii of Other Worlds”, The Astrophysical Journal, Vol. 834, No. 1, Article id. 17, January 2017

Teruyuki Hirano et al., “K2-155: A Bright Metal-Poor M Dwarf with Three Transiting Super-Earths”, Astronomical Journal, Vol. 155, No. 3, Article ID. 124, March 2018 [Preprint]

R.K. Kopparapu et al., “Habitable zones around main-sequence stars: new estimates”, The Astrophysical Journal, Vol. 765, No. 2, Article ID. 131, March 10, 2013

Ravi Kumar Kopparapu et al., “Habitable zones around main-sequence stars: dependence on planetary mass”, The Astrophysical Journal Letters, Vol. 787, No. 2, Article ID. L29, June 1, 2014

Ravi Kumar Kopparapu et al., ” Habitable Moist Atmospheres on Terrestrial Planets near the Inner Edge of the Habitable Zone around M Dwarfs”, The Astrophysical Journal, Vol. 845, No. 1, Article ID. 5, August 2017

Leslie A. Rogers, “Most 1.6 Earth-Radius Planets are not Rocky”, The Astrophysical Journal, Vol. 801, No. 1, Article id. 41, March 2015

Jun Yang et al., “Strong Dependence of the Inner Edge of the Habitable Zone on Planetary Rotation Rate”, The Astrophysical Journal Letters, Vol. 787, No. 1, Article id. L2, May 2014

“15 new planets confirmed around cool dwarf stars”, Tokyo Institute of Technology Press Release, march 12, 2018 [Press Release]