Extrasolar planets have captured the public imagination in recent years for reasons ranging from the joy of scientific discovery to the desire of finding a new home for humanity in the distant future. And without a doubt, NASA’s Kepler mission has been the most prolific hunter of extrasolar planets in history. By monitoring the brightness of hundreds of thousands of stars, Kepler is able to detect the telltale dip in brightness indicating the transit of an exoplanet across the face of its host star. Most exciting of all is that the design of the Kepler hardware and mission has been tailored to detect Earth-size planets orbiting inside of the habitable zones of their suns.

Even after the loss of a second reaction wheel ended its primary mission of monitoring almost 200,000 stars in a patch of sky straddling the border of the constellations Cygnus and Lyra, Kepler team members were able to devise an ingenious new observation strategy for an extended mission known as “K2”. Instead of staring at a single patch of sky as was done for four years as part of the primary mission, Kepler now observes a sequence of star fields along the ecliptic for periods of about 80 days each. With this new observation scheme, Kepler can track each star field with sufficient accuracy to continue planet hunting using its remaining pair of operating reaction wheels in conjunction with the slight pressure of sunlight reflecting off of the spacecraft. Although this approach is not as sensitive as that used in the primary mission and can only detect planets with orbital periods shorter than about a month instead of about a year or more, this approach promises to sample a broader range of stars looking for extrasolar planets.

By the beginning of 2015, members of the Kepler mission science team started announcing discoveries from the K2 mission. Among the first was a trio of planets orbiting the red dwarf called EPIC 201367065 (now also known as K2-3) announced on January 16, 2015 by a team led by Ian Crossfield (Lunar & Planetary Laboratory – University of Arizona). Crossfield and his team believed that the outermost planet they found in this system is potentially habitable, although an independent assessment suggests that this claim may be somewhat overstated (see “Habitable Planet Reality Check: Kepler’s New K2 Finds”). Another find that some have claimed may be among the most Earth-like extrasolar planets is called EPIC 201912552b (or better known now as K2-18b) whose discovery was announced in a paper with Benjamin T. Montet (Harvard-Smithsonian Center for Astrophysics) as the lead author. Unfortunately given the measured size of this world, even Montet et al. believe it is much more likely that this planet is a volatile-rich mini-Neptune (see “Habitable Planet Reality Check: EPIC 201912552b”). Despite the dubious prospects for these worlds being habitable, future study of these and other Kepler finds promises to shed much light on the limits of planetary habitability and demonstrate the promise of the K2 mission.

Crossfield et al. have continued their search for new exoplanets and recently submitted for publication the results of their analysis of the first year’s worth of K2 data as well as their follow up observations of promising candidates from ground-based observatories. In addition to detecting exoplanets which had been identified by earlier analyses (see “The First Year of Kepler’s K2 Mission”), Crossfield et al. have validated 64 more exoplanets and uncovered another 63 candidates requiring more observations to confirm. Out of this haul, 37 exoplanets have radii smaller than twice that of the Earth. One of the more interesting systems announced in this latest work is a four-planet system associated with the star know as K2-72 some of which may be potentially habitable planets.

Background

The star K2-72 (also known as EPIC 206209135 and 2MASS J22182923-0936444) is a red dwarf star with a V magnitude of 15.31 in the constellation of Aquarius located, according to a NASA press release, about 181 light years away. Kepler observed this star as part of its K2 Campaign 3 which ran from November 14, 2014 to February 3, 2015. Based on the star’s proper motion and multi-band photometry, Huber et al. estimated that K2-72 has a surface temperature of 3497±150 K, a radius of 0.23 times that of the Sun and a mass 0.22 times. Huber et al. note, however, that these latter two values are likely underestimated due to known limitations of the data and models they used.

The first planet found orbiting this star, K2-72b with a period of 5.58 days, was revealed publicly in November 2015 by Vandenberg et al. in a preprint of their first-look analysis of the first four K2 observing campaigns. Three more exoplanet candidates with longer periods were confirmed by the new analysis by Crossfield et al.. Assuming the host star’s properties of Huber et al., all four of the planets appeared to be slightly smaller than the Earth while two have effective stellar flux values (i.e. how much energy a planet receives from its sun) comparable to Earth’s opening the possibility that they might be potentially habitable planets. The properties of the four planets currently known to be orbiting K2-72 are summarized in Table 1 below.

Table 1: Properties of Planets Orbiting K2-72 (Assuming Host Star Properties of Huber et al.)

Planet b c d e Radius (Earth=1) 0.75±0.20 0.86±0.22 0.73±0.20 0.82±0.22 Orbit Period (days) 5.577 15.19 7.760 24.17 Orbit Radius (AU) 0.037 0.072 0.046 0.098 S eff (Earth=1) 5.4±3.2 1.41±0.85 3.5±2.1 0.76±0.46

Because of the dimness of the host star, determining the masses of these planets using precision radial velocity measurements will be difficult. However, since the orbits of K2-72c and d seem to be in a 2:1 mean motion resonance while K2-72b and c are close to a 5:7 resonance, it is expected that the orbits of these exoplanets will vary noticeably over time due to strong mutual gravitational interactions. This opens the possibility that the masses could be derived eventually by analyzing transit timing variations (TTV). The masses combined with the radii would allow the planets’ mean density to be calculated giving some indication of their bulk composition.

Potential Habitability

While a full assessment of the habitability of any exoplanet would require very detailed information about all of its properties, obtaining such information is simply beyond the reach of our current technology. At this early stage in our search for other Earth-like worlds, the best we can do is compare what properties we can derive to our current expectations of the range of properties for habitable worlds to determine if a new find is potentially habitable. And by “habitable”, I mean habitable in an Earth-like sense where the surface conditions allow for the existence of liquid water on the planet’s surface. While there may be other worlds that might possess environments that could support life (e.g. Mars or the tidally heated oceans on the moons Europa and Enceladus), these would not be Earth-like habitable worlds of the sort being considered here.

One of the important properties we can estimate with Kepler finds is the effective stellar flux, S eff . This can be calculated using the orbital period derived from Kepler data in combination with stellar properties determined by the analysis of various ground-based observations. Looking at the latest models for the conservative limits of the habitable zone (HZ) for Earth-like planets from Kopparapu et al., the HZ of K2-72 would have S eff values which range from about 0.93 times that of Earth for the inner edge (corresponding to the runaway greenhouse limit) out to about 0.25 (corresponding to the maximum greenhouse limit). Given the rather large uncertainties currently associated with these exoplanets’ S eff values, I roughly estimate that there is about a 20% and 50% probability that K2-72c and e, respectively, orbit inside of the HZ defined this way. The S eff values for the other pair of exoplanets is too high for them to have any reasonable chance of orbiting inside the conservatively defined HZ even with the large uncertainties involved in their S eff values.

However, the actual inner edge of the HZ might have a higher S eff value than the conservatively defined value for Earth-like planets. Since the planets in this system orbit so close to their host star, it is probable that tidal effects have slowed their rotation or even made them to rotate synchronously. Increasingly detailed climate models for slow and synchronous rotators over the last two decades has shown that such planets can maintain habitable conditions globally with higher S eff values than fast rotators like the Earth could. Using recent climate models of Yang et al., the inner limit of the HZ of K2-72 for a slowly rotating planet would correspond to an S eff as high as 1.65. Given this higher S eff value for the inner limit of the HZ, I estimate that there is roughly a 55% and 85% probability that K2-72c and e, respectively, orbit inside of this expanded definition of the HZ. Combined with their radii that are smaller than Earth’s (which virtually guarantees that these exoplanets are predominantly rocky in composition like the Earth), it seems fairly likely that K2-72b and especially K2-72e could be considered potentially habitable extrasolar planets.

Unfortunately, this optimistic assessment comes with a huge caveat. Aside from the orbital period and the size of the planet relative to its host star which are derived from the analysis of Kepler’s transit observations, the properties of these planets were determined using the host star’s properties from Huber et al. which were already suspected of being low. Based on their own analysis, Crossfield et al. estimated that K2-72 probably has a radius closer to 0.40 +0.12/-0.07 times that of the Sun instead of 0.232 ±0.056 found by Huber et al.. Crossfield et al. note that analyses of new spectral data for this star that are currently being prepared for publication seem to support this larger radius value.

One of the effects of this change in stellar properties is that the calculated absolute sizes of the four planets orbiting K2-72b would now be larger. Instead of being smaller than the Earth, Crossfield et al. believe that these exoplanets probably have radii in the 1.2 to 1.5 R E range. Based on earlier work by Rogers (see “Habitable Planet Reality Check: Terrestrial Planet Size Limit”), it is now known that planets transition from being predominantly rocky like the Earth to volatile-rich mini-Neptunes at a radius no larger than 1.6 R E and probably closer to 1.5 R E . A more recent analysis of the mass-radius relationship with a much larger collection of exoplanetary data by Jingjing Chen and David Kipping (Columbia University) suggests that this transition begins at 1.2 R E with the probability that a planet has a rocky composition decreasing quickly with increasing radius. With this in mind, the probability has increased that the four planets known to be orbiting K2-72 with radii in the 1.2 to 1.5 R E range are mini-Neptunes with little chance of being habitable in the conventional sense. While certainly not a “deal breaker” at this early stage, the probabilities that K2-72c and e are potentially habitable are certainly diminished somewhat.

By far the greatest impact of a larger host star on the assessment of the potential habitability of the planets orbiting K2-72 would be in the S eff values. The larger radius of K2-72 implies a larger mass – perhaps on the order of a factor of two larger. With an orbital period fixed by Kepler’s transit observations, this means the orbital radii of these exoplanets would increase by something on the order of 25%. At the same time, the larger mass results in an even greater increase in the star’s luminosity. The net result is that the S eff values for these four exoplanets could be on the order of half again as high as those listed above in Table 1. This would make it less likely that K2-72e is potentially habitable and possibly force K2-72c out of consideration entirely. We will have to wait until better data become available for the host star properties before we can more accurately assess the potential habitability of the planets in this system. Crossfield et al. seem to suggest that will be soon.

Conclusion

Based on the properties listed in the paper by Crossfield et al. which were derived from cataloged host star parameters, it appears likely that K2-72c and e are Earth size planets orbiting inside the habitable zone of their host star. However, these quantities are admittedly based on values of key host star properties with known shortcomings. Crossfield et al. believe that the star K2-72 is actually much larger resulting in planets with radii in the 1.2 to 1.5 R E range along with higher effective stellar flux levels, S eff whose values have yet to be determined with any certainty. While the larger planet radii increases the chances these worlds are mini-Neptunes with little chance of being habitable like the Earth, by far the greatest impact on the potential habitability of these planets would be from an increase in S eff . As we wait for a better characterization of the key properties of the host star to resolve the current uncertainties, it does seem likely that K2-72e and possibly even K2-72c could be potentially habitable.

Regardless of how the issue with the properties of K2-72 and its planets is resolved, it should be remembered that these finds represent just the tip of the proverbial iceberg of K2 discoveries. Depending on how long Kepler can continue to operate as well as the efficiency of ongoing data processing and analysis efforts, Crossfield et al. estimate that Kepler’s K2 mission should be able find a total of 500 to 1,000 new exoplanets. With only 104 validated exoplanetary discoveries made to date from just the first year of K2 mission data (a mission that has already exceeded two years in length), we could eventually see up to another dozen or more systems like K2-72 identified some of which could host much more promising candidates for being potentially habitable.

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Related Reading

“The First Year of Kepler’s K2 Mission”, Drew Ex Machina, November 29, 2015 [Post]

“Habitable Planet Reality Check: Terrestrial Planet Size Limit”, Drew Ex Machina, July 24, 2014 [Post]

General References

Jingjing Chen and David Kipping, “Probabilistic Forecasting of the Masses and Radii of Other Worlds”, arXiv 1603.08614, March 29, 2016 [Preprint]

Ian J.M. Crossfield et al., “197 Candidates and 104 Validated Planets in K2’s First Five Fields”, arXiv 1607.05263 (accepted for publication in The Astrophysical Journal Supplement Series), July 18, 2016 [Preprint]

Daniel Huber et al., “The K2 Ecliptic Plane Input Catalog (EPIC) and Stellar Classifications of 138,600 Targets in Campaigns 1-8”, The Astrophysical Journal Supplement Series, Vol. 224, No. 1, Article ID. 2, May 2016

Andrew Vanderburg et al., “Planetary Candidates from the First Year of the K2 Mission”, The Astrophysical Journal Supplement Series, Vol. 222, No. 1, Article ID. 14, January 2016

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

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

“NASA’s Kepler Confirms 100+ Exoplanets During its K2 Mission”, NASA Press Release, July 18, 2016 [Press Release]