I have said it before but it certainly bears repeating: without a doubt, the most prolific planet hunter to date has got to be NASA’s Kepler mission. Launched into solar orbit on March 7, 2009, Kepler spent four years continuously observing the brightness of almost 200,000 stars in a single patch of sky straddling the border of the constellations Cygnus and Lyra looking for small dips in star brightness indicative of planetary transits. With the loss of a second reaction wheel in May 2014, Kepler was no longer able to point with the required stability at this field of stars. To get around this loss, a new extended mission with a different mode of operations was devised. Kepler now observes 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. With this new observation strategy, Kepler can track each star field using its remaining pair of reaction wheels and the slight pressure of sunlight reflecting off of the spacecraft with sufficient accuracy to continue planet hunting. Called “K2”, Kepler’s extended mission with its new observation strategy officially started in May 2014.

By the beginning of 2015, the Kepler team started announcing discoveries from the K2 mission including EPIC 201367065d (now also known as K2-3d) which some claimed is potentially habitable (see “Habitable Planet Reality Check: Kepler’s New K2 Finds”). As follow up observations have been made of Kepler’s K2 and earlier finds, astronomers working with the Kepler science team are continuing to refine the derived properties of these extrasolar planets. In a paper recently accepted for publication in The Astrophysical Journal with Joshua Schlieder (NASA Ames Research Center) as the lead author, the latest findings on two small “temperate” transiting planets found during the first two K2 campaigns were announced. One of these planets, called K2-9b (the first planet found orbiting the ninth K2 target discovered to have a confirmed planet), just might be potentially habitable.

Background

The red dwarf star K2-9 (also known as EPIC 201465501) is located about 360 light years away in the constellation Virgo and was observed by Kepler during “Campaign 1” of its extended K2 mission which ran from May 30 to August 21, 2014 (see “The First Year of Kepler’s K2 Mission”). Based on detailed follow up observations by Schlieder et al., K2-9 is an M2.5V red dwarf with an estimated radius of 0.31±0.11 times that of the Sun, a mass of 0.30±0.14 times and a surface temperature of 3390±150 K resulting in a luminosity of just 0.012±0.010 times that of the Sun’s. Although the uncertainties in the derived star parameters are still fairly large, K2-9 seems fairly typical of the red dwarf stars that dominate our galactic neighborhood.

The discovery of the planet K2-9b was announced along with 30 other planets found during Kepler’s K2 Campaign 1 in a preprint with Benjamin Montet (Harvard-Smithsonian Center for Astrophysics) as the lead author originally submitted for publication to The Astrophysical Journal in March 2015. In this same paper, it was claimed that another one of their extrasolar planetary finds, EPIC 201912552b (now also known as K2-18b), was a potentially habitable planet (see “Habitable Planet Reality Check: EPIC 201912552b”). Like many other Kepler finds, K2-9 was the subject of extensive follow up observations by Schlieder et al. to refine the properties of the star which in turn helps to pin down the planet’s properties more accurately.

According to the analyses by Montet et al. and Schlieder et al., K2-9b has an orbital period of 18.45 days. Based on the stellar properties they derived, Schlieder et al. estimate that K2-9b has an orbital radius of 0.091 AU and a radius of 2.25 +0.53/-0.96 Earth radii (or R E ). This is larger than the radius of 1.60±0.42 R E derived by Montet et al. but, given the large measurement uncertainties, the two values are consistent with each other to within their error bars. Based on their analysis, Schlieder et al. estimate that the effective stellar flux, S eff , is 1.36 +1.59/-0.81 times that of the Earth.

Follow up ground-based observations as well as a search of archival images by Schlieder et al. shows only a single star in the region of sky where K2-9 is today eliminating the possibility that this detection is a false positive resulting from a background eclipsing binary. This imaging data combined with the results of radial velocity measurements which showed no significant variation, the possibility that K2-9 has a companion that might generate a false positive or otherwise complicate the interpretation of the Kepler data has effectively been eliminated with a calculated false positive probability of only 2.4X10-5.

Potential Habitability

The first criterion that needs to be satisfied if K2-9b is to be considered potentially habitable is that it must be a rocky planet like the Earth as opposed to a volatile-rich world like Neptune with little likelihood of being habitable in the conventional sense. Based on the analysis of Kepler results for small extrasolar planets by Leslie Rogers (Hubble Fellow at Caltech), planets larger than 1.6 R E are increasingly unlikely to be rocky planets and are more likely to be mini-Neptunes instead (see “Habitable Planet Reality Check: Terrestrial Planet Size Limit”). Based on their 2.25 R E radius estimate, Schlieder et al. calculate that there is only ~21% probability that K2-9b is a rocky world. However, the uncertainties associated with the radius derived for K2-9b are rather large and span the transition from rocky to volatile-rich planets found by Rogers. Assuming that the smaller radius of 1.60 R E found by Montet et al. is closer to the actual value, Schlieder et al. find that there is about a 50% probability that K2-9b is a rocky planet. Because of this low probability that K2-9b is a rocky planet, Schlieder et al. have downplayed this planet’s potential habitability in their paper.

While it seems to be more probable that K2-9 is a mini-Neptune, the possibility that it is a rocky planet can not be entirely eliminated especially in light of the large uncertainties in its derived radius. A more definitive determination of the kind of planet K2-9b is will require improvements in the radius knowledge as well as a measurement of its mass. Based on studies of the mass-radius relationship performed by others, Schlieder et al. estimate that their 2.25 R E value of the radius implies a mass of 7.6±4.1 times that of the Earth (or M E ) while the 1.60 R E radius found by Montet et al. leads to a mass estimate of 4.7±2.9 M E . Assuming the larger mass, Schlieder et al. calculate that K2-9b should cause a ~4.1 meter/second variation in the radial velocity of the star it orbits. While this is well within the demonstrated capability of ground-based, high precision radial velocity measurement systems like HARP, unfortunately K2-9 with a V magnitude of only 15.6 is too faint at visible wavelengths to be observed by these instruments. Like many other faint red dwarfs found by Kepler to have planets, we will have to wait until the introduction of the next generation of precision radial velocity instruments which operate in infrared (IR) wavelengths (where cool red dwarfs are brighter) like CARMENES, SPIRou, IRD or HPF to get the data required to determine the mass of K2-9b or at very least set upper limits.

In the mean time, scientists are hoping to get more data about K2-9b in the near future. Photometric observations currently planned using NASA’s Spitzer IR telescope should help refine our knowledge of the properties of this planet. More observations will also allow its orbit to be more precisely characterized and could reveal transit time variations (TTV) as a result of interactions of K2-9b with non-transiting planets that are likely to be present in the system (see “The Architecture of M-Dwarf Planetary Systems“). This might allow the TTV method to be used to estimate the mass of this extrasolar planet. A better distance measurement from ESA’s Gaia mission combined with additional ground-based observations should also help pin down the properties of this new world and its sun more accurately.

The next criterion to consider in determining the potential habitability of K2-9b is its position relative to the habitable zone (HZ). According to the conservative limits defined by Kopparapu et al., the inner edge of the HZ of K9-2 for a planet with a mass of 5 M E , as defined by the runaway greenhouse limit, is at a distance of 0.11 AU corresponding to an S eff value of 1.00. With a calculated S eff of 1.36, K2-9b is beyond this but, when the uncertainty in the S eff is taken into account, I estimate that there is still about one chance in three that K9-2b orbits within the conservatively defined HZ.

Of course there are other, less conservative definitions of the HZ that can be considered. Using the “early Venus” limit with an S eff of 1.49, Schlieder et al. find that their nominal S eff value is comfortably inside this definition of the HZ. Since it is likely that K2-9b is a synchronous rotator, the inner limit of the HZ for such worlds found by Yang et al. may apply. For a star like K2-9, the inner limit of the HZ for a slow or synchronous rotator corresponds to an S eff value of 1.62, assuming that this model is correct, I estimate that there is less than about one chance in four that K2-9b orbits outside of this much more optimistic limit of the HZ. So K2-9b might orbit inside the HZ depending on its actual S eff value and the definition of the HZ that might apply.

When dealing with planets orbit red dwarfs, there are a range of other issues that have been raised that might affect the potential habitability of an orbiting extrasolar planet. Schlieder et al. note that measurements made by NASA’s GALEX (Galaxy Evolution Explorer) satellite found that K9-2 has a rather high UV flux suggesting a high level of activity. Whether GALEX measurements are typical or K2-9, by chance, just happened to be observed during a transient flare is not known. But high UV flux could affect the atmosphere of any planet orbiting K2-9 and breakup key molecules like H 2 O, CO 2 and CH 4 high in the atmosphere impacting the potential habitability of K2-9b. If the high UV flux observed by GALEX was just a transit phenomenon, the atmosphere of K2-9b would be less affected. Only future observations to probe the composition of the atmosphere of K2-9b will tell what, if any effects, this UV flux has no matter its true nature. Unfortunately, given the dimness of K9-2 and the small size of its planet, JWST will not be able to provide such observations. We will have to wait for the introduction of a new generation of 30-meter class ground-based telescopes to get the required observations.

Conclusion

The initial impression is that K2-9b does not seem to be an especially promising a candidate for being a potentially habitable extrasolar planet. Even Schlieder et al. acknowledge that its measured radius suggests it is more likely to be a mini-Neptune. In addition, its S eff is rather high placing outside at least the conservatively defined HZ. The apparently elevated UV flux of K2-9 could also present problems for planetary habitability. But given the large uncertainties in the derived properties of K2-9b as well as the questions surrounding the true limits of the HZ especially for synchronous rotators, this extrasolar planet still has a possibility of being potentially habitable. More data will be required to characterize this newly discovered extrasolar planet better.

No matter which way the question of the potential habitability of K2-9b is resolved, Schlieder et al. rightfully point out that continued study of this extrasolar planet “can provide key insights into trends in bulk composition and atmospheric properties at the transition from silicate dominated to volatile rich bodies”. Such insights are vital as scientists begin to probe the true limits of planetary habitability and eventually validate the sometimes conflicting HZ definitions found by various theoretical studies over the past couple of decades.

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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: Kepler’s New K2 Finds”, Drew Ex Machina, January 20, 2015 [Post]

“Habitable Planet Reality Check: EPIC 201912552b”, Drew Ex Machina, May 12, 2015 [Post]

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

General References

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

Benjamin T. Montet et al., “Stellar and Planetary Properties of K2 Campaign 1 Candidates and Validation of 18 Systems, Including a Planet Receiving Earth-like Insolation”, The Astrophysical Journal, Vol. 809, No. 1, Article id. 25, August 2015

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

Joshua E. Schlieder et al., “Two Small Temperate Planets Transiting Nearby M Dwarfs in K2 Campaigns 0 and 1”, arXiv 1601.02706 (Accepted for publication in The Astrophysical Journal), January 12, 2016 [Preprint]

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