Habitable Planet Reality Check: Kepler’s New K2 Finds

The New Year is certainly proving to be a very productive one for NASA’s Kepler mission. With the detailed analysis of the data from its primary mission well underway, there has been a steady stream of discoveries already announced this year. Among these was the announcement of the discovery of eight new planets orbiting in the habitable zone (HZ) (see “Habitable Planet Reality Check: 8 New Habitable Zone Planets”), the quieter announcement of a pair of Kepler planet candidates orbiting inside the HZ of two Sun-like stars (see “Earth Twins on the Horizon?”) and an analysis on the prevalence of potentially habitable planets orbiting red dwarfs just to name just a few (see “Occurrence of Potentially Habitable Planets Orbiting Red Dwarfs”). With such a strong start and more discoveries sure to come, 2015 should prove to be a historic year for extrasolar planetary studies.

But as scientists continue to pore over the huge data base generated during Kepler’s primary mission, the initial results from its extended K2 mission are just now beginning to trickle in. Among the first discoveries to be announced from the K2 mission are three planets orbiting an obscure red dwarf star designated EPIC 201367065 and later better known simply as K2-3. The discovery of these three super-Earth-size planets was announced by a team led by Ian Crossfield (Lunar & Planetary Laboratory – University of Arizona) on January 16, 2015. Some have even claimed that the smallest of these new planets, K2-3d, is potentially habitable. In fact, the University of Puerto Rico at Arecibo Planetary Habitability Laboratory already lists EPIC 201367065d in their “Habitable Exoplanet Catalog” ranking it as the seventh most Earth-like planet currently known.

Diagram depicting potentially habitable planets currently in the “Habitable Exoplanet Catalog” (assuming very optimistic definitions) maintained by the Planetary Habitability Laboratory. Included in the top row is EPIC 201367065d also known as K2-3d. Click on Image to enlarge. (PHL)

Background

The star K2-3 (referred to as EPIC 201367065 in the original discovery paper) is an M0V red dwarf located an estimated 150 light years away in the constellation of Leo. Crossfield et al. have determined that it has an effective surface temperature of 3896 K, a mass 0.60 times that of the Sun and a radius of 0.56 that of the Sun. Although these parameters imply an luminosity of about 6.5% that of the Sun, its V-band magnitude of 12.2 makes it one of the brighter red dwarf stars Kepler has observed during its primary or extended K2 mission.

For Kepler’s K2 mission, the satellite 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. This observation strategy was required so that Kepler could accurately track each star field using its remaining pair of reaction wheels and the slight pressure of sunlight reflecting off of the spacecraft. K2-3 is located in the first star field observed during K2 designated Campaign 1 which ran from May 30 to August 21, 2014.

K2-3 came to the attention of Crossfield et al. early because of its brightness and high proper motion which hinted that it was relatively nearby. Using the data processing algorithm called TERRA developed to process K2 data, Crossfield et al. easily found the periodic transit signature of a planet with an orbital period of 10.1 days. After removing its effects from the Kepler photometric data, a second planet with a period of 24.6 days was found. A visual inspection of the data then revealed an additional pair of transits implying the presence of a third planet with an orbital period of 44.6 days which had been missed by TERRA because it requires three transits for a candidate to be detected.

Ground-based telescopic observations of EPIC 201367065 or simply K2-3. The images are from a) the Digital Sky Survey, b) the Sloan Digital Sky Survey, c) the Keck/NIRC2 K-band adaptive optics and d) a contrast curve showing the sensitivity of the latter observations to faint companions. Click on image to enlarge. Crossfield et al.)

Follow-up observations allowed Crossfield et al. to determine the basic properties of K2-3 and eliminate the possibility of the presence of any close companions or other issues that would generate false positives or affect the determination of the planets’ properties. Combining the data from Kepler photometry and the results of their analysis of the follow-up observations, Crossfield et al. were able to derive the size and basic orbital properties of the three planets they found. Some of the key parameters are listed below in Table 1.

Table 1: Properties of Planets Orbiting EPIC 201367065 (K2-3)

At this time, the masses of the three new planets found orbiting K2-3 have yet to be determined. But Crossfield et al. have been able to estimate that they have masses about in the 4 to 5 M_E range using some reasonable assumptions about the possible range of compositions of these worlds. Based on dynamical simulations using these assumed masses, Crossfield et al. have determined that this system appears to be stable.

Based on these assumed masses, Crossfield et al. estimate that these planets should produce variations in their sun’s radial velocity with amplitudes in the 1.2 to 2.3 meter/second range. Given the relative brightness of K2-3, current extrasolar planet search programs using the radial velocity technique should be able to measure the masses of these worlds with fair accuracy in the near future so long as the natural noise or jitter in the radial velocity of K2-3 is fairly low. With an estimated age in excess of one billion years and no signs in the Kepler photometry indicating rotation-related changes in brightness (the first sign of surface activity), K2-3 is likely not to be too active and promises to provide fairly accurate radial velocity data. Since the radii of these planets span the transition between rocky and more Neptune-like planets which was determined by Rogers to occur at radii no larger than 1.6 R_E, the determination of these planet’s masses will aid in the study of the mass-radius relationship of extrasolar planets (see “Habitable Planet Reality Check: Terrestrial Planet Size Limit”).

Potential Habitability

Using a model based on recent work by Torres et al. who just announced the discovery of eight HZ planets among stars observed during Kepler’s primary mission (see “Habitable Planet Reality Check: 8 New Habitable Zone Planets”), I estimate that there is about a 13%, 37% and 50% chance that K2-3b, c and d, respectively, are rocky planets. Based on these estimates, it seems likely that only planet d has a reasonable probability of being a rocky planet with any potential for being habitable.

Looking at the latest models for the conservative limits of the habitable zone (HZ) from Kopparapu et al., the habitable zone of K2-3 for a 5 M_E planet would have insolation or effective stellar flux values, S_eff, ranging from about 1.01 times that of Earth for the inner edge (corresponding to the runaway greenhouse limit) out to about 0.26 (corresponding to the maximum greenhouse limit). With  the S_eff values for K2-3b and c, it is apparent that they readily exceed the maximum value corresponding to the conservative inner limit of the HZ for this star. Coupled with the likelihood that these two planets are mini-Neptunes instead of rocky super-Earths, it is highly improbable that these are potentially habitable planets.

As mentioned earlier, there have been claims already made about the potential habitability of K2-3d. With an S_eff of 1.51 (+0.57/-0.43), the nominal value of the effective stellar flux for this planet readily exceeds the conservative HZ limit as defined by Kopparapu et al.. But given the still relatively large uncertainty associated with the S_eff value for this planet at this time, I estimate that there is still about a 12% probability that K2-3d actually orbits inside the conservatively defined HZ. Coupled with the about even odds of not being a rocky planet, K2-3d seems to have poor prospects of being a potentially habitable planet despite the claims being made by some.

Given the disagreement in the definition of the inner limits of the HZ, there are other less conservative definitions of the HZ that might still be worth considering. Yang et al. have studied the effects of slow or even synchronous rotation of planets on HZ limits. While the rotation state of K2-3d is unknown, it could easily be a slow or even a synchronous rotator as a result of tidal interactions with its sun. Using the models of Yang et al., the inner limit of the HZ of K2-3 for a slowly rotating planet would correspond to an S_eff as high as 1.74. I estimate that there is about a 66% chance that K2-3d would orbit inside this more optimistic definition of the HZ assuming it is a slow rotator.

While we can debate about which definition of the inner limit of the HZ is correct and whether or not K2-3d and similar worlds should be considered “potentially habitable”, future observations of this planet might actually provide hard data to help resolve the issue. As was mentioned earlier, all three of the new planets found orbiting K2-3 should produce radial velocity variations that will allow their masses to be determined or at very least tightly constrained. Combined with the radii determined from K2 observations, the densities of these worlds can be calculated and their bulk compositions determined. Follow-up observations by NASA’s TESS (Transiting Exoplanet Survey Satellite) should help to further refine the properties of the inner two planets and there are better than even odds of detecting at least one transit of the third planet during its scans of this part of the sky.

A depiction of NASA Transiting Exoplanet Survey Satellite (TESS) which should be able to make follow-up observation of the planets of K2-3. (NASA)

More importantly, calculations by Crossfield et al. indicate that it should be possible for the James Webb Space Telescope (JWST) to study these planets spectroscopically due to their large size and the relative brightness of K2-3. This should allow the composition of their atmospheres above any cloud layers present to be determined. These additional data along with comparable follow-up observations of similar planets should help scientists begin to resolve the potential habitability of these worlds and provide better definitions for the limits of the HZ.

Conclusions

Contrary to the claims being made by some, it does not appear that K2-3d (called EPIC 201367065d in the original discovery paper) is a good candidate for being a potentially habitable planet given what we currently know about this find. While it has about an even chance of being a rocky planet, its effective stellar flux is about 50% higher than the conservative inner limit of the HZ as defined by the onset of a runaway moist greenhouse effect as described by Kopparapu et al.. It is only by invoking much more optimistic definitions for the HZ that K2-3d seems to have a reasonable chance of being a potentially habitable planet.

Although this initial assessment is not very promising, it could change. Additional follow-up observations should help to reduce the uncertainties in the properties of this planet especially for its effective stellar flux. It should also be possible to measure the masses of all three of the new planets found orbiting K2-3 using existing precision radial velocity techniques. Combined with data from the Kepler mission (and further refinements using additional TESS observations that will be available in a few years), it should prove possible to calculate these planets’ densities and determine whether they are rocky super-Earths of volatile-rich mini-Neptunes. Most importantly, calculations strongly suggest that future spectroscopic studies using the JWST should reveal the compositions of the atmospheres of these three planets. These observations should not only be able to resolve the question about the potential habitability of K2-3d, but also provide data needed to assess the often conflicting models for the inner limits of the HZ.

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Related Reading

“Habitable Planet Reality Check: 8 New Habitable Zone Planets”, Drew Ex Machina, January 8, 2015 [Post]

“Occurrence of Potentially Habitable Planets around Red Dwarfs”, Drew Ex Machina, January 12, 2015 [Post]

“Earth Twins on the Horizon?”, Drew Ex Machina, January 9, 2015 [Post]

“Composition of Super-Earths”, Drew Ex Machina, January 3, 2015 [Post]

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

“The Architecture of M-Dwarf Planetary Systems”, Drew Ex Machina, October 24, 2014 [Post]

General References

Ian J.M. Crossfield et al., “A Nearby M Star with Three Transiting Super-Earths Discovered by K2”, arVix 1501.03798 (submitted to The Astrophysical Journal), January 16, 2015 [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

Leslie A. Rogers, “Most 1.6 Earth-Radius Planets are not Rocky”, arVix 1407.4457 (submitted to The Astrophysical Journal), July 16, 2014 [Preprint]

Guillermo Torres et al., “Validation of Twelve Small Kepler Transiting Planets in the Habitable Zone”, arVix 1501.01101 (submitted to The Astrophysical Journal), January 6, 2015 [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

“Habitable Exoplanet Catalog”, Planetary Habitability Laboratory web site [Link]