Habitable Planet Reality Check: Kepler 1649c & The Implications for Earth-Size Exoplanets in Red Dwarf Habitable Zones

Andrew Vanderburg who led the team which found Kepler 1649c. (University of Texas at Austin)

Even though NASA’s Kepler spacecraft was officially retired on October 30, 2018 after it finally exhausted its propellant used for attitude control, teams of scientists around the globe continue to study its data looking for new exoplanets missed by the Kepler science team’s initial analysis. In February of 2020, a group led by graduate student Michelle Kunimoto (University of British Columbia) announced the discovery of 17 new transiting exoplanets which had been previously overlooked in the Kepler database including a potentially habitable world (see “Habitable Planet Reality Check: The Student-Discovered KIC-7340288b”).

On April 15, 2020 NASA announced the discovery of yet another potentially habitable exoplanet orbiting the star Kepler 1649. Found by a team led by NASA Sagan Fellow, Andrew Vanderburg (University of Texas at Austin), this new find is not only of interest in its own right but has provided data for a new statistical estimate for how common Earth-size exoplanets are in the habitable zones of red dwarfs.

Background

The star Kepler 1649 is a G-magnitude 16.3 star located on the border between the constellations of Lyra and Cygnus. Also known as 2MASS J19300092+4149496, Gaia’s latest parallax measurements place the star at a distance of 301.5±1.8 light years from us. Using this distance in combination with existing photometry, models indicate that Kepler 1649 is a mid-M-type dwarf (i.e. M3V to M5.5V) with the properties listed in the table below.

Properties of Kepler 1649 (Vanderburg et al.)

Kepler 1649 was observed for a total of 735 days during Quarters 6 through 9 and again for Quarters 12 through 17 as part of Kepler’s primary mission from 2010 to 2013 with the Kepler Input Catalog designation of KIC 6444896. An analysis of the first three quarters of photometry for this star showed periodic dips in brightness indicative of a transiting exoplanet candidate in a 8.689-day orbit which was given the initial designation of KOI 3138.01. With seven more quarters of data available for a more thorough analysis and validation of the signal, the candidate was confirmed in 2017 and given the designation Kepler 1649b.

Diagram showing the major components of NASA’s Kepler spacecraft. (NASA/Kepler Mission/Ball Aerospace)

A second signal with a period of 19.535 days was also noted in the analysis but Kepler’s automated validation software characterized the signal, initially identified as KOI 3138.02, as a false positive. Members of Vanderburg et al. set up the Kepler False Positive Working Group to visually inspect Kepler’s false positives for mischaracterized finds. They realized that unlike most false positives, KOI-3138.02 was in a short period orbit with dozens of observed transits with consistent shapes and depths. After an in depth reevaluation with light curves Vanderburg et al. generated themselves from Kepler’s photometric data, the team found that the transits passed a series a validation tests consistent with a planetary origin for the photometric signal giving the new exoplanet candidate the designation of Kepler 1649c.

The phase-folded light curves for Kepler 1649b (left) and c (right). Individual data points are in gray, averaged data points are in purple while the red curve gives the best fit transit model. Click on image to enlarge. (Vanderburg et al.)

The properties of the exoplanets orbiting Kepler 1649 based on the latest stellar parameters and Kepler data analysis are summarized in the table below. Included is the effective stellar flux of S_eff which provides a measure of the amount of energy a planet receives from its sun compared to the Earth. Kepler 1649c appears to be an Earth-size exoplanet with an Earth-like S_eff value opening the prospect that it is a potentially habitable exoplanet.

Properties of Kepler 1649 Exoplanets (Vanderburg et al.)

One of the curious properties of this system noted by Vanderburg et al. was that the orbital periods of the two exoplanets was almost exactly a 9:4 ratio. Much more common among packed planetary systems found by Kepler are ratios of 2:1 or 3:2. Vanderburg et al. speculate that there may be a planet orbiting between Kepler 1649b and c with a period of 13.030 days and a semimajor axis of about 0.063 AU creating a more likely 3:2 resonance chain in the system. Vanderburg et al. inspected their light curves and found no evidence for transits with this period down to the 600 parts per million level. Either this possible exoplanet is smaller than Mars and therefore too tiny for Kepler to detect or the inclination of its orbit to the plane of the sky is less than 89.0° so that the exoplanet fails to transit its primary as viewed from Earth. Future observations of transits may provide indirect evidence for the presence of this third exoplanet through transit timing variations (TTV) in the pair of transiting planets as a result of gravitational interactions between the trio.

Potential Habitability

So, what are the habitability prospects for Kepler 1649c? 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 biocompatible 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 Kepler 1649c 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 radius of Kepler 1649c has been derived from Kepler measurements, unfortunately there are no mass measurements currently available. And given the low apparent magnitude of the host star, it may take the next generation of precision radial velocity instruments to detect the reflex motion of the orbiting exoplanet.

Without any information on the mass of Kepler 1649c, 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.

This artist’s depiction illustrates the relative sizes of the Earth and Kepler 1649c. (NASA/Ames Research Center/Daniel Rutter)

A subsequent 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. With a radius of 1.06 +0.15/-0.10 R_E, the odds seem to favor Kepler 1649c being a rocky world like the Earth. Similarly, with a radius of 1.02±0.05 R_E, the earlier discovered Kepler 1649b is slightly more likely to be rocky as well.

Habitable Zone of Kepler 1649

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 outer limit of the HZ is conservatively defined by the maximum greenhouse limit beyond which a CO₂-dominated greenhouse is incapable of maintaining a planet’s surface temperature. Instead of helping to heat the atmosphere, the addition of more CO₂ beyond this point makes the atmosphere more opaque causing the surface temperatures to drop instead of increase. Kopparapu et al. (2013, 2014) suggests an S_eff value of about 0.24 for the outer limit of the HZ of an Earth-sized exoplanet orbiting Kepler 1649 corresponding to a mean orbital distance of 0.15 AU. Kepler 1649c, with a S_eff value of 0.75, orbits comfortably inside this outer limit.

Kopparapu et al. (2013, 2014) conservatively define the inner edge of the HZ 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 its water in a geologically brief time in the process. For a fast-rotating, Earth-mass exoplanet orbiting Kepler 1649, this happens at an S_eff value of 0.93  which corresponds to a mean orbital distance of 0.075 AU. Even by this conservative definition, Kepler 1649b would seem to orbit comfortably inside the HZ bolstering the claim that this new find is potentially habitable.

There are other definitions of the inner edge of the HZ  worth considering. Because of the tight orbits of these planets, they would be expected to be synchronous rotators which keep the same side pointing towards their sun during the course of their orbits. 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. But there are limits besides a runaway greenhouse effect to consider when defining the inner edge of the HZ.

This plot shows the orbits of Kepler 1649b (inner) and c (outer) compared to definitions of the habitable zone (HZ) in gray. Click on image to enlarge. (Vanderburg et al.)

A 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 orbiting Kepler 1649 at an S_eff of around 1.21 (corresponding to a orbital semimajor axis of 0.065 AU) even before a runaway greenhouse effect sets in. 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. Kepler 1649b clearly orbits well inside this limit and is likely to be a slightly larger and hotter version of Venus. The third undetected exoplanet hypothesized by Vanderburg et al. should orbit right at the edge of this more liberal definition of the HZ making it possible that the Kepler 1649 system has two potentially habitable planets (assuming the third exoplanet exists and is massive enough to hold onto its volatiles).

Conclusions

On the surface, it appears that Kepler 1649c has fairly good prospects of being considered potentially habitable. Of course this assessment comes with the usual caveats about how the volatile inventories of exoplanets orbiting red dwarfs evolve especially during the first couple of billion years when stellar activity is much higher. Future observations of exoplanets like Kepler 1649c should help to resolve these questions. Unfortunately, given the low apparent brightness of Kepler 1649, it may be quite some time before we learn more about these exoplanets. In the mean time, the visual inspection of Kepler data by Vanderburg et al. along with other groups holds the promise of unveiling additional overlooked finds which will help to complete the exoplanet census conducted by Kepler.

An artist’s fanciful depiction of the view from the surface of Kepler 1649c. (NASA/Ames Research Center/Daniel Rutter)

The discovery of Kepler 1649c has also provided an important data point to help estimate how common such exoplanets are orbiting mid-M-type dwarfs. Earlier work by Dressing and Charbonneau published in 2015 found that larger and brighter early-M-type dwarfs observed by Kepler have 0.16 +0.17/-0.07 exoplanets with radii in the 1.0 to 1.5 R_E range orbiting inside of their HZ (for an earlier analysis, see “Occurrence of Potentially Habitable Planets around Red Dwarfs”). With the discovery of Kepler 1649c, Vanderburg et al. recalculated the occurrence rate of HZ exoplanets in this size range based on the 466 smaller mid-M-type stars observed by Kepler during its primary mission. Based on their statistical analysis, Vanderburg et al. estimate that these smaller red dwarfs have 0.64 +0.41/-0.19 Earth-size exoplanets orbiting in their HZ. This result implies that smaller red dwarfs are about four times more likely to have Earth-size HZ planets than larger red dwarfs. Given the great number of these stars in the galaxy, it could be that such exoplanets dominate the potentially habitable worlds in our neighborhood.

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Related Reading

Other articles about the planetary finds of NASA’s Kepler mission and the K2 extended mission can be found on the 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

Courtney Dressing and David Charbonneau, “The Occurrence of Potentially Habitable Planets Orbiting M Dwarfs Estimated from the Full Kepler Dataset and an Empirical Measurement of the Detection Sensitivity”, The Astrophysical Journal, Vol. 807, No. 1, Article id. 45, July 2015

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

Andrew Vanderburg et al., “A Habitable-zone Earth-sized Planet Rescued from False Positive Status”, The Astrophysical Journal Letters, Vol. 893, No. 1, Article id.L27, April 2020

Earth-Size, Habitable-Zone Planet Found Hidden in Early NASA Kepler Data, JPL Press Release 2020-072, April 15, 2020 [Post]