While NASA’s Kepler spacecraft was shutdown well over a year ago, there are still teams of scientists around the globe combing through its huge, nine-year database trying to tease out more exoplanetary discoveries that were missed by the mission’s initial analysis. Using updated data processing techniques to sift through Kepler’s primary mission database, graduate student Michelle Kunimoto (University of British Columbia) announced the discovery of 17 previously missed transiting exoplanets with the publication of a peer-reviewed paper in The Astronomical Journal on February 25, 2020. Among these new finds was a super-Earth-size world that appears to orbit in its parent star’s habitable zone (HZ). So what are the prospects that this newly discovered exoplanet is potentially habitable?

Background

The new HZ find by Kunimoto et al. orbits the star known by its Kepler Input Catalog designation, KIC-7340288. Also known as 2MASS J18523595+4255114, it is a dim G magnitude 15.8 star located in the constellation of Lyra inside the field of view of Kepler’s four-year primary mission. Analysis of existing photometry for this star suggests that it is an early K-type star with Gaia pegging its distance at 1,070±12 light years. Kunimoto et al. found a periodic variation in the brightness of KIC-7340288 of about 13.4 days which they take as the rotational period of this star – one half of the value found in earlier survey work. This short period of rotation suggests that the star is still comparatively young. The properties of KIC-7340288 are summarized in the table below.

Properties of KIC-7340288 (Kunimoto et al.)

Spectral Type Late K Surface Temperature 3949 +79/-52 K Mass (Sun=1) 0.57 +0.02/-0.01 Radius (Sun=1) 0.55±0.01 Luminosity (Sun=1) 0.066 Rotation Period (days) ~13.4 Distance (LY) 1070±12

In order to find new exoplanets, Kunimoto et al. searched through the entire Kepler primary database from Q1 to Q17, unlike earlier efforts independent of the Kepler program which only looked at small subset of the ~200,000 stars observed by this space telescope. While they used the same three-transit detection threshold used by Kepler mission scientists to identify exoplanet candidates, Kunimoto et al. employed a series of new data processing and vetting techniques in combination with manual inspections to spot transits through the noise in the data. In the end, Kunimoto et al. were able to recover 98.8% of Kepler’s confirmed exoplanet finds. But in addition to these, Kunimoto et al. also spotted 17 previously undetected exoplanets including five which appear to have smaller radii than the Earth.

Among the handful of new super Earths discovered is KIC-7340288b. Kunimoto et al. identified three transits of this exoplanet with individual signal to noise ratios better than 7 indicating an orbital period of 142.5 days. The properties of this new exoplanet derived by Kunimoto et al. based on an analysis of the transit photometry are 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. Because of its radius and S eff , Kunimoto et al. claim that KIC-7340288b is a rocky planet orbiting inside the optimistic habitable zone (HZ) of this system.

Properties of KIC-7340288b (Kunimoto et al.)

Planet b Orbit Period (days) 142.5 Orbit Semimajor Axis (AU) 0.444±0.004 Radius (Earth=1) 1.51 +0.13/-0.11 S eff (Earth=1) 0.33 +0.03/-0.02

Kunimoto et al. used a series of vetting processes to help eliminate the possibility that this detection is a false positive. In addition to these tests, Kunimoto et al. secured high resolution imagery of KIC-7340288 to eliminate the possibility that the detection was a false positive of astrophysical origins (e.g. a background eclipsing binary) or that there was a companion present which would complicate the interpretation of the results. On July 1, 2019, observations were made of KIC-7340288 in the K s -band using the adaptive optics assisted NIRI (Near InfraRed Imager and spectrograph) instrument on the 8.1-meter Gemini North telescope on Mauna Kea in Hawaii.

The images revealed a companion star just 3.87 arc seconds away from KIC-7340288 which is 5.20 magnitudes dimmer in the K S band. While this is likely just a visual companion not physically associated with KIC-7340288, it is close enough to present issues with the interpretation of the photometric transit data. Fortunately because of its dimness, this visual companion does not substantially impact the derived size of the new exoplanet and it can not be the source of the 0.06% dimming observed in the Kepler photometry. The adaptive optics images acquired by Kunimoto et al. lower the astrophysical false positive probability to 7.8×10-4. While this find remains an exoplanet candidate requiring further confirmation, the discovery appears to be on a very firm footing.

Potential Habitability

So, what are the habitability prospects for KIC-7340288b? 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 KIC-7340288b 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 KIC-7340288b 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 KIC-7340288b, 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 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. While Kunimoto et al. have claimed that KIC-7340288b is a rocky planet, in reality its radius of 1.5 R E places it right in the middle where the exoplanet population transitions from primarily rocky to a volatile-rich composition. This strongly suggests that there is only about a 50-50 chance that KIC-7340288b has a rocky composition – not quite as good as claimed but still a promising candidate.

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 CO 2 -dominated greenhouse is incapable of maintaining a planet’s surface temperature. Instead of helping to heat the atmosphere, the addition of more CO 2 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 0.26 for the outer limit of the HZ of an Earth-sized exoplanet orbiting KIC-7340288 corresponding to a mean orbital distance of 0.50 AU. KIC-7340288b, with a S eff value of 0.33, 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 CO 2 present in its atmosphere resulting in the loss of all its water in a geologically brief time in the process. For a fast-rotating, 5 M E super-Earth orbiting KIC-7340288, this happens at an S eff value of about 1.01 which corresponds to a mean orbital distance of 0.26 AU. Even by this conservative definition, KIC-7340288b would seem to orbit comfortably inside the HZ bolstering the claim that this new find is a potentially habitable world.

Summary

While it can be argued that Kunimoto et al. have overstated the likelihood that KIC-7340288b is a rocky planet, overall the claim that this newly discovered exoplanet is potentially habitable certainly has merit. But before we get too invested in any particular interpretation of the available data, it should be remembered that follow up observations to refine the characterization of the dim host star will inevitably lead to changes in the derived exoplanet properties – changes which can bolster or weaken the case that KIC-7340288b is potentially habitable.

Unfortunately, because of the dimness of the host star, it may be a while before we get new transit observations. KIC-7340288 is too dim for NASA’s TESS (Transiting Exoplanet Survey Satellite) to observe and other instruments, like the recently launched ESA CHEOPS (CHaracterising ExOPlanets Satellite), have hundreds if not thousands of higher priority targets to observe. Likewise, observations of this system using other techniques will need to wait for the availability of more sensitive instruments. Still, the work by young scientists like Michelle Kunimoto combing through the databases of Kepler and other exoplanet surveys will help fill out our exoplanet census especially for smaller sub- to super-Earth size exoplanets allowing a more complete statistical characterization of the exoplanetary systems in our part of the galaxy.

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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

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

Michelle Kunimoto, Jaymie M. Matthews and Henry Ngo, “Searching the Entirety of Kepler Data. I. 17 New Planet Candidates Including One Habitable Zone World”, The Astronomical Journal, Vol. 159, No. 3, ID 124, February 25, 2020 [Preprint]

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

“UBC student discovers 17 new planets, including potentially habitable, Earth-sized world”, University of British Columbia Press Release, February 28, 2020 [Press Release]