Habitable Planet Reality Check: Kepler’s Latest Finds

On May 10, 2016 NASA held a press conference to announce officially the discovery of the latest group of extrasolar planets found by the Kepler mission. While such announcements have become almost commonplace over the last few years, this one was notable because of the sheer number of new finds announced: a record number totaling 1,284 confirmed extrasolar planets from Kepler’s primary mission more than doubling the mission’s already impressive (and ever-increasing) tally. Among this record haul of exoplanets were also quite a few smaller worlds including some that were found to orbit inside the habitable zones (HZ) of their systems. Among this group is a fairly promising candidate for being “potentially habitable” that was specifically identified during NASA’s press conference called Kepler 1229b. We will now take a closer look at what we know about these newly discovered extrasolar planets and their prospects for being potentially habitable.

Background

NASA’s Kepler mission monitors the brightness of stars looking for periodic dips in brightness indicating the presence of a transiting extra solar planet. In the past, members of the Kepler science team were required to perform follow up observations (typically from ground-based observatories) of transiting planet candidates in order to eliminate the various possible sources of “false positives” and confirm their planetary nature. The potential causes of false positives include transits by brown dwarfs or low-mass stars, grazing transits of larger stars or background eclipsing binary stars which might be blended with the image of the target star in Kepler’s images. Many of these false positives can be eliminated with measurements of the stars’ radial velocity and high-resolution imagery to look for nearby stars.

In the past, Kepler planet candidates required extensive follow up observations to verify their planetary nature. (NASA)

Once these potential false positives are eliminated and statistical calculations of the FPP (False Positive Probability) from remaining sources of error dropped below the 1% threshold (indicating that there is a greater than 99% chance that the candidate is a bona fide exoplanet), the discovery is considered “confirmed”. But with literally thousands of exoplanet candidates requiring confirmation using astronomical instrumentation with limited availability, the process can take a lot of time. But with a September 2017 deadline looming for the Kepler science team to wrap up their analysis of data from Kepler’s primary mission, a faster method to verify planet candidates accurately was needed.

A scatter plot showing the size of exoplanet candidates the the apparent brightness of the stars they orbit. Click on image to enlarge. (NASA Ames/W. Stenzel; Princeton University/T. Morton)

In order to overcome this bottle neck, Timothy Morton (Princeton University) developed a new data analysis program to automatically sift through the huge Kepler data set to identify exoplanet candidates with low FPP values without the need for time consuming follow up observations. Called VESPA (Validation of Exoplanet Signals using a Probabilistic Algorithm), this program takes a two-part approach to the problem of validating exoplanet candidates: first, the photometric data from the transits is analyzed in detail looking for subtle features in the light curves that can differentiate a true planetary transit from the various types of false positive. Second, the program determines how common various scenarios are that could cause false positives. Based on this analysis, VESPA calculates an FPP value for each candidate. If the probability that a candidate is a false positive is less than 1%, it is considered verified and its status is changed to a “confirmed extrasolar planet”.

Pie chart illustrating the analysis results from VESPA for the 4,302 potential planets from the Kepler mission’s July 2015 planet candidate catalog. In addition to the 984 previously announced exoplanets, another 1,284 newly validated planets were also found in Kepler’s primary mission data set. (NASA Ames/W. Stenzel; Princeton University/T. Morton)

A team of scientists used VESPA to analyze a version of Kepler’s complete primary mission data set as of July 2015 designated DR24. In addition to finding the 984 exoplanets which had been previously confirmed and validated using the more time consuming methods involving follow up observations, a total of 1284 new finds were identified with a greater than 99% probability of being extrasolar planets. Another 1307 candidates were identified as being more likely to be exoplanets and another 707 are more likely to be imposters. More traditional follow up observations will be required to definitively categorize these 2014 candidate one way or the other. The details of how VESPA works and a synopsis of what was found was published by Morton et al. in the May 10, 2016 issue of The Astrophysical Journal.

A histogram showing the numbers of various sized exoplanets discovered to date using the transit method. Kepler’s newest finds are highlighted in orange. Click on image to enlarge. (NASA Ames/W. Stenzel)

Because of the sensitivity of VESPA in uncovering extrasolar planets, the majority of the new finds are smaller than Neptune and a substantial number are close to Earth in size including a handful of extrasolar planets in or near the habitable zone (HZ) of their respective systems. The table below lists the key properties of nine exoplanets identified in Morton et al. as having radii less than twice that of the Earth and within the “optimistic” HZ of their suns – the type of extrasolar planets which deserve particular attention in the future. The values for key properties and their associated measurement uncertainties are taken directly from Table 3 in Morton et al. with the orbit semimajor axis values calculated using data in that same table. This list represents the largest such haul of HZ planets announced by Kepler project participants since January 2015 (see “Habitable Planet Reality Check: 8 New Habitable Zone Planets”).

Newly Validated Planets in the “Optimistic” Habitable Zone

Potential Habitability

Because the follow up observations required to confirm earlier discoveries were not systematically performed for this latest group of exoplanetary discoveries, the information about the parent stars tends to be fairly basic compared to earlier confirmed Kepler finds. And given the quality of data available on these stars, the derived planet properties frequently have larger than usual measurement uncertainties as well. More detailed follow up observations and analysis will help rectify this in the future and these values are almost sure to change. But in the mean time, it is still possible to make an initial assessment of these worlds’ potential habitability with the limited data available.

The first issue that would affect the potential habitability of these exoplanets is their bulk composition: are they rocky worlds like the Earth or volatile-rich mini-Neptunes with hot deep atmospheres and little prospect of being habitable in the conventional sense. An analysis of the mass-radius relationship for extrasolar planets smaller than Neptune performed by Leslie Rogers (Hubble Fellow at Caltech) strongly suggests that planets transition from being predominantly rocky planets to predominantly volatile-rich worlds at radii no greater than 1.6 times that of the Earth or R_E (for a detailed description of this work, see “Habitable Planet Reality Check: Terrestrial Planet Size Limit”). A more recent analysis of how exoplanetary properties change with radius and mass by Jingjing Chen and David Kipping (Columbia University) suggests that this transition starts at about 1.2 R_E with smaller planets most likely to be rocky and the proportion of mini-Neptunes increasing rather quickly with planetary radius.

An initial inspection of the list of new HZ exoplanets by Morton et al. shows that eight of the nine Kepler finds have radii greater than 1.5 R_E and are therefore much more likely to be mini-Neptunes. Two of the borderline cases, Kepler 560b with a radius of 1.57±0.26 R_E and Kepler 1410b with a radius of 1.56±0.15 R_E, have radii small enough with measurement uncertainties large enough that they might have better chances of being rocky planets instead. Future follow up observations of the stars these exoplanets orbit will help to refine their properties allowing a better assessment of their potential habitability.

By far the most promising new HZ planet identified by Morton et al. is Kepler 1229b. With a measured radius of 1.12 (+0.13/-0.22) R_E, it is comfortably below Rogers threshold of no greater than 1.6 R_E. It is also a bit smaller than Kepler 186f with a radius of 1.17±0.08 R_E which Chen and Kipping found has a 59% probability of being a rocky planet (see “Habitable Planet Reality Check: Kepler 186f Revisited”). Even when factoring in the larger uncertainty in its radius, it seems that the odds favor Kepler 1229b being a rocky planet. Unfortunately, the star that this exoplanet orbits is so dim, it is beyond the capability of current instruments to measure its mass using precision radial velocity measurements. While follow up observations will help to refine our knowledge of this planet’s properties, it may be a while before a definitive mass is obtained that will allow the bulk composition of this promising extrasolar planet to be estimated.

Diagram showing Kepler’s discoveries with radii less than twice that of the Earth as a function of mean stellar flux and the temperature of the star they orbit. The green band shows the approximate limits of the habitable zone while the size of the planets is proportional to their measured radii. Click on image to enlarge. (NASA Ames/N. Batalha and W. Stenzel)

The next property we have available which can be used to assess the potential habitability of a planet is its effective stellar flux or S_eff or the amount of energy the exoplanet receives from its Sun compared to the Earth. For the borderline case (in terms of size) of Kepler 560b, it has a S_eff estimated to be 1.26 (+0.58/-0.43) times that of the Earth. The conservative inner limit of the HZ is defined by Kopparapu et al. as corresponding to the runaway greenhouse limit. For a star like Kepler 560 with a temperature of 3387 K, the inner edge of the HZ has an S_eff value of 0.92. Even with the large uncertainties associated with the S_eff value of Kepler 560b, it is more likely that Kepler 560b orbits well beyond the inner limit of the conservatively defined HZ even when the rather large uncertainty in the stellar flux is taken into account.

However, given the tight orbit of this exoplanet, it seems likely that Kepler 560b is a synchronous rotator. Work by Yang et al. on the habitability of slow and synchronous rotating planets suggests that the inner edge of the HZ may correspond to a higher S_eff value of 1.62 greatly improving the probability Kepler 560b orbits inside the HZ. Kepler 1410b, which is likely to be a synchronous rotator as well, also shows some promise with an S_eff of 0.93 (+0.25/-0.21). According to Yang et al., the S_eff of the inner edge of the HZ for a synchronous or slow rotator orbiting Kepler 1410 with a temperature of 3903 K is 1.74 – well above the current S_eff estimate for Kepler 1410b.

The situation with Kepler 1229b, which is likely to be a rocky planet, is even more promising. With an S_eff of 0.35 (+0.12/-0.14), it orbits well away from the inner edge of the HZ and closer to its outer bounds. According to Kopparapu et al., the conservative outer limit of the HZ corresponds to the maximum greenhouse limit where the addition of more CO₂ to the atmosphere will no longer increase a planet’s surface temperature. For a star like Kepler 1229 with a surface temperature of 3724 K, the S_eff of the outer edge of the HZ is about 0.25. Based on the currently available data, it seems that Kepler 1229b orbits comfortably inside the HZ. In fact, Kepler 1229b seems to have properties broadly similar to those of Kepler 186f which is still considered to be one of the best candidates for being a potentially habitable planets currently known.

But before we get too excited about the potential habitability of Kepler 1229b, it needs to be remembered that these listed values for key properties are preliminary and will undoubtedly change after more detailed follow up observations are secured. In addition, the currently listed properties of Kepler 1229b and Kepler 1410b come with a rather important caveat. A search by Morton et al. of Kepler’s CFOP (Community Follow-up Observation Program) archive has uncovered high-resolution images of these stars which indicate the presence of close stellar companions. The presence of these companions might be affecting the derived properties of the stars and the planets that circle them by an amount that is greater than the current formal measurement uncertainties. Additional detailed follow up observations of these stars will help resolve the issue and lead to more accurate properties being derived that will almost certainly differ from those listed in Morton et al..

Conclusions

Based on an initial look at the new group of Kepler’s extrasolar planetary finds, it appears that Kepler 1229b is a very promising case for being considered potentially habitable. It is comparable to Kepler 186f (which is still considered to be one of the most promising potentially habitable planets currently know) in many key properties and on the surface may even be a superior candidate: it is smaller than Kepler 186f, increasing the chance it is a rocky world, and it has a higher effective stellar flux closer to Earth’s. Unfortunately, Kepler 1229 has a nearby star which might be affecting the assessment of its potential habitability. More detailed follow up observations will be required to resolve the situation one way or the other.

In addition to Kepler 1229b, which was explicitly mentioned during the NASA press conference, there are also two additional Kepler finds that are worth watching in the future: Kepler 560b and Kepler 1410b. While their estimated radii of greater than 1.5 R_E makes it more likely that they are mini-Neptunes with little prospect of being habitable, the measurement uncertainties are rather large and the possibility that they might be rocky planets orbiting inside the HZ can not be excluded with any certainty. In addition, a close companion of Kepler 1410 might also be affecting the derived properties of its exoplanet. Once again, more detailed follow up observations will be required for a more definitive determination of these planets’ properties and an assessment of their potential habitability.

No matter how the issue is resolved, Kepler continues to provide a torrent of data that is allowing scientists to gain a better picture of extrasolar planetary systems and the issues that affect planetary habitability.

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Related Reading

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

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

“Habitable Planet Reality Check: Kepler 186f Revisited”, Drew Ex Machina, April 17, 2016 [Post]

In addition to the articles cited above, there is an ever-growing list of articles on Drew Ex Machina related to the results from NASA’s Kepler mission. A complete list of these articles can be found on this web site’s Kepler mission page.

General References

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

Timothy D. Morton et al., “False positive probabilities for all Kepler Objects of Interest: 1284 newly validated planets and 428 likely false positives”, The Astrophysical Journal, Vol. 822, No. 2, Article ID 86, May 10, 2016 [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

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