The main objective of NASA’s Kepler mission and the primary driver of its design was to find Earth-sized planets in Earth-like orbits around Sun-like stars – the type of worlds that scientists want to study to understand our world better as well as some dreamers interested in finding new homes for humanity in the distant future. While finding these “Earth twins” has been an important goal of Kepler, the program’s initial batch of potentially habitable planets, like the famous Kepler 186f discovered in 2014, have been found orbiting dimmer K and M-dwarf stars (see “Habitable Planet Reality Check: Kepler 186f Revisited”). This is not because potentially habitable planets orbiting more Sun-like stars are necessarily rarer, it simply that potentially habitable planets orbiting smaller stars are much easier to detect using the transit method employed by Kepler. Finding true “Earth twins” requires more time to analyze the available data and acquire the needed follow up observations.
The situation with the lack of Earth twins among Kepler’s finds began to change in early 2015 with the identification of an increasing number of very promising extrasolar planet candidates that were about the size of the Earth found in Earth-like orbits around more Sun-like stars (see “Earth Twins on the Horizon?”). Finally at a NASA press conference on July 23, 2015, a team led by Jon Jenkins (NASA Ames Research Center) announced the discovery of Kepler 452b – the first confirmed, nearly Earth-sized extrasolar planet found orbiting within the habitable zone (HZ) of a Sun-like star. While the press hailed the discovery of what some in the media had dubbed “Earth 2.0”, there was some healthy skepticism of some of the more guarded claims being made by NASA and Jenkins et al. about the potential habitability of this extrasolar planet. With another year’s worth of new findings about this world and the nature of similarly-sized exoplanets, it is time to revisit Kepler 452b and the claims of its potential habitability.
The star Kepler 452 (also known by the catalog designations WISE J194400.89+441639.2, 2MASS J19440088+4416392 and earlier by the Kepler project designation KIC 8311864) is a G2 type star like the Sun. It is located an estimated 1,400 light years away in the constellation Cygnus – part of the star field observed during Kepler’s four-year primary mission. An analysis of the available data on this star by Jenkins et al. shows that Kepler 452 has a surface temperature of 5757±85 K, a mass estimated to be 1.04±0.05 times that of the Sun and a radius of 1.11 +0.15/-0.09 times the Sun’s. Based on these data, Kepler 452 has a calculated luminosity 1.21 times that of the Sun. Kepler 452 is basically a slightly heavier and brighter version of the Sun. Comparison of the known properties of this star with standard models of stellar evolution yields an age of 6±2 billion years or about 1½ billion years older than the Sun and its system of planets.
The first mention in print of the extrasolar planet which would become known as Kepler 452b can be found in a paper by members of the Kepler science team with Shawn Seader (SETI Institute/NASA Ames Research Center) as the lead author which was initially submitted on January 15, 2015 for publication in The Astrophysical Journal Supplement Series and subsequently published in March. In this paper, Seader et al. summarized their work to process the entire data set from Kepler’s primary mission (for a complete description of this work, see “First Look at Kepler’s Complete Primary Mission Data Set”). Seader et al. also reported some details of eight new transit event sequences that had not been reported previously. Based on an initial analysis, six of these appeared to be nearly Earth-size planet candidates which seemed to be in or near the habitable zones (HZ) of their host stars. The other two candidates were of interest because they are smaller than the Earth. One of these potential Earth-like candidates was identified orbiting KIC 8311864 which we now know a Kepler 452b.
Seader et al. reported that KIC 8311864b was orbiting a Sun-like star with a period of 384.85 days. Based on an initial determination of the properties of its host star, KIC 8311864b appeared to have a radius of 1.19 times that of the Earth (or RE) with an effective stellar flux (i.e. the amount of energy a planet receives from its sun denoted as Seff) estimated to be 0.56 times that of the Earth. While on the surface these properties made KIC 8311864b look like an excellent candidate for being a true Earth-twin, it was already known to Seader et al. that these latter properties were likely in error. Team members were already aware of the results of detailed follow up observations of KIC 8311864 that indicated that the host star was actually a bit larger and brighter than had been originally thought. As a result the exoplanet would be larger with a higher Seff.
These follow up observations, once completed and analyzed, were the basis of the discovery announcement by Jon Jenkins (NASA Ames Research Center) and his team on July 23, 2015. Using the new properties they derived for KIC 8311864, now known as Kepler 452, Jenkins et al. were able to calculate revised values of its planet’s properties: Kepler 452b is in a 384.84-day orbit with an average orbital radius of 1.046 +0.019/-0.015 AU. This orbital radius combined with the new stellar luminosity value yielded an effective stellar flux for Kepler 452b that is 1.10 +0.29/-0.22 times that the Earth receives from the Sun. The updated radius for Kepler 452b was revised upwards as well to 1.63 +0.23/-0.20 RE. While the discovery paper and NASA’s accompanying press release were somewhat restrained, Jenkins et al. did claim that this newly discovered exoplanet orbits inside the HZ and was more than likely a rocky planet making Kepler 452b a bigger and older cousin of the Earth. Much less restrained media outlets hyped this as the long awaited discovery of “Earth 2.0”. But was it really?
While a full assessment of the habitability of any exoplanet would require very detailed information about all of its properties, obtaining such information is simply beyond the reach of our current technology. At this early stage in our search for other Earth-like worlds, the best we can do is compare what properties we can derive to our current expectations of the range of properties for habitable worlds to determine if a new find is potentially habitable. And by “habitable”, I mean habitable in an Earth-like sense where the surface conditions allow for the existence of liquid water on the planet’s surface. While there may be other worlds that might possess environments that could support life (e.g. Mars or the tidally heated oceans on the moons Europa and Enceladus), these would not be Earth-like habitable worlds of the sort being considered here.
One of the important properties of an extrasolar planet that can be used indirectly to assess its potential habitability is the orbital period. Combined with a knowledge of the host star’s mass and luminosity, the size of the planet’s orbit and its effective stellar flux, Seff, can be determined. According to the work by Kopparapu et al. on the limits of the HZ based on decades of increasingly detailed climate modeling, the inner limit of the HZ is conservatively defined as corresponding to the runaway greenhouse limit where even a slight increase in Seff results in temperature spike which would effectively sterilize a planet. For a Sun-like star like Kepler 452, this conservative inner limit corresponds to an Seff of 1.10 for an Earth-mass planet – essentially the same Seff reported by Jenkins et al. for Kepler 452b of 1.10 +0.29/-0.22. Based on the probable distribution of Seff values for this exoplanet, Jenkins at al. estimated that there is only a 28.0% probability that Kepler 452b actually orbits inside of the conservatively defined HZ. However, they also calculated that there is a 96.8% chance that it orbits inside a more optimistic definition of the HZ corresponding to early conditions on Venus with an Seff value of 1.77.
However, it should be noted that Jenkins et al. used a definition of the HZ limits for an Earth mass planet. If Kepler 452b has a mass closer to five times that of the Earth (or 5 ME), which is likely to be the case for a rocky planet with a radius of 1.63 RE, the Seff for the inner edge of the HZ increases from 1.10 to 1.18 times that of the Earth because of the compression of the atmospheric column due to the planet’s higher surface gravity. This modest increase in Seff raises the chances that Kepler 452b orbits inside of the HZ to something like better-than-even odds. And since Kepler 452, like all stars, would have been dimmer in its youth, Kepler 452b would have been orbiting even more comfortably inside the HZ for billions of years before now. Considering all these facts combined with the limitations of current models in defining the true inner boundary of the HZ, this is close enough even for this skeptic to consider Kepler 452b as potentially habitable at least in terms of its orbit and Seff value.
The next vital piece of information that can be derived using the transit method employed by the Kepler mission is the radius of the planet. But this alone is insufficient to constrain the bulk composition of a planet – it could be a rocky planet like the Earth or a volatile-rich mini-Neptune with no chance of being habitable in the conventional sense. What is required to help constrain the bulk properties of an exoplanet is an accurate mass. Unfortunately, methods like the use of precision radial velocity measurements to measure the reflex motion of the host star are not currently sensitive enough to detect the effects of potentially Earth-like worlds such as Kepler 452b. In lieu of a measured mass value to constrain the bulk composition of Kepler 452b, we do have statistical arguments based on the observed properties of other extrasolar planets.
It is here where I feel the claim that Kepler 452b is potentially habitable runs into trouble. Using distributions of host star properties from two different models in combination with two different published exoplanetary mass-radius relationships, Jenkins et al. estimated that the chances that Kepler 452b was a rocky planet ranged from about 40% to 64% with an average that was greater than 50%, which is the value they adopted. While it was certainly a clever approach to a difficult problem, unfortunately there were questions about how accurate this derived value actually was (for more details on how these estimates were made, see “Is Kepler 452b a Rocky Planet or Not?”).
A better approach for determining the probability that Kepler 452b is a rocky planet would be to compare its properties directly to the population of exoplanets with known radii and accurately determined masses. Unfortunately, the most authoritative paper available until recently on the subject by Leslie Rogers (Hubble Fellow at Caltech) does not include a simple function that others can use to calculate such a probability since this was outside the scope of her work (see “Habitable Planet Reality Check: Terrestrial Planet Size Limit”). Despite this shortcoming, the title of her paper published in The Astrophysical Journal in March 2015 really says it all: “Most 1.6 Earth-Radius Planets are not Rocky” (my emphasis). In other words, Kepler 452b with a radius of 1.63 RE is most likely not a rocky planet but is a mini-Neptune instead, contrary to the claim made by Jenkins et al.. At the time, it seemed more likely that the probability that Kepler 452b was rocky was more towards the low end of the “40% to 64%” range of values calculated by Jenkins et al..
A more recent analysis of the mass-radius relationship with a much larger collection of exoplanetary data by Jingjing Chen and David Kipping (Columbia University) was submitted for publication in March 2016. They included details of their derivation of an algorithm which allows them to estimate the probability that an exoplanet is a rocky planet given, for example, its radius and the measurement uncertainty of that radius. Chen and Kipping found that Kepler 452b with a radius of 1.63 +0.23/-0.20 RE has a likely mass of 3.9 +2.9/-1.5 ME with only 13% of the possible mass values being consistent with a rocky or Earth-like composition. In contrast to the claim by Jenkins et al., Chen and Kipping predict that Kepler 452b is highly unlikely to be a rocky planet. Until an accurate mass value becomes available to demonstrate otherwise, it seems likely that Kepler 452b is an non-habitable mini-Neptune despite its location on the apparent edge of the HZ.
The prospects that Kepler 452b is a potentially habitable planet have dimmed considerably since its discovery announcement on July 23, 2015 when it was described as a bigger and older cousin of the Earth and even as “Earth 2.0” by some. Given what we know about this exoplanet, it seems fairly likely that it orbits near the inner edge of the habitable zone (HZ) and would have orbited even more comfortably in the HZ in the past when its host star was younger and dimmer. However, based on our improving knowledge of the nature of extrasolar planets somewhat larger than the Earth, the chances that Kepler 452b is a rocky planet appear to have plummeted far below the initial optimistic estimate of better than 50% by Jenkins et al. at the time of discovery. Instead it now appears that the probability that Kepler 452b is a rocky planet is closer to 13% – far lower than even the more pessimistic predictions of 40% at the time of discovery. Based on what we know about other exoplanets of similar radius and until an accurate mass measurement becomes available that suggests otherwise, it seems much more probable that Kepler 452b is a volatile-rich mini-Neptune with poor prospects of being habitable in the conventional sense.
While this turn of events is a bit disappointing, it should be noted that Kepler 452b and other exoplanets like it still deserve much additional study in the years and decades ahead as new techniques and technologies become available. Even the study of “near misses” and otherwise non-habitable worlds can reveal new insights about planetary habitability and how such worlds evolve over time. It should also be remembered that the huge data set from Kepler’s primary mission is still being actively analyzed with increasingly sophisticated tools. There are already a number of exoplanet candidates that appear to be Earth-size with Earth-like orbits around Sun-like stars that are the subject of intensive follow up observations. It is just a matter of time before the discovery of a true Earth twin is finally announced.
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“First Look at Kepler’s Complete Primary Mission Data Set”, Drew Ex Machina, January 26, 2015 [Post]
“Habitable Planet Reality Check: Kepler 452b”, Drew Ex Machina, July 24, 2015 [Post]
“Is Kepler 452b a Rocky Planet or Not?”, Centauri Dreams, August 7, 2015 [Post]
Jingjing Chen and David Kipping, “Probabilistic Forecasting of the Masses and Radii of Other Worlds”, arXiv 1603.08614, March 29, 2016 [Preprint]
Jon M. Jenkins et al., “Discovery and Validation of Kepler-452b: A 1.6-RE Super Earth Exoplanet in the Habitable Zone of a G2 Star”, The Astronomical Journal, Vol. 150, No. 2, Article ID. 56, August 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
Leslie A. Rogers, “Most 1.6 Earth-Radius Planets are not Rocky”, The Astrophysical Journal, Vol. 801, No. 1, Article id. 41, March 2015
Shawn Seader et al., “Detection of Potential Transit Signals in 17 Quarters of Kepler Mission Data”, The Astrophysical Journal Supplement Series, Vol. 217, No. 1, Article ID. 18, March 2015