Without a doubt, NASA’s Kepler mission is the most prolific extrasolar planet hunter in the history of astronomy. As of July 23, 2015, a total of 1,030 extrasolar planet discoveries have been confirmed with 4,696 more considered unconfirmed and currently under study. As the data from Kepler’s primary mission continues to be analyzed and with more data being gathered as part of the “K2” extended mission, the final tally of Kepler find could total in the tens of thousands (see “First Look at Kepler’s Complete Primary Mission Data Set”).

While the mission objective of Kepler and the primary driver of its design was to find Earth-sized planets in Earth-like orbits around Sun-like stars, to date all of the potentially habitable planets discovered by Kepler like the well known Kepler 186f have been found orbiting dimmer K and M-dwarf stars (see “Habitable Planet Reality Check: Kepler 186f” and “A Review of the Best Habitable Planet Candidates”). This is not because potentially habitable planets orbiting more Sun-like stars are necessarily rarer, it simply that planets orbiting smaller stars in tighter orbits are easier to detect using the transit method employed by Kepler. Finding “Earth twins” requires more time to analyze the available data and acquire the needed follow up observations.

All of this is now changing. At a NASA press conference on July 23, 2015, a team led by Jon Jenkins (NASA Ames Research Center) announced the discovery of a newly confirmed and 12 yet to be confirmed extrasolar planets found using Kepler with radii less than twice that of the Earth orbiting in or near their stars’ habitable zone. One of these planets, called Kepler 452b, turns out to be the first confirmed, nearly Earth-sized extrasolar planet orbiting within the habitable zone of a Sun-like star. While the press has hailed the discovery of what some have dubbed “Earth 2.0”, I wanted look beyond the all too often inaccurate media hype and make an independent assessment of this world’s potential for being habitable given our current limited knowledge of the properties of Kepler 452b.

Background

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 which was originally discussed in “The First Look at Kepler’s Complete Primary Mission Data Set“) 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 primary mission. According to the available data on this star summarized in the Kepler data archive, Kepler 452 has a surface temperature of 5757±85 K, a mass of 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, I calculate that Kepler 452 has a 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 planet Kepler 452b (previously known as KOI 7016.01 before its planetary nature was confirmed) is in a 384.84-day orbit around its sun. Based on the presumed properties of Kepler 452, the planet has an orbital radius of 1.05 AU and a planetary radius of 1.6±0.2 times that of the Earth (R E ). Using the data on the planet and the star it orbits, I calculate that the effective stellar flux, S eff , of Kepler 452b is 1.11 times that of the Earth with an uncertainty that I estimate to be on the order of 1% or so. This is a touch higher than Earth’s S eff but is comparable to what we could expect when our home planet is about six billion years old.

Potential Habitability

The first order of business in assessing the potential habitability of Kepler 452b is to determine what sort of world it is: is it a rocky planet like the Earth or is it a volatile-rich mini-Neptune with little prospects of being habitable in an Earth-like sense. While an analysis of Kepler data has provided us with a good measure of its radius, it is too small to be detected using precision radial velocity measurements. As a result, we have no measure of its mass which, when combined with the radius, would provide scientists with its bulk density and some idea of its composition. Fortunately, a statistical analysis of earlier Kepler finds allows us to estimate the probability that Kepler 452b is a rocky planet.

A recently published analysis of the mass-radius relationship for extrasolar planets smaller than Neptune performed by Leslie Rogers strongly suggests that planets transition from being predominantly rocky planets like the Earth to predominantly volatile-rich worlds like Neptune at radii no greater than 1.6 R E (see “Habitable Planet Reality Check: Terrestrial Planet Size Limit”). While rocky planets larger than this are possible, they become more uncommon with increasing radius. Using a model based on recent work by Torres et al. (see “Habitable Planet Reality Check: 8 New Habitable Zone Planets”), I estimate that there is something like a 40% chance that Kepler 452b with a radius of 1.6 R E is a rocky planet. This is somewhat less than the claim of “greater than 50%” chance found in the discovery paper and repeated in media reports.

Unfortunately, the chance that Kepler 452b is a terrestrial planet might not be as good as even 40%. Recent work by Dawson et al. strongly suggests that planets with masses greater than about 2 times that of the Earth (or 2 M E which would have a radius of about 1.2 R E , assuming an Earth-like bulk composition) which orbit stars with a high metallicity are more likely to be mini-Neptunes. This is because stars with higher metallicities tend to have more solid material available to form planetary embryos more quickly making it more likely for them to acquire some gas directly from the protoplanetary disk before it dissipates. Stars with lower metallicity tend to form planetary embryos more slowly and they might not reach the required 2 M E mass threshold fast enough to begin to acquire more than trace amounts of gas before the it has already dissipated from the disk. Only 1% or 2% of a planet’s total mass of hydrogen and helium is sufficient to puff up its observed radius and make it a mini-Neptune. With a iron-to-hydrogen ratio about 60% higher than the Sun, Kepler 452 has a slightly higher metallicity than the Sun increasing the odds somewhat that Kepler 452b is a mini-Neptune.

While the odds seem to favor Kepler 452b being a mini-Neptune, it is still quite possible that it is a rocky terrestrial planet like the Earth. If it is, studies to date suggest that it probably has a bulk composition very similar to the Earth (see “The Composition of Super-Earths”). With a radius of 1.6 R E and an Earth-like composition, models suggest Kepler 452b would have a mass of about 5 M E when the effects of compression from the extra mass are taken into account. This would yield a surface gravity of about twice that of the Earth. While this might be a bit high for terrestrial life forms, by itself this surface gravity value should not preclude the existence of life on this world.

Another major factor involved in assessing a planet’s potential habitability is it effective stellar flux, S eff . Looking at the latest models for the conservative limits of the habitable zone (HZ) from Kopparapu et al., the HZ of Kepler 452 for a 5 M E planet would have effective stellar flux values ranging from about 1.18 for the inner edge (corresponding to the runaway greenhouse limit) out to about 0.36 (corresponding to the maximum greenhouse limit). With the S eff of 1.11, the orbit of Kepler 452b is just inside the conservatively defined HZ for this system even with the uncertainties associated with its properties. Given that main sequence stars tend to brighten as they age, Kepler 452b would have resided much more comfortably inside its system’s HZ earlier in its six billion year life. At least in terms of its orbit and its resulting effective stellar flux, Kepler 452b is likely to be warmer than the Earth but still habitable, given current models and assuming the planet is not a mini-Neptune.

Conclusion

Based on what we currently know about Kepler 452b, it is indeed an extrasolar planet that orbits just inside of the habitable zone of a Sun-like star. Given the estimated six billion year age of this system, Kepler 452b would have orbited even more comfortably inside of the habitable zone earlier in its history. Unfortunately, with a radius of 1.6 R E , Kepler 452b has maybe no more than a 40% chance of being a rocky planet given what we have seen of earlier Kepler finds. It seems a bit more likely that Kepler 452b is a mini-Neptune instead with poor prospects of being habitable in the conventional sense. We will have to wait to get more data for a more definitive determination of the true nature of this newly discovered world.

Despite the outstanding issue of the nature of Kepler 452b, it still has very real prospects of be potentially habitable. But even if it proves not to be, future studies of its properties will provide scientists with vital information on the limits of planetary habitability. And while some might be disappointed by this assessment, it should be remembered that scientists are still actively analyzing the Kepler data set and performing follow up observations. There have already been reports of unconfirmed extrasolar planets closer in size to the Earth orbiting inside the habitable zones of other Sun-like stars (see “Earth Twins on the Horizon?”). According to NASA’s latest press release, another dozen new habitable zone planet candidates ranging from one to two times the size of Earth including nine orbiting Sun-like stars have been identified. Information of these and other still to be confirmed planets are included in a separate paper to be submitted to The Astrophysical Journal. It is only a matter of time before the discovery of more convincing “Earth twins” is announced.

Postscript

After the publication of this article, I reviewed the discovery paper by Jenkins et al. and their assessment that Kepler 452b is a rocky planet in more detail. A full discussion of this question can be found in my article in Centauri Dreams entitled “Is Kepler 452b a Rocky Planet or Not?“.

A French translation of “Habitable Planet reality Check: Kepler 452b” by Alexandre Lomaev is also available: “Planètes habitables: le cas Kepler-452 b”, Extrasolar.fr – Encylopédie des Mondes Extérieurs, July 29, 2015 (in French) [Post]

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Related Reading

“Is Kepler 452b a Rocky Planet or Not?”, Centauri Dreams, August 7, 2015 [Post]

“Habitable Planet Reality Check: Kepler 186f”, Drew Ex Machina, April 20, 2014 [Post]

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

“First Look at Kepler’s Complete Primary Mission Data Set”, Drew Ex Machina, January 26, 2015 [Post]

“A Review of the Best Habitable Planet Candidates”, Centauri Dreams, January 30, 2015 [Post]

General References

Rebekah I. Dawson, Eugene Chiang, Eve J. Lee, “A Metallicity Recipe for Rocky Planets”, arXiv 1506.06867 (submitted for publication in Monthly Notices of the Royal Astronomical Society), June 23, 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”, The Astrophysical Journal, Vol. 801, No. 1, Article id. 41, March 2015 [Preprint]

Guillermo Torres et al., “Validation of Twelve Small Kepler Transiting Planets in the Habitable Zone”, The Astrophysical Journal, Vol. 800, No. 2, Article id. 99, February 2015 [Preprint]

Confirmed Planet Overview Page: Kepler 452b, NASA Exoplanet Archive [Link]

“NASA’s Kepler Mission Discovers Bigger, Older Cousin to Earth”, NASA Press Release 15-156, July 23, 2015 [Press Release]