Announcements about the discovery of habitable extrasolar planets have regularly dominated the news from the astronomical community over the last several years. This naturally reflects the public’s interest in finding Earth-like planets for reasons ranging from the joy of scientific discovery to desire of finding a new home for humanity in the distant future. While the majority of these claims found in the media about habitable worlds are, at best, simply overstated or, at worst, just blatantly inaccurate hype, there have been a handful of newly discovered exoplanets which actually have scientifically justifiable prospects for being considered potentially habitable. With the rate of exoplanet discoveries increasing in recent years, it is now possible to compile a realistic “top five” list of potentially habitable exoplanets.

Basic Habitability Criteria

While a full assessment of the habitability of any exoplanet would require very detailed information about all of its properties, obtaining such information about even the nearest known exoplanets 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 biocompatible environments that could support life (e.g. Mars or the tidally heated oceans on some icy body like Europa and Enceladus), these would not be Earth-like habitable worlds of the sort being considered here.

One of the key pieces of information we typically have available for newly discovered extrasolar planets to assess their potential habitability is their effective stellar flux (or S eff where Earth’s value is defined as 1). This can be readily calculated using information about a planet’s orbit and the luminosity of the star it orbits. If an exoplanet’s S eff falls within a range corresponding to the limits of a sun’s habitable zone (HZ), this planet has met one of the basic criteria for potential habitability.

One of the better definitions for the limits of the HZ is based on the work of James Kasting (Pennsylvania State University) starting a quarter of a century ago. These predictions are based on an extrapolation of our knowledge of the processes that have kept our own planet habitable over the last several billion years despite a 30% increase in the Sun’s luminosity yet have rendered our neighbors non-habitable. The latest refinements of this work by Kopparapu et al. (2013, 2014) define the inner limit of the HZ to correspond to the S eff where a moist runaway greenhouse effect sets in. At higher S eff values, skyrocketing surface temperatures and the permanent loss of a planet’s allotment of water in a geologically brief period of time will result. For an Earth-size planet orbiting a Sun-like star, this limit corresponds to an S eff of about 1.11. The S eff corresponding to this inner limit of the HZ would be slightly higher for planets more massive than the Earth and slightly lower for stars cooler than the Sun.

There have been models proposed with higher S eff values for the inner limit of the HZ in cases of synchronous rotation where an exoplanet always presents the same face to its sun – a condition which would be common for planets orbiting inside the small HZs of dim red dwarfs. While synchronous rotation was believed at one time to compromise a planet’s habitability, increasingly detailed climate models developed over the last two decades have shown that this is not the case after all. In fact, it has been found that the inner edge of the HZ for slowly or synchronously rotating planets has a higher S eff value than that for fast rotators like the Earth resulting in a larger HZ for slow and synchronous rotators. This is because various feedback mechanisms can increase the coverage of high-albedo clouds on the sun-facing hemisphere of a synchronous rotator increasing the amount of energy reflected back into space. Recent models by Yang et al. suggest that the S eff for the inner limit of the HZ can be on the order of 1.5 to 1.7 or even higher for cool red dwarf stars, depending on the star’s effective surface temperature.

More recent work by Kopparapu et al. (2016) largely confirms the findings of Yang et al. and others about the higher S eff values for the inner limit of the HZ for synchronous rotators. However, they found that earlier models neglected to include the effects of much shorter orbit and rotation periods on cloud formation for planets orbiting the smallest red dwarfs. These faster spinning slow rotators produce enhanced Coriolis forces which inhibit the formation of reflective clouds on the sun-facing hemisphere. While the S eff for the inner limit of the HZ for synchronous rotators found by Kopparapu et al. (2016) can be lower than those found by Yang et al., especially for the coolest and dimmest red dwarfs, they still tend to be much higher than for exoplanets with faster, more Earth-like rotation periods.

There have also been models proposed over the past decade and more with higher S eff values for the inner limit of the HZ for a range of other special conditions. Such definitions have been attractive to some hoping to maximize the chances that a new exoplanetary find might be considered to be habitable. However, these sometimes involve extreme extrapolations from conditions here on Earth or contrived special circumstances. In general, these definitions require more study and some reliable empirical observations to be on a firmer theoretical footing. In a review paper by Kasting et al., it was argued that while there is certainly genuine uncertainty on the precise inner limit of the HZ as a result of limitations of the comparatively simple models used to date, some of the most optimistic inner limit definitions involve scenarios that are physically unrealistic. As result, many including myself, tend to favor the more conservative definitions of the inner limits of the HZ.

The outer limit of the HZ, as defined by Kopparapu et al. (2013, 2014), corresponds to the maximum greenhouse limit beyond which a CO 2 -dominated greenhouse is incapable of maintaining a planet’s surface temperature. In fact, the addition of more CO 2 makes the atmosphere more opaque and can decrease surface temperatures. The latest work suggests an S eff value of about 0.36 for a Sun-like star with cooler stars having lower values because more of their energy is radiated in the infrared. As with the inner limit of the HZ, there are some slightly more optimistic definitions of the outer edge of the HZ such as the early-Mars scenario or evoking some sort of super-greenhouse where the ad hoc addition of gases other than just CO 2 contribute to warming a planet. But these more optimistic definitions tend not change the S eff for the outer limit of the HZ significantly.

Another important parameter we have available today to gauge the potential habitability of an extrasolar planet is its size. In the case of exoplanets observed using the transit method like that employed by NASA’s highly successful Kepler mission, it is the radius or R P value that is measured. Precision radial velocity measurements are also capable of detecting the reflex motion of Earth-size exoplanets in the HZ especially for smaller red dwarf stars. In the case of the radial velocity measurements, we actually only know the planet’s M P sini value where i is the inclination of the orbit with respect to the plane of the sky. Since the inclination can not be determined directly from radial velocity measurements alone, we can only know the planet’s minimum mass or the probability that the actual mass is within some range of interest. By definition, the actual mass of a planet with an unconstrained orbit inclination is most likely larger than this minimum mass – in some cases it can be much larger. By combining the radius and mass information for an exoplanet, its bulk composition can be constrained allowing scientists to determine if an exoplanet is rocky like the Earth or a volatile-rich mini-Neptune with no prospect of being habitable in the conventional sense.

Unfortunately, we rarely know the radius and mass of an exoplanet. In these cases, we need to rely on statistical arguments based on the observed mass-radius relationship of exoplanets. A series of analyses of Kepler data and follow-up observations published over the last couple of years have 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 a Neptune-like world. Rogers has shown that planets with radii greater than no more than 1.6 times that of the Earth (or 1.6 R E ), corresponding to a mass of about 6 times that of the Earth (or 6 M E ) assuming an Earth-like composition, are most likely mini-Neptunes (see “Habitable Planet Reality Check: Terrestrial Planet Size Limits”).

A more recent analysis of the mass-radius relationship with a much larger collection of exoplanetary data by Chen and Kipping 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 has a rocky composition given, for example, its radius and the measurement uncertainty of that radius. Their work suggests that that the gradual transition from rocky to volatile-rich exoplanets starts at about 1.2 R E or 2 M E with the probability that a planet is rocky decreasing with increasing radius or mass.

With these basic criteria available, it is possible to start to gauge the potential habitability of an extrasolar planet. After reviewing the available scientific literature on various exoplanets, most of the known exoplanets which have been proclaimed by some to be potentially habitable are in fact too large to be rocky planets or have S eff too high to be considered inside the more conservatively defined HZ. And there are cases where even the existence of the exoplanet is in doubt because of concerns that natural stellar noise or jitter modulated by the rotation of the observed star and irregularly sampled in time has been mistaken for the radial velocity signature of an orbiting planet (for access to all of the Drew Ex Machina essays with habitability assessments of various exoplanets, good and bad, see the Planetary Habitability page). In the end, a total of five currently known exoplanets were identified with having the best (or at least the most reasonable) prospects of being habitable given our current limited knowledge of these distant worlds.

Kepler 1229b

Choosing the number five spot on this list turned out to be more difficult than I anticipated. The uncertainties associated with the values of many key properties of some exoplanets and the stars they orbit can be quite large rendering the process of choosing among a list of borderline candidates equally uncertain. While others may weigh the available evidence differently, I have chosen Kepler 1229b – the most promising of a batch of HZ exoplanets announced on May 10, 2016 (see “Habitable Planet Reality Check: Kepler’s Latest Finds”).

Kepler 1229b is in a 87-day orbit around a dim red dwarf star located about 770 light years away. According to the original preprint of the discovery paper by Morton et al., this exoplanet has a radius of 1.12 R E and a S eff value of 0.35. This small radius makes it more likely that this exoplanet is a rocky world like the Earth. In addition, since the S eff value corresponding to the outer limit of the HZ for a star like Kepler 1229 is estimated by Kopparapu et al. (2013) to be 0.26, this exoplanet seems to be orbiting comfortably in the outer part of the HZ.

A more recent paper by Kane et al. combined the best available data for Kepler’s exoplanets from Data Release 24 (DR24) with the best estimates of the star properties from DR 25 and found a larger radius of 1.25 R E and higher S eff of 0.37 (see “Habitable Zone Exoplanets from NASA’s Kepler Mission”). This places Kepler 1229b more comfortably inside the HZ but increases the probability somewhat that Kepler 1229b is not a rocky planet. In the discovery paper for Kepler 1229b by Morton et al., it was noted that Kepler’s CFOP (Community Follow-up Observation Program) archive included high-resolution images of this system’s host star which indicates the presence of close stellar companion. The presence of this companion might be affecting the derived properties of the star and its planet. It might turn out that Kepler 1229b is much larger with a higher S eff increasing the probability that this is a mini-Neptune or that it even orbits beyond the inner edge of the HZ. In fact, the NASA Exoplanet Archive quotes a radius of about 1.4 R E for Kepler 1229b which would substantially decrease the probability that this is a potentially habitable rocky planet but a mini-Neptune instead. Only additional follow up observations to refine the properties of this star and its planet will resolve the current uncertainties surrounding the potential habitability of this exoplanet.

Kepler 62f

Kepler 62f is part of a multi-planet system discovered in 2013 associated with a type K2V star about 1,200 light years away. The planet completes an orbit in 267 days and, with an orbital semi major axis of about 0.72 AU, has a S eff of 0.39 – comfortably inside the HZ whose outer edge is estimated by Kopparapu et al. (2013) to correspond to an S eff of 0.31. With a calculated radius of 1.41 R E , the size of Kepler 62f is clearly in the transition zone between rocky and volatile-rich planets identified in recent work. As such, Kepler 62f has a moderately high probability of being a mini-Neptune but the odds seem to slightly favor it being a predominantly rocky world like the Earth. While its size enhances the probability it is not habitable, this is offset to a certain extent by the fact that Kepler 62f orbits a comfortable distance away from its sun thus avoiding some of the potential issues which would compromise habitability identified for dimmer red dwarf stars. Future follow up observations could help to determine (or at very least constrain) the mass of Kepler 62f to provide more definitive insights into its bulk density and its potential habitability.

Another planet in this system, Kepler 62e, has been thought by some to be potentially habitable but a closer look casts grave doubts on this optimistic assessment. With an S eff of 1.11, it would appear that this world orbits just outside the HZ whose inner limit is defined by Kopparapu et al. (2013, 2014) to have an S eff of 1.08, assuming a larger 5 M E exoplanet. The bigger issue is the size of Kepler 62e. With a radius of 1.61 R E , it is much more likely that this exoplanet is a mini-Neptune with no chance of being habitable.

Kepler 442b

Kepler 442b was the most promising find in a batch of HZ exoplanets announced during the first week of 2015 (see “Habitable Planet Reality Check: 8 New Habitable Zone Planets”). The sun of this system is a relatively young K-dwarf star about 1,100 light years away. The exoplanet Kepler 442b, with an orbital period and semi major axis of 112 days and 0.41 AU, respectively, has a radius of 1.34 R E and a S eff of 0.66. This exoplanet and its sun are broadly similar to Kepler 62f except that the planet is slightly smaller (improving the odds somewhat that it is a rocky planet) and it has a higher S eff which places it even more comfortably inside the HZ, whose outer limit is estimated by Kopparapu et al. (2013) to have an S eff of about 0.29. As a result, Kepler 442b places a bit higher than Kepler 62f in its potential habitability.

Kepler 186f

Probably one of the most famous potentially habitable planets identified so far by NASA’s Kepler mission is Kepler 186f whose discovery was announced on April 17, 2014 (see “Habitable Planet Reality Check: Kepler 186f Revisited”). While hardly the “Earth twin” that many have been waiting for, it does seem to meet the existing basic criteria for being potentially habitable. The star Kepler 186 is a type M1V red dwarf located about 560 light years away. Kepler 186f, with a orbital period of 130 days and a semi major axis of 0.43 AU, was the last discovered of the five exoplanets currently known in this system. With a radius of 1.17 R E , Chen and Kipping estimate that Kepler 186f has a 59% probability of being a rocky planet. Its S eff of 0.30 places Kepler 186f comfortably inside the outer part of the HZ which Kopparapu et al. (2013) sets at a S eff of 0.26. Although it is expected to be a synchronous rotator, various climate models predict that Kepler 186f should be globally habitable given a realistic range of atmospheric conditions. Overall, Kepler 186f is currently the best of the Kepler mission finds in terms of its potential habitability.

Proxima Centauri b

The most recently discovered, the closest and possibly the best potentially habitable exoplanet currently known is Proxima Centauri b. Unlike the other exoplanets on this list which were discovered by NASA’s Kepler mission, Proxima Centauri b was discovered using precision radial velocity measurements as part of a series of increasingly intense observation campaigns culminating with the Pale Red Dot campaign during the first quarter of 2016 (see “Habitable Planet Reality Check: Proxima Centauri b”). Unlike all of the other exoplanets on this “top five” list which are many hundreds of light years away, Proxima Centauri is the closest known star at a distance of only 4.24 light years. This intrinsically dim type M5.5V red dwarf star is the smallest member of the α Centauri (or Rigil Kentaurus) star system locked in a distant 591,000-year orbit around the pair of brighter Sun-like stars (see “The Orbit of Proxima Centauri”).

On August 24, 2016 an international team of astronomers led by Guillem Anglada-Escudé (Queen Mary University of London) announced the discovery of Proxima Centauri b. It has a M p sini or minimum mass of 1.27 M E , an orbital period of 11.2 days and an S eff of 0.65. While the actual mass of Proxima Centauri b is currently not known, the odds heavily favor that it is well below the 2 M E threshold found by Chen and Kipping where exoplanets start to transition from being predominantly rocky to volatile-rich. Generic climate models like those of Kopparapu et al. (2016) and others specifically to study Proxima Centauri b all indicate that this world could be habitable over a range of assumed initial volatile inventories and atmospheric properties despite being a synchronous rotator. However, because of the small size of the HZ and the known flare activity of this small red dwarf, there are a number of unresolved concerns about how they might affect habitability.

Fortunately, Proxima Centauri b is ideally situated to be observed by future instruments including direct imaging and even spectroscopy by the next generation of ground and space-based telescopes. With it being so easily observed compared to the other exoplanets on this list, it will provide scientists with valuable data about its properties and environment to test our current understanding of planetary habitability – especially for Earth-size exoplanets orbiting red dwarfs which are the most common star type in the galaxy.

Conclusion

Barely two decades since the first extrasolar planets were discovered orbiting main sequence stars like the Sun, we are now finding exoplanets whose properties are consistent with being potentially habitable even when conservatively defined. But it must be remembered that this process has only just begun. This “top five” list of potentially habitable planets currently consists only of the more easily detected worlds orbiting K and M-type stars which are dimmer than our Sun and would probably not be considered very Earth-like – certainly not “Earth twins”. That is not to say that habitable planets around Sun-like star are necessarily rare. Extrapolations from recent analyses of Kepler results suggest that maybe one in ten Sun-like stars has a rocky Earth-size planet orbiting inside its HZ (see “The Prevalence of Earth-Size Planets Arnd Sun-Like Stars”). The simple fact of the matter is that true “Earth-twins” (i.e. Earth-size planets in Earth-like orbits around Sun-like stars) have proved to be more difficult to detect than originally anticipated. However, the ongoing analysis of Kepler data with increasingly sophisticated tools along with ever-improving telescopes, instruments and techniques are guaranteed to reveal even more promising potentially habitable planets in the years and decades to come.

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Related Reading

“Habitable Zone Exoplanets from NASA’s Kepler Mission”, Drew Ex Machina, August 7, 2016 [Post]

“Habitable Planet Reality Check: Proxima Centauri b”, Drew Ex Machina, August 29, 2016 [Post]

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

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

“Habitable Planet Reality Check: Kepler’s Latest Finds”, Drew Ex Machina, May 14, 2016 [Post]

General References

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

Stephen R. Kane et al., “A Catalog of Kepler Habitable Zone Exoplanet Candidates”, The Astrophysical Journal, Vol. 830, No. 1, Article ID 1, October 2016

James F. Kasting et al., “Remote life-detection criteria, habitable zone boundaries, and the frequency of Earth-like planets around M and late K stars”, Proceedings of the National Academy of Sciences, Vol. 111, No. 35, pp. 12641-12646, September 2, 2014

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., “The Inner Edge of the Habitable Zone for Synchronously Rotating Planets around Low-mass Stars Using General Circulation Models”, The Astrophysical Journal, Vol. 819, No. 1, Article ID. 84, March 2016

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