Another Earth 2.0? Not So Fast

1. The Meaning of the Habitable Zone (HZ)

If a planet is located in the HZ, it does not mean it is habitable. The term was coined to denote the zone in which a planet similar in mass to Earth would retain an atmosphere and liquid water on its surface (Kasting et al., 1993). The HZ is quite narrow for most stars, and the position and extent of the HZ depends primarily on the total amount of energy emitted by the star, usually measured as its luminosity. However, even for the same type of star, the exact boundaries of the circumstellar HZ depend also on the planet's geophysical and geochemical characteristics, in particular its mass, albedo (amount of reflected energy), and the thickness and chemical composition of its atmosphere. The concept of the HZ is useful when we search for a second Earth and as a first screening tool for finding habitable planets outside our solar system. However, its relevance has often been misinterpreted as that stellar region in which we would expect to find life. A pointed example is our Moon, which is located in the HZ of our solar system, but no one would argue that our Moon is habitable. A second problem with blurring the meaning of the HZ from its original intent is that the concept of a HZ does not take into account other possible habitats such as subsurface oceans (e.g., Europa, Enceladus, Titan), potential life-forms based on a different biochemistry (e.g., using a different solvent), or life that simply utilizes adaptations and mechanisms that do not require permanent liquid water on a planetary surface (Schulze-Makuch et al., 2015). To be so “Earth-centric” bears the danger that we find nothing other than Earth-like life even though the diversity of life in the Universe (and its habitats) is probably much greater. The Viking life-detection experiments should remind us how difficult it is to determine whether life is present on a planet.

The “blurring” of meaning applies also to recently developed indices to describe similarities of other planets to Earth, particularly the Earth Similarity Index (ESI). It was originally developed to quantify the similarity of an exoplanet to Earth as a function of basic geophysical parameters such as mass, radius, and temperature of an exoplanet that is knowable from current remote sensing technology (Schulze-Makuch et al., 2011). ESI was suggested as an initial screening tool that is similar to the HZ, but with no relationship to habitability in general or the presence of life (a case in point, again, our Moon, which has a relatively high ESI). Nevertheless, the index has frequently been used both by scientists and the media as a measure of habitability, and it has been related to the possible presence of life.

By analogy to the circumstellar HZ, the concept of a galactic habitable zone (GHZ) has been suggested as well (Gonzalez et al., 2001). The GHZ is defined as that region in our galaxy (or any other galaxy) where biogenic elements are available and any life would be far enough away from the galactic center (and its super black hole) such that it is not exposed to disruptive gravitational forces or too much high-energy radiation. The GHZ has been modeled and was quantified as that region between 7 and 9 kiloparsecs (kpc) from the galactic center that widens with time [for reference, the Sun is about 8.0 ± 0.5 kpc (∼26 ± 1.6 kilo-light-years) from the galactic center (Eisenhauer et al., 2003)]. The GHZ is composed of stars typically between 4 and 8 billion years old (Lineweaver et al., 2004). The underlying assumptions for a GHZ were based on (1) the presence of a suitable host star, (2) enough heavy elements to form terrestrial planets, (3) sufficient time for biological evolution, and (4) an environment free of life-extinguishing supernovae. The concept of a GHZ has some similar advantages and drawbacks as the concept of a circumstellar HZ. While these concepts are useful as a first step in prioritizing astrobiology targets, many exceptions are likely the case, with the result of being too overly restrictive for the search for life in the Universe.

2. Planetary Habitability and Life

Habitability has evolved to become a buzzword, and more and more scientific meetings and sessions are organized around this topic, especially in respect to planetary habitability. A few recent examples are the IAU Symposium 293: Formation, Detection and Characterization of Extrasolar Habitable Planets, Beijing, China, 2012 ¹ ; the Habitability of Icy Worlds Workshop, Pasadena, United States, 2014 ² ; Pathways Towards Habitable Planets, Bern, Switzerland, 2015 ³ ; Missions to Habitable Worlds, Budapest, Hungary, 2015 ⁴ ; and the scientific meeting on the Astrophysics of Planetary Habitability, Vienna, Austria, 2016 ⁵ . There appears to be a common misconception that a habitable planet means the presence of life. This is not so. However, habitability is a necessary requirement for life. A recent review of the many factors that contribute to a planet's suitability and potential for life has been provided by Cockell et al. (2016). The HZ concept relies on the presence of liquid surface water, which is beneficial, though likely not required; instead, there are additional important parameters that need to be considered such as the presence of polymeric chemistry, a suitable energy source, a sufficient amount of time, an efficient recycling mechanism such as plate tectonics, stellar location, orbital parameters, and many others (e.g., Schulze-Makuch et al., 2011). This, in particular, will be the case for a planet with a biosphere with significant biomass and a higher degree of biological complexity such as would be expected for Earth 2.0 (Irwin et al., 2014). Given these constraints, it becomes clear that Earth 2.0 may be exceedingly rare, even more so than originally anticipated by the Rare Earth hypothesis (Ward and Brownlee, 2000). On the other hand, during the last 15 years, thousands of exoplanet candidates have been found, mostly by the Kepler mission, and statistical analyses of the data indicate that about 10–25% of G-K-M stars should have Earth-sized planets orbiting inside their HZs (Petigura et al., 2013; Dressing and Charbonneau, 2015) at the time of writing, with 42 of them having been claimed to be potentially habitable planets (Planetary Habitability Laboratory, 2016). Further, it has been suggested that “superhabitable” planets exist, planets that are more suitable to life than Earth (Heller and Armstrong, 2014). In all this understandable and justifiable excitement, we have to be careful with our nomenclature such that we do not oversell our findings, which is tempting for those in the sciences, particularly in the form of public lectures, press releases, and interviews. The media thrives on exciting and exceptional news, and we as scientists may be a little too willing to give in and savor the rare public attention of our work, the details of which are usually of little interest to the public.

Science hype is not necessarily bad as long as researchers are simply publicizing and promoting their results, but it becomes unethical if the claims are exaggerated or unsubstantiated. The latter occurs most often when those in the news media, such as bloggers or news agency or university PR representatives, are eager to garner attention by way of commissioning the next news story. Luckily, exaggerated (or overhyped) claims by scientists are not very prevalent in exoplanet research, and it is to be hoped that this will remain so. An advantage of exoplanet research in this regard is that the science to assess the habitability of an exoplanet is experimentally reproducible and verifiable, which can and should be used to assess whether a new discovery is overhyped or not. We will accrue more confidence with new methods and missions that provide information as to whether a certain candidate planet (a qualifier often dropped in recent discourse) is potentially habitable or even exists. Gliese 581d and 581g exemplify these uncertainties by showing that the previously thought potentially habitable terrestrial planets (Vogt et al., 2010), particularly the promising 581g (Von Bloh et al., 2011), may not exist at all, their putative detection possibly being the result of an artifact of stellar activity (Gregory, 2011; Robertson et al., 2014).

3. Observations Needed for Earth 2.0

It is still very difficult and often very time-consuming to find Earth-sized planets (∼ 0.5–10 Earth masses) in stellar HZs. Although we currently can determine important orbital and physical properties of exoplanets with current instrumentation, such as HARPS and HIRES-Keck, among others (Table 1), additional critical planetary properties that are needed to determine how “Earth-like” a planet actually is will require more powerful and sophisticated instrumentation. Planned space missions such as the James Webb Space Telescope (JWST), Transiting Exoplanet Survey Satellite (TESS), PLAnetary Transits and Oscillations of stars (PLATO), Gaia, Wide Field Infrared Survey Telescope (WFIRST), New Worlds Mission (NWM), and others will address these previous shortcomings. In addition, from the ground the Giant Magellan Telescope (GMT) and the 39 m European Extremely Large Telescope (E-ELT) are under construction and should be operational within the next decade. Instrumentation planned for these huge telescopes can return images and spectra of nearby potentially habitable planets and measure parameters relevant to habitability. One of the most interesting possibilities is to deploy a large occulting “star shade” in combination with JWST (or WFIRST or a dedicated 4 m class space telescope) to isolate planets from their host stars. With the help of spectroscopy, the chemical composition of an isolated planet's atmosphere and surface could be determined, while photometry would allow for the detection of oceans, continents, polar caps, and clouds (NASA Exoplanet Exploration, 2016). A recent review of instruments and methods for studying atmospheres of Earth-sized exoplanets and their suitability for life was given by Billings (2016).

Nevertheless, a word of caution is appropriate. Even if we could establish without doubt that a candidate Earth 2.0 planet is habitable at the time of observation, this would not mean that life necessarily developed or exists there. Moreover, it can also be said that a candidate planet that exhibits fewer Earth-like conditions than the “ideal” candidate may, in fact, have hosted life or still host life. Life, in such a case, may use different biochemical building blocks or a solvent other than water (Schulze-Makuch and Irwin, 2008). In addition, it is critical that observers know the age and history of a candidate planet, though for an extrasolar planet such information may not be available for quite some time. Without deeper and more detailed observations (such as imaging or spectroscopy of a planet that is not currently available), we would not know, for example, whether a given candidate “Earth-like planet” in its recent past was impacted by a collision with a large sterilizing comet, asteroid, or rogue planet; blasted by a nearby supernovae explosion or gamma-ray burst; or possibly exposed to a superflare or perhaps even a megaflare (flares with energies of tens of thousands of times the Sun's largest flares).

4. The Way Forward

Is harm being done when results are extensively promoted and the public misunderstands the implications of our research and has unrealistic expectations? We believe so. With a continual stream of announcements regarding the “most Earth-like planet” found yet, the importance of long-term funding for the next generation of space-based telescopes may not be fully understood. With the discovery of apparently more and more Earth-like planets that exhibit no signs of life, we could experience a similar type of backlash that occurred after the controversial Viking life-detection mission on Mars. Government officials, and the public at large, were puzzled that after providing NASA with a generous budget, including the involvement of some of the best scientists of the time, the question of life on our neighboring planet still appeared as unresolved as ever. We think that this experience contributed to a hiatus of more than 20 years before the next lander mission to Mars was realized and that Viking has remained the only life-detection mission to date. Such misconceptions could also result in a disservice to the scientists who eventually accomplish the monumental achievement of the discovery of a true Earth-like planet, because it would seem to many that this had already been accomplished many times before.

How do we prevent this from happening? First, we must avoid misnomers and adhere to the definition of terms as they were originally conceived. This might be tedious or even unexciting, but it would prevent the blurring that could result in misconceptions when different people mean different things with the same terms. Certainly, we will never fully prevent exaggeration in the media, given that we have a free press and the media thrives on exceptional results. However, as scientists we have a responsibility to accurately state in press releases, conferences, and public lectures what our results indicate and what they mean in the larger context. It is not only important that we do our research according to the highest scientific standards, but we must also communicate and present our results such that they are not misunderstood. We must clearly and honestly state not only what our research findings are but also their implications and, in particular, their limitations.

Even without over-interpretation of new results from exoplanet research, there is plenty of justification for the search for Earth 2.0, which should remain one of the highest scientific research priorities in the coming years. The latest exciting example of such research is the discovery of a potentially habitable planet in the temperate orbit around Proxima Centauri (Anglada-Escudé et al., 2016). These authors' cautious wording that the planet is located in the “temperate zone” rather than within the HZ is especially laudable, given that we do not know whether this planet is habitable. Proxima Centauri b appears to have lost less water over time than that of an Earth ocean, but it is not clear what the initial water endowment might have been (Ribas et al., 2016). The planet might also be a Mercury-, or Venus- or Mars-type planet (or even an entirely different kind of planet) rather than Earth-like. Although Earth-like planets appear to be very rare, we are naturally most interested in them because they may harbor evidence as to how we came into being on our rather exceptional planet and whether we are alone in the Universe.