This section makes some final points about the HZ and synthesizes some of the main points made in previous sections.

Nevertheless, the debate continues as to how useful the HZ really is. A common critique is that the HZ can only search for “Earth-like” life or “life as we know it” (e.g., [ 270 271 ]). It is first worth noting that “Earth-like” is a vague and commonly-used term with no consensus as to what it means nor to what degree a planet can differ from the Earth and still be considered Earth-like (alternate terms include “habitable planets” or “potentially habitable planets”). Although the classical HZ includes several assumptions that are consistent with such “Earth-like” conditions (e.g., planets orbiting main-sequence stars, key greenhouse gases are COand HO), the habitable zone, even in its most restrictive classical definition, allows for a wide variety of atmospheric compositions that are strictly not like what we see on our planet. For instance, a potentially habitable planet near the inner edge of our solar system may have orders of magnitude more water vapor in its atmosphere than does the Earth. A planet near our outer edge would contain ~8 bars of atmospheric CO(e.g., [ 1 26 ]), while possessing a drier atmosphere. Around a late M-star, enhanced absorption arising from the red shifted stellar energy distribution could allow the buildup of ~20 bars of COfor outer edge planets (e.g., [ 1 26 ]). As also explained in Section 4 , volcanic rates would need to be many times higher than Earth’s to sustain such dense COatmospheres ([ 2 79 ]). The small sampling of planetary environments just mentioned exhibit conditions that are clearly unlike Earth, and so assuming that any emergent life would be like ours is unsubstantiated. In fact, alien vegetation on planets orbiting other star types may photosynthesize at different wavelengths and manifest different colors than terrestrial plants ([ 272 273 ]). Previous work has modeled what the expected biosignatures over geologic history for a habitable planet might be, but these were done for an Earth clone with a 1 bar atmosphere orbiting the Sun and not for the diverse HZ planets just mentioned [ 108 261 ]. In other words, there is no a priori reason to suspect that biosignatures for many classical HZ planets would be similar to those an extraterrestrial observer might detect for the Earth. Altogether, and especially including the various different HZ formulations and recent advances discussed in previous sections (e.g., [ 2 200 ]), the HZ is thus equipped to assess potentially habitable planets that may not be “Earth-like” or “life as we know it”.

However, to ensure that these missions are ultimately successful in the search for life, it is vital to employ a navigational tool that is capable of distinguishing promising targets from those that are not. For this task, the HZ currently remains far and away the best and most viable option. Despite its shortcomings and uncertain assumptions (discussed below), the classical definition is and has been in wide usage by the Kepler team (e.g., [ 269 ]) and continues to be a major influence in target selection for upcoming missions.

One of the most important questions that mankind can attempt to answer is: “Are we alone?” In the drive to answer this age-long question, our civilization has made major progress in the last few decades, from wondering whether planets outside of our solar system really exist, to confirming the existence of nearly 4000 exoplanets (with ~4500 candidates) today [ 268 ]. As a result, the future of the search for extraterrestrial life has never been brighter. Following the success of Kepler and the recently-launched TESS (transiting exoplanet survey satellite), the (JWST) James Webb space telescope will soon follow suit and plans are afoot for proposed direct imaging (HabEX; habitable exoplanet imaging mission and LUVOIR; large UV/O/IR surveyor) and transiting (OST; origins space telescope and PLATO; planetary transits and oscillations of stars) missions. Very large ground-based telescopes (TMT, thirty meter telescope; GMT, giant magellan telescope; and ELT, extremely large telescope) are also on the horizon.

In contrast, indirect (although also unproven) evidence is available for terrestrial planets composed of atmospheric compositions that are consistent with alternate HZ formulations. For instance, close-in terrestrial planets with dense primordial hydrogen atmospheres likely exist, according to Kepler observations (e.g., [ 269 ]). Martian meteorites and models also favor a highly-reduced, possibly H- or CH-rich early atmosphere on early Mars ([ 146 ]), perhaps within a relatively dense, although not multi-bar, COatmosphere ([ 68 141 ]). This is because the addition of secondary greenhouse gases, like CHand H, can significantly reduce the COpressures required to achieve warm conditions, while also providing the additional heating to counter the atmospheric collapse mentioned above ([ 2 132 ]). Thus, such secondary greenhouse gases help keep the HZ wide. Moreover, as mentioned in Section 7.1 , scale heights for CO–Hatmospheres are larger than those for classical HZ dense COatmospheres, which facilitates the extraction of spectral information. After all, can we actually probe useful spectral information from a dense 10-bar COatmosphere near the outer edge? All such ideas, along with a universal carbonate-silicate cycle, should be considered as working hypotheses. Only through observations can the ideas found to be supported in nature be refined and improved for follow-up missions.

One potential drawback of the HZ’s flexibility is the uncertainty over whether the planets it predicts actually exist. Such arguments have recently been used to cast doubt on alternative HZ definitions ([ 8 129 ]), including the existence of potentially habitable planets with dense primordial hydrogen envelopes or desert worlds [ 153 ]. However, such arguments are troublesome because the same skepticism can (and should) be applied to classical outer edge HZ planets with multiple bars (up to ~20) of COin their atmospheres. Despite the widespread usage of the classical HZ in recent decades, there is no evidence that such worlds exist either. Their existence is inferred from extrapolating the carbonate-silicate (or equivalent) cycle on Earth to suggest that atmospheres of habitable planets near the outer edge may contain many bars of atmospheric CO(e.g., [ 1 ]). However, no direct observations exist of such a long-term cycle [ 274 ], as its existence has only been indirectly inferred from solubility experiments and theoretical models (e.g., [ 33 275 ]). Although a multi-bar COatmosphere may have existed on the early Earth following accretion, it was brief as most of that COwas quickly subducted (within ~10–100 Myr) as the atmosphere cooled from an uninhabitable runaway greenhouse state [ 276 ]. Moreover, even should a universal long-term carbonate–silicate cycle operate on other habitable planets, it is not clear to what extent it does. For instance, perhaps the cycle “shuts down” beyond a certain outgassing rate or atmospheric pressure level before planets can accumulate the multi-bar COatmospheres that characterize the outer edge. Alternatively, as has been argued recently [ 94 ], multi-bar COatmospheres closer to the outer edge would collapse under the reduced sunlight, possibly rendering such planets uninhabitable. If true, this would substantially decrease HZ width in the absence of secondary greenhouse gases ([ 94 ]). Thus, the prediction that all habitable outer edge planets should have very thick (>~5 bar) COatmospheres is an untested assumption that needs to be verified by observations.

Once the direct imaging telescope is sized, the HZ can then be used to guide search efforts [ 153 ]. However, the classical HZ is limited to finding potentially habitable planets with CO–HO atmospheres orbiting main-sequence stars. Instead, recent HZ formulations (e.g., [ 2 216 ]) should be used, possibly in conjunction with the classical HZ. The newer definitions are better-equipped to find planets with both CO/HO-rich or H- or CH-rich atmospheres (e.g., [ 2 132 ]), potentially habitable worlds during the pre- or post-main-sequence phases of stellar evolution [ 51 131 ], possibly life-bearing ocean worlds [ 216 ], habitable worlds around A-stars [ 2 132 ], potential habitats in the white dwarf HZ [ 200 ], or even habitable planets orbiting binary stars (e.g., [ 225 229 ]). By observing a wide variety of plausibly habitable planets, we can maximize our chances of finding extraterrestrial life. The classical HZ should not be used as our only navigational tool.

However, maximizing success in the search for life will also require input from indirect detection methods, including the radial velocity and transit techniques. For example, the Kepler mission has been a resounding success, demonstrating the power of the transit technique (which finds exoplanets by measuring the slight dimming of a star as an orbiting planet passes between it and Earth) in spite of the early failure of two of its reaction wheels (e.g., [ 281 ]). The recently-launched TESS mission will also be using the transit technique to target 200,000 K- and M-stars located across the entire sky [ 282 ]. TESS stars will be significantly brighter than those surveyed by Kepler, facilitating planetary characterization for follow-up missions, like JWST. Ground-based radial velocity measurements will continue to confirm planets initially found with the transit technique. Radial velocity observations can also find planets (by measuring the wobble in a star’s orbit) that can be followed up by direct imaging.

Moreover, determining which are the “best” HZ limits to size direct imaging telescopes remains a judgement call. The angular separation between the star and the planet (θ) is ~Cλ/D ~ a/d, where λ is the observing wavelength, D is the telescope diameter, d is the distance from Earth to the star (in parsecs), a is the planetary orbital separation (in AU), and C is a constant derived from the telescope design. For the inner working angle (e.g., inner edge), C is a smaller number, whereas it is larger for the outer working angle. Therefore, the closest stars have the largest separations (θ) and only smaller telescopes are needed to resolve their planetary systems (e.g., [ 153 ]). Designing a telescope around a pessimistic HZ will then require a larger aperture because ηwould be calculated to be small and the search would be confined to a smaller region of orbital space. In contrast, designing the telescope around a wider HZ implies a larger η, which would require a smaller aperture, possibly resulting in more HZ planets, although perhaps fewer that are similar to the Earth (e.g., more desert worlds and planets located beyond the classical outer edge) [ 135 153 ]. As we learn more about the characteristics of habitable planets, proposed next generation telescopes (HabEx [Jet Propulsion Lab, Pasadena, USA], LUVOIR [NASA Goddard, Greenbelt, USA], PLATO [European Space Agency/OHB System, Oberpfaffenhofen, Germany]) could provide further information on the ideal mission architectures for subsequent missions.

Kasting et al. [ 153 ] suggest using the more pessimistic (narrower HZ) moist and maximum greenhouse limits of the classical HZ to determine the telescope diameter (or aperture). This is because ηwould be small and the telescope would then be as big as necessary according to Kasting et al. [ 153 ]. However, this assumes similar exoplanet yields in both cases. At a given telescope diameter, HZ exoplanet yields can be ~1.5–2× greater if more optimistic definitions are used (see Figure 2 in Stark et al. [ 135 ]). Exoplanet yields should be even larger than these if outer edge extensions, like those in Ramirez and Kaltenegger [ 132 ] or Pierrehumbert and Gaidos [ 129 ], are employed instead. For example, if the telescope can observe the outer edge of the CO–HHZ (2.4 AU), the planet–star contrast ratio there would, according to the inverse-square law, only worsen by a factor of ((2.4/1.67)) two relative to the classical HZ outer edge. Designing a telescope capable of observing such distances may not be a bad idea because the CO–HHZ includes additional terrestrial planets missed by the classical definition, like TRAPPIST-1h [ 149 ]. Striving to achieve better contrast ratios may also allow us to observe sub-Earths, like Mars, which show abundant evidence of a once habitable planet (e.g., [ 68 ] and see Section 3.2 ). Such goals may justify the extra expense. Although achieving these contrast ratios near the outer edge and beyond is currently a technical challenge, it is not unreasonable to expect that technology will continue to improve in the upcoming decades.

Thus, the HZ size itself remains uncertain despite numerous climate modeling efforts over the years (e.g., [ 1 132 ]). An accurate estimate of HZ size is useful for determining the telescope aperture required for direct imaging. Missions use a quantity called η, which is the estimated fraction of stars that host at least one terrestrial planet in the habitable zone (e.g., [ 277 ]). Estimates of this quantity have been made for different spectral classes (e.g., [ 278 279 ]), although large error bars exist. This is partially because observations will miss some planets, requiring statistical corrections that enable a more accurate estimate of planetary occurrence around different stars (e.g., [ 279 280 ]. If ηis computed to be big, then potentially habitable worlds are common, which suggests that only a smaller (and cheaper) telescope is needed to find life. The opposite is true if ηis found to be small.

15.4. The HZ as a Navigational Filter

Although the HZ is used to determine the region where planetary surfaces may reliably support standing bodies of liquid water, some might question the concept’s utility given that life itself is not a requirement. However, I argue that the HZ has evolved from just being a mere navigational tool for finding planets that may have surface liquid water to a more complete “navigational filter” capable of determining which of these are most likely to harbor life. This is for two mean reasons. First, the HZ, possibly in addition with other rubrics, can predict bulk planetary process that may be occurring or have occurred, yielding first order evaluations of planetary habitability. Secondly, the HZ is a great tool for testing scientific predictions. I explain what I mean by all of this below.

For instance, the HZ can be used to test whether HZ theory itself is valid. First, a planet near the conservative HZ inner edge should undergo enhanced water vapor photolysis in its upper atmosphere, triggering a moist greenhouse. If these calculations coincide with observations, it bolsters support for the HZ concept. Moreover, these atmospheres may also produce significant levels of abiotic oxygen [ 24 ], although a careful consideration of atmospheric composition and surface temperature conditions would be needed to infer whether life really exists or not, and if it does, what type it is ( Section 5 ).

2 pressures on habitable planets should increase at farther distances from the star, with maximum pressures at the outer edge (e.g., [ 2 pressure would allow estimates of the planetary volcanic outgassing rate required to support it. Alternatively, if, after observing many systems, including planets inferred to be potentially habitable (if not inhabited), no evidence of such trends is ever found, then this aspect of the HZ concept could be falsified. It is worth noting that inferring a simple pCO 2 gradient from inner to outer edge may be difficult in practice because real atmospheres contain secondary greenhouse gases that may confound such predictions (e.g., [ The classical HZ also predicts that atmospheric COpressures on habitable planets should increase at farther distances from the star, with maximum pressures at the outer edge (e.g., [ 1 ]). Future observations would be able to infer if such a gradient exists [ 274 ]. If it does, this would provide strong support for a long-term carbonate–silicate cycle (or equivalent carbon cycling mechanism) that operates universally on habitable planets [ 274 ]. Knowing the COpressure would allow estimates of the planetary volcanic outgassing rate required to support it. Alternatively, if, after observing many systems, including planets inferred to be potentially habitable (if not inhabited), no evidence of such trends is ever found, then this aspect of the HZ concept could be falsified. It is worth noting that inferring a simple pCOgradient from inner to outer edge may be difficult in practice because real atmospheres contain secondary greenhouse gases that may confound such predictions (e.g., [ 2 132 ]). Nevertheless, observations should be able to tell if the behavior is generally correct or not.

77, Some models predict that some outer edge planets, or those with low volcanic outgassing rates, exhibit limit cycles [ 76 79 ]. If true, that would suggest that some glaciated planets near the outer edge may still be habitable upon deglaciation, although the long timescales involved may make it difficult to infer this remotely.

114, Although most HZ work has focused on planetary habitability during the main-sequence phase of stellar evolution (e.g., [ 1 26 ]), recent years have seen a growing appreciation of the temporal evolution of the HZ, and the pre-main-sequence in particular (e.g., [ 51 130 ]). This is because the pre-main-sequence evolution determines whether planets are still habitable during their host star’s main sequence phase. This is particularly important for planets orbiting M-dwarfs (e.g., [ 51 183 ]). For example, although Proxima Centauri b orbits a main-sequence M-star and receives an Earth-like level of stellar insolation today, unless the planet had migrated inward later, it is likely to have been in a runaway greenhouse state for over 100 Myr ([ 51 ] and Figure 11 ). Depending on the size of the initial ocean inventory, this may or may not be enough to completely desiccate the planet, but the (highly likely) irradiated surface should make its potential habitability dubious [ 181 ]. Similar arguments can be made for the TRAPPIST-1 planets [ 208 ], even though 3–4 of them may be in the current day habitable zone [ 149 206 ]. To survive, life would have to evolve extreme resistance or tolerance to such high radiation levels or find safe haven within underground or ocean habitats. None of these insights would have been possible by simply assessing main-sequence habitability. Thus, the pre-main-sequence HZ, in conjunction with the main-sequence HZ, should be used to rank which worlds are most likely to support large bodies of standing water. All else equal, a planet that was not located within the pre-main-sequence HZ, even if located in the HZ today, should be ranked lower than a planet that has always been inside the HZ.

2 –CH 4 atmospheres near the outer edge of hotter stars are suggestive of inhabitance [ 2 gradient from the inner and outer edge exists, we could infer that the outer edge distance is smaller in M-star systems that contain planets with dense CO 2 –CH 4 atmospheres [ 2 in dense CO 2 atmospheres is likely to be of volcanic [ 2 dominates the planetary atmosphere, then the source is probably primordial instead. Further, if we were to find big terrestrial planets with dense CO 2 –H 2 atmospheres, then we may conclude that they must have very high volcanic outgassing rates, low escape rates, or potent magnetic fields (ibid). Plus, higher scale heights for hydrogen-rich atmospheres can distinguish them from other types of rocky planets ([132, Moreover, if dense CO–CHatmospheres near the outer edge of hotter stars are suggestive of inhabitance [ 2 ], we should observe that a relatively large fraction of these planets are inhabited, as explained in Section 7.2 and Section 7.3 . Supposing that an observable pCOgradient from the inner and outer edge exists, we could infer that the outer edge distance is smaller in M-star systems that contain planets with dense CO–CHatmospheres [ 2 ]. Also, the Hin dense COatmospheres is likely to be of volcanic [ 132 ] rather than primordial [ 129 ] origin. Alternatively, if Hdominates the planetary atmosphere, then the source is probably primordial instead. Further, if we were to find big terrestrial planets with dense CO–Hatmospheres, then we may conclude that they must have very high volcanic outgassing rates, low escape rates, or potent magnetic fields (ibid). Plus, higher scale heights for hydrogen-rich atmospheres can distinguish them from other types of rocky planets ([ 129 152 ]). Thus, the point here is that many planetary processes can be inferred simply by considering the implications of observations as predicted by HZ theory.