Initial stages of habitability

Necessary conditions for the developing of life are thought to include rocky surface and liquid water; however, the aspects connected with the stages preceding the onset of biological era are usually left out of consideration. Planetary age as a necessary condition for life to emerge was first stressed by Huang (Reference Huang1959) and implicitly mentioned by Crick & Orgel (Reference Crick and Orgel1973) in their concept of a Directed Panspermia.

In order to understand the importance of planetary age for the evolution of a detectable biosphere we will consider, as an example, the development of cyanobacteria and the related atmospheric oxidation (Irwin et al. Reference Irwin, Méndez, Fairén and Schulze-Makuch2014). This process involves several endothermic reactions and requires sufficiently high temperatures to be activated. In general, the temperature dependence of the photosynthetic rate is rather complicated and conditionally sensitive, with the effective activation energy being of the order of tens of kJ mol⁻¹ (Hikosaka et al. Reference Hikosaka, Ishikawa, Borjigidai, Muller and Onoda2006), much higher than the typical equilibrium temperature on habitable planets. Thus small variations in atmospheric and crust properties can considerably inhibit photosynthesis and increase the growth time of the mass of cyanobacteria. This conclusion may be illustrated through the consideration of the elementary process of carboxylation of RuBP (ribulose-1,5-bisphosphate: C₅H₁₂C₁₁P₂) in the dark Benson–Bassham–Calvin cycle of photosynthesis (Benson et al. Reference Benson, Bassham, Calvin, Goudate, Haas and Stepka1950; Farquhar et al. Reference Farquhar, von Caemmerer and Berry1980). These photosynthetic reactions, controlled by enzymes, are known to be very sensitive to ambient temperatures with an optimum rate at about 40°C, and a practically zero rate outside the temperature range of 0° < t < 60°C (Toole & Toole Reference Toole and Toole1997). Amongst other fundamental factors RuBP carboxylation is probably the most relevant one, determining the optimal temperature of photosynthesis, and is characterized by the activation energy V _C ≃ 30 − 60 kJ mol⁻¹ at the growth temperature (Hikosaka et al. Reference Hikosaka, Ishikawa, Borjigidai, Muller and Onoda2006). We can roughly characterize the RuBP carboxylation by the Arrhenius law

(1) $${k_{\rm C}} = A{{\rm e}^{ - {V_{\rm C}}/T}},$$

where k _C is the rate constant, A is the prefactor, and T is the absolute temperature. The characteristic time of RuBP carboxylation is ${{\rm \tau} _{\rm C}} \propto k_{\rm C}^{ - 1} \propto \exp ({V_{\rm C}}/T)$ . Since the RuBP carboxylation is one of the main processes optimizing photosynthetic reactions, τ_C can roughly characterize the rate of photosynthesis on a planet.

The range of variation in τ_C on a habitable planet due to the uncertainty in the equilibrium temperature T _e is

(2) $$\displaystyle{{\vert{{\rm \delta} {\rm \tau}} \vert} \over {{{\rm \tau} _{\rm e}}}} = \displaystyle{{{V_{\rm C}}} \over {{T_{\rm e}}}}\displaystyle{{\vert{\rm \delta} T\vert} \over {{T_{\rm e}}}}, $$

with τ _e being a characteristic time of photosynthesis at T _e. The equilibrium temperature T _e, in turn, is calculated using planetary parameters inferred from the observations,

(3) $${T_{\rm e}} = {\left[ {\displaystyle{{L(1 - a)} \over {16{{\rm \pi} {\rm \sigma} {\rm \varepsilon}} {r^2}}}} \right]^{1/4}}, $$

where the uncertainties in the parameters determine the uncertainty in its estimate,

(4) $$\displaystyle{{\vert{\rm \delta} T\vert} \over {{T_{\rm e}}}} = \displaystyle{1 \over 4}\left( {\displaystyle{{\vert{\rm \delta} L\vert} \over L} + \displaystyle{{\vert{\rm \delta} a\vert} \over {1 - a}} + \vert{{\rm \delta} {\rm \varepsilon}} \vert + 2\displaystyle{{\vert{\rm \delta} r\vert} \over r}} \right). $$

Here L is the luminosity of the central star, a and ε the planet's albedo and emissivity, r the orbital radius, and σ the Stephan–Boltzmann constant. It is readily seen that the actual time of the onset of photosynthesis for a given habitable planet might differ significantly from the value calculated from largely uncertain parameters that were, in turn, derived from observables. Indeed, uncertainties in estimates of the equilibrium temperature |δT|/T _e are heavily amplified for habitable planets with V _C/T _e ≃ 10 − 20 for V _C ≃ 30 − 60 kJ mol⁻¹ and T _e ~ 300 K, such that even relatively low observational errors in deriving the parameters in equation (4), of 5% each, might result in 50−100% error in the estimates of the overall photosynthesis rate. If one considers oxidation of the Earth atmosphere as a process tracing the developing photosynthesis, the characteristic time for the growth of biota on early Earth can be estimated as the oxidation time, ${{\rm \tau} _{{\rm O}_2}} \sim 2$  Gyr (Kasting Reference Kasting1993; Wille et al. Reference Wille, Kramers, Nägler, Beukes, Schröder, Meisel, Lacassie and Voegelin2007; Fomina & Biel Reference Fomina, Biel, Allakhverdiev, Rubin and Shuvalov2014). Therefore, a 50% error in τ_C may delay the possible onset of biological evolution on a planet by 1 Gyr, i.e. biogenesis might not start earlier than 3 Gyr from the planetary formation. In general, however, the problem of the photosynthetic process is much more complex, depending on many factors determined by thermal and non-thermal processes on a planet (Shizgal & Arkos Reference Shizgal and Arkos1996; Hikosaka et al. Reference Hikosaka, Ishikawa, Borjigidai, Muller and Onoda2006), and might be even more sensitive to variations in physical conditions. Even on the early Earth, physical conditions could have been such as to preclude the onset of biogenesis over a long time (Sagan Reference Sagan1974; Maher & Stevenson Reference Maher and Stevenson1988; Solomatov Reference Solomatov and Canup2000).

From this point of view, the planetary habitability index (PHI) recently proposed by Schulze-Makuch et al. (Reference Schulze-Makuch, Méndez, Fairén, von Paris, Turse, Boyer, Davila, António, Catling and Irwin2011) in the form

(5) $${\rm PH}{{\rm I}_0} = {(S \cdot E \cdot C \cdot L)^{1/4}},$$

can be generalized with explicit inclusion of the age of the planet t as

(6) $${\rm PHI}(t) = {\rm PH}{{\rm I}_0}\prod\limits_i {(1 - {{\rm e}^{ - t/{t_i}}})}. $$

In equation (5), S defines a stable substrate, E the necessary energy supply, C the polymeric chemistry and L the liquid medium; all the variables here are in general vectors, while the corresponding scalars represent the norms of these vectors. In equation (6), the index i denotes a chemical chain relevant for further biochemical evolution, and t _i is its characteristic time. It is obvious that the asymptotic behaviour – approaching the maximum habitability – is controlled by the slowest process with the longest t _i .

Other factors delaying the onset of habitability

Sagan (Reference Sagan1974) was the first to stress that harmful endogenous and exogeneous processes in the early Earth could postpone emergence of life on it. Such processes could be important even in the very initial primitive episodes of biogenesis and delay the formation of biota for up to billions of years. It is known from the ¹⁸²W isotope dating that the late heavy bombardment of Earth, Moon and Mars lasted till about 3.8 Ga (Schoenberg et al. Reference Schoenberg, Kamber, Collerson and Moorbath2002; Moynier et al. Reference Moynier, Koeberl, Quitté and Telouk2009; Robbins & Hynek Reference Robbins and Hynek2012). The Martian primitive atmosphere is believed to have been lost through catastrophic impacts about 4 Ga (e.g., Melosh & Vickery Reference Melosh and Vickery1989, Webster et al. Reference Webster2013). Evidence of a heavy bombardment in other exoplanet systems exists: collision-induced hot dust was detected in several young planetary systems. Spectral signatures of warm water- and carbon-rich dust in the HZ of a young ~1.4 Ga MS star η Corvi (Lisse et al. Reference Lisse, Wyatt, Chen, Morlok, Watson, Manoj, Sheehan, Currie, Thebault and Sitko2012), and of host dust in seven sun-like stars (Wyatt et al. Reference Wyatt, Smith, Greaves, Beichman, Bryden and Lisse2007) indicate recent frequent catastrophic collisions between asteroids, planetesimals or even possible planets (Song et al. Reference Song, Zuckerman, Weinberger and Becklin2005). Out of these seven stars, five are young systems within their first Gyr of life.

It is also well-known that solar-type stars remain very active in the first billion years of their life, sustaining conditions that are hostile to the survival of the atmosphere and to the planetary habitability. G-type stars, within the first 100 Myr of reaching ZAMS, produce continuous flares of extreme-UV (EUV) radiation up to 100 times more intense than the present Sun, and have much denser and faster stellar winds with an average wind density of up to 1000 times higher. Low-mass K- and M-type stars remain X-ray and EUV-active longer than solar-type stars, where EUV emission can be up to 3–4 times and 10–100 times, respectively, higher than G-type stars of the same age; and active M-type stars could keep stellar winds in the HZ that are at least 10 times stronger than that of present Sun (France et al. Reference France2013).

In recent simulations by Schaefer and Sasselov (Reference Schaefer and Sasselov2015) of the development of oceans on super-Earths, it was shown that though these planets keep their oceans for longer than the Earth (up to 10 Gyr), it also takes longer for them to develop the surface ocean due to the delayed start of volcanic outgassing that returns water back to the surface from the mantle. For super-Earths 5 times the Earth's mass, that would take about a billion years longer. The oceans are believed to have established on Earth 750 Myr after formation, therefore super-Earths would have their surface water established at only 2 Gyr after formation. After all, the Great Oxygenation Event about 2.5 Ga (Anbar et al. Reference Anbar, Duan, Lyons, Arnold, Kendall, Creaser, Kaufman, Gordon, Scott, Garvin and Buick2007) was most likely induced by oceanic cyanobacteria, which allowed life to emerge on land about 480 − 360 Ma (Myr ago) (Kenrick and Crane, Reference Kenrick and Crane1997).

Potentially habitable planets

At the time of writing, more than 1900 exoplanets have been confirmed (Extrasolar Planets Encyclopaedia, June 2015) with another 4000 waiting for confirmation (NASA Exoplanet Archive). The majority of detected planets are in the vicinity of the Sun, and their hosts are mostly young Population I (Pop I) stars with ages of hundreds of Myr to a few Gyr. The age distribution of the host stars with measured ages is shown in Fig. 1. 58% of the host stars have ages of 4.5 Gyr and less, and more than one-third (~38%) are younger than 3 Gyr. Simple statistics shows the median age of ~3.8 Gyr.

Fig. 1. Age distribution of the stars hosting confirmed planets (total 583 hosts with known ages at time of writing). We highlight the number of stars with ages below 3 Gyr in black, and between 3 and 4.5 Gyr as hatched. The predominance of young host stars is clearly seen, which could be the effect of observational selection (Shchekinov et al. Reference Shchekinov, Safonova and Murthy2013). This figure was made using the Extrasolar Planets Encyclopaedia data.

The fact that more than one-third of the planetary systems, discovered by ongoing exoplanetary missions, are younger than ~3 Gyr is not surprising, because the continuous star formation (SF) in the Galactic disk supplies young stars, and the fraction of hosts younger than 3 Gyr represents that very fraction of Pop I stars that would be born provided the SF rate is nearly constant during the whole period of the thin disk formation. Most of the current exoplanet missions suffer from an observational bias – they mostly detect systems that are younger than the age at which life is presumed to have appeared on the EarthFootnote ³ .

Incidentally, Fig. 1 shows a deficit of stars with ages t > 6 Gyr. Assuming that Pop I stars, i.e. the thin Galactic disc, have started forming at about 10 Ga (Chen et al. Reference Chen, Hou and Wang2003; Carraro et al. Reference Carraro, Geisler, Villanova, Frinchaboy and Majewski2007), one might expect the presence of such old stars in our vicinity in the proportion corresponding to the SF history in the early Galaxy. The most conservative assumption implies a constant SF rate, in which case one should expect the number of planet-hosting stars with ages t > 6 Gyr of about 40%. It is, however, believed that the SF was more active in the early epochs (Bouwens et al. Reference Bouwens, Illingworth, Franx and Ford2007), therefore, the fraction of hosts older than 6 Gyr should be correspondingly higher. The reason for the decline in the number of the hosts in this age range is unclear and might, in particular, indicate that planetary systems lose planets with age.

About 40 PHPs are currently documentedFootnote ⁴ , though extrapolation of Kepler's data shows that in our Galaxy alone there could be as many as 40 billion PHPs (Petigura et al. Reference Petigura, Howard and Marcy2013). In Table 1, we show the data for PHPs for which the host ages were available in the literature. The fraction of young planetary systems is nearly consistent with the age distribution of Pop I stars: among the 33 confirmed habitable planets with known ages more than half are ≲3.5 Gyr old.

Table 1. Host ages for confirmed PHPs

^a The radii of these planets are >1.7 Earth's, however, it is still too soon to exclude them from the list, according to Torres et al. (Reference Torres2015), since there are many uncertainties in the modelling of the transition from rocky to hydrogen/helium planets, and these planets may be rocky.

References to ages: 1. The Extrasolar Planet Encyclopaedia (http://exoplanet.eu); 2. Angalada-Escudé et al. (Reference Anglada-Escudé2012); 3. Anglada-Escudé et al. (Reference Anglada-Escudé2013); 4. Borucki et al. (Reference Borucki2013); 5. Anglada-Escudé et al. (Reference Anglada-Escudé2014); 6. Mamajek & Hillenbrand (Reference Mamajek and Hillenbrand2008); 7. Tuomi & Anglada-Escudé (Reference Tuomi and Anglada-Escudé2013); 8. Open Exoplanet Catalogue (http://www.openexoplanetcatalogue.com); 9. Nordström et al. (Reference Nordström, Mayor, Andersen, Holmberg, Pont, Jørgensen, Olsen, Udry and Mowlavi2004); 10. Tuomi et al. (Reference Tuomi, Anglada-Escudé, Gerlach, Jones, Reiners, Rivera, Vogt and Butler2013a); 11. Pepe et al. (Reference Pepe, Lovis, Ségransan, Benz, Bouchy, Dumusque, Mayor, Queloz, Santos and Udry2011); 12. Metcalfe (Reference Metcalfe2013); 13. Wittenmyer et al. (Reference Wittenmyer2014); 14. Torres et al. (Reference Torres2015); 15. NASA Exoplanet Archive at http://exoplanetarchive.ipac.caltech.edu; 16. Gaidos (Reference Gaidos2013); 17. Crossfield et al. (Reference Crossfield2015); 18. Tuomi et al. (Reference Tuomi2013b).

It seems reasonable to update the definitions in footnote 1 on page 1 as

1. 1. PHP – a rocky, terrestrial-size planet in an HZ of a star.

2. 2. Habitable planet – a rocky, terrestrial-size planet in an HZ with detected surface water and some of the biogenic gases in atmosphere.

3. 3. Inhabited planet – the best case scenario: a rocky, terrestrial-size planet in an HZ with simultaneous detection of species such as water, ozone, oxygen, nitrous oxide or methane in atmosphere, as proposed by e.g. Sagan et al. (Reference Sagan, Thompson, Carlson, Gurnett and Hord1993) or Selsis et al. (Reference Selsis, Despois and Parisot2002).

We may expect only a primitive form of biota on the youngest planets (≲2 Gyr) in Table 1, which would not be detectable. Biogenesis could have started, or even progressed to more advanced stages with an oxidized atmosphere, on older planets with ages from 2 to 4 Gyr. In the former case, one can expect that methane from metabolic reactions has already filled the atmosphere, while in the latter case, oxygen molecules at some level can appear in the atmosphere – though atmospheric oxygen on Earth appeared about 2.5 Ga, the Earth itself became visibly habitable only about 750–600 Ma, when the biosphere became active and complex enough to modify the environment to be noticed from space (e.g., Méndez et al. Reference Méndez, González, Jimenez, Pérez and Bracero2013). The traces of these gases may, in principle, be observed in sub-mm and micron wavelengths, provided the planets are orbiting low-mass stars (0.5–0.8 M _⊙). Even if one-third of the low-mass stars in the sky host planets (Tutukov & Fedorova Reference Tutukov and Fedorova2012), there may be as many as a thousand planets within a 10 pc vicinity with ages ranging from Myr to a few Gyr.

The age of a planet is of primary importance for developing the future strategy of looking for life on PHPs. Since space programs are extremely expensive and require extensive valuable telescope time, it is crucial to know in advance which planets are more likely to host detectable life. Young planets will not have atmospheres abundant in products of photosynthetic processes, and many planets, though residing in the HZ, may not actually be habitable for life as we know it. For example, the host stars in the Degenerate Objects around Degenerate Objects (DODO) direct imaging search for sub-solar mass objects around white dwarfs (Hogan et al. Reference Hogan, Burleigh and Clarke2009) are rather young with an average age of only 2.25 Gyr. The target star selection of the Darwin (ESA) mission is restricted to stars within 10 − 25 pc (Kaltenegger & Fridlund Reference Kaltenegger and Fridlund2005), and two space missions that are currently under study, the NASA Transiting Exoplanet Survey Satellite (TESS) mission and ESA's PLAnetary Transits and Oscillations of stars (PLATO) mission, will only survey bright F, G, K stars and M stars within 50 pc (e.g., Lammer et al. Reference Lammer2013), sampling therefore only the thin Galactic disc stars – young Pop I hosts. The main focus of Exoplanet Characterization Observatory (EChO) (Drossart et al. Reference Drossart2013) is the observation of hot Jupiter and hot Neptune planets, limited due to the mission lifetime constraints to bright nearby M stars (Tinetti et al. Reference Tinetti2012). Most known habitable planets cannot have an existing complex biosphere although they may develop it in the future, because most currently known PHPs are found around relatively young Pop I stars. We feel that it is reasonable to fix a period of ~4 Gyr as the minimum necessary time for the formation of complex life forms under optimal conditions, as evidenced by the Earth's biosphere. Direct observations of planetary atmospheres in IR and sub-mm wavebands would be a promising method for tracing biogenesis. Planned future IR and sub-mm observatories could provide such observations (see the discussion in Section ‘Observational prospects’ below.).

In this context, we have undertaken the project of updating the catalogue of Nearby Habitable Systems (HabCat) constructed for SETI by Turnbull & Tarter (Reference Turnbull and Tarter2003a) for the search for potentially habitable hosts for complex life. A complete characterization of all the stars within a few hundred (or even a few tens of) parsecs, including their masses, ages, and whether they have planetary systems (including terrestrial planets), was not realizable at that time. Our aim was to find out the information on these stars: their ages and whether they have planets and if they could be potentially habitable.

To begin with, we have taken the HabCat II, a ‘Near 100’ subset – a list of the nearest 100 star systems of the original HabCat (Turnbull & Tarter Reference Turnbull and Tarter2003b), as a basis for our project. These stars were scrutinized for information on their age, nearby planets etc., which were missing in the original catalogue but are important now due to their impact on selecting the targets for future space missions. Out of 100 nearby (within 10 pc) objects in the HabCat II, we have found the age data for 50 stars. This list is being cross-correlated with the Hypatia Catalogue, which is a project to find abundances for 50 elements, specifically bio-essential elements, for the stars in the HabCat (Hinkel et al. Reference Hinkel, Timmes, Young, Pagano and Turnbull2014). Our goal is to compile a list of the most probable planets that may allow future missions to search our neighbourhood for habitable/inhabited planets more efficiently. The preliminary result of this project is presented in Table A.2 in Appendix B.

Old planetary systems

Observational prospects

Recently, a 13.6 Gyr star was detected placing it as the oldest star in the Universe (SMSS J031300.36–670839.3, Keller et al. Reference Keller2014); the age was estimated by its metallicity [Fe/H] ≤ −7.41. In spite of that, this star, believed to have formed from the remnants of the first-generation SN, was found to contain carbon, metals such as lithium, magnesium, calcium, and even methylidyne (CH). It is quite possible that such stars have planets that are directly observable in micron wavelength range. Such EMP stars are known to have low masses and, as such, the orbiting planets could be seen directly in the IR.

The number of EMP stars is estimated to be about 250,000 within 500 pc in SDSS database (Aoki et al. Reference Aoki2006), so the mean distance between them is about 10 pc. If each EMP star hosts an Earth-size planet, the flux from the planet at a distance d in the IR range (λ ~ 10 µm) evaluated at the peak frequency (Wien's law) ν _T = αkT/h, is

(7) $$F_\nu ^{{\rm pl}} = {\rm \sigma} T_{{\rm pl}}^4 {\rm \pi} {\left( {\displaystyle{{{R_{{\rm pl}}}} \over d}} \right)^2}. $$

we can rewrite this flux as

(8) $$F_\nu ^{{\rm pl}} = 0.73{\left( {\displaystyle{{{T_{{\rm eq}}}} \over {300\;{\rm K}}}} \right)^3}{\left( {\displaystyle{d \over {10\;{\rm pc}}}} \right)^{ - 2}}{\left( {\displaystyle{R \over {{R_{_{\rm E}}}}}} \right)^2}\;{\rm mJy}, $$

where T _eq is the equilibrium temperature of a planet and R is its radius. For the Sun/Earth system, the ratio of the fluxes at a distance of 10 pc is

(9) $$\displaystyle{{F_\nu ^{{\rm pl}}} \over {{F_{\ast}}}} = \displaystyle{{{T_{\rm E}}} \over {{T_ \odot}}} {\left( {\displaystyle{{{R_{\rm E}}} \over {{R_ \odot}}}} \right)^2} \sim 4 \times {10^{ - 6}}. $$

However, if we consider a super-Earth with M ~ 5M _E, R ~ 2R _E and T _eq = 300 K, orbiting the star with T = 3000 K and R ~ 0.1 R _⊙ – an M dwarf, we get an improvement of

(10) $$\displaystyle{{F_\nu ^{{\rm pl}}} \over {{F_{\ast}}}} = 3.4 \times {10^{ - 3}}. $$

It seems challenging to detect such a weak contribution to a total flux from a planet even in the IR. There is, however, a possibility to distinguish the emission from the planet in IR molecular features, such as CH₄ or O₂, tracing either initiated biogenesis or developed metabolism. Detection of direct IR emission from O₂ on exoplanets going through the initial epoch of biogenesis, or which are already at a stage with developed biota, was discussed in Churchill & Kasting (Reference Churchill and Kasting2000) and Rodler & Lopez-Morales (Reference Rodler and Lopez-Morales2014), respectively. Rich IR to sub-mm spectra of methane (Niederer Reference Niederer2012; Hilico et al. Reference Hilico, Loete and Champion1987) also allow to optimistically view the future detection of this biosignature. Even at the low temperatures of EMP stars, T _* ~ 3000 K, these molecules are unlikely to survive in sufficient amount in their atmospheres. Therefore, if such emission is observed from an EMP star, it should be considered as a direct indication of an orbiting rocky planet that has already entered the habitable epoch with growing PHI (equation (6)). The most promising way to identify habitable (inhabited) planets seems to look for simultaneous presence of water, O₂, O₃, CH₄ and N₂O in atmospheric spectra (e.g., Selsis et al. Reference Selsis, Despois and Parisot2002; Kaltenegger et al. Reference Kaltenegger, Traub and Jucks2007; Kiang et al. Reference Kiang, Segura, Tinetti, Blankenship, Cohen, Siefert, Crisp and Meadows2007). Though such observations can be used to detect planets with highly developed habitability orbiting old EMP stars, the expected fluxes in the IR are still below current sensitivity limits and might be only possible in the future. For example, the future Millimetron space observatory planned for launching in next decade (estimated launch 2025) will have the detection limit of 0.1 µJy in 1 h observation in 50–300 µm range (Kardashev et al. Reference Kardashev2014). A molecular CH₄ absorptions at ~50 µm can be detected by Millimetron in 3 h observations (Equation (10)) if a nearby (within 10 pc) habitable super-Earth planet transits an M-dwarf.

Summary

• • Age of a planet is an essential attribute of habitability along with such other factors as liquid water (or an equivalent solvent), rocky mantle, appropriate temperature, extended atmosphere, and so forth. The knowledge of the age of a ‘habitable’ planet is an important factor in developing a strategy to search for complex (developed) life.

• • Nearly half of the confirmed PHPs are young (with ages less than ~3.5 Gyr) and may not have had enough time for evolution of sufficiently complex life capable of changing its environment on a planetary scale;

• • Planets do exist around old Pop II stars, and recently discovered EMP stars (belonging presumably to an intermediate Pop II.5) are good candidates for direct detection of orbiting planets in the IR and sub-mm wavelengths. Though currently only very few such PHPs are known, old giant planets may have habitable worlds in the form of orbiting moons.

• • IR and sub-mm observations of terrestrial planets orbiting low-mass old stars are a promising way to trace biogenetic evolution on exoplanets in the solar neighbourhood.