ASTROBIOLOGY, EXOPLANETS AND THIER HABITABILITY

Brief Overview

To weave that tapestry story of our place in the universe, astrobiology starts at the first ce of hydrogen - the simplest element. Over time, nuclear fusion in the cores of the first stars turned that hydrogen into many of the rest of the elements, especially the ones that make up all life on Earth, including our very selves. Astrobiology follows those elements into planets and moons, studying the process of planetary system formation, in which the star and all the bodies that orbit it form together, from the same source material and at the same time.

Astrobiology investigates the relationship between life and its planet (or moon). It champions the idea of a planet as a container that facilitated the emergence of life as a transition from geochemistry to biochemistry, supplying the required raw materials and energies. Astrobiology goes on to explore the dance between planet and life - the co-evolution wherein planet influences life’s evolutionary trajectory; then life innovates, adapts, and influences back - and on and on. It even looks into the relationship of all life at the molecular level, seeking to define the Last Universal Common Ancestor, at once appreciating the diversity - yet common ancestry - of all life, and the shared provenance of all things in the universe.

Definition

Astrobiology, formerly known as exobiology or xenobiology, is an interdisciplinary scientific study of the origins, early evolution, distribution, and future of life in the universe. A multidisciplinary field dealing with the nature, existence, and search for extraterrestrial life (life beyond Earth). Astrobiology encompasses areas of biology, astronomy, biophysics, biochemistry, chemistry, physical cosmology, exoplanetology and geology.

Although no compelling evidence of extraterrestrial life has yet been found, the possibility that biota might be a common feature of the universe has been strengthened by the discovery of extrasolar planets (planets around other stars), by the strong suspicion that several moons of Jupiter and Saturn might have vast reserves of liquid water, and by the existence of microorganisms called extremophiles that are tolerant to extremes environments. The first development indicates that habitats for life may be numerous. The second suggests that even in the solar system there may be other worlds on which life evolved. The third suggests that life can arise under a wide range of conditions.

The principal areas of astrobiology research can be classified as:

1. Understanding the conditions under which life can arise
2. Looking for habitable worlds
3. Searching for evidence of life, and
4. When and how the research on had Astrobiology started?

These are what the article is going to highlight for us. And will conclude with a discussion on a milestone in the field of astrobiology. Astrobiology concerns itself with interpretation of existing scientific data, and although speculation is entertained to give context, astrobiology concerns itself primarily with hypotheses that fit firmly into existing scientific theories.

Extremophiles and Their Natural Position on Earth

Life Under Extreme Conditions

One of the rapidly developing areas in astrobiology is the study of life under extreme conditions, including that in space. Extremophiles are microbes that live under extreme conditions on Earth, which are generally hostile to most other life. Extreme conditions include extreme heat or cold, high concentration of salts, and high pressure. Many extremophiles are polyextremophiles. They are adapted to multiple forms of stress. These microbes are good models for the survival under extraterrestrial extreme conditions. Progress in space missions allowed for the study of such microbes in space. They were found to be able to survive in space under a specific limited exposure. Adaptation of life to extreme environment includes viruses.

There is hardly any place on our planet, however extreme, that is not inhabited by at least some types of living organisms adapted to the environment. Thus, the physico-chemical boundaries for life on Earth are very broad. One can almost state that any place where a minimal amount of liquid water is available can support life. Microbial life was found in very cold and very hot environments, even at temperatures exceeding 100°C in undersea springs, where the boiling point of water is elevated because of the hydrostatic pressure. Microorganisms was known to be adapted to life in concentrated acids and in highly alkaline lakes. Life is possible in salt-saturated brines, under high levels of ionizing radiation and under pressure in the deepest parts of the ocean. Microorganisms even live in deserts, hot as well as cold, thriving on very small amounts of water that become available occasionally.

The lowest water activity that still allows active growth of microorganisms appears to be around 0.63–0.64, but many types can survive prolonged periods of desiccation until conditions again become suitable for growth. There are also many types of microorganisms that can be call polyextremophiles - organisms adapted to more than one environmental extreme. Thus, there is haloalkaliphiles that grow only at salt concentrations approaching saturation and at PH greater than 8.5 - 9; there are thermoacidophiles that thrive only in hot acid solutions, thermophiles that prefer the hydrostatic pressure of the deep sea, and even haloalkaliphilic thermophilic anaerobes that inhabit the sediments of some shallow soda lakes in tropical area.

1 - High-Temperature Environments and Thermophilic Microorganisms

There is no lack of hot environments on planet Earth. The best studied ones are associated with volcanic activity - hot springs are abundant, for example, in the western USA (in Yellowstone National Park), New Zealand, Iceland, Japan, and Italy. Many hot springs are also highly acidic due to the oxidation of reduced sulfur compounds. Even higher temperatures prevail in the deep-sea hydrothermal vents, where the in situ temperature often exceeds 100°C and the ambient pressure prevents the water from boiling.

The anoxic, sulfide, and mineral-loaded waters emitted by the so-called black smoker chimneys in marine hydrothermal vents can reach temperatures of ~350°C. Surface soils in hot deserts may heat up to temperatures as high as 70°C during daytime. Similar temperatures were also measured in compost piles and silage, heated as a result of the microbial activity of fermentative bacteria. Microbial life at high temperature may also abound deep within the crust of the Earth. All these environments have become rich sources of isolates of thermophilic (generally defined as organisms with an optimum growth temperature >45°C) and hyperthermophilic microorganisms (with optimum growth temperatures >80°C) of many kinds. Some of these have found interesting biotechnological applications.

Some chemoorganotrophic and chemolithotrophic Bacteria can grow at temperatures up to 95°C –100°C. At the highest temperatures enabling life (the highest value recorded today being 122°C), only members of the domain Archaea survive. A short time after the discovery of the deep-sea hot vents and the life associated with them, a report was published, claiming that black smoker bacteria may be capable of chemolithotrophic growth under in situ vent pressure of ~27 MPa at temperatures of at least 250°C. This observation is today considered an artifact. Based on the limited thermal stability of metabolites in aqueous solution, the theoretical maximum temperature for life was recently estimated at 150°C–180°C.

2 - Cold Environments and Psychrophilic Microorganisms

Permanently cold environments are also abundant on planet Earth. Most of the deep waters of the oceans have a constant temperature of 1°C–3°C. The Arctic and the Antarctic are permanently cold, and parts of northern countries such as Siberia-Russia and Canada are permanently frozen - “permafrost”. All these environments support life of cold-adapted, psychrophilic, and psychrotolerant microorganisms.

True psychrophiles are generally defined as such organisms that grow optimally below 15°C and that still can grow below 0°C. Organisms that show growth below 0°C but have their optimum above 15°C–20°C are called psychrotolerant or psychrotrophs. The ability of psychrophiles and psychrotrophs to grow at low temperatures depends on adaptive changes in their cellular proteins and lipids.

The lowest growth temperature recoded for the heterotrophic gas vacuolate Psychromonas ingrahamii (Gammaproteobacteria), isolated from sea ice of Alaska, is −12°C, with a generation time of 240h.

Other species able to grow at low temperatures are Psychrobacter cryohalentis and Psychrobacter arcticus isolated from Siberian permafrost - they grow at temperatures from −10°C to +3°C and salinities up to 1.7M. Planococcus halocryophilus, isolated from permafrost, even grows slowly at −15°C, the lowest growth temperature documented for any bacterium. Microbial activity may be possible at even lower temperatures. During respiration measurements in cultures of the Antarctic glacial isolates Sporosarcina sp. B5 and Chryseobacterium sp. V3519-10 within ice at temperatures from −4°C to −33°C.

The terrestrial permafrost environment and its microorganisms can be considered as a model for conditions for life on other cold planets. The deep cold biosphere is the most stable environment for microorganisms, and possible fluctuations should be explained by geological events only. Cold-adapted microorganisms may survive within permafrost at subzero temperatures over geological time and resume their activities upon thawing. Studies of microbial communities in permafrost sediments of different lithology and age suggest that the temperature and the length of exposure define the ratio between cells that can rapidly resume activity and deep-resting “viable but non-culturable” cells.

Microorganisms may survive within a film of unfrozen water enveloping soil particles that protects them from freezing. Brine lenses called cryopegs are found in Siberian permafrost, and these were formed and isolated from ancient marine sediment layers of the Arctic Ocean ~100,000–120,000 years ago. Owing to its high salinity (170–300 g/L), the water remains liquid at the in situ temperature of −10°C, but even at −15°C, uptake of [14C]-glucose could be measured. Different types of psychrophilic prokaryotes, including aerobes, fermentative anaerobes, sulfate reducers, acetogens, and methanogens, could be retrieved from these cryopegs.

3 - Hot and Cold Deserts and Their Microbial Communities

Life depends on water. Therefore, areas with low water availability are harsh habitats for life. This is true for hot deserts as well as for very cold desert environments such as the Dry Valleys in Antarctica. Because of the relevance of such environments as model systems to evaluate the possibilities for life in space, relatively much research has been devoted to the presence and activities of microorganisms in the driest deserts on Earth.

Mars-like soils in the most arid region of the Atacama Desert in Chile contain very little organic material, and in most samples, the numbers of colony-forming heterotrophic bacteria were below the detection limits of dilution plating. Decomposition of organic material in these soils is probably due mainly to non-biological processes. Solar ultraviolet-B (UVB) radiation kills even the most resistant microorganisms within a few hours of exposure to the conditions of the Atacama.

However, endospores of Bacillus subtilis (Firmicutes) and conidia of Aspergillus niger (Ascomycota) incubated in the dark in the Atacama Desert for up to 15 months survived to a significant extent. Microorganisms can survive in deserts, hot as well as cold ones, as endolithic communities below the surface of sandstone and other porous rocks. There, they are relatively protected from extreme desiccation and from excess light. The organisms rapidly switch their metabolic activities on and off in response to environmental changes.

In hot desert rocks, the endolithic community consists entirely of cyanobacteria (mainly Chroococcidiopsis) and other prokaryotes. Growth is slow, but the communities can survive for very long times. Production of pigments, exopolysaccharides, and osmoprotectants may also reduce the level of stress. Characterization of hypolithic communities of Chroococcidiopsis and associated heterotrophs colonizing translucent stones in Atacama revealed that each stone supported a number of unique 16S rRNA gene - defined genotypes. In the most arid zone of the Atacama Desert, such hypolithic cyanobacteria are rarely found.

The Dry Valleys of Eastern Antarctica are the coldest and driest deserts on Earth. Yet, a narrow subsurface zone of certain rock types is colonized by microorganisms. Lichens, growing between the crystals of porous rocks, are the dominant compound of this community.

The endolithic communities of the cold deserts of Antarctica thus differ from those of the hot deserts by dominance of eukaryotes rather than prokaryotes. A large number of black, mostly meristematic fungi were isolated from these lichen-dominated communities. Most isolates were affiliated with the Dothideomycetidae, genera Friedmanniomyces and Cryomyces. All had thick melanized cell walls and could produce exopolysaccharides.

4 - Microbial Life at High Hydrostatic Pressure

The mean depth of the oceans is about 4km, equivalent to a pressure of 400 atmospheres or 40MPa, and the deepest parts of the oceans are more than 10km deep. Microorganisms living in such environments must withstand pressures of >100 MPa. The deep sea is inhabited not only by microorganisms - a variety of marine animals is adapted to life at high pressures. As most of the deep sea is also cold (typically 1°C–3°C), microorganisms living there must possess both psychrophilic and piezophilic/barophilic) or piezotolerant/barotolerant properties.

Deep-sea bacteria adapted to life at high pressure and low temperature generally grow slowly, so that the overall microbial activity in the deep-sea environment is low. Because of the complex technical demands of performing experiments at hundreds of atmospheres of pressure, without decompression that kills obligately piezophilic microorganisms, understanding of the special adaptations of such organisms is limited.

The deep-sea hot vents are hotspots of life on the sea bottom, such hot vents have been a treasure trove for the discovery of interesting thermophilic Archaea and Bacteria. As such hot vents are typically found at depths of several kilometers below the sea surface, the organisms inhabiting them must also be barophilic or at least barotolerant.

5 - Life at High Levels of Ionizing Radiation

Another extreme environmental condition to which some microorganisms show a surprisingly high level of resistance is ionizing radiation, including radioactivity. The study of the mechanisms enabling microorganisms to tolerate high radiation levels is highly relevant for astrobiology.

The genus best known for its extreme radiation tolerance is Deinococcus, which forms a deep phylogenetic lineage with the thermophilic Thermus and relatives. The first member of the genus, Deinococcus radiodurans, is still unsurpassed with respect to its tolerance toward high radiation levels - it survives exposure to greater than 5kGy of gamma radiation and to 1500 Jm−2 of UV radiation. For comparison, exposure to 5Gy is lethal for humans. Strains of D. radiodurans were originally isolated from canned meat that had been irradiated. But the species has also been found in radioactive waste. Deinococcus strains have been isolated from desert soil and from many other environments. 16S rRNA gene sequences of Deinococcus were also found in South Pole snow.

One of the secrets of Deinococcus is its extraordinary ability to repair damaged DNA, not only single-strand breaks but also double-strand breaks. It also has special ways to package its DNA and to protect its proteins against damage. Studies on the desiccation resistance of ionizing-radiation resistant organisms suggest that the ability of microorganisms to repair their DNA might be a response to DNA damage caused by prolonged desiccation rather than ionizing radiation. This is supported by the higher proportions of ionizing-radiation-resistant bacteria found in arid soils as compared with soils from less arid regions. However, cells of D. radiodurans incubated in the dark in the Atacama Desert for up to 15 months poorly survived, because they were inactivated at relative humidity between 40% and 80%, which typically occurs during desert nights.

The microorganisms on Earth can colonize at nearly every place on the planet, thanks not only to the fact that they can utilize almost any energy source available but also to the fact that the microbial world can adapt to extremes of temperature (hot as well as cold), pH (from pH <0 to pH >10–11), salt concentration and presence of chaotropic ions, hydrostatic pressure, radiation, and often combinations of one or more of these potentially stressful environmental factors.

METHODS FOR DETECTING EXOPLANETS AND THEIR HABITABILITY POTENTIALS

We have seen different extreme conditions that can supports life of microbes, like extreme heat or cold, high concentration of salts, and high pressure. Now, it is time for us to study how outer planets are been detected and the process of determining whether life is habitable in these planets.

Extrasolar planets, or exoplanets, are planetary-mass bodies in orbit around stars other than the Sun. As of September 3rd 2020, there are confirmed detections of 4,330 exoplanets, ranging from objects with masses more than 10 times that of Jupiter to the ones with masses, and radii, less than that of the Earth.

Discovering what are very faint objects around very bright host stars is, of course, challenging. As an illustration, Figure 1 shows the 2M1207 system. It consists of a planetary-mass object (red object in lower-left region) in orbit around what is known as a brown dwarf. A brown dwarf is an object that does not become massive enough to ignite nuclear fusion in its core and, therefore, does not become a star.

Figure 1: Image showing the 2MASS J12073346-3932539 system, which includes a planetary-mass body (red object in the lower-left region of the figure) and a brown dwarf host.

(Courtesy of European Southern Observatory [ESO].)

In this case, the brown dwarf host has a mass 25 times that of Jupiter and a luminosity 500 times less than that of the Sun. The planetary companion has a mass of about 4 Jupiter masses and, because it is still young, is considerably brighter than Jupiter. The separation between the planet and its host is also more than 10 times greater than the separation between Jupiter and the Sun.

Now imagine trying to detect a planet that is fainter than that shown in Figure 1 and more than 10 times closer to a host that is 500 times brighter. It is clearly hugely a challenging task, and although it is sometimes possible to directly image an exoplanet around a Sun-like star, most of the exoplanets detected to date have been detected using indirect, rather than direct, methods.

In the texts below, our attention will be focus on the various methods that have been used to detect exoplanets and also what we can learn about these planets from those different methods. We’ll focus, initially, on methods that indirectly detect exoplanets but will also explore direct detection at the end, which is becoming more and more successful. The discussion will also talk on what have been known for now about the properties and characteristics of exoplanets and what is expected to discover in the coming years, and will examine the prospects for detecting planets that may potentially be habitable.

DETECTION METHODS

1 - Pulsar Timing

Even though, the terms of planets orbiting their parent stars was often known, in reality, the star and planets all orbit the common center of mass of the system. In most planetary systems, the star has, by far, the most mass, and so, the center of mass will be located near, or sometimes inside, the star. The motion of the star around the center of mass is a consequence of the gravitational influence of the planets on their parent star. Therefore, one way to detect exoplanets is to observe stars to see if they are moving in a way consistent with them having planetary companions. The first known exoplanets were detected in this way but were not actually detected around a Sun-like star - they were detected around a pulsar.

A pulsar is a rapidly spinning compact object, typically the remnant core of a dead, high-mass star, a star that originally had a mass at least eight times that of the Sun. Pulsars are highly magnetized and can emit a beam of electromagnetic radiation. Because the radiation beam is not aligned with the spin axis, the beam can sweep past the Earth, producing what we observe as pulses, hence the name. If the pulsar is an isolated object, then the interval between these pulses will be very regular. If, however, it is the host to a planet, or planets, then because it will also be in orbit around the center of mass of the system, it will sometimes be slightly further from the Earth than at other times.

Consequently, there will be variations in the pulse timings that can then be used to infer the presence of a planetary companion. The first such detection was announced in 1992 and was a planetary system composed of three planets orbiting a 1.4 solar-mass pulsar. The innermost planet has a mass only slightly larger than that of the Earth’s moon, while the other two have masses a few times that of the Earth. They orbit the pulsar with orbital periods of 25, 66, and 98 days. In this case, however, the pulsar likely formed from the merger of two white dwarfs, the remnant cores of stars with masses similar to that of the Sun. Therefore, rather than the planets being primordial, they probably formed from a disc of material generated during the merger of these two white dwarfs.

So, not only was this detection not of planets around a star like the Sun, these planets were also probably never in orbit around a main-sequence star, having formed well after the death of the stars that later became their pulsar host. Such planets also turn out to be rare, with very few others having been found. They are, however, still the first planets to be found outside our solar system and demonstrate how they can be detected via their influence on their parent object.

Figure 2: Artists impression of extrasolar planets in the pulsar, PSR B1257+12

Image credit: By NASA/JPL-Caltech/R. Hurt (SSC) - http://photojournal.jpl.nasa.gov/catalog/PIA08042, Public Domain, https://commons.wikimedia.org/w/index.php?curid=705149

2 - Astrometry

Given that, in a planetary system, both the planets and the host star orbit the common center of mass, one expected way to be able to detect planets would be to actually observe the motion of the star around the center of mass of the system. Doing so would require measuring the angular motion of a planetary host star by using high-precision astrometry.

However, the angular motion of a planetary host star is typically very small. For example, the Sun executes an orbit about the center of mass (due to the gravitational influence of the planets in the solar system), with a radius of about 0.005AU. When viewed from a distance of 10 parsecs (1 parsec being 206,265 times the distance from the Sun to the Earth), the Sun would appear to have an angular motion of about 0.001arcsecs, or 2.8 × 10⁻⁷ degrees. Even though, such measurements have not been make. However, European Space Agency (ESA’s) Gaia mission, which launched in 2013, will collect enough data that it will be possible to detect planets via astrometric measurements of the motion of their host stars.

Figure 3: Motion of the center of mass (barycenter) of solar system relative to the Sun

Image credit: By Solarsystembarycenter.gif: Carl Smithderivative work: Rubik-wuerfel (talk) - http://s173.photobucket.com/user/CarlSmith_2007/media/Sun%20SSB/ssb-orbit-col.gif.html > GIF, CC BY-SA 3.0, https://commons.wikimedia.org/w/index.php?curid=9952653

3 - Radial Velocity/Doppler Wobble

The first exoplanet found in orbit around a Sun-like star was announced in Mayor and Queloz (1995). The method used to detect this planet is known as the radial velocity, or Doppler wobble, method. Stars have a spectrum with absorption lines that depend on the various atomic, and molecular species in their atmospheres.

If a star is moving relative to us, then the wavelengths of these spectral lines shift in a way that depends on the relative line-of-sight velocity of the star. This is known as the Doppler effect. If the star is moving away from us, the lines will shift to longer wavelengths (red-shifted). If the star is moving toward us, they will shift to shorter wavelengths (blue-shifted).

However, if a star has a companion - such as a planet, it will orbit the common center of mass of the system. This means that at some times, it will be coming toward us, while at other times, it will be moving away from us. If the radial velocity of the Sun-like star HD32963 in meters per second (m/s), were plotted against time in days, it will shows an amplitude of just over 10m/s, with a period of 2372 days (6.5 years). In this case, the observations were taken over a period of more than 6.5 years, but the data has been phase-folded, so as to represent it as a single orbital period.

The period of the orbit of Sun-like star HD32963, can be determined directly from the radial velocity curve. The radial velocity measurements are also estimated using the spectrum of the star itself. Hence, the stellar spectra can be use to determine what type of star it is and it’s mass. From the mass of the star, and the period of the orbit, Kepler’s laws can be use to infer the orbital radius of the planet, typically called the semi-major axis.

The mass of the companion can also be determine using conservation of linear momentum. The radial velocity measurements give the line-of-sight velocity of the star. The Companion’s orbital radius and orbital period can be used to determine its velocity. Combining this with the mass of the star can be used to estimate the companion’s mass.

As already noted with Sun-like star HD32963, it shows an orbital period of 6.5 years. The central star has a mass of 1.03 solar masses, which indicates that the companion must have orbital radius (semi-major axis) of 3.4AU, where 1AU is an astronomical unit, the average distance from the Sun to the Earth. This means that this planet orbits its parent star in between where Mars and Jupiter orbit the Sun. The amplitude of the radial velocity curve indicates that the mass of the companion is around 0.7 Jupiter masses.

Figure 4: This artist’s concept shows the relative sizes and separation of the star HD 32963 and its newly discovered Jupiter-mass planet.

Credit: Stefano Meschiari/McDonald Observatory

There is, however, one caveat. The radial velocity method only allows to determine the radial, or line-of-sight, velocity of the star. The actual inclination of the orbit is not known. If it is inclined relative to the line of sight, then the radial velocity measured will be smaller than the actual orbital velocity of the star. The mass estimated for the companion will therefore be smaller than its actual mass, and so, this estimate is typically taken to be a lower limit to the mass of the companion. However, it doesn’t depend very strongly on inclination. Hence, the estimated mass will often be reasonably close to the actual mass. One final thing that can be determined using radial velocity measurements is the eccentricity of the orbit. The eccentricity indicates how circular, or non-circular, the orbit is. An eccentricity close to 0 would indicate an almost circular orbit, while an eccentricity close to 1 would indicate an extremely eccentric orbit.

4 - The Transit Method

The radial velocity method can be used to determine a number of properties of a planetary system, in particular, orbital period, orbital radius, planetary mass and orbital eccentricity. Another method for detecting, and characterizing, exoplanets is the transit method. This involves observing a large number of stars, with the goal of measuring small dips in brightness caused by something passing between us and the star being observed.

Figure 5: Transit method of detecting extrasolar planets. The graph below, demonstrates the light levels received over time by Earth.

Image credit: By User:Nikola Smolenski - Inspired by image at http://www.iac.es/proyect/tep/transitmet.html, CC BY-SA 3.0, https://commons.wikimedia.org/w/index.php?curid=487277

Figure 6 By Поташев Роман Евгеньевич - Сам посчитал, CC BY 3.0, https://commons.wikimedia.org/w/index.php?curid=9063546

Figure 5 illustrates the basics of the transit method. Most stars have a reasonably constant brightness. Hence, if there is a small dip in brightness, this may indicate that something has passed between the star and us. If this dip repeats periodically, then it can indicate that something is in orbit around the star, potentially a planetary-mass companion. One might expect that it is pretty straightforward to detect planetary companions by using the transit method. It turns out, however, that there are various complications. As stated, a brown dwarf is an object that is more massive than something regarded as a planet but isn’t massive enough to become a star. Its radius, however, is very similar to Jupiter’s radius. Therefore, a transit of a brown dwarf can look similar to that of a massive planet. Similarly, a grazing eclipse by a stellar companion can block as much light as a full planetary transit. Very distant binary stars that happen to almost align with the star being observed can also produce apparent periodic dips of the target star.

However, it is possible to eliminate these false positives, and the transit method has become the most successful of the exoplanet detection methods, accounting for more than 2500 of the 4330 known exoplanets.

One final thing to consider here is that a planet will only transit if it happens to pass between us (the observer) and its parent star. The probability of a transit depends on the radius of the star and on the distance of the planet from the star - the further a planet is from a star, the less likely it is to transit. Detecting transits therefore requires observing many stars. The first phase of NASA’s Kepler mission, for example, observed more than 100,000 stars and detected just over 2500 planets.

This does not, however, mean that only 2500 of Kepler’s target stars host planets. Given that the transit probably decreases with increasing planet orbital radius, it is very unlikely that a planet with an orbital radius greater than about 1AU will be detected. Similarly, a large number of closer-in planets may simply not transit, because their orbit means that they will not pass between their parent star and us.

5 - Gravitational Microlensing

The final indirect detection method that will be briefly discuss here is based on Einstein’s Theory of General Relativity. Gravity is the force between two objects that depends on their masses and on the distance between them. The original formulation, presented by Isaac Newton, essentially suggested that this force acted instantaneously. This violates Einstein’s Theory of Special Relativity, which suggests that nothing can travel faster than the speed of light.

Einstein’s Theory of General Relativity proposed that objects with mass act to distort spacetime (which is really just the fusion of the three spatial dimensions with the time dimension) and that gravity is simply a manifestation of this curved spacetime. Essentially, massive objects in the universe will tend to fall toward other massive objects because of this curvature of spacetime.

Similarly, light will also be affected by this curvature of spacetime. Light follows a path that takes the least time and, as a consequence of the curvature of spacetime, will appear to be bent if it passes near an object that has mass. This can be used to detect planets around stars.

If a distant star happens to pass behind a nearer star, when viewed from the Earth, the nearer star can act like a lens, focusing some of the more distant star’s light onto the Earth. In principle, this will produce two images of the more distant (source) star. However, telescopes today do not have the resolution to actually observe this. What actually happens is that as the source star moves behind the lens star, more and more of its light will be focused onto the Earth and it will appear to get much brighter. As it passes out from behind the lens star, it will then get fainter and fainter, until it returns to its original brightness. This can take many days, potentially even a few months.

If however, the lens star happens to have a planet and this planet happens to be in the right place, it can provide an additional magnification that will last for a relatively short amount of time, typically a few hours to a day.

For example, the light-curve for the OGLE-2005-BLG-390 microlensing event, which occurred in 2005. The event lasted for about 50 days and was due to the light from the distant star being magnified by a closer star. In this case, the lens star does have a planet, which causes the additional magnification - lasting for approximately 1 day - about 10 days after July 31, 2005. This event was analyzed, and it indicated that the planet has a mass of about 5.5 Earth masses and an orbital radius of about 2.6AU (i.e., about 2.6 times further from its parent star than the Earth is from the Sun). What makes this an interesting method is that it is able to detect planets with masses similar to that of the Earth.

Figure 7: Gravitational microlensing.

Image Credit: By Gravitational_micro_rev.jpg: created by NASAderivative work: Malyszkz (talk) - Gravitational_micro_rev.jpg, Public Domain, https://commons.wikimedia.org/w/index.php?curid=15006820

However, because such alignments are very rare, it is typically search in the direction of the center of our galaxy (the Milky Way), because there are lots of stars in that direction and therefore such chance alignments become more likely. A consequence of this, however, is that such events will typically show a planetary signature only if the planet lies a few astronomical units from its host star (i.e., if the orbital radii is greater than that of the Earth around the Sun). Hence, this method can potentially find Earth-mass planets that are cool.

A downside of this method, though, is that such an event will at most happen once for a particular system, and these stars are faint, so there is not really any possibility of doing follow-up observations. So, the estimates for the planet’s mass and orbital radius can be gotten but cannot really characterize the system any further. However, they do provide an additional sample of exoplanets and also probe a region of parameter space (cool, low-mass planets) that cannot currently be probed by other methods.

6 - Direct Imaging

All the previous methods discussed earlier, talked on the indirect methods for detecting exoplanets. For this method, the direct method for detecting exoplanets will be discussed. An advantage of this is that rather than inferring the planets’ properties from their influence on their parent stars, they can be infer through direct observations. In particular, direct observations will help in understanding the properties of their atmospheres. As will be discuss later, we can also see some atmospheric properties by using spectroscopic observations of transiting exoplanets.

However, the direct observation is likely to play a key role in the characterization of exoplanet atmospheres. Directly imaging exoplanets involves surveying nearby young (less than 200 million years old) stars to look for faint companions. Typically, observations are taken at different times to establish if the star and its potential companion are moving together and also to establish if the companion appears to be orbiting the star.

Typically, when making observations with a telescope, the telescope is rotated so as to fix the orientation of what is being observed in the resulting image. However, when trying to directly detect planets, rotation is often turned off. If the star is placed in the center of the image, any other sources in the image will appear to move as the telescope tracks the target star. However, the noise, which comes from the structure, and optics, of the telescope itself, will remain fixed. The star, and this noise, can then be removed, enhancing any other real objects in the image. These can then be analyzed to see if they are planets in orbit around the target star. This process is known as angular differential imaging (ADI).

The HR8799 planetary system was one of the first to be directly detected. It was found to initially have three and then four planetary companions with masses of between 5 and 7 Jupiter masses. The closest orbits at about 14.5AU, while the furthest orbits at about 68AU. The central star has a mass about 1.5 times that of the Sun and is probably about 30 million years old.

Figure 8: This image shows the light from three planets orbiting a star 120 light-years away. The planets' star, called HR8799, is located at the spot marked with an "X." This picture was taken using a small, 1.5-meter (4.9-foot) portion of the Palomar Observatory's Hale Telescope, north of San Diego, Calif. This is the first time a picture of planets beyond our solar system has been captured using a telescope with a modest-sized mirror -- previous images were taken using larger telescopes. The three planets, called HR8799b, c and d, are thought to be gas giants like Jupiter, but more massive. They orbit their host star at roughly 24, 38 and 68 times the distance between our Earth and sun, respectively (Jupiter resides at about 5 times the Earth-sun distance).

Image credit: By NASA/JPL-Caltech/Palomar Observatory - http://www.nasa.gov/topics/universe/features/exoplanet20100414-a.html, Public Domain, https://commons.wikimedia.org/w/index.php?curid=10016784

This quite nicely illustrates the constraints associated with directly imaging extrasolar planets. Currently, it can only really detect relatively massive planets (masses above that of Jupiter) on wide orbits (orbital radii greater than about 10AU) and that are still quite young. This is because it is typically observed in the infrared, where thermal emission detected from the planet itself, rather than reflected light from the parent star. Planets with masses below that of Jupiter are simply not bright enough to be detected. Also, these planets tend to cool as they age and hence become fainter. Therefore, they can be only detected when they are relatively young (younger than a few hundred million years). Also, even though much of the bright central star can be remove from the resulting images, it is still difficult to find planetary companions that are closer than about 10AU.

The above discussions covered the techniques used to detects exoplanets, where human thought may find other organisms that have lived there. In most cases, exoplanets are detected via indirect methods (astrometry, radial velocity, transits, timing, and microlensing), but also, the Astrobiologists have now been able to use direct method of detecting exoplanet to directly image some planetary-mass objects. Each method, however, typically works in some circumstances but not in all methods.

Characteristics of Exoplanet

As discussed previously, the different detection methods provide different information about the planets that they detect. For example, both the transit and radial velocity methods can determine the planet’s orbital period (and, hence, orbital radius), but the former also determines the planet’s physical radius, while the latter can be used to estimate the planet’s mass. If you combine the two, you can then get an estimate for the planet’s density and, hence, internal composition.

There are now many exoplanets for which both the mass and radius are estimated. Initially, these were mostly planets that had masses and radii consistent with them being gas giants, like Jupiter or Saturn. In some cases, however, these turned out to have radii somewhat bigger than expected, with some having radii almost twice that of Jupiter. In the absence of any additional energy sources, gas giant planets more than a few billion years old should not have radii more than 1.2 times that of Jupiter.

However, most of these inflated gas giant exoplanets are very close to their parent stars and, hence, are heavily irradiated. Being so strongly irradiated might either reduce the rate at which these planets cool and, hence, shrink, or actually deposit energy in the planetary interior, inflating their radii.

With the advent of NASA’s Kepler mission, numerous planets with radii very similar to that of the Earth are now also known. In some cases, there have been able, using ground based radial velocity measurements, to estimate their masses. All of those with Earth-like internal compositions orbit very close to their parent stars and therefore will have surface temperatures much higher than that of the Earth. For example, K-78b (or, Kepler-78b) orbits a star similar to the Sun, has an Earth-like internal composition, but has an orbital period of only 8.5 hours. It therefore, probably has a surface temperature in excess of 2000K.

There are a number of super Earths that have substantial amounts of water. However, if water makes up more than 1% of the planet’s mass, then the pressure at the bottom of a water layer will be high enough to form high-pressure ice polymorphs, which means that there will be no direct contact between the liquid ocean and the planetary interior. This will probably have a substantial impact on the chemical composition of the liquid ocean, which will almost certainly impact the potential habitability of such planets.

There also appears to be an approximate gap in the exoplanet radius distribution. Planets tend to have radii below about 1.5 Earth radii, or above 2 Earth radii - there are currently very few with radii between 1.5 and 2 Earth radii. Those below 1.5 Earth radii tend to be rocky, while those above 2 Earth radii are either water-rich or have substantial volatile envelopes. If the habitability was assumed to requires a predominantly rocky planet, then that would suggest that such planets will preferentially have radii less than 1.5 Earth radii.

The Habitable Zone

As we have already read from the beginning of this article, the habitable zone is typically taken to be the region, around a star, in which a planet could have liquid water on its surface and, hence, could potentially support life. There is have to be slightly careful when using this, because being in the habitable zone doesn’t mean that a planet will support life, and being outside it does not necessarily mean that it cannot support life.

The exact region in which a planet could have liquid water on its surface depends on several factors, including the type of star, the atmospheric composition of the planet, and how much of the incoming stellar flux it reflects back into space.

NOTE: Flux is the total amount of energy intercepted by the detector divided by the area of the detector.

A simple estimate might suggest that it extends - around a Sun-like star - from about 0.5AU to about 2AU (i.e., from half the distance between the Sun and the Earth to twice the distance between the Sun and the Earth). However, this would imply that both Venus and Mars (in our solar system) lie within the habitable zone, yet neither is regarded as currently habitable. A more accurate estimate might suggest that it is somewhat narrower. However, if a planet is highly reflective, or has a weak greenhouse effect, it could sustain liquid water on its surface even when quite close to its parent star. Similarly, a planet with a low reflectivity and/or a strong greenhouse effect could sustain liquid water on its surface even if quite far from its parent star, in extreme cases potentially out to ~10AU around a Sun-like star. This would, however, require a hydrogen rich atmosphere. For a planet on which carbon dioxide drives the greenhouse effect, the outer edge is probably well within 2AU.

Determining Habitability

As already mentioned, simply finding rocky planets in the habitable zones of other stars does not mean that they are indeed habitable. This will require other observations to try and identify biosignatures. However, it is clear that detecting biosignatures will require characterizing the atmospheres of these potentially habitable exoplanets.

NOTE:

Biosignatures

Biosignatures are used to detect life, past or present, on Earth or elsewhere. The list of the life features that are used as biosignatures is long. It includes:

• Metabolic gases
• Spectral signatures of biology, such as the red edge of vegetation in earthlight, reflected from the Moon
• Biomolecules and/or their degradation products
• Homochiral organic molecules
• Isotopic fractionation of bioelements, such as C, S, N, and others
• Biominerals formed by microbial metabolism, such as carbonate and phosphate
• Morphological features produced by microorganisms and their activities
• Bacterial fossils,
• and many more.

It’s possible to however, already say something about exoplanet atmospheres. For close-in exoplanets, transit spectroscopy and secondary eclipse spectroscopy can be used. Both of these, look for changes in the stellar spectrum as a planet move in front of, and behind, its parent star. Such observations have indicated the presence of sodium, water vapor, and clouds in the atmospheres of hot Jupiter. It is also possible sometimes to extend this to lower-mass planets. There are observations suggesting the presence of clouds in the atmosphere of a super-Earth, and observations indicating that some of the planets in the TRAPPIST-1 system do not have cloud-free, hydrogen dominated atmospheres. This suggests that these planets are most likely rocky.

TRAPPIST-1 is a seven planet system orbiting a very-low-mass star and in which a number of the planets lie within the star’s habitable zone. With JWST, it may even be possible to use transit spectroscopy to detect ozone in the atmospheres of some of the TRAPPIST-1 planets.

As already mentioned, phase-curve variations can also be use to infer something about an exoplanet’s atmosphere. As a close-in planet orbits its host star, the side facing toward us will go from being the hot, irradiated dayside to the much colder night side. This can produce a detectable thermal phase variation. The form of this phase curve depends on the presence of an atmosphere and on its properties and so can be used to infer-or rule out-if a close-in exoplanet has an atmosphere.

The recently selected ESA’s Ariel mission will use transit, eclipse, and phase-curve spectroscopy to characterize the atmospheres of hundreds of exoplanets. This will include gas giants (Jupiter-like), Neptune-like, super-Earths, and Earth-sized planets around a range of host-star types. It will primarily focus on hot, or warm, exoplanets, with a particular goal of understanding how atmospheres form and how the chemical composition of these exoplanet atmospheres relates to the properties of the host star.

Since planets move much faster than their parent stars, their spectral lines will also be shifted with respect to those from the star. The Astrobiologists can therefore use high-resolution spectroscopy to try to identify the planet’s spectral lines and hence learn something about its atmospheric composition. Doing so for rocky planets may, however, require them to wait for the European Extremely Large Telescope (E-ELT), which will start operation next year, 2024.

Ultimately, however, they would probably like to directly image planets, so as to determine their spectra, and to infer something about their potential habitability. This, however, remains extremely challenging, and the ability to do so depends on both the contrast ratio between the planet and its parent star and their angular separation. Currently, they can directly observe young, giant planets on wide orbits, both because they are quite bright (planet-to-star contrast ratios of about 10−4) and because the angular separation between the planet and its host star can be quite large. There are already indications, for example, that clouds are present in the atmospheres of some of the directly imaged giant planets.

Imagers on 30-m class telescopes (due to beginning of the operating in the next decade) will probably be able to directly image young Jupiter analogues (i.e. Jupiter-mass planets on orbits a few astronomical units from their parent star) and, potentially, rocky planets in the habitable zones of very-low mass stars. However, to directly observe a true Earth analogue (Earth mass/size planet at about 1AU around a Sun-like star) may require large space telescopes that are currently only being planned, such as NASA’s Luvoir mission, which has yet to be formally selected and would only launch in the mid-2030s, at the earliest.

History of Astrobiology

Not long after NASA was established in 1958, the agency began a broad-based effort to learn how to look for the presence – both ancient and current – of life beyond Earth. Joining the agency’s human and robotic space programs with an offshoot of biology has not always been an easy or accepted fit, especially since no actual samples of life have ever been found elsewhere. But by now the two programs have become so interwoven, so interdependent, that each would be deeply damaged without the other.

Some of that initial pairing stemmed from unintended timing, the positioning of two historic advances. First came surprising discoveries and follow-on theories about how life organizes itself, and how it might have started on Earth. That was followed soon after by NASA’s Astrobiologists first successes in space travel, and the implicit promise of much more to come.

So, the scientist’s ability to reach into space came at a time when people were open, eager even, to learn more about the dynamics and origins of life on Earth - and possibly beyond.

Figure 9: The first humans to walk on another world - Neil Armstrong and Buzz Aldrin - flying the ascent stage of their Lunar Module back to the Moon-orbiting Command and Service Module. Apollo photographs of Earth, such as this one taken by Command Module pilot Michael Collins, helped launch the environmental movement and got us wondering about the habitability of other worlds.

Image Credit: Apollo 11 / NASA

The connection between space exploration and astrobiology (then called exobiology) was highlighted and given early legitimacy by molecular biologist-turned-exobiologist Joshua Lederberg. Even before NASA was formally established, he was reaching out to colleagues about the possibilities of finding life beyond Earth. He won the Nobel Prize (at age 33, for discoveries about the genetics of bacteria) the same year NASA was founded.

By 1960 he was writing in the journal Science that: “Exobiology is no more fantastic than the realization of space travel itself, and we have a grave responsibility to explore its implications for science and for human welfare with our best scientific insights and knowledge.”

While the 1960s were defined within NASA primarily by the efforts to land humans on the Moon, all during that period the agency was also supporting a robust effort to prepare for a mission to Mars. Its core goal: To search for signatures of life beyond Earth.

So, while hunting for present or past life on Mars was a very popular idea, it opened a Pandora’s box of extremely difficult questions about the still-mysterious nature and origins of life. Nonetheless, the possibility of actually finding extraterrestrial life reached a fever pitch of excitement during the Viking landing in 1976. Many predicted that life would be found on Mars – including Carl Sagan, who looked forward to encountering, via Viking, visible, perhaps floating creatures.

Figure 10: This picture was taken by the Viking Lander 1 on February 11, 1978 on Sol 556. The large rock just left of the center is about two meters wide. This rock was named "Big Joe" by the Viking scientists. The top of the rock is covered with red soil. Those portions of the rock not covered are similar in color to basaltic rocks on Earth. Therefore, this may be a fragment of a lava flow that was ejected by an impact crater. The part of the Lander that is visible in the lower left is the cover of the nuclear power supply.

Image credit: By Roel van der Hoorn Van der Hoorn

But those predictions gave way to first images of a bleak and barren martian landscape, and then to negative but also confusing scientific conclusions about whether signs of life, or even of organic compounds, had been detected.

The experience was sufficiently sobering that the study of Mars took an abrupt backseat, and it would be decades before interest recovered. And while orbiters, landers, and rovers returned to Mars in the 1990s and 2000s, it wasn’t until the 2012 landing of Curiosity that another astrobiology (though not life detection) mission began. Fortunately, a great deal had been learned in the intervening years.

For instance, previously unknown microbial communities were discovered on Earth that survive – thrive, even – in what were previously considered dead, uninhabitable environments. The first major “extremophile” discovery was made in the blackness of the deep ocean off the Galapagos Islands, alongside the hydrothermal vents that dot the seafloor. Not only were microbes and later tube worms found living in the total dark, but they were living in water made scaldingly hot by the vents.

That 1977 discovery led researchers to extreme environments around the world, where they found microbes living in bitter cold, in highly acidic and salty water, in the rock of goldmines dug miles underground, in the atmosphere high above ground, and in surroundings with high levels of radioactivity.

This explosion of often NASA-sponsored research told scientists a great deal about life on Earth, but it also quite clearly suggested that life can exist beyond Earth in conditions long deemed unsurvivable – such as the frozen-over oceans of Jupiter’s moon Europa.

Researchers have also found all the chemicals needed for life in space, and many of the key building blocks in meteorites and even comets. Amino acids, for instance, were found in samples of the comet Wild 2 after NASA’s Stardust spacecraft passed through the comet’s dusty coma in 2004, and nucleotides have been discovered by NASA scientists in meteorites. These results from the field of “astrochemistry” have told scientists that the ingredients presumed to be needed for life are actually falling on planets, moons, and asteroids everywhere.

Figure 11: Illustration of the Kepler Space Telescope on a transparent background.]

Image credit: By NASA

The water story on Mars has been especially promising, with the identification of deep river channels, valley systems, alluvial fans, and, more recently, lakes and suggestions of a once-grand northern ocean. The dwarf planet Ceres and Jupiter’s moon Ganymede now also appear to hold inner oceans, and the possibilities for finding more water worlds seem endless.

That’s because the past twenty years have witnessed a revolution in our understanding of exoplanets – bodies that orbit distant suns. Scientists have long suspected that other stars produce solar systems, but it wasn’t until 1995 that the first was detected. Since then thousands more have been identified, especially by NASA’s Kepler Space Telescope, but also through ground-based observations.

Headlines in 1996 told of a NASA research team, led by David McKay, that had found six indicators of past life in a meteorite from Mars. The famous ALH84001 meteorite, uncovered in the Allan Hills region of Antarctica in 1984, was presented as containing clear signs that microbial life once existed on Mars. There were even images of what was interpreted to be the fossil remains of a bacterium-like life form.

As with the Viking results, however, many in the Mars and astrobiology communities were not convinced. While the authors of both the Viking results and the Mars meteorite results stand by their work, the scientific consensus has largely rejected them — concluding that the findings could be explained without the presence of biology.

Nonetheless, the Mars meteorite and the excitement surrounding it gave a jumpstart to NASA’s renewed search for life beyond Earth. The NASA Astrobiology Institute was founded two years after the Mars meteorite paper was released, with Nobel laureate Baruch Blumberg as its director, and the organization has been funding wide-ranging research ever since.

Some of the work involves studying environments on Earth to better understand potentially similar ones beyond Earth (so-called “analogue environments”). Other work goes into technology development for use on other planets and moons, while other research explores the origins and early development of life on our planet.

The 2000s saw a renewed interest in exploring Mars with NASA orbiters, landers, and rovers. None had specifically astrobiological missions, but all contributed to better understanding pathways into the discipline’s goals. The Phoenix lander, for instance, found water ice in the north of Mars, ground-truthing the theory that Mars had substantial ice deposits just under its surface. The MER rovers, Opportunity and Spirit, detected carbonates and other minerals important to understanding the potential for biology in the martian past.

And then came Curiosity, which has had an explicitly astrobiological mission – to determine whether ancient Mars was habitable. The rover does not have the capacity to assess whether the planet was actually once inhabited by microbial life, but the results it has collected have convinced its science team that portions of the Gale Crater landing site were once perfectly capable of supporting life. It was the first formal identification of a habitable environment beyond Earth.

As is always the case with astrobiology, it was a combination of results — gathered by way of geology, geochemistry, minerology, sedimentology, super-high temperature chemistry and precision photography — that led to the conclusion. These findings support the theory that Mars was warmer and much wetter during its earliest days, even though climate modelers can’t figure out how an ancient Mars could have been warm enough, and had an atmosphere thick enough, to keep that water liquid for potentially tens of millions of years.

As technologies and scientific understandings have progressed, astrobiology has entered ever more fields. Moving beyond the astronomical detections of a cosmic menagerie of exoplanets, efforts are now underway to analyze the atmospheres, and ultimately the surfaces, of those bodies.

Carbon dioxide, water, and other compounds have already been detected in exoplanet atmospheres, but the ultimate goal is to find concentrations of oxygen, ozone and perhaps methane – gases which are associated with biology. Because oxygen and ozone quickly bond with other elements, the presence of large reservoirs of elemental oxygen, for instance, would tell scientists that it is constantly being produced. On Earth, the production of oxygen is largely a function of life.

Where there is liquid water on Earth, virtually no matter what the physical and chemical conditions are, there is life. There are only few exceptions. When in deep-sea hydrothermal vents, the extremely high temperature of the water is incompatible with the stability of proteins, nucleic acids, lipid membranes, and other biomolecules, or when the concentration of chaotropic ions in the absence of stabilizing kosmotropic ions is too high for stability of biological structures, life cannot be sustained, in spite of the presence of water in the liquid state.

Understanding of the possibilities and limitations of life as we know it on Earth and of the properties of extremophilic microorganisms of all kinds must be the basis for any search for similar life elsewhere in space.

There is now known a very large sample of exoplanets in orbit around stars other than the Sun. These range from planets larger than Jupiter to ones smaller than the Earth. The various detection methods allow the Astrobiologists to probe a wide range of parameter space, in both planet mass and orbital radius, but the one region that they are not yet able to probe is Earth-sized planets orbiting at about 1AU around a Sun-like star. This region will become accessible through future space-based exoplanet missions.

However, they now have a small sample of planets orbiting within their stars’ habitable zone, defined as being the region in which liquid water could exist on the planet’s surface. Most of these planets are, however, orbiting stars less massive than the Sun. The habitability of such planets is still unclear, but Astrobiologists will start uncovering atmospheric characteristics of such planets with the next generation of both ground-based and space-based instruments.

Guided by the mantra “follow the water,” NASA missions in our solar system have discovered a surprising variety of astrobiology targets. First came Jupiter’s moon Europa, with an ocean beneath its icy crust. On-going research suggests that the water is salty, a brine with apparent parallels to our oceans. And most recently plumes of that water may have been detected leaking from the moon – similar in some ways to those spurting out of Saturn’s moon Enceladus.

Earth-based research has been essential to astrobiology and has significantly changed our understanding of Earth and what might be possible on other worlds. But NASA and European robotic missions and space telescopes have most often been the engines that drive the field.

Determining if an exoplanet is habitable, or not, may well turn out to be even beyond the next generation of astrophysical instruments. However, we will almost certainly, in the near future, have a much better understanding of exoplanet atmospheres, what will be required in order to further improve this understanding, and what would be required to actually detect potential biosignatures.

With so many lines of research underway, NASA leaders are optimistic about finding life beyond Earth in the not too distant future.

REFERENCES

1. Astrobiology: Encyclopaedia Britannica - Encyclopaedia Britannica, Inc. Archived From https://www.britannica.com/science/astrobiology on 14 February 2019. Retrieved April 16, 2020.
2. Astrobiology: Astrobiology – Wikipedia. Retrieved from https://en.wikipedia.org/wiki/Astrobiology on April 15, 2020.
3. HISTORY OF ASTROBIOLOGY: ASTROBIOLOGY at NASA – LIFE IN THE UNIVERSE. Retrieved from https://astrobiology.nasa.gov/about/history-of-astrobiology/ on April 14, 2020.
4. Vera M. Kolb (June, 2018). Handbook of Astrobiology – ISBN: 13: 978-1-1380-6512-3