Thousands of exoplanets have been discovered in the last two decades, uncovering a wide diversity of planets that are very different from those in our own solar system. Now the push is to smaller and smaller planets down to those of Earth-size that orbit in the so-called “habitable zone” of their host stars. Ideas for how to detect signs of life in the variety of planetary possibilities, by way of biosignature gases, are expanding, although they largely remain grounded in study of familiar gases produced by life on Earth and how they appear in Earth’s spectrum as viewed as an exoplanet. What are the chances we will be able to observe and identify biosignature gases on exoplanets in the coming two decades? I review the upcoming ground- and space-based telescopes and their opportunities in the search for habitable planets and biosignature gases.
1. Motivation
A longstanding goal in modern astronomy is to discover an exoplanet with suitable conditions to host life. That is, a planet with surface temperatures suitable for liquid water (needed for all life as we know it). An even more grand vision is to find an exoplanet with atmospheric gases that might be attributed to life, “biosignature gases”. New ground- and space-based telescopes coming online in the next decade or two will give astronomers the capability to realize the grand goal, but only if Nature has cooperated in the form of providing a multitude of nearby habitable worlds.
2. Upcoming Telescopes at the Frontiers
An organizational framework describing each of three categories of concepts to search for habitable exoplanets is shown in Figure 1, and described in the below subsections. The categories are: transiting exoplanet transmission spectroscopy; direct imaging from large ground-based telescopes now under construction; and sophisticated direct imaging space-based telescopes. Each category of techniques includes capabilities of observing exoplanet atmospheres, needed to estimate the planetary surface temperature and search for biosignature gases by way of spectral features.
2.1 Transiting Planets with the James Webb Space Telescope
Transiting exoplanets are incredibly valuable [1]. A transiting planet’s size and mass and hence average density can be measured or constrained. Furthermore, a transiting planet’s atmosphere can be measured, given a bright enough host star and suitable planet characteristics. These values are in contrast to many other planet-finding techniques where only a planet mass can be measured or estimated. Transiting exoplanets are those that pass in front of their parent star as seen from the telescope. Only a small fraction of planets have the required fortuitous alignment; stars’ rotation axes (and to some extent the planet orbits) are randomly oriented with respect to our line of sight.
The dominant transit survey mission operating today is the MIT-led NASA mission the Transiting Exoplanet Survey Satellite (TESS) [2], currently in its two-year prime mission (August 2018 through June 2020). TESS carries four identical, specialized wide-field CCD cameras, each with a 100-mm aperture and covering 24° × 24° on the sky. In a two-year, nearly all-sky survey, TESS will cover 400 times as much sky as NASA’s pioneering Kepler space telescope [3] did. In the process, TESS will examine millions of stars at 30-minute cadence and 200,000 stars at two-minute cadence to likely find thousands of exoplanets with orbital periods (i.e., years) up to about 50 days. TESS will not be able to detect true Earth analogs (that is, Earth-sized exoplanets in 365 day orbits about sun-like stars), but it will be capable of finding Earth-sized and super Earth-sized exoplanets (defined as planets up to 1.75 times Earth’s size) transiting M dwarf stars, stars that are significantly smaller, cooler, and more common than our Sun. The TESS search for habitable planets that may be followed up for signs of life is, therefore, focused on M dwarf host stars. (Several other ground- and space-based transit surveys are ongoing or planned for host stars of all types.)
TESS may be called a “finder telescope” for the James Webb Space Telescope (JWST [4]; launch scheduled for 2021). The JWST will be capable of observing small exoplanet atmospheres for planets transiting small M dwarf stars. As the planet transits the star, some of the starlight passes through the planet atmosphere – but not all of it. At some wavelengths the planet atmosphere absorbs starlight more than at other wavelengths. By carefully observing wavelength-by-wavelength, we can identify which gases and in some instances how much of them are present in the planet atmosphere. This technique is called transit transmission spectroscopy [5]. Most transiting planets also go behind their parent star, called a secondary eclipse. As the planet disappears behind the host star, the combined planet and starlight drops by a tiny amount related to the planets thermal emission which can be converted to an estimated top-of-the-atmosphere brightness temperature. Dozens of exoplanets have been observed in transmission and thermal emission [6], though most are limited to giant and or hot exoplanets, which are more favorable to observation with the Hubble Space Telescope and with some large ground-based telescopes.
The JWST will bring the capability to observe small rocky planet atmospheres via transmission spectroscopy and secondary eclipse spectroscopy, but challenges remain. With an optimal target, a large amount of observing time, and some luck – the JWST could detect habitable conditions on a rocky transiting exoplanet in the habitable zone of a nearby mid-to-late-type red dwarf star. The habitable zone is the zone around the star where a rocky planet with a thin atmosphere, heated by its star, may have liquid water on its surface. Liquid water is needed for all life as we know it, and detection of atmospheric water vapor as a proxy for surface liquid water on a small exoplanet is key.
Small planets transiting small red dwarf stars for this first category already exist: the Trappist 1 planets [7] and LHS 1132b [8].
Despite their extensive current popularity, transits are only the first part of a long story. Transiting planets must be fortuitously aligned, which eliminates most planetary systems. While atmospheres of small planets transiting small M dwarf stars are within reach, the same small planet transiting a larger sun-like star is out of reach.
2.2 Direct Imaging with Extremely Large Telescopes
Very large ground-based telescopes are now under construction and anticipating “first light” in the 2020s. The USA is pursuing two telescopes, the 20 m mirror diameter Giant Magellan Telescope [2] [9] in Chile, and the Thirty Meter Telescope [3] [10] on Mauna Kea in Hawaii. The European Southern Observatory is building the 40 m mirror diameter European Extremely Large Telescope [4] [11] in Chile. With the right instrumentation and extreme adaptive optics to correct for the blurring effects of Earth’s atmosphere, these telescopes will be able to study rocky planets around M dwarf stars by direct imaging.
Direct imaging is a planet-finding technique whereby the starlight is blocked out, enabling any orbiting planets to be detected. To date direct imaging has succeeded for self-luminous planets (that is very young and/or massive planets) orbiting far from the star (top right in Figure 2).
Planets orbiting in the habitable zone of a red dwarf star will shine in reflected light at the tiny level of about 10-7 to10-8 planet-star flux ratio, about thousands of times more challenging than current capabilities. Equally challenging will be the small projected separation from the host star, on order of 0.1 arcseconds or smaller. A planet in the habitable zone of a red dwarf star orbits much closer to the star than a planet in the habitable zone of a sun-like star because the red dwarf star gives off lower energy than a sun-like star. The ELTs will have capability on the middle, left side of Figure 2. A couple of hundred M dwarf stars are suitable for the ELT search for and atmospheric characterization of small rocky exoplanets.
Nearby red dwarf stars with a suitable exoplanets separation already exist: Proxima Centauri b [12] and Ross 128 b [13].
2.3 Direct Imaging with Space-Based Telescopes
The ultimate goal to many working in exoplanet astronomy is to discover a true Earth analog – that is an Earth-size planet in an Earth-like orbit about a sun-like star. While other planet finding techniques (namely radial velocity and transits) are pushing down to the sensitivities to uncover Earth analog, only direct imaging affords the possibility of atmosphere study. Atmospheres are critical to identifying a habitable world – Venus and Earth would appear the same to all planet-finding techniques that have no atmosphere study capability. Venus and Earth are about the same size and same mass, yet the Venusian surface is completely inhospitable to life because a massive greenhouse atmospheres makes the surface temperatures far too hot for life.
NASA recently concluded two studies on sophisticated space telescopes with starlight-blocking capability to operate in space above the blurring effects of Earth’s atmosphere. The challenge for direct imaging an Earth analog is tremendous, the planet-star flux ratio is one part in ten billion. Two decades of serious work of lab demonstrations have shown this astonishing starlight blocking capability is within reach. One study is called Large UV Optical IR Surveyor (LUVOIR) [5] [14] where UV means ultraviolet wavelengths, optical means visual wavelengths, and IR means infrared wavelengths. LUVOIR uses a coronagraph, an internal starlight-blocking device first invented in the 1920s by the French astronomer Lyot. LUVOIR would be huge, one version has an effective 8 m mirror diameter and a second version 16 m. LUVOIR would be the most ambitious space telescope ever built, with the ability to search a few hundred of sun-like stars.
The second concept studied by NASA is the Habitable Exoplanet Observatory (HabEx) [6] [15]. HabEx houses a coronagraph but also includes a starshade. A starshade has its own spacecraft and is a giant specially-shaped screen tens of meters in diameter that would fly in formation tens of thousands of km from a space telescope, blocking out starlight to the required 1 part in 10 billion so that only planet light enters the telescope. The starshade’s technology development is anchored in NASA’s Exoplanet Exploration Program’s effort called “S5” (Starshade to Technology Readiness Level 5), [7] an activity to address all critical starshade technologies and raise them to TRL 5 by 2023. The fiducial HabEx design has a 4 meter diameter “off axis” mirror and a 52 m diameter starshade. HabEx would closely survey about fifty of the nearest neighboring sun-like stars.
In addition to HabEx is a smaller starshade mission with the starshade at 26 m diameter, the Starshade Rendezvous Probe [8] [16]. The Starshade Rendezvous Probe would be built and launched on its own, then rendezvous on orbit with NASA’s planned WFIRST telescope (itself anticipated to launch in the mid 2020s).
3. The Frontiers of Biosignature Gas Research
The material in this section is adapted from [17].
3.1 Earth’s Biosignature Gases
Life on Earth has completely reengineered Earth’s atmosphere, specifically making it 20% oxygen (O2) by volume. Without plants and photosyntheric bacteria, Earth would have almost no O2, about orders of magnitude less than present today [18]. O2 is so reactive it should not be present in the atmosphere in any significant quantity unless continually generated. It is therefore not surprising that O2 has been considered a biosignature gas for nearly a century [19]. If we observe O2 in a small rocky exoplanet atmosphere spectrum, we will want to attribute it to life, with the caveat of false positives in the form of exotic geological or atmospheric processes [20]. We should keep in mind, however, that while O2 may be unique in being both abundant and non-geological, life on other worlds may not make O2.
Life on Earth produces not only oxygen but literally hundreds of thousands of other chemicals (see review in [21]), estimated from plant natural products, microbial natural products, and marine natural products. A subset of hundreds are both in gas form and present in Earth’s atmosphere in at least trace concentrations. A further subset of tens of gases are present in the atmosphere in high enough concentrations to consider as being remotely detectable for astronomical purposes. A further subset of gases are spectroscopically active enough to be considered as potential biosignature gases on an Earth-like exoplanet.
Out of the promising numbers and variety of gases produced by life on Earth, there appears to be a conundrum when considering their potential as biosignature gases for exoplanets [22]. On Earth, the most abundant biosignature gases (e.g. CH4) can also be produced from abiological sources, and will therefore have false positives in an exoplanet context. These are gases produced when life exploits a chemical potential energy gradient that is geochemically stable. In more detail, two materials (such as hydrogen and carbon dioxide) are produced by different geochemical processes and come together in one place. Their reaction is thermodynamically favored but kinetically inhibited – their reaction cannot happen at ambient temperatures and pressures. Life catalyzes the reaction. Gas products of such reactions include CH4, N2O, and H2S. Such gases are abundant because they are created from chemicals that are plentiful in the environment. However, not only does geology have the same molecules to work with as life does, but in one environment where a given redox reaction will be kinetically inhibited and thus will proceed only when activated by life’s enzymes, in another environment with the right conditions (temperature, pressure, concentration, and acidity) the same reaction might proceed spontaneously. Small common molecules produced as biosignature gases will therefore be fraught with false positives.
In contrast to the category of simple gases produced by life exploiting chemical potential energy gradients, a second category of biosignature gases are larger more complex molecules that appear to be unique to life. In other words, they are unlikely to have false positives. For example, dimethylsulfide released by oceanic plankton. These unique-to-life gases are present only in tiny quantities that in most imaginable exoplanet scenarios may be too low for the gas to accumulate to detectable levels. This category of biosignature gases appear to be special to particular species or groups of organisms, and require energy for their production). They are produced for organism-specific reasons and are highly specialized chemicals not directly tied to the local chemical environment and thermodynamics. Some biosignature gases in this category of energy-requiring specialized byproduct gases include methanethiol CH3SH [23], methyl chloride CH3Cl [24], and organic sulfur compounds (CS2, OCS, CH3SH, CH3SCH3, and CH3S2CH3 [25]). The thought is that some of these gases, under the right conditions of excess production or favorable UV flux conditions, might accumulate to hypothetically detectable levels.
3.2 Thousands of Biosignature Gas Possibilities
My research team aims for as general a view as possible – so as to explore all possibilities to increase our chances of identifying signs of life on planets beyond Earth (Figure 3). We have curated a detailed database of all volatile molecules, not just those that are produced by life on Earth, but all of those that are both stable and volatile at a wide range of habitable atmospheric temperatures and pressures [21].
The appeal of the “all small molecule” approach is that it is independent of terrestrial biochemistry. The only assumption is that life beyond Earth uses chemistry and metabolism to store energy and outputs metabolic byproduct gases. After all, astronomers will only be able to see the byproducts of metabolism and not any life forms themselves.
To proceed, we are taking molecules, or classes of molecules based on functional groups, and assessing first how likely they are to accumulate in an exoplanet atmosphere of a specified composition, and second whether or not they are spectroscopically detectable by a remote space telescope (Figure 3). The first point depends on the UV radiation environment from the host star that drives photochemistry, the atmospheric composition and atmospheric mass, and the surface chemistry (including sources and sinks). In other words, we take classes of molecules and aim to determine if they are stable and can accumulate in different planetary environments by integration into existing computer models of planetary chemistry and photochemistry. The spectroscopic detectability relies on molecular line lists, which for many molecules do not exist yet. We have therefore developed a complimentary approach to quantum mechanical model generated precise line lists, by estimated functional group spectral features [26]. In the assessment we must also consider false positives, that is how a gas could be produced by non-biogenic processes, such as in volcanoes or via various chemical reactions in the atmosphere. We could, in some cases be highly confident that a gas might be produced by life, and in others less so, perhaps even assigning a probability after detailed atmosphere calculations.
4. Outlook
Astronomers are exuberant about the extensive progress in exoplanetary science and in the promise of the future for finding and characterizing other Earths. The new telescopes coming online have the capability of detecting biosignature gases, if they exist on a planet favorable for observation around a nearby star.
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