Briefing | Astrobiology

The search for ET hots up

If life exists beyond Earth, science may find it soon

TEEGARDEN’S STAR is a tiny, dim object in the zodiacal constellation of Aries. It has a tenth of the sun’s mass and emits most of its light in the infrared part of the spectrum. That makes it too faint to see with the naked eye, even though it is only 12 light-years away. So far, so unremarkable. But when astronomers at Calar Alto Observatory, in Spain, started scrutinising it, they spotted tiny wobbles in its motion. In 2019, after three years of careful measurement, they concluded that these are a consequence of the gravitational fields of two planets tugging the star around. The innermost, Teegarden b, has roughly the same mass as Earth, receives a similar amount of illumination from its host star and is probably rocky.

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At the moment, Teegarden b tops the Habitable Exoplanets Catalogue (HEC), a list of planets beyond the solar system maintained by Abel Méndez and his colleagues at the University of Puerto Rico, Arecibo. They are looking for Earthlike worlds which, among other things, lie in the zones around stars that might support terrestrial-type life. This means, in practice, planets that are the correct distance from their parent stars to be able to maintain liquid water on their surfaces without that water either freezing or boiling. For water, the medium in which biochemical reactions take place on Earth, is assumed to be a precondition for the existence of life elsewhere, too.

Warm bodies

The existence of the HEC, compilation of which began in 2011, is one manifestation of a renewed surge of interest in answering, once and for all, the question of whether life is a phenomenon that exists only on Earth, or is widespread. Until recently, that question was unanswerable. But astrobiologists, as the diverse band of scientists now trying to do so describe themselves, are increasingly confident that an answer will be found within a couple of decades.

There are, broadly, three ways of doing this. One is to look from a distance, using telescopes to examine systems such as that orbiting Teegarden’s star. The second, if the object of interest is close enough, is to visit it, as is happening with the arrival this month of Mars-bound craft launched by America, China and the United Arab Emirates. And the third is to search for radio signals or other signs of technology, on the assumption that at least some life elsewhere has followed the trajectory of life on Earth and generated technically adept species.

The first two exoplanets were discovered in 1992. Now, more than 4,000 are known. It seems likely that every one of the billions of stars in the Milky Way, Earth’s home galaxy, is the centre of a system that includes one or more planets, and also that lots of these planets orbit in habitable zones. Because they are big, and therefore easy to spot, a lot of the planets discovered so far are gas giants larger even than Jupiter. These seem unlikely places for life to establish itself. But an increasing number of bodies close in size to Earth are turning up, too (see chart).

A study published recently in the Astronomical Journal suggests that about half of the sun-like stars in the Milky Way are circled by at least one rocky planet capable of sustaining liquid water on its surface. This amounts to 300m potentially habitable worlds. That calculation implies the presence of at least four such favoured orbs within 30 light-years of the solar system, with the closest at most 20 light-years away. There is, then, plenty to study.

Just because a planet is a rocky world orbiting within its star’s habitable zone does not, though, automatically make it a good candidate. Some such, for instance, are known to be “water worlds”, with deep oceans and no continents. These might support life if it arrived from elsewhere, but the chances that biology could start from scratch in such a place seem slim. How life begins is unknown. But it is a fair bet that it needs the concentration of certain chemicals in a way that is difficult to achieve in the swirling volume of an ocean.

Similarly, just because an object could have liquid water on its surface in a habitable zone does not mean such water actually exists. Earth’s moon, for example, is within the habitable zone of the sun and yet is almost waterless.

An important task, then, is to understand not only where a planet is in its star system, but what it is made of—whether it has an all-covering ocean, an ocean interspersed with continents, a covering of ice, or is all dry land. Also, whether it has an atmosphere. And that will be done by a new generation of instruments.

Chart toppers

The first of these, the James Webb Space Telescope, is the creation of NASA, America’s space agency. It has a mirror 6.5 metres across, which gives it a light-collecting area more than six times greater than that of the Hubble Space Telescope, currently in orbit, which has a mirror a mere 2.4 metres in diameter. The plan is to launch it into orbit this October and to use it for detailed spectrographic observations of exoplanetary atmospheres. It will hunt, in particular, for molecules such as oxygen and methane that are produced by biological processes on Earth.

Teegarden b is a prime candidate for such study, but is by no means the only one. Potentially habitable planets within telescope range are turning up thick and fast. In 2020 alone, four new Earth-size exoplanets entered the HEC top ten.

In January of that year, for example, astronomers using another NASA instrument, the Transiting Exoplanet Survey Satellite (TESS), announced the discovery of that probe’s first entry into the HEC. This planet, TOI-700d, is one of three orbiting a star called TOI-700, which has 40% of the mass of the sun and is 100 light-years away. TOI-700d is 20% bigger than Earth, completes an orbit every 37 days and receives 86% of the energy from its star that Earth gets from the sun. It currently sits in second place on the HEC list. And in March two further top-tenners were added by astronomers using the European Southern Observatory’s instruments at La Silla, in Chile. They were orbiting in the habitable zone of a star called GJ 1061, which, like Teegarden’s, is 12 light-years from Earth.

The fourth of 2020’s additions to the HEC top ten was announced in April. It was the result of reanalysis of measurements taken by Kepler, a now-defunct NASA space telescope (it operated from 2009 to 2018) that, in its heyday, found more than 2,000 new planets. This re-examination uncovered a hitherto-overlooked body, dubbed Kepler-1649c, which is almost the same size as Earth, takes 19.5 days to orbit its star, and receives 75% of the light that Earth gets from the sun. Kepler-1649c jumped straight into the charts at number five.

Other top-tenners have been on the list for longer. One of particular interest is Proxima Centauri b (number six). It weighs in at 1.3 times Earth’s mass and has an 11-day orbit around its star, Proxima Centauri, which, at a mere 4.2 light-years away, is the sun’s closest stellar neighbour. But the most striking star to supply HEC candidates is TRAPPIST-1. This is 41 light-years from Earth and is orbited by seven potentially habitable planets of similar density to Earth, one of which, TRAPPIST-1d, is in the HEC top ten, at number four.

At its most recent update, the HEC listed 60 worlds, some two dozen of which are thought to be of rocky composition and similar in size to Earth. The rest are so-called “super Earths”, which have masses larger than Earth’s but less than those of the solar system’s ice giants, Uranus and Neptune. There are doubts about the habitability of super Earths, since they probably have thick atmospheres and may even be composed almost entirely of gas. For the moment, though, it seems sensible to keep them under scrutiny.

Because of the limits of current observing technology, which finds it easier to see signs of planets when they circle dimmer stars, almost all these promising exoplanets orbit what are known as M-type stars—or, colloquially, red dwarfs. Red dwarfs are smaller and dimmer than F-, G- and K-types, known as orange and yellow dwarfs (the sun is G-type). They are by far the most common stars in the Milky Way (some estimates suggest they make up three-quarters of the total), so the easy success in finding so many potentially habitable worlds circling them suggests that these sorts of planets are abundant.

Atmospheric conditions

Astrobiologists are already making simple follow-up measurements of some planets. In 2015 a super Earth called K2-18b turned up. It is 124 light-years away, has nine times Earth’s mass and orbits its M-type star once every 33 days. In 2019 the Hubble Space Telescope looked at starlight streaming through this planet’s atmosphere. Spectroscopic analysis indicated the presence there of a fair amount of water vapour—a first for an exoplanet in a habitable zone.

That was a useful start, says Giovanna Tinetti, an astrophysicist at University College, London who led the study. But, as she observes, “current instrumentation is just not good enough really to go beyond saying, ‘Oh, there is some water vapour in the atmosphere’.” That leaves plenty of questions. What type of object is K2-18b? Is it a world covered by an ocean, or perhaps a thick layer of ice like the icy moons of Jupiter and Saturn? Is the atmosphere pure water vapour or mostly hydrogen with a dash of water and perhaps some other elements?

Future observations with the James Webb telescope will fill in some of these gaps. However, Dr Tinetti highlights the need for catalogues of the properties of exoplanetary atmospheres, against which astronomers can compare their latest finds. To this end she is working on ARIEL, a mission planned by the European Space Agency (ESA) to characterise the properties of a set of around 1,000 diverse exoplanets.

Once a potentially habitable planet is found, life on Earth provides clues about how to detect life on it. For example, as James Lovelock, a British chemist, proposed in 1965, the presence of gases in chemical disequilibrium with their surroundings could be a sign of life. Oxygen is a reactive gas that would not build up in Earth’s atmosphere in normal conditions. As a by-product of photosynthesis, however, it is being replenished continuously. Given the presence of so much oxygen, the simultaneous persistence of methane in Earth’s atmosphere is also inexplicable without lifeforms that keep producing the gas. Normal abiotic chemistry would otherwise quickly deplete it.

Other gases likely to be biosignatures include nitrous oxide, methyl chloride, isoprene, ammonia and phosphine. Indeed, Sara Seager, an astrobiologist and astrophysicist at the Massachusetts Institute of Technology, has identified more than 14,000 small, volatile molecules, of which a quarter are produced by life and others potentially so. This greatly increases the number of potential quarry for future astrobiologists to hunt. Meanwhile, laboratory experiments and computer models that can characterise the sources and life cycles of these gases in different types of atmospheres will help the understanding of future data collected about exoplanets.

The edge of reason

Other biosignatures might come from a planet’s surface. On Earth, a phenomenon called the red edge is a sign of oxygenic photosynthesis. Chlorophyll, the plant pigment that captures the light which provides the energy for photosynthesis, absorbs most visible frequencies emitted by the sun but reflects the longer wavelengths of infrared light. This sharp change in reflectance can be spotted easily from space.

With due acknowledgment that photosynthetic life on other planets would almost certainly employ other pigments, tuned to absorb the electromagnetic frequencies emitted from their parent stars in the way that chlorophyll is tuned to sunlight, this method could be adopted to look for “plants” elsewhere. Near an M-type star, for example, some astrobiologists’ models suggest that planetary vegetation tuned to local conditions could reflect yet longer wavelengths than those reflected by Earth. Alternatively, a planet might be dominated by light-harvesting organisms similar to Earth’s purple bacteria, which thrive in anoxic conditions and produce sulphur as the waste product of their photosynthesis, rather than oxygen. Yet other pigments, each with its own spectral signature, might have jobs beyond photosynthesis, such as protection against harsh radiation.

None of this study will be easy, particularly when the molecules under investigation are dozens or hundreds of light-years away. The James Webb telescope will begin by looking for biosignatures in the atmospheres of planets around M-type stars, but may struggle to do the same for those orbiting brighter G-types. Examining the surfaces of planets, understanding atmospheric dynamics, looking for continents and detecting surface biosignatures will have to wait until direct-imaging technology is sensitive enough to reach across the light-years and record something useful.

That might happen by the mid 2030s, when three ground-based telescopes with mirrors 25-40 metres wide, which should start operating later this decade, get into full swing. These are the Giant Magellan Telescope, in Chile, the Extremely Large Telescope, also in Chile, and the Thirty Metre Telescope, proposed for Hawaii. Their observations may be complemented by images taken by two proposed NASA spacecraft, LUVOIR and HabEx. If approved, these could fly in the late 2030s. LUVOIR would be a general-purpose successor to the James Webb. HabEx would be designed specifically to take pictures of habitable planets.

Finding one particular chemical on another planet will never be a clear cut indicator of life. Volcanoes also produce some of the molecules associated with biology, so the risk of false positives is high. Even oxygen is not foolproof. It can be generated abiotically when water molecules are split into their constituents by high-energy radiation from a parent star. Conversely, a planet with life on it may not yet have detectable levels of oxygen in its atmosphere (which was indeed the case with Earth for much of its early history). For astronomers, this means placing potential detections of biomolecules into the wider context of the planet under study.

Building catalogues of non-biological sources of gases will help to fine-tune models and give astrobiologists a better chance of weeding out false positives. But Charles Cockell, an astrobiologist at Edinburgh University, says a more robust approach would be to collect spectrographic data from lots of exoplanets and thereby create better statistical confidence in individual detections. If astronomers had atmospheric data from tens of thousands of them, for example, and a thousand showed strong signals for oxygen, that would build confidence about the oxygen being from biological sources, rather than the results simply being false positives.

This, though, is to assume the process of detection itself is robust. An instructive tale here is the recent debate over whether or not there is phosphine in the atmosphere of Venus—for this is a gas which, on Earth, is created only by living organisms (some of them, admittedly, human chemists). In September, a group of astronomers announced that Venusian air contained 20 parts per billion of phosphine. Others who subsequently scrutinised these results raised red flags. Some questioned the way the original team had processed the data. Some tried to find evidence for phosphine in independent data sets, and failed. Partly in response to those criticisms, the original team later reanalysed the data themselves, and concluded there was, after all, only a tentative detection of one part per billion of phosphine present on Venus.

Venus sky trap

Spectroscopic analysis of Venus’s atmosphere in this way can be viewed as a test-bed for the harder task of doing the same to the atmospheres of exoplanets. In the case of Venus, though, if the signs do end up looking good, it will be possible to go and check directly—the second of the broad approaches to astrobiology. That the Venusian atmosphere may have a biomarker in it came as a surprise even to those optimistic about finding life elsewhere. Most such eyes are turned to Mars, with a side-bet on the icy moons of Jupiter and Saturn.

Mars has already generated a couple of intriguing results. One was from the so-called labelled-release experiments carried on board Vikings 1 and 2, which landed successfully on Mars in 1976. These mixed samples of Martian regolith with organic compounds of the sort consumed by microbes on Earth. Those compounds were labelled with radioactive carbon atoms, and any gas evolving from the experimental mixture was tested for radioactivity, to see if those compounds were being metabolised. The results suggested metabolic activity in the regolith at both landing sites, which were over 6,000km apart. They were discounted at the time as being the product of some unknown abiotic reaction, because other experiments showed no sign of organic compounds in the regolith. But they have never been explained.

The other result that has not quite gone away was the analysis in 1996 of a meteorite blown off the surface of Mars by an asteroid impact and collected in Antarctica. This contained several features, including putative microfossils, unusual grains of magnetite of a type made biologically on Earth, and organic compounds, that were taken at the time as possible signs of life. All are now known to have alternative, and more widely believed, explanations. But, at the time, the meteorite’s discovery stimulated interest in the possibility of life on Mars.

Wild rovers

And there has, indeed, been an intensive programme of investigation of the place since the 1990s, using orbiters, landers and wheeled surface vehicles known as rovers. This has followed up the discovery by the Viking missions of what looked like water-carved topography, by showing that parts of Mars are covered with sedimentary rocks, and that these include clay minerals, which, on Earth, often form in the presence of water. This demonstrates, to most people’s satisfaction, that the planet did once play host to bodies of liquid water.

NASA’s next Mars rover, Perseverance, scheduled to arrive there on February 18th, will look in some of the rocks it encounters for fossilised remnants of microbial mats called stromatolites. It will also, by collecting and bottling for future retrieval the most interesting samples it finds, be the first step in a decade-long multi-agency mission to bring samples of Martian rock back to Earth. Then, in 2023, Perseverance will be followed by a rover from ESA called Rosalind Franklin. This will drill a few metres under the Martian surface to search for microbes, both fossilised and living.

The idea of life on Mars goes back at least as far as 1877, when Giovanni Schiaparelli, an Italian astronomer, thought he saw dark lines on the planet, which he interpreted as water-carrying channels with vegetation growing along their banks. These turned out to be optical illusions, but the idea stuck. That there might be life on some of the moons of Jupiter and Saturn is a more recent suggestion, stimulated by the discovery by probes sent to orbit those planets that some of their ice-covered moons appear to have subsurface oceans of liquid water.

This thought is taken seriously enough for NASA to be laying plans for missions to Europa, a moon of Jupiter, and Enceladus, a moon of Saturn. The idea is to sample water in the plumes of geysers erupting from these moons (see picture), and test it for chemicals that might indicate life. NASA also plans to launch a drone called Dragonfly on Titan, another moon of Saturn, to search its surface for molecules that might be life’s precursors.

The geysers of Enceladus

If it turns out that somewhere else in the solar system either has, or had, life, one further question needs to be answered. This is, is it the same as life on Earth? One school of thought, known as panspermia, suggests that life might not have evolved in situ everywhere that it is found, but could instead spread from place to place. This would be easier within systems of planets than between them. If organisms live on other bodies in the solar system it should be possible to work out from their biochemistry whether they share an ancestor with those on Earth—and it might even be possible to do this with fossils, if enough of their chemical structure is preserved.

Either answer to the panspermia question would be interesting. If any life on Venus, Mars or the Jovian or Saturnian moons had separate origins from life on Earth, it would suggest that biology starts up easily. If it started on only one of them, and then spread from place to place within the solar system, that would give a boost to the idea of it spreading between star systems, too, since two examples are now known of rocks from elsewhere entering and then departing from the solar system, either of which might have carried bacteria-like organisms deep within it, in cold storage.

The third approach to astrobiology, besides looking for biosignatures and visiting promising planets and moons, is to scan the cosmos for signs of technology elsewhere. This is, perhaps, the most flaky tactic. But it is also the most far reaching. It is flaky because the history of life on Earth, at least, suggests that the path from primordial soup to engineering prowess is a long and winding one. It is far reaching because such prowess might manifest itself in ways such as radio signals that can be detected at greater distances than any natural sign of life.

People have been listening for radio signals from ET almost since the invention of radio telescopes. There have been a couple of false alarms, but nothing definite. The use of lasers for communication on Earth has raised suggestions of looking for signals from these, too—or even taking the initiative and beaming messages by laser towards promising nearby planetary systems. And real optimists wonder whether super-advanced civilisations might engage in engineering projects large enough to leave a footprint in the spectrum of their home stars. Again, there have been false alarms provoked by this thought. But nothing concrete.

Hope springs eternal

For the moment, then, sheer weight of numbers suggests that the most likely place to find evidence of alien life remains the Habitable Exoplanets Catalogue or some future, similar, compilation. Numbers increase the chance of a lucky strike. They also allows the application of statistics to the problem, for it is likely that, rather than looking for yes/no answers to the question of life elsewhere, researchers will have to search for probabilities. This could mean measuring the levels of several gases, each with a different likelihood of having come from a biological source, and combining those data with an understanding of environmental context. The hope would be that life-bearing anomalies would stick out like sore thumbs, in the manner of Dr Cockell’s putative oxygen-rich planets.

As to whether astrobiologists actually believe life exists elsewhere, that is not exactly a scientific question, but it is pertinent to their motives. David Grinspoon, a veteran of the field who is part of the Planetary Sciences Institute, an American research organisation, puts it thus: “I think there’s widespread belief in extraterrestrial life now among scientists, even though we don’t have specific evidence for it.”

Dr Grinspoon observes that astrobiology is where exoplanetology was when he was a student in the 1980s—patiently waiting for the right tools to become available. Exoplanets made the transition from belief to reality with aplomb. Whether ET will follow suit remains to be seen.

Dig Deeper

The search for life elsewhere in the universe is heating up and may even yield an answer soon. In addition to our coverage, learn about Avi Loeb, the alien hunter of Harvard, in our sister publication, 1843, and read our review of his book, Extraterrestrial.

This article appeared in the Briefing section of the print edition under the headline "Come out, come out, wherever you are!"

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