On Valentine’s Day 1990, as the Voyager 1 space probe pushed towards the outer edge of our solar system, its cameras looked back to where it had come from to snap a series of photographs. Taken at a distance of about four billion miles from Earth, the images form the first and only “family portrait” of six of the eight planets arrayed around their parent star. In one, the Earth appears as a pinprick of light just one 12th of a pixel in size, a “mote of dust suspended in a sunbeam”, as the astronomer Carl Sagan observed.

This image is still the furthest-away photograph ever taken of our “blue dot”, underscoring its beauty and fragility, imploring us to take care of the only place we know that harbours life: our home. It is infinitely precious — and yet, in the three decades since Voyager’s backward glance, we have learnt that our neighbourhood scatter of worlds is far from unique.

It was in the early 1990s that scientists started to discover planets orbiting stars other than our sun. Among the thousands of other solar systems we have catalogued since then, we have identified more than 4,000 “exoplanets” — in fact, it seems that just about every star in our galaxy has at least one planet. With approximately 200 billion stars in our galaxy, that makes 200 billion planets in our galaxy alone — at least. And our galaxy is just one among billions of galaxies.

The planets orbiting other stars range from huge gas balls such as Jupiter, covered with raging storms, to extremely hot worlds that have oceans of boiling magma, to small rocky worlds that may be like our own.

Earth, described by astronomer Carl Sagan as a ‘mote of dust suspended in a sunbeam’, photographed by the Voyager 1 space probe at a distance of more than four billion miles in February 1990
Earth, described by astronomer Carl Sagan as a ‘mote of dust suspended in a sunbeam’, photographed by the Voyager 1 space probe at a distance of more than four billion miles in February 1990 © Wikicommons

We can’t find all the small planets yet, because they leave only a tiny signal for astronomers to spot, requiring long observation runs on large telescopes and dedicated space missions such as Nasa’s Transiting Exoplanet Survey Satellite. But the data we have accumulated suggests that at least every fifth star harbours a rocky planet at a distance from its star where liquid water could exist on the surface.

This distance, where it is not too hot and not too cold for liquid water, is called the “habitable” or “Goldilocks” zone. Astronomers are interested in worlds that orbit within that zone because liquid water is essential for life as we know it. It was exciting to be part of the team that, a few weeks ago, announced the discovery of one such planet, GJ 357d, and led the work on characterising it. The star it orbits, which lies in the constellation of Hydra, is 31 light years away — relatively close, given our galaxy’s 100,000-light-year diameter.

The challenge now is to refine our observations to the point where we can answer the question that fuels many people’s fascination with these worlds: do any of them actually harbour life?

The distances in the cosmos are vast and our telescopes are limited in what they can discover. Without the means (yet) to travel, we use other ways to explore the universe. In particular, we try to wring as much information as we can from light, which travels the cosmos at about 671,000,000 miles per hour, or just under 5,900 billion miles a year — the distance we call one light year.

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The light that reaches our telescopes from distant stars carries the imprint of the chemicals it has encountered en route. If it has reflected off the surface and/or atmosphere of the planets that comprise their solar systems, we can learn about the composition of those worlds. And just as every person’s fingerprint is slightly different, it seems so far that every planet and moon has its own unique fingerprint, encoded in the reflected light.

At the Carl Sagan Institute at Cornell University, a team of scientists and engineers is building a forensic toolkit to find life beyond Earth, both inside our solar system and around other stars. One of these tools is a “light fingerprint catalogue” for planets and moons in our own solar system, against which we can compare exoplanets as a first step in identifying those most likely to host life.

From left to right, Titan, Saturn and Earth, with their respective light fingerprints
From left to right, Titan, Saturn and Earth, with their respective light fingerprints © Cornell University

In essence, we measure the intensity of light at each wavelength — its spectrum from blue to infrared — and analyse it to see which chemicals are present on each world. The light that’s reflected from Earth, for example, shows water, oxygen and methane in its spectral fingerprint. From a distance — from the vantage point of Voyager 1 when it captured that blue dot, say — we could tell that the Earth was not only within its sun’s habitable zone but also that it did indeed have water; the additional presence of oxygen and methane in combination, meanwhile, would be strong evidence of life. Venus’s spectrum, on the other hand, lacks these signs, indicating a world devoid of life.

The challenge is to detect these fingerprints from greater distances — from the 31 light years that separate us from GJ 357d, for example. At present, our instruments are able to obtain them only from a few hot, big planets; the signal from small, rocky worlds is swamped by the glare of the parent star. (We detect the existence of these planets indirectly, by their subtle effects on the movement and apparent brightness of their respective stars.) But the next generation of telescopes will be able to distinguish finer details: the James Webb Space Telescope, to be launched in 2021, and the Extremely Large Telescope in Chile, which will come online in 2025, will be the first instruments that can collect light from the Earth-scale worlds that loom so large in our imaginations.

Even with billions of planets out there, finding a carbon-copy of the Earth seems unlikely. Such a planet would have to have undergone the same steps in its evolution to get to a similar state to our “now”. And though an exoplanet could still be habitable if it was a bit hotter, drier or colder, we’d expect life to have evolved and adapted for that specific environment, just as it has for Earth’s niches.

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Lisa Kaltenegger: 'When I look up at the night sky and gaze at all those distant suns, I often wonder if someone is also looking up from an exoplanet at the same time'
Lisa Kaltenegger: ‘When I look up at the night sky and gaze at all those distant suns, I often wonder if someone is also looking up from an exoplanet at the same time’

When I started in this field, I assumed we knew what conditions are needed for life to start — but we don’t. Earth’s geological record gives us a mere glimpse of what the planet was like when life emerged more than 3.5 billion years ago. We don’t know which of our planet’s grand variety of environments encouraged chemicals to coalesce into self-perpetuating life forms. What is needed to provide that spark? Is it shallow water, in some inland lake, where high levels of ultraviolet radiation from the sun enable more energetic chemical reactions, or is it in the deep ocean, where no light penetrates but volcanic vents provide a rich source of heat and nutrients?

An open question is what broader conditions need to obtain for life to start — whether, in other words, it needs to evolve as it has on Earth at all. It seems likely that the building blocks will be the same, since hydrogen, carbon, oxygen and nitrogen, which combine to form the amino acids essential to terrestrial life, are among the most abundant elements in the universe.

Carbon’s ability to form — and re-form — long and complex molecules makes it the most likely backbone for life anywhere. Water may be less of a prerequisite: if a planet or moon is too cold for liquid water, life may be able to rely on different solvents to mediate its processes. Saturn’s frigid moon Titan has lakes of ethane and methane that would fit the bill; in June, Nasa announced it would launch a probe in 2026 to take a closer look.

Meanwhile, my team is using our home planet as a Rosetta Stone, a key to interpreting the light fingerprints of different exoplanets. Earth’s own fingerprint has changed over time; the emergence of life, for one thing, has produced oxygen in large amounts, which we see in its spectrum. But oxygen alone is not conclusive evidence of life. There are geological reactions that slowly produce oxygen molecules. If nothing reacts with that oxygen, then eventually — after four and a half ­billion years, say, roughly the age of the Earth — there will be a large concentration of it in the atmosphere, but it won’t mean there is life.

Detecting it would certainly be interesting, though — and finding it in combination with a gas such as methane would be very interesting indeed. That is because oxygen reacts with methane to form carbon dioxide and water; the two gases don’t hang around long before they react with each other. So if we detected oxygen in an atmosphere in the presence of methane, as on Earth, it would mean the oxygen was being produced in large quantities — perhaps by something living.

Life may also influence the colour of a planet as seen from space. We have created a “colour catalogue” of life on Earth as part of our forensic toolkit (available at carlsaganinstitute.org); it includes 137 different kinds of organism, such as lichen and cyanobacteria, taken from a wide range of environments. Extreme conditions sometimes give rise to vivid colours: for example, some of the beautiful hues seen around the hot sulphur springs in Yellowstone National Park are due to the microscopic life that thrives there.

These various pigments provide a point of comparison with planets that could have different dominant life forms from ours — a yellow, green or orange “dot” to contrast with our blue version. One intriguing possibility is biofluorescence, in which some types of coral become luminous as protection against ultraviolet light from the sun. Might we be able to detect such a glow from planets whose parent stars are prone to UV outbursts? (Such stars include the sun’s nearest neighbour, Proxima Centauri, which has a planet in its habitable zone.)

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Our search also has the potential to give us insights into our own world. We know how our sun was born and will die because we observe stars like it at different stages in their evolution. Similarly, if we could find dozens of Earth-like planets at various evolutionary stages, we could piece together more precisely how Earth has formed and glimpse a possible future. For example, if we were to see that all older Earths show high concentrations of sulphur dioxide — which we can’t breathe — in their atmospheres, it might be smart to develop a technology that could filter it out of our own atmosphere, just in case. Understanding other planets could allow us to take better care of our own — though scientific knowledge does not necessarily translate in to political will.

When I look up at the night sky and gaze at all those distant suns, I often wonder whether someone is also looking up from an exoplanet at the same time and likewise wondering whether they are alone in the universe. On Earth, for the first time in history, we are in a position to start to answer this question. Even with at least 40 billion possible other Earths in our galaxy, the search will be extremely challenging. As so often in astronomy, we are at the very limits of what our technology can see. But I like our chances.

Lisa Kaltenegger is director of the Carl Sagan Institute and associate professor in astronomy at Cornell University

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Via Financial Times