Earth-like planets undoubtedly orbit Trappist-1, the star in the background here – and there might even be water on them. | Illustration: Eso / M. Kornmesser

In July 1995, the telescope of the Observatoire de Haute Provence in south-east France was pointing at the constellation Pegasus in outer space. A tiny anomaly appeared in the data it was collecting from a sun-like star we today call Helvetios – a minimal, periodic shift in the light it was emitting. This is how the first-ever exoplanet was discovered. The two men observing the heavens from this plateau, 100 km north of Marseille, were the Swiss researchers Didier Queloz and Michel Mayor. “It was a sensation at the time”, says Sascha Quanz of ETH Zurich. And it brought the two men a Nobel Prize.

The planet was later christened ‘Dimidium’. And it once more brought to the fore the age-old question: Does life exist somewhere else out there? The list of candidates for a ‘second Earth’ is growing longer every week right now. Researchers have discovered more than 5,600 exoplanets thus far, including strange worlds whose surface is completely covered in water, or that consist mostly of diamonds. A good 50 of them are located in the so-called ‘habitable zone’ where life as we know it is theoretically possible. Our own Solar System is our blueprint for this search, even though other types of world are conceivable. “Over the past 30 years, we’ve learnt that there’s an incredible variety of planets and star systems far beyond anything we could have imagined. And yet our search for life remains in its infancy”, says Quanz. We know practically nothing about most exoplanets except for their radius, and have more details only for about 100 of them.

Two stars in focus

Researchers have identified five criteria that are necessary for life: liquid water, an energy source, the chemical building blocks of life, an atmosphere that can protect life against intense radiation, and environmental conditions stable enough for life to have sufficient time to develop. Here on Earth, it took less than a billion years for the first life forms to emerge. Our only source of information on traces of life in other star systems is the light from them that reaches us. If molecules are present in distant atmospheres – such as oxygen, ozone, methane, certain carbon compounds and water vapour – they will absorb specific wavelengths of the light emitted by their parent star, and thereby enable us to identify their spectral fingerprint.

“Ultimately, it depends on how well our instruments can break down their signals”, says Christoph Mordasini, the head of Space Research & Planetary Sciences at the University of Bern.

Until now, researchers have often only been able to determine the radius of an exoplanet when it passes directly in front of its star. But if it’s possible to measure its orbital speed at the same time, then we can also determine its mass. And once we know an exoplanet’s mass and radius, we can calculate its average density – this enables researchers to categorise planets as either gas planets, Neptunian planets (also known as ‘ice giants’) or Earth-like planets. This is important in our search for life, because not all types of planets can hold water on their surface.

“For a long time, we were only able to study the gas giants”, says Sascha Quanz. “But now the Earth-like exoplanets are gradually coming into focus”. Two stars are getting particular attention at the moment: Trappist-1 and Proxima Centauri, which Quanz calls “golden targets”. Trappist-1 is located 40 light years away from us, and with its seven known planets it holds the record as the biggest solar system that we know about, outside our own. These seven include three rocky, Earth-like planets in the habitable zone. But the closest exoplanet to Earth is actually Proxima Centauri b, which is just over four light years away from us.

Christophe Lovis is a professor at the University of Geneva who specialises in the spectroscopy of exoplanets. He uses his earth-bound instruments to evaluate the light from a star after it’s travelled through the atmospheres of its planets. “We have to filter out the tiny signal of a planet from that of its parent star that is often millions of times brighter”. Thus far, this has only been possible for smaller, fainter stars such as Trappist-1 and Proxima Centauri. But it’s also only possible when planets pass directly in front of their star. If Lovis wanted to do the same for Earth-like planets passing in front of big, bright stars, he’d need more powerful telescopes and spectrographs.

But researchers have to be patient. It often takes years for the right measuring instruments and the necessary telescopes to be built. Lovis is developing his spectrographs for a large-scale project that is currently under construction here on Earth: the Extremely Large Telescope of the European Southern Observatory (ESO) in the Atacama Desert in Chile, whose mirror will have a diameter of almost 40 metres. That’s some four times the diameter of the most powerful telescope currently in use. This new telescope ought to be able to find trace gases in the atmosphere of Earth-like, extrasolar planets.

The smell of the sea in a telescope

There was a stir recently when the James Webb Space Telescope detected faint traces of dimethyl sulphide on an exotic planet called K2-18b. However, how these data are being interpreted is very controversial. This substance could come from plankton in an ocean. “That would make it like the smell of the sea”, says Mordasini. “But it’s difficult to interpret the spectroscopic signals of life properly”. For example, here on Earth, oxygen is a clear indication of life. But an oxygen signal could also come from a water-rich exoplanet that has no life, but is exposed to harsh UV radiation that could split the water into hydrogen and oxygen. At low gravity, the lighter hydrogen would escape into space, leaving behind the heavier oxygen.

The boom in research into the origins of life ought to provide pointers towards new target molecules that could have played a role on Earth. Much interdisciplinary research is needed for this, involving all manner of fields from astrobiology to geochemistry to physics. It’s also important to simulate alien worlds – life, after all, might exist there in forms very different from those on Earth. Recent studies have shown that it would also be conceivable on a planet whose atmosphere comprises hydrogen and helium. These gases would be as dense as an impenetrable fog, and since such an atmosphere would be highly insulating, the heat from the interior of the planet would suffice to keep water in its liquid state. “Such a state of affairs could last for many billions of years”, says Mordasini. That would be enough time for life to develop there.

Is there life on Earth?

Mordasini’s colleague Quanz is also researching into the origins of life – a branch of research that is currently being driven forward by Queloz. The plan is for their findings to be incorporated into the planned exoplanet mission ‘LIFE’ – the Large Interferometer for Exoplanets. This is an ambitious international initiative, led by ETH Zurich, which aims to search for habitable terrestrial exoplanets from space. Five satellites will be launched that will together form a large telescope to capture the thermal emissions of exoplanets; they are due to be stationed where the James Webb Space Telescope is currently functioning.

Quanz is using Venus, Earth and Mars as three model planets that the new telescope ought to be able to recognise reliably, and distinguish from each other, from a distance of several light years away. In the case of Earth, he is also using variants from different epochs. “Two billion years ago, the Earth looked very different”, he says. “And if we find a young Earth out there, we also have to be able to recognise it. After all, we don’t know what phase any planet might be in”. Doctoral students in his research group recently tested the LIFE mission’s proposed methods using atmospheric data gathered from Earth. They are essentially honing a famous idea that originated in the teeming mind of the astrophysicist Carl Sagan. Thirty years ago, the Galileo probe passed Earth on its way to Jupiter, and when Quanz’s team analysed the measurements it made of Earth, these conformed to what would be expected from a planet where life was widespread. “That was a clever experiment”, says Quanz. Today’s technological possibilities are more advanced – which is necessary because we’re going to be observing exoplanets that are light years away from us. “It’s like getting a planet as a pixel in which all the averaged signs of life and geology have been compressed”.

When he’s asked whether we’ll ever be able to fly to a ‘second Earth’ if we find one, Quanz just laughs. “For me, that’s a philosophical question. Twenty years from now, LIFE will have analysed between 30 and 50 Earth-like planets that exist in habitable regions. If there are no global, atmospheric signatures of life on any of these planets, then we’ll know that planets like Earth are very rare indeed. Such a realisation would change our perspective in a way last achieved by Copernicus”.