Life beyond Earth : Farewell to the habitable zone

For a long time the earth was regarded as a model for a life-friendly world. But this dogma crumbles: microorganisms could have developed in completely different places.

© janezvolmajer / stock.adobe.com

For astronomers it has long been a kind of dogma: life can only thrive on worlds that are not too close to their star and not too far. Only in this "habitable zone" can water flow, which is regarded as a prerequisite for life. If a planet is further away from its sun, it freezes to thick glaciers. If it is closer, any microorganisms pass away in prodigious heat. Only when a world remains billions of years in the habitable zone, life has a chance, so the assumption of the researchers was. Meanwhile, it is overhauled: the variety of thousands of newly discovered exoplanets and finds in our solar system tend to suggest that there are far more life-friendly niches in the universe than long thought.

The history of the habitable zone began with a very conservative view of life. In 1977, US astronomer Michael Hart carried out computer simulations that led him to a pessimistic conclusion: although it seemed to be around many stars a life-friendly area, but that was extremely narrow. Earth-like conditions could develop exclusively on planets in this narrow band, which corresponds approximately to the Earth's orbit in a sun-like star. Beyond that, death awaits, Hart suspects.

Mushrooms grow even in damaged nuclear reactors

When his work appeared, it was not much more than a thought experiment: until the discovery of the first exoplanet in 1995, only a few researchers believed that there are any worlds in the orbit of strange stars. Thus, for the time being, there were discoveries on the earth that allowed the narrow strip of habitable zones to grow. Biologists found microorganisms in many extreme places: in the deep sea in absolute darkness, in hot springs with up to 122 degrees Celsius, hundreds of meters under the glacial ice of Antarctica and kilometer deep in the earth's crust, where many microbes acquire chemical energy directly under immense pressure from the rock. Even in the damaged nuclear reactors of Chernobyl mushrooms sprout.

© NASA Ames/SETI Institute/JPL-Caltech
For a long time, astronomers have assumed that exoplanets like Kepler-186f must lie in the habitable zone of their star

An insight into geosciences helped to increase the habitable zone. According to this, the climate of the Earth continually fell into the margins of habitability throughout history, but never went beyond it. Once huge glaciers covered the entire surface, sometimes a supercontinent near the tropics caused a global dryness. But life survived: it lasted so long until the ice melts or drifting continents ended the drought.

A planet can regulate its climate

Scientists have shown that the Earth counteracts such changes through geological processes: when the dryness is low, carbon dioxide sinks from the atmosphere, causing the air temperature to drop. During cold epochs, on the other hand, our planet releases CO2 via volcanoes - and thawing the frozen earth by means of the greenhouse effect. For worlds with such a climate-regulating plate tectonics, the width of the habitable zones should also grow, the researchers realized. The geoscientist James Kasting made detailed calculations for this in 1993. According to this, the habitable zone in our solar system reaches as far as the Mars' orbit.

© NASA/JPL-Caltech
The planetary system of TRAPPIST-1

The fact that our red neighbor is today a life-hostile desert world is explained by researchers with their comparatively small size: Because Mars is less mass than the Earth it can never build up a global magnetic field. This is necessary in order to prevent the solar wind from blowing the air molecules little by little into space. Mars lost a great amount of its atmosphere three billion years ago. And without this protective cover, the air pressure sank so far that water either freezes or evaporates on the surface of Mars.

Microbes could travel via meteorite

Kastings habitable zone has long been regarded as the gold standard of exoplanet research. Several dozen of the nearly 3,500 exoplanets known today are orbiting their star in this region. Over time, however, scientists made discoveries that opened up new opportunities for life: for even if planets like Mars are life-friendly for only a few hundred million years, their lives are not necessarily lost.

Organisms would have to be thrown into space with a meteorite impact and land relatively quickly on another planet. Thus even Venus, which is now completely overheated, would no longer be a hopeless case in exobiology, but would be questioned as the cradle of life. In its youth phase, it might have possessed liquid water and offered homes to simple organisms which could have been thrown into space by a meteorite impact.

While microbes in our solar system are very difficult to travel from planet to planet because of the large distances, this is perhaps more often the case elsewhere: The recently discovered rock planes around the star TRAPPIST-1 are much closer together than the planets in our solar system. The inverted case would also be conceivable here: microorganisms could develop on a life-friendly planet, reach the neighboring habitats in the course of the eons at the edge of the habitable zone and persist in niches until the environmental conditions become better.

A warm atmosphere of hydrogen

The abundance of the exoplanets found to this day has also brought down another assumption of exobologists: a planet suitable for life does not have to circle near a star, but could also move its course at a great distance - where in our solar system large gas and ice giants are at home. A new class of planets, which does not occur in our solar system but is very frequent in the Milky Way, contributed to this realization.

These super-earths are up to ten times as heavy as the earth and have a solid surface under their atmosphere. The gas envelop of many of these worlds is likely to contain significant amounts of hydrogen left over during the formation of the mother's star. This would make super-earths fundamentally different from our home planet, which can not bind the very volatile gas for a long time. The Earth's primordial atmosphere instead consisted mainly of the gases nitrogen and carbon dioxide, which yield the geological processes inside the planet.

There is so much in our own solar system what we can learn

Athena Coustenis, Observatoire de Paris

For regulating the temperature, a hydrogen-containing atmosphere could have advantages because hydrogen is a much more potent greenhouse gas than CO2. Linda Elkins-Tanton and Sara Seager from the Massachusetts Institute of Technology calculated in 2008 that a super-earth, the atmosphere of which is partly enriched with hydrogen, would be warm enough for liquid oceans even away from the Jupiter track. With sufficient internal heat, even a lonely planet without a star could get its water - and thus carry organisms in the sea across the galaxy.

There could be life on icy moons

Not least, discoveries in our solar system have shattered the concept of the habitable zone. One of Saturn's smaller ice probe was the precedent for this: The interior of Enceladus would have to be deeply frozen by minus 190 degrees Celsius, about 1.4 billion million kilometers away from the sun. The Cassini space probe, however, had an average ice depth of 37 kilometers, which is almost ten times deeper than that of the earth. Probably because a solid nucleus in the water is actually thrown through the effects of Saturn, the ocean can remain fluid - and through cracks near the South Pole, even in thousands of kilometers high steam docks emerge.

© NASA/JPL/Space Science Institute
The moving surface of the Saturn moon Enceladus suggests that it has a violent geological past and perhaps a similar present.

Cassini found in the steam mixture methane, ethane, butane and pentane, biologically available nitrogen compounds and salts, all ingredients which are considered necessary for the development of life, even if they do not prove that Enceladus houses life. In the depths of the ocean microorganisms might feel comfortable, researchers conclude:

Enceladus has shown us a very favorable climate, very far from the sun, says Athena Coustenis from the Observatoire de Paris, which has been dealing with the Saturn's moons for two decades.

This is not yet an extraterrestrial life discovered. However, Enceladus is by no means the only place in the outer solar system, where biological activity seems conceivable. The ice masses of Jupiter are also very interesting for exobiologists with their oceans hidden deep under the ice. For Athena Coustenis it would therefore be a long time to realign the search for life-friendly worlds. For Enceladus, water samples could be taken and analyzed in the not too distant future, while such accurate measurements on worlds in the orbit of other stars would probably never be possible.

I find exoplanets very interesting, says Coustenis. But there is still so much in our own solar system what we can learn.

The habitable zone is obsolete

From the point of view of some researchers, the concept of the habitable zone has become superfluous at the latest with the discovery of Enceladus geysers. Finally, they suggest that the building blocks of life can come together almost everywhere: close to a hot venus-like orbit, in the cold regions of gas giants such as Saturn and on exotic superiors with dense hydrogen atmosphere. Until researchers have found first alien microbes, the most pessimistic conclusion is still possible: life on earth could owe its existence to an extremely rare - or even unique - coincidence. However, the more astronomers are more likely to encounter life-friendly conditions in still unfamiliar places, the more arguments speak against it.

Source: Coustenis, A., Encrenaz, Th., 2013. Life beyond Earth: the search for habitable worlds in the Universe. Cambridge Univ. Press (book). ISBN: 9781107026179.