杏吧原创

(Image: Logan Brinkley)

We thought that the search for life on other planets meant finding Earth鈥檚 twin. But now it looks like some bizarre worlds could still be fit for life

鈥淓PPUR si muove鈥� 鈥� 鈥淎nd yet it moves鈥�. Galileo鈥檚 reported utterance following his trial for heresy in 1633 is perhaps science鈥檚 most famous muttered riposte. Through the newly invented telescope Galileo had seen many things that couldn鈥檛 be explained by the dominant cosmology of the time, rooted in the idea that all things revolved around Earth: things like moons crossing the face of Jupiter, or the changing phases of Venus as sunlight caught it at different angles 鈥� an impossibility if Venus鈥檚 orbit encircled Earth. All this had made Galileo a champion of the Copernican revolution: the idea that all the planets, including Earth, orbit the sun.

Nearly four centuries on, we are on the cusp of another revolution in our cosmological understanding. From our vantage point on this orbiting world we have now spied almost 2000 others doing the same around far-flung stars. A flood of information from planet-hunters such as NASA鈥檚 , coupled with improved models of how planets and solar systems work, is forcing us to reconsider another set of geocentric views 鈥� this time about what a planet capable of harbouring life should look like. Increasingly it seems all our assumptions about Earth鈥檚 鈥渢win鈥� are wrong. As the search continues, we may need to bear in mind that it will not look anything like Earth at all.

We take many things for granted when assessing a planet鈥檚 suitability for life. First and foremost is the idea that, if biology elsewhere works anything like it does on Earth, life will be carbon-based 鈥� carbon chemistry has an unmatched complexity 鈥� and need liquid water as its essential solvent. That assumption leads directly to the concept of the habitable zone, first introduced in the 1950s. This is defined as the narrow region around a star where liquid water can exist. Too close to a star, and any water on a planet boils away; too far away, and it freezes. Only in the middling 鈥淕oldilocks zone鈥� 鈥� neither too hot nor too cold, but just right 鈥� can life thrive.

Our sun is well placed to host a planet that hosts life. Three-quarters of the stars in our Milky Way are fainter red dwarfs that pump out significantly less heat. The habitable zone of a red dwarf would be very close in, so close in that any planet orbiting within it would be 鈥渢idally locked鈥�: the gravitational grasp of the star would hold one side of the planet constantly facing towards it. That side would bake in perpetual daylight and searing temperatures, while the other would freeze in constant darkness 鈥� scarcely ideal conditions for the development of life. The closest thing we know in our solar system is Mercury. It is not quite tidally locked, but still experiences a temperature swing of 600 掳C between the day and night side.

As it is, Earth is sufficiently far away to rotate stably on its axis, with the sun鈥檚 heat distributed evenly to all sides of the planet. As Earth spins, it traces out a near perfectly circular orbit entirely within the Goldilocks zone 鈥� albeit, according to the latest researches, rather more precariously close to its sultry inner edge than we had thought (New 杏吧原创, 8 June 2013, p 40). Our unusually large moon is a further boon: its tug ensures that the tilt of Earth鈥檚 axis 鈥� its obliquity 鈥� changes very little. All those factors add up to an invitingly constant environment in which life has thrived.

It also meant that when the Kepler telescope launched in March 2009, it had a goal: to track down Earth鈥檚 doppelg盲nger. The telescope was named after the astronomer Johannes Kepler, who in 1619 first published the mathematical relationship between orbital distance and orbital time. That the mission was originally scheduled to run for three years is no coincidence. Earth takes a year to orbit the sun, so a similar planet in a similar position in a sunlike star鈥檚 habitable zone will orbit in the same time. Three years is enough to spot such a planet crossing the face of its star three times, confirming its existence beyond reasonable doubt.

That is not what Kepler has seen. It did see a planet in the habitable zone, Kepler 22b, as far back as 2011, and Kepler 61b and Kepler 62e joined the club in 2013. But they are not at all like Earth 鈥� all three are significantly larger 鈥渟uper-Earths鈥�. Most models suggest that the stronger gravity at these planets鈥� surfaces flattens them, making it easier for water to engulf their landscapes. Land warms up and cools down more readily than water, so Earth鈥檚 exposed continents play a pivotal part in regulating our climate. 鈥淥cean worlds may be more prone to climate instability and so be less habitable,鈥� says , an astrobiologist at the University of Leicester, UK.

Eccentric worlds

of the University of California, Berkeley, and his colleagues have also analysed changes in the host stars鈥� light caused by the gravitational tug of their orbiting planets. Around three-quarters of Kepler鈥檚 finds, although larger in size than Earth, are not massive enough to be watery, rocky worlds at all. Instead, they must be light planets rather like miniature Neptunes, their rocky cores surrounded by a puffed-up atmosphere containing large amounts of hydrogen and helium. 鈥淭his opens up a concern that the predominant type of planet may not be suitable for biology,鈥� says Marcy.

Rather than other Earths, Kepler has in fact turned up all kinds of strange and unusual things. The Kepler 47 system, for instance, has two stars and at least three orbiting planets. There are also many instances of highly 鈥渆ccentric鈥� planets, with orbits that deviate significantly from the circular orbits typical of our solar system. Instead, the planets behave like comets, plunging into the warmer inner zone from the frostier outer climes. Perhaps the most unexpected find is Kepler 11 鈥� a miniature version of our solar system, with five of its six planets huddling closer to their star than Mercury does to the sun. 鈥淚t seems anything is possible within the rules of physics,鈥� says planetary scientist of the Massachusetts Institute of Technology.

Anything except, on the face of it, a planet with those qualities we believe provide a safe haven for life. But that鈥檚 just where the facts are prodding us towards a rethink. 鈥淲e might have been making a similar mistake to the pre-Copernican era, of thinking that we鈥檙e special and that all habitable planets must share the same attributes as Earth,鈥� says Marcy.

鈥淲e are making the mistake of thinking all habitable planets must look like Earth鈥�

Oddly, some of the motivation for a change of heart comes from closer studies of life on our own planet. Over the past few years we have discovered organisms eking out an existence in Earth鈥檚 depths 鈥� most startlingly, nematode worms 3.6 kilometres down at the bottom of South Africa鈥檚 deepest gold mine (New 杏吧原创, 27 April 2013, p 36). Because only a fraction of possible subsurface habitats have been studied, Dartnell is one researcher reaching a conclusion alien to our conceptions even of a few years ago. 鈥淟ife in the deep biosphere on Earth grossly outnumbers the ecosystems that we鈥檙e familiar with on the surface,鈥� he says.

That has consequences for life elsewhere. 鈥淢any more planets can be considered habitable once the subsurface is taken into account,鈥� says from the University of Aberdeen, UK. In September last year, McMahon and two colleagues published a paper examining the potential for life in underground water kept liquid by heat from a planet鈥檚 core (). The deeper the water lies, the better shielded it is from the outside temperature, so the further the planet can be away from its star. Life 5 kilometres down could survive on a planet with an orbit at three times the radius of the traditional habitable zone. If life were sunk as low as 10 kilometres, the habitable zone would rocket out past the orbit of Saturn 鈥� 14 times the accepted distance around a sunlike star (see graphic).

Similar considerations mean we can鈥檛 rule out subsurface life in the outer reaches of our solar system. The tidal friction generated on Jupiter鈥檚 moon Europa as it orbits its massive host may have melted water beneath its crust, perhaps creating a subsurface ocean with the conditions necessary for life to get started.

This January, at a meeting of the American Astronomical Society in Washington DC, of Weber State University in Ogden, Utah, highlighted another scenario with the potential to shift the habitable zone. Earth鈥檚 moon-stabilised tilt might be a boon, but a changing tilt need not be a disadvantage: it just changes the rules of the game. 鈥淥bliquity variations can suppress the build-up of polar ice sheets, meaning less heat is reflected away into space,鈥� he says. According to his models, a wobbling planet might be able to retain liquid water at almost twice the distance from its star as an Earth-like non-wobbler.

Future missions might glimpse this changing obliquity by looking for variations in a planet鈥檚 brightness as light bounces off wobbling polar caps. Not that such a planet would necessarily harbour Earth-like life, however 鈥� at least not visibly. 鈥淚t鈥檚 unclear how complex ecosystems of multicellular animals and plants would cope with the wildly swinging climate of a wobbling world,鈥� says Dartnell. 鈥淏ut for bacteria below the surface it probably makes no difference at all.鈥�

It would be significant if habitable zones can extend out further from their stars, not least because fainter stars such as red dwarfs would be back in the game: life-bearing planets could orbit at distances large enough to keep them from being tidally locked. But even tidal locking might not be so much of problem, according to more advanced climate models developed over the past decade. They show that if a tidally locked planet were to have the same sort of nitrogen-rich atmosphere as Earth, heat could be efficiently transported to the perpetually dark night side, creating a more balanced, balmier climate.

Close to home

The potential for red-dwarf habitability was underscored last year by the first 3D atmospheric model of a tidally locked planet, created by of the University of Chicago and a couple of colleagues. It threw up a tantalising possibility: that clouds form more readily directly under the unmoving spotlight of the star鈥檚 glare ().

Clouds reflect more radiation back into space, meaning a cloudy planet can get even closer to its star while maintaining a reasonable temperature 鈥� extending the habitable zone inwards this time. 鈥淭his doubles the number of possible habitable planets around red dwarfs,鈥� Abbot says. Given the preponderance of red dwarfs in our galaxy, that is a huge boost to the number of potentially habitable planets, and for our chances of finding them (see 鈥�Kepler revived鈥�).

鈥淭he number of red dwarfs in our galaxy makes the findings a huge boost for life鈥�

In fact, habitable zones around some stars might be rather movable feasts. Modelling by , an astronomer at the University of California at Santa Cruz, indicates that liquid water, and hence life, might even survive on a planet on a highly elliptical, comet-like orbit. During its closest approach to a star, the heating rate on such a world would go up 20 times in a matter of hours 鈥� but the planet might 鈥渢ake the insult鈥�, says Laughlin. Although the equatorial temperature would reach that of an oven, heat would be dissipated quickly enough at higher latitudes for any water to stay liquid rather than boiling off. Meanwhile the extreme tidal forces inflicted by the star during close approach would inject gravitational energy into the planet鈥檚 core, providing it with a source of warmth even during its icy passage far from the star.

Whether such a world could host multicellular creatures is still debatable: again, it is hard to know how photosynthetic life would cope with such wildly changing amounts of sunlight. 鈥淚t would be very different from the Earth, but we need to keep an open mind,鈥� says Dartnell. At least such planets are easier to spot: they get much closer to their stars, making for a more noticeable dip in brightness as they transit across its face. 鈥淭he probability of detecting these worlds can go up by a factor of ten,鈥� says Laughlin.

One by one, then, all the 鈥渞ules鈥� we have established that say life-harbouring planets must look and act like Earth seem to be falling away. Armstrong goes a step further, and says unearthly planets might even be more conducive to life than our home. Along with at McMaster University in Hamilton, Ontario, Canada, he has recently set out the characteristics of 鈥渟uperhabitable鈥� worlds, concluding that a planet more massive than Earth orbiting a star smaller than the sun would boast advantages such as slower tectonic activity and reduced exposure to high-energy stellar radiation ().

That might be a blow to our earthly self-esteem. But just as centuries ago we discarded Earth as the centre of the universe, now might just be the time to dispense with our planet as the poster child for habitability. In the search for life elsewhere we need to keep an open mind, says Seager. 鈥淚f we limit ourselves to just Earth鈥檚 twin, then we are dead in the water.鈥�

This article appeared in print under the headline 鈥淚deal homes鈥�