BREAKING

dimanche 30 mars 2014

Ideal homes

EPPUR si muove” – “And yet it moves”. Galileo’s reported utterance following his trial for heresy in 1633 is perhaps science’s most famous muttered riposte. Through the newly invented telescope Galileo had seen many things that couldn’t 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 – an impossibility if Venus’s 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 planethunters such as NASA’s Kepler space telescope, coupled with improved models of how planets and solar systems work, is forcing us to reconsider another set of geocentric views – this time about what a planet capable of harbouring life should look like. Increasingly it seems all our assumptions about Earth’s “twin” 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’s 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 – carbon chemistry has an unmatched complexity – 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 “Goldilocks zone” – neither too hot nor too cold, but just right – 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 “tidally locked”: 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 – scarcely ideal conditions for the development of life. The closest

No place like home Our ideas of where life can thrive depend on the idea of a “habitable zone” where liquid water can exist on an Earth-like planet. But that might not be the whole story Life at depth A rocky planet at Saturn’s distance from the sun would be way outside such a star’s habitable zone – but life might still exist in a subsurface ocean warmed by heat from the planet’s core WATER CORE HABITABLE ZONE KEPLER REVIVED When NASA’s Kepler space telescope was launched in 2009, it was equipped with four “reaction wheels”, flywheels that rotated the craft and helped point the telescope accurately enough at stars to detect evidence of distant planets. Three reaction wheels are needed for stable positioning, so when first one reaction wheel failed, followed by a second early last year, it seemed to mark the end of the mission. But an ingenious idea has prompted Kepler’s Lazarus-like revival. The gentle nudging of radiation pressure from sunlight can act like a third wheel and stabilise the telescope. That allows Kepler to look for planets in one patch of sky for around 80 days at a time. After that time, Earth’s passage around the sun means it has to be repositioned again to avoid being blinded by the sun’s glare. That’s not enough time to spot Earth-like planets crossing the face of their stars if their orbits take a year. But if habitable planets really can hug in close to red dwarf stars (see main story), it would be more than enough to see them completing several orbits. The proposed pared-down mission, dubbed K2, has already proved it has what it takes, by independently spotting a planet already seen by another telescope. Wobbly worlds The tilt of Earth’s axis is stabilised by our large moon – but a moonless, wobbling planet would develop no ice caps, and retain liquid-water temperatures beyond the habitable zone MOON EARTH ICE CAPS thing we know in our solar system is Mercury. It is not quite tidally locked, but still experiences a temperature swing of 600 °C between the day and night side. As it is, Earth is sufficiently far away to rotate stably on its axis, with the sun’s 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 – albeit, according to the latest researches, rather more precariously close to its sultry inner edge than we had thought (New Scientist, 8 June 2013, p 40). Our unusually large moon is a further boon: its tug ensures that the tilt of Earth’s axis – its obliquity – 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’s doppelgänger. 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’s 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 – all three are significantly larger “super-Earths”. Most models suggest that the stronger gravity at these planets’ surfaces flattens them, making it easier for water to engulf their landscapes. Land warms up and cools down more readily than water, so Earth’s exposed continents play a pivotal part in regulating our climate. “Ocean worlds may be more prone to climate instability and so be less habitable,” says Lewis Dartnell, an astrobiologist at the University of Leicester, UK. Eccentric worlds Geoffrey Marcy of the University of California, Berkeley, and his colleagues have also analysed changes in the host stars’ light caused by the gravitational tug of their orbiting planets. Around three-quarters of Kepler’s 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. “This opens up a concern that the predominant type of planet may not be suitable for biology,” 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 “eccentric” planets, with orbits that deviate significantly from the circular orbits typical of Central heating Temperature swings on planets with elliptical orbits were thought to make them unsuitable for life – but when close to their star, gravitational distortions can store additional energy within them, warming them for the outer reaches 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 – a miniature version of our solar system, with five of its six planets huddling closer to their star than Mercury does to the sun. “It seems anything is possible within the rules of physics,” says planetary scientist Sara Seager 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’s just where the facts are prodding us towards a rethink. “We might have been making a similar mistake to the preCopernican era, of thinking that we’re special and that all habitable planets must share the same attributes as Earth,” says Marcy. 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’s depths – most startlingly, nematode worms 3.6 kilometres down at the bottom of South Africa’s deepest gold mine (New Scientist, 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. “Life in the deep biosphere on Earth grossly outnumbers the ecosystems that we’re familiar with on the surface,” he says. That has consequences for life elsewhere. “Many more planets can be considered habitable once the subsurface is taken into Homely dwarfs A planet orbiting a faint red dwarf at liquid-water distance would have one face gravitationally “locked in” to the star. But cloud formation in an Earth-like atmosphere can help redistribute heat, perhaps making a habitable planet HALF-SCORCHED PLANET account,” says Sean McMahon 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’s core (Planetary and Space Science, vol 85, p 312). 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 “ We are making the mistake of thinking all habitable planets must look like Earth” CLOUD ATMOSPHERE RED DWARF zone. If life were sunk as low as 10 kilometres, the habitable zone would rocket out past the orbit of Saturn – 14 times the accepted distance around a sunlike star (see graphic). Similar considerations mean we can’t rule out subsurface life in the outer reaches of our solar system. The tidal friction generated on Jupiter’s 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,  John Armstrong of Weber State University in Ogden, Utah, highlighted another scenario with the potential to shift the habitable zone. Earth’s moon-stabilised tilt might be a boon, but a changing tilt need not be a disadvantage: it just changes the rules of the game. “Obliquity variations can suppress the build-up of polar ice sheets, meaning less  heat is reflected away into space,” 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’s brightness as light bounces off wobbling polar caps. Not that such a planet would necessarily harbour Earth-like life, however – at least not visibly. “It’s unclear  how complex ecosystems of multicellular animals and plants would cope with the  wildly swinging climate of a wobbling world,” says Dartnell. “But for bacteria below the
surface it probably makes no difference at all.” 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 Dorian Abbot 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’s glare (Astrophysical Journal Letters, vol 771, p L45). Clouds reflect more radiation back into space, meaning a cloudy planet can get even closer to its star while maintaining a reasonable temperature – extending the habitable zone inwards this time. “This doubles the number of possible habitable planets around red dwarfs,” 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 “ The number of red dwarfs in our galaxy makes the findings a huge boost for life” them (see “Kepler revived”, page 40). In fact, habitable zones around some stars might be rather movable feasts. Modelling by Greg Laughlin, 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 – but the planet might “take the insult”, 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’s 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. “It would be very different from the Earth, but we need to keep an open mind,” 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. “The probability of detecting these worlds can go up by a factor of ten,” says Laughlin. One by one, then, all the “rules” 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 René Heller at McMaster University in Hamilton, Ontario, Canada, he has recently  set out the characteristics of  “superhabitable” 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 (Astrobiology, vol 14, p 50). That might be a blow to our earthly selfesteem. 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. “If we limit ourselves to just Earth’s twin, then we are dead in the water.”  ■

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