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1. RESEARCH NEWS & VIEWS AST RONO MY a period of hundreds of days. The transits ofA new class of planet such a body will not only be infrequent but will cause the light from the star to drop by a puny 0.01%. Such a signal is much too mea- gre to pick up with ground-based telescopes, which suffer from the blurring effect of Earth’sThree examples of a new family of planets, which orbit a pair of stars rather than atmosphere, as well as inevitable interruptionsa single one, have been discovered. The Milky Way may contain millions of these due to daylight and bad weather.circumbinary planets. See Letter p .475 Finding a habitable planet requires a larger — and much more expensive — tele- scope outside Earth’s atmosphere. Enter theJOHN SOUTHWORTH blocked during transit. This means that their Kepler spacecraft, the primary aim of which is surface gravities and mean densities can be to use the transit method to discover Earth-likeA lthough more than 700 extrasolar calculated, ultimately allowing the investiga- planets. It monitors 150,000 stars in the con- planets have been detected, none was tion of their internal structure and formation stellations Cygnus and Lyra, and has already known to orbit more than one star until process. found more than 2,000 candidate transitingthe recent discovery1 of a circumbinary planet, The transit method has led to the discovery planets. Three of these have been confirmed towhich orbits a pair of stars. This concept was of more than 200 planets, predominantly by be circumbinary planets: Kepler-34 b (Fig. 1)previously confined to theory — and to sci- teams that operate small wide-field robotic and Kepler-35 b, which Welsh et al. describe inence fiction, for example the planet Tatooine survey telescopes at observatories spread their study2, and Kepler-16 b (ref. 1). Not onlyin Star Wars. On page 475 of this issue, Welsh around the world, such as HATNet4 and does each of these three planets transit both ofet al.2 describe the discovery of two more such SuperWASP5. These surveys are heavily biased its parent stars, but the stars themselves eclipseplanets and provide insight into their fre- towards large planets with small orbits. As a each other.quency of occurrence. The previously discov- result, they are unparalleled sources of odd- Although the discovery1 of Kepler-16 b 1ered planet and the new ones, each of which balls such as WASP-17 (ref. 6), the biggest and revealed that it was possible for such an objectorbits its own system of two stars, were found most rarefied planet known (up to twice the to exist, Welsh and colleagues’ identificationusing NASA’s Kepler space telescope*. radius of Jupiter and only 6% as dense), and of two more circumbinary planets not only For hundreds of years, scientists assumed WASP-18 (ref. 7), which is ten times the mass shows that such a planet is no freak object,that the Solar System is a typical example of a of Jupiter and whirls around its host star every but also allows an estimate of their prevalenceplanetary system. That assumption was chal- 23 hours (Jupiter’s orbital period is 11.9 years). to be made. The authors2 find that, for short-lenged in 1995 by the discovery3 of 51 Pegasi b, The overriding aim of planetary research is period binary star systems, the frequencythe first planet to orbit a normal star other than to find one that might support life. Habitability of occurrence of circumbinary planets is atthe Sun. Although this planet is probably a gas most probably requires a rocky surface with least 1%. Taking into account the fractiongiant (the lower limit on its mass is 0.47 Jupiter liquid water, which, in turn, demands a planet of stars that are short-period binaries, thismasses), it orbits at only 0.052 astronomi- no bigger than two Earth radii on an orbit with result implies that there are millions of suchcal units (au) from its star (1 au is the planets distributed throughout theaverage distance between Earth and Galaxy. This analysis does not accountthe Sun). This means that 51 Pegasi b is for longer-period binary star systems,100 times closer to its star than Jupiter which are similarly plentiful in theis to the Sun. Galaxy. Planet 51 Pegasi b was discovered Some circumbinary planets may eventhrough precise measurements of the be habitable, although the three knownvelocity of its parent star, which revealed ones are not. Kepler-16 b is slightly toothe motion induced in the star by the cold, and Kepler-34 b and Kepler-35 bpresence of the orbiting planet. This A are too hot. They also have extrememethod has proved very successful for B seasons because the light received fromspotting planets, and can be credited their parent stars changes not only dur-with the discovery of roughly 400 so ing the stars’ orbital periods (tens offar. As observational programmes con- days) and the planetary orbital periodtinue, they become sensitive to planets (hundreds of days), but also on muchon wider orbits (longer orbital periods). b 0.5 AU longer timescales through precession ofThe dominant population of extraso- the orbits due to three-body effects.lar planets currently consists of objects What common characteristics dothat are more massive than Jupiter and these three planets have? The centralare separated from their host stars by Figure 1 | Orbital configuration of the Kepler-34 system. The binary systems have orbital separationsseveral astronomical units; many of outer ellipse represents the orbital motion of the circumbinary of between 0.18 and 0.22 au, and thethese are in multi-planet systems. planet Kepler-34 b, labelled b, around its host binary star system, planets orbit their hosts at distances of The other very successful method for which is composed of stars A and B in orbit around one another between 0.6 and 1.1 au. They are thusdiscovering planets is to look for those (as indicated by arrows). The plus sign shows the system’s centre all close to the smallest possible stablethat periodically transit (eclipse) their of mass. Spheres denote the orbital positions of the three bodies. orbits, but the fact that such planets One astronomical unit (au) is the average distance between Earthparent star. These transiting planets are and the Sun. Gravitational effects between the three bodies mean were the first to be found is at leasta gold mine of information: they are the that this orbital configuration is gradually changing, so the bodies partly an effect of the detection method.only ones whose size can be obtained, follow different paths on successive orbits. This is why the orbit As Kepler continues to observe, it willby measuring the amount of starlight of the planet shows a discontinuity in the upper part of the figure. become sensitive to planets on longer*This article and the paper 2 under discussion Kepler-34 b is one of two circumbinary planets discovered by periods: these three systems maywere published online on 11 January 2012. Welsh and colleagues2. (Modified from ref. 2.) represent only the tip of the iceberg. ■4 4 8 | N AT U R E | VO L 4 8 1 | 2 6 JA N UA RY 2 0 1 2 © 2012 Macmillan Publishers Limited. All rights reserved
NEWS & VIEWS RESEARCHJohn Southworth is in the Astrophysics 3. Mayor, M. & Queloz, D. Nature 378, 355–359 of cooperation, ranging from the sharingGroup, Keele University, Newcastle-under-Lyme (1995). of meat within bands of hunter-gatherers to 4. Bakos, G. Á., Lázár, J., Papp, I., Sári, P. & Green, E. M.ST5 5BG, UK. Publ. Astron. Soc. Pacif. 114, 974–987 (2002). territorial defence, cannot be easily accountede-mail: email@example.com 5. Pollacco, D. L. et al. Publ. Astron. Soc. Pacif. 118, for by these models1. 1407–1418 (2006). To address this gap, researchers began to1. Doyle, L. R. et al. Science 333, 1602–1606 6. Anderson, D. R. et al. Astrophys. J. 709, 159–167 (2011). (2010). develop and apply new models. Gene–culture2. Welsh, W. F. et al. Nature 481, 475–479 (2012). 7. Hellier, C. et al. Nature 460, 1098–1100 (2009). co-evolutionary models propose that, because social strategies are culturally learned, rapid cultural change tends to generate substantial S OC IA L SCIENCE variation in cooperation among groups while reducing variation within groups. In theseHunter-gatherer models, cooperation is sustained by a process of cultural learning and the sanctioning of norm violators, which leads to the continuouscooperation reassortment of groups6. More cooperative groups tend to endure and expand, whereas less cooperative groups gradually break down. Another class of models, based on social selec-A study of social networks in the hunter-gatherer Hadza people in Tanzania tion, proposes that individuals cooperate com-illuminates the evolutionary origins of humans’ unique style of cooperation petitively, as a means of attracting an inflow ofin groups. See Letter p .497 partners who bring benefits7. A third approach proposes that cooperation can be sustained as individuals seek out those with different skills,JOSEPH HENRICH sustain such assortative interactions against resources or abilities. Here, assortment is based invasion by ‘free-riders’ — non-cooperators on complementarity rather than similarity8.T he social behaviour of humans poses a who siphon off collective benefits. To illuminate how the Hadza tackle the significant evolutionary puzzle. Influ- Through the 1970s and 1980s, many core dilemma of cooperation, Apicella et al.4 enced by ‘prosocial’ motivations, we researchers assumed that hunter-gatherers gathered data on assortment and coopera-routinely help our relatives and friends in tackle this core dilemma by relying on a com- tive tendencies. The authors studied assort-ways big and small, from donating kidneys to bination of kinship and direct reciprocity. By ment within two social networks. To assemblesharing food. Perhaps most puzzlingly, and targeting kin on the basis of shared genetic the first (a campmate network), they askedunlike other primates, we also help strangers inheritance, cooperators are more likely to adult Hadza from 17 different bands whoand cooperate in large groups by, for exam- deliver benefits to fellow cooperators. Simi- they wanted to camp with when their nextple, giving blood, going to war, recycling and larly, by reciprocating help with help, unrelated band formed. For the second network (a giftpaying taxes. Yet human prosocial behaviour individuals can sustain tit-for-tat coopera- network), individuals received three honeyvaries dramatically between groups — from tion. However, by the twenty-first century sticks — Hadza love honey — and were askedsocieties with little cooperation beyond it had become clear that although kinship to secretly specify who should get each stick.extended kin to the vast scales of cooperation and direct reciprocity can each explain some Finally, to measure cooperativeness, thefound in many modern states1,2. aspects of human prosociality, many domains researchers gave individuals from each band Among the key challenges to understand-ing the origins of human cooperation are the PHOTOSTOCK-ISRAEL/ALAMYdifficult questions of what the social lives ofour Palaeolithic ancestors were like, and howthey shaped our psychology. Studying mod-ern foraging populations who depend on toolsand resources similar to those of our ances-tors is one of the few means we have of glean-ing certain kinds of insight into the past3. Onpage 497 of this issue, Apicella et al.4 give us aglimpse into the social dynamics of one of thefew remaining populations of nomadic hunter-gatherers, the Hadza of Tanzania (Fig. 1). With its practical implications, human coop-eration stands as a central question that spansthe behavioural sciences. From an evolutionaryperspective, the conundrum is how individu-als sustain cooperation in the face of the ever-present forces of self-interest; in other words,how could natural selection favour actionsthat benefit others, or one’s group, but thatalso incur a personal cost? Theorists generallyagree that the solution to this core dilemmarequires assortative interaction, such that Figure 1 | Helping hands. The Hadza people of Tanzania, such as these young men who arecooperators benefit other cooperators more roasting birds they have caught, rely on hunting and gathering to obtain most of their food. By studyingthan non-cooperators5. The challenge arises Hadza social networks, Apicella et al.4 illuminate the population dynamics that underpin the evolutionwhen one tries to delineate the processes that of human cooperation. 2 6 JA N UA RY 2 0 1 2 | VO L 4 8 1 | N AT U R E | 4 4 9 © 2012 Macmillan Publishers Limited. All rights reserved