1. RESEARCH NEWS & VIEWS
AST RONO MY a period of hundreds of days. The transits of
A 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’s
Three examples of a new family of planets, which orbit a pair of stars rather than atmosphere, as well as inevitable interruptions
a 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 the
JOHN 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-like
A
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 transiting
the recent discovery1 of a circumbinary planet, The transit method has led to the discovery planets. Three of these have been confirmed to
which 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 in
ence fiction, for example the planet Tatooine survey telescopes at observatories spread their study2, and Kepler-16 b (ref. 1). Not only
in Star Wars. On page 475 of this issue, Welsh around the world, such as HATNet4 and does each of these three planets transit both of
et al.2 describe the discovery of two more such SuperWASP5. These surveys are heavily biased its parent stars, but the stars themselves eclipse
planets 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
1
ered 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 object
orbits its own system of two stars, were found most rarefied planet known (up to twice the to exist, Welsh and colleagues’ identification
using 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 prevalence
planetary 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 frequency
the first planet to orbit a normal star other than to find one that might support life. Habitability of occurrence of circumbinary planets is at
the Sun. Although this planet is probably a gas most probably requires a rocky surface with least 1%. Taking into account the fraction
giant (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, this
masses), it orbits at only 0.052 astronomi- no bigger than two Earth radii on an orbit with result implies that there are millions of such
cal units (au) from its star (1 au is the planets distributed throughout the
average distance between Earth and Galaxy. This analysis does not account
the 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 the
is to the Sun. Galaxy.
Planet 51 Pegasi b was discovered Some circumbinary planets may even
through precise measurements of the be habitable, although the three known
velocity of its parent star, which revealed ones are not. Kepler-16 b is slightly too
the motion induced in the star by the cold, and Kepler-34 b and Kepler-35 b
presence of the orbiting planet. This A are too hot. They also have extreme
method has proved very successful for B seasons because the light received from
spotting 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 of
far. As observational programmes con- days) and the planetary orbital period
tinue, they become sensitive to planets (hundreds of days), but also on much
on wider orbits (longer orbital periods). b 0.5 AU longer timescales through precession of
The dominant population of extraso- the orbits due to three-body effects.
lar planets currently consists of objects What common characteristics do
that are more massive than Jupiter and these three planets have? The central
are separated from their host stars by Figure 1 | Orbital configuration of the Kepler-34 system. The binary systems have orbital separations
several astronomical units; many of outer ellipse represents the orbital motion of the circumbinary of between 0.18 and 0.22 au, and the
these 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 thus
discovering planets is to look for those (as indicated by arrows). The plus sign shows the system’s centre all close to the smallest possible stable
that 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 Earth
parent star. These transiting planets are and the Sun. Gravitational effects between the three bodies mean were the first to be found is at least
a 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 will
by 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 may
were 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

2. NEWS & VIEWS RESEARCH
John Southworth is in the Astrophysics 3. Mayor, M. & Queloz, D. Nature 378, 355–359 of cooperation, ranging from the sharing
Group, 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 accounted
e-mail: jkt@astro.keele.ac.uk 5. Pollacco, D. L. et al. Publ. Astron. Soc. Pacif. 118, for by these models1.
1407–1418 (2006). To address this gap, researchers began to
1. 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–culture
2. 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 these
Hunter-gatherer models, cooperation is sustained by a process
of cultural learning and the sanctioning of
norm violators, which leads to the continuous
cooperation
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 of
in 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 assemble
sharing food. Perhaps most puzzlingly, and targeting kin on the basis of shared genetic the first (a campmate network), they asked
unlike other primates, we also help strangers inheritance, cooperators are more likely to adult Hadza from 17 different bands who
and cooperate in large groups by, for exam- deliver benefits to fellow cooperators. Simi- they wanted to camp with when their next
ple, giving blood, going to war, recycling and larly, by reciprocating help with help, unrelated band formed. For the second network (a gift
paying taxes. Yet human prosocial behaviour individuals can sustain tit-for-tat coopera- network), individuals received three honey
varies dramatically between groups — from tion. However, by the twenty-first century sticks — Hadza love honey — and were asked
societies 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, the
found 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/ALAMY
difficult questions of what the social lives of
our Palaeolithic ancestors were like, and how
they shaped our psychology. Studying mod-
ern foraging populations who depend on tools
and 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. On
page 497 of this issue, Apicella et al.4 give us a
glimpse into the social dynamics of one of the
few 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 spans
the behavioural sciences. From an evolutionary
perspective, 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 actions
that benefit others, or one’s group, but that
also incur a personal cost? Theorists generally
agree that the solution to this core dilemma
requires assortative interaction, such that Figure 1 | Helping hands. The Hadza people of Tanzania, such as these young men who are
cooperators benefit other cooperators more roasting birds they have caught, rely on hunting and gathering to obtain most of their food. By studying
than non-cooperators5. The challenge arises Hadza social networks, Apicella et al.4 illuminate the population dynamics that underpin the evolution
when 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