A new class_of_planet

A new class_of_planet

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  NEWS & VIEWS doi:10.1038/nature10818 AST RO NO MY A new class of planet Three examples of a new family of planets, which orbit a pair of stars rather than a single one, have been discovered. The Milky Way may contain millions of these circumbinary planets. JOHN SOUTHWORTH The overriding aim of planetary research is to find one that might support life. Habit- A lthough more than 700 extrasolar ability most probably requires a rocky planets have been detected, none surface with liquid water, which, in turn, was known to orbit more than one demands a planet no bigger than two Earth star until the recent discovery1 of a circum- radii on an orbit with a period of hundreds binary planet, which orbits a pair of stars. of days. The transits of such a body will not This concept was previously confined to only be infrequent but will cause the light theory — and to science fiction, for exam- A from the star to drop by a puny 0.01%. Such ple the planet Tatooine in Star Wars. In a B a signal is much too meagre to pick up with paper published on Nature’s website today, ground-based telescopes, which suffer from Welsh et al.2 describe the discovery of two the blurring effect of Earth’s atmosphere, as more such planets and provide insight well as inevitable interruptions due to day- into their frequency of occurrence. The light and bad weather. previously discovered planet1 and the new Finding a habitable planet requires a b 0.5 AU ones, each of which orbits its own system larger — and much more expensive — tele- of two stars, were found using NASA’s scope outside Earth’s atmosphere. Enter the Kepler space telescope. Kepler spacecraft, the primary aim of which For hundreds of years, scientists assumed Figure 1 | Orbital configuration of the Kepler-34 is to use the transit method to discover that the Solar System is a typical example system. The outer ellipse represents the orbital motion Earth-like planets. It monitors 150,000 stars of a planetary system. That assumption of the circumbinary planet Kepler-34 b, labelled b, around in the constellations Cygnus and Lyra, and was challenged in 1995 by the discovery3 of its host binary star system, which is composed of stars has already found more than 2,000 candi- 51 Pegasi b, the first planet to orbit a nor- A and B in orbit around one another (as indicated by date transiting planets. Three of these have mal star other than the Sun. Although this arrows). The plus sign shows the system’s centre of mass. been confirmed to be circumbinary planets: planet is probably a gas giant (the lower Spheres denote the orbital positions of the three bodies. Kepler-34 b (Fig. 1) and Kepler-35 b, which One astronomical unit (au) is the average distance limit on its mass is 0.47 Jupiter masses), it between Earth and the Sun. Gravitational effects between Welsh et al. describe in their study2, and orbits at only 0.052 astronomical units (au) the three bodies mean that this orbital configuration is Kepler-16 b (ref. 1). Not only does each of from its star (1 au is the average distance gradually changing, so the bodies follow different paths these three planets transit both of its parent between Earth and the Sun). This means on successive orbits. This is why the orbit of the planet stars, but the stars themselves eclipse each that 51 Pegasi b is 100 times closer to its star shows a discontinuity in the upper part of the figure. other. than Jupiter is to the Sun. Kepler-34 b is one of two circumbinary planets discovered Although the discovery1 of Kepler-16 b Planet 51  Pegasi  b was discovered by Welsh and colleagues2. (Modified from ref. 2.) revealed that it was possible for such an through precise measurements of the veloc- object to exist, Welsh and colleagues’ identi- ity of its parent star, which revealed the motion starlight blocked during transit. This means fication of two more circumbinary planets not induced in the star by the presence of the orbit- that their surface gravities and mean densi- only shows that such a planet is no freak object, ing planet. This method has proved very suc- ties can be calculated, ultimately allowing the but also allows an estimate of their prevalence cessful for spotting planets, and can be credited investigation of their internal structure and to be made. The authors2 find that, for short- with the discovery of roughly 400 so far. As formation process. period binary star systems, the frequency observational programmes continue, they The transit method has led to the discovery of occurrence of circumbinary planets is at become sensitive to planets on wider orbits of more than 200 planets, predominantly by least 1%. Taking into account the fraction of (longer orbital periods). The dominant popu- teams that operate small wide-field robotic stars that are short-period binaries, this result lation of extrasolar planets currently consists of survey telescopes at observatories spread implies that there are millions of such planets objects that are more massive than Jupiter and around the world, such as HATNet4 and distributed throughout the Galaxy. This analy- are separated from their host stars by several SuperWASP5. These surveys are heavily biased sis does not account for longer-period binary astronomical units; many of these are in multi- towards large planets with small orbits. As a star systems, which are similarly plentiful in planet systems. result, they are unparalleled sources of odd- the Galaxy. The other very successful method for balls such as WASP-17 (ref. 6), the biggest and Some circumbinary planets may even be discovering planets is to look for those that most rarefied planet known (up to twice the habitable, although the three known ones periodically transit (eclipse) their parent star. radius of Jupiter and only 6% as dense), and are not. Kepler-16 b is slightly too cold, and These transiting planets are a gold mine of WASP-18 (ref. 7), which is ten times the mass Kepler-34 b and Kepler-35 b are too hot. information: they are the only ones whose size of Jupiter and whirls around its host star every They also have extreme seasons because the can be obtained, by measuring the amount of 23 hours (Jupiter’s orbital period is 11.9 years). light received from their parent stars changes | NAT U R E | 1 © 2012 Macmillan Publishers Limited. 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  RESEARCH NEWS &VIEWS not only during the stars’ orbital periods orbits, but the fact that such planets were the 1. Doyle, L. R. et al. Science 333, 1602–1606 (tens of days) and the planetary orbital period first to be found is at least partly an effect of (2011). (hundreds of days), but also on much longer the detection method. As Kepler continues 2. Welsh, W. F. et al. Nature http://dx.doi.org/ nature.10768 (2012). timescales through precession of the orbits due to observe, it will become sensitive to planets 3. Mayor, M. & Queloz, D. Nature 378, 355–359 to three-body effects. on longer periods: these three systems may (1995). What common characteristics do these represent only the tip of the iceberg. ■ 4. Bakos, G. Á., Lázár, J., Papp, I., Sári, P. & Green, E. M. three planets have? The central binary sys- Publ. Astron. Soc. Pacif. 114, 974–987 (2002). 5. Pollacco, D. L. et al. Publ. Astron. Soc. Pacif. 118, tems have orbital separations of between 0.18 John Southworth is in the Astrophysics 1407–1418 (2006). and 0.22 au, and the planets orbit their hosts Group, Keele University, Newcastle-under-Lyme 6. Anderson, D. R. et al. Astrophys. J. 709, 159–167 at distances of between 0.6 and 1.1 au. They ST5 5BG, UK. (2010). are thus all close to the smallest possible stable e-mail: jkt@astro.keele.ac.uk 7. Hellier, C. et al. Nature 460, 1098–1100 (2009). 2 | NAT U R E | © 2012 Macmillan Publishers Limited. All rights reserved