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<jats:p> <jats:italic>Science</jats:italic> presents an overview of planet-hunting techniques and representative efforts for the nearly 900 confirmed exoplanets and the hundreds of fresh candidates that are turning up every month. </jats:p>

<jats:p>Observational surveys for extrasolar planets probe the diverse outcomes of planet formation and evolution. These surveys measure the frequency of planets with different masses, sizes, orbital characteristics, and host star properties. Small planets between the sizes of Earth and Neptune substantially outnumber Jupiter-sized planets. The survey measurements support the core accretion model, in which planets form by the accumulation of solids and then gas in protoplanetary disks. The diversity of exoplanetary characteristics demonstrates that most of the gross features of the solar system are one outcome in a continuum of possibilities. The most common class of planetary system detectable today consists of one or more planets approximately one to three times Earth’s size orbiting within a fraction of the Earth-Sun distance.</jats:p>

<jats:p>The search for exoplanets includes the promise to eventually find and identify habitable worlds. The thousands of known exoplanets and planet candidates are extremely diverse in terms of their masses or sizes, orbits, and host star type. The diversity extends to new kinds of planets, which are very common yet have no solar system counterparts. Even with the requirement that a planet’s surface temperature must be compatible with liquid water (because all life on Earth requires liquid water), a new emerging view is that planets very different from Earth may have the right conditions for life. The broadened possibilities will increase the future chances of discovering an inhabited world.</jats:p>

<jats:title>Magnetic Frustration</jats:title> <jats:p> The study of magnetic frustration has a long history in solid-state physics, but cold-atom systems now offer the possibility of simulating the problem with exquisite control. <jats:bold> Islam <jats:italic>et al.</jats:italic> </jats:bold> (p. <jats:related-article xmlns:xlink="http://www.w3.org/1999/xlink" ext-link-type="doi" issue="6132" page="583" related-article-type="in-this-issue" vol="340" xlink:href="10.1126/science.1232296">583</jats:related-article> ) study a system of 16 trapped ions, using the Coulomb interactions between the ions to simulate exchange interactions present in magnetic systems. The measured spin correlations provide a window into the behavior of the system, as the effective magnetic field and the range of the interactions are tuned. </jats:p>

<jats:title>Two Small Habitable Planets</jats:title> <jats:p> NASA's Kepler space telescope was launched in 2009 with the goal of detecting planets the size of Earth in the habitable zone of Sun-like stars and determining the frequency of these planets. Using data from Kepler, <jats:bold> Borucki <jats:italic>et al.</jats:italic> </jats:bold> (p. <jats:related-article xmlns:xlink="http://www.w3.org/1999/xlink" ext-link-type="doi" issue="6132" page="587" related-article-type="in-this-issue" vol="340" xlink:href="10.1126/science.1234702">587</jats:related-article> , published online 18 April) report the detection of a five-planet system where all the planets are smaller than twice the size of Earth and where the two outermost planets orbit in the habitable zone of their star, defined as the region where a rocky planet can host liquid water on its solid surface. The star, Kepler-62, is smaller and cooler than the Sun. </jats:p>

<jats:title>Closing the Cycle</jats:title> <jats:p> Cyclic hydrocarbons that incorporate nitrogen in the ring are among the most heavily investigated compounds in medicinal chemistry. <jats:bold>Hennessy and Betley</jats:bold> (p. <jats:related-article xmlns:xlink="http://www.w3.org/1999/xlink" ext-link-type="doi" issue="6132" page="591" related-article-type="in-this-issue" vol="340" xlink:href="10.1126/science.1233701">591</jats:related-article> ) demonstrate an iron catalyst that forms a range of such cyclic compounds by inducing linear alkyl azides to curl back on themselves, inserting the nitrogen at one end into a carbon-hydrogen bond further down the chain. The reaction furthers a trend of C-H bond activation chemistry that forms elaborate products from relatively simple precursors, without the need to install activating groups at unreactive sites. </jats:p>

<jats:title>From Coils to Cages</jats:title> <jats:p> Self-assembly strategies that mimic protein assembly, such as the formation of viral coats, often begin with simpler peptide assemblies. <jats:bold> Fletcher <jats:italic>et al.</jats:italic> </jats:bold> (p. <jats:related-article xmlns:xlink="http://www.w3.org/1999/xlink" ext-link-type="doi" issue="6132" page="595" related-article-type="in-this-issue" vol="340" xlink:href="10.1126/science.1233936">595</jats:related-article> , published online 11 April; see the Perspective by <jats:bold> <jats:related-article xmlns:xlink="http://www.w3.org/1999/xlink" ext-link-type="doi" issue="6132" page="561" related-article-type="in-this-issue" vol="340" xlink:href="10.1126/science.1237708">Ardejani and Orner</jats:related-article> </jats:bold> ) designed two coiled-coil peptide motifs, a heterodimer, and a homotrimer. Both peptides contained cysteine residues and could link through disulfide bonds, so that the trimer could form the vertices of a hexagonal network and the dimer its edges. However, these components are flexible and, rather than form extended sheets, they closed to form particles ∼100 nanometers in diameter. </jats:p>

<jats:title>Biological Transistor</jats:title> <jats:p> A transistor is a device that amplifies and switches electronic signals. <jats:bold> Bonnet <jats:italic>et al.</jats:italic> </jats:bold> (p. <jats:related-article xmlns:xlink="http://www.w3.org/1999/xlink" ext-link-type="doi" issue="6132" page="599" related-article-type="in-this-issue" vol="340" xlink:href="10.1126/science.1232758">599</jats:related-article> , published online 28 March; see the Perspective by <jats:bold> <jats:related-article xmlns:xlink="http://www.w3.org/1999/xlink" ext-link-type="doi" issue="6132" page="554" related-article-type="in-this-issue" vol="340" xlink:href="10.1126/science.1237738">Benenson</jats:related-article> </jats:bold> ) engineered a genetic circuit to behave like a transistor in individual living cells. Instead of regulating messenger RNA levels, which has been used previously in designing such systems, the approach relied on changing the state of double-stranded DNA. Six basic logic gates were designed and constructed that were based on the activity of two serine recombinases. </jats:p>

<jats:title>Lord of the Robotic Flies</jats:title> <jats:p> While small-scale flying objects are ubiquitous in nature, they are quite hard to engineer. As sizes get smaller, fixed-winged flight becomes less efficient because of increased drag resistance. <jats:bold> Ma <jats:italic>et al.</jats:italic> </jats:bold> (p. <jats:related-article xmlns:xlink="http://www.w3.org/1999/xlink" ext-link-type="doi" issue="6132" page="603" related-article-type="in-this-issue" vol="340" xlink:href="10.1126/science.1231806">603</jats:related-article> ) developed a tethered robotic fly with wings that flap through the use of piezo-electric materials. Control of the flight motion involved a feedback process, which allowed the tethered robotic fly to hover and make controlled flight maneuvers. </jats:p>

<jats:title>Seeing Below the Surface</jats:title> <jats:p> Marte Vallis represents the largest of the young outflow channels on Mars. Thought to be the result of an ancient megaflood that occurred during a period that is otherwise known to be cold and dry, this channel system extends more than approximately 1000 kilometers in length and 100 kilometers in width. The channels' depths are unknown, however, because they have been covered by lava flows. By using the SHARAD radar sounder on the Mars Reconnaissance Orbiter, <jats:bold> Morgan <jats:italic>et al.</jats:italic> </jats:bold> (p. <jats:related-article xmlns:xlink="http://www.w3.org/1999/xlink" ext-link-type="doi" issue="6132" page="607" related-article-type="in-this-issue" vol="340" xlink:href="10.1126/science.1234787">607</jats:related-article> , published online 7 March) produced a three-dimensional reconstruction of the subsurface channels. The channels are twice as deep as previously thought and the floodwaters came from the Cerberus Fossae extensional fracture system. </jats:p>