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Theory and observation for the search for life on exoplanets via atmospheric ‘biosignature gases’ is accelerating, motivated by the capabilities of the next generation of space- and ground-based telescopes. The most observationally accessible rocky planet atmospheres are those dominated by molecular hydrogen gas, because the low density of H 2 gas leads to an expansive atmosphere. The capability of life to withstand such exotic environments, however, has not been tested in this context. We demonstrate that single-celled microorganisms ( Escherichia coli and yeast) that normally do not inhabit H 2 -dominated environments can survive and grow in a 100% H 2 atmosphere. We also describe the astonishing diversity of dozens of different gases produced by E. coli , including many already proposed as potential biosignature gases (for example, nitrous oxide, ammonia, methanethiol, dimethylsulfide, carbonyl sulfide and isoprene). This work demonstrates the utility of laboratory experiments to better identify which kinds of alien environments can host some form of possibly detectable life. Escherichia coli bacteria and yeast cultures (representative prokaryotes and eukaryotes) have been tested under laboratory conditions in a 100% H 2 atmosphere. They can reproduce normally, with lower growth rates, producing a range of biosignature gases. Exoplanets with a H 2 -dominated atmosphere might thus not be totally hostile to some forms of life.

The prospects for finding transiting exoplanets in the range of a few to20 M ⊕ 20     M ⊕ is growing rapidly with both ground-based and spaced-based efforts. We describe a publically available computer code to compute and quantify the compositional ambiguities for differentiated solid exoplanets with a measured mass and radius, including the mass and radius uncertainties.

Photometry and spectroscopy of extrasolar planets provides information about their atmospheres and surfaces. From extrasolar planet spectra and photometry we can infer the composition and temperature of the atmospheres as well as the presence of molecular species, including biosignature gases or surface features. So far photometry has been published for three different transiting hot Jupiters (gas giant planets in short-period orbits), opening the era of comparative exoplanetology.

The detection of massive planets orbiting nearby stars has become almost routine 1 , 2 , but current techniques are as yet unable to detect terrestrial planets with masses comparable to the Earth's. Future space-based observatories to detect Earth-like planets are being planned. Terrestrial planets orbiting in the habitable zones of stars—where planetary surface conditions are compatible with the presence of liquid water—are of enormous interest because they might have global environments similar to Earth's and even harbour life. The light scattered by such a planet will vary in intensity and colour as the planet rotates; the resulting light curve will contain information about the planet's surface and atmospheric properties. Here we report a model that predicts features that should be discernible in the light curve obtained by low-precision photometry. For extrasolar planets similar to Earth, we expect daily flux variations of up to hundreds of per cent, depending sensitively on ice and cloud cover as well as seasonal variations. This suggests that the meteorological variability, composition of the surface (for example, ocean versus land fraction) and rotation period of an Earth-like planet could be derived from photometric observations. Even signatures of Earth-like plant life could be constrained or possibly, with further study, even uniquely determined.

An amendment to this paper has been published and can be accessed via a link at the top of the paper.

Open clusters potentially provide an ideal environment for the search for transiting extrasolar planets, since they feature a relatively large number of stars of the same known age and metallicity at the same distance. With this motivation, over a dozen open clusters are now being monitored by four different groups. We review the motivations and challenges for open cluster transit surveys for short‐period giant planets. Our photometric monitoring survey of Galactic southern open clusters, the Extrasolar Planet Occultation Research/Open Clusters (EXPLORE/OC) project, was designed with the goals of maximizing the chance of finding and characterizing planets and of providing a statistically valuable astrophysical result in the case of no detections. We use the EXPLORE/OC data from two open clusters, NGC 2660 and NGC 6208, to illustrate some of the largely unrecognized issues facing open cluster surveys, including severe contamination by Galactic field stars (>80%) and the relatively low number of cluster members for which high‐precision photometry can be obtained. We discuss how a careful selection of open cluster targets under a wide range of criteria such as cluster richness, observability, distance, and age can meet the challenges, maximizing chances to detect planet transits. In addition, we present the EXPLORE/OC observing strategy to optimize planet detection, which includes high‐cadence observing and continuously observing individual clusters rather than alternating between targets.

Planets with radii between that of the Earth and Neptune (hereafter referred to as ‘sub-Neptunes’) are found in close-in orbits around more than half of all Sun-like stars 1 , 2 . However, their composition, formation and evolution remain poorly understood 3 . The study of multiplanetary systems offers an opportunity to investigate the outcomes of planet formation and evolution while controlling for initial conditions and environment. Those in resonance (with their orbital periods related by a ratio of small integers) are particularly valuable because they imply a system architecture practically unchanged since its birth. Here we present the observations of six transiting planets around the bright nearby star HD 110067. We find that the planets follow a chain of resonant orbits. A dynamical study of the innermost planet triplet allowed the prediction and later confirmation of the orbits of the rest of the planets in the system. The six planets are found to be sub-Neptunes with radii ranging from 1.94 R ⊕ to 2.85 R ⊕ . Three of the planets have measured masses, yielding low bulk densities that suggest the presence of large hydrogen-dominated atmospheres. Observations of six transiting planets around the bright nearby star HD 110067 show that they follow a chain of resonant orbits, with three of the planets inferring the presence of large hydrogen-dominated atmospheres.

Light from an alien planet For the first time, light from a planet outside our Solar System has been detected on Earth. The planet is HD 209458b, previously identified by the wobble its gravity induces in its host star's orbit. It is slightly larger than Jupiter, but orbits its star at less than a twentieth of the distance between the Earth and the Sun, making it a so-called ‘hot Jupiter’ planet. As HD 209458b passes behind the star, the amount of infrared light coming from the area drops slightly: that drop represents the planet's light contribution. A class of extrasolar giant planets—the so-called ‘hot Jupiters’ (ref. 1 )—orbit within 0.05  au of their primary stars (1  au is the Sun–Earth distance). These planets should be hot and so emit detectable infrared radiation 2 . The planet HD 209458b (refs 3 , 4 ) is an ideal candidate for the detection and characterization of this infrared light because it is eclipsed by the star. This planet has an anomalously large radius (1.35 times that of Jupiter 5 ), which may be the result of ongoing tidal dissipation 6 , but this explanation requires a non-zero orbital eccentricity (∼ 0.03; refs 6 , 7 ), maintained by interaction with a hypothetical second planet. Here we report detection of infrared (24 µm) radiation from HD 209458b, by observing the decrement in flux during secondary eclipse, when the planet passes behind the star. The planet's 24-µm flux is 55 ± 10 µJy (1 σ ), with a brightness temperature of 1,130 ± 150 K, confirming the predicted heating by stellar irradiation 2 , 8 . The secondary eclipse occurs at the midpoint between transits of the planet in front of the star (to within ± 7 min, 1 σ ), which means that a dynamically significant orbital eccentricity is unlikely.