The sheer range of possible outcomes in a planetary system is something we’re beginning to appreciate with each new exoplanet. Not long ago we looked at a possible collision between two large worlds in the young system Kepler 107, and the knowledge of how violent an evolving system can be informs our thinking about the formation of our own Moon and other Solar System phenomena. Now we’re learning to look for signs of another kind of early cataclysm, the migration of a planet caused by the close passage of one or more nearby stars. None of this should be surprising when we think about the outer system today. We have a vast cloud made up of trillions of comets encircling a more disk-like belt of debris in the Kuiper Belt, and a host of small objects moving on orbits that challenge our theories of how they formed. Indeed, the orbits of ‘scattered disk’ objects influenced by Neptune and, even more intriguing, unusual trans-Neptunian objects like Sedna may implicate a yet undiscovered...

If you had a hot new instrument like the Habitable Zone Planet Finder (HPF) now mounted at the Hobby-Eberly Telescope (McDonald Observatory, University of Texas), how would you run it through its paces for fine-tuning and verification of its performance specs? The team behind HPF has chosen to deploy the instrument during its commissioning phase on a nearby target, Barnard's Star, which for these purposes we can consider something of an M-dwarf standard. Working at near-infrared wavelengths, HPF uses radial velocity methods to identify low-mass planets around nearby M-dwarf stars. The choice of wavelength is determined by the mission: M-dwarfs (also known as 'red dwarfs') are prey to substantial magnetic activity that shows up as spots and flares that disrupt instruments working in visible light, not to mention the fact that they are small to begin with and thus faint on the sky. In the near-infrared, close to but not in the visible spectrum, this category of star appears brighter...

It seems increasingly clear that the factors that govern what kind of a planet emerges where in a given stellar system are numerous and not always well understood. Beyond the snowline, planets draw themselves together from the ice and other volatiles available in these cold regions, so that we wind up with low-density gas or ice-giants in the outer parts of a stellar system. Sometimes. Rocky worlds are made of silicates and iron, elements that, unlike ice, can withstand the much warmer temperatures inside the snowline. But consider: While we now have 2,000 confirmed exoplanets smaller than three Earth radii, the spread in their densities is all over the map. We’re finding that other processes must be in play, and at no insubstantial level. Low-density giant planets can turn up orbiting close to their stars. Planets not so dissimilar from Earth in terms of their radius may be found with strikingly different densities in the same system, and at no great distance from each other. Which...

It's hard to fathom when we look at a globe, but our planet Earth's substantial covering of ocean is relatively modest. Alternative scenarios involving 'water worlds' include rocky planets whose silicate mantle is covered in a deep, global ocean, with no land in sight. Kilometer after kilometer of water covers a layer of ice on the ocean floor in these models, making it unlikely that the processes that sustain life here could develop -- how likely is a carbon cycle in such a scenario, and without it, how do we stabilize climate and make an inhabitable world? These are challenging issues as we build the catalog of exoplanets and try to figure out local conditions. But it's also intriguing to ask what made Earth turn out as dry as it is. Tim Lichtenberg developed a theory while doing his thesis at the Eidgenössische Technische Hochschule in Zürich (he is now at Oxford), and now presents it in a paper in collaboration with colleagues at Bayreuth and Bern, as well as the University of...