Figuring out how planets form is an old occupation, with the basic ideas of planetary accretion going back several centuries, though tuned up, to be sure, in the 1970s and tweaked ever since. In a disk of gas and dust orbiting a young central star, dust grains begin to clump together, eventually forming planetesimals. Accretion models assume that these small planetesimals bang into each other and gradually grow. The assumption is that in the inner system at least temperatures are hot and the era of planet formation occurs well after the central star has formed. Image: Artist's conception of a protoplanetary disk. Credit: NASA/JPL-Caltech/T. Pyle. Adjust for distance from the star and subsequent planetary migration in the gas/dust disk and you can come up with a system more or less like ours, with rocky inner worlds and gas giants out beyond the snow line, the latter being the distance from the star where it is cool enough for volatile icy compounds to remain solid. But Anne...

One of these days we're going to have a new generation of telescopes, some in space and some on the Earth, that can analyze the atmosphere of a terrestrial world around another star. It's not enough to find individual gases like oxygen and ozone, carbon dioxide or methane. Any of these can occur naturally without ramifications for life. But finding all of these gases in the same atmosphere is telling, because without life to replenish them, some would disappear. Getting the data is going to be hard, which is why new work using the European Southern Observatory's Very Large Telescope is so interesting. The work involves 'Earthshine,' the reflection of sunlight off the Earth that is in turn reflected off the surface of the Moon. It's faint, to be sure, but Earthshine is visible in a crescent Moon when the light of the entire lunar disc is visible although only the crescent is brightly lit. Michael Sterzik (ESO) and team have used Earthshine to analyze our own planet's biosignature, and...