What exactly does the word 'habitable' mean? The question comes to mind because of two things, the first being the media buzz over Gliese 581g, now widely described as the first potentially habitable planet we've found. The second is Paul Davies' presentation yesterday at the International Astronautical Congress in Prague, where Davies was careful to differentiate between 'habitable' and 'inhabited.' More on the latter in a moment. Let's look first at this outstanding find, two new planets in the Gliese 581 system discovered through the unflagging efforts of the Lick-Carnegie team. A World in the Zone? The beauty of Gl 581, of course, is not only that it has yielded a storehouse of planets (six known at present), but that these worlds are on nearly circular orbits, and several have caught our eye re habitability before. The current buzz seems a bit tamer than the one that greeted the announcement of Gl 581c, at the time thought to be capable of sustaining liquid water on its surface,...

While we're this early in the game of detecting life signs from distant planets, it makes sense to focus on surface habitability, which is why oceans are so interesting. Sure, we can imagine potential biospheres under the ice of a Europa or even an Enceladus, but given the state of our instrumentation and the distance of our target, going after the most likely catch makes sense, and that means looking for oceans. Significant work from the EPOXI mission has given us some of the parameters for studying a planet like ours using multi-wavelength photometry. EPOXI, you'll recall, is the extended mission of the Deep Impact spacecraft that drove an impactor into Comet Tempel 1 in 2005 and is now enroute to Comet Hartley 2. Its views of Earth are being used to help scientists prepare for studies of terrestrial worlds around other stars. Planets with large bodies of water should reflect light from their star differently than dry planets, and as the observed planet goes through its phases as...

Globular clusters held an early fascination for me, and I guess anyone who encounters these rich cities of stars for the first time wonders what it would be like to be on a planet deep inside one of them. The clusters appear to be distributed in a spherical halo around the galactic center, ancient collections of stars much lower in heavy elements than stars in the galactic disk (although globular clusters in some other Local Group galaxies seem younger). The thought of the night sky on a planet embedded in such a place makes the mind reel, star upon star upon star filling the view. Image: The globular cluster 47 Tucanae, the second brightest globular cluster orbiting the Milky Way (behind Omega Centauri). Imagine the night sky deep within such a cluster. Credit: South African Astronomical Observatory. But a new paper suggests that at least one category of planets may be rare in such clusters. It follows up on an earlier survey of the cluster 47 Tucanae which examined some 34,000...

The ExoClimes 2010 conference ("Exploring the Diversity of Planetary Atmospheres") is well in progress in Exeter (UK) as I write, with its talks now being posted online and the hope that video of the presentations will soon be available on the conference site. Already the latest lingo is in the air, as in 'Hermean,' a term used by Brian Jackson (NASA GSFC) to describe hot, rocky exoplanets with tenuous atmospheres. The analogy is with Mercury, though these are even hotter places with magma oceans and melted surfaces, leading to what Jackson calls a 'rock vapor atmosphere' that just might be visible given sufficient spectral resolution. But what catches my eye this morning, as I survey the ongoing conference buzz online from an ocean away, is Franck Selsis (Laboratoire d'Astophysique de Bordeaux) and his work on the atmospheres of short-period terrestrial exoplanets. Selsis is interested in the habitability of planets around M-dwarfs, noting their strong tidal interactions with their...

We're entering the era of the 'super-Earths,' when rocky planets larger than our own will pepper the lists of new discoveries. These smaller worlds will occasionally make a transit of their star, as does CoRoT-7b, and that's when things really get interesting. After all, we know that secondary eclipses, in which a transiting exoplanet swings behind its star as seen from Earth, can be used to study distant atmospheres. The method collects light from both star and planet and, when the planet is hidden, subtracts the starlight to get the planetary signature. Now Lisa Kaltenegger (Harvard-Smithsonian Center for Astrophysics) and colleagues Wade Henning and Dimitar Sasselov are advancing the idea that we can use near-term instrumentation like the James Webb Space Telescope to spot volcanic eruptions using these same methods. Their model is based on eruptions on an Earth-like planet, extrapolating from what happens on our world to suggest that sulfur dioxide from a major volcanic event on...