Yesterday's post on exomoons and their possibilities as abodes for life leads naturally to new work from René Heller (Leibniz Institute for Astrophysics, Potsdam) and Rory Barnes (University of Washington). We're finding planets much larger and more massive than Earth in the habitable zone, as the recent findings of the Planet Hunters project attest. What can we say about the habitability of any large moons these planets may have? In their paper, Heller and Barnes look at the issues that separate exomoon habitability from habitability on an exoplanet itself. If Earth-sized satellites of giant planets exist, they may have certain advantages over terrestrial planets in the same orbit, depending on the host star. We know that M-class dwarfs are by far the most common kind of star in the galaxy, and that habitable zone planets around one of these will probably be tidally locked, with one hemisphere permanently facing the star and the other in permanent darkness. Extreme weather...

Because the sky is full of surprises, we can’t afford to be too doctrinaire about what tomorrow’s discovery might be. After all, ‘hot Jupiters’ were considered wildly unlikely by all but a few, and even here in the Solar System, probes like our Voyagers have turned up one startling thing after another -- volcanoes on Io were predicted just before Voyager arrived, but who thought we'd actually see them in the act of erupting? So I don’t think we can rule out the idea of habitable moons around a gas giant in the habitable zone, but there are reasons to question how numerous they would be. We’ve had this discussion before on Centauri Dreams, and while I love the idea of a huge 'Jupiter' hanging in the sky of a verdant, life-bearing planet, there are some factors that argue against it, as reader FrankH pointed out recently. One problem is that moons around a gas giant will probably be made largely of ice and rock, because the planet itself would have formed beyond the snow line and...

Plenty of interesting news is coming out of the American Astronomical Society meeting in Long Beach CA, enough that I'll want to spread our look at it out over the next few days. I want to start with Geoff Marcy's investigations with grad student Erik Petigura at UC-Berkeley, the two working in tandem with Andrew Howard (University of Hawaii) on the question of Earth-sized planets and their distribution in the galaxy. But I can't help noting before I begin how science fictional all these exoplanets are starting to seem as each day brings a new paper or announcement. For me, science fiction has always been as much about landscape as it is about science, and exoplanets are the ultimate exercise in imagining exotic places. When exoplanet announcements were still new and we had only a small catalog of these worlds, I would find myself pondering each and thinking about what it would be like to orbit one, or stand on it. Now we're getting hard data on potentially habitable places that...

Exactly what kind of planets can form around M-class dwarf stars is a major issue. After all, these stars, comprising 70 percent or more of the stars in the galaxy, are far more common than stars like the G-class Sun. About 5500 of the 160,000 stars the Kepler mission is looking at are M-dwarfs, and of these, 66 had been found to show at least one planetary transit signal at the time a new paper on M-dwarf planets was in preparation. That paper, the work of John Johnson and postdoc Jonathan Swift (Caltech) and team, homes in on the Kepler-32 system, whose five transiting planets offer a chance to study planet formation and frequency around such stars. Kepler-32 is about half as massive as the Sun and has half its radius, with about 5 percent of its luminosity. The planets here have radii that range from 0.8 to 2.7 times that of the Earth, all of them orbiting within about a tenth of an astronomical unit from the star, a distance that is about a third of the radius of Mercury's orbit...

As the number of confirmed planets and planet candidates has grown, we've gone through a variety of techniques for exoplanet hunting, as Michael Lemonick's new book Mirror Earth: The Search for Our Planet's Twin (Walker & Co., 2012) makes clear. I'm only a third of the way into the book but I bring it up because it's germane to today's discussion in two ways. The first is purely administrative. Readers of Centauri Dreams are used to seeing information about the book I'm reading on the front page, but as many emails have reminded me, lately it's been absent. What's happening is this: The software I use to display the book cover and progress bar is no longer being maintained by its creator, and the program has become flaky. I've discovered more and more that certain books will not display properly, so that although I can enter them in the configuration file, nothing shows up in the sidebar on the home page. As a result, I'm searching for alternatives that will display titles like...

In his new article on Alpha Centauri in Astronomy & Geophysics, Martin Beech (Campion College, University of Regina) noted that the Alpha Centauri stars seem to go through waves of scientific interest. Beech used Google's Ngram Viewer to look for references to the system in both the scientific literature as well as general magazines and newspapers, finding that there is a 30-year interval between peaks of interest. The figure is suspiciously generational, and Beech wonders whether it reflects an awakening of interest in this nearby system as each generation of scientists and publishers arises. I mentioned on Christmas Eve that the Beech paper was a real gift for the holidays, and for those of us who try to track developments about Alpha Centauri, it certainly is, drawing together recent work and commenting with care on the findings. The big issue for now is the existence of planets around these stars, a question Centauri B b will begin to answer if it can be confirmed. Everyone from...

We saw yesterday how a newly refined radial velocity technique allowed researchers to identify five planet candidates around the nearby star Tau Ceti. The latter has long held fascination for the exoplanet minded because it’s a G-class star not all that different from the Sun, and one of the planets around it -- if confirmed -- appears to be in its habitable zone. But smaller stars remain much in the news as well, as witness Gl 667C, a red dwarf (M-class) star in a triple system that also contains two closely spaced K-class stars with a semimajor axis of 1.82 AU. M-class stars offer a lot to planet hunters, as new work using the HARPS spectrograph at La Silla is making clear. For one thing, a planet of a given size induces more radial velocity variation around a low-mass star than around a larger one, making the planet easier to spot. For another, red dwarfs are dimmer than G and K-class stars, with a habitable zone much closer to the star. Here again we get a larger radial velocity...

I discovered while trying to get to my copy of Stephen Dole's Habitable Planets for Man that my office was so choked with stacks of books mixing with Christmas gifts about to be wrapped that I couldn't reach the necessary shelf. Thus space studies end inevitably in office cleaning, the only benefit of which is that there is now a clear path to the most distant of the bookshelves and Dole's book (this is the 1964 edition written with Isaac Asimov) now sits before me. I was feeling nostalgic and wanted the Dole volume to remind myself of my early enthusiasm for the nearby star Tau Ceti. The news that five planet candidates have been identified around this star -- one of them in the habitable zone -- brings back the fascination that was piqued when Frank Drake made Tau Ceti one of his two targets in 1960's Project Ozma, a search for extraterrestrial radio signals from Green Bank, WV. And in fact what I found in Dole's book on the subject of Tau Ceti was mostly about Drake's interest in...