How large a planet is depends upon its composition and mass. Earth is largely made of silicates, with a diameter of 7,926 miles at the equator. Imagine an Earth mass planet made of iron and you're looking at a diameter of a scant 3000 miles. Interestingly, the relationship between mass and diameter follows a similar pattern no matter what material makes up the planet. Running the numbers, an Earth mass planet made of pure water will be 9500 miles across. Sara Seager (Massachusetts Institute of Technology) has been studying these things as part of a project to model the kind of Earth-size planets we're likely to find around nearby stars. About the mass/diameter pattern, she says this: "All materials compress in a similar way because of the structure of solids. If you squeeze a rock, nothing much happens until you reach some critical pressure, then it crushes. Planets behave the same way, but they react at different pressures depending on the composition. This is a big step forward in...

Our assumptions about terrestrial planets seem pretty straightforward. We're only now reaching the level where detecting such worlds becomes a possibility, with advances in ground- and space-based telescopes imminent that will begin to give us an idea how common such planets are. Hoping for the best, we assume Earth-sized worlds in relatively comfortable places are common and even extend our search from G and K-type stars to the much dimmer (and more numerous) M-dwarfs. But what do we mean by a terrestrial planet? Size is an obvious criterion, but so is placement in the kind of habitable zone we would find conducive to our kind of life. That means liquid water at the surface. So far so good, but keep a sharp eye on the wild card in all this: Orbital ecccentricity. It's a measure of how far the orbit of a planet deviates from a circle, and we need to know more about it. Obviously a highly eccentric orbit could swing a planet through the habitable zone and right back out again, never...

Neon isn't an unusual find in spectroscopic studies of massive stars or, for that matter, in observations of novae or the galactic core. Energetic X-ray or ultraviolet emissions can ionize the gas, at which point it produces infrared light at characteristic wavelengths. Not expecting to find it around low-mass stars like our Sun, researchers have been surprised to find four Sun-like stars showing neon in their disks as measured by a Spitzer Space Telescope project run by the University of Arizona. That project, called Formation and Evolution of Planetary Systems (FEPS) is run out of Steward Observatory. The idea is to study planet-forming gas around 35 young, solar-type stars. Before this work, none of these stars would have been thought energetic enough to radiate the amount of X-ray and ultraviolet light needed to ionize neon. Unexpected though they might be, the observations are useful because neon, while hardly abundant, offers a precise spectral signature that makes it easier...

I, for one, wouldn't want to be around to witness what happens when the Earth is faced with an ever expanding Sun that has exhausted its hydrogen fuel. Conventional wisdom has it that the planet will likely be engulfed by what will then become a red giant. Certainly Mercury and Venus will, and the Earth's orbit is close enough that it may meet the same fate. But it's intriguing to learn that other outcomes are possible. Thus news out of Iowa State that the planet known as V 391 Pegasi b has evidently survived just such an encounter with its own star. Larger than Jupiter, the distant world in the constellation Pegasus was once situated at roughly the same distance from its parent that the Earth is from the Sun. That distance has changed over time as the star lost its outer regions in the helium flash, the onset of helium fusion that is produced as hydrogen is exhausted and contraction heats the stellar core. Image: An artist's conception of V 391 Pegasi b as it survives the red giant...

Figuring out planetary habitable zones gets a little less theoretical when we start talking about known systems. And when that system is Gliese 581, the interest level rises considerably. After the initial announcements of a possibly habitable planet around that star, Gliese 581c was later analyzed (in a paper by Werner von Bloh and team) as being too close to its star for liquid water to exist. But another planet, the more distant GL 581d, seemed to hold distinct promise of being in the habitable zone. Now a new paper tackles the question with intriguing results. Petr Chylek and Mario Pérez (Los Alamos National Laboratory) find some reason to think that both inner planets in this system may, under special but feasible conditions, have become suitable for life. The thinking here depends upon analyzing planetary environments as they evolve, with reference to our own Solar System in terms of that evolution. Start with this: Early on, Venus, Earth and Mars lost their original,...

We've found our share of unusual planets in the short time since actual observations could be made. A decade ago, it would have been hard to come up with anything more unexpected that a 'hot Jupiter,' orbiting so close to its parent star that its orbital period is measured in scant days. Add in 'super Earths' around dim red dwarfs and pulsar planets (actually the first type of exoplanets to be discovered) like those around the pulsar PSR 1257+12, and you have a bestiary of odd objects in the making. And now an outburst of gamma and X rays from the direction of the galactic center, one first detected with the Swift satellite's Burst Alert Telescope, gives promise of yet another kind of object. Pulsing in X rays 182.07 times per second, the source is clearly a 'millisecond' pulsar, a neutron star spinning at fantastic rates. Precise studies of the X-ray timing data have revealed the existence of a low-mass companion with a minimum mass of seven Jupiters, but there is wide play in that...