Habitable zones, and our idea of what constitutes them, change over time. We know, for example, that the habitable zone around a given star should migrate outward as main sequence stars become brighter with age. Thus the notion of the 'continuously habitable zone' (CHZ) has emerged, the region where a planet remains in habitable conditions for a specified period of time. If you want to look for technological civilizations, that time frame might be 4 billion years, paralleling the experience of life on our own planet. If you're content to look for microbes, as little as a billion years might suffice, perhaps less. Among the numerous factors involved in creating the CHZ, ultraviolet radiation is significant. A new paper points out the need to assess UV and the limits it places upon emerging biospheres. Get too much of it and you inhibit photosynthesis, as well as damaging DNA and various proteins. Get too little and you dampen a primary energy source for the synthesis of biochemical...

Three new planets have just been announced, but contrary to expectations, the number of planetary detections has not been picking up in recent times. The peak rate was 34 planetary discoveries in 2002, with the years since showing about 25 planets per year. There are a number of reasons for the slowdown, among them the fact that readily detectible short-period planets for the most obvious candidate stars have already been detected, and it will take years for enough data to accumulate to snare the presumably numerous outer planets in these systems. Greg Laughlin's Systemic site provides the details. But we do have a nice set of three new worlds delivered to us courtesy of the radial velocity method, which remains the primary detection scheme as we tune up transit and microlensing searches. HD 224693 offers a 0.7 Jupiter mass world in a 27-day orbit and HD 33283 shows a Saturn-class world in an 18-day orbit. HD 86081 is the most intriguing; it seems to have a 1.5 Jupiter mass planet in...

Recent exoplanet detections like the 'super Earth' found orbiting a red dwarf 9000 light years away have put the spotlight on gravitational microlensing. The phenomenon occurs when light from a background star is deflected by the gravity of an intervening object; in other words, one star passing quite near or in front of a far more distant one (as seen from Earth) will cause a lensing effect that can be studied. We've seen that a distant quasar can be lensed by a foreground galaxy, producing eerie, multiple images of the same quasar. But things get trickier when it comes to microlensing within our own galaxy using individual stars. We can't resolve the images created by these events with current telescopes, but the lensing does produce a measurable amplification of the distant star's light. And any planets in orbit around the intervening star can perturb that lensing effect enough to signal their presence. The beauty of this is that microlensing is sensitive to planets down to...

A brief heads-up on the ongoing work on 51 Peg, which Greg Laughlin and team are studying to see if additional planets can be found in the voluminous data. 51 Peg, remember, was the first example of an extrasolar planet being found around a main-sequence star. The dataset goes back ten years and is far more extensive than those used in most other planet discoveries via radial velocity measurements. Using the Systemic Console, Laughlin sees evidence of a possible second planetary companion to 51 Peg, and it's a beauty: a Saturn-class world in an Earth-like orbit. "Does it really exist, this room-temperature Saturn?," Laughlin writes. "Is it really out there? Do furious anticyclonic storms spin through its cloud bands? Does it have rings? Does it loom as enormous white crescent in the deep blue twilight sky of a habitable moon?" 51 Peg c is a breaktaking, beautiful thought, but Laughlin is quick to caution that this cannot yet be described as an exoplanet discovery. Plenty of work...

It was not all that long ago that binary star systems were thought to preclude planets, and I can remember reading as a boy that stars like Tau Ceti and Epsilon Eridani were far more likely to have planetary systems than the close binaries Centauri A and B. Now, of course, all that has changed, and we know from theoretical work that stable planetary orbits are possible around both Centauri A and B, though naturally constrained to orbits in the inner systems of both (which includes, satisfyingly enough, their habitable zones). But what can we make out not just from theory but observation? A new study of the 131 planetary systems detected by radial-velocity measurements (as of July 1, 2005) has come up with interesting results. 23 percent of these exoplanetary systems have stellar companions. Many of these had been recognized before as binary systems, but the international team behind this work, led by Deepak Raghavan and Todd Henry (Georgia State) also found six stars in five systems...

Imagine the supernova leading to the formation of a pulsar. Also called a 'neutron star,' a pulsar is the remnant that survives the catastrophe. The explosion of a star at least 1.4 times the mass of the Sun leaves behind an object with a diameter of 10 miles, so dense that a teaspoon of its matter would weigh about two billion tons. It's hard to see such a pulsar as the forming ground for a new planetary system. But we know that such things happen, as at least one pulsar planetary system has already demonstrated. The pulsar PSR B1257+12 was found to have three planets orbiting it back in the early 1990s, two of them the size of the Earth. These were the first exoplanets ever discovered, and it seemed even then that they must have been created out of some kind of debris disk. After all, any planets originally orbiting a star that goes supernova will doubtless be incinerated, so pulsar planets are not survivors but entirely new worlds. Image: This artist's concept depicts the pulsar...