Last Saturday's image of Proxima Centauri raised questions for several readers, who asked where Centauri A and B were in the photograph. The answer is that they are not in the field of view. To get a broader perspective, let's step back a bit. In the image below, I'm using a photograph taken by Noël Cramer at the Observatoire de Genève. I've cropped the image to show the relative position of the primary Centauri stars and Proxima Centauri. If you look to the upper right of the image, you'll see the tip of the red arrow that Cramer used to point to Proxima, which is otherwise indistinguishable. Now ponder the bright 'star' at lower left. It is actually not one but two stars, Centauri A and B. The two are so close to each other, and so close to us, that they effectively merge into a single image, which is why we talk about 'Alpha' Centauri -- it was once thought to be simply the single brightest star in the Centaur constellation. Now, of course, we know it is a triple system, with...

Astronomy can be a time machine, taking us back to the era when the light we are observing left its source. Looking at a galaxy ten billion light years away thus tells us what galaxies looked like in that epoch. But the dizzying number of stars in our galaxy alone also lets us see into our own past, by showing us stellar systems much like ours once was. Such a system is that around HD 12039, a Sun-like star about 137 light years away. By Sun-like, I mean a yellow G-type star, and that equates to surface temperatures between 5,000 and 7,000 degrees Fahrenheit. But HD 12039 is also much younger than the Sun, perhaps 30 million years old. In other words, it's about the age that the Sun was when the Earth and Moon probably formed. Like our Sun in that era, the star has not yet settled into the main sequence, which marks mature nuclear-burning. It's a bit brighter, a bit cooler, and a bit more massive than Sol. The interesting news comes when we consider HD 12039's debris disk. A team led...

Proxima Centauri is not exactly an imposing star. In fact, this tiny M-class red dwarf would not be noticeable even in the skies near Centauri A and B except for its huge parallax, an indication to local sky-watchers that it was in the vicinity and moving fast. Some astronomers have suggested that Alpha Centauri may, in fact, not be a triple-star system after all, that Proxima is simply an independent star passing through the neighborhood. But the jury is still out on that one. Image credit & copyright: David Malin, UK Schmidt Telescope, DSS, AAO. Nonetheless, the interest Proxima exerts is almost hypnotic, because at 4.22 light years, it is the closest of all known stars (Centauri Dreams leaves open the possible, and in my view likely, discovery of a red or brown dwarf even closer). The image above (click for a closeup) was selected by the good people at Goddard Space Flight Center as Astronomy Picture of the Day last week, and it's the best image I've ever seen of the tiny star....

Seeing a planet around another star means finding a way to mask the overwhelming glare that swamps the faint image. The job is, as a news release from the American Institute of Physics reminds us, something like trying to see the light of a match held next to an automobile's headlight from a distance of 100 meters. Consider that the Earth is ten billion times less bright than the Sun at optical wavelengths and you see the enormity of the problem. Among the possible solutions is an approach taken by Grover Swartzlander and his colleagues at the University of Arizona. Swartzlander eliminates excessive starlight by feeding it through a helical 'mask' -- a kind of lens. The result is what the team calls an optical vortex coronagraph. From the news release: The process works in the following way: light passing through the thicker and central part of the mask is slowed down. Because of the graduated shape of the glass, an "optical vortex" is created: the light coming along the axis of the...

The search for planets around small stars seems to be gaining in intensity. Now a team of French and Swiss astronomers has announced the discovery of a Neptune-class planet around the star Gliese 581. The Gliese catalog lists all known stars within 25 parsecs (81.5 light years) of the Sun, making its listings of significant interest in the hunt for exoplanets and, long-term, in our thinking about robotic interstellar probes. Also catching the eye is the fact that this star is a red dwarf, confirming the notion that such stars are ripe for exoplanetary investigation. Gliese 581 is 20.5 light years away in the constellation Libra. Red dwarfs like it make up about 70 percent of the galactic population; in fact, of the 100 stars closest to the Sun, fully 80 are red dwarfs. Earlier surveys of red dwarfs have revealed few with planets, but Stéphane Udry (Geneva Observatory), a co-author of the paper on the new find, believes the earlier surveys may have operated with insufficient...

As if we didn't already have enough trouble defining what a planet is, astronomers have now discovered a brown dwarf only eight times the mass of Jupiter. Surrounded by a dusty disk, the object is actually smaller than a number of planets already known to be orbiting other stars. Any miniature solar system that formed around the brown dwarf would be roughly 100 times smaller than our own. All of which raises the question of what to call objects that might be found around this tiny dwarf: planets or moons? The question has obvious resonance in an era marked by repeated discoveries in the Kuiper Belt that could be considered of planetary size. And another sign of the ambiguity in definition is that worlds like Titan, Ganymede and Callisto are large enough in their own right to qualify as planets, if we overlook the inconvenient fact that they orbit massive planets of their own. The question may seem insignificant, but how we define things is ultimately a measure of how extensive our...

M-class red dwarfs have never figured prominently in the SETI search. The reason for this is apparent: such stars, of which Proxima Centauri, Earth's nearest stellar neighbor, is one, are flare stars. The intense radiation from solar flares should cleanse a planetary surface of life, especially given the close proximity of such a planet to its star. Remember, the habitable zone around a red dwarf is going to occur well inside the orbit of Mercury. And there's a second reason. By virtue of having to orbit the host star so tightly, a planet around a red dwarf is going to be tidally locked. One side would be baked, the other frozen, which makes the odds on liquid water look slim. But assumptions are made to be questioned, which is why work at Ames Research Center in the late 1990s remains so interesting. One implication of the Ames work, for example, is that there are conceivable weather patterns that could circulate heat to the dark side of a tidally locked world, keeping it warm...

If you're going to have a conference on exoplanets, there is no better venue than l'Observatoire de Haute-Provence. It was here, just ten years ago, that Mayor and Queloz discovered the first planet orbiting a main sequence star outside our own Solar System. The star was 51 Pegasi, a name that will surely be recalled for generations as the first confirmation that planets exist around other stars. And attendees at a late August conference celebrating the discovery had much to say about the course of future exoplanetary developments. David Charbonneau (Harvard-Smithsonian Center for Astrophysics) summarizes the conference findings in his paper "Hot Jupiters: Lands of Plenty," now available on the arXiv site. A major issue stands out: the huge leap in precision for radial velocity observations of the sort that bagged 51 Pegasi's planet, allowing researchers to monitor a wider group of stars than the F, G, K and early M-class dwarfs that have been the focus heretofore. The new precision...

M-class red dwarf stars are of increasing interest in terms of astrobiology. If we can devise weather models that allow for regions of relative stability, a planet locked tidally to its star at a fraction of the distance from Mercury to the Sun could produce conditions suitable for life. The National Geographic show 'Extraterrestrial,' shown again the other night, projects just such an environment, and imagines life forms that might evolve there. But red dwarfs are tricky because they're flare stars. In their early lives, they spin more quickly than they will when they enter their dotage; the rapid spin can produce magnetic fields that, in turn, create flares. Life on a planet circling a younger red dwarf would have to adapt to flares that can double the star's brightness within a matter of seconds. Some believe this makes Proxima Centauri an unlikely candidate for life-bearing planets. And what about Barnard's Star, so tantalizingly close (5.9 light years) to our own Sun? Study a...

Centauri Dreams has been a champion of Webster Cash's New Worlds Imager for several years now. The proposal, whose initial study was funded by NASA's Institute for Advanced Concepts, offered a way to find terrestrial planets around other stars and, in its most fully developed configuration, to create startlingly sharp images of such worlds down to the level of continents and weather patterns moving across their surfaces. Now two new developments -- related in a phone call from Cash last week -- bring New Worlds Imager to the fore as NASA weighs strategies for its Terrestrial Planet Finder mission. First, Cash has changed the basic design of New Worlds Imager to move away from the enormous 'pinhole camera' concept discussed earlier in these pages to an occulter -- a design that blocks the starlight from the central star to allow its planetary companions to be visible. The problem with occulters has always been that no matter how scientists worked with their design, they could not get...

If brown dwarfs, those 'failed stars' that never make it to the stage of full nuclear burning, can have planets around them, then the speculations of Karl Schroeder's novel Permanence (New York: Tor Books, 2002) may be closer to reality than Centauri Dreams once thought. Schroeder imagines human colonies, artificially sustained through extraordinary technologies, on planets surrounding a variety of brown dwarf stars, an entire civilization of humans living in the spaces between the 'lit' stars we see in the night sky. Now the Spitzer Space Telescope has found the signs of early planet formation around six young brown dwarfs located some 520 light years away in the Chameleon constellation. Ranging in size from between 40 to 70 times the mass of Jupiter, the brown dwarfs are between 1 and 3 million years old. And five of them have disks made up of dust particles that are clearly sticking together, in what looks suspiciously like the early stages of planet formation. The astronomers...

It was in 1999 that former NASA administrator Dan Goldin spoke to the American Astronomical Society about what future telescopes might be able to see around distant stars. He imagined a classroom filled with images of exoplanets. "When you look on the walls, you see a dozen maps detailing the features of Earth-like planets orbiting neighboring stars," Goldin said. "Schoolchildren can study the geography, oceans, and continents of other planets and imagine their exotic environments, just as we studied the Earth and wondered about exotic sounding places like Banghok and Istanbul . . . or, in my case growing up in the Bronx, exotic far-away places like Brooklyn." Is a telescope that could take such pictures remotely conceivable? The most innovative proposal I've heard to achieve these goals is Webster Cash's New Worlds Imager concept. The University of Colorado at Boulder astronomer knows how tricky the project would be. As astronomer looking at the Earth from Alpha Centauri would face...

Of the 161 planets so far detected around other stars, eight have been discovered by the transit method as they moved between that star and the line of sight to Earth. Such transits, effective as planet finders in themselves, are also useful because they allow scientists to study the properties of the atmospheres around these worlds. The first planet found by transit methods orbits the star HD 209458 and is the object of intense atmospheric study. Can such methods be applied to transiting Earth-size planets? A new paper studies the question in terms of the kind of signatures that might be expected, and the near-term technologies that could make such detections possible. The paper focuses on terrestrial worlds orbiting K, G or F-type stars, and notes that the best targets will be K-type stars, which are in any case more abundant than the other types as well as smaller. According to the analysis, the strongest signatures in the atmosphere of such worlds could be water, ozone and carbon...

We've developed many techniques for planetary detection since the first discovery of a planet orbiting a main sequence star in 1995, but a recent addition to the repetoire is looking in systems already known to have planets. By studying stars that display a transiting planet -- a planet moving in front of the star as seen from Earth -- any variation in time between the transits can be detected. From that data, information about any unseen planet perturbing the transits can be inferred. The new method is called TTV, for 'transiting timing variations,' and here's the most exciting thing about it: a planet comparable to the size of Earth should be detectable using these methods, giving TTV a sensitivity in advance of any other current detection method. And we do expect to find many a multi-planet system out there, with data that can provide insights into the formation of our own Solar System. But to use the method we have to discover and monitor transiting planets. All of this is...

Brown dwarfs are often called 'failed stars,' objects without enough mass to sustain the hydrogen-to-helium fusion reaction that powers the Sun. They're dim enough that it was only in 1995 that the first brown dwarf, Gliese 229B, was discovered and the spectral classes L and T created to accomodate the category. The nearest known brown dwarfs are found around the star Epsilon Indi, a main sequence K-5 dwarf star; intriguingly, the first brown dwarf discovered in the system was subsequently found to have another brown dwarf orbiting it. Image: An artist's impression of a brown dwarf, a 'failed star' too cool to sustain nuclear fusion. Credit: Douglas Pierce-Price, Joint Astronomy Centre (Hilo, HI). Now a team from Arizona State University led by Russell Ryan has used date from the Hubble Space Telescope to look for brown dwarfs above and below the galactic plane, with an eye toward determining their total population in the Milky Way. The team's near infrared data tracked down 28 stars...

The Project Daedalus starship, designed by members of the British Interplanetary Society in the 1970s, was the first full-scale attempt to work out the parameters of a realistic interstellar mission. The target of this unmanned probe was Barnard's Star, a red dwarf some 5.9 light years from Earth. The Daedalus team leader, Alan Bond, made a key assumption: technology had reached the point where an interstellar mission could be designed without assuming any further breakthroughs in physics. The inertial confinement fusion techniques Daedalus would use have been the subject of much refinement in the days since and continue to be fertile ground for study. But the question I most often hear about Daedalus is, why Barnard's Star, when the Alpha Centauri system is considerably closer? The answer is addressed in an article by astronomer Alan Boss (Carnegie Institution of Washington) that appears on the Astrobiology Magazine Web site. And it involves a planetary detection that only failed...