People sometimes ask why we are spending so much time searching for planets that are so far away. The question refers to the Kepler mission and the fact that the distance to its target stars is generally 600 to 3,000 light years. In fact, fewer than one percent of the stars Kepler is examining out along the Orion arm are closer than 600 light years. The reason: Kepler is all about statistics, and our ability to learn how common exoplanets and in particular terrestrial planets are in the aggregate. The last thing the Kepler team is thinking about is targets for a future interstellar probe. Studies of closer stars continue -- we have three ongoing searches for planets around the Alpha Centauri stars, for example. But there is so much we still have to learn about the overall disposition of planets in our galaxy. New work by an international team of astronomers involves gravitational microlensing to answer some of these questions, and the results suggest that planets -- even warm,...

The annual meeting of the American Astronomical Society is now in session in Austin, sure to provide us with interesting fodder for discussion in coming days. Just coming off embargo yesterday was news of further study of the interesting Kepler-16 system. This one made quite a splash last fall when the planet known as Kepler-16b was discovered to orbit two stars, with the inevitable echoes of Star Wars and the twin suns that warmed the planet Tatooine. This planet, though, was a gas giant more reminiscent of chilly Saturn than a cozily terrestrial world. Image: An artist's conception of the Kepler-16 system (white) from an overhead view, showing the planet Kepler-16b and the eccentric orbits of the two stars it circles (labeled A and B). For reference, the orbits of our own solar system's planets Mercury and Earth are shown in blue. New work out of the University of Texas at Arlington explores the question of habitability in a system like this. Credit: NASA/Ames/JPL-Caltech. You'll...

If you want a glimpse of how remarkable technology continues to transform the exoplanet hunt, look no further than the Subaru telescope and its SEEDS project. SEEDS (Strategic Exploration of Exoplanets and Disks with Subaru Telescope/HiCIAO) works with data from the 8.2-meter telescope that the National Observatory of Japan runs on Mauna Kea (Hawaii). In its most recent finding, Subaru was used with the adaptive optics system HiCIAO (High Contrast Instrument for the Subaru Next Generation Adaptive Optics) to image the dust ring around HR 4796 A, a relatively young star (8-10 million years old) some 240 light years from Earth. Working with evanescent clouds of dust and debris over these distances is no easy matter, but adaptive optics can correct for atmospheric blurring to produce images that rival the Hubble Space Telescope's in terms of clarity. Add to that an advanced image processing technique called angular differential imaging, which suppresses the glare of the central star and...

The most interesting thing about the worlds known as KOI 55.01 and KOI 55.02 is not just the fact that they are -- if current thinking holds -- the smallest planets yet detected around an active star other than our Sun, but that they are evidently survivors of the most extreme kind of experience. KOI 55, their host star, is of subdwarf B class, the exposed core of a red giant that has lost most of its gaseous envelope. The two planets that circle it are in such tight orbits that they would have been engulfed when the central star went through its red giant expansion. What a scenario, one we've often contemplated in these pages as we look toward the future of our own Sun. We tend to think in terms of planets that survive the red giant phase by orbiting far enough from the primary not to be swallowed up in it -- smaller worlds like Mercury, Venus and the Earth would not survive the experience. But KOI 55.01 and KOI 55.02 evidently were swallowed, and probably represent the remains of...

I'm delighted that we keep finding solar systems so different from our own. The discovery of two new planets that are roughly the size of the Earth just confirms the feeling -- in a galaxy of dazzling fecundity, every system we look at has its own peculiarities to instruct and delight us. The system around the star called Kepler-20 (from its designation by the space observatory studying planetary transits) is a case in point. Yes, it has small, rocky worlds, but it also has three larger planets, and all five orbit closer than the orbit of Mercury in our own system. Kepler-20 is a G-class star somewhat cooler than the Sun located some 950 light years from Earth in the constellation Lyra. Moreover, while we once assumed that smaller planets orbited close to stars while larger gas giants orbited further out in the system (again based on our own system and our assumptions about it), our new discoveries point to different scenarios. In Kepler-20 we have a system where the larger planets...

So-called 'super-Earths,' planets larger than the Earth but smaller than Neptune, pose problems to our theories of planet formation. The most recent illustration of this came in the announcement that the candidate planets found by Kepler had now reached 2,326. Remember, many of these will not be confirmed -- they're candidates -- but taken in the aggregate, what is interesting here is that one-third to one-half of these candidates fit the super-Earth category. And just as we had a problem with 'hot Jupiters' in trying to figure out their orbital position, many of these new planets are likewise in orbits close to their parent star, where the models say they shouldn't be. Things seemed so much simpler when we just had a single solar system to worry about, our own. Then, the idea of core accretion could readily account for everything we saw. The dust in the protoplanetary disk was thought to have aggregated into small planetesimals which, in the course of time and numerous collisions,...

It's fun to see Kepler-22b -- an intriguing new world that lies 600 light years from us toward Lyra and Cygnus -- being referred to as the 'Christmas planet' in the newspapers this morning, the latter a nod to Kepler chief scientist William Borucki, who said he thought of the planet that way, as a seasonal gift to the team. Borucki's enthusiasm is understandable, and it's echoed by Geoff Marcy (UC-Berkeley), who called the Kepler-22b work a 'phenomenal discovery in the course of human history.' I can't argue with scientists of this calibre -- with a surface temperature not so different from an April afternoon where I live, Kepler-22b can lay claim to being the smallest planet we've found orbiting in the habitable zone of a star like our Sun. The host star is, in fact, a G5-class object with mass and radius only slightly less than that of our Sun, which is a G2, and the planet in question orbits it with a period of 289 days, some 15 percent closer to its star than we are to ours....

Our notions of habitability are built around environments like our own, which is why the search for planets with temperatures that support liquid water at the surface is such a lively enterprise. But as we saw yesterday, it is not beyond possibility that many places in our Solar System could have sub-surface oceans, even remote objects in the Kuiper Belt. And that raises the question of how we assess astrobiological environments, an issue studied by Dirk Schulze-Makuch (Washington State University) and Abel Mendez (University of Puerto Rico at Arecibo), working with an international team of researchers in a paper suggesting a new approach. Schulze-Makuch makes the situation clear: "Habitability in a wider sense is not necessarily restricted to water as a solvent or to a planet circling a star. For example, the hydrocarbon lakes on Titan could host a different form of life. Analog studies in hydrocarbon environments on Earth, in fact, clearly indicate that these environments are...

Although I can't make the journey just then, I wish I could attend an upcoming conference at Arecibo (Puerto Rico) called Planets around Stellar Remnants. The meeting takes place twenty years after the discovery of the first exoplanets, the worlds orbiting the millisecond pulsar PSR B1257+12. I've been interested in the fate of planets around older stars ever since reading H.G. Wells' The Time Machine as a boy and encountering his image of a swollen, dying Sun. We also have interesting questions to ask about the kind of planets that exist around white dwarfs, and whether new planets (and chances for life) may eventually occur in their systems. It's appropriate that the conference be organized by Penn State, for it was that university's Alexander Wolszczan, working with Dale Frail of the National Radio Astronomy Observatory, who made the discovery of those two and possibly three planets that launched the modern exoplanet era. Nor has Wolszczan slowed in his efforts. The most recent...

Long-time Centauri Dreams readers will be familiar with the work of Manoj Joshi and Robert Haberle. Back in the 1990s when both were at NASA Ames (Joshi is now at the University of Reading), the scientists went to work on the question of whether planets around red dwarf stars could be habitable, given the problem of close orbits and tidal lock. Simulating the atmosphere of such a planet, they found even a thin atmosphere would circulate globally, moving enough heat to prevent the air on the darkside from freezing out. The prospect of a planet with oceans and a climate mild enough to support life began to look more promising. Joshi and Haberle have a new paper out that looks once again at planets around red dwarfs, this time extending the possible habitable zone to a greater distance from the star. M-class red dwarfs are smaller and cooler than G-class stars like the Sun, and emit a much larger fraction of their radiation at longer wavelengths where the reflectivity of ice and snow...

Imaging an Earth-like planet in the habitable zone may happen some time in the next decade if the James Webb Space Telescope can make its way through its budgetary hurdles and achieve a 2018 liftoff. But the word 'imaging' is a bit deceptive when you consider that we won't be getting anything remotely like the view of a planet in our own Solar System through Webb's instruments. No small disc with discernible features, in other words, but a single dot useful not because of what we can pick out visually, but because we can use its light to take a spectrum. And if we're really lucky, we'll find a dot that's blue and a spectrum that shows the signature of a living world. The JWST works at infrared wavelengths (covering a range from 0.6 to 28 micrometers), which is why shielding it from heat is so important, and why the design is marked by a large sunshield. These wavelengths should allow the instrument to study stars and galaxies from the early universe, but the addition of a starshade...

An orange dwarf star a bit smaller than our Sun is giving us valuable clues about how water-covered planets like Earth may evolve. TW Hydrae is 176 light years away, so young (5 to 10 million years) that it is still in the early stages of forming a planetary system. Working with data from ESA's Herschel space observatory, astronomers have found cold water vapor in the disc of dust and gas that surrounds the star. It's a significant find, because while we've found warmer water vapor in proto-planetary discs closer to their star, we now see evidence for much larger amounts of water in the outer disc, where the material for icy comets is found. Current theory holds that water will be far scarcer in the inner solar nebula around a coalescing system, meaning extensive oceans would have to be delivered by impacting objects from the outer regions. The Herschel data show the distinct signature of water vapor, probably produced when ultraviolet radiation from the central star warms ice-coated...

I've put off writing about Wesley Traub's paper on the frequency of planets in the habitable zone because I knew Adam Crowl had reservations about Traub's method. We talked about this at the 100 Year Starship Symposium, which led to Adam's agreeing to writing this piece for Centauri Dreams. How you define a habitable zone is, of course, a critical matter, especially when you're dealing with a topic as compelling as extrasolar planets that can support life. Adam places Traub's work in the context of earlier attempts at defining the habitable zone and finds HZ estimates different from Traub's, though one is surprisingly similar to a much earlier study. by Adam Crowl The recent paper by Wesley Traub [reference below] has estimated the frequency of terrestrial ("Earth-like") planets in the Habitable Zone (HZ) of their stars based on statistical analysis of the recent Kepler data release, but the frequency computed, of ~34(+/-14)% around FGK stars, is dubious due to the assumption of...

Three planets recently discovered through Kepler data provide an interesting take on how we look at smaller planets. Not that the planets around the star designated Kepler-18 are all that small -- two of them are Neptune-class and one is a super-Earth. But what is becoming clear is that given the state of our current technology, we'll have to get used to a process different from planet verification as we move to ever smaller worlds. The technique is being referred to as planet validation -- it helps us determine the probability that the detected object could be something other than a planet. Image: The orbits of the three known planets orbiting Kepler-18 as compared to Mercury's orbit around the Sun. Credit: Tim Jones/McDonald Obs./UT-Austin. The new system shows how this works. Kepler-18 is a star similar to ours, about 10 percent larger than the Sun and with 97 percent of the Sun's mass. Around it we have Kepler-18 c and d, which turn up through transits. Planet c has a mass of...

Because the financing for missions like Kepler is supported by tax dollars, it's gratifying to see the public getting actively involved in working with actual data from the Kepler team. That's what has been going on with the Planet Hunters site, where 40,000 users from a wide variety of countries and backgrounds have been analyzing what Kepler has found. Planet hunter Debra Fischer (Yale University), a key player in the launch of Planet Hunters, has this to say: "It's only right that this data has been pushed back into the public domain, not just as scientifically digested results but in a form where the public can actively participate in the hunt. The space program is a national treasure -- a monument to America's curiosity about the Universe. It is such an exciting time to be alive and to see these incredible discoveries being made." So far, so good on the citizen science front. Using publicly available Kepler data, Planet Hunters has found two new planets, both of them discarded...

The confirmation of a planet circling two stars, recounted in these pages yesterday, is actually the result of a long process. Jean Schneider (CNRS/LUTH - Paris Observatory) noted in a follow-up comment to the Kepler-16b story that investigation of such systems dates back to 1990 (see citation below), while Alex Tolley has pointed out that the great space artist Chesley Bonestell was painting imaginary planets orbiting binary stars fully sixty years ago. So the idea isn't new, but the confirmation was obviously useful, and in more ways than we might have expected. For one thing emerging from the Kepler-16b paper is that the smaller of the two stars in this binary system, an M-dwarf, is now the smallest low-mass star to have both its mass and radius measured at such precision. The question of stellar mass and M-dwarfs is significant because a new paper by Philip Muirhead (Cornell University) and colleagues goes to work on the parameters of low-temperature Kepler planetary host stars...

I'm just back from a long trip and am only now catching up on some of the news stories from late last week. Among these I should mention the discovery of the world with the double sunset, identified through Kepler data and reminiscent of the famous scene from Star Wars, where Luke Skywalker stands on the soil of Tatooine and looks out at twin suns setting. I remember carping about the scene when it first came out because it implied a planet that orbited two stars at once. Now we have confirmation that such a configuration is stable and that planets can exist there. Kepler-16 isn't a habitable world by our standard definition, but much more like Saturn, cold and gaseous. One of the stars it circles is a K dwarf of about 69 percent the mass of our Sun, while the other is a red dwarf of about 20 percent solar mass. Some 220 light years away in the direction of the constellation Cygnus, the system is fortuitously edge-on as seen from Earth, allowing Kepler scientists to identify the...

The largest variations in brightness ever seen on a cool brown dwarf have turned up on the brown dwarf 2MASS 2139 (known as 2MASS J21392676+0220226 to its friends). The findings, reported at the Extreme Solar Systems II conference in Jackson Hole, Wyoming, show a remarkable 30 percent change in brightness in a period of just under eight hours. The assumption is that brighter and darker patches of atmosphere are periodically moving into view as the brown dwarf rotates. In fact, Ray Jayawardhana (University of Toronto), co-author of the paper on this work, thinks one possibility is something similar to what we see in our own Solar System. “We might be looking at a gigantic storm raging on this brown dwarf, perhaps a grander version of the Great Red Spot on Jupiter in our own solar system,” says Jayawardhana, “or we may be seeing the hotter, deeper layers of its atmosphere through big holes in the cloud deck.” Image: Astronomers have observed extreme brightness changes on a nearby brown...

This is a big week for exoplanet news with the continuing presentations at the Extreme Solar Systems II conference in Wyoming. But I'm going to have to be sporadic with posts this week because of ongoing commitments. The papers for the upcoming 100 Year Starship Symposium are due within days, which is a major driver, but I've also got even more important matters unrelated to my interstellar work to attend to. I'll probably be able to get another post off this week, and then we can catch up a bit next week. For now, here's a story I want to get in that involves things we can't see. Remember 'Invisible Invaders'? This 1959 drive-in classic involved aliens you can't see in spaceships that are likewise transparent, arriving on Earth to take over the bodies of the recently deceased. John Agar and Robert Hutton spent a lot of this movie chasing a comely physicist (Jean Byron) when they weren't working out a way to foil the aliens' plans to take over our planet in three days. Knowing my...

With the online press conference re new results from the HARPS spectrograph (High Accuracy Radial Velocity Planet Searcher) now being discussed, I want to pause for a moment before getting into them to mention the ongoing Extreme Solar Systems II conference, which runs until the 17th at quite a venue, Jackson Lake Lodge in Wyoming. The tentative program is available online, with the welcome news of new HARPS and Kepler results and any number of intriguing talks on everything from debris disk imaging around nearby stars to core accretion models. We’ll doubtless be talking about some of these findings in coming weeks. But for now, on to the HARPS discussion at the Wyoming conference. The take-away quote from today’s news was this, from Michel Mayor (University of Geneva): “The harvest of discoveries from HARPS has exceeded all expectations and includes an exceptionally rich population of super-Earths and Neptune-type planets hosted by stars very similar to our Sun. And even better —...