We always have to watch our preconceptions, an early one in the exoplanet game being that solar systems around other stars would look pretty much like our own. Then we started the whole exoplanet discovery binge by finding planets around a pulsar, of all things, and went on to the terrifically odd world of 'hot Jupiters,' whose existence had not been predicted by most theorists. Now we've gotten used to the idea that solar systems come in huge variety, but finding one that looks more or less like ours would still be comforting, and would make it seem more likely that there are other 'Earths' out there, perhaps teeming with life. Today and tomorrow we look at two such finds, noting the resemblance to what we have around Sol and pondering the implications for the broader planet search. First up is not a single but a double planetary find, two worlds that inhabit a place much like that of Jupiter and Saturn in our Solar System. Not only is this an intriguing discovery in itself, but...

We have a long way to go in the study of circumstellar disks, especially around smaller stars. Given the difficulty of making such observations, work at the Subaru Telescope has focused on stars more massive than the Sun in hopes of studying the more apparent structure of the disks around such stars. But FN Tauri is an exception. The young star is a tenth of the Sun's mass, its disk seven times lighter than the lowest mass disk previously imaged, which was around the star TW Hydrae. The hope is to extend our knowledge of planetary formation more broadly across stellar types to learn what kind of worlds they form and where. The team of Japanese researchers performing this work used the Coronagraphic Imager with Adaptive Optics (CIAO) at the Subaru Telescope. What they've learned about FN Tauri is that the thick, roughly circular disk, with a radius of 260 AU, is relatively featureless at this point in the star's evolution (FN Tauri is thought to be a mere 100,000 years old). Thus far...

When you have assets in space, the thing to do is redeploy them as needed. That creates what's called an 'extended mission,' and the latest spacecraft to get one is Deep Impact, the vehicle whose impactor made such a splash when it was driven into comet Tempel 1 in the summer of 2005. That July 4 explosion was memorable enough, but under the name EPOXI the doughty craft leaves its vaporized impactor behind and moves on to two other missions, one of which has direct extrasolar applications. For one of EPOXI's twin goals is to observe five nearby stars known to have transiting exoplanets. Observations began on January 22. The 'hot Jupiters' around the five stars have been confirmed previously, but EPOXI's mission is to see whether any of these transiting gas giants is accompanied by other worlds in the same stellar system. Perhaps the most intriguing aspect of the investigation is summed up by Drake Deming (NASA GSFC): "We're on the hunt for planets down to the size of Earth, orbiting...

Watching two suns over Tatooine's sky in the original Star Wars movie was a breathtaking experience, particularly given where most science fiction films were at the time. Here was an attempt to convey a truly alien landscape. But a second thought quickly came unbidden. Was this planet not in an extremely unstable orbit, moving around both stars simultaneously in an obvious habitable zone? The suspicion was that a planet could orbit one or the other members of a binary system, but surely not both unless its orbit were extended so far out into the planetary nether regions as to make life doubtful. Image: The twin suns of Tatooine. Are planetary orbits like this possible? Credit: © Lucasfilm Ltd. & TM. All Rights Reserved. That was back in the 1970s, of course, but take a look at the situation today. The 'hot Jupiter' in the triple system HD 188753 is interesting, but the planet in question orbits but one of the stars. The early discussion of HD 188753 Ab was quick to raise the Tatooine...

We're still arguing about how giant planets form around Sun-like stars, but terrestrial planets seem to be less controversial. Assuming the model is right, we start with a swarm of planetesimals in the range of one kilometer in size. As these objects grow, out to a range of at least 2 AU, the largest bodies at some point go through a runaway period of chaotic growth marked by collisions. Emerging from the debris should be terrestrial worlds, some in Earth-like orbits. Add to this the fact that gas and dust disks seem to be relatively routine outcomes of star formation and you have an indication that small rocky planets may be widespread. The problem with all this is that theory has to be matched with observation. On that score, new work by Mike Meyer (University of Arizona) and colleagues Lynne Hillenbrand and John Carpenter (California Institute of Technology) is instructive. The researchers chose to look at mid-range infrared emissions at the 24 micron level, a range chosen because...

The closest we've come so far to identifying Earth-like planets around other stars is in the identification of so-called 'super Earths.' Calculations designed to model the composition of such planets say that worlds up to about ten Earth masses are rocky rather than gaseous. Some of these, as we have in the case of Gliese 581, have even excited interest in their possible habitability. We'd like to find ways beyond the now conventional radial velocity and transit studies to identify more such worlds. Now a new planet may have been found around GJ 436, a red dwarf already known to host a Neptune-mass planet in a tight 2.6 day orbit. This is interesting work because of the methods used. Ignasi Ribas (Institut de Ciències de l'Espai, Spain) and team have taken a close look at the known planet and are arguing it is possible to identify a second world, a super-Earth, through the telltale variations in the transit duration of GJ 436b, the already known 'hot Neptune.' Giving the game away is...

Planets around other stars are too faint to be imaged directly, and although claims have been made for such detections (2M1207b is a case in point), it's safe to say that our current techniques need significant upgrading to achieve reliable images of such distant worlds. But studying terrestrial planets is a long-term objective and numerous studies have gone into concepts like Terrestrial Planet Finder and Darwin. One day and with some instrument we will indeed be looking at an exoplanet as small as the Earth, working with estimates of surface temperatures and checking its atmosphere for biomarkers that flag the presence of life. So let's suppose that in fifteen years or so we're looking at actual reflected light from a terrestrial world. What else can we learn about the place? The brightness of a planet like this can be affected by many things, including the presence of deserts on the surface or bright clouds above it. An active weather pattern would indicate the presence of a...

I'm late getting to this one, because I wanted to get Mike Gruntman's paper on interstellar instrumentation finished. But for exoplanet enthusiasts like myself, the best news to come out of the recent American Astronomical Society meeting may have been the announcement of a new planet around the star HD 74156. So let's talk about it, an interesting find because we haven't had a new planet turn up just where predicted since Urbain Le Verrier and John C. Adams (independently) worked out the existence of Neptune by noting its effects on the motion of Uranus. Thus were calculations turned into observations and thence discovery. Rory Barnes (University of Arizona) has been working on a theory that led to the HD 74156 discovery for some time. His computer simulations (begun with Thomas Quinn while both were at the University of Washington) on the stability of extrasolar planetary systems showed a key fact: All systems whose planets were close enough to affect each other gravitationally...

Another youthful star makes the news today, the eight million year old HR 4796A in Centaurus, some 220 light years from Earth. As we saw yesterday, we have much to learn about how planets form around young stars. This one hasn't yielded a planet, but its dust disk, discovered in 1991, seems to derive from a planetary system in formation, the evident product of collisions between small bodies called planetesimals. The latest work on HR 4796A draws on observations made by the Near-Infrared Multi-Object Spectrometer aboard the Hubble Space Telescope. The spectra that John Debes and Alycia Weinberger (Carnegie Institution, Washington) studied in visible and infrared light scattered by the star's disk look red and imply the existence of the large organic carbon molecules called tholins. These are organic aerosols, complex molecules that, on Titan at any rate, remain suspended in the atmosphere and may contain chemical precursors to life. Image: Red and near infrared wavelengths from the...

People seem to be getting younger all the time. I'm told this is a common perception as you get older. In any case, it wasn't so long ago that I met the son of an acquaintance at an informal gathering. He looked to me to be about fourteen years old, but something warned me not to assume this. I said "What do you do? Are you in school?" His reply: "No, I've got my own dental practice downtown." I don't know how old you have to be to become a dentist, but I do know it's a lot older than fourteen! Exoplanets and the stars they circle, on the other hand, seem to be mostly of a certain age, the denizens of relatively mature systems. Which is why TW Hydrae is so interesting. It's an infant in stellar terms, at eight to ten million years old only a fraction of the Sun's age. Like other stars in its age group, it is surrounded by a circumstellar disk of gas and dust, the sort of place where planets can form. And indeed, what seems to be the youngest planet yet detected has now been located...

Apropos of Saturday's article on the Subaru Telescope -- and the optical improvements that may help it detect an exoplanet image -- comes news that scattered light from a planet orbiting another star has been detected by an international team. The planet is our old friend HD189733b, some sixty light years away in the constellation Vulpecula. The transiting world is a 'hot Jupiter,' orbiting its star in 2.2 days at a blindingly close 0.03 AU. Thanks to Hans Bausewein for passing along the link to a news release on this work. The scientists involved -- Svetlana Berdyugina (ETH Zurich & Tuorla Observatory), Andrei Berdyugin and Vilppu Piirola (Tuorla Observatory), and Dominique Fluri (ETH Zurich) did their study using polarimetry, examining starlight that is scattered in the distant planet's atmosphere. Polarimetry measures the angle of rotation of the plane of polarized light that occurs when it moves through particular materials. It's a useful technique for exoplanetary work because...

How much information can you extract from a single pixel? That's a key question for exoplanet studies as we look to the day when advanced telescopes can actually see a planet orbiting another star. But a single point of light seems to offer scant value, which is where Enric Pallé (Instituto de Astrofísica de Canarias) and colleagues go to work. They've been looking at how that single pixel changes over time, and what we might glean from it in terms of planetary details. A key factor is cloud cover. Using data from Earth's weather satellites, the scientists have been able to discover consistent patterns associating clouds with arid or rainy landmasses. Pallé explains: "The trick lies in interpreting the movement of the Earth's surface and the clouds as periodical signals, just as if we were to observe the spots on a spinning ball appearing and disappearing...[O]n a global scale clouds aren't as random and chaotic as is generally believed, but instead follow a pattern...

How long did it take for the planets in our Solar System to form? Much depends upon the surface density of the solar nebula protoplanetary disk, the gas and dust from which the planets emerged. And the problem with surface density -- mass per area -- in these settings is that it's hard to observe with our current instrumentation. Looking at distant systems in the process of formation, we see mostly dust and miss larger objects. Thus an estimate based on known factors is called into play. It produces the so-called minimum mass solar nebula. Using it, scientists can estimate solar nebula mass by starting with the rocky components of each planet, adding hydrogen and helium until the composition resembles that of the Sun. Spread that mass over the area of each planet's orbit and you get disk masses that look like what we see in systems around other stars. But there's a problem. The low surface densities this model produces aren't sufficient to allow the planets to form in a reasonable...

Gliese 581 is back in the news with a flourish. Astronomy & Astrophysics is publishing two independent studies of the system asserting that at least one of the inner planets is indeed located within the habitable zone of that star. Gliese 581 c and d are noteworthy every time they're mentioned. Of five and eight Earth masses respectively, they are the first exoplanets ever considered serious candidates for habitability. M dwarfs have inherent problems in terms of habitability, not the least of which is the tidal lock that planets in the HZ of such stars presumably experience, keeping one side perpetually dark. But models of atmospheric circulation exist that overcome that obstacle, and the intense magnetic activity of early M star life (producing dangerous flares) is no longer considered a necessary disqualifier for all forms of life. It's going to be a while before we have any definitive answers, but current thinking is that habitable M dwarf planets are very much in the picture,...

We're learning more and more about HD 189733b, an extrasolar planet some 63 light years from Earth in the direction of the constellation Vulpecula. This transiting 'hot Jupiter' orbits once every two days about three million miles out from its primary. David Charbonneau (Harvard-Smithsonian Center for Astrophysics) and team recently measured an unusual spectrum from the planet's atmosphere using the Spitzer Space Telescope. Looking for water, carbon dioxide and methane, they found instead a flat spectrum that Charbonneau thinks may indicate the presence of dark silicate clouds. Now we have further work, this time using Hubble Space Telescope data, that points to the presence of haze in the atmosphere of HD 189733b. That's an interesting finding to which we can add another result: Studying how light varies when the planet makes its transit indicates that this world has neither Earth-sized moons or a discernible ring system. Moreover, we've got a fairly good read on the temperature of...

With Hubble's Space Telescope Imaging Spectrograph now out of commission, the study of exoplanetary atmospheres becomes a bit more problematic. But Seth Redfield (University of Texas at Austin) has now used a ground-based instrument to detect the atmosphere of a planet orbiting the star HD189733, some 63 light years away in the constellation Vulpecula. Discovered in 2004, this transiting world is about twenty percent more massive than Jupiter, orbiting its parent ten times closer than Mercury orbits our Sun. Working from the ground is tricky but the odds go up when you observe more than a single transit. Redfield worked with eleven transits observed over the course of a year, using the Hobby-Eberly Telescope (HET) at McDonald Observatory in Austin. Studying the chemical composition of a distant atmosphere involves taking a spectrum during a transit and another when no transit is occurring. Working with the difference and comparing results over multiple transits helps you put together...

Since we've just been looking at young stars -- protostars, at that -- the news from Ann Arbor seems timely. Astronomers at the University of Michigan are announcing systems around UX Tau A and Lk Ca 15, young stars each, located about 450 light years away in the Taurus star formation region. What they're actually observing at infrared wavelengths are gaps in the protoplanetary disks around these stars, the assumed result of planets sweeping the area clear of debris. Unlike the infant star-in-the-making we looked at yesterday, UX Tau A and Lk Ca 15 are old enough -- about a million years each -- for planetary formation. Both are still pre-main sequence, deriving their energy from gravitational contraction instead of hydrogen-to-helium burning. To reach any conclusion about what's happening around them, the Michigan team has to rule out photoevaporation, which is what happens when the dust and gas of a protoplanetary cloud heats up, evaporates and begins to dissipate. Catherine...

A flattened envelope of gas and dust surrounding the young protostar L1157 gives us some idea of what our Solar System may have looked like as it began to form. The object is only a few thousand years old, the central star hidden, with its envelope detectable in silhouette as a black bar. The view from the Spitzer Space Telescope (below) shows how infrared can look within the dust to see structure. While the telescope cannot penetrate the envelope (itself hard to see in this image), enormous jets whose hottest points appear in white are clearly defined. These jets are interesting. They're being emitted from the protostar's two magnetic poles, and are approximately one and one half light years from end to end. The envelope of material is too thick for Spitzer to penetrate and appears in black, its thickest part visible as a black line crossing the jets. The envelope is roughly centered on the polar jets and perpendicular to them, showing up more clearly in the grayscale image below,...

One thing we'd like to know about exoplanets is where they are likely to be found. We've located more than 250 of them, but most are confined within about 650 light years. That's very much in the local neighborhood by galactic standards -- our methods have led us to nearby, bright stars. We do have a small number of planets detected through microlensing, some as far away as 6000 parsecs (about 19,500 light years), but our radial velocity detections, which form the great bulk of the current catalog, tend to be confined to relatively close higher mass stars. Other similarities? The exoplanet host stars we know about are generally metal rich. And because they're nearby, they're located in the galactic disk. This leaves us with some key questions, among them whether planets are equally abundant elsewhere in the galaxy. Other issues: Do planets occur with the same frequency around lower mass stars? Does the presence of heavy elements favor particular parts of the galaxy for planet...

Having just written about dust formation around HD 23514, a Sun-like star in the Pleiades, I was drawn to this quote by Nadya Gorlova (University of Florida, Gainesville), whose recent work suggests that if moons like our own were common, we'd be seeing more dust than we do around other stars. "When a moon forms from a violent collision, dust should be blasted everywhere," says Gorlova. "If there were lots of moons forming, we would have seen dust around lots of stars -- but we didn't." By contrast, the UCLA study on the Pleiades sees major collisions as common in young solar systems, though to be sure it didn't focus its conclusions on the 30 million year age range, as the Florida study did. Gorlova's team used data from the Spitzer Space Telescope and operated under current assumptions about lunar formation, in which an impactor the size of Mars is thought to have struck the Earth, creating a vast debris field that fell into Earth orbit and eventually became the Moon. The theory...