With the question of habitable planets on my mind following Andrew LePage's splendid treatment of Kepler-452b on Friday, I want to turn to the interesting news out of San Diego State, where astronomer William Welsh and colleagues have been analyzing a new transiting circumbinary planet, a find that brings us up to a total of ten such worlds. Planets like these, invariably likened to the planet Tatooine from Star Wars, have two suns in their sky. Now we have Kepler-453b to study, a world that presented researchers with a host of problems. Transits of the new world occur only nine percent of the time because of changes in the planet's orbit. Precession -- the change in orientation of the planet's orbital plane -- meant that Kepler couldn't see the planet at the beginning of its mission, but could after it swung into view about halfway through the mission's lifetime, allowing three transits. Clearly, this is a system we could easily have missed, says William Welsh (San Diego State), who...

Where is the dividing line between a large, rocky planet and a 'mini-Neptune?' It's a critical issue, because life is at least possible on one, unlikely on the other. But while we're getting better at figuring out planetary habitable zones, the question of how large a planet can be and remain 'terrestrial' is still unresolved. As Andrew LePage explains below, our view of potentially habitable planets like Kepler-452b depends upon how we analyze this matter -- clearly, just being in or near the habitable zone isn't enough. A prolific essayist with over 100 articles in venues like Scientific American and Sky & Telescope, LePage writes the excellent Drew ex Machina site, where his scrutiny of recent exoplanet finds is intense. The work seems a natural fit given his day job at Visidyne, Inc. near Boston, where he specializes in the processing and analysis of remote sensing data. by Andrew LePage A couple of weeks ago, the media was filled with reports about the discovery of Kepler 452b....

Detecting planets around brown dwarfs is tricky business, but it's worth pursuing not only for its own sake but because planetary systems around brown dwarfs can tell us much about planet formation in general. A new paper from Andrzej Udalski (Warsaw University Observatory) and colleagues makes this point while noting four brown dwarf planets we've thus far found, all of them much larger than Jupiter. An extremely large planet well separated from a brown dwarf suggests a scaled-down binary star system rather than one growing out of an accretion disk. Fortunately, we can use gravitational microlensing to go after much smaller worlds around brown dwarfs, a method that is not compromised by the faintness of both planet and dwarf. In microlensing we don't 'see' the planet but can infer its presence by observing how light from a more distant star is affected as a brown dwarf system passes in front of it. Udalski and team have used microlensing to discover OGLE-2013-BLG-0723LB/Bb, which...

The discovery and confirmation of the exoplanet HD 219134b give us a useful touchstone relatively close to the Solar System. At 21 light years away in the constellation Cassiopeia, HD 219134b distinguishes itself by being the closest exoplanet to Earth to be detected using the transit method. That's useful indeed, because we'll be able to use future instruments like the James Webb Space Telescope to learn about the composition of any atmosphere there. Image: This sky map shows the location of the star HD 219134 (circle), host to the nearest confirmed rocky planet found to date outside of our solar system. The star lies just off the "W" shape of the constellation Cassiopeia and can be seen with the naked eye in dark skies. It actually has multiple planets, none of which are habitable. Credit: NASA/JPL-Caltech/DSS. Too close to its star to be considered a candidate for life, the new world is a 'super-Earth,' sighted by the HARPS-North instrument using radial velocity techniques, which...

I've come to dislike the term 'Earth 2.0.' It's not so much the idea of a second Earth as the use of 2.0, which in our technological era invariably recalls software updates. Windows 2.0 was better than Windows 1.0, but Windows 3.0 was the one that really took off -- the idea here is that progressive iterations improve the product. I'd rather see us use 'Earth 2' than 'Earth 2.0,' for the latter implies a new and improved Earth, and I'm not sure just what that would be. Speculating about that is, I suppose, a key activity of philosophers. But Earth 2.0 has stuck as a way of designating a planet much like our own. Here too we have to be careful. A planet with liquid water on at least parts of its surface might exist around a red dwarf, packed into a tidally-locked orbit and divided between a frigid night side and a day side with, perhaps, only a few zones where life might flourish. It's not Earth 2.0 because it has a star that never moves in its sky and its susceptibility to solar...

Not long ago we looked at a new paper from Alan Boss that modeled interactions in young protoplanetary disks (A Disruptive Pathway for Planet Formation). The idea here is that as dust grains and larger objects bump into each other on the way to forming planetesimals, a mechanism must exist to keep them from spiraling into their star. Boss' models show explosive phases in young stars that lead to gravitational instabilities of the sort needed to scatter these small objects outward and preserve their prospects for forming into planetesimals, and perhaps one day, planets. Watching infant solar systems form is akin to studying embryology in animal species, a chance to understand the myriad interactions that affect growth and set it in particular directions. Now we have work out of the University of St. Andrews, recently presented at the National Astronomy Meeting in Llandudno, Wales, that announces the discovery of a ring of small rocks circling the star DG Tauri, a 2.5 million year old...

As I write, we're thirteen and a half days out from the Pluto/Charon encounter. New Horizons will make its closest approach to Pluto at 0749 EDT (1149 UTC) on July 14. All of which has had me reading Pluto-related science fiction that I missed along the way, including most recently Wilson Tucker's "To the Tombaugh Station." The story, which ran in the July, 1960 issue of Fantasy and Science Fiction, is a murder investigation that includes a journey to the station of the title, which had been established to investigate a 'Planet X' still further out in the system. Isaac Asimov has an essay on Pluto in this issue as well. Image: The cover of the July, 1960 Fantasy and Science Fiction shows a generic moon landing scene by artist Mel Hunter. But if you look at it with our post-1978 (discovery of Charon) eyes, it could be seen as an imaginative take on a Pluto landing, with the Earth on the horizon being replaced by Charon. Given the prominence of "To the Tombaugh Station" on the cover, I...

Planet formation can be tricky business. Consider that our current models for core accretion show dust grains embedded in a protoplanetary disk around a young star. Mixing with rotating gas, the dust undergoes inevitable collisions, gradually bulking up to pebble size, then larger. As the scale increases, we move through to planetesimals, bodies of at least one kilometer in size, which are large enough to attract each other gravitationally. Some planetesimals break apart through subsequent collisions, but a few grow into protoplanets, then planets themselves. It's a reasonable theory that fits what we see around young stars as solar systems take hold. But what Alan Boss (Carnegie Institution for Science) has been working on is a question raised by the process: How do the dust grains and objects smaller than planetesimals keep from being drawn into the protostar before they can become large enough to attract the materials they need to grow? The pressure gradient of the gas in the disk...

Objects that seem younger than they ought to be attract attention. Take the so-called 'blue stragglers.' Found in open or globular clusters, they're more luminous than the cluster stars around them, defying our expectation that stars that formed at about the same time should develop consistent with their neighbors. Allan Sandage discovered the first blue stragglers back in 1953 while working on the globular cluster M3. Because blue stragglers are more common in the dense core regions of globular clusters, they may be binary stars that have merged, but a number of theories exist, most of them focusing on interactions within a given cluster. Image: The center of globular cluster NGC 6397, in an image taken by the Hubble Space Telescope. Credit: Francesco Ferraro (Bologna Observatory), ESA, NASA. Now we may have found a planet that seems to be younger than it ought to be. Michael Jura (UCLA) and team report on the results in the Astrophysical Journal Letters, making the case that a...

Astronomers at Penn State, NASA Ames, the University of Chicago and the SETI Institute are publishing news of an exoplanetary first: A planet smaller than Earth whose mass and size have both been measured. Kepler-138b is a Mars-sized world orbiting a red dwarf about 200 light years from Sol in the constellation Lyra. This is transit work, focusing on a system with two other transiting worlds, all three of which are too close to their parent star to make life a likely possibility. If we look back at how far exoplanet research has come in the last fifteen years, it's startling to realize that Kepler-138b, with a mass of about 6.7 percent that of the Earth, is 3000 times less massive than the first planet whose density was measured. That's the word from Eric Ford (Penn State), a co-author on the study, which is being published today in Nature, and I assume he's talking about HD 209458 b, whose size and density were first measured in 1999. Previous work on the Kepler-138 system had...

A question in a grad school astrobiology seminar at the University of Washington prompted Amit Misra to go to work on plate tectonics. The movement of huge blocks of a planetary surface is beneficial to life because it prompts recycling, as materials move back and forth between the inside of the planet and the atmosphere. We've learned a lot about plate tectonics on Earth, but the seminar question stuck with Misra. How could we detect plate tectonics on an exoplanet? The result is a paper in press at Astriobiology. Misra and colleagues make the case that transient sulfate aerosols produced by volcanic outgassing could provide just the signature scientists need. Explosive volcanic events produced by subduction at the edges of tectonic plates inject such aerosols directly into the atmosphere, where they can persist over periods of months to years. The paper argues that future instruments like the James Webb Space Telescope or the European Extremely Large Telescope (E-ELT) will be able...

The Scorpius-Centaurus OB association is a collection of several hundred O and B-class stars some 470 light years from the Sun. Although the stars are not gravitationally bound, they are roughly the same age -- 10 to 20 million years -- their formation triggered by a series of supernovae explosions in large molecular clouds. Now the Gemini Planet Imager on the Gemini South instrument in Chile has uncovered a young planetary system within the association, one with solid similarities to our own Solar System in its infancy. In fact, says lead author Thayne Currie (Subaru Telescope), the ring orbiting the star HD 115600 could be a Solar System clone. “It’s kind of like looking at [our] outer solar system when it was a toddler,” the astronomer adds, noting that the ring is about the same distance from its host star as the Kuiper Belt is from the Sun, receiving about the same amount of light from an F-class star that is about fifty percent more massive than our own G-class Sol. Image: The...

We've recently looked at the role of small spacecraft, inspired in part by The Planetary Society's LightSail, a CubeSat-based sail mission that launched last week. It's interesting in that regard to consider small missions in the exoplanet realm. ExoplanetSat, for example, is a 3-unit CubeSat designed at MIT as a mission to discover Earth-sized exoplanets around nearby stars. Here the beauty of the CubeSat is obvious: The platform is low-cost, the development time is relatively short, and there are frequent launch opportunities. Up to 100 ExoplanetSats are planned. Pulling big benefits from small packages is not new, as the example of the Canadian MOST mission (Microvariability and Oscillations of STars) reminds us. MOST was the first mission dedicated to asteroseismology, to be followed by CoRoT (COnvection ROtation and planetary Transits) and then Kepler. Now we have a proposal for what is being called the United Quest for Exoplanets (UniQuE), which grows out of work performed by...

The cascading numbers of exoplanet discoveries raise questions about how to interpret our data. In particular, what do we do about all those transit finds where we can work out a planet's radius and need to determine its mass? Andrew LePage returns to Centauri Dreams with a look at a new attempt to derive the relationship between mass and radius. Getting this right will be useful as we analyze statistical data to understand how planets form and evolve. LePage is the author of an excellent blog on exoplanetary science called Drew ex Machina, and a senior project scientist at Visidyne, Inc. specializing in the processing and analysis of remote sensing data. By Andrew LePage As anyone with even a passing interest in planetary studies can tell you, we are witnessing an age of planetary discovery unrivaled in the long history of astronomy. Over the last two decades, thousands of extrasolar planets have been discovered using a variety of techniques. The most successful of these to date in...

If you're looking for planets that may be habitable, eccentric orbits are a problem. Vary the orbit enough and the surface goes through extreme swings in temperature. In our own Solar System, planets tend to follow circular orbits. In fact, Mercury is the planet with the highest degree of eccentricity, while the other seven planets show a modest value of 0.04 (on a scale where 0 is a completely circular orbit -- Mercury's value is 0.21). But much of our work on exoplanets has revealed gas giant planets with a wide range of eccentricities, and we've even found one (HD 80606b) with an eccentricity of 0.927. As far as I know, this is the current record holder. These values have been measured using radial velocity techniques that most readily detect large planets close to their stars, although there is some evidence for high orbital eccentricities for smaller worlds. Get down into the range of Earth and 'super-Earth' planets, however, and the RV signal is tiny. But a new paper from...

Finding small and possibly habitable worlds around M-dwarfs has already proven controversial, as we've seen in recent work on Gliese 581. The existence of Gl 581d, for example, is contested in some circles, but as Guillem Anglada-Escudé argues below, sound methodology turns up a robust signal for the world. Read on to learn why as he discusses the early successes of the Doppler technique and its relevance for future work. Dr. Anglada-Escudé is a physicist and astronomer who did his PhD work at the University of Barcelona on the Gaia/ESA mission, working on the mission simulator and data reduction prototype. His first serious observational venture, using astrometric techniques to detect exoplanets, was with Alan Boss and Alycia Weinberger during a postdoctoral period at the Carnegie Institution for Science. He began working on high-resolution spectroscopy for planet searches around M-stars during that time in collaboration with exoplanet pioneer R. Paul Butler. In a...

Roughly twice the radius and eight times as massive as Earth, 55 Cancri e is a ‘super-Earth’ in the interesting five-planet system some 41 light years away in the constellation Cancer. No habitable conditions here, at least not for anything remotely like the kind of life we understand: 55 Cancri e orbits its G-class primary every 18 hours (55 Cancri is actually a binary, accompanied by a small red dwarf at a separation of 1000 AU). The closest super-Earth we’ve yet found, this is a tidally locked world that, helpfully for our purposes, transits its host. What we find in a just announced study of the planet’s thermal emissions out of the University of Cambridge is an almost threefold change in temperature over a two year period. Although we’ve done it before with gas giant atmospheres, this is the first time any variability in atmosphere has been observed on a rocky planet outside our own Solar System. No other super-Earth has yet given us signs of possible surface activity, and...

We've found a lot of planets far away from the Sun but know comparatively little about what may be circling nearby stars. The rationale is clear: The Kepler mission's field of view was carefully chosen to provide a large sample (over 145,000 main sequence stars) that could be studied for transits by the spacecraft's photometer. Looking out along the Cygnus arm of the Milky Way, far enough from the ecliptic plane to avoid the Sun, the Kepler stars have been providing statistical data to help us understand how common planets actually are in the galaxy. But as we saw with the announcement of a candidate planet around Alpha Centauri B, the news of planets closer to home excites interest. These are places close enough to us that they could conceivably be the targets of future interstellar probes. As we continue to look at the Kepler inflow, we're also anticipating missions like TESS (Transiting Exoplanet Survey Satellite), scheduled for a 2017 launch, and PLATO (PLAnetary Transits and...

It's the twentieth anniversary of the discovery of 51 Pegasi b, a 'hot Jupiter' that was the first planet to be discovered around a normal star. I always have to throw in that 'normal' qualifier because it was in 1992 that Aleksander Wolszczan and Dale Frail announced their discovery of planets around the pulsar PSR 1257+12, the first extrasolar planets ever found, and an extraordinary discovery in themselves. Michel Mayor and Didier Queloz announced the 51 Pegasi b discovery in 1995, and it was quickly confirmed by Geoff Marcy and Paul Butler. 51 Pegasi b, some 50 light years from Earth in the constellation Pegasus, is the prototypical 'hot Jupiter,' a gas giant orbiting in tight proximity to its star. A new paper from Jorge Martins (Universidade do Porto, Portugal) and team announces another first for this world, the first detection of the spectrum of visible light reflected off an exoplanet. The detection was made by painstakingly removing the host star's spectrum to reveal the...

Yesterday's look at NExSS (the Nexus for Exoplanet System Science), NASA's new 'virtual institute,' focused on the multidisciplinary nature of the effort. The work I'm looking at today, an analysis of the planets around Tau Ceti performed at Arizona State University, only emphasizes the same point. To get a read on whether two planets that are possibly in Tau Ceti's habitable zone are likely to be terrestrial worlds like Earth, the ASU team brought the tools of Earth science into play, in particular the work of Sang-heon Shim. Shim is a mineral physicist who worked with astrophysicists Michael Pagano, Patrick Young and Amanda Truitt in the Tau Ceti analysis. His perspective was vital because early work had already suggested that Tau Ceti has an unusual balance between the rock-forming minerals magnesium and silicon. In fact, the ratio of magnesium to silicon here is 1.78, about 70% more than we find in the Sun. That casts long-standing views of Tau Ceti as Sol's twin into doubt, and...