The Titius-Bode law has always been a curiosity, one often attributed to little more than happenstance. But recently this numerological curiosity, which predicts that planets in a solar system appear with a certain ratio between their orbital periods, has been the subject of renewed investigation. Francois Graner (Ecole Normale Superieure, Paris) and Berengere Dubrulle (Observatoire Midi Pyrenees, Toulouse) revisited Titius-Bode in the 1990s, asking whether it actually flagged symmetry properties that most solar systems should exhibit. And now continuing work out of Australian National University and the University of Copenhagen has made predictions using a modified version of the law that can be tested against observation of known exoplanetary systems. So we need to refresh our memory on the formulation, which shows us a relationship that predicts planetary orbits. Take a sequence where each number is double the number that preceded it. Thus 0, 3, 6, 12 and so on. Add 4 to each of...

One of these days we'll have the instruments in place to examine light from a terrestrial-class world around another star. This opens up the possibility of identifying atmospheric gases like oxygen, ozone, carbon dioxide and methane. All of these can occur in an atmosphere in the absence of life, but if we find them existing simultaneously in great enough quantities, we will have detected a possible biosignature, for without life's activity to replenish them, these gases would recombine and leave us with a much less tantalizing atmospheric mix. But tackling planetary atmospheres for biosignatures is only one way to proceed. An interdisciplinary team led by Cornell University's Lisa Kaltenegger and Siddharth Hegde (Max Planck Institute for Astronomy), is examining life detection based on the characteristic tint of lifeforms. An alien organism covering large parts of the planet -- think forests, for example, on Earth -- would reflect light at particular wavelengths, light that could be...

As we get the next generation of space-based telescopes into operation, one of our more significant problems is going to be knowing where to look. After all, once we've identified potentially interesting planets for follow-up with spectroscopic analysis of their atmospheres, we're still faced with the need to focus on the most likely targets. Telescope time is precious, and the ability to rule out planets so as to whittle down our list is a necessary skill to refine. On that score, Rory Barnes (University of Washington) and colleagues have weighed in with a particular type of planetary configuration we may want to avoid. Barnes is interested in solar systems where gravity plays a significant role in disrupting what might otherwise be a circular orbit in the habitable zone. Some of these effects may be relatively small, but if. over time, we elongate the orbit of an otherwise habitable planet by these small interactions, we can all but eliminate its chances for life. The particular...

Planets in multiple star systems intrigue us particularly when we try to imagine the view from the surface. Call it the 'Tatooine Effect,' made to order for visual effects specialists and cinematographers. But planets like these also raise interesting issues. Lewis Roberts (JPL) and colleagues have just published a new study of the 30 Ari system, demonstrating that it is a quadruple star system with a gas giant of about four times the mass of Jupiter in a 335 day orbit around its primary star. We already knew about the planet in the 30 Ari system. What's new is the discovery of the additional star. At 23 AU from the planet, the newly discovered fourth star would seem to be a factor in the orbital dynamics of the gas giant, but just what effects it has remain to be studied. The paper, which also reports the detection of a stellar companion to the exoplanet host system HD 2638, notes that 30 Ari is the second quadruple system known to host an exoplanet. And interestingly, both HD 2638...

It's been years since I've written about laser frequency comb (LFC) technology, and recent work out of the Max Planck Institute of Quantum Optics, the Kiepenheuer Institute for Solar Physics and the University Observatory Munich tells me it's time to revisit the topic. At stake here are ways to fine-tune the spectral analysis of starlight to an unprecedented degree, obviously a significant issue when you're dealing with radial velocity readings of stars that are as tiny as those we use to find exoplanets. Remember what's happening in radial velocity work. A star moves slightly when it is orbited by a planet, a tiny change in speed that can be traced by studying the Doppler shift of the incoming starlight. That light appears blue-shifted as the star moves, however slightly, towards us, while shifting to the red as it moves away. The calibration techniques announced in the team's paper show us that it's possible to measure a change of speed of roughly 3 cm/s with their methods, whereas...

Our discovery of the interesting disk around Beta Pictoris dates back all the way to 1984, marking the first time a star was known to host a circumstellar ring of dust and debris. But it’s interesting how far back thinking on such disks extends. Immanuel Kant’s Universal Natural History and Theory of the Heavens (1755) proposed a model of rotating gas clouds that condensed and flattened because of gravity, one that would explain how planets form around stars. Pierre-Simon Laplace developed a similar model independently, proposing it in 1796, after which the idea of gaseous clouds in the plane of the disk continued to be debated as alternative theories on planet formation emerged. Today we can view debris disks directly and learn from their interactions. Out of the Beta Pictoris discovery have grown numerous observations including the new visible-light Hubble images shown below. The beauty of this disk is that we see it edge-on and, because of the large amount of light-scattering dust...

Now it's really getting interesting. Here are the two views of Ceres that the Dawn spacecraft acquired on February 12. The distance here is about 83,000 kilometers, the images taken ten hours apart and magnified. As has been true each time we've talked about Ceres in recent weeks, these views are the best ever attained, with arrival at the dwarf planet slated for March 6. What I notice and really enjoy about watching Dawn in action is the pace of the encounter. Dawn is currently moving at a speed of 0.08 kilometers per second relative to Ceres, which works out to 288 kilometers per hour. The distance of 83,000 kilometers on the 12th of February has now closed (as of 1325 UTC today, the 18th) to 50,330 kilometers. Its quite a change of pace from the days when we used to watch Voyager homing in on a planetary encounter. Voyager 2 reached about 34 kilometers per second as it approached Saturn, for example, then slowed dramatically as it climbed out of the giant planet's gravitational...

One of the big arguments against habitable planets around low mass stars like red dwarfs is the likelihood of tidal effects. An Earth-sized planet close enough to a red dwarf to be in its habitable zone should. the thinking goes, become tidally locked, so that it keeps one face toward its star at all times. The question then becomes, what kind of mechanisms might keep such a planet habitable at least on its day side, and could these negate the effects of a thick dark-side ice pack? Various solutions have been proposed, but the question remains open. A new paper from Jérémy Leconte (Canadian Institute for Theoretical Astrophysics, University of Toronto) and colleagues now suggests that tidal effects may not be the game-changer we assumed them to be. In fact, by developing a three-dimensional climate model that predicts the effects of a planet's atmosphere on the speed of its rotation, the authors now argue that the very presence of an atmosphere can overcome tidal...

A small satellite designed to study and characterize exoplanet atmospheres is being developed by University College London (UCL) and Surrey Satellite Technology Ltd (SSTL) in the UK. Given the engaging name Twinkle, the satellite is to be launched within four years into a polar low-Earth orbit for three years of observations, with the potential for an extended mission of another five years. SSTL, based in Guildford, Surrey and an experienced hand in satellite development, is to build the spacecraft, with scientific instrumentation in the hands of UCL. The method here is transmission spectroscopy, which can be employed when planets transit in front of their star as seen from Earth. Starlight passing through the atmosphere of the transiting world as it moves in front of and then behind the star offers a spectrum that can carry the signatures of the various molecules there, a method that has been used on a variety of worlds like the Neptune-class HAT-P-11b and the hot Jupiter HD...

The fascination with finding habitable planets -- and perhaps someday, a planet much like Earth -- drives media coverage of each new, tantalizing discovery in this direction. We have a number of candidates for habitability, but as Andrew LePage points out in this fine essay, few of these stand up to detailed examination. We're learning more all the time about how likely worlds of a given size are to be rocky, but much more goes into the mix, as Drew explains. He also points us to several planets that do remain intriguing. LePage is Senior Project Scientist at Visidyne, Inc., and also finds time to maintain Drew ex Machina, where these issues are frequently discussed. by Andrew LePage The past couple of years have been eventful ones for those with an interest in habitable extrasolar planets. The media have been filled with stories about the discovery of many new extrasolar planets that have been billed as being "potentially habitable". Unfortunately follow-up observations and new...

Not long ago we looked at a paper from Rodrigo Luger and Rory Barnes (University of Washington) making the case that planets now in a red dwarf’s habitable zone may have gone through a tortured history. Because of tidal forces causing surface volcanism and intense stellar activity in young stars, a planet’s supply of surface water may be lost entirely. As the red dwarf slowly settles into the main sequence, the upper atmosphere of a planet in what will eventually become its habitable zone can be heated enough to cause its hydrogen to escape into space. Remember that M-dwarfs have a long, slow contraction phase, one that can last as long as a billion years. That exposes planets formed in what will ultimately become the habitable zone to extreme radiation, with hydrogen loss leading to a dessicated surface inimical to life. In such worlds, a dense oxygen envelope could remain, in which case we might detect oxygen and mistakenly take it for a bio-signature (see Enter the ‘Mirage Earth’...

Finding planets around stars that are two and a half times older than our own Solar System causes a certain frisson. Our star is four and a half billion years old, evidently old enough to produce beings like us, who wonder about other civilizations in the cosmos. Could there be truly ancient civilizations that grew up around stars as old as Kepler-444, a K-class star in the constellation Lyra that is estimated to be fully 11.8 billion years old? It's a tantalizing speculation, and of course, nothing more than that. But the discovery of planets here still catches the eye. The just announced discovery and accompanying paper are the work of Tiago Campante (University of Birmingham, UK), who led a large team in the investigation. What we learn is that five planets have been discovered using Kepler data around a star that is 117 light years from Earth. These are not habitable worlds by our standards -- all five planets complete their orbits in less than ten days, making them hotter than...

Might there be gas giant planets somewhere with moons as large as the Earth, or at least Mars? Projects like the Hunt for Exomoons with Kepler (HEK) are on the prowl for exomoons, and the possibility of large moons leads to astrobiological speculation when a gas giant is in its star's habitable zone. Interestingly, we may be looking at evidence of an extremely young -- and very large -- moon in formation around a planet that circles the young star J1407. That would be intriguing in itself, but what researchers at Leiden Observatory (The Netherlands) and the University of Rochester have found is an enormous ring structure that eclipses the young star in an epic way. The diameter of the ring system, based on the lightcurve the astronomers are getting, is nearly 120 million kilometers, which makes it more than two hundred times larger than the rings of Saturn. This is a ring system that contains about an Earth's mass of dust particles, with a marked gap that signals the possibility of...

About a year ago we looked at a young star called HD 142527 in the constellation Lupus (see HD 142527: An Unusual Circumstellar Disk). A T Tauri star about five million years old, HD 142527 has drawn attention because it shows evidence of both an inner and an outer disk, each of which may be capable of producing planets. These are disks with a twist, as astronomers at the Millennium ALMA Disk Nucleus project at the Universidad de Chile demonstrate in a new paper that explains the three-dimensional geometry of this unusual system. HD 142527's two disks are striking because no other star shows a gap this large between an inner and outer disk, a gap that spans a region from 10 AU out to 120 AU. Two dark regions stand out in observations of the outer disk that break its continuity. The new study reveals these outer disk features to be caused by the shadow of the inner disk. The shape and orientation of the shadows thus become a measure of the inner disk's orientation. Using radiative...

Working at near-infrared wavelengths, the Gemini Planet Imager, now entering regular operations at the Gemini South Telescope in Cerro Pachon (Chile), is producing striking work, including images of exoplanets and circumstellar disks. Have a look at the image below, which highlights the instrument's ability to achieve high contrast at small angular separations. Such capabilities make it possible to image exoplanets around nearby stars, as seen here in the case of the star HR 8799. Image: GPI imaging of the planetary system HR 8799 in K band, showing 3 of the 4 planets. (Planet b is outside the field of view shown here, off to the left.) These data were obtained on November 17, 2013 during the first week of operation of GPI and in relatively challenging weather conditions, but with GPI's advanced adaptive optics system and coronagraph the planets can still be clearly seen and their spectra measured. Credit: Christian Marois (NRC Canada), Patrick Ingraham (Stanford University) and the...

The eight ‘habitable zone’ planets we discussed yesterday appear today in a much broader context. The Kepler mission has verified its 1000th planet, and with the detection of 554 more planet candidates, the total candidate count has now reached 4175. According to this NASA news release, six of the new planet candidates are near-Earth size and orbit in the habitable zone of stars similar to the Sun. These all require follow-up observation to confirm their status as planets, but with confirmed planets like Kepler-438b and Kepler-442b, along with these further candidates in the habitable zone, the numbers keep inching us closer to an Earth 2.0. “Kepler collected data for four years -- long enough that we can now tease out the Earth-size candidates in one Earth-year orbits,” says Fergal Mullally, a SETI Institute Kepler scientist at Ames who led the analysis of a new candidate catalog. “We’re closer than we’ve ever been to finding Earth twins around other sun-like stars. These are the...

One way to confirm the existence of a transiting planet is to run a radial velocity check to see if it shows up there as a gravitationally induced 'wobble' in the host star. But in many cases, the parent stars are too far away to allow accurate measurements of the planet's mass. What Guillermo Torres (Harvard-Smithsonian Center for Astrophysics) did in the case of eight new candidates possibly in their stars' habitable zones was to use BLENDER, a software program he and Francois Fressin developed that runs at NASA Ames on the Pleiades supercomputer. A BLENDER analysis can determine whether candidates are statistically likely to be planets. Torres and Fressin have applied it before in the case of small worlds like Kepler 20e and Kepler 20f, important finds because both were exoplanets near the size of the Earth. Using the software allowed the researchers to create a range of false-positive scenarios to see which could reproduce the observed signal. A nearby binary star system, for...

We often think about how thin Earth's atmosphere is, imagining our planet as an apple, with the atmosphere no thicker than the skin of the fruit. That vast blue sky can seem all but infinite, but the great bulk of it is within sixteen kilometers of the surface, always thinning as we climb toward space. Now a presentation by graduate student Laura Schaefer (Harvard-Smithsonian Center for Astrophysics) at the 225th meeting of the American Astronomical Society in Seattle points out that, like the atmosphere, water is also a tiny fraction of what makes up our planet. A small enough fraction, in fact, that although water does cover seventy percent of the Earth's surface, it makes up only about a tenth of one percent of the overall bulk of a world that is predominantly rock and iron. Dimitar Sasselov (CfA), co-author of the paper on this work, thinks of Earth's oceans as a film as thin as fog on a bathroom mirror. But we've seen recently that water isn't strictly a surface phenomenon. The...

As we start thinking ahead to the TESS mission (Transiting Exoplanet Survey Satellite), currently scheduled for launch in 2017, the exoplanet focus sharpens on stars closer to home. The Kepler mission was designed to look at a whole field of stars, 156,000 of them extending over portions of the constellations Cygnus, Lyra and Draco. Most of the Kepler stars are from 600 to 3000 light years away. In fact, fewer than one percent of these stars are closer than 600 light years, while stars beyond 3000 light years are too faint for effective transit signatures. Kepler has proven enormously useful in helping us develop statistical models on how common planets are, with the ultimate goal, still quite a way off, of calculating the value of ?Earth (Eta_Earth) -- the fraction of stars orbited by planets like our own. Looking closer to home will be the mandate of TESS, which will be performing an all-sky survey rather than the ‘long stare’ Kepler has used so effectively. We should wind up with...

Looking forward from winter into spring in North America -- unfortunately still a few months out -- I can thank Earth's obliquity for a seasonal change I enjoy more every year. Obliquity is the angle that our planet's rotational axis makes as it intersects the orbital plane, which in the case of Earth is 23.5°, so that when we reach the summer solstice, the north pole of the planet tilts toward the Sun by this angle. At winter solstice, the pole is tilted away by the same amount. Our Solar System's most extreme case of obliquity is Uranus, where the angle is a whopping 97.8°. Imagine a planet where the north pole points all but directly at the Sun, cycling through a year where the southern pole will eventually do the same. I'm reminded of Stephen Baxter's novel Ark (Roc, 2011). Here, interstellar travelers come to a planet they hopefully designate Earth II (82 Eridani is its primary). Alas, the obliquity turns out to be 90 degrees, kicking off extreme seasonality. In this...