by Paul Gilster | Oct 12, 2015 | Outer Solar System |
Kepler-47 is an eclipsing binary some 4900 light years from Earth in the direction of the constellation Cygnus. It’s a system containing two transiting circumbinary planets, meaning the planets orbit around the binary pair rather than around one or the other star. That configuration caught the eye of Simon Porter, a postdoc at the Southwest Research Institute, because the configuration is so similar to another circumbinary system, the one involving four small moons around Pluto/Charon. In both cases, we have a binary at the center of the orbit. Porter writes about the configuration in this post from the New Horizons team.
In the case of Pluto, the binary could be considered a binary planet, with Charon the other half of the duo. Both are orbited by a system of four moons, each of them less than 50 kilometers in diameter, the moons orbiting around the system’s center of mass. New Horizons, the gift that keeps on giving, has already sent some striking images of these small moons, but we have even better imagery yet to come as we continue to download data from the craft’s Long Range Reconnaissance Imager (LORRI), the high resolution camera that has given us so so many unforgettable images already. But I’ll open the week with an image not from LORRI but from the Multispectral Visible Imaging Camera, just to provide a sense of context and a bit of awe.
Image: Pluto’s haze layer shows its blue color in this picture taken by the New Horizons Ralph/Multispectral Visible Imaging Camera (MVIC). The high-altitude haze is thought to be similar in nature to that seen at Saturn’s moon Titan. The source of both hazes likely involves sunlight-initiated chemical reactions of nitrogen and methane, leading to relatively small, soot-like particles (called tholins) that grow as they settle toward the surface. This image was generated by software that combines information from blue, red and near-infrared images to replicate the color a human eye would perceive as closely as possible. Credit: NASA/JHUAPL/SwRI.
Blue atmospheric haze in the Kuiper Belt is not something anyone was expecting. SwRI’s Carly Howett offers a read on what we’re seeing:
“That striking blue tint tells us about the size and composition of the haze particles. A blue sky often results from scattering of sunlight by very small particles. On Earth, those particles are very tiny nitrogen molecules. On Pluto they appear to be larger — but still relatively small — soot-like particles we call tholins.”
This JHU/APL news release has more, explaining current thinking that tholins form in the upper atmosphere as ultraviolet light breaks nitrogen and methane molecules apart, allowing them to form increasingly complex negatively and positively charged ions that recombine to form macromolecules. Small particles can grow out of the process, with volatile gases condensing to coat their surfaces before they fall back to the surface, adding to its reddish hue.
But back to the system of moons. The closest to New Horizons during the July encounter was Nix, of which LORRI has delivered three close-ups so far. Have a look at the object as, in the second view, it reveals its ‘potato-like’ aspect — the elongation is lost in the first image because we’re looking down the long axis. What stands out here is the size of that crater. Are we looking at a fragment of an older moon, as Porter speculates, or was Nix just lucky to have survived a shot that could leave a crater of that size on such a small surface? A crescent Nix shows up on the far right, which may yield information on the surface of the diminutive moon.
Image: Pluto’s moon Nix is viewed at three different times during the New Horizons July 2015 flyby. Credit: NASA/JHUAPL/SwRI.
Now have a look at Nix as seen through the Ralph/Multispectral Visible Imaging Camera. Here we’re working with only a quarter of LORRI’s resolution, but we’ve got color now and can discern that most of Nix is white, while that provocative crater and the ejecta it produced show up as reddish. It’s a natural assumption that Nix’s interior is made up of much darker material than the surface. “We don’t actually know what either the dark or the light material is,” writes Porter, “nor will we be able to tell until we download the Nix data from the Ralph-Linear Etalon Imaging Spectral Array (LEISA) composition mapping spectrometer.”
Image: Pluto’s moon Nix is shown in high-resolution black-and-white and lower resolution color. Credit: NASA/JHUAPL/SwRI.
Below is Hydra as seen through LORRI, with the caveat that this moon was on the other side of Pluto during close approach, so we don’t have the same level of resolution we had for Nix. Porter notes a certain similarity in aspect with another object that caught our attention this summer: Comet 67P/Churyumov-Gerasimenko, around which the ESA’s Rosetta spacecraft continues its operations. In both cases, we have the possibility of a low-speed collision which melded two originally separate objects. The images of Styx and Kerberos that we’ll get later in the year, by the way, should be of roughly the same resolution as this image of Hydra.
Image: Pluto’s moon Hydra as seen from NASA’s New Horizons spacecraft, July 14, 2015. Credit: NASA/JHUAPL/SwRI
New Horizons also detected surface water ice on Pluto, with areas showing the most apparent water ice signatures corresponding to areas that appear red in other recent images of Pluto. Figuring out how water ice interacts with the reddish tholins is going to take some work.
Image: Regions with exposed water ice are highlighted in blue in this composite image from New Horizons’ Ralph instrument, combining visible imagery from the Multispectral Visible Imaging Camera (MVIC) with infrared spectroscopy from the Linear Etalon Imaging Spectral Array (LEISA). The strongest signatures of water ice occur along Virgil Fossa, just west of Elliot crater on the left side of the inset image, and also in Viking Terra near the top of the frame. A major outcrop also occurs in Baré Montes towards the right of the image, along with numerous much smaller outcrops, mostly associated with impact craters and valleys between mountains. The scene is approximately 450 kilometers across. Note that all surface feature names are informal. Credit: NASA/JHUAPL/SwRI.
In addition to the sheer thrill of seeing places as tiny as Nix at some level of detail, not to mention the often startling and mesmerizing views of Pluto and Charon themselves, I note the fact that exoplanets have become so common that we can draw analogies from our catalogues to describe what we see in our own system, as Simon Porter did in his description of Pluto’s moons. The world has changed so much in the past twenty years of exoplanet hunting, meaning that our view of ourselves and our place in the universe has been much enriched, and we have a panoply of planetary configurations to draw on as we consider how solar systems are made.
by Paul Gilster | Oct 9, 2015 | Astrobiology and SETI |
We often conceive of SETI scenarios in which Earth scientists pick up a beacon-like signal from another star, obviously intended to arouse our attention and provide information. But numerous other possibilities exist. Might we, for example, pick up signs of another civilization’s activities, perhaps through intercepting electromagnetic traffic, or their equivalent of planetary radars? Even more interesting, as Brian McConnell speculates below, is the idea of listening in on a galactic network that contains information not just from one civilization but many. As Centauri Dreams readers know, McConnell and Alex Tolley have been developing the ‘spacecoach’ concept of interplanetary travel, discussed in the just published A Design for a Reusable Water-Based Spacecraft Known as the Spacecoach (Springer, 2015). It’s a shrewd and workable way to get us deep into the Solar System. Today McConnell turns his attention to a SETI network whose detection could offer a big payoff for a young civilization.
by Brian McConnell
With the revival of SETI funding, it’s interesting to contemplate what we might find if SETI succeeds. One possibility that is especially tantalizing is that first contact would not be with an individual civilization but rather a large scale network of civilizations that is organized not unlike the Internet. This is not a new idea (Timothy Ferris and others have explored this concept) but it is one that should be considered seriously. Assuming that communicative civilizations are commonplace, a big if of course, a decentralized or mesh network will be the most time and energy efficient way for them to organize their communications.
Consider the energy cost of sending a unit of information from one edge of the galaxy to the other (~ 100,000 light years) via direct means versus a peer-to-peer relay system. The savings ratio can be estimated as:
daverage / wgalaxy
If communicative civilizations are separated by an average distance of, say, 1000 light years, the energy cost of sending a unit of information across the galaxy via relay will be about 1/100th that of direct communication. The energy requirement per link drops off by the ratio of (daverage / wgalaxy)2 but as more hops are required with shorter links, the overall energy requirement drops by daverage / wgalaxy. This is an approximation, but it highlights the order of magnitude improvements in economy, and suggests that if communicative civilizations are widespread, energy economics and other considerations will favor this type of arrangement.
Reliability and redundancy are another important feature of a mesh network. When sending information across such great distances, and with such long transit times, a sender may want to protect especially important information against loss or corruption by sending it repeatedly or by sending it via multiple paths between endpoints. This technique can virtually guarantee that information is eventually transmitted even if the network is damaged, even without the use of sophisticated forward error correction codes. This isn’t to say that an extraterrestrial intelligence will copy the TCP/IP protocol, but it’s safe to assume that someone who is sophisticated enough to build an interstellar communication link will probably be familiar with the characteristics and benefits of decentralized mesh networks.
There will also be benefits to receivers, especially newcomers, as contact with one node will be the same as contacting many nodes, since any node in the network can function as a relay for others. The cost of joining the network is also reduced, as a new node need only establish communication with its nearest neighbors, and can relay messages to and receive information from any other site on the network. Such a network would not merely be a communication system, but also a long term repository of knowledge as important information from long dead civilizations could continue to circulate throughout the network in perpetuity.
Image: The Milky Way as seen from the mountains of West Virginia. Could the galaxy be filled with the traffic of networked civilizations? Credit: ForestWander.
Fermi Implications
The existence of such a system might also help explain the Fermi Paradox, as the most energy efficient mode of operation, in terms of detecting new civilizations, will be for each node to concentrate its detection efforts on its immediate neighborhood using a listen and reply strategy. There would be little point in building powerful omnidirectional beacons that are detectable over great distances, as they would cost far more energy to operate, would have to wait millennia for a response, and would be plagued by duty cycle issues. Better for peripheral nodes to listen for microwave leakage from nearby civilizations as they develop early communication technology, and then target those for active communication soon after they are detected. This sort of strategy would be cheap both in terms of energy and the number of radio-telescopes required at each node in the network, and would offer a high probability of success in detecting new nodes just as they become active, while not wasting energy by transmitting in the blind.
An important point to consider here is that an emerging technological civilization would become detectable independently of any intent to attempt interstellar communication. Indeed here on Earth, the vast vast majority of energy expended on electromagnetic signaling has been for purposes other than Active SETI. It seems likely that most technological civilizations would go through a period where they are microwave bright, even if they later go dark due to transitioning to other technology, fear of ETI contact, etc.
As we would just now be detectable to nodes within about 100LY (80LY is probably a better estimate), we would just now expect to be receiving a response from a node within 40-50LY. It’s possible that rapid changes in atmospheric spectra, as Earth has experienced with the sudden increase in carbon dioxide, might also serve as a early tripwire for attempting active communication, but those could be ambiguous signals with natural explanations like volcanism, whereas a sudden spike in monochromatic microwave transmission points definitively to a technological origin. Viewed from the network’s perspective, this decentralized strategy would enable detection of new sites with the least energy expenditure and the shortest possible lag time between detection and active communication, with the added bonus feature that the first nodes to establish contact could relay stored information from nodes far beyond the initial radius of communication. On the other hand, if the average distance between nodes is large, it may be a long time before the nearest nodes are aware of us, and it may be hard for such a network to become established in the first place.
Choice of Encoding Schemes
Another interesting aspect of a long running galactic communication system is that there will be a natural selection of sorts that favors the message encoding schemes that are most likely to be mimicked. The selection pressure in this case will favor an encoding scheme that is broadly comprehensible (easy to understand the basic design pattern) and flexible (able to accommodate many different types of information via that framework). A transmission that is extremely difficult to parse, for example because of strong encryption or sophisticated forward error correction codes, is less likely to be mimicked than one whose basic design pattern is comprehensible to many receivers, even if it is less than optimal in terms of capacity or error resistance. This leads to the fittest message being more likely to replicate (be mimicked in retransmission) than its competitors. This is also an incentive for civilizations wishing to project influence through remote communication to design messages that peer sites will want to and be able to copy.
The point is not to speculate about what would be in such a message, but how it is organized at a low level. To build a mesh network that can handle many data types, you don’t need a very sophisticated message format, even if some of the data types sent within the message are extremely complex or difficult to comprehend. Typically you break a large amount of data, be it a file or communication stream, into smaller predictably organized subunits which are labeled with metadata, which might include:
- a frame or packet number : identifies a message segment’s position within a collection, file, stream, etc
- a collection or file number : to identify a larger grouping of frames, pages, packets, etc
- an author or sender number : to identify the author or sender of a particular segment.
- a receiver number : to identify the intended recipient, if any
- a content identifier : to identify the type of content represented by the frame or packet
- a blob of data, or payload, that is described by the above metadata
While one could design more complex schemes, the above defines the minimal set of metadata needed to describe something like a mesh network or file system with many files, authors and varying file types. What someone decides to convey with such a system is a different matter entirely, but a mesh network in its simplest form consists of a long chain of | meta data | blob of data | meta data | blob of data | segments with obvious repeating structures.
Implications for SETI
While the basic design pattern of a system like this can be rather simple, it will be capable of delivering data that varies widely in content type and “difficulty level”, and also offers a high degree of durability (important message fragments can be resent out of sequence or sent via multiple paths). Some content types such as rasterized or bitmapped images will probably be nearly universally understood due to their utility in astronomy and space photography, while others that are based on advanced math may be unrecognizable to many recipients. It’s not unlike DNA, whose basic encoding scheme has just four letters, yet can encode for something as simple as an isolated protein or as complex as a human being. That’s one of the interesting characteristics of the fittest message — it should be easy to parse at a low level, yet capable of conveying data types representing a wide range of complexity.
All of this suggests that SETI surveys should be concentrating a portion of their observing time on nearby targets. This also suggests that a large scale network will probably need to find us before we can find it, but will also be relatively easy to spot once it does. This doesn’t exclude other possibilities, and indeed SETI should be trying many strategies in parallel, from looking for distant beacons to Bracewell probes.
Should we encounter a network like this, the implications of that would be nothing short of staggering because of the volume and variety of information that could flow through a system like this. It’s possible that much of that communication will be over our heads. On the other hand, the quasi Darwinian selection pressure on message formats may favor those that are broadly comprehensible, or at least contain elements like rasterized photos that virtually any astronomically communicative receiver can understand, including us.
by Paul Gilster | Oct 8, 2015 | Exoplanetary Science |
With almost 2000 exoplanets now confirmed, not to mention candidates in the thousands, it’s amazing to recall that it was just twenty years ago that the first planet orbiting a main sequence star beyond the Solar System was found. Continued work on the world revealed that 51 Pegasi b is about half as massive as Jupiter, though 50 percent larger. Orbiting its star in roughly four days, the planet is some fifty light years from Earth. Thus we began to learn not just that exoplanets were out there, but that their environments could be truly extreme — remember that it was just in 1992 that planets were found around the pulsar PSR 1257+12.
Without any evidence other than my imagination, I grew up believing that most stars should have planets, and just assumed that their stellar systems were more or less like our own. There should be a few planets too close to their star for life to exist, and several gas giants out at the outskirts of the system, and somewhere in between there should be a world not so different from Earth. It was a naive view, but not completely implausible, and anyway, we lacked data.
Discovering ‘hot Jupiters’ is one way we began to realize that other configurations could exist, and the number of ‘super-Earths’ has made the same case. Just how ‘normal’ is our Solar System in the first place? A new paper from Rebecca Martin (University of Nevada, Las Vegas) and Mario Livio (Space Telescope Science Institute) tackles the question, comparing what we see in our Solar System to our growing database of exoplanetary information.
Obviously, this is a work in progress, for we’re not only still examining abundant Kepler and K2 data but continuing a robust planet hunt that looks forward to space-based missions like TESS (Transiting Exoplanet Survey Satellite) and PLATO (PLAnetary Transits and Oscillations of stars), surveys of considerable scope that should build our catalog of ‘nearby’ planets. Nonetheless, we can draw some conclusions based on what we see in the image below.
Image (click to enlarge): Model of all the multi-planet systems found by Kepler as of November 2013; our terrestrial planets are shown in grey at the top left for comparison. A new study examines how our solar system compares to the exoplanetary systems we’ve found. Credit: NASA/Kepler/Dan Fabricky.
Our Solar System is composed of a good deal more than planets, of course, which leads to an important caveat, one that Martin and Livio mention early on in the paper. We have two belts orbiting the Sun, the main asteroid belt and the Kuiper Belt. The problem is that although we can see a number of debris and dust belts around other stars, belts with as little mass as ours would not be observable to us around other stars. So a study like this one has to base its findings solely on planets. It can be mentioned, though, that hundreds of debris disk candidates are now in play, and about two-thirds of these are best modeled as two component disks.
That’s a plus for the idea that our Solar System isn’t all that atypical. What about the planets? The authors use a mathematical transformation that allows them to set up a statistical comparison. The low mean eccentricity of planets around our Sun is one area where we differ from other multi-planet systems — our planets move in largely circular orbits — but as the paper notes, our observation methods are biased toward finding high eccentricity planets. Circular orbits work to our benefit, for planets with low eccentricity are more likely to be dynamically stable. Indeed, the terrestrial planets in our system are thought to be stable until that distant time when the Sun becomes a red giant and disrupts the entire inner system.
What about age? The Sun is about half the age of the Milky Way disk, hardly setting up our system as special, and at least one study has found that about 80 percent of existing Earth-like planets were already formed when the Earth came into existence. We also know that terrestrial planets in the habitable zone of their host star appear to be common. The paper notes, for example, the work of Courtney Dressing and David Charbonneau, which uses Kepler data for M-dwarfs and finds an occurrence rate for Earth-sized planets in the habitable zone of 18% to 27%, a conservative estimate that Martin and Livio say could be as high as 50 percent (see How Common Are Potential Habitable Worlds in Our Galaxy?)
If we’re not unusual in terms of age or habitability, we do differ considerably from other systems in two respects. First, we have no planets inside the orbit of Mercury, in contrast to systems with rocky worlds on far closer orbits. Moreover, the Solar System lacks a super-Earth, a category of planet now turning out to be common.
The paper summarizes its findings this way:
We find that the properties of the planets in our solar system are not so significantly special compared to those in exosolar systems to make the solar system extremely rare. The masses and densities are typical, although the lack of a super-Earth-sized planet appears to be somewhat unusual. The orbital locations of our planets seem to be somewhat special but this is most likely due to selection effects and the difficulty in finding planets with a small mass or large orbital period. The mean semi-major axis of observed exoplanets is smaller than the distance of Mercury to the Sun. The relative depletion in mass of the solar system’s terrestrial region may be important. The eccentricities are relatively low compared to observed exoplanets, although the observations are biased toward finding high eccentricity planets. The low eccentricity, however, may be expected for multi-planet systems. Thus, the two characteristics of the solar system that we find to be most special are the lack of super-Earths with orbital periods of days to months and the general lack of planets inside of the orbital radius of Mercury.
So while we’ve had quite a few surprises in the past twenty-five years, going from no exoplanets known to planets around pulsars and then main sequence stars, and moving from those early detections to thousands of candidates, we’re not seeing anything that would peg us as being unique. In terms of habitability, the authors see nothing in the Solar System that would make it especially conducive to life’s formation as opposed to other planetary systems:
If exosolar life happens to be rare it would probably not be because of simple basic physical parameters, but because of more subtle processes that are related to the emergence and evolution of life. Since at the moment we do not know what those might be, we can allow ourselves to be optimistic about the prospects of detecting exosolar life.
That lack of a super-Earth troubles me, though. Systems that have a super-Earth generally have more than one. The authors ask a good question: Does the presence of a super-Earth affect terrestrial planet formation? Several studies have looked at a migrating super-Earth moving slowly through the habitable zone, finding that a terrestrial planet that forms there later will tend to be rich in volatiles. Many observed super-Earths are found in orbits where they were unlikely to have formed, so scenarios of super-Earth migration surely deserve further study.
The paper is Martin and Livio, “The Solar System as an Exoplanetary System,” The Astrophysical Journal Vol. 810, No. 2 (3 September 2015). Full text.
by Paul Gilster | Oct 7, 2015 | Exoplanetary Science |
The European Southern Observatory’s SPHERE instrument is turning up interesting things around the star AU Microscopii. Surrounded by a large dusty disk, the star is young enough to raise the interest of those studying how planets form. What has turned up are structures that Anthony Boccaletti (Observatoire de Paris) describes as ‘arch-like, or wave-like,’ a structure that his research team has never seen before. The issue is addressed in a new paper in Nature, which discusses five wave-like arches at different distances from the star.
Fortunately, AU Mic is a well studied star, with abundant Hubble imagery taken in 2010 and 2011 available for comparison. The results of that comparison are striking: The features do indeed show up on the Hubble imagery, but they show distinct change with time, meaning they are in rapid motion. “We reprocessed images from the Hubble data and ended up with enough information to track the movement of these strange features over a four-year period,” explains team member Christian Thalmann (ETH Zürich, Switzerland). “By doing this, we found that the arches are racing away from the star at speeds of up to about 40 000 kilometres/hour!”
Image: Using images from ESO’s Very Large Telescope and the NASA/ESA Hubble Space Telescope, astronomers have discovered never-before-seen structures within a dusty disc surrounding a nearby star. The fast-moving wave-like features in the disc of the star AU Microscopii are unlike anything ever observed, or even predicted, before now. The origin and nature of these features present a new mystery for astronomers to explore. The results are published in the journal Nature on 8 October 2015. Credit: ESO.
Disturbances in a protoplanetary disk are always intriguing — think of the star Beta Pictoris, another object whose disk is well studied, with observations showing disk anomalies that flagged the presence of at least one fledgling planet. But what’s happening at AU Mic is challenging to understand. The arch-like features furthest from the star appear to be moving faster than those closer in, with at least three of them moving fast enough to escape AU Mic entirely. The team believes that they could not be caused by planets disturbing disk material as they orbit, nor do spiral waves triggered by disk instabilities offer an explanation.
Complicating the observations is the fact that we see AU Mic’s disk edge-on, making an examination of its three-dimensional structure all the more difficult. Is stellar activity the answer? Perhaps, says co-author Glenn Schneider (Steward Observatory, University of Arizona):
“One explanation for the strange structure links them to the star’s flares. AU Mic is a star with high flaring activity — it often lets off huge and sudden bursts of energy from on or near its surface. One of these flares could perhaps have triggered something on one of the planets — if there are planets — like a violent stripping of material which could now be propagating through the disc, propelled by the flare’s force.”
But the range of possibilities is large. From the paper:
Another case of interest may involve the presence of a planetary magnetosphere. Planetary magnetic field lines reconnect with the magnetized stellar wind creating a magnetotail which can lead to cyclical ejections of plasma out of the tail. Dust particles interacting with the ejected plasma are accelerated by the Lorentz force which is dominant over gravity. This would yield an outflow pointed away from the star, and may explain the super-Keplerian velocities of a few tens of km/s and possibly vertical transport with respect to the disk midplane. Such magnetotail ejections are observed in the solar system from Jupiter and Saturn 1-2 au away from the planet.
Just 32 light years from Earth, AU Mic is obviously going to receive a great deal of further observation, with the Boccaletti team planning to extend its work to the Atacama Large Millimeter/submillimeter Array (ALMA), located in the Atacama desert of northern Chile. SPHERE, meanwhile, is quickly proving itself in its first year of observation. An acronym for Spectro-Polarimetric High-contrast Exoplanet REsearch instrument, SPHERE takes as its domain the realm of direct exoplanet imaging, which of course includes debris disks.
To make these observations, the instrument, which is mounted on Unit Telescope 3 of the Very Large Telescope at ESO’s Paranal Observatory, uses a coronagraph to block out the bright central star as it examines planets or disks. The system also uses differential imaging, which plays off the fact that starlight is unpolarized, while starlight reflected off the surface of a planet or a dusty disk is partially polarized. SPHERE is designed to pick out the polarized signal, isolating it from the stream of unpolarized light by using special filters. Throw in adaptive optics to correct for turbulence in the Earth’s atmosphere and you get an instrument that can provide a sharp view of objects close to the central star, such as the now mysterious disk around AU Mic.
The paper is Boccaletti et al., “Fast-Moving Structures in the Debris Disk Around AU Microscopii,” Nature 8 October 2015.
by Paul Gilster | Oct 6, 2015 | Exoplanetary Science |
If we had a space-based instrument fully capable of analyzing an exoplanet’s atmosphere in place right now, where would we find our best targets? The goal, of course, is to pluck out the signature of biological activity, which means we’re looking at planets in the habitable zone of their stars, that region where liquid water can exist on the surface. Right now there aren’t many planets that fit the bill, but the day is coming when there will be hundreds, then thousands. How we optimize our search time and choose the targets with the most likely pay-off is a major issue.
Which is where a new metric called the ‘habitability index for transiting planets’ comes into play. Developed by Rory Barnes and Victoria Meadows (University of Washington), working with research assistant Nicole Evans, the index is an attempt to prioritize the selection process, looking at those exoplanets that should be at the top of our list. Says Barnes:
“Basically, we’ve devised a way to take all the observational data that are available and develop a prioritization scheme, so that as we move into a time when there are hundreds of targets available, we might be able to say, ‘OK, that’s the one we want to start with.'”
Kepler has already given us a taste of what’s ahead, with its datasets rich in planets, but when we launch TESS (Transiting Exoplanet Survey Satellite) and later PLATO (PLAnetary Transits and Oscillations of stars), we’ll be looking at a much broader field. Bear in mind that Kepler took its ‘long stare’ out along the Orion Arm, where 156,000 stars were in range to be analyzed for the lightcurves that identify planetary transits. That’s a way of using brute force methods to get a statistical reading on how common various kinds of planets are in the galaxy.
But most of the stars in the Kepler field of view are hundreds if not thousands of light years away — fewer than one percent of them are closer than 600 light years. So the starfield in Cygnus and Lyra that gave us such sensational results with Kepler will now be supplanted with the search for planets closer to home, and a TESS field of view that will cover 400 times as much sky. TESS should be a good target-finder for the James Webb Space Telescope, whose launch in 2018 will open up the possibility of studying the atmospheres of such worlds.
Image: UW astronomers Rory Barnes and Victoria Meadows of the Virtual Planetary Laboratory have created the “habitability index for transiting planets” to compare and rank exoplanets based on their likelihood of being habitable. Credit: Rory Barnes.
Barnes and Meadows presented their habitability index in a paper for The Astrophysical Journal called ‘Comparative Habitability of Transiting Exoplanets.’ The goal is to move well beyond what has thus far been the ‘first cut’ for habitable planets, the question of whether they can have liquid water on the surface. The index takes particular account of the planet’s ‘eccentricity-albedo degeneracy,’ which examines the energy reflected off its surface (albedo) and weighs it against the circularity of its orbit.
This gets into an interesting balancing act. High albedo means a great deal of light and energy being reflected back into space, so that less is available at the surface. And eccentric orbits can create intense periods of energy influx, during which the planet passes closest to its star. As the paper notes, there is an equilibrium that marks the most life-friendly zone given these factors. In the excerpt below, H refers to the habitability index number:
Our approach to comparative habitability assessments parameterizes the eccentricity-albedo degeneracy by calculating the fraction of all possible (A, e) [albedo, eccentricity] pairs that could produce a clement climate. We must also assess “rockiness” probabilistically based on inferences from the few planets with well-known masses and radii. Despite these assumptions, H can provide more insight into a planet’s potential to support life than simply comparing its orbit to that of its host star’s HZ.
Applying the index to the Kepler dataset produces some interesting results:
We ranked the known Kepler and K2 planets for habitability and found that several have larger values of H than Earth. This does not mean these planets are “more habitable” than Earth – it means that an Earth twin orbiting a solar twin that is observed by Kepler would not have the highest probability of being habitable. The best candidates have incident radiation levels, assuming circular orbits, of 60-90% that of Earth’s. These levels are about in the middle of the HZ, which is not surprising as our flux limits are derived from the models that produced the HZ. Our method is grounded in the fundamental observables and can, when applicable, fold in eccentricity constraints, and is therefore more powerful than the HZ for transiting exoplanets.
The age of the star can also be a useful data point when it comes to habitability. Here we look at the timescale on which life arises, a challenging issue because all we have to work with is what we know of life’s development on Earth. In general, the paper notes, planets in the habitable zone of F, G or K stars should see the last wave of giant impacts on their young surfaces at around 100 million years. Given that impacts likely sterilized our planet for several million years, this is an important issue, and stellar age, if known, can factor into the question of habitability.
A habitability index like this one isn’t designed for a single mission, but anticipates creating a target list we’ll want to investigate with a variety of technologies. The James Webb Space Telescope will be the first spacecraft that can perform transit transmission spectroscopy, but the paper notes that planets with high values of H could well be unobservable by JWST because of its detection capabilities. Barnes and Meadows examined four candidates for JWST spectroscopy — these were all Kepler or K2 detections — and found that even detecting water bands on these would be challenging for JWST. What TESS may identify as the best target for JWST won’t necessarily be the best target for later missions with different instrumentation.
The paper is Barnes, Meadows & Evans, “Comparative Habitability of Transiting Exoplanets,” accepted at The Astrophysical Journal (preprint). A UW press release is also available.
