Centauri Dreams
Imagining and Planning Interstellar Exploration
Emergence of the ‘Venus Zone’
In terms of habitability, it’s clear that getting a world too close to its star spells trouble. In the case of Gliese 581c, we had a planet that some thought would allow liquid water at the surface, but subsequent work tells us it’s simply too hot for life as we know it. With the recent dismissal of Gl 581d and g (see Red Dwarf Planets: Weeding Out the False Positives), that leaves no habitable zone worlds that we know about in this otherwise interesting red dwarf system.
I’m glad to see that Stephen Kane (San Francisco State) and his team of researchers are working on the matter of distinguishing an Earth-like world from one that is more like Venus. We’ve made so much of the quest to find something roughly the same size as the Earth that we haven’t always been clear to the general public about what that implies. For Venus is Earth-like in terms of size, but it’s clearly a far cry from Earth in terms of conditions.
Indeed, you would be hard-pressed to find a more hellish place than Venus’ surface. Kane wants to understand where the dividing line is between two planetary outcomes that could not be more different. Says the scientist:
“We believe the Earth and Venus had similar starts in terms of their atmospheric evolution. Something changed at one point, and the obvious difference between the two is proximity to the Sun.”
Kane and company’s paper on this will appear in Astrophysical Journal Letters and is already available on the arXiv server (citation below). At stake here is solar flux, the incoming energy from the planet’s star, which can be used to define an inner and an outer edge to what Kane calls the ‘Venus Zone.’ Venus is 25 percent closer to the Sun than the Earth, but it gets twice the amount of solar flux. Get close enough to the star to trigger runaway greenhouse effects — the results of which make Venus the distinctive nightmare at the surface that it is — and you are at the outer edge of the Venus Zone.
Go further in toward the star and you can pick out the point where a planet’s atmosphere would begin to be eroded by the incoming flux. This is the inner edge of the Venus Zone, and by understanding the boundaries here, we are helping future attempts to characterize Earth-sized worlds in the inner systems of their stars. Find an Earth-sized planet in the Venus Zone and there is reason to suspect that a runaway greenhouse gas effect is in play.
Image: This graphic shows the location of the “Venus Zone,” the area around a star in which a planet is likely to exhibit atmospheric and surface conditions similar to the planet Venus. Credit: Chester Harman, Pennsylvania State University.
The broader picture is an attempt to place our Solar System in context. The Kepler results have consistently demonstrated that any thought of our Solar System being a kind of template for what a system should look like must be abandoned. From the paper:
A critical question that exoplanet searches are attempting to answer is: how common are the various elements that we find within our own Solar System? This includes the determination of Jupiter analogs since the giant planet has undoubtedly played a significant role in the formation and evolution of our Solar System. When considering the terrestrial planets, the attention often turns to atmospheric composition and prospects of habitability. In this context, the size degeneracy of Earth with its sister planet Venus cannot be ignored and the incident flux must be carefully considered.
The study identifies 43 potential Venus analogs from the Kepler data, with occurrence rates similar to those for Earth-class planets, though as the paper notes, with smaller uncertainties. After all, Kepler is more likely to detect shorter-period planets in the Venus Zone than Earth-class planets with longer orbital periods. Overall, the team estimates based on Kepler data that approximately 32% of small low-mass stars have terrestrial planets that are potentially like Venus, while for G-class stars like the Sun, the figure reaches 45%.
Kane notes that future missions will be challenged by the need to distinguish between the Venus and Earth model. We’ll also be looking at the question of carbon in a planet’s atmosphere and its effects on the boundaries of the Venus Zone, the assumption being that more carbon in the atmosphere would push the outer boundary further from the star.
The paper is Kane, Kopparapu and Domagal-Goldman, “On the frequency of potential Venus analogs from Kepler data,” accepted for publication in The Astrophysical Journal Letters and available as a preprint. Be aware as well of the team’s Habitable Zone Gallery, which currently identifies 51 planets as likely being within their star’s habitable zone.
Space Telescopes Beyond Hubble and JWST
Ashley Baldwin tracks developments in astronomical imaging with a passion, making him a key source for me in keeping up with the latest developments. In this follow-up to his earlier story on interferometry, Ashley looks at the options beyond the James Webb Space Telescope, particularly those that can help in the exoplanet hunt. Coronagraph and starshade alternatives are out there, but which will be the most effective, and just as much to the point, which are likely to fly? Dr. Baldwin, a consultant psychiatrist at the 5 Boroughs Partnership NHS Trust (Warrington, UK) and a former lecturer at Liverpool and Manchester Universities, gives us the overview, one that hints at great things to come if we can get these missions funded.
by Ashley Baldwin
Hubble is getting old.
It is due to be replaced in 2018 by the much larger James Webb Space Telescope. This is very much a compromise of what is needed in a wide range of astronomical and cosmological specialties, one that works predominantly in the infrared. The exoplanetary fraternity will get a portion of its (hoped for) ten year operating period. The JWST has coronagraphs on some of its spectrographs which will allow exoplanetary imaging but as its angular resolution is actually lower than Hubble, its main contribution will be to characterise the atmospheres of discovered exoplanets.
It is for this reason that the designers of TESS (Transiting Exoplanet Survey Satellite) have made sure a lot of its most prolonged viewing will overlap with that of the JWST. Its ability to do this will depend on several factors such as the heat (infrared) the planet is giving out, its size and critically its atmospheric depth (the deeper the better) and the proximity of the planet in question. The longer the telescope has to “stop and stare” at its target planet the better, but we already know lots of other experts want some of the telescope’s precious time, so this will be a big limiting factor.
Planet Hunting in Space
The big question is, where are the dedicated exoplanet telescopes? NASA had a mission called WFIRST planned for the next decade, with the predominant aim of looking at dark matter. There was an add on for “micro-lensing” discovery of exoplanets that happened to pass behind further stars, getting magnified by the stars’ gravity and showing up as “blips” in the star’s spectrum. When the National Reconnaissance Mirrors (NRO) were recently donated to NASA, it was suggested that these could be used for WFIRST instead.
Being 2.4 m in diameter they would be much larger than the circa 1.5 m mirror originally proposed and would therefore make the mission more powerful, especially because by being “wide field” they would view far bigger areas of the sky, further increasing the mission’s potency. It was then suggested that the mission could be improved yet further by adding a “coronagraph” to the satellite’s instrument package. The savings made by using one of the “free” NRO mirrors would cover the coronagraph cost.
Coronagraphs block out starlight and were originally developed to allow astronomers to view the Sun’s atmosphere (safely!). Subsequently they have been placed in front of a telescope’s focal plane to cut out the light of more distant stars, thus allowing the much dimmer light of orbiting exoplanets to be seen. A decade of development at numerous research testbeds such as the Jet Propulsion Laboratory, Princeton and at the Subaru telescope on Mauna Kea has refined the device to a high degree. When starlight of all wavelengths strikes a planet it can be reflected directly into space, or absorbed to be re-emitted as thermal infrared energy. The difference between the amount of light emitted by planets versus stars is many orders of magnitude in the infrared compared to the visible, so for this reason telescopes looking to visualise exoplanets do so in the infrared. The difference is still a billion times or so.
Thus the famous “firefly in the searchlight “metaphor. Any coronagraph must cut out infrared to the tune of a billion times or more for an exoplanet to first be seen and then analysed spectroscopically. The latter is crucial as it tells us about the planet and its atmosphere according to the factors described above. This light reduction technique is called “high contrast imaging” with the reduction described according to negative powers of ten. Typically a billion times reduction is simplified to 10e9. This level of reduction should allow Jupiter size planets, ice giants like Neptune and, at a push, “super earths”. To visualise Earth like, terrestrial planets, an extra order of magnitude, 10e10 or better is necessary.
Image: A coronagraph at work. This infrared image was taken at 1.6 microns with the Keck 2 telescope on Mauna Kea. The star is seen here behind a partly transparent coronagraph mask to help bring out faint companions. The mask attenuates the light from the primary by roughly a factor of 1000. The young brown dwarf companion in this image has a mass of about 32 Jupiter masses. The physical separation here is about 120 AU. Credit: B. Bowler/IFA.
The Emergence of WFIRST AFTA
Telescope aperture is not absolutely critical (with a long enough view), with even small metre-sized scopes able to see exoplanets with the correct coronagraph. The problem is the inner working angle or IWA. This represents how close to the parent star its light is effectively blocked, allowing imaging with minimal interference. Conversely, the outer working angle, OWA , determines how far away from the star a planet can be seen. The IWA is particularly important for seeing and characterising planets in the habitable zone (HBZ) of sun-like stars. By necessity it will need to shrink as the HBZ shrinks, as with M dwarfs, which would obviously make direct imaging of any terrestrial planets discovered in the habitable zones of TESS discoveries very difficult. For bigger stars with wider HBZs obviously the IWA will be less of an issue.
So all of this effectively made a new direct imaging mission, WFIRST AFTA. Unfortunately the NRO mirror was not made for this sort of purpose. It is a Cassegrain design, a so-called “on axis” telescope with the focal plane in line with the primary mirror’s incoming light, with the secondary mirror reflecting its light back through a hole in the primary to whatever science analysis equipment is required. In WFIRST AFTA this would mainly be a spectrograph.
The coronagraph would have to be at the focal plane and along with the secondary mirror, would further obscure the light striking the primary. It would also need squeezing between the “spider’ wires that support the secondary mirror (these give the classic ‘Christmas tree star’ images we are all familiar with in common telescopes).
Two coronagraphs are under consideration that should achieve an image contrast ratio of 10 to the minus 9, which is good enough to view Jupiter-sized planets. Every effort is being made to improve on this and to get down to a level where terrestrial planets can be viewed. Difficult and expensive, but far from impossible. Obviously, WFIRST has quite easily the biggest mirror of the options under consideration by NASA and hence the greatest light intake and imaging range. It could also be possible to put the necessary equipment on board to allow it to use a starshade at a later date. The original WFIRST budget came in at $1.6 billion but that was before NASA came under increasing political pressure on the JWST’s (huge) overspend.
An independent review of cost suggested WFIRST would come in at over $2 billion. Understandably concerned about the potential for “mission creep”, seen with the JWST development, NASA put the WFIRST AFTA design on hold until the budgetary statement of 2017, with no new building commencing until JWST launched. So whatever is eventually picked, 2023 will be the earliest launch date. Same old story, but limited costs sometimes lead to innovation. In the meantime, NASA commissioned two “Probe” class alternative back up concepts to be considered in the “light” of the budgetary statement.
Exoplanet Telescope Alternatives
The first of these is EXO-C. This consists of a 1.5 m “off axis” telescope ( the primary mirror is angled so that the focal plane and secondary mirror are at the side of the telescope and don’t obscure the primary, thus increasing its light gathering ability). There are potential imaging issues with such scopes so they cost more to build. EXO-C has a coronagraph and a spectrograph away from the optical plane. The issue for this concept is which coronagraph to choose. There are many designs, tested over a decade or more with the current “high contrast imaging”( see above) level between 10e9 and 10e10. So EXo-C is relatively low risk and should at a push be able to even see some Earth or Super-Earth planets in the HBZs of some nearby stars, as well as lots of “Jupiters”.
The other Probe mission, even more exciting, is EXO-S. This involves combining a self propelled “on axis” 1.1m telescope with a “starshade”. The starshade is a flower-like (it even has petals) satellite — the choice of a flower shape rather than a round configuration reduces image-spoiling “Fresnel” diffraction from the starshade edges. The shade sits between the telescope and the star to be examined for planets. It casts a shadow in space within which the telescope propels itself to the correct distance for observation (several thousands of kms).
Like a coronagraph, the starshade cuts out the star’s light, but without the difficulty of squeezing an extra device into the telescope. The hard bit is that both telescope and shade need to have radio or laser communication to achieve EXACT positioning throughout the telescopic “stare” to be successful, requiring tight formation flying. The telescope carries propellant for between 3 and 5 years. With several days for moving into position, this is around 100 or so separate stop and stares. The shade concept means two devices instead of one although they can be squeezed into one conventional launch vehicle, to separate at a later point in the mission.
Image: The starshade concept in action. Credit: NASA/JPL.
The good news is that with a starshade the inner working angle is dependent on the telescope starshade distance rather than the telescope. The price of this is that the further apart the two are, the greater the precision placement required. The distances involved depend on the size of the starshade. For EXO-S’ 35 m starshade, this is in excess of thirty seven thousand kilometres. EXO-S, despite its small mirror size, will be able to view and spectrographically characterise terrestrial planets around suitable nearby stars and Jupiter-sized planets considerably further out.
Achieving Space Interferometry
“Formation flying” of telescopes is an entirely new concept that hasn’t been tried before, so potentially more risky, especially as its development is way behind that of coronagraph telescopes. If it works, though, it opens the gate to fantastic discovery in a much wider area than EXO-S. This is just the beginning. If you can get two spacecraft to fly in formation, why not 3 or 30 or even more? In the recent review I wrote for Centauri Dreams on heterodyne interferometers, I described how 30 or so large telescopes could be linked up to deliver the resolution of an telescope with an aperture equivalent to the largest gap between the unit scopes of the interferometer (a diluted aperture). The number of scopes increases light intake ( the brightness of the image) and “baselines” , the gap between constituent scopes in the array, delivering detail across the diluted aperture of the interferometer.
We’re in early days here, but this is heading in the direction of an interferometer in space with resolution orders of magnitude larger than any New Worlds telescope. A terrestrial planet finder yes, but more important, with a good spectrograph, a terrestrial planet characteriser interferometer. TPC-I. To actually “see” detail on an exoplanet would require hundreds of large space telescopes spread over hundreds of kilometers, so that’s one for Star Trek. Detailed atmospheric characterisation, however, is almost as good and not so far in the future if EXO-S gets the go ahead and the Planet Formation Imager evolves on the ground before migrating into space. All roads lead to space.
As an addendum, EXO-S has a yet to be described back-up that could best be seen as WFIRST AFTA-S. Here the starshade has the propulsive system, but the telescope is made from the NRO 2.4 m mirror, thus making the device potentially the most potent of the three designs. Having the drive system on the starshade, along with a radio connector to the telescope, is a concept even newer than the conventional EXO-S . But it is potentially feasible. We await a cost from the final reports the design concept groups need to submit.
In the meantime, various private ventures such as the BoldlyGo Institute run by Jon Morse, formerly of NASA, are hoping to fund and launch a 1.8 m off-axis telescope with BOTH an internal coronagraph AND a starshade. Sadly, the two methods have been found not to work in combination, but obviously a coronagraph telescope can look at stars while its starshade moves into position, increasing critical viewing time over a 3 year mission.
By way of comparison, coronagraphs can and have been used increasingly effectively on ground-based scopes such as Gemini South. It is believed that thanks to atmospheric interference the best contrast image achievable, even with one of the new ELTs being built, will be around 10 to the minus 9, so thanks to their huge light gathering capacity, they too might just discover terrestrial planets around nearby stars but probably not in the HBZ.
The future holds exciting developments. Tantalisingly close. In the meantime, it is important to keep up the momentum of development. The two Probe design groups recognise that their ideas, whilst capable of exciting science as well as just “proof of concept”, are a long way short of what could and should be done. The JWST for all its overspend will hopefully be a resounding success and act as a pathfinder for a large, 16 m plus New Worlds telescope that will start the exoplanet characterisation that will be completed by TPC-I. Collapsible, segmented telescopes will be shown to fit into and work from available launch vehicles, such as the upcoming Space Launch system (SLS), or one of the new Falcon Heavy rockets. New materials such as silicon carbide will reduce telescope costs. The lessons learned from JWST will make such concepts economically viable and deliver ground-shaking findings.
How ironic if would be if we discover other life in another star system before we find it in our own !
Evidence for Plate Tectonics on Europa
It was the Galileo mission, which ended in 2003 when the probe descended into the depths of Jupiter’s atmosphere, that brought us the first solid evidence of an ocean beneath the ice of Europa. Galileo made multiple flybys of the Jovian moon, the first spacecraft to do so, with the closest pass being a scant 180 kilometers on October 15, 2001. As you would imagine, the radiation environment near Europa is hazardous, which is why the flybys were reserved for Galileo’s extended mission. We’ve been mining the Galileo data on Europa ever since.
You may remember that Galileo was unable to open its high-gain antenna on the way to Jupiter, so we had to rely on the ingenuity of mission controllers to get the maximum performance out of the low-gain antenna. That 70 percent of the mission’s science goals were still met, and that we are making new discoveries with the Galileo data today, still amazes me. Now we have new work on Europa that flags the evidence for plate tectonics on the distant moon, which would be the first sign of such activity on any world other than our own.
Simon Kattenhorn (University of Idaho) and Louise Prockter (Johns Hopkins University Applied Physics Laboratory) led this work, which offers visual evidence of the expansion of Europa’s icy crust. A look at Europa’s cracked and ridged surface as sent back by Galileo calls into question how the terrain formed, because while new crust is visible, the mechanism for destroying older crust is not apparent. Kattenhorn and Prockter suggest that this ‘missing terrain’ was absorbed into Europa’s ice shell rather than breaking through it into the ocean that lies beneath. But the evidence for plate tectonics is compelling, and the thickness of the ice shell remains controversial.
Image: Scientists have found evidence of plate tectonics on Jupiter’s moon Europa. This false-color image of the trailing northern hemisphere on Jupiter’s moon Europa — the hemisphere that faces away from Jupiter — shows numerous ridges (red) and band (light-colored) features. Subduction zones — regions where two tectonic plates converge and one is forced beneath the other — may also be present in the study area and are identified by arrows. Image credit: NASA/JPL/University of Arizona.
Plate tectonics describes the motion of large plates in the Earth’s outermost shell, causing earthquakes and volcanic activity as well as mountain-building and the formation of trenches in the oceans as the plates meet. Subduction can carry plate material back into the mantle, while new crust can emerge from seafloor spreading. On Europa’s surface, the break up of crustal material and its replacement by bands of fresh ice from below is apparent. The new material fills in broad bands that are kilometers wide. Kattenhorn and Prockter reconstructed what areas of the surface would have looked like before these disruptions occurred.
Just where was the old crust being destroyed so that the new crust could form? When the researchers looked at areas where subduction similar to Earth’s might be occurring on Europa, they found ice volcanoes on the overriding plate. The smoothness of the surface in these areas implied that older material was forced below rather than remaining as crumpled mountainous terrain on the surface. So now we have evidence not only of material moving up through the ice crust but a mechanism for moving surface material back into the shell.
Simon Kattenhorn comments on the significance of the finding:
“Europa may be more Earth-like than we imagined, if it has a global plate tectonic system. Not only does this discovery make it one of the most geologically interesting bodies in the solar system, it also implies two-way communication between the exterior and interior — a way to move material from the surface into the ocean — a process which has significant implications for Europa’s potential as a habitable world.”
Image: Scientists have found evidence of plate tectonics on Jupiter’s moon Europa. This conceptual illustration of the subduction process (where one plate is forced under another) shows how a cold, brittle, outer portion of Europa’s 20-30 kilometer (roughly 10-20 mile) thick ice shell moved into the warmer shell interior and was ultimately subsumed. A low-relief subsumption band was created at the surface in the overriding plate, alongside which cryolavas may have erupted. Image credit: Noah Kroese, I.NK.
Bear in mind the reason for Galileo’s fiery plunge into the Jovian atmosphere. The spacecraft, its systems degrading in the high-radiation environment, its fuel largely spent, was crashed into the giant planet so that there would be no possibility it might contaminate Europa at some point in the future with bacteria from Earth. Europa remains a target of high astrobiological interest, and preventing even the faintest possibility of contamination kept this fascinating moon pristine. We now ponder what kinds of equipment it might take to explore near-Europa space and the surface itself in hopes of finding evidence of life from below.
The paper is Kattenhorn and Prockter, “Evidence for subduction in the ice shell of Europa,” Nature Geoscience, published online 7 September 2014 (abstract). See also this JHU/APL news release.
Binary Stars: The Likelihood of Planets
In Greg Bear’s novel Queen of Angels
“In the past few weeks, AXIS has returned images of three planets circling Alpha Centauri B. As yet these worlds have not been named, and are called only B-1, B-2, and B-3. B-3 was already known to moonbased astronomers; it is a huge gas giant some ten times larger than Jupiter in our own solar system. Like Saturn, it is surrounded by a thin rugged ring of icy moonlets. B-1 is a barren rock hugging close to Alpha Centauri B, similar to Mercury. But the focus of our attention is now on B-2, a justright world slightly smaller than Earth. B-2 possesses an atmosphere closely approximating Earth’s, as well as continents and oceans of liquid water. It is orbited by two moons each about a thousand kilometers in diameter.”
It’s a tale that is only partially devoted to interstellar matters, but those with an interest in artificial intelligence of a high order indeed and its possibilities in future probes will want to become familiar with it. As you can see, Bear’s guess about Centauri Bb is about right, at least based on what little we know about the candidate world located in a scorching inner orbit. We can rule out the gas giant based on subsequent work which has whittled down the possibilities for large worlds, but we do have the region within 2 AU in which to hope for a stable orbit for another planet (outside of that, planetary orbits according to our simulations are quickly disrupted).
Are we likely to find another Alpha Centauri planet, a hypothetical Centauri Bc? We can certainly hope so, but while we await the lengthy period of data acquisition and analysis that may tell us, we can look at recent work from Elliott Horch (Southern Connecticut State) and team, which has shown, using Kepler data, that 40% to 50% of host stars for exoplanets are binary stars. Says Horch: “It’s interesting and exciting that exoplanet systems with stellar companions turn out to be much more common than was believed even just a few years ago.”
Image: The Kepler field of view, located between two bright stars in the summer triangle, rising over the WIYN telescope in southern Arizona. Credit: NOAO.
Indeed, there was a time not all that long ago when the idea of planets around multiple star systems was considered unlikely because of the gravitational disruptions such systems — at least relatively close binaries — would experience. But a number of studies since the 1990s have demonstrated stable orbits even in systems as close as Alpha Centauri, where the separation between Centauri A and B closes from 40 AU down to a tight 11 AU. That 2 AU of breathing room I mentioned above re Centauri B gives us a planet possibility perhaps as far out as the asteroid belt in our own system if we throw in a fudge factor, but not much further.
As to the work of Horch and company, the researchers used speckle imaging using data from the WIYN telescope located on Kitt Peak in southern Arizona and the Gemini North telescope (Mauna Kea) to look at targets at a rate of 15 to 25 times per second. The resolution achieved through this method, combining the images with suitable algorithms, can detect companion stars that are as much as 125 times as faint as the target star and only 0.05 arcseconds away. The occurrence rate of binaries in this work yields the high percentage of exoplanet host stars that turn out to be binaries, or at least appear to be. From the paper:
After a distance-limited subsample of these objects is constructed, the known statistics concerning binarity among stars near the Sun is added. The simulations predict that the very large majority of sub-arcsecond companions will be physically bound to the Kepler star.
The needed simulations are there to rule out objects that may only be in line of sight with the Kepler Object of Interest star being studied. As this National Optical Astronomy Observatory news release explains, the simulation relies on known statistical properties of binary star systems and line of sight ‘companions.’ Continuing from the paper:
This result suggests that, over the separation range to which we are sensitive, exoplanet host stars have a binary fraction consistent with that of field stars. Our speckle imaging program has identified a sample of candidate binary-star exoplanet systems in which only a modest number of false positives are likely to exist.
Thus the large majority of stellar companions revealed around KOI stars turn out to be actual companion stars rather than line of sight stars not connected with the system. And because we are talking about companion stars with separations between several AU out to no more than 100 AU, we may not always be sure around which star a given planet orbits. Now that binaries are thought to account for about half of known stars, these results suggest that the presence of the companion star does not not adversely affect the formation of planets.
The paper is Horch et al., “Most Sub-Arcsecond Companions of Kepler Exoplanet Candidate Host Stars are Gravitationally Bound,” accepted at The Astrophysical Journal (preprint).
A Deep Probe of Planet Formation
Surrounding the star HD100546, some 335 light years from Earth in the southern hemisphere constellation Musca (The Fly), is a cloud of gas and dust in the shape of a disk. The young star is 30 times brighter than the Sun and about 2.5 times as large. Sean Brittain (Clemson University) and team have now discovered a newly forming planet within the disk, one believed to be a gas giant about three times the size of Jupiter, 13 AU from the host star. They may also have discovered a circumplanetary disk around the newly forming planet.
At work here is a technique called spectro-astrometry, about which a few words. Spectroscopic observations can tell us much about what is happening around young stars, producing data on their motion and helping to resolve close binaries. What becomes problematic with spectroscopy, though, is the need being to improve angular resolution and find ways around the problems created by observing through the Earth’s atmosphere. We don’t yet have the resolution to see how jets form in young stars, for example.
Spectro-astrometry gets around this problem by allowing astronomers to work on scales below the normal limit on resolution set by their equipment. First developed in the early 1980s, the technique compares the positions of objects through different filters, teasing out information at smaller scales by combining the angular position at two different wavelengths. In a paper on the method by Emma Whelan and Paolo Garcia (citation below), the authors describe it as ‘a combination of spectroscopy and astrometry,’ spectroscopy being the analysis of radiation intensity as a function of wavelength, and astrometry the measurement of the precise movements of stars.
Sean Brittain and team used spectro-astrometry by studying tiny changes in the position of carbon monoxide emissions, finding a source of excess carbon monoxide that varies in position and velocity. Because the changes are consistent with orbital motion around the star, the team believes it is seeing emission from a circumplanetary disk of gas orbiting the forming planet. “Another possibility,” adds Brittain, “is that we’re seeing the wake from tidal interactions between the object and the circumstellar disk of gas and dust orbiting the star.”
Joan Najita (National Optical Astronomy Observatory), a member of Brittain’s team, places the method in context:
“We stumbled onto this project when a paper in the literature predicted that forming planets would induce a detectable signature in the CO emission from disks. Because we had studied HD100546 for many years, we could immediately test this idea in one system. It was uncanny that the first system we studied actually showed the signature of orbital motion. It’s not every day that you look for something exciting and actually find it! But the test of any interpretation is to make a prediction and see if it is verified. We are thrilled that the data recently reported confirm the signature of orbital motion that we predicted based on our earlier work.”
Image: An artist’s conception of the young massive star HD100546 and its surrounding disk. A planet forming in the disk has cleared the disk within 13AU of the star, a distance comparable to that of Saturn from the sun. As gas and dust flows from the circumstellar disk to the planet, this material surrounds the planet as a circumplanetary disk (inset). These rotating disks are believed to be the birthplaces of planetary moons, such as the Galilean moons that orbit Jupiter. While they are theoretically predicted to surround giant planets at birth, there has been little observational evidence to date for circumplanetary disks outside the solar system. Brittain et al. (2014) report evidence for an orbiting source of carbon monoxide emission whose size is consistent with theoretical predictions for a circumplanetary disk. Observations over 10 years trace the orbit of the forming planet from behind the near side of the circumstellar disk in 2003 to the far side of the disk in 2013. These observations provide a new way to study how planets form. Credit: P. Marenfeld & NOAO/AURA/NSF.
The idea of a circumplanetary disk around a young gas giant is not unusual, as it would act as the breeding ground for systems of planetary moons like those around Jupiter and Saturn. To my knowledge, however, this would be the first time one has been observed. HD100546 has previously produced evidence of another planet in formation, one at about the distance of Pluto from the Sun that appears to be a gas giant of roughly Jupiter mass. John Carr (Naval Research Laboratory) is a co-author on the paper describing these findings:
“The possibility that we have caught a planet in the act of formation is an exciting result. What makes this work doubly interesting is the evidence that we are seeing gas as it swirls around and flows onto the planet to feed its continuing growth. This could be observational confirmation for the existence of circumplanetary disks that are predicted to surround giant planets at birth. An important point in this research is that we were able to track the object over a period of several years and show that it is indeed orbiting around the star as expected for a planet.”
So we are evidently looking at a solar system engaged in the birth of multiple planets, showing signs of the disk formation that may one day result in stable worlds, each circled by its own system of moons. Up next for HD100546 will be close inspection through instruments like the European Southern Observatory’s Very Large Telescope or the Gemini South Telescope as we probe this useful celestial laboratory of planet formation.
The paper is Brittain et al., “NIR Spectroscopy of the HAeBe Star 100546. III. Further Evidence of an Orbiting Companion?” The Astrophysical Journal 791 (2014), 136 (preprint). The Whelan and Garcia paper on spectro-astrometry is “Spectro-astrometry: The Method, its Limitations and Applications,” in Jets from Young Stars II, Lecture Notes in Physics Volume 742 (2008), pp. 123-149. A Clemson University news release is also available.
Jim Benford: Final Comments on Particle Beam Propulsion
Our recent discussion of deep space magsails propelled by neutral particle beams inspired a lot of comments and a round of comment response from author Jim Benford. For those just joining us, Benford had studied a magsail concept developed by Alan Mole and discussed by Dana Andrews, with findings that questioned whether interstellar applications were possible, though in-system work appeared to be. The key issue was the divergence of the beam, sharply reducing its effectiveness at the sail. Today we’ll wrap up the particle beam sail story for now, with Jim’s thoughts on the latest round of comments. The full paper on this work is headed for one of the journals for peer review there and eventual publication. I’ll be revisiting particle beam propulsion this fall, and of course the comments on the current articles remain open.
by James Benford
Eric Hughes wrote in the comments that my work had shown only that one method of neutralizing the neutral particle beam would produce divergence. Specifically, his comment read: “I think it’s important to recall that Benford’s article last Friday only addresses one class of methods for making a neutral particle beam. He acknowledges that himself in the last sentence of the article, when he speaks of “much more advanced beam divergence technology than we have today.”
Are there other methods of producing these beams that don’t produce divergence? Let me re-state my basic argument:
- Accelerating low-energy particles in electromagnetic fields produces high-energy particle beams.
- For those electromagnetic fields to interact with the particles, the particles must be charged. Only charged particles interact with electromagnetic fields.
- Therefore, accelerating charged particles to high-energy to produce the final beam, which is then neutralized, produces neutral beams.
- I showed that the neutralization process itself would produce an irreducible divergence. This applies to all methods for producing neutral beams.
- The only possible exception would be to produce high-energy neutral particles by nuclear reactions. But nuclear reactions are not highly directional and won’t produce a narrowly collimated beam.
- Consequently, the argument I made is quite general and fundamentally limits the properties of neutral beams.
On the other comments, these remarks: James Essig is certainly correct that the Sun provides plenty enough power for thrusters to maintain the Beamer in place. A more demanding problem is how to operate such powerful thrusters while not disturbing the microradian pointing of the beam. The beam has to stay on the sail for a long time and variations in the thrusters’ sideways motion could easily direct it away from the sail.
Electrostatic and magnetic forces never cancel no matter how relativistic the beam is; certainly they are far from cancellation for the example, where gamma is only 1.02.
Eniac hopes that gravity will provide a restoring force to the momentum of the beam generator. No such thing happens. Gravity is an attractive force. There will be a restoring force only in a potential well such as a Lagrange point, but these are noticeably weak and not up to the scale of these forces.
Eniac also writes: “Would the beam be dense enough to tear the field right off the loop and carry it away, leaving the craft behind? Yes, I think moving plasma does wreak havoc on fields that way.”
But the answer is no. The magnetic field won’t depart unless the current leaves the conductor. What does it flow in then?
The transform of the magnetic field to the moving frame of the beam is given by the product of gamma, beta and the field strength. My estimate is that ionization will be easy. Eniac’s 10 GV/m for ionization, when only 13 eV is needed, would mean that there would never be ionization in the universe, so this number is ridiculously far off.
Michael and others seem to think that the charged particles will not interact strongly if they are far apart. But they cannot be far apart and part of a beam going out to hit this 270 m sail. Divergence inevitably follows.
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