‘Hot Jupiters’: Explaining Spin-Orbit Misalignment

by Paul Gilster on September 16, 2014

Bringing some order into the realm of ‘hot Jupiters’ is all to the good. How do these enormous worlds get so close to their star, having presumably formed much further out beyond the ‘snowline’ in their systems, and what effects do they have on the central star itself? And how do ‘hot Jupiter’ orbits evolve so as to create spin-orbit misalignments? A team at Cornell University led by astronomy professor Dong Lai, working with graduate students Natalia Storch and Kassandra Anderson, has produced a paper that tells us much about orbital alignments and ‘hot Jupiter’ formation.

It’s no surprise that large planets — and small ones, for that matter — can make their stars wobble. This is the basis for the Doppler method that so accurately measures the movement of a star as affected by the planets around it. But something else is going on in ‘hot Jupiter’ systems. In our own Solar System the rotational axis of the Sun is more or less aligned with the orbital axis of the planets. But some systems with ‘hot Jupiters’ have shown a misalignment between the orbital axis of the gas giants and the rotational axis of the host star.

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Image: ‘Hot Jupiters,’ large, gaseous planets in inner orbits, can make their suns wobble after they wend their way through their solar systems. Credit: Dong Lai/Cornell University.

The Cornell team went to work on simulations of such systems, working with binary star systems separated by as much as hundreds of AU. Their work shows that gas giants can be influenced by partner binary stars that cause them to migrate closer to their star. At play here is the Lidov-Kozai mechanism in celestial mechanics, an effect first described by Soviet scientist Michael Lidov in 1961 and studied by the Japanese astronomer Yoshihide Kozai. The effect of perturbation by an outer object is an important factor in the orbits of planetary moons, trans-Neptunian objects and some extrasolar planets in multiple star systems.

Thus the mechanism for moving a gas giant into the inner system, as described in the paper:

In the ‘Kozai+tide’ scenario, a giant planet initially orbits its host star at a few AU and experiences secular gravitational perturbations from a distant companion (a star or planet). When the companion’s orbit is sufficiently inclined relative to the planetary orbit, the planet’s eccentricity undergoes excursions to large values, while the orbital axis precesses with varying inclination. At periastron, tidal dissipation in the planet reduces the orbital energy, leading to inward migration and circularization of the planet’s orbit.

As the planet approaches the star, interesting things continue to occur. From the paper:

It is a curious fact that the stellar spin axis in a wide binary (~ 100 AU apart) can exhibit such a rich, complex evolution. This is made possible by a tiny planet (~ 10-3 of the stellar mass) that serves as a link between the two stars: the planet is ‘forced’ by the distant companion into a close-in orbit, and it ‘forces’ the spin axis of its host star into wild precession and wandering.

Moreover, “…in the presence of tidal dissipation the memory of chaotic spin evolution can be preserved, leaving an imprint on the final spin-orbit misalignment angles.”

The approach of the ‘hot Jupiter’ to the host star can, in other words, disrupt the previous orientation of the star’s spin axis, causing it to wobble something like a spinning top. The paper speaks of ‘wild precession and wandering,’ a fact that Lai emphasizes, likening the chaotic variation of the precession to chaotic phenomenon such as weather systems. The spin-orbit misalignments we see in ‘hot Jupiter’ systems are thus the result of the evolution of changes to the stellar spin caused by the migration of the planet inward.

The paper goes on to mention that we see examples of chaotic spin-orbit resonances in our own Solar System. Saturn’s satellite Hyperion experiences what the paper calls ‘chaotic spin evolution’ because of resonances between its spin and orbital precession periods. Even the rotation axis of Mars undergoes chaotic variation due to much the same mechanism.

The paper is Storch, Anderson & Lai, “Chaotic dynamics of stellar spin in binaries and the production of misaligned hot Jupiters,” Science Vol. 345, No. 6202 (12 September 2014), pp. 1317-1321 (abstract / preprint)

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Emergence of the ‘Venus Zone’

by Paul Gilster on September 15, 2014

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.

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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.

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Space Telescopes Beyond Hubble and JWST

by Paul Gilster on September 12, 2014

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

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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.

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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.

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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 !

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Evidence for Plate Tectonics on Europa

by Paul Gilster on September 11, 2014

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.

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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.”

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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.

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Binary Stars: The Likelihood of Planets

by Paul Gilster on September 10, 2014

In Greg Bear’s novel Queen of Angels (Gollancz, 1990), a robotic probe called AXIS (Automated eXplorer of Interstellar Space) has used antimatter propulsion to make a fifteen-year crossing to Alpha Centauri. The world’s various networks of the future begin to feast on reports of what it finds, like this one:

“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.”

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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).

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A Deep Probe of Planet Formation

by Paul Gilster on September 9, 2014

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.”

Circumplanetary-Disk-PR

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.

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Jim Benford: Final Comments on Particle Beam Propulsion

by Paul Gilster on September 8, 2014

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

James-Benford-starship-255x300

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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Andreas Hein is a familiar figure in these pages, having written on the subject of worldships as well as the uploading of consciousness. He is Deputy Director of the Initiative for Interstellar Studies (I4IS), as well as Director of its Technical Research Committee. He founded and leads Icarus Interstellar’s Project Hyperion: A design study on manned interstellar flight. Andreas received his master’s degree in aerospace engineering from the Technical University of Munich and is now working on a PhD there in the area of space systems engineering, having conducted part of his research at MIT. He spent a semester abroad at the Institut Superieur de l’Aeronautique et de l’Espace in Toulouse and also worked at the European Space Agency Strategy and Architecture Office on future manned space exploration. Today’s essay introduces the Initiative for Interstellar Studies’ Project Dragonfly Design Competition.

by Andreas Hein

Hein_official_LRT_picture_v2

2089, 5th April: A blurry image rushes over screens around the world. The image of a coastline, waves crashing into it, inviting for a nice evening walk at dawn. Nobody would have paid special attention, if it were not for one curious feature: Two suns were mounted in the sky, two bright, hellish eyes. The first man-made object had reached another star system.

Is it plausible to assume that we could send a probe to another star within our century? One major challenge is the amount of resources needed for such a mission. [1, 2]. Ships proposed in the past were mostly mammoths, weighing ten-thousands of tons: the fusion-propelled Daedalus probe with 54,000 tonnes and recently the Project Icarus Ghost Ship with over 100,000 tonnes. All these concepts are based on the rocket principle, which means that they have to take their propellant with them to accelerate. This results in a very large ship.

Another problem with fusion propulsion in particular is the problem of scalability. Most fusion propulsion systems get more efficient when they are scaled up. There is also a critical lower threshold for how small you can go. These factors lead to large amounts of needed propellant and large engines, for which you need a large space infrastructure. A Solar System-wide economy is probably needed, as the Project Daedalus report argues [3].

Icarus Ghost Ship

Image: The Project Icarus Ghost Ship: A colossal fusion-propelled interstellar probe
http://www.spaceanswers.com/futuretech/ghost-ship-to-alpha-centauri/

However, there is a different avenue for interstellar travel: going small. If you go small, you need less energy for accelerating the probe and thus less resources. Pioneers of small interstellar missions are Freeman Dyson with his Astrochicken; a living, one kilogram probe, bio-engineered for the space environment [4]. Robert Forward proposed the Starwisp probe in 1985 [5]. A large, ultra-thin sail which rides on a beam of microwaves. Furthermore, Frank Tipler and Ray Kurzweil describe how nano-scale probes could be used for transporting human consciousness to the stars [6, 7].

At the Initiative for Interstellar Studies (I4IS), we wanted to have a fresh look at small interstellar probes, laser sail probes in particular. The last concepts in this area have been developed years ago. How did the situation change in recent years? Are there new, possibly disruptive concepts on the horizon? We think there are. The basic idea is to develop an interstellar mission by combining the following technologies:

  • Laser sail propulsion: The spacecraft rides on a laser beam, which is captured by an extremely thin sail [8].
  • Small spacecraft technology: Highly miniaturized spacecraft components which are used in CubeSat missions
  • Distributed spacecraft: To spread out the payload of a larger spacecraft over several spacecraft, thus, reducing the laser power requirements [9, 10]. The individual spacecraft would then rendezvous at the target star system and collaborate to fulfill their mission objectives. For example, one probe is mainly responsible for communication with the Solar System, another responsible for planetary exploration via distributed sensor networks (smart dust) [11].
  • Magnetic sails: A thin superconducting ring’s magnetic field deflects the hydrogen in the interstellar medium and decelerates the spacecraft [12].
  • Solar power satellites: The laser system shall use space infrastructure which is likely to exist in the next 50 years. Solar power satellites would be temporarily leased to provide the laser system with power to propel the spacecraft.
  • Communication systems with external power supply: A critical factor for small interstellar missions is power supply for the communication system. As small spacecraft cannot provide enough power for communicating over these vast distances. Thus, power has to be supplied externally, either by using laser or microwave power from the Solar System during the trip and solar radiation within the target star system [5].

Size Comparison

Image: Size comparison between an interplanetary solar sail and the Project Icarus Ghost Ship. Interstellar sail-based spacecraft would be much larger. (Courtesy: Adrian Mann and Kelvin Long)

Bringing all these technologies together, it is possible to imagine a mission which could be realized with technologies which are feasible in the next 10 years and could be in place in the next 50 years: A set of solar power satellites are leased for a couple of years for the mission. A laser system with a huge aperture has been put into a suitable orbit to propel the interstellar, as well as future planetary missions. Thus, the infrastructure can be reused for multiple purposes. The interstellar probes are launched one-by-one.

After decades, the probes start to decelerate by magnetic sails. Each spacecraft charges its sails differently. The first spacecraft decelerates slower than the follow-up probes. Ideally, the spacecraft then arrive at the target star system at the same point in time. Then, the probes start exploring the star system autonomously. They reason about exploration strategies, exchange and share data. Once a suitable exploration target has been chosen, dedicated probes descend to the planetary surface, spreading dust-sized sensor networks onto the pristine land. The data from the network is collected by other spacecraft and transferred back to the spacecraft acting as a communication hub. The hub, powered by the light from extrasolar light sends back the data to us. The result could be the scenario described at the beginning of this article.

Artistic impression

Image: Artist’s impression of a laser sail probe with a chip-sized payload. (Courtesy: Adrian Mann)

Of course, one of the caveats of such a mission is its complexity. The spacecraft would have to rendezvous precisely over interstellar distances. Furthermore, there are several challenges with laser sail systems, which have been frequently addressed in the literature, for example beam collimation and control. Nevertheless, such a mission architecture has many advantages compared to existing ones: It could be realized by a space infrastructure we could imagine to exist in the next 50 years. The failure of one or more spacecraft would not be catastrophic, as redundancy could easily be built in by launching two or more identical spacecraft.

The elegance of this mission architecture is that all the infrastructure elements can also be used for other purposes. For example, a laser infrastructure could not only be used for an interstellar mission but interplanetary as well. Further applications include an asteroid defense system [20]. The solar power satellites can be used for providing in-space infrastructure with power [18].

spacecraft swarm

Image: Artist’s impression of a spacecraft swarm arriving at an exosolar system (Courtesy: Adrian Mann)

How about the feasibility of the individual technologies? Recent progress in various areas looks promising:

  • The increased availability of highly sophisticated miniaturized commercial components: smart phones include many components which are needed for a space system, e.g. gyros for attitude determination, a communication system, and a microchip for data-handling. NASA has already flown a couple of “phone-sats”; Satellites which are based on a smart phone [13].
  • Advances in distributed satellite networks: Although a single small satellite only has a limited capability, several satellites which cooperate can replace larger space systems. The concept of Federated Satellite Systems (FSS) is currently explored at the Massachusetts Institute of Technology as well as at the Skolkovo Institute of Technology in Russia [14]. Satellites communicate opportunistically and share data and computing capacity. It is basically a cloud computing environment in space.
  • Increased viability of solar sail missions. A number of recent missions are based on solar sail technology, e.g. the Japanese IKAROS probe, LightSail-1 of the Planetary Society, and NASA’s Sunjammer probe.
  • Greg Matloff recently proposed use of Graphene as a material for solar sails [15]. With an areal density of a fraction of a gram and high thermal resistance, this material would be truly disruptive. Currently existing materials have a much higher areal density; a number crucial for measuring the performance of solar sails.
  • Material sciences has also advanced to a degree where Graphene layers only a few atoms thick can be manufactured [16]. Thus, manufacturing a solar sail based on extremely thin layers of Graphene is not as far away as it seems.
  • Small satellites with a mass of only a few kilograms are increasingly proposed for interplanetary missions. NASA has recently announced the Interplanetary CubeSat Challenge, where teams are invited to develop CubeSat missions to the Moon and even deeper into space (NASA) [17]. Coming advances will thus stretch the capability of CubeSats beyond Low-Earth Orbit.
  • Recent proposals for solar power satellites focus on providing space infrastructure with power instead of Earth infrastructure [18, 19]. The reason is quite simple: Solar power satellites are not competitive to most Earth-based alternatives but they are in space. A recent NASA concept by John Mankins proposed the use of a highly modular tulip-shaped space power satellite, supplying geostationary communication satellites with power.
  • Large space laser systems have been proposed for asteroid defense [20]

In order to explore various mission architectures and encourage participation by a larger group of people, I4IS has recently announced the Project Dragonfly Competition in the context of the Alpha Centauri Prize [21]. We hope that with the help of this competition, we can find unprecedented mission architectures of truly disruptive capability. Once this goal is accomplished, we can concentrate our efforts on developing individual technologies and test them in near-term missions.

If this all works out, this might be the first time in history that there is a realistic possibility to explore a near-by star system within the 21st or early 22nd century with “modest” resources.

References

[1] Millis, M. G. (2010). First Interstellar Missions, Considering Energy and Incessant Obsolescence. Journal of the British Interplanetary Society, 63(11), 434.

[2] Hein, A. M. (2012). Evaluation of Technological-Social and Political Projections for the Next 100-300 Years and the Implications for an Interstellar Mission. Journal of the British Interplanetary Society, 65, 330-340.

[3] Martin, A. R. (Ed.). (1978). Project Daedalus: The Final Report on the BIS Starship Study. British Interplanetary Soc.

[4] Dyson, F. J. (1979). Disturbing the universe. Basic Books.

[5] Forward, R. L. (1985). Starwisp-An ultra-light interstellar probe. Journal of Spacecraft and Rockets, 22(3), 345-350.

[6] Tipler, F. (1994), The Physics of Immortality, Chapter 2, Doubleday, New York.

[7] Kurzweil, R. (2005). The singularity is near: When humans transcend biology. Penguin.

[8] Forward, R. L. (1984). Roundtrip interstellar travel using laser-pushed lightsails. Journal of Spacecraft and Rockets, 21(2), 187-195.

[9] Mathieu, C., & Weigel, A. L. (2005, August). Assessing the flexibility provided by fractionated spacecraft. In Proc. of AIAA Space 2005 Conference, Long Beach, CA, USA.

[10] Brown, O., & Eremenko, P. (2006). Fractionated space architectures: a vision for responsive space. Defense Advanced Research Projects Agency, Arlington, VA.

[11] Colombo, C., & McInnes, C. (2011). Orbital Dynamics of” Smart-Dust” Devices with Solar Radiation Pressure and Drag. Journal of Guidance, Control, and Dynamics, 34(6), 1613-1631.

[12] Andrews, D., & Zubrin, R. (1990). Magnetic sails and interstellar travel. Journal of the British Interplanetary Society 43, 265-272.

[13] Wikipedia, Phonesat: http://en.wikipedia.org/wiki/PhoneSat

[14] Golkar, A. (2013, April). Federated Satellite Systems: an Innovation in Space Systems Design. In 9th IAA Symposium on Small Satellites for Earth Observation, IAA, Berlin, Germany.

[15] Matloff, G. L. (2012). Graphene, the Ultimate Interstellar Solar Sail Material? Journal of the British Interplanetary Society, 65, 378-381.

[16] Paton, K. R., Varrla, E., Backes, C., Smith, R. J., Khan, U., O’Neill, A., … & Coleman, J. N. (2014). Scalable production of large quantities of defect-free few-layer graphene by shear exfoliation in liquids. Nature Materials, 13(6), 624-630.

[17] NASA Interplanetary Cubesat Challenge: http://sservi.nasa.gov/articles/interplanetary-cubesat-challenge/

[18] Mankins, J., Kaya, N., & Vasile, M. (2012). Sps-alpha: The first practical solar power satellite via arbitrarily large phased array (a 2011-2012 nasa niac project). In 10th International Energy Conversion Engineering Conference.

[19] Mankins, J.C. (2014). The Case for Space Solar Power, Virginia Edition Publishing.

[20] Hughes, G. B., Lubin, P., Bible, J., Bublitz, J., Arriola, J., Motta, C., … & Pryor, M. (2013, September). DE-STAR: Phased-array laser technology for planetary defense and other scientific purposes. In SPIE Optical Engineering+ Applications (pp. 88760J-88760J). International Society for Optics and Photonics.

[21] I4IS Project Dragonfly Design Competition: http://i4is.org/news/dragonfly

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Laniakea: Milky Way’s Address in the Cosmos

by Paul Gilster on September 4, 2014

Science fiction writers have a new challenge this morning: To come up with a plot that takes in not just the galaxy and not just the Local Group in which the Milky Way resides, but the far larger home of both. Laniakea is the name of this supercluster, after a Hawaiian word meaning ‘immense heaven.’ And immense it is. Superclusters are made up of groups like the Local Group — each of these contain dozens of galaxies — and clusters that contain hundreds more, interconnected by a filamentary web whose boundaries have proven hard to define.

Where does one supercluster begin and another end? As explained in a cover story in the September 4 issue of Nature, an emerging way to tune up our cosmic maps is to look at the effect of large-scale structures on the movements of galaxies. A team under R. Brent Tully (University of Hawaii at Manoa) has been using data from radio telescopes to study the velocities of 8000 galaxies, adjusting for the universe’s accelerating expansion to create a map of the cosmic flow of these galaxies as determined by gravitational effects.

The boundaries between superclusters, such as those between Laniakea and the Perseus-Pisces Supercluster, are where the galactic flows diverge and neighboring structures shear apart. As this National Radio Astronomy Observatory news release points out, within the boundaries of the Laniakea Supercluster, the motions of galaxies are directed inward. In other superclusters, the flow of galaxies goes toward a different gravitational center.

This is how our horizons get adjusted. Previously we thought of the Milky Way as part of the Virgo Supercluster, but now we see even this region as just part of the far larger Laniakea Supercluster. We’re talking about a structure some 520 million light years in diameter that contains the mass of one hundred million billion suns across a staggering 100,000 galaxies. And just as the Sun is in the galactic ‘suburbs’ of the Milky Way, a long way from the galaxy’s teeming center, so the Milky Way itself lies on the outskirts of the Laniakea Supercluster.

GBTSupercluster2_nrao

Image: A slice of the Laniakea Supercluster in the supergalactic equatorial plane — an imaginary plane containing many of the most massive clusters in this structure. The colors represent density within this slice, with red for high densities and blue for voids — areas with relatively little matter. Individual galaxies are shown as white dots. Velocity flow streams within the region gravitationally dominated by Laniakea are shown in white, while dark blue flow lines are away from the Laniakea local basin of attraction. The orange contour encloses the outer limits of these streams, a diameter of about 160 Mpc. This region contains the mass of about 100 million billion suns. Credit: SDvision interactive visualization software by DP at CEA/Saclay, France.

Those of us with an interest in Polynesia will love the name Laniakea, which was chosen to honor the Polynesian sailors who used their deep knowledge of the night sky to navigate across the Pacific. If you look through the essays in Interstellar Migration and the Human Experience (University of California Press, 1985), you’ll find several that dwell on the historical example of the Polynesian navigators as a way of examining future migration into the stars. The theme resonates and I invariably hear it mentioned at the various conferences on interstellar flight.

The diagrams below offer another way of viewing the gravitational interactions that pull together the immense supercluster. You’ll notice the Great Attractor, a gravitational focal point that influences the motion of galaxy clusters including our own Local Group. The NRAO refers to it as a ‘gravitational valley’ whose effects can be felt across the Laniakea Supercluster.

GBTSupercluster_nrao_2

Image: Two views of the Laniakea Supercluster. The outer surface shows the region dominated by Laniakea’s gravity. The streamlines shown in black trace the paths along which galaxies flow as they are pulled closer inside the supercluster. Individual galaxies’ colors distinguish major components within the Laniakea Supercluster: the historical Local Supercluster in green, the Great Attractor region in orange, the Pavo-Indus filament in purple, and structures including the Antlia Wall and Fornax-Eridanus cloud in magenta. Credit: SDvision interactive visualization software by DP at CEA/Saclay, France.

Have a look at this video from Nature to see the whole supercluster set in motion.

So now we know that our home supercluster is actually 100 times larger in volume and mass than we previously thought. In an article summarizing these findings in Nature, Elizabeth Gibney points out that a somewhat different definition of a supercluster is being used by Gayoung Chon (Max Planck Institute for Extraterrestrial Physics, Germany) and colleagues, who base their definition on structures that will one day collapse into a single object, something that cannot be said for Laniakea because some of its galaxies will always move away from each other. Clearly, the definition of a supercluster is a work in progress, but let’s hope the name sticks.

The paper is Tully et al., “The Laniakea supercluster of galaxies,” Nature 513 (4 September 2014), 71-73 (abstract).

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Red Dwarf Planets: Weeding Out the False Positives

by Paul Gilster on September 3, 2014

For those of you who, like me, are fascinated with red dwarf stars and the prospects for life around them, I want to mention David Stevenson’s Under a Crimson Sun (Springer, 2013), with the caveat that although it’s on my reading list, I haven’t gotten to it yet. More about this title after I’ve gone through it, but for now, notice that the interesting planet news around stars like Gliese 581 and GJ 667C is catching the eye of publishers and awakening interest in the public. It’s easy to see why. Planets in the habitable zone of such stars would be exotic places, far different from Earth, but possibly bearing life.

At the same time, we’re learning a good deal more about both the above-mentioned stars. A new paper by Paul Robertson and Suvrath Mahadevan (both at Pennsylvania State) looks at GJ 667C with encouraging — and cautionary — results. The encouraging news is that GJ 667Cc, a super-Earth in the habitable zone of the star, is confirmed by their work. The cautionary note is that stellar activity can mimic signals that we can interpret, wrongly, as exoplanets, and not every planet thought to be in this system may actually be there.

gj667cc

Image: The view from GJ 667Cc as presented in an artist’s impression. Note the distant binary to the right of the parent red dwarf. New work confirms the existence of this interesting world in the habitable zone. Credit: ESO/L. Calçada.

Remember that this is a system that was originally thought to have two super-Earths: GJ 667Cb and GJ 667Cc. The complicated designation is forced by the fact that the red dwarf in question, GJ 667C, is part of a triple star system, a distant companion to the binary pair GJ 667AB. It was just last year that the first two planets around GJ 667C were announced, followed by results from a different team showing five more super-Earths around the same star. See Gliese 667C: Three Habitable Zone Planets for my discussion of the apparent result, which at the time seemed spectacular.

The new paper from Robertson and Mahadevan takes a critical look at this system, examining the amount of stellar activity found in the host star and finding ways to study the average width of the star’s spectral absorption lines, which should flag changes to the spectrum being produced by magnetic features like starspots. Using these methods the team was able to remove the stellar activity component from the observed signals, allowing the signature of the real planets to remain while suggesting problems with the other candidates.

GJ 667Cc survives the test, a happy outcome for those interested in the astrobiological prospects here. GJ 667Cb also makes the cut, but Robertson and Mahadevan believe that planet d in this system, originally thought to be near the outer edge of the habitable zone, is a false positive created by stellar activity and the rotation of the star. As for the other planet candidates in this system, Paul Robertson has this to say in an online post:

The signals associated with them are so small that they cannot be seen with “industry-standard” analysis techniques, regardless of whether we have corrected for activity. However, considering how successful our activity correction has been at boosting the signals of real planets, the fact that we see no sign of any of these planet candidates after the activity correction leads us to strongly doubt their existence.

All of this should remind us not to jump too swiftly to conclusions about planet candidates, particularly given the sensitivity involved with the spectrographs used in our planet-finding work. Radial velocity data that looks strong can actually be the result of magnetic events on the surface of the star being observed. When Robertson and Mahadevan looked at Gliese 581, another highly interesting system because of planets possibly in the habitable zone, they found no sign of Gliese 581g, a controversial candidate whose existence is still being debated. In Gliese 581 and the Stellar Activity Problem, Robertson has this to say:

With an orbital period of 33 days, the controversial “planet g” also lies at an integer ratio of the stellar rotation period. Sure enough, no sign of g remains after our activity correction, revealing that it too was an artifact of magnetic activity. While this outcome is certainly disappointing for anyone hoping to find signs of life in the GJ 581 system, it is heartening to finally put the confusion and dispute surrounding this system to rest.

Moreover, another candidate potentially in the habitable zone, Gl 581d, falls back into the measurement noise, meaning that it was another signature of stellar activity rather than an actual planet. The red dwarf Gliese 581 is thus reduced to three planets in its system, and we’ve lost the best candidates for life. That may sound discouraging, but I think we can take heart from the fact that work like this shows we’re getting much better at eliminating false positives. Using these methods, real planets stand out in the data, which means we can more readily identify habitable zone planets as our spectrographic instrumentation improves.

The paper is Robertson and Mahadevan, “Disentangling Planets and Stellar Activity for Gliese 667C,” accepted for publication at Astrophysical Journal Letters (preprint). For Gliese 581, see Robertson and Mahadevan, “Stellar Activity Masquerading as Planets in the Habitable Zone of the M dwarf Gliese 581,” published in Science Express (3 July 2014).

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