by Paul Gilster | Sep 9, 2014 | Deep Sky Astronomy & Telescopes |
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.
by Paul Gilster | Sep 8, 2014 | Sail Concepts |
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.
by Paul Gilster | Sep 5, 2014 | Sail Concepts |
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
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].
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].
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.
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].
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
by Paul Gilster | Sep 4, 2014 | Deep Sky Astronomy & Telescopes |
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.
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.
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).
by Paul Gilster | Sep 3, 2014 | Exoplanetary Science |
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.
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).
