by Paul Gilster | Jan 15, 2015 | Outer Solar System |
How objects in the sky get named is always interesting to me. You may recall that the discovery of Uranus prompted some interesting naming activity. John Flamsteed, the English astronomer who was the first Astronomer Royal, observed the planet in 1690 and catalogued it as 34 Tauri, thinking it a star, as did French astronomer Pierre Lemonnier when he observed it in the mid-18th Century. William Herschel, seeing Uranus in 1781, thought at first that it was a comet, and reported it as such to the Royal Society.
By 1783, thanks to the work of the Russian astronomer Anders Lexell and Berlin-based Johann Elert Bode, Herschel came to agree that the new object was indeed a planet. Herschel, asked by then Astronomer Royal Nevil Maskelyne to name the new world, declared it to be Georgium Sidus, the ‘Georgian Planet,’ a name honoring King George III. The unpopular name soon met with alternative suggestions, including Herschel, Neptune and (Bode’s own idea) Uranus.
Image: Sir William Herschel (1738-1822), whose idea about naming a new planet met with scant approval. Credit: Lemuel Francis Abbott – National Portrait Gallery.
Herschel had worried that naming planets after the ‘ principal heroes and divinities’ of ancient eras would be out of place in his time, suggesting that naming them after the era they were discovered (hence, the reign of George III) would be the more satisfactory method. But of course we haven’t followed the suggestion, and now look not only for the names of ancient beings both human and divine as well as names related to specific cultures. The geography of Ceres, for example is to be named after mythology associated with agriculture and vegetation, a nod to Giuseppe Piazzi, its discoverer, who knew Ceres as the Roman goddess of agriculture.
The problem with all this is that we’re making so many discoveries that we’re taxing our ability to come up with the best nomenclature. Some 24 craters on Saturn’s moon Phoebe have been named by classical reference to the Argonauts, the intrepid adventurers who sailed with Jason to find the Golden Fleece. But the Gazetteer of Planetary Nomenclature also notes that future craters on Phoebe may have names associated with the goddess, who was, according to ancient lore, a Titan, the daughter of Uranus and Gaea. As mapping continues, features other than craters may acquire names based on Appollonius Rhodius’ 3rd C. text The Argonautica.
Titan, much in our thoughts with the 10th anniversary of the landing of the Huygens probe, gets plenty of attention from the International Astronomical Union, the U.S. Geological Survey, and NASA, all of whom have a hand in determining the names of features. Craters on the Saturnian moon take the names of gods and goddesses of wisdom, while a variety of surface features are open to names drawn from characters from Tolkien’s Middle Earth, characters from the Foundation series by Isaac Asimov and the names of planets from Frank Herbert’s Dune novels, surely a nod to science fictional interests among researchers.
And let’s not forget Xanadu, a plateau-like, highly reflective region on Titan, a name deriving ultimately from the Yuan Dynasty’s summer capital as established by Kublai Khan and immortalized in the West by Samuel Taylor Coleridge. Interesting places, these new worlds, and full of so many features that need names! When Makemake was discovered soon after Easter in 2005, it was immediately nicknamed Easterbunny, but later yielded to an IAU-sanctioned monicker based on fertility mythology on Rapa Nui, which most of us know as Easter Island.
I could go on with this entertaining subject indefinitely, even sticking within our own Solar System. The Uranian satellite Miranda, for instance, draws feature names from characters in Shakespeare’s plays, as do all the major moons of Uranus, though small satellites can draw on names from the poetry of Alexander Pope. We’ll doubtless have plenty of suggestions for features on Pluto once New Horizons gets close enough to see them. The theme there will be underworld deities. New moons like Nyx and Hydra have already received names according to this convention.
What happens when we turn to exoplanets? With so many being discovered, it’s no surprise that the International Astronomical Union has organized a global contest to name selected exoplanets. The NameExoWorlds contest is already open, with a first round that will allow nominations for ExoWorlds (by this, the IAU means the entire exoplanetary system and its host star) to be made available for the next stage of the contest, where names can be proposed.
Image: An artist’s impression of Alpha Centauri Bb. How many place names will we eventually have to come up with for places like this? Credit: ESO/L. Calçada/Nick Risinger.
The IAU, which goes about assigning scientifically recognized names to newly discovered objects, says that the NameExoWorlds contest will be the first opportunity for the public to name both exoplanets and the stars around which they orbit. To participate, clubs and non-profit organizations have to register with the IAU Directory of World Astronomy by May 15, 2015. The deadline for the first stage of the contest is February 15, 2015, when the nominating process for the first 20 ExoWorlds is to close. After that, each club or organization will be allowed to submit names, with a later worldwide public vote that will presumably take place over the Internet.
If you’d like to get involved, this IAU news release has all the details. News of the contest had me thinking about new categories for names, and I immediately thought about drawing ideas from Arthurian romances of the Middle Ages. But alas, I learn from the Gazetter of Planetary Nomenclature that this one has already been taken, on Mimas, of all places, where craters are to be named after people from Malory’s Morte d’Arthur. Malory scooped up most of the major characters in earlier English and French Arthurian tales, but maybe there are a few he missed. It’s worth a look, because as we keep discovering new worlds, names are going to be in short supply.
by Paul Gilster | Jan 14, 2015 | Exoplanetary Science |
About a year ago we looked at a young star called HD 142527 in the constellation Lupus (see HD 142527: An Unusual Circumstellar Disk). A T Tauri star about five million years old, HD 142527 has drawn attention because it shows evidence of both an inner and an outer disk, each of which may be capable of producing planets. These are disks with a twist, as astronomers at the Millennium ALMA Disk Nucleus project at the Universidad de Chile demonstrate in a new paper that explains the three-dimensional geometry of this unusual system.
HD 142527’s two disks are striking because no other star shows a gap this large between an inner and outer disk, a gap that spans a region from 10 AU out to 120 AU. Two dark regions stand out in observations of the outer disk that break its continuity. The new study reveals these outer disk features to be caused by the shadow of the inner disk. The shape and orientation of the shadows thus become a measure of the inner disk’s orientation. Using radiative transfer methods that analyze the absorption, emission and scattering of light, the researchers find that the inner disk has to be tilted by about 70 degrees to produce the known features.
Image: Comparison between the the observed infrared image of HD142527 (left) and the result of the warped inner disk model (right) as predicted by radiative transfer calculations. Shadows correspond to the two dark regions or intensity nulls seen in the upper and bottom sections of the outer disk. Credit: Henning Avenhaus/Millenium ALMA Disk Nucleus.
We’re left with the puzzle of how this system came to be in this unusual configuration. One possibility, suggested in 2014, is a relatively recent close encounter with another star, although lead author Sebastian Marino argues in the paper that no candidate has yet been identified. Interactions between the circumstellar disks and possible proto-planets are more likely, and we have an example in the debris disk around Beta Pictoris where an inner disk was apparently influenced by a young planet whose orbit is aligned with an inclined warped disk component.
But that answer raises questions as well, notes Sebastian Perez, a co-author of the paper:
“The astounding fact is that this planet would most likely need to be in a highly inclined orbit, just like the inner parts of the disk. Which poses more interesting questions about the dynamical stability of such arrangement.”
It’s interesting to speculate on how shadowing like this might affect planet formation, given that the shadows create regions colder and denser than the rest of the outer disk. And can we generalize from the HD 142527 disks? This finding, notes the paper, “… poses a challenge to understand the dynamics of the HD 142527 system, and is an invitation to interpret scattered light images of gapped protoplanetary disks from the perspective of inner warp shadows.”
The paper is Marino et al., “Shadows cast by a warp in the HD 142527 protoplanetary disk,” Astrophysical Journal Letters Vol. 798, No. 2, L44 (abstract / preprint). A news release from the Millenium ALMA Disk Nucleus project is available.
by Paul Gilster | Jan 13, 2015 | Deep Sky Astronomy & Telescopes |
Figuring out how fast a star spins can be a tricky proposition. It’s fairly simple if you’re close by, of course — in our Solar System, we can observe sunspot patterns on our own star and watch as they make a full rotation, the spin becoming obvious. From such observations we learn that how fast the Sun spins depends on where you look. At the equator, the rotation period is 24.47 days, but this rotation rate decreases as you move toward the poles. Differential rotation means that some regions near the Sun’s poles can take as much as 38 days to make a rotation.
Because of these issues, astronomers have chosen an area about 26 degrees from the equator, where large numbers of sunspots tend to appear, as the point of reference, giving us a rotation of 25.38 days. You can imagine how complicated solar rotation gets once we look at other stars. We can’t resolve them to begin with, much less their ‘starspots,’ but what we can do is measure the decrease in light that starspots cause as they rotate around the star. You can see why studying a lightcurve like this resembles searching for transiting exoplanets, and in fact a mission like Kepler has to take the possibility of false positives from starspots into account.
Determining the Age of a Star
Stars between 80 and 140 percent of the Sun’s mass are the subject of new work on stellar spin by Søren Meibom (Harvard-Smithsonian Center for Astrophysics) and team. Using Kepler data, the researchers homed in on a 2.5 billion year old cluster called NGC 6819, part of a continuing investigation of stellar spin rates. Meibom’s work is part of the broader Kepler Cluster Study, for which he is the principal investigator. The work uses data from the original Kepler mission, the four years of data before the current K2 mission, to study star clusters.
The goal is to develop a precise method of obtaining the ages of stars like these, a method co-author Sydney Barnes (Leibniz Institute for Astrophysics) calls ‘gyrochronology.’ Stellar spin is the critical factor because stars slow down as they age, while other indicators, like size, temperature, and brightness, stay relatively constant. So far we’ve been able to use spin rate to determine stellar ages only with fast-spinning stars in young clusters. Starspot activity is prominent on young stars and thus easier to detect. We’re also helped by the fact that the pattern of color and brightness in a cluster can give us a good read on the overall cluster’s age.
Image: It’s easier to tell the age of a young star because they rotate more quickly and have larger starspots. Credit: David A. Aguilar (CfA).
Thus we can correlate spin rate with age in specific instances. Get beyond about 600 million years in stellar age, though, and there is a huge gap between these young stars and stars as old as our 4.6 billion year old Sun. How fast do stars of intermediate ages spin? In this video on his work, Meibom refers to the ‘four billion year gap’ created.by our inability to measure spin for older stars. Young, rapidly spinning stars with large starspot regions, where brightness changes are pronounced, show a far more marked lightcurve than older stars, where starspot activity has dropped and teasing out the spin rate takes us to the limit of our instruments’ capabilities.
The Kepler telescope’s exquisitely sensitive measurements of stellar brightness are what has allowed us to fill in the gap. Studies of the cluster NGC 6811 produced the first measurements for the spin rate of one billion year old stars in 2011. Analyzing NGC 6819 data, Meibom and team have now been able to measure spin rates for 30 2.5 billion year old stars. We can thus plot spin against age on a far more meaningful graph, allowing us to extrapolate that when the Sun was 2.5 billion years old, its spin rate was roughly 18 days, with the rate dropping to eleven days when the Sun was 1 billion years old. A stellar ‘clock’ like this can help us determine which stars have planets that have had time for complex life to evolve. Looking forward, studying planets like our own around much older stars will give us clues to what lies ahead for the Earth.
“Now we can derive precise ages for large numbers of cool field stars in our Galaxy by measuring their spin periods,” says Meibom. “This is an important new tool for astronomers studying the evolution of stars and their companions, and one that can help identify planets old enough for complex life to have evolved.”
The paper is Meibom et al., “A spin-down clock for cool stars from observations of a 2.5-billion-year-old cluster,” published online in Nature 5 January 2015 (abstract). Links to the full text, and to video of the AAS press conference where these findings were announced, are available along with much more material on stellar age here. A CfA news release is also available.
by Paul Gilster | Jan 9, 2015 | Advanced Rocketry |
How do you go about creating a straightforward, highly durable design for a spacecraft, one that is readily refuelable and offers manifest advantages for crew comfort and safety? Alex Tolley and Brian McConnell have been asking that question for some time now, coming up with an ingenious solution that could open up large swathes of the Solar System. Alex tells me he is a former computer programmer now serving as a lecturer in biology at the University of California, where he hopes to inspire the next generation of biologists. He’s also a Centauri Dreams regular who was deeply influenced by 2001: A Space Odyssey and the Apollo landings. Below, he fills us in on the details in a narrative that imagines an early trip on such a vessel.
by Alex Tolley
The covered wagon or prairie schooner is one of the iconic images of the 19th century westward migration of the American pioneers. The wagon was simple in construction, very rugged, and repairable. They were powered most often by oxen that lived on the food and water found along the trail. The cost of a wagon, oxen and supplies was about 6 months of family wages.
In 2009 my colleague Brian McConnell and I were thinking about how to open up the exploration of space in an analogous way to the opening up of the American West during the 19th century pioneering era. We were looking for an approach that, like the covered wagon, was affordable, relatively low tech, provided safety in the case of emergencies and the space environment, could “live off the land” for propulsion like oxen, and preferably was reusable so that costs could be amortised over a number of flights.
What follows is a description of the “spacecoach” from the perspective of a new crew member making a first visit to the ship that will be on a Phobos return mission.
Image: ‘Ships of The Plains’ by Samuel Colman.
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Our transfer vehicle docked gently with the Martian Queen airlock. On approach, the Martian Queen resolved into 4 fat sausages, linked end to end. On either side, from bow to stern, were solar PV arrays, partially unfurled. She looked like no spaceship seen since the dawn of the space age.. There was no gleaming metal hull, and she was devoid of all the encrustations of antennae and dishes of those earlier ships. Neither were there any signs of fuel tanks holding liquid cryofuels. Instead, the hull looked dull and somewhat like an old blimp, those non-rigid airships of the early 20th century. The only sign of exterior equipment were those solar PV panels. These were lightweight, moderate performance thin film arrays, extended out on booms to face the sun and drink her rays to power the ship. They looked more like square rigged sails as they fluttered every so gently in the tenuous atmosphere remaining at her orbit.
I knew from the briefing that the Martian Queen needed about 160KW of power, requiring about 800 m2 of arrays at Mars orbit. There was also talk of the next generation “spacecoaches” replacing the PV panels with lightweight rectennas, to convert microwave beams from the orbital transmitters. Most crews didn’t trust that idea yet, but adding a lightweight rectenna was considered a good idea to back up the PVs and also compensate for the lower intensity of sunlight as the newer ships were about to explore Jupiter space. So this was the Martian Queen, the “spacecoach” that would be my home, about to make her 2nd voyage to Phobos.
Following my crew mate Vicki, I passed through the airlock and entered a large space, nearly 60 m3 in volume, shaped like a large cylinder. The interior diameter was about 4.5 meters, about the same as the mothballed Orion I’d seen back at the Cape museum.. But with a length of 10 meters, the volume was 3x larger. The Martian Queen was composed of 4 modules, providing over 200 m3 of full sea level atmosphere pressurized volume, about 2/3rds that of the old Mir space station. Touching the inner skin of the hull it felt flexible, and slightly cool to the touch. A few light taps and the resonant sounds confirmed that there was liquid behind the skin.
Vicki answered my unspoken question about the liquid in the hull. Water was sandwiched between several layers of impermeable Kevlar in the hull. The primary, and ultimately end, use of all the water was for propellant. The spacoach had originally been folded for launch in a standard Falcon 9 fairing. Each module, without any propellant, weighed just 4 tonnes including payload. This was very little and reduced the deadweight mass of the ship. Once in orbit, the interior had been inflated and the hull filled with water. Most of that water had been launched by dumb, low cost boosters, but some was being supplied from extra-terrestrial resources. Supplies from the lunar south pole were becoming increasingly available as Chevron-Petrobras’ Shackleton base was building up mining production. Exploratory vessels were also initiating operations on asteroids, with 24 Themis looking promising with confirmed surface water. In a few decades, it was expected that all water would be supplied from extra-terrestrial sources.
“Why do you put all the water in the hull, rather than in separate tanks?” I asked.
Vicki explained that the water had a number of roles, not just as propellant. The primary reason was radiation protection. The water acted as a good radiation shield, with a halving of the radiation flux with every 18 cm. Starting with about 25 cm of water in the hull, the radiation level inside the module was just 40 percent of that striking the hull. In the event of a major solar flare, the crew could also redirect the water to an interior tube to provide the best radiation shielding for the crew. It looked like that space could get very cozy for the crew, but better than suffering radiation burns.
But it didn’t end there. Micrometeoroids are a rare, but important hazard. The water acted as a shield, absorbing the energy of these grains and preventing penetration inside the hull. The tiny holes in the outer layers quickly heal too. The outer layers of water could be allowed to freeze, trapping a dense forest of fine fibers between the 2 outer fabric layers. This made a strong material, very much like pykrete [1] that offered a stiff outer hull to protect against larger impacts. At Earth’s 1 AU from the sun, reflective foils deployed over the hull allowed passive freezing of the outer layers providing both protection and a large heat sink for the engines.
A noticeable side effect of the hull architecture was the silence. There are no clicks and bangs from thermal heating stresses. Nor did the sunward side of the interior feel noticeably warmer. Thus the water was going to offer very good thermal control of the interior, with pumps in the hull circulating the water providing dynamic thermal control.
Vicki indicated that I should follow her forward to another module. This included the kitchen and dining space. There was a freezer of dried food packages that was being organized by Pieter. Enough for a long trip with a fair variety of meals.
“You seem to have ordered a lot of Boeuf Bourguignon”, joked Pieter.
I wondered when the taste of Boeuf Bourguignon would become rather tiresome after some months. Perhaps more spicy meals like curries would have been more appropriate. I noted that the water supply for rehydrating the food and drinks was connected to the hull too. Of course, I reminded myself, the hull was a huge reservoir of water, effectively inexhaustible are far as the crew was concerned, at least on the outward bound flight.
The facilities were oriented so that “down” was towards the end of the module. This was because during cruise the Martian Queen was going to be rotated, providing some artificial gravity. This made the flight much more comfortable and familiar. We could even eat off regular plates.
Vicki quickly showed me the crew quarters and bathroom in the next module. The inner skin of the hull had been moulded into shapes that could contain water. The baths and showers were also connected to the hull’s water supply. The clean water input was connected to heaters and pumps to the various faucets and shower heads. The grey water from the drains was routed to the main purifier and returned to the hull. I inquired how frequently I could take a shower? Once, twice even three times a week?
“As much as you like”, said Vicki. “There is ample water supply for a single pass through the purifier for all the crew to shower once or twice a day. If the crew is particularly extravagant, even this can be increased with greater recycling. Hygiene is a huge morale booster on these trips.”
The toilet was apparently a composting type, although suitably modified for space. This made sense. The nitrogen and phosphorus was going to be needed for the plants growing in the interior, as well as the Phobos base agricultural areas. Nitrogen and phosphorus were still valuable elements with no rich, off-Earth supplies available. Ducking back into the kitchen space, it was clear that much of the interior was given over to growing plants. They provided the needed psychological connection with Earth, helped recycle the CO2, and freshened the air, removing unpleasant volatiles. The stale, locker room smell of most spaceships was almost absent. Some plants were also growing some fresh foods. I could just imagine the value of a fresh tomato after 6 months of spaceflight!
Image: The Genesis 2 space module. An inflatable habitat launched in 2007 and still operational. A design concept similar to the spacecoach. Credit: Bigelow Aerospace (http://www.bigelowaerospace.com).
Image: Inside Bigelow Aerospace Space Station Alpha mockup. This is similar to the spacecoach basic module before addition of specialized fixtures and fittings. Credit: Bigelow Aerospace.
Pulling ourselves back through the leafy interior of the modules, I looked for the engine compartment in the last module. The engines were not obvious on docking, and I wondered where they were. At the rear of the last module, an airlock was currently open, showing an enclosed space beyond. Inside, Hans, the engineer was taking apart one of the engines. He was removing a metal liner from the engine and replacing it with a fresh one. He handed the old one to me and said “carbon deposits”.
I looked closely and saw what he was talking about. Carbon deposition from contaminants in the water supply could build up in the engines, reducing performance. The engines were not much more complex than microwave ovens, although they were fitted with electric grids to further accelerate the microwave heated water plasma.
The exhaust exited via the rear, when the bay doors were opened. Now they were closed, allowing the shirt sleeve repair of the engines. I asked how frequent engine repairs were. Hans informed me that an engine needed some rework after 3-6 hours of operation. The microwave electrothermal engine performance had an Isp of about 800s, although the secondary electric grids could double that by drawing on reserve energy from the solar arrays. Vicki thanked Hans and we drifted back to the main module.
I was a little surprised at the lack of windows, but pleased that there were many flat screens where windows should have been. I looked “out” and saw that I had missed the vernier and maneuvering jets on the hull.
“How are these powered?” I asked Vicki.
“Hydrogen Peroxide, H2O2” she replied.
“Where’s the fuel?”.
“There isn’t any yet. It’s made during the flight. Some of the water in the hull is tapped off, run through that off-the-shelf, standard unit over there. We store the peroxide in hull pockets to wait for the next use. The peroxide engines aren’t very efficient, having an Isp of about 160s, but they provide higher thrust than the main engines and can be used to boost the ship for a faster departure, or land the ship on low gravity worlds with orbital delta-Vs of 0.5 km/s or less. The peroxide has other uses too. It can be decomposed to provide oxygen [3] more quickly than the main ESS electrolyzers, act as an energy store for emergency power [4] and finally as an excellent bactericide to keep the interior clean and remove the bacterial slimes and molds that grow on the inner skin, often in difficult to reach spaces. And before you ask, yes, we have rotating cleaning duties on the Martian Queen.”
So the water in the hull fulfilled a range of uses, before being finally consumed as propellant. Major uses included bathing, direct consumption, rehydrating food, growing plants and, of course, the main oxygen supply. It was converted to peroxide for the high thrust engines, for energy storage and for another emergency O2 supply.
“Vicki, a quick mental calculation seems to come up short on the water requirement for the flight. Is what I see all that is needed?”
Vicki smiled: “The impact of using water as propellant on performance is significant. The total water budget for the trip is about 4 times the total mass of the ship and payload, compared to about 14 times for a conventional liquid hydrogen and LOX chemical rocket, primarily because of the higher Isp of the electrothermal engines. But the low hull mass and reduced consumables payload reduces the main mass of the the Martian Queen allowing a much smaller, more efficient spaceship. She is also a lot roomier, more comfortable and much safer. An Apollo 13 type accident would not be survivable in a conventional ship, but we have very large reserves of consumables and oxygen for the crew to survive until a rescue or the return trajectory was complete. In addition, even without water supplies at Phobos, the baseline mission cost to Phobos and return is on the order of a $100m dollars. That is why your institution can afford to pay for your slot on this mission. Reusability of the Martian Queen for multiple missions, fresh water at Phobos, and better performing solar panels and electric engines will eventually reduce that cost perhaps another order of magnitude.”
I pondered that for a moment. While not a cheap solution for interplanetary travel, it put the cost well within the realm of the super-rich and wealthy institutions. A mere decade earlier, a simple lunar flyby and return in an adapted Soyuz craft was priced at around $100m per passenger by Space Adventures. Spaceflight was definitely getting cheaper and safer.
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If interplanetary travel is initially based around the design concepts of water propellant craft, then the economics and infrastructure requirements will be dependent on available supplies of water already in space at suitable locations for fuel dumps. Bodies that may harbor economically useful quantities of accessible water include the moon (shadowed polar regions), water rich asteroids and dead comets. A tantalizing possibility is Ceres, that Dawn is expected to rendezvous with this year (2015). Ceres is expected to have prodigious quantities of frozen water, possibly even a subsurface ocean. A mining operation to extract pure water from the brew of ice and chemicals might offer the opportunity to open up the inner solar solar system. Once at Jupiter, the icy moons offer an almost inexhaustible supply of water.
References
1. Pykrete http://en.wikipedia.org/wiki/Pykrete
2. Bigelow Aerospace B330 http://bigelowaerospace.com/b330/
3. 47kg O2/1000 kg H2O2 (10%)
4. ~2 MJ, kg.
5. J E Brandenburg, J Kline and D Sullivan, “The microwave electro-thermal (MET) thruster using water vapor propellant,” Plasma Science, IEEE Transactions on (Volume:33, Issue:2) pp 776-782 (2005).
6. E. Wernimont, M. Ventura, G. Garboden and P. Mullens. “Past and Present Uses of Rocket Grade Hydrogen Peroxide”, http://www.hydrogen-peroxide.us/history-US-General-Kinetics/H2O2_Conf_1999-Past_Present_Uses_of_Rocket_Grade_Hydrogen_Peroxide.pdf
by Paul Gilster | Jan 8, 2015 | Exoplanetary Science |
Working at near-infrared wavelengths, the Gemini Planet Imager, now entering regular operations at the Gemini South Telescope in Cerro Pachon (Chile), is producing striking work, including images of exoplanets and circumstellar disks. Have a look at the image below, which highlights the instrument’s ability to achieve high contrast at small angular separations. Such capabilities make it possible to image exoplanets around nearby stars, as seen here in the case of the star HR 8799.
Image: GPI imaging of the planetary system HR 8799 in K band, showing 3 of the 4 planets. (Planet b is outside the field of view shown here, off to the left.) These data were obtained on November 17, 2013 during the first week of operation of GPI and in relatively challenging weather conditions, but with GPI’s advanced adaptive optics system and coronagraph the planets can still be clearly seen and their spectra measured. Credit: Christian Marois (NRC Canada), Patrick Ingraham (Stanford University) and the GPI Team.
This work, presented at the American Astronomical Society meeting in Seattle, is especially promising given that the Gemini Planet Imager (GPI) has until recently been in its commissioning phase, observing a wide range of targets as the instrument was integrated into the Gemini Observatory software. Patrick Ingraham (Stanford University), lead author of the upcoming paper on HR 8799, notes that even these early observations were productive, revealing spectra for two of the planets that differed markedly from what had been expected given their similar colors.
“Current atmospheric models of exoplanets cannot fully explain the subtle differences in color that GPI has revealed,” Ingraham added..”We infer that it may be differences in the coverage of the clouds or their composition. The fact that GPI was able to extract new knowledge from these planets on the first commissioning run in such a short amount of time, and in conditions that it was not even designed to work, is a real testament to how revolutionary GPI will be to the field of exoplanets.”
Image: GPI spectroscopy of planets c and d in the HR 8799 system. While earlier work showed that the planets have similar overall brightness and colors, these newly-measured spectra show surprisingly large differences. The spectrum of planet d increases smoothly from 1.9-2.2 microns while planet c’s spectrum shows a sharper kink upwards just beyond 2 microns. These new GPI results indicate that these similar-mass and equal-age planets nonetheless have significant differences in atmospheric properties, for instance more open spaces between patchy cloud cover on planet c versus uniform cloud cover on planet d, or perhaps differences in atmospheric chemistry. These data are helping refine and improve a new generation of atmospheric models to explain these effects. Credit: Patrick Ingraham (Stanford University), Mark Marley (NASA Ames), Didier Saumon (Los Alamos National Laboratory) and the GPI Team.
Also revealed at AAS 225 was imagery from the dusty disk around the young star HR 4796A, seen below. Here the striking feature is the instrument’s ability to analyze the polarized light of the disk, which reveals that it must be partially opaque. That would suggest a disk composition much denser than the dust found in the outer regions of our own Solar System. The HR 4796A dust ring is about twice the diameter of the planetary orbits in our system, while its star is twice the mass of the Sun.
Image: GPI imaging polarimetry of the circumstellar disk around HR 4796A, a ring of dust and planetesimals similar in some ways to a scaled up version of the solar system’s Kuiper Belt. These GPI observations reveal a complex pattern of variations in brightness and polarization around the HR 4796A disk. The western side (tilted closer to the Earth) appears brighter in polarized light, while in total intensity the eastern side appears slightly brighter, particularly just to the east of the widest apparent separation points of the disk. Reconciling this complex and apparently-contradictory pattern of brighter and darker regions required a major overhaul of our understanding of this circumstellar disk. Credit: Marshall Perrin (Space Telescope Science Institute), Gaspard Duchene (UC Berkeley), Max Millar-Blanchaer (University of Toronto), and the GPI Team.
The illustration below explains the team’s current thinking on the HR 4796A disk.
Image: Diagram depicting the GPI team’s revised model for the orientation and composition of the HR 4796A ring. To explain the observed polarization levels, the disk must consist of relatively large (> 5 µm) silicate dust particles, which scatter light most strongly and polarize it more for forward scattering. To explain the relative faintness of the east side in total intensity, the disk must be dense enough to be slightly opaque, comparable to Saturn’s optically thick rings, such that on the near side of the disk our view of its brightly illuminated inner portion is partially obscured. This revised model requires the disk to be much narrower and flatter than expected, and poses a new challenge for theories of disk dynamics to explain. GPI’s high contrast imaging and polarimetry capabilities together were essential for this new synthesis. Credit: Marshall Perrin (Space Telescope Science Institute).
The Gemini Planet Imager is now open to proposals for early observations in 2015. The core science program for the instrument is to be the Gemini Planet Imager Exoplanet Survey (GPIES), which will look at 600 young and relatively nearby stars over the next several years. The planets of such stars will offer the best targets for the GPI’s infrared wavelength operations. All told, the GPIES intends to make 890 hours of observations over the next three years. This Gemini Observatory news release offers additional background.
Be aware that Marshall Perrin (Space Telescope Science Institute), who is one of the team leaders on the Gemini Planet Imager (and the scientist who presented these results at the AAS meeting), used to write a highly readable astronomy column for the online science fiction magazine Strange Horizons. It’s been a bit over five years since his last, and I for one would love to see the column resumed. Consider this a gentle nudge in that direction.
