by Paul Gilster | Mar 17, 2010 | Exoplanetary Science |
Claire Moutou, one of an international team of astronomers behind the discovery of the planet CoRoT-9b, says the distant world will become a ‘Rosetta stone in exoplanet research.’ And perhaps it will, for this is a transiting gas giant, but not a ‘hot Jupiter.’ In an orbit not dissimilar to that of Mercury, CoRoT-9b transits its star every 95 days, each transit lasting about eight hours. We’ve identified approximately 70 planets by transit methods, but this one is ten times farther from its host star than most gas giants previously discovered by this technique.
We may be jumping the gun a bit to call the climate here ‘temperate,’ as this European Southern Observatory news release does, because temperatures here will depend on layers of highly reflective clouds that may or may not exist on CoRoT-9b. ESO cites temperatures between 160 degrees and minus twenty degrees Celsius beneath those assumed clouds, but we should be able to learn much more because of the lengthy transit periods. Didier Queloz, a co-author of the Nature paper announcing the find, sees it this way:
“Our analysis has provided more information on CoRoT-9B than for other exoplanets of the same type. It may open up a new field of research to understand the atmospheres of moderate- and low-temperature planets, and in particular a completely new window in our understanding of low-temperature chemistry.”
The planet is about eighty percent the mass of Jupiter and was identified by CoRoT after 145 days of observations in 2008, with follow-up observations confirming the find at the 3.6-meter ESO telescope at La Silla (Chile). Team member Tristan Guillot says that it is mostly hydrogen and helium, with as much as 20 Earth masses of other elements, including water and rock at high-temperatures and pressures. The host star is similar to the Sun and is located some 1500 light years away from Earth in the direction of the constellation Serpens.
From the paper:
…Corot-9b is the first transiting planet among those with longer periods that does not represent a case of extreme eccentricity with associated extreme temperature changes (e.g. HD80806b’s temperature is estimated to rise from ~800K to ~1,500K over a six-hour period near periastron). On the contrary, it is the first transiting planet whose general properties coincide with the largest known population of planets, those of longer periods and low- to-moderate eccentricities, but which previously were known only from RV surveys. Our results on Corot-9b show that these planets may be expected to be rather similar to the giants of our Solar System.
Image: The orbital parameters of CoRoT-9b among extrasolar planets. Shown is the eccentricity and period of all 339 exoplanets for which both values are known as of Nov 1, 2009. Solid dots are the 58 transiting planets among them – most of them have short periods of < ~5 days and zero or low eccentricities. Only two further transiting planets have orbital periods longer than 10 days; they are HD17156b with 21.2 and HD80806b with 111.4 days. However, both of them also have the highest eccentricity among planets of similar periods. Open dots are the remaining exoplanets, known only from radial velocity observations. Credit: www.exoplanet.eu/Deeg et al.
So we have a transiting gas giant that is not a hot Jupiter, allowing us to study a category of 'temperate' gas giants in much greater depth now that we can apply both transit observations (revealing the planet's diameter) and radial velocity measurements, which can help us figure out its mass and hence its density. That should produce information that will illuminate the large number of temperate gas giants found thus far by non-transit methods.
The paper is Deeg et al., "A Transiting Giant Planet with a Temperature Between 250 K and 430 K," Nature 464 (18 March 2010), pp. 384-387.
by Paul Gilster | Mar 16, 2010 | Outer Solar System |
One of the things we need to learn about the Alpha Centauri stars is whether Proxima Centauri is gravitationally bound to Centauri A and B. Much hinges on the issue, for if Proxima is merely passing in the night, then whatever disruptive effect it may have upon an outer halo of comets around the Centauri stars would be a one-shot affair. On the other hand, if Proxima is a stable part of this system, then it may send comets laden with volatiles into whatever planetary systems are around Centauri A and B. Proxima might be, in other words, the difference between dry rocky worlds and planets with abundant water, with all that implies about the possibilities for life.
We’ve seen the same kind of thinking applied to our own Solar System in the form of the star dubbed ‘Nemesis.’ As the theory goes, Nemesis could be a red or perhaps a brown dwarf that could account for what seems to be a periodicity in terms of extinction events on Earth. Disrupting cometary orbits in the Oort Cloud, such an object might send comets into the inner system on a 26-million year cycle. The Telegraph is one of a variety of media looking into the story (Astrobiology Magazine is another), and speculating on whether or not some evidence for Nemesis has turned up in the orbit of Sedna, the dwarf planet discovered in 2003 whose orbit continues to cause ripples in the planetary sciences community.
The Telegraph quotes the object’s discoverer, Mike Brown (Caltech), on the matter:
“Sedna is a very odd object – it shouldn’t be there! It never comes anywhere close to any of the giant planets or the sun. It’s way, way out there on this incredibly eccentric orbit. The only way to get on an eccentric orbit is to have some giant body kick you – so what is out there?”
Did someone say ‘eccentric’? Sedna gives new meaning to the term, with a perihelion of 76 AU and an aphelion of a whopping 975 AU, making for an orbital period lasting as long as 12,000 years. Brown himself says his own surveys would not have picked up anything as distant and slow-moving as Nemesis would have to be, all of which leaves the prospects for an unseen companion still very much alive.
In a 2006 paper, Rodney Gomes (Observatório Nacional, Brazil) and colleagues note that Sedna and the object 2000 CR105 stand out from the thousand or so trans-Neptunian objects thus far discovered because their trajectories cannot be accounted for by the planetary configuration we know about. The authors call them ‘the first discovered true inner Oort cloud objects.’ They may be evidence of a distant planetary-mass solar companion, one the authors characterize as follows:
A small mass companion (roughly Earth to Neptune mass) could have formed within the planetary region, been ejected to its current heliocentric distance by gravitational scatterings of Jupiter and Saturn. Such a body may have been ejected from the Solar System after producing the DDP [distant detached population], or it could remain as the largest member of the standard scattered disk population. Or its perihelion could have been raised by perturbations of passing stars which would have needed to have been passing at a closer distance than would be reasonable to hypothesize subsequent to the dispersal of the Sun’s birth cloud/cluster, but not as close as required for direct stellar perturbations producing the observed high perihelion scattered disk objects. A Jupiter mass or larger object on a highly inclined orbit beyond 5000 AU would most likely have formed as a small, distant binary-star like companion, e.g., by fragmentation during collapse or capture.
With WISE (Wide-Field Infrared Explorer) on the case, it may not be too long before we have evidence one way or the other. Chances are a red dwarf no more than 25,000 AU out would have been spotted by now, but a brown dwarf might still elude detection. John Matese (University of Louisiana at Lafayette), one of the authors of the above paper, has been studying Nemesis possibilities for more than two decades, and now opts for an object about three to five times the size of Jupiter as a possible cause of the cometary commotion. WISE will need until perhaps mid-2013 before two sky scans and telescopic follow-ups of any Nemesis-like detection can be completed.
For more, see Gomes et al., “A Distant Planetary Mass Solar Companion May Have Produced Distant Detached Objects,” Icarus 184, No. 2 (October, 2006), where the authors say the object in question could be an Earth-mass planet at 1000 AU, a Neptune-mass planet at 2000 AU, or a Jupiter-mass object at 5000 AU or farther (abstract). So maybe we’re not talking about a ‘death star’ at all, but a huge Planet X of the kind Percival Lowell was once so intent on finding. Lowell, thanks to the indefatigable Clyde Tombaugh, had to settle for tiny Pluto, but we’ve yet to rule out much larger objects in the dark realms beyond the Kuiper Belt. WISE should settle the matter conclusively in about three years.
by Paul Gilster | Mar 15, 2010 | Asteroid and Comet Deflection |
Why mount a mission to an asteroid? For one thing, some of them cross the Earth’s orbit, and that makes gathering knowledge about their composition essential to any future trajectory-altering operation. For another, the science return could be immense. These are unprepossessing objects, no more than chunks of rock and dust, but they can tell us much about the early Solar System. Moreover, getting to an asteroid, as NASA GSFC is now proposing to do with a mission called OSIRIS-REx, would allow us to examine samples in situ, something mission proponent Bill Cutlip finds more valuable than studying chunks of asteroids that fall to Earth in the form of meteorites:
“[Meteorites] are toasted on their way through Earth’s atmosphere. Once they land, they then soak up the microbes and chemicals from the environment around them.”
No, pristine is better, for we’re trying to learn about the earliest days of our system, the period of planetary formation and the origins of the organic compounds that resulted in life. An asteroid like 1999 RQ36, some 580 meters in diameter and the target of OSIRIS-REx, is debris from the solar nebula that gave birth to the Sun and planets about 4.5 billion years ago. Usefully, it seems to have undergone little alteration since that era, unlike asteroids that have suffered from collisions or grown large enough that their interiors became molten. Moreover, RQ36 is rich in carbon. Does it also contain organic molecules of the kind found in other meteorite and comet samples?
If the mission is approved, we’ll learn the answer to this and more, generating a complete map of the asteroid and its topography. The mission will include two infrared spectrometers, a light detection and ranging (LIDAR) instrument to bounce laser pulses off the surface, a mass spectrometer to separate and identify atoms and molecules, and a laser altimeter. Following evaluation from orbit, OSIRIS-REx will collect a surface sample that will be returned to Earth. The plan is to orbit the asteroid for about a year before selecting the sample site, allowing a thorough study of the surface. Sampling an asteroid could be tricky because of the fast rotation, making the operation more like the meeting of two spacecraft:
“Gravity on this asteroid is so weak, if you were on the surface, held your arm out straight and dropped a rock, it would take about half an hour for it to hit the ground,” says Joseph Nuth (NASA GSFC). “Pressure from the sun’s radiation and the solar wind on the spacecraft and the solar panels is about 20 percent of the gravitational attraction from RQ36. It will be more like docking than landing.”
But learning how to maneuver in proximity to an asteroid could be useful for future dealings with near-Earth objects. RQ36 has a slight but real chance of striking the Earth in the year 2170. One motivation for OSIRIS-REx is to measure the Yarkovsky effect, the slight push that occurs when an asteroid soaks up sunlight and emits heat. The uneven surface and variation in asteroid composition makes the Yarkovsky effect difficult to calculate, but if we ever need to change the trajectory of such an object, we’ll need to know how the effect changes its orbit. Adds Nuth: “It’s like trying to make a complex, banking shot in a game of pool with someone shaking the table and kicking the legs.”
If selected by NASA, OSIRIS-REx would be launched no later than December of 2018, but right now it’s one of three proposals chosen in late 2009 under NASA’s New Frontiers program. As for the acronym, it’s torturous indeed. O stands for origins (i.e., the origin of life), SI for spectral interpretation, RI for resource identification, S for security, and REx for regolith explorer. All of that in a single package, but hey, it’s not easy to choose a mission name, and once cobbled together, this one has a nice ring to it.
by Paul Gilster | Mar 12, 2010 | Exotic Physics |
Yesterday’s trip to the dark side involved the so-called ‘dark flow,’ the apparent motion of galactic clusters along a path in the direction of the constellations Centaurus and Hydra. Today we look at two other dark conjectures — dark matter and dark energy. Are both a part of the universe we observe, or can we do away with them by clever manipulation of Einstein’s theory of general relativity? The latest word, from an international team of researchers studying the clustering of more than 70,000 galaxies, is that GR seems to have passed yet another test. This is useful stuff, because one of the implications is that dark matter is the most likely explanation of the movement of galaxies and galaxy clusters as they seem to respond to an unseen mass.
The possibility of dark matter was noted as long ago as 1933 by Fritz Zwicky, who studied the average mass of galaxies within the Coma cluster and obtained a value much higher than expected from their luminosity. Later studies of individual galaxies made it clear that a halo of dark matter would explain anomalous galactic rotations. But all that assumed no changes to general relativity at cosmological scales.
Image: A partial map of the distribution of galaxies in the Sloan Digital Sky Survey, going out to a distance of 7 billion light years. The amount of galaxy clustering that we observe today is a signature of how gravity acted over cosmic time, and allows us to test whether general relativity holds over these scales. Credit: M. Blanton, Sloan Digital Sky Survey.
Confirming general relativity is old news on the level of the Solar System, but tests on the galactic level has proven inconclusive. Theories like tensor-vector-scalar gravity (TeVeS) have emerged that avoid the presence of dark matter by applying changes to general relativity. The theory was developed by Jacob Bekenstein and could account for observed galactic rotations as well as gravitational lensing, but it has remained controversial and the new work seems to rule it out. According to this news release from the University of California at Berkeley, TeVeS posits that acceleration caused by the gravitational force from a body depends not only on the mass of that body, but also on the value of acceleration caused by gravity.
Uros Seljak (UC Berkeley), a co-author of the paper on this work, notes the value of testing general relativity at cosmological distances:
“The nice thing about going to the cosmological scale is that we can test any full, alternative theory of gravity, because it should predict the things we observe. Those alternative theories that do not require dark matter fail these tests.”
As to the tests themselves, they revolve around a quantity known as EG, which is based on the amount of clustering in observed galaxies and the distortion of light produced by its passage through intervening matter. Pengjie Zhang (Shanghai Observatory) explains EG this way:
“Put simply, EG is proportional to the mean density of the universe and inversely proportional to the rate of growth of structure in the universe. This particular combination gets rid of the amplitude fluctuations and therefore focuses directly on the particular combination that is sensitive to modifications of general relativity.”
Seljak notes that cosmological experiments usually involve measuring fluctuations in space, while gravity theories predict relationships between density and velocity, or between density and gravitational potential:
“The problem is that the size of the fluctuation, by itself, is not telling us anything about underlying cosmological theories. It is essentially a nuisance we would like to get rid of,” Seljak said. “The novelty of this technique is that it looks at a particular combination of observations that does not depend on the magnitude of the fluctuations. The quantity is a smoking gun for deviations from general relativity.”
The new study also questioned theories like f(R), a mechanism for explaining the accelerated expansion of the universe without resorting to dark energy. The Wikipedia offers up a look at these alternate theories in its section on dark matter, also going into variations of Modified Newtonian Dynamics (MOND), one of which is Bekenstein’s TeVeS. The researchers worked with data from the Sloan Digital Sky Survey to calculate EG and compare it to the predictions of TeVeS as well as f(R) and the cold dark matter model of general relativity as enhanced with a cosmological constant to explain the universe’s accelerated expansion.
The result: General relativity fits within the experimental error, while the EG predicted by f(R) is also within the margin of error, but TeVeS is not. We seem to be making progress, but it’s worth remarking that Seljak is looking toward expanding the analysis to as many as a million galaxies with the advent of the Sloan Digital Sky Survey III’s Baryon Oscillation Spectroscopic Survey, which will be finished in about five years. We can also hope for data from ESA’s Euclid mission along with NASA’s Joint Dark Energy Mission, but for both we’ll have a wait of at least a decade. That gives us time as well for the unfolding of direct detection experiments, which may one day tell us more about the exact identity of dark matter and dark energy.
The paper is Reyes et al., “Confirmation of general relativity on large scales from weak lensing and galaxy velocities,” Nature 464 (11 March 2010), pp. 256-258 (abstract).
by Paul Gilster | Mar 11, 2010 | Deep Sky Astronomy & Telescopes |
When you’re studying galaxy clusters, it doesn’t pay to be in a hurry. Harald Ebeling (University of Hawaii) is an expert on the matter, working with a catalog of over a thousand such clusters in a new study of the so-called ‘dark flow,’ the apparent motion of galaxy clusters along a path centered on the southern constellations Centaurus and Hydra. Says Ebeling:
“It takes, on average, about an hour of telescope time to measure the distance to each cluster we work with, not to mention the years required to find these systems in the first place. This is a project requiring considerable followthrough.”
The study, led by Alexander Kashlinsky (NASA GSFC), relies on hot X-ray emitting gas within a cluster, which scatters photons from the cosmic microwave background. The wavelength of scattered photons then tells us something about the motion of individual clusters. This tiniest of shifts in the CMB’s background temperature in the cluster’s direction, known as the Sunyaev-Zel’dovich effect, is small enough that it has never been measured in a single galaxy cluster. That’s where the large number of clusters comes in. The researchers used a catalog of 700 clusters from a 2008 study and folded in another 700, along with results from the Wilkinson Microwave Anisotropy Probe. All of which is complicated enough without throwing in unexpected results.
You would assume that in relation to the CMB, all large-scale motion would be random, but the ‘dark flow’ appeared in the data in 2008, and the new study extends the motion to twice the distance previously reported. Kashlinsky says it persists to as far as 2.5 billion light years away, even if the direction of motion is now considered unresolved. “We detect motion along this axis, but right now our data cannot state as strongly as we’d like whether the clusters are coming or going,” says Kashlinsky, but the motion itself is what is controversial, suggesting structure beyond the visible universe that is pulling on matter that we can see.
Image: The colored dots are clusters within one of four distance ranges, with redder colors indicating greater distance. Colored ellipses show the direction of bulk motion for the clusters of the corresponding color. Images of representative galaxy clusters in each distance slice are also shown. Credit: NASA/Goddard/A. Kashlinsky, et al.
So now we can add the ‘dark flow’ to the other great ‘dark’ imponderables, like dark energy and dark matter. All, of course, are the subject of further investigation, and more on that tomorrow. For now, I see that the dark flow will be the subject of new studies with the latest WMAP data, along with information gleaned from the European Space Agency’s Planck mission, which is giving us another look at the microwave background.
The paper is Kashlinsky et al., “A New Measurement of the Bulk Flow of X-Ray Luminous Clusters of Galaxies,” Astrophysical Journal Letters 712 (March 20, 2010), pp. L81-L85 (abstract). See also this GSFC news release.
