The SOCIETY for POPULAR ASTRONOMY
Electronic News Bulletin No. 392 2015 February 1
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ASTEROID THAT FLEW PAST THE EARTH HAS A MOON
NASA/Jet Propulsion Laboratory
Scientists working with the 70-m Deep Space Network antenna at Goldstone, California, obtained radar images of asteroid 2004 BL86 when it made its close approach on Jan. 26 at a distance of about 1.2 million km, or 3.1 times the distance from the Earth to the Moon. The 20 individual images show that the primary body is about 325 m across and has a moon about 70 m across. In the near-Earth population, about 16% of asteroids 200 m across or larger are binary or even triple systems (two moons). The orbit of asteroid 2004 BL86 is well determined; the recent fly-by was the closest it will make to the Earth for at least the next two centuries. It is also the closest that a known asteroid of that size will come until asteroid 1999 AN10 passes by in 2027.
COMET 67P SHOWS MORE SURPRISES
Southwest Research Institute
Scientists using data from the Rosetta spacecraft now orbiting omet 67P/Churyumov-Gerasimenko have discovered that the comet's atmosphere, or coma, is much less homogeneous than expected and that out-gassing varies significantly over time. Cometary nuclei are composed of ice, dust, and small rocky particles. As they approach the Sun, they heat up and out-gas, displaying visible atmospheres and often tails. Comets contain some of the best-preserved material from the formation of the planetary system, offering clues about physical and chemical conditions that existed in the early Solar System. After the Rosetta spacecraft reached 67P, it landed a probe on the comet's surface last November. The lander is now in hibernation, but the Rosetta orbiter itself continues conducting 11 experiments that may help our understanding of comets in general and of comet 67P specifically, as it approaches the Sun.
From a telescope, images of a comet's atmosphere suggest that the coma is uniform and does not vary over short periods of hours or days. It was certainly a surprise when Rosetta observed time variations from 200 km away. More surprising was that the composition of the coma was also varying by very large amounts. It has been supposed that comets are made mostly of water ice. For this comet, the coma sometimes contains much more carbon dioxide than water vapour. The Rosetta mission is providing an opportunity for long-term study of a comet during its Sunward approach; the data being discussed are from the initial two-month period of the encounter. Measuring the in-situ coma composition at the position of the spacecraft, ROSINA data indicate that the H2O signal is strongest overall; however, there are periods when the CO and CO2 rival that of H2O. The large fluctuations in composition in a heterogeneous coma indicate diurnal or day-night and possibly seasonal variations in the major out-gassing species. Astronomers believe 67P has a complex coma-nucleus relationship, with seasonal variations possibly driven by temperature differences just below the comet surface. The nucleus of 67P consists of two lobes of different sizes, connected by a neck region. That complex shape may play a role in the variations, as different portions of the nucleus face the Sun during the comet's 12-plus-hour diurnal rotation cycle. If the coma composition reflects the composition of the nucleus, variations suggest that the nucleus may have formed by collision of smaller bodies that originated from very different regions of the early Solar System. As the comet continues on its 6.5-year journey round the Sun, Rosetta scientists will hope to recognise seasonal variations as well.
MORE DWARF PLANETS IN OUR SOLAR SYSTEM LIKELY
There could be at least two unknown dwarf planets hidden well beyond Pluto. That has been suggested by numerical calculations made at the Complutense University of Madrid and the University of Cambridge. If confirmed, the hypothesis would affect Solar-System models. Astronomers have debated whether some dark trans-Plutonian planet remains to be discovered within the Solar System. According to the new calculations not one, but at least two, planets must exist to explain the orbital behaviour of extreme trans-Neptunian objects (ETNO). The most accepted theory says that the orbits of those objects should be distributed randomly, and by an observational bias, their paths should fulfil a series of characteristics: have a semi-major axis with a value close to 150 AU, an inclination of almost zero and an argument or angle of perihelion (closest point of the orbit to the Sun) also close to 0 or 180°. Yet what is observed in a dozen of these bodies is quite different: the values of the semi-major axis are very diverse (between 150 AU and 525 AU), the average inclination of their orbits is around 20° and argument of perihelion -31°, without appearing in any case close to 180°. The excess of objects with unexpected orbital parameters makes researchers believe that some invisible forces are altering the distribution of the orbital elements of the ETNO, and they consider that the most probable explanation is that other unknown planets exist beyond Neptune and Pluto.
To carry out the study, the researchers have analysed the effects of the so-called 'Kozai mechanism', related to the gravitational perturbation that a large body exerts on the orbit of another much smaller and more distant object. As a reference they have considered how that mechanism works in the case of comet 96P/Machholz1 under the influence of Jupiter. Despite their surprising results, the authors recognise that their data come up against two problems. On the one hand, their proposal goes against the predictions of current models on the formation of the Solar System, which state that there are no other planets moving in circular orbits beyond Neptune. However, the recent discovery by the ALMA radio telescope of a planet-forming disc more than 100 astronomical units from the star HL Tauri, which is younger than the Sun and more massive, suggests that planets can form several hundred astronomical units away from the centre of the system. On the other hand, the team recognises that the analysis is based on a sample with few objects (specifically 13), but they point out that in the coming months more results are going to be published, making the sample larger. Some time ago researchers from the United States discovered a dwarf planet called 2012 VP113 in the Oort cloud, just beyond the Solar System. The discoverers consider that its orbit is influenced by the possible presence of a dark and icy super-Earth, up to ten times larger than our planet.
EXO-PLANET DISPLAYS SATURN-TYPE RINGS
University of Rochester
Astronomers have discovered that the ring system that they see eclipse the very young Sun-like star J1407 is of enormous proportions, much larger and more massive than the ring system of Saturn. The ring system -- the first of its kind to be found outside the Solar System -- was discovered in 2012. The astronomers used data from the SuperWASP project -- a survey that is designed to detect gas giants that move in front of their parent star. Adaptive optics and Doppler spectroscopy were used to estimate the mass of the ringed object. Their conclusion is that J1407 has a companion that is likely to be a giant planet -- not yet seen -- with a gigantic ring system responsible for the repeated dimming of J1407's light. Analysis of the data shows that the system consists of over 30 rings, each of them tens of millions of kilometres in diameter. Furthermore, there are gaps in the rings, which indicate that satellites ('exo-moons') may have formed. The eclipse lasts for several weeks, but rapid changes on time-scales of tens of minutes can be seen as a result of fine structures in the rings. The star is much too far away to observe the rings directly, but astronomers could make a detailed model based on the rapid brightness variations in the star light passing through the ring system. If we could replace Saturn's rings with the rings around J1407b, they would be easily visible at night and be many times larger than the full Moon. The J1407 planet is much larger than Jupiter or Saturn, and its ring system is roughly 200 times larger than Saturn's. It is estimated that the ringed companion J1407b has an orbital period roughly a decade in length. The mass of J1407b has been difficult to constrain, but it is most likely to be in the range of about 10 to 40 Jupiter masses.
ANCIENT STAR SYSTEM REVEALS EARTH-SIZE PLANETS
University of Sydney
A Sun-like star with orbiting planets, dating back to the dawn of the Galaxy, has been discovered by an international team of astronomers. At 11.2 billion years old, it is the oldest star known to have Earth-sized planets, and proves that such planets have formed throughout the history of the Universe. The discovery used observations made by the Kepler satellite. The star, named Kepler-444, hosts five planets smaller than the Earth, with sizes between those of Mercury and Venus. The planets formed when the Universe was a fifth its current age. Kepler-444 is much older than the Solar System, which is 4.5 billion years old. The team used asteroseismology to determine the age of the star and planets. That technique measures oscillations -- the natural resonances of the host star caused by sound waves trapped within it. They lead to miniscule changes or pulses in the star's brightness and allow researchers to measure its diameter, mass, and age. The presence and size of the planets is detected by the dimming that occurs when the planets pass across the face of the star. The fading enables scientists to estimate the sizes of the planets relative to the size of the star. When asteroseismology was developed about two decades ago, it could be used only on the Sun and a few bright stars, but thanks to Kepler it can now be applied to literally thousands of stars. In the case of Kepler-444 the planets orbit their parent star in less than 10 days, at less than one-tenth the Earth's distance from the Sun. They must be unpleasantly hot and dry!
DIMMING QUASAR OBSERVED
Astronomers have been watching a variable quasar. Quasars are massive, luminous objects that draw their energy from black holes. Scientists have been unable to study both bright and dim phases of a quasar in a single source until now, when they have observed a quasar that has dimmed by a factor of six or seven, compared with observations from a few years ago. The phenomenon was noticed during an ongoing watch of Stripe 82 -- a sliver of sky near the celestial equator. Stripe 82 has been scanned in numerous astronomical surveys, including the Sloan Digital Sky Survey. The fading of the quasar was accompanied by a change in the broad emission lines in its spectrum. Those lines are signatures of gas that is too distant to be consumed by a black hole, yet close enough to be excited by energy from material that does fall into the hole, and most of them have disappeared in the quasar concerned.
COSMIC RADIO BURST CAUGHT IN ACTION
Astronomers using the 64-m Parkes radio telescope in eastern Australia have for the first time seen a 'fast radio burst' -- a short, sharp flash of radio waves from an unknown source -- happening live. Lasting only milliseconds, the first such radio burst was discovered in 2007 by astronomers combing the Parkes data archive for unrelated objects. Six more bursts, apparently from outside our Galaxy, have now been found with Parkes and a seventh with the Arecibo telescope in Puerto Rico. The bursts were generally discovered weeks or months after they happened, but now one has been caught in real time. An international team was poised to make rapid follow-up observations, at wavelengths from radio to X-rays. As soon as Parkes saw the burst go off the team swung into action on twelve telescopes around the world -- in Australia, California, the Canary Islands, Chile, Germany, Hawaii, and India -- and in space. No optical, infrared, ultraviolet or X-ray counterpart showed up. That in itself ruled out some possible candidates, such as long gamma-ray bursts and nearby supernovae, but short or low-energy gamma-ray bursts and giant flares from distant magnetars (the most magnetic stars in the Universe) are still contenders.
One of the big unknowns of fast radio bursts is their distances. The characteristics of the radio signal (how it is smeared out in frequency from travelling through space) indicate that the source of the new burst was up to 5.5 billion light-years away. That implies that it could have given off as much energy in a few milliseconds as the Sun does in a day. The burst left another clue as to its identity, but a puzzling one. Parkes' real-time detection system measured its polarisation -- something that had not been recorded for previous bursts. The emission from the new fast radio burst was more than 20% circularly polarised, which hints that there are magnetic fields near the source. Identifying the origin of fast radio bursts remains an urgent problem.
NEW EXO-PLANET-HUNTING TELESCOPES
University of Warwick
The Next-Generation Transit Survey (NGTS) has achieved first light at Paranal Observatory in northern Chile. That project will search for transiting exo-planets -- planets that pass in front of their parent star and hence produce a slight dimming of the star's light. The telescopes will focus on discovering Neptune-sized and smaller planets, with diameters between two and eight times that of the Earth. NGTS is a wide-field observing system made up of an array of twelve telescopes, each with an aperture of 20 cm. It was built by a UK, Swiss and German consortium. It is designed to operate in a robotic mode and will continuously monitor the brightness of hundreds of thousands of comparatively bright stars in the southern skies. It is searching for transiting exo-planets and will reach a level of accuracy in measuring the brightness of stars (one part in a thousand) that has never before been attained with a ground-based wide-field survey instrument.
The discoveries of NGTS will be studied further with larger telescopes, including the VLT. One goal is to find small planets that are bright enough for the planetary mass to be measured. That will allow planetary densities to be deduced, providing clues about the composition of the planets. It may also be possible to investigate the atmospheres of the planets whilst they are in transit. During the transit some of the star's light passes through the planet's atmosphere, if it has one, which leaves a tiny, but detectable, signature. So far only a few such very delicate observations have been made, but NGTS should provide many more opportunities. This is the first telescope project hosted, but not operated, by ESO on Paranal. Several projects operating under similar arrangements have long been at work at the older La Silla Observatory. The NGTS data will flow into the ESO archive system and will be available to astronomers worldwide.
UNIVERSE TO BE MAPPED WITH LARGEST RADIO TELESCOPE
An international team of scientists has joined forces to lay the foundations for an experiment to put together the biggest map of the Universe ever made. The experiment will combine signals from hundreds of radio dishes to make the cosmic atlas. Researchers from the Cosmology Science Working Group of the Square Kilometre Array (SKA) have worked out how to use that, the world's largest telescope, for the task. The SKA will be a collection of thousands of radio receivers and dishes spread across two sites in South Africa and Western Australia. The first phase is due to be completed in 2023, and a second and still larger phase in the late 2020s. By the late 2020s, the researchers hope to have mapped almost a billion galaxies; in comparison, the largest galaxy surveys to date have mapped the positions of 'only' around a million.
Bulletin compiled by Clive Down
(c) 2015 the Society for Popular Astronomy
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