GIANT COMETS COULD ENDANGER EARTH
RAS
A team of astronomers reports that the discovery of hundreds of giant comets in the outer regions of the Solar System over the last two decades implies that those objects pose a much greater hazard to life than asteroids. The giant comets, termed centaurs, have unstable orbits crossing the paths of the massive outer planets Jupiter, Saturn, Uranus and Neptune. The planetary gravitational fields can occasionally deflect the objects towards the Earth. Centaurs are typically 50 to 100 kilometres across, or larger, and a single such body has more mass than the entire population of Earth-crossing asteroids found to date. Calculations of the rate at which centaurs enter the inner Solar System indicate that one will be deflected onto a path crossing the Earth's orbit about once every 40,000 to 100,000 years. Whilst in near-Earth space, they are expected to disintegrate into dust and larger fragments, flooding the inner Solar System with cometary debris and making impacts on our planet inevitable.
Known severe upsets of the terrestrial environment and interruptions in the progress of ancient civilizations, together with our growing knowledge of interplanetary matter in near-Earth space, indicate the arrival of a centaur around 30,000 years ago. That giant comet would have strewn the inner planetary system with debris ranging in size from dust all the way up to lumps several kilometres across. Specific episodes of environmental upheaval around 10,800 BCE and 2,300 BCE, identified by geologists and palaeontologists, are also consistent with the new understanding of cometary populations. Some of the greatest mass extinctions in the distant past, for example the death of the dinosaurs 65 million years ago, may similarly be explained on the giant-comet hypothesis. In the last three decades scientists have invested a lot of effort in tracking and analysing the risk of a collision between the Earth and an asteroid. But we need to look beyond our immediate neighbourhood too, and look out beyond the orbit of Jupiter to find centaurs. Those distant comets could be a serious hazard. The researchers have also uncovered evidence from disparate fields of science in support of their model. For example, the ages of the sub-millimetre craters identified in lunar rocks returned in the Apollo programme are almost all younger than 30,000 years, indicating a vast enhancement in the amount of dust in the inner Solar System since then.
'HABITABLE ZONE' PLANET ORBITS NEARBY STAR
University of New South Wales
Astronomers have discovered the closest potentially habitable planet found outside our Solar System so far, orbiting a star just 14 light-years away. The planet, more than four times the mass of the Earth, is one of three that the team detected around a red dwarf star called Wolf 1061. It is an interesting find because all three planets are of low enough mass to be potentially rocky and have solid surfaces, and the middle planet, Wolf 1061c, is within the zone where it might be possible for liquid water to exist. While a few other planets have been found that orbit stars closer to us than Wolf 1061, those planets are not considered to be remotely habitable. The three newly detected planets orbit that small, relatively cool and stable star about every 5, 18 and 67 days. Their masses are at least 1.4, 4.3 and 5.2 times that of the Earth, respectively. The larger outer planet falls just outside the outer boundary of the habitable zone and is also likely to be rocky, while the smaller inner planet is too close to the star to be habitable. The team made the discovery using observations of Wolf 1061 collected by the HARPS spectrograph on ESO's 3.6-m telescope at La Silla in Chile. Small rocky planets like our own are now known to be abundant in our galaxy, and multi-planet systems also appear to be common. However most of the rocky exoplanets discovered so far are hundreds or thousands of light-years away. An exception is Gliese 667Cc which is 22 light-years away. It orbits a red dwarf star every 28 days and is at least 4.5 times as massive as the Earth. The close proximity of the planets around Wolf 1061 means that there is a good chance that they may pass across the face of the star. If they do, then it may be possible to study their atmospheres in future to see whether they would be conducive to life.
MASSIVE PLANET IN BINARY-STAR SYSTEM
Carnegie Institution
A team of astronomers has discovered an unusual planetary system comprised of a Sun-like star, another smaller star, and a planet belonging to the primary star. The planet, discovered in 2011 orbiting the star HD 7449, is about eight times the mass of Jupiter and has one of the most eccentric orbits ever found. The large eccentricity was a clue that another object -- something bigger than the known planet -- might also belong in the system. To ascertain whether it is a planet or another star, the team used the Magellan adaptive-optics instrument to image the object directly. It was clear that the third object had to be a star. It has only a fifth of the mass of the Sun, and is 18 astronomical units (AU) away from the primary. (1 AU is the distance from the Earth to the Sun.) Not long ago, binaries (two co-orbiting stars) were thought incapable of hosting planets, but over the past few years the number of known circumbinary planets detected has been growing. The HD 7449 system, though, is one of only a few that consist of a Sun-like star, a lesser one, and a gas-giant planet -- all within 20 AU. Among them, the planet HD 7449Ab is by far the most massive and has the most eccentric orbit. The secondary star and the planet must influence one another gravitationally. The planet's orbital eccentricity and inclination (the angle of its orbital plane relative to the equatorial plane of the primary star) must have been oscillating back and forth in a process known as Kozai oscillations -- and will continue to do so for the foreseeable future. The team will continue to monitor the system over the coming years using both the radial-velocity technique and direct imaging.
NuSTAR FINDS CLUMPY DOUGHNUT AROUND BLACK HOLE
RAS
Some of the most massive black holes in the Universe are encircled by thick doughnut-shaped discs of material. That material ultimately feeds and nourishes the growing black holes. Until recently, some of the doughnuts were too opaque for any telescopes to penetrate, but now a team of astronomers has used the NuSTAR and XMM-Newton X-ray observatories to see inside. Originally, astronomers thought that some black holes were hidden behind walls or screens of material that could not be seen through. NuSTAR recently looked inside one of the densest doughnuts known around a supermassive black hole. That black hole lies at the centre of a well-studied spiral galaxy called NGC 1068, located 47 million light-years away in the direction of the constellation Cetus. The observations revealed a clumpy doughnut. Doughnuts around supermassive black holes were first proposed in the mid-80s to explain why some black holes are hidden behind gas and dust, while others are not. The idea is that the orientation of the doughnut relative to the Earth affects the way we perceive a black hole and its intense radiation. If the doughnut is viewed edge-on, the black hole is blocked. If the doughnut is viewed face-on, the black hole and its surrounding, blazing materials can be detected. This idea is referred to as the unified model because it joins together the different black-hole types, solely upon the basis of orientation.
In the past decade, astronomers have been finding hints that such doughnuts are not as smoothly shaped as once thought. They are more like defective doughnuts with lumps. The new discovery is the first time that such lumpiness has been observed in an ultra-thick doughnut, but the phenomenon may be common. The research is important for understanding the growth and evolution of massive black holes and their host galaxies. Astronomers do not fully understand why some supermassive black holes are so heavily obscured, or why they are clumpy. Both NuSTAR and XMM-Newton observed the supermassive black hole in NGC 1068 simultaneously on two separate occasions. On one of those occasions, last August, NuSTAR observed a spike in brightness. NuSTAR observes higher-energy X-rays than XMM-Newton, and those high-energy X-rays can pierce thick clouds around the black hole. The scientists say that the spike in high-energy X-rays was due to a clearing in the thickness of the material surrounding the black hole. NGC 1068 is well known to astronomers as the first black hole which gave birth to the unification idea. But now astronomers have a direct, albeit fleeting, glimpse of its black hole through such clouds, allowing a better test of the unification concept. Future research will address the question of what causes the unevenness in the doughnut material. There could be various different answers. It is possible that a black hole generates turbulence as it stirs up nearby material. The energy given off by young stars could also create turbulence, which would then percolate outward through the doughnut. Or the clumps may be from material that falls onto the doughnut from outside it. As galaxies form, material migrates toward the centre, where the density and gravity is greatest. The material tends to fall in clumps, almost like a falling stream of water condensing into droplets as it hits the ground. Astronomers would like to know if the unevenness of the material is being generated from outside the doughnut, or within it.
FIRST-EVER PREDICTED EXPLODING STAR
ESA/Hubble Information Centre
The Hubble telescope has captured the image of the first-ever predicted supernova explosion. (See ENB No. 411). The reappearance of the Refsdal supernova was calculated from different models of the galaxy cluster whose immense gravity is warping the supernova's light. Many stars end their lives with a with a bang, but only a few such stellar explosions have been caught in the act. When they are, observing them successfully has been down to pure luck -- until now. On December 11 astronomers not only imaged a supernova in action, but saw it when and where they had predicted it would be. The supernova, nicknamed Refsdal, has been observed in the galaxy cluster MACS J1149.5+2223. While the light from the cluster has taken about five billion years to reach us, the supernova itself exploded much earlier, nearly 10 billion years ago. Refsdal's story began in 2014 November when scientists saw four separate images of the supernova in a rare arrangement known as an Einstein Cross around a galaxy within MACS J1149.5+2223. The cosmic optical illusion was due to the mass of a single galaxy within the cluster warping and magnifying the light from the distant stellar explosion in a process known as gravitational lensing. While studying the supernova, astronomers realised that the galaxy in which it exploded was already known to be a galaxy that is being lensed by the cluster. The supernova's host galaxy appears to us in at least three distinct images caused by the warping mass of the galaxy cluster. The multiple images of the galaxy presented a rare opportunity. As the matter in the cluster -- both dark and visible -- is distributed unevenly, the light creating each of the images takes a different path with a different length. Therefore the images of the host galaxy of the supernova refer to different times.
Using other lensed galaxies within the cluster and combining them with the discovery of the Einstein Cross event in 2014, astronomers tried to make predictions for the reappearance of the supernova. Their calculations also indicated that the supernova should have appeared once before in a third image of the host galaxy in 1998 -- an event not observed by any telescope. Researchers used seven different models of the cluster to calculate when and where the supernova was going to appear in the future. It was a big effort from the community to gather the necessary input data using Hubble, VLT-MUSE, and Keck and to construct the lens models, but all seven models predicted approximately the same time for when the new image of the exploding star would appear. Since the end of October Hubble has been periodically observing MACS J1149.5+2223, hoping to observe a re-run of the distant explosion and prove the models correct. On December 11 Refsdal finally made its predicted reappearance. Testing predictions through observations provides a means of improving our understanding of the cosmos. The detection of Refsdal's reappearance served as an opportunity for astronomers to test their models of how mass -- especially that of mysterious dark matter -- is distributed within that galaxy cluster.
INSIGHT MISSION TO MARS SUSPENDED
NASA
NASA managers have decided to suspend the planned 2016 March launch of the 'Interior Exploration using Seismic Investigations Geodesy and Heat Transport' (InSight) mission. The decision follows unsuccessful attempts to repair a leak in a section of the prime instrument in the payload. Learning about the interior structure of Mars has been a high-priority objective for planetary scientists since the Viking missions, but the fact is that NASA is not ready to launch in the 2016 window. The instrument involved is the 'Seismic Experiment for Interior Structure' (SEIS), a seismometer designed to measure ground movements as small as the diameter of an atom. The instrument requires a vacuum seal around its three main sensors to withstand the harsh conditions of the Martian environment. InSight's investigation of Mars's interior is designed to increase understanding of how all rocky planets, including the Earth, formed and evolved. Mars retains evidence about the rocky planets' early development that has been erased on the Earth by internal churning that Mars lacks. Gaining information about the core, mantle and crust of Mars is a high priority for planetary science, and InSight was built to accomplish that.
A leak earlier this year that previously had prevented the seismometer from retaining vacuum conditions was repaired, and the mission team was hopeful that the most recent fix also would be successful. However, during recent testing at -45°C the instrument again failed to hold a vacuum. There is now insufficient time to mend the new leak and to complete the work and thorough testing required to ensure a successful mission. The relative positions of the planets are most favourable for launching missions from Earth to Mars for only a few weeks every 26 months. For InSight, the 2016 launch window exists from March 4 to March 30. In 2008, NASA made a decision to postpone the launch of the Mars Science Laboratory mission for two years to improve the likelihood of success. The successes of that mission's rover, Curiosity, have outweighed any disappointment about that delay.
Topic: Early January Astronomy Bulletin (Read 2382 times)