POSSIBLE MISSION TO TRITON
NASA
When NASA's Voyager 2 spacecraft flew by Neptune's strange moon Triton three decades ago, it wrote a planetary science cliffhanger. Voyager 2 is the only spacecraft ever to have flown past Neptune, and it left a lot of unanswered questions. The views were as stunning as they were puzzling, revealing massive, dark plumes of icy material spraying out from Triton's surface. But how? Images showed that the icy landscape was young and had been resurfaced over and over with fresh material. But what material, and from where? How could an ancient moon six times farther from the Sun than Jupiter still be active? Is there something in its interior that is still warm enough to drive this activity? A new mission competing for selection under NASA's Discovery Program aims to untangle these mysteries. Called Trident, it is one of four that is developing concept studies for new missions. Up to two will be selected by summer 2021 to become a full-fledged mission and will launch later in the decade. Investigating how Triton has changed over time would give scientists a better understanding of how solar system bodies evolve and work. The oddities of Triton could fill an almanac: As Neptune rotates, Triton orbits in the opposite direction. No other large moon in the solar system does that. And Triton's orbit lies at an extreme tilt, offset from Neptune's equator by 23 degrees. About three-quarters the diameter of our own Moon, Triton isn't where it used to be, either. It likely migrated from the Kuiper Belt, a region beyond Neptune of icy bodies left over from the early solar system. Triton has an unusual atmosphere, too: Filled with charged particles, a layer called the ionosphere is 10 times more active than that of any other moon in the solar system. That last trait is especially strange, because ionospheres generally are charged by solar energy. But Triton and Neptune are far from the Sun - 30 times farther from the Sun than Earth, so some other energy source must be at work. (It takes 165 Earth years for Neptune to complete one orbit around the Sun.) And Triton's climate is dynamic and changing, with a steady flow of organic material, likely nitrogen, snowing onto the surface.
Those mysterious plumes Voyager 2 spotted are especially intriguing. Plumes seen on Saturn's moon Enceladus, and possibly present on Jupiter's moon Europa, are thought to be caused by water from the interior being forced through thick, icy crusts. If an ocean is the source of the plumes on Triton (which lies much farther out in the solar system than Europa and Enceladus), the discovery would give scientists new information about how interior oceans form. Unlike other known ocean worlds, Triton's potential ocean likely developed after it was captured by Neptune's gravity. It would also expand scientists' understanding of where we might find water. Figuring out what factors lead to a solar system body having the necessary ingredients to be habitable, which include water, is one Trident's three major goals. The spacecraft would carry an instrument to probe the moon's magnetic field to determine if an ocean lies inside, while other instruments would investigate the intense ionosphere, organic-rich atmosphere and bizarre surface features. A second goal is to explore vast, unseen lands. Triton offers the largest unexplored solid surface in the solar system this side of the Kuiper Belt. Most of what we know of the moon came from Voyager 2 data, but we've only seen 40% of the moon's surface. Trident would map most of the remainder. Trident's third major goal is to understand how that mysterious surface keeps renewing itself. The surface is remarkably young, geologically speaking (possibly only 10 million years old in a 4.6-billion-year-old solar system) and has almost no visible craters. There's also the question of why it looks so different from other icy moons, and features unusual landforms like dimpled "cantaloupe terrains" and protruding "walled plains." The answers could shed light on how landscapes develop on other icy bodies. The proposed launch date in October 2025 (with a backup in October 2026) would take advantage of a once-in-a-13-year window, when Earth is properly aligned with Jupiter. The spacecraft would use the gravitational pull of Jupiter as a slingshot straight to Triton for an extended 13-day encounter in 2038. And it may seem that time moves slowly in the outer reaches of the solar system, where Neptune's years are long. Ironically for Triton, the long timeline presents limitations. If Trident arrives before 2040, the team could perform its test of what's powering the plume activity. Any later, and the Sun moves too far north ... for the next hundred years.
REGULAR RHYTHM OF RADIO WAVES DETECTED
MIT
A team of astronomers has picked up on a curious, repeating rhythm of fast radio bursts emanating from an unknown source outside our galaxy, 500 million light years away. Fast radio bursts, or FRBs, are short, intense flashes of radio waves that are thought to be the product of small, distant, extremely dense objects, though exactly what those objects might be is a longstanding mystery in astrophysics. FRBs typically last a few milliseconds, during which time they can outshine entire galaxies. Since the first FRB was observed in 2007, astronomers have catalogued over 100 fast radio bursts from distant sources scattered across the Universe, outside our own galaxy. For the most part, these detections were one-offs, flashing briefly before disappearing entirely. In a handful of instances, astronomers observed fast radio bursts multiple times from the same source, though with no discernible pattern. This new FRB source, which the team has catalogued as FRB 180916.J0158+65, is the first to produce a periodic, or cyclical pattern of fast radio bursts. The pattern begins with a noisy, four-day window, during which the source emits random bursts of radio waves, followed by a 12-day period of radio silence. The astronomers observed that this 16-day pattern of fast radio bursts reoccurred consistently over 500 days of observations. The team analyzed data from the Canadian Hydrogen Intensity Mapping Experiment, or CHIME, a radio telescope in British Columbia that was the first to pick up signals of the new periodic FRB source. In 2017, CHIME was erected at the Dominion Radio Astrophysical Observatory in British Columbia, where it quickly began detecting fast radio bursts from galaxies across the Universe, billions of light years from Earth. CHIME consists of four large antennas, each about the size and shape of a snowboarding half-pipe, and is designed with no moving parts. Rather than swivelling to focus on different parts of the sky, CHIME stares fixedly at the entire sky, using digital signal processing to pinpoint the region of space where incoming radio waves are originating.
From September 2018 to February 2020, CHIME picked out 38 fast radio bursts from a single source, FRB 180916.J0158+65, which the astronomers traced to a star-churning region on the outskirts of a massive spiral galaxy, 500 million light years from Earth. The source is the most active FRB source that CHIME has yet detected, and until recently it was the closest FRB source to Earth. As the researchers plotted each of the 38 bursts over time, a pattern began to emerge: One or two bursts would occur over four days, followed by a 12-day period without any bursts, after which the pattern would repeat. This 16-day cycle occurred again and again over the 500 days that they observed the source. Exactly what phenomenon is behind this new extragalactic rhythm is a big unknown, although the team explores some ideas in their new paper. One possibility is that the periodic bursts may be coming from a single compact object, such as a neutron star, that is both spinning and wobbling -- an astrophysical phenomenon known as precession.
Assuming that the radio waves are emanating from a fixed location on the object, if the object is spinning along an axis and that axis is only pointed toward the direction of Earth every four out of 16 days, then we would observe the radio waves as periodic bursts. Another possibility involves a binary system, such as a neutron star orbiting another neutron star or black hole. If the first neutron star emits radio waves, and is on an eccentric orbit that briefly brings it close to the second object, the tides
between the two objects could be strong enough to cause the first neutron star to deform and burst briefly before it swings away. This pattern would repeat when the neutron star swings back along its orbit. The researchers considered a third scenario, involving a radio-emitting source that circles a central star. If the star emits a wind, or cloud of gas, then every time the source passes through the cloud, the gas from the cloud could periodically magnify the source's radio emissions. Perhaps the most exciting possibility is the idea that this new FRB, and even those that are not periodic or even repeating, may originate from magnetars -- a type of neutron star that is thought to have an extremely powerful magnetic field. The particulars of magnetars are still a bit of a mystery, but astronomers have observed that they do occasionally release massive amounts of radiation across the electromagnetic spectrum, including energy in the radio band. Very recently, the same group made a new observation that supports the idea that magnetars may in fact be a viable source for fast radio bursts. In late April, CHIME picked up a signal that looked like a fast radio burst, coming from a flaring magnetar, some 30,000 light years from Earth. If the signal is confirmed, this would be the first FRB detected within our own galaxy, as well as the most compelling evidence of magnetars as a source of these mysterious cosmic sparks.
BLACK HOLE’S HEART STILL BEATING
Durham University
The first confirmed heartbeat of a supermassive black hole is still going strong more than ten years after first being observed. X-ray satellite observations spotted the repeated beat after its signal had been blocked by our Sun for a number of years. Astronomers say this is the most long lived heartbeat ever seen in a black hole and tells us more about the size and structure close to its event horizon -- the space around a black hole from which nothing, including light, can escape. The black hole's heartbeat was first detected in 2007 at the centre of a galaxy called RE J1034+396 which is approximately 600 million light years from Earth. The signal from this galactic giant repeated every hour and this behaviour was seen in several snapshots taken before satellite observations were blocked by our Sun in 2011. In 2018 the European Space Agency's XMM-Newton X-ray satellite was able to finally re-observe the black hole and to scientists' amazement the same repeated heartbeat could still be seen. Matter falling on to a supermassive black hole as it feeds from the accretion disc of material surrounding it releases an enormous amount of power from a comparatively tiny region of space, but this is rarely seen as a specific repeatable pattern like a heartbeat. The time between beats can tell us about the size and structure of the matter close to the black hole's event horizon. The main idea for how this heartbeat is formed is that the inner parts of the accretion disc are expanding and contracting. The only other system we know which seems to do the same thing is a 100,000 times smaller stellar-mass black hole in our Milky Way, fed by a binary companion star, with correspondingly smaller luminosities and timescales. This shows that simple scalings with black hole mass work even for the rarest types of behaviour. It proves that such signals arising from a supermassive black hole can be very strong and persistent. It also provides the best opportunity for scientists to further investigate the nature and origin of this heartbeat signal.
GALAXIES CONTAINING TWO BLACK HOLES
Clemson University
An international team of astronomers has identified periodic gamma-ray emissions from 11 active galaxies, paving the way for future studies of unconventional galaxies that might harbour two supermassive black holes at their centres. Among astronomers, it has long been well-established that most galaxies host a black hole at their centre. But galaxies hosting a pair of black holes has remained theoretical. In general, supermassive black holes are characterized by masses of more than a million masses of that of our Sun. Some of these supermassive black holes, known as active galactic nuclei (AGN) have been found to accelerate particles to near the speed of light in collimated beams called jets. The emission from these jets is detected throughout the entire electromagnetic spectrum, but most of their energy is released in the form of gamma rays. Gamma rays, which are the most extreme form of light, are detected by the Large Area Telescope onboard NASA's Fermi Gamma-ray Space Telescope. AGN are characterized by abrupt and unpredictable variations in brightness. The team accomplished the first difficult step of identifying a large number of galaxies that emits periodically over years and is trying to address the question of what is producing that periodic behaviour in these AGN. Several of the potential explanations are fascinating.
The team has a few possibilities in mind -- from lighthouse effects produced by the jets to modulations in the flow of matter to the black hole -- but one very interesting solution would be that periodicity is produced by a pair of supermassive black holes rotating around each other. Understanding the relation of these black holes with their environment will be essential for a complete picture of galaxy formation. Thanks to a decade of Fermi-LAT observations, the team was able to identify the repetition of gamma-ray signals over cycles of a few years. On average, these emissions repeated about every two years. Enlarging the limited sample of periodic emitters constitutes an important leap forward for understanding the underlying physical processes in these galaxies. Previously only two blazars were known to show periodic changes in their gamma-ray brightness. Thanks to the study, the team can confidently say that this behaviour is present in 11 other sources. In addition, the study found 13 other galaxies with hints of cyclical emission. But to confidently confirm this, astronomers need to wait for Fermi-LAT to collect even more data.
QUASAR JETS ARE GIANT PARTICLE ACCELERATORS
CNRS
A collaboration bringing together over 200 scientists from 13 countries has shown that the very high-energy gamma-ray emission from quasars, galaxies with a highly energetic nucleus, is not concentrated in the region close to their central black hole but in fact extends over several thousand light-years along jets of plasma. This discovery shakes up current scenarios for the behaviour of such plasma jets. Over the past few years, scientists have observed the Universe using gamma rays, which
are very high-energy photons. Gamma rays, which form part of the cosmic rays that constantly bombard the Earth, originate from regions of the Universe where particles are accelerated to huge energies unattainable in human-built accelerators. Gamma rays are emitted by a wide range of cosmic objects, such as quasars, which are active galaxies with a highly energetic nucleus. The intensity of the radiation emitted from these systems can vary over very short timescales of up to one minute. Scientists therefore believed that the source of this radiation was very small and located in the vicinity of a supermassive black hole, which can have a mass several billion times that of the Sun's. The black hole is thought to gobble up the matter spiralling down into it and eject a small part of it in the form of large jets of plasma, at relativistic speeds, close to the speed of light, thus contributing to the
redistribution of matter throughout the Universe.
Using the H.E.S.S. Observatory in Namibia, an international astrophysics collaboration observed a radio galaxy (a galaxy that is highly luminous when observed at radio wavelengths) for over 200 hours at unparalleled resolution. As the nearest radio galaxy to Earth, Centaurus A is favourable to scientists for such a study, enabling them to identify the region emitting the very high-energy radiation while studying the trajectory of the plasma jets. They were able to show that the gamma-ray source extends over a distance of several thousand light-years. This extended emission indicates that particle acceleration does not take place solely in the vicinity of the black hole but also along the entire length of the plasma jets. Based on these new results, it is now believed that the particles are reaccelerated by stochastic processes along the jet. The discovery suggests that many radio galaxies with extended jets accelerate electrons to extreme energies and might emit gamma-rays, possibly explaining the origins of a substantial fraction of the diffuse extragalactic gamma background radiation. These findings provide important new insights into cosmic gamma-ray emitters, and in particular about the role of radio galaxies as highly efficient relativistic electron accelerators. Due to their large number, it would appear that radio galaxies collectively make a highly significant contribution to the redistribution of energy in the intergalactic medium. The results of this study required extensive observations and optimized analysis techniques with H.E.S.S., the most sensitive gamma-ray observatory to date. Next-generation telescopes (Cherenkov Telescope Array, or CTA) will no doubt make it possible to observe this phenomenon in even greater detail.
SIGNAL FROM THE END OF THE UNIVERSE’S ‘DARK AGE’
University of Washington
Today, stars fill the night sky. But when the Universe was in its infancy, it contained no stars at all. And an international team of scientists is closer than ever to detecting signal from this era that has been traveling through the cosmos ever since that starless era ended some 13 billion years ago. The team reported last year that it had achieved an almost 10-fold improvement of radio emission data collected by the Murchison Widefield Array. Team members are currently scouring the data from this radio telescope in remote Western Australia for a telltale signal from this poorly understood "dark age" of our Universe. Learning about this period will help address major questions about the Universe today. Before this dark age, the Universe was hot and dense. Electrons and photons regularly snared one another, making the Universe opaque. But when the Universe was less than a million years old, electron–photon interactions became rare. The expanding Universe became increasingly transparent and dark, beginning its dark age. The starless era lasted hundreds of millions of years during which neutral hydrogen —hydrogen atoms with no overall charge—dominated the cosmos. For this dark age, of course there's no light-based signal we can study to learn about it—there was no visible light. But there is a specific signal we can look for which comes from neutral hydrogen. This signal has never been measured, but we know it's out there. And it's difficult to detect because in the 13 billion years since that signal was emanated, our Universe has become a very busy place, filled with other activity from stars, galaxies and even our technology that drown out the signal from the neutral hydrogen.
The 13 billion-year-old signal that the team is after is electromagnetic radio emission that the neutral hydrogen emanated at a wavelength of 21 centimetres. The Universe has expanded since that time, stretching the signal out to nearly 2 metres. That signal should harbour information about the dark age and the events that ended it. When the Universe was just 1 billion years old, hydrogen atoms began to aggregate and form the first stars, bringing an end to the dark age. The light from those first stars kicked off a new era—the Epoch of Reionization—in which energy from those stars converted much of the neutral hydrogen into an ionized plasma. That plasma dominates interstellar space to this day. The Epoch of Reionization and the dark age preceding it are critical periods for understanding features of our Universe, such as why we have some regions filled with galaxies and others relatively empty, the distribution of matter and potentially even dark matter and dark energy. The Murchison Array is the team's primary tool. This radio telescope consists of 4,096 dipole antennas, which can pick up low-frequency signals like the electromagnetic signature of neutral hydrogen. But those sorts of low-frequency signals are difficult to detect due to electromagnetic "noise" from other sources bouncing around the cosmos, including galaxies, stars and human activity. The team has developed increasingly sophisticated methods to filter out this noise and bring them closer to that signal. In 2019, the researchers announced that they had filtered out electromagnetic interference—including from our own radio broadcasts—from more than 21 hours of Murchison Array data. Moving forward, the team has about 3,000 hours of additional emission data collected by the radio telescope. The researchers are trying to filter out interference and get even closer to that elusive signal from neutral hydrogen—and the dark age it can illuminate.
SIX BILLION EARTH-LIKE PLANETS IN OUR GALAXY
University of British Columbia
There may be as many as one Earth-like planet for every five Sun-like stars in the Milky way Galaxy, according to new estimates by astronomers using data from NASA's Kepler mission. To be considered Earth-like, a planet must be rocky, roughly Earth-sized and orbiting Sun-like (G-type) stars. It also has to orbit in the habitable zones of its star -- the range of distances from a star in which a rocky planet could host liquid water, and potentially life, on its surface. Calculations place an upper limit of 0.18 Earth-like planets per G-type star. Estimating how common different kinds of planets are around different stars can provide important constraints on planet formation and evolution theories, and help optimize future missions dedicated to finding exoplanets. Our Milky Way has as many as 400 billion stars, with seven per cent of them being G-type. That means less than six billion stars may have Earth-like planets in our Galaxy. Previous estimates of the frequency of Earth-like planets range from roughly 0.02 potentially habitable planets per Sun-like star, to more than one per Sun-like star. Typically, planets like Earth are more likely to be missed by a planet search than other types, as they are so small and orbit so far from their stars. That means that a planet catalogue represents only a small subset of the planets that are actually in orbit around the stars searched. The research also shed more light on one of the most outstanding questions in exoplanet science today: the 'radius gap' of planets. The radius gap demonstrates that it is uncommon for planets with orbital periods less than 100 days to have a size between 1.5 and two times that of Earth. She found that the radius gap exists over a much narrower range of orbital periods than previously thought. Her observational results can provide constraints on planet evolution models that explain the radius gap's characteristics.
NEW LIGHT SHED ON INTELLIGENT LIFE ACROSS GALAXY
University of Nottingham
One of the biggest and longest-standing questions in the history of human thought is whether there are other intelligent life forms within our Universe. Obtaining good estimates of the number of possible extraterrestrial civilizations has however been very challenging. A new study has taken a new approach to this problem. Using the assumption that intelligent life forms on other planets in a similar way as it does on Earth, researchers have obtained an estimate for the number of intelligent communicating civilizations within our own galaxy -- the Milky Way. They calculate that there could be over 30 active communicating intelligent civilizations in our home Galaxy. There should be at least a few dozen active civilizations in our Galaxy under the assumption that it takes 5 billion years for intelligent life to form on other planets, as on Earth. The idea is looking at evolution, but on a cosmic scale. We call this calculation the Astrobiological Copernican Limit. The classic method for estimating the number of intelligent civilizations relies on making guesses of values relating to life, whereby opinions about such matters vary quite substantially. The new study simplifies these assumptions using new data, giving us a solid estimate of the number of civilizations in our Galaxy. The two Astrobiological Copernican limits are that intelligent life forms in less than 5 billion years, or after about 5 billion years -- similar to on Earth where a communicating civilization formed after 4.5 billion years. In the strong criteria, whereby a metal content equal to that of the Sun is needed (the Sun is relatively speaking quite metal rich), the study calculates that there should be around 36 active civilizations in our Galaxy."
The research shows that the number of civilizations depends strongly on how long they are actively sending out signals of their existence into space, such as radio transmissions from satellites, television, etc. If other technological civilizations last as long as ours which is currently 100 years old, then there will be about 36 ongoing intelligent technical civilizations throughout our Galaxy. However, the average distance to these civilizations would be 17,000 light-years away, making detection and communication very difficult with our present technology. It is also possible that we are the only civilization within our Galaxy unless the survival times of civilizations like our own are long. The new research suggests that searches for extraterrestrial intelligent civilizations not only reveals the existence of how life forms, but also gives us clues for how long our own civilization will last. If we find that intelligent life is common then this would reveal that our civilization could exist for much longer than a few hundred years, alternatively if we find that there are no active civilizations in our Galaxy it is a bad sign for our own long-term existence. By searching for extraterrestrial intelligent life -- even if we find nothing -- we are discovering our own future and fate.
Topic: Late June Astronomy Bulletin (Read 1216 times)