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Late March Astronomy Bulletin
« on: March 21, 2021, 08:55 »

On April 13, 2029 asteroid 99942 Apophis will fly past Earth so close you can see it with your naked eye. In early March, Apophis made a "pre-flyby" of Earth about 16 million km away, the closest it will be before the big event in 2029. Asteroid Apophis is about 370 metres wide. That's big enough to punch through Earth's atmosphere, devastating a region the size of, say, Texas, if it hit land, or causing widespread tsunamis if it hit ocean. Fortunately, Apophis will not hit Earth in 2029. Back in 2004 when the asteroid was first discovered, astronomers thought there might be a collision. Improved observations of Apophis's orbit have since ruled out a strike. The asteroid will skim Earth's belt of geosynchronous satellites, but come no closer than 31,900 km to Earth itself. Observations in the past year have reduced the uncertainty of the flyby distance to ±20 km. At such close range, Earth's gravity could stretch the asteroid, change the way it spins, and trigger small avalanches. Radar observations during the hours of closest approach will be able to image the asteroid's surface with few-metre resolution, potentially revealing the changes. Shining like a 3rd magnitude star, Apophis will be plainly visible to the naked eye from rural areas and an easy (albeit fast-moving) target for small telescopes. No one in recorded history has ever seen an asteroid in space so bright. NASA, China, the Planetary Society and others are planning or contemplating missions to Apophis. The more we know about it the better. The next two flybys in 2029 and 2036 are safe, but analysts still haven't completely ruled out a low-probability impact in 2068.


Look up to the night sky just before dawn, or after dusk, and you might see a faint column of light extending up from the horizon. That luminous glow is the zodiacal light, or sunlight reflected toward Earth by a cloud of tiny dust particles orbiting the Sun. Astronomers have long thought that the dust is brought into the inner solar system by a few of the asteroid and comet families that venture in from afar. But now, a team of Juno scientists argues that Mars may be the culprit. An instrument aboard the Juno spacecraft serendipitously detected dust particles slamming into the spacecraft during its journey from Earth to Jupiter. The impacts provided important clues to the origin and orbital evolution of the dust, resolving some mysterious variations of the zodiacal light. Onboard cameras snap photos of the sky every quarter of a second to determine Juno’s orientation in space by recognizing star patterns in its images – an engineering task essential to the magnetometer’s accuracy. But researchers hoped the cameras might also catch sight of an undiscovered asteroid. So one camera was programmed to report things that appeared in multiple consecutive images but weren’t in the catalogue of known celestial objects. The camera showed that dust grains had smashed into Juno at about 16,000 kilometres per hour, chipping off submillimeter pieces of spacecraft. The spray of debris was coming from Juno’s expansive solar panels – the biggest and most sensitive unintended dust detector ever built. Each piece of debris records the impact of an interplanetary dust particle, allowing astronomers to compile a distribution of dust along Juno’s path. The majority of dust impacts were recorded between Earth and the asteroid belt, with gaps in the distribution related to the influence of Jupiter’s gravity. According to the scientists, this was a radical revelation. Before now, scientists have been unable to measure the distribution of these dust particles in space. Dedicated dust detectors have had limited collection areas and thus limited sensitivity to a sparse population of dust. They mostly count the more abundant and much smaller dust particles from interstellar space. In comparison, Juno’s expansive solar panels have 1,000 times more collection area than most dust detectors. Juno scientists determined that the dust cloud ends at Earth because Earth’s gravity sucks up all the dust that gets near it. That’s the dust we see as zodiacal light.

As for the outer edge, around 2 astronomical units (AU) from the Sun, it ends just beyond Mars. At that point, the scientists report, the influence of Jupiter’s gravity acts as a barrier, preventing dust particles from crossing from the inner solar system into deep space. This same phenomenon, known as orbital resonance, also works the other way, where it blocks dust originating in deep space from passing into the inner solar system. The profound influence of the gravity barrier indicates that the dust particles are in a nearly circular orbit around the Sun, and the only object we know of in almost circular orbit around 2 AU is Mars, so the natural thought is that Mars is a source of this dust. The researchers developed a computer model to predict the light reflected by the dust cloud, dispersed by gravitational interaction with Jupiter that scatters the dust into a thicker disk. The scattering depends only on two quantities: the dust inclination to the ecliptic and its orbital eccentricity. When the researchers plugged in the orbital elements of Mars, the distribution accurately predicted the telltale signature of the variation of zodiacal light near the ecliptic. While there is good evidence now that Mars, the dustiest planet we know of, is the source of the zodiacal light, the team cannot yet explain how the dust could have escaped the grip of Martian gravity.

University of Minnesota

In early 2016, an icy visitor from the edge of our solar system hurtled past Earth. It briefly became visible to stargazers as Comet Catalina before it slingshotted past the Sun to disappear forevermore out of the solar system. Among the many observatories that captured a view of this comet, which appeared near the Plough, was the Stratospheric Observatory for Infrared Astronomy (SOFIA), NASA's telescope on an airplane. Using one of its unique infrared instruments, SOFIA was able to pick out a familiar fingerprint within the dusty glow of the comet's tail -- carbon. Now this one-time visitor to our inner solar system is helping explain more about our own origins as it becomes apparent that comets like Catalina could have been an essential source of carbon on planets like Earth and Mars during the early formation of the solar system. Originating from the Oort Cloud at the farthest reaches of our solar system, Comet Catalina and others of its type have such long orbits that they arrive on our celestial doorstep relatively unaltered. This makes them effectively frozen in time, offering researchers rare opportunities to learn about the early solar system from which they come. SOFIA's infrared observations were able to capture the composition of the dust and gas as it evaporated off the comet, forming its tail. The observations showed that Comet Catalina is carbon-rich, suggesting that it formed in the outer regions of the primordial solar system, which held a reservoir of carbon that could have been important for seeding life.

While carbon is a key ingredient of life, early Earth and other terrestrial planets of the inner solar system were so hot during their formation that elements like carbon were lost or depleted. While the cooler gas giants like Jupiter and Neptune could support carbon in the outer solar system, Jupiter's jumbo size may have gravitationally blocked carbon from mixing back into the inner solar system. So how did the inner rocky planets evolve into the carbon-rich worlds that they are today? Researchers think that a slight change in Jupiter's orbit allowed small, early precursors of comets to mix carbon from the outer regions into the inner regions, where it was incorporated into planets like Earth and Mars. Comet Catalina's carbon-rich composition helps explain how planets that formed in the hot, carbon-poor regions of the early solar system evolved into planets with the life-supporting element. All terrestrial worlds are subject to impacts by comets and other small bodies, which carry carbon and other elements. We are getting closer to understanding exactly how these impacts on early planets may have catalysed life.

University of New South Wales

A newly discovered planet could be our best chance yet of studying rocky planet atmospheres outside the solar system. The planet, called Gliese 486b (pronounced Glee-seh), is a 'super-Earth': that is, a rocky planet bigger than Earth but smaller than ice giants like Neptune and Uranus. It orbits a red dwarf star around 26 light-years away, making it a close neighbour -- galactically speaking. With a piping-hot surface temperature of 430 degrees Celsius, Gliese 486b is too hot to support human life. But studying its atmosphere could help us learn whether similar planets might be habitable for humans -- or if they're likely to hold other signs of life. Astronomers have known for a long time that rocky super-Earths must exist around the nearby stars, but haven't had the technology to search for them until recently. Like Earth, Gliese 486b is a rocky planet -- but that's where the similarities end. Our neighbour is 30 per cent bigger and almost three times heavier than Earth. It's possible that its surface -- which is hot enough to melt lead -- may even be scattered with glowing lava rivers. Super-Earths themselves aren't rare, but Gliese 486b special for two key reasons: firstly, its heat 'puffs up' the atmosphere, helping astronomers take atmospheric measurements; and secondly, it's a transiting planet, which means it crosses over its star from Earth's perspective -- making it possible for scientists to conduct in-depth analysis of its atmosphere. A planet's atmosphere can reveal a lot about its ability to support life. For example, a lack of atmosphere might suggest the planet's nearby star is volatile and prone to high stellar activity -- making it unlikely that life will have a chance to develop. On the other hand, a healthy, long-lived atmosphere could suggest conditions are stable enough to support life. Both options help astronomers solve a piece of the planetary formation puzzle.

Astronomers think Gliese 486b could have kept a part of its original atmosphere, despite being so close to its red dwarf star. As a transiting planet, Gliese 486b gives scientists two unique opportunities to study its atmosphere: first when the planet passes in front of its star and a fraction of starlight shines through its atmospheric layer (a technique called 'transmission spectroscopy'); and then when starlight illuminates the surface of the planet as it orbits around and behind the star (called 'emission spectroscopy'). In both cases, scientists use a spectrograph -- a tool that splits light according to its wavelengths -- to decode the chemical makeup of the atmosphere. This is the single best planet for studying emission spectroscopy of all the rocky planets we know. It's also the second-best planet to study transmission spectroscopy. Gliese 486b is a great catch for astronomers -- but you wouldn't want to live there. With a surface of 430 degrees Celsius, you wouldn't be able to go outside without some kind of spacesuit. The gravity is also 70 per cent stronger than on Earth, making it harder to walk and jump. Someone who weighed 50 kilograms on Earth would feel like they weighed 85 kilograms on Gliese 486b. Red dwarfs are the most common type of star, making up around 70 per cent of all stars in the universe. They are also much more likely to have rocky planets than Sun-like stars.

University of Colorado at Boulder

Astronomers have discovered new hints of a giant, scorching-hot planet orbiting Vega, one of the brightest stars in the night sky. The work focuses on an iconic and relatively young star, Vega, which is part of the constellation Lyra and has a mass twice that of our own Sun. This celestial body sits just 25 light-years, or about 150 trillion miles, from Earth -- pretty close, astronomically speaking. Scientists can also see Vega with telescopes even when it's light out, which makes it a prime candidate for research. Despite the star's fame, researchers have yet to find a single planet in orbit around Vega. That might be about to change: Drawing on a decade of observations from the ground, astronomers unearthed a curious signal that could be the star's first-known world. If the team's findings bear out, the alien planet would orbit so close to Vega that its years would last less than two-and-a-half Earth days. (Mercury, in contrast, takes 88 days to circle the Sun). This candidate planet could also rank as the second hottest world known to science -- with surface temperatures averaging a searing 2,977 degrees Celsius. Scientists have discovered more than 4,000 exoplanets, or planets beyond Earth's solar system, to date. Few of those, however, circle stars that are as bright or as close to Earth as Vega. That means that, if there are planets around the star, scientists could get a really detailed look at them.

Vega is what scientists call an A-type star, the name for objects that tend to be bigger, younger and much faster-spinning than our own Sun. Vega, for example, rotates around its axis once every 16 hours -- much faster than the Sun with a rotational period of 27 Earth days. Such a lightning-fast pace can make it difficult for scientists to collect precise data on the star's motion and, by extension, any planets in orbit around it. The team pored through roughly 10 years of data on Vega collected by the Fred Lawrence Whipple Observatory in Arizona. In particular, the team was looking for a tell-tale signal of an alien planet -- a slight jiggle in the star's velocity. The team discovered a signal that indicates that Vega might host what astronomers call a "hot Neptune" or maybe a "hot Jupiter. That close to Vega, the candidate world might puff up like a balloon, and even iron would melt into gas in its atmosphere. The researchers have a lot more work to do before they can definitively say that they've discovered this sizzling planet. The easiest way to look for it might be to scan the stellar system directly to look for light emitted from the hot, bright planet.

University of Tokyo

Red supergiants are a class of star that end their lives in supernova explosions. Their lifecycles are not fully understood, partly due to difficulties in measuring their temperatures. For the first time, astronomers develop an accurate method to determine the surface temperatures of red supergiants. Stars come in a wide range of sizes, masses and compositions. Our Sun is considered a relatively small specimen, especially when compared to something like Betelgeuse which is known as a red supergiant. Red supergiants are stars over nine times the mass of our Sun, and all this mass means that when they die they do so with extreme ferocity in an enormous explosion known as a supernova, in particular what is known as a Type-II supernova. Type II supernovae seed the cosmos with elements essential for life; therefore, researchers are keen to know more about them. At present there is no way to accurately predict supernova explosions. One piece of this puzzle lies in understanding the nature of the red supergiants that precede supernovae. Despite the fact red supergiants are extremely bright and visible at great distances, it is difficult to ascertain important properties about them, including their temperatures. This is due to the complicated structures of their upper atmospheres which leads to inconsistencies of temperature measurements that might work with other kinds of stars.

In order to measure the temperature of red supergiants, astronomers needed to find a visible, or spectral, property that was not affected by their complex upper atmospheres. Chemical signatures known as absorption lines were the ideal candidates, but there was no single line that revealed the temperature alone. However, by looking at the ratio of two different but related lines -- those of iron -- we found the ratio itself related to temperature. And it did so in a consistent and predictable way. Researchers observed candidate stars with an instrument called WINERED which attaches to telescopes in order to measure spectral properties of distant objects. They measured the iron absorption lines and calculated the ratios to estimate the stars' respective temperatures. By combining these temperatures with accurate distance measurements obtained by the European Space Agency's Gaia space observatory, the researchers calculated the stars luminosity, or power, and found their results consistent with theory.


In the first all-sky survey by the eROSITA X-ray telescope onboard SRG, astronomers have identified a previously unknown supernova remnant, dubbed "Hoinga." The finding was confirmed in archival radio data and marks the first discovery of a joint Australian-eROSITA partnership established to explore our Galaxy using multiple wavelengths, from low-frequency radio waves to energetic X-rays. The Hoinga supernova remnant is very large and located far from the galactic plane -- a surprising first finding -- implying that the next years might bring many more discoveries. While the supernova itself is only observable on a timescale of months, their remnants can be detected for about 100,000 years. These remnants are composed of the material ejected by the exploding star at high velocities and forming shocks when hitting the surrounding interstellar medium. About 300 such supernova remnants are known today -- much less than the estimated 1200 that should be observable throughout our home Galaxy. So, either astrophysicists have misunderstood the supernova rate or a large majority has been overlooked so far. An international team of astronomers are now using the all-sky scans of the eROSITA X-ray telescope to look for previously unknown supernova remnants. With temperatures of millions of the degrees, the debris of such supernovae emits high-energy radiation, i.e. they should show up in the high-quality X-ray survey data.

After the astronomers found the object in the eROSITA all-sky data, they turned to other resources to confirm its nature. Hoinga is -- although barely -- visible also in data taken by the ROSAT X-ray telescope 30 years ago, but nobody noticed it before due to its faintness and its location at high galactic latitude. However, the real confirmation came from radio data, the spectral band where 90% of all known supernova remnants were found so far. The eROSITA X-ray telescope will perform a total of eight all-sky surveys and is about 25 times more sensitive than its predecessor ROSAT. Both observatories were designed, build and are operated by the Max Planck Institute for Extraterrestrial Physics. The astronomers expected to discover new supernova remnants in its X-ray data over the next few years, but they were surprised to identify one so early in the programme. Combined with the fact that the signal is already present in decades-old data, this implies that many supernova remnants might have been overlooked in the past due to low-surface brightness, being in unusual locations or because of other nearby emission from brighter sources. Together with upcoming radio surveys, the eROSITA X-ray survey shows great promise for finding many of the missing supernova remnants, helping to solve this long-standing astrophysical mystery.

Harvard-Smithsonian Center for Astrophysics

Scientists have long theorized that supermassive black holes can wander through space -- but catching them in the act has proven difficult. For their search, the team initially surveyed 10 distant galaxies and the supermassive black holes at their cores. They specifically studied black holes that contained water within their accretion disks -- the spiral structures that spin inward towards the black hole. As the water orbits around the black hole, it produces a laser-like beam of radio light known as a maser. When studied with a combined network of radio antennas using a technique known as very long baseline interferometry (VLBI), masers can help measure a black hole's velocity very precisely. The technique helped the team determine that nine of the 10 supermassive black holes were at rest -- but one stood out and seemed to be in motion. Located 230 million light-years away from Earth, the black hole sits at the centre of a galaxy named J0437+2456. Its mass is about three million times that of our Sun. Using follow-up observations with the Arecibo and Gemini Observatories, the team has now confirmed their initial findings. The supermassive black hole is moving with a speed of about 110,000 miles per hour inside the galaxy J0437+2456. But what's causing the motion is not known. The team suspects there are two possibilities. But there's another, perhaps even more exciting possibility: the black hole may be part of a binary system. Further observations, however, will ultimately be needed to pin down the true cause of this supermassive black hole's unusual motion.


Using the Very Large Telescope (VLT), astronomers have discovered and studied in detail the most distant source of radio emission known to date. The source is a “radio-loud” quasar — a bright object with powerful jets emitting at radio wavelengths — that is so far away its light has taken 13 billion years to reach us. We see it as it was when the Universe was just around 780 million years old. The discovery could provide important clues to help astronomers understand the early Universe. Quasars are very bright objects that lie at the centre of some galaxies and are powered by supermassive black holes. As the black hole consumes the surrounding gas, energy is released, allowing astronomers to spot them even when they are very far away. The newly discovered quasar is nicknamed P172+18. While more distant quasars have been discovered, this is the first time astronomers have been able to identify the tell-tale signatures of radio jets in a quasar this early on in the history of the Universe. Only about 10% of quasars — which astronomers classify as “radio-loud” — have jets, which shine brightly at radio frequencies. P172+18 is powered by a black hole about 300 million times more massive than our Sun that is consuming gas at a stunning rate. The astronomers think that there’s a link between the rapid growth of supermassive black holes and the powerful radio jets spotted in quasars like P172+18. The jets are thought to be capable of disturbing the gas around the black hole, increasing the rate at which gas falls in. Therefore, studying radio-loud quasars can provide important insights into how black holes in the early Universe grew to their supermassive sizes so quickly after the Big Bang.

P172+18 was first recognised as a far-away quasar, after having been previously identified as a radio source, at the Magellan Telescope at Las Campanas Observatory in Chile. However, owing to a short observation time, the team did not have enough data to study the object in detail. A flurry of observations with other telescopes followed, including with the X-shooter instrument on ESO’s VLT, which allowed them to dig deeper into the characteristics of this quasar, including determining key properties such as the mass of the black hole and how fast it’s eating up matter from its surroundings. Other telescopes that contributed to the study include the National Radio Astronomy Observatory's Very Large Array and the Keck Telescope in the US.

University of Southern Denmark

The Universe was created by the Big Bang 13.8 billion years ago, and then it started to expand. The expansion is ongoing: it is still being stretched out in all directions like a balloon being inflated. Physicists agree on this much, but something is wrong. Measuring the expansion rate of the Universe in different ways leads to different results. So, is something wrong with the methods of measurement? Or is something going on in the Universe that physicists have not yet discovered and therefore have not taken into account? Scientists propose the existence of a new type of dark energy in the Universe. If you include it in the various calculations of the expansion of the Universe, the results will be more alike. When physicists calculate the expansion rate of the Universe, they base the calculation on the assumption that the Universe is made up of dark energy, dark matter and ordinary matter. Until recently, all types of observations fitted in with such a model of the Universe's composition of matter and energy, but this is no longer the case. Conflicting results arise when looking at the latest data from measurements of supernovae and the cosmic microwave background radiation; the two methods quite simply lead to different results for the expansion rate. In the new model, physicists find that if there was a new type of extra dark energy in the early Universe, it would explain both the background radiation and the supernova measurements simultaneously and without contradiction. They believe that in the early Universe, dark energy existed in a different phase. You can compare it to when water is cooled and it undergoes a phase transition to ice with a lower density. In the same way, dark energy in this model undergoes a transition to a new phase with a lower energy density, thereby changing the effect of the dark energy on the expansion of the Universe. According to the calculations, the results add up if you imagine that dark energy thus underwent a phase transition triggered by the expansion of the Universe. Today we know approx. 20 per cent of the matter that the Universe is made of. It is the matter that you and I, planets and galaxies are made of. The Universe also consists of dark matter, which no one knows what is. In addition, there is dark energy in the Universe; it is the energy that causes the Universe to expand, and it makes up approx. 70 pct. of the energy density of the Universe.

National Institute for Space Research

Observations of galactic rotation curves give one of the strongest lines of evidence pointing towards the existence of dark matter, a non-baryonic form of matter that makes up an estimated 85% of the matter in the observable Universe. Current assessments of galactic rotation curves are based upon a framework of Newtonian accounts of gravity. A new paper suggests that if this is substituted with a general relativity-based model, the need to recourse to dark matter is relieved, replaced by the effects of gravitomagnetism. The main role of dark matter has historically been to resolve the disparity between astrophysical observations and current theories of gravity. Put simply, if baryonic matter -- the form of matter we see around us every day which is made up of protons, neutrons and electrons -- is the only form of matter, then there shouldn't be enough gravitational force to prevent galaxies from flying apart. By disregarding general relativistic corrections to Newtonian gravity arising from mass currents, and by neglecting these mass currents, astronomers assert that these models also miss significant modifications to rotational curves -- the orbital speeds of visible stars and gas plotted against their radial distance from their galaxy's centre. This is because of an effect in general relativity not present in Newton's theory of gravity -- frame-dragging or the Lense Thirring effect. This effect arises when a massive rotating object like a star or black hole 'drags' the very fabric of spacetime along with it, in turn giving rise to a gravitomagnetic field. The paper presents a new model for the rotational curves of galaxies which is in agreement with previous efforts involving general relativity. The researcher demonstrates that even though the effects of gravitomagnetic fields are weak, factoring them into models alleviates the difference between theories of gravity and observed rotational curves -- eliminating the need for dark matter. The theory still needs some development before it is widely accepted, with the author particularly pointing out that the time evolution of galaxies modelled with this framework is a complex problem that will require much deeper analysis. The team concludes by suggesting that all calculations performed with thin galactic disk models performed up until this point may have to be recalculated, and the very concept of dark matter itself, questioned.

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