HUBBLE SEES BREAKUP OF COMET ATLAS
ESA/Hubble Information Centre
The Hubble Space Telescope has provided astronomers with the sharpest view yet of the breakup of Comet C/2019 Y4 (ATLAS). The telescope resolved roughly 30 fragments of the fragile comet on 20 April and 25 pieces on 23 April. The comet was first discovered in December 2019 by the ATLAS (Asteroid Terrestrial-impact Last Alert System) robotic astronomical survey system in Hawaiʻi, USA. It brightened quickly until mid-March, and some astronomers initially anticipated that it might be visible to the naked eye in May to become one of the most spectacular comets seen in the last two decades. However, the comet abruptly began to get dimmer, leading astronomers to speculate that the icy core may be fragmenting, or even disintegrating. ATLAS's fragmentation was confirmed by amateur astronomer Jose de Queiroz, who photographed around three pieces of the comet on 11 April. The Hubble Space Telescope's new observations of the comet's breakup on 20 and 23 April reveal that the broken fragments are all enveloped in a sunlight-swept tail of cometary dust. The images provide further evidence that comet fragmentation is probably common and might even be the dominant mechanism by which the solid, icy nuclei of comets die. Because comet fragmentation happens quickly and
unpredictably, reliable observations are rare. Therefore, astronomers remain largely uncertain about the cause of fragmentation. One suggestion is that the original nucleus spins itself into pieces because of the jet action of outgassing from sublimating ices. As this venting is likely not evenly dispersed across the comet, it enhances the breakup. The disintegrating ATLAS comet is currently located inside the orbit of Mars, at a distance of approximately 145 million kilometres from Earth when the latest Hubble observations were taken. The comet will make its closest approach to Earth on 23 May at a distance of approximately 115 million kilometres, and eight days later it will skirt within 37 million kilometres of the Sun.
ATMOSPHERIC TIDAL WAVES MAINTAIN VENUS’ SUPER-ROTATION
Hokkaido University
Venus rotates very slowly, taking 243 Earth days to rotate once around its axis. Despite this very slow rotation, Venus' atmosphere rotates westward 60 times faster than its planetary rotation. This super-rotation increases with altitude, taking only four Earth days to circulate around the entire planet towards the top of the cloud cover. The fast-moving atmosphere transports heat from the planet's dayside to nightside, reducing the temperature differences between the two hemispheres. Since the super-rotation was discovered in the 1960s, however, the mechanism behind its forming and maintenance has been a long-standing mystery. A team of Japanese astronomers developed a new, highly precise method to track clouds and derive wind velocities from images provided by ultraviolet and infrared cameras on the Akatsuki spacecraft, which began its orbit of Venus in December 2015. This allowed them to estimate the contributions of atmospheric waves and turbulence to the super-rotation. The group first noticed that atmospheric temperature differences between low and high latitudes are as small as it cannot be explained without a circulation across latitudes. Further analyses revealed that the maintenance is sustained by the thermal tide -- an atmospheric wave excited by the solar heating
contrast between the dayside and the nightside -- which provides the acceleration at low latitudes. Earlier studies proposed that atmospheric turbulence and the waves other than the thermal tide may provide the acceleration. However, the current study showed that they work oppositely to weakly decelerate the super-rotation at low latitude, even though they play an important role at mid- to high latitudes. Their findings uncovered the factors that maintain the super-rotation while suggesting a dual circulation system that effectively transports heat across the globe: the meridional circulation that slowly transports heat towards the poles and the super -rotation that rapidly transports heat towards the planet's nightside. The study could help better understand atmospheric systems on tidally-locked exo-planets whose one side always facing the central stars, which is similar to Venus having a very long solar day,
POPULATION OF INTERSTELLAR ASTEROIDS
RAS
A new study has identified the first known permanent population of asteroids originating from outside our Solar System. The objects are believed to have been captured from other stars billions of years ago, and have been orbiting our Sun in disguise ever since. The first interstellar visitor, the asteroid known as ‘Oumuamua, hit the headlines in 2017, however it was just passing through. The newly-identified asteroids on the other hand are thought to have been present almost since the birth of our Solar System, 4.5 billion years ago in a star cluster where each sun had its own planets and asteroids. The close proximity of the stars meant that they felt each others’ gravity much more strongly in those early days than they do today. This enabled asteroids to be pulled from one star system to another. In the new work, astronomers ran numerical simulations to turn back the clock to the earliest days of the Solar System, producing a snapshot that allowed them to see where the asteroids were originally located. At the time of the snapshot, the asteroids were orbiting the Sun in a distant region beyond the reach of the original Solar System disc, and also moving perpendicular to the orbital plane shared by the planets and other asteroids. These two observations indicate that the new group did not originally belong to our Solar System, but must have been captured from the interstellar medium during planet formation. Being able to tell apart interstellar asteroids from native asteroids born in the Solar System has long eluded astronomers, but the team’s results identified 19 asteroids of interstellar origin. These are currently orbiting as part of the group of asteroids known as Centaurs, which roam the space in between the giant planets of the Solar System. This population will give us clues about the Sun’s early birth cluster, how interstellar asteroid capture occurred, and the role that interstellar matter had in chemically enriching the Solar System and shaping its evolution.
INTERSTELLAR COMET REVEALS ORIGINS
NASA
On Aug. 30, 2019, when amateur astronomer Gennady Borisov gazed upward with his homemade telescope, he spotted an object moving in an unusual direction. Now called 2I/Borisov, this runaway point of light turned out to be the first confirmed comet to enter our solar system from some unknown place beyond our Sun’s influence. Astronomers everywhere rushed to take a look with some of the most powerful instruments in the world, hoping to learn as much as they could about the mysterious visitor. Now, thanks to observations with the Hubble Space Telescope and the National Radio Astronomy Observatory’s Atacama Large Millimeter/submillimeter Array (ALMA), astronomers have discovered that 2I/Borisov has an unusual composition. Specifically, it has a higher concentration of carbon monoxide than any comet seen at a similar distance; that is, within about 300 million kilometres
of the Sun. This suggests that the comet could have formed around a red dwarf — a smaller, fainter type of star than the Sun — though other kinds of stars are possible. Another idea is that 2I/Borisov could be a carbon monoxide-rich fragment of a small planet. Comets are snowballs of ice, dust and frozen gas. When totally frozen (or “inactive”), they’re approximately the diameter of a small town, but when heated by the Sun their tails can extend for millions of miles. 2I/Borisov is about 0.98 kilometres, making it relatively small. All comets form in the primordial disk of material that encircles a young star, preserving remnants of a planetary system’s ancient past. Comets from our own solar neighbourhood reveal the history of materials, including water, that made Earth the planet we know today, as well as our other planetary neighbours. An interstellar comet, on the other hand, is a chemical ambassador from an entirely different star system — containing a treasure trove of clues to worlds too far to reach with modern technology.
Astronomers used Hubble to look at 2I/Borisov from Dec. 11, 2019 to Jan. 13, 2020. Separately, a team of international scientists studied the comet on December 15 and 16, 2019, with ALMA, an array of radio telescopes in northern Chile. Radio telescopes are especially useful for looking at cold, low-energy gas in objects like comets. Results from both Hubble and ALMA estimate that 2I/Borisov’s carbon monoxide concentration is higher than that of the average solar system comet. A high carbon-monoxide-to-water ratio suggests that the comet has travelled from a very cold place — as cold as the area where Pluto is in relation to our Sun, called the Kuiper Belt. The group using Hubble additionally theorizes 2I/Borisov may have originated around the most common type of star in the Milky Way: a red dwarf. Red dwarfs are much smaller and dimmer than the Sun, so the planet-forming material around them would be colder than the building blocks of our solar system. Scientists using ALMA say it’s possible that 2I/Borisov could be a fragment of a dwarf planet that had a lot of carbon monoxide near its surface, regardless of which type of star it came from. But 2I/Borisov may have simply formed as a comet with a high concentration of carbon monoxide, the ALMA team points out. Alternatively, it may have an unusually thick outer layer that insulates frozen gases like hydrogen cyanide and water. As the more volatile carbon monoxide evaporates or “outgasses,” it may appear more abundant than other cometary gases. 2I/Borisov’s unusual properties may also suggest a wider diversity of carbon monoxide in comets in our own solar system than previously thought. In our own solar system, there are two places where most comets reside: The Kuiper Belt, an area that includes Pluto; and the Oort Cloud, which is much farther away. All of these comets likely formed closer to the Sun, but may have been booted outward by the erratic movements of Jupiter and Saturn billions of years ago.
These giant planets, because of their immense gravity, could have even sent comets flying toward other stars, escaping the influence of the Sun’s gravity altogether. Given this history, scientists using Hubble theorize that a massive planet in a red dwarf system, in an environment with frozen carbon monoxide, may have punted 2I/Borisov our way. The team using ALMA agrees that a young moving planet likely sent the comet on its way. 2I/Borisov is only the second object astronomers have detected that definitely came from a different star system. The first was 'Oumuamua, discovered in October 2017, that whizzed by too quickly for scientists to pin down its chemistry. Whether it too is a comet, an asteroid, or some thing else — we may never know. 2I/Borisov is continuing on its path through the solar system, and will eventually head out. As more advanced telescopes and other instruments turn on and gaze out in the coming years, astronomers expect to find more interstellar objects, though they will still be rare.
EXOPLANET SEEMINGLY VANISHES
NASA/Goddard Space Flight Center
What astronomers thought was a planet beyond our solar system has now seemingly vanished from sight. One interpretation is that, rather than being a full-sized planetary object, which was first photographed in 2004, it could instead be a vast, expanding cloud of dust produced in a collision between two large bodies orbiting the bright nearby star Fomalhaut. Potential follow-up observations might confirm this extraordinary conclusion. The object, called Fomalhaut b, was first announced in 2008, based on data taken in 2004 and 2006. It was clearly visible in several years of Hubble observations that revealed it was a moving dot. Until then, evidence for exoplanets had mostly been inferred through indirect detection methods, such as subtle back-and-forth stellar wobbles, and shadows from planets passing in front of their stars. Unlike other directly imaged exoplanets, however,
nagging puzzles arose with Fomalhaut b early on. The object was unusually bright in visible light, but did not have any detectable infrared heat signature. Astronomers conjectured that the added brightness came from a huge shell or ring of dust encircling the planet that may possibly have been collision-related. The orbit of Fomalhaut b also appeared unusual, possibly very eccentric. The team emphasizes that the final nail in the coffin came when their data analysis of Hubble images taken in 2014 showed the object had vanished, to their disbelief. Adding to the mystery, earlier images showed the object to continuously fade over time.
The interpretation is that Fomalhaut b is slowly expanding from the smashup that blasted a dissipating dust cloud into space. Taking into account all available data, astronomers think the collision occurred not too long prior to the first observations taken in 2004. By now the debris cloud, consisting of dust particles around 1 micron (1/50th the diameter of a human hair), is below Hubble's detection limit. The dust cloud is estimated to have expanded by now to a size larger than the orbit of Earth around our Sun. Equally confounding is that the team reports that the object is more likely on an escape path, rather than on an elliptical orbit, as expected for planets. This is based on the researchers adding later observations to the trajectory plots from earlier data. Because Fomalhaut b is presently inside a vast ring of icy debris encircling the star, colliding bodies would likely be a mixture of ice and dust, like the comets that exist in the Kuiper belt on the outer fringe of our solar system. The team estimate that each of these comet-like bodies measured about 200 kilometres across (roughly half the size of the asteroid Vesta). According to the author's calculations, the Fomalhaut system, located about 25 light-years from Earth, may experience one of these events only every 200,000 years.
EARTH-SIZE PLANET HIDDEN IN EARLY KEPLER DATA
NASA/Jet Propulsion Laboratory
A team of scientists, using reanalyzed data from the Kepler space telescope, has discovered an Earth-size exoplanet orbiting in its star's habitable zone, the area around a star where a rocky planet could support liquid water. Scientists discovered this planet, called Kepler-1649c, when looking through old observations from Kepler, which NASA retired in 2018. While previous searches with a computer algorithm misidentified it, researchers reviewing Kepler data took a second look at the signature and recognized it as a planet. Out of all the exoplanets found by Kepler, this distant world -- located 300 light-years from Earth -- is most similar to Earth in size and estimated temperature. This newly revealed world is only 1.06 times larger than our own planet. Also, the amount of starlight it receives from its host star is 75% of the amount of light Earth receives from our Sun -- meaning the exoplanet's
temperature may be similar to our planet's as well. But unlike Earth, it orbits a red dwarf. Though none have been observed in this system, this type of star is known for stellar flare-ups that may make a planet's environment challenging for any potential life. There is still much that is unknown about Kepler-1649c, including its atmosphere, which could affect the planet's temperature. Current calculations of the planet's size have significant margins of error, as do all values in astronomy when studying objects so far away. But based on what is known, Kepler-1649c is especially intriguing for scientists looking for worlds with potentially habitable conditions.
There are other exoplanets estimated to be closer to Earth in size, such as TRAPPIST-1f and, by some calculations, Teegarden c. Others may be closer to Earth in temperature, such as TRAPPIST-1d and TOI 700d. But there is no other exoplanet that is considered to be closer to Earth in both of these values that also lies in the habitable zone of its system. Kepler-1649c orbits its small red dwarf star
so closely that a year on Kepler-1649c is equivalent to only 19.5 Earth days. The system has another rocky planet of about the same size, but it orbits the star at about half the distance of Kepler-1649c, similar to how Venus orbits our Sun at about half the distance that Earth does. Red dwarf stars are among the most common in the galaxy, meaning planets like this one could be more common than we previously thought. Previously, scientists on the Kepler mission developed an algorithm called Robovetter to help sort through the massive amounts of data produced by the Kepler spacecraft. Kepler searched for planets using the transit method, staring at stars, looking for dips in brightness as planets passed in front of their host stars. Most of the time, those dips come from phenomena other than planets -- ranging from natural changes in a star's brightness to other cosmic objects passing by -- making it look like a planet is there when it's not. Robovetter's job was to distinguish the 12% of dips that were real planets from the rest. Those signatures Robovetter determined to be from other sources were labeled "false positives," the term for a test result mistakenly classified as positive. With an enormous number of tricky signals, astronomers knew the algorithm would make mistakes and would need to be double-checked -- a perfect job for the Kepler False Positive Working Group. That team reviews Robovetter's work, going through each false positive to ensure they are truly errors and not exoplanets, ensuring fewer potential discoveries are overlooked. As it turns out, Robovetter had mislabeled Kepler-1649c. Even as scientists work to further automate analysis processes to get the most science as possible out of any given dataset, this discovery shows the value of double-checking automated work. Even six years after Kepler stopped collecting data from the original Kepler field -- a patch of sky it stared at from 2009 to 2013, before going on to study many more regions -- this rigorous analysis uncovered one of the most unique Earth analogs discovered yet.
Kepler-1649c not only is one of the best matches to Earth in terms of size and energy received from its star, but it provides an entirely new look at its home system. For every nine times the outer planet in the system orbits the host star, the inner planet orbits almost exactly four times. The fact that their orbits match up in such a stable ratio indicates the system itself is extremely stable and likely to survive for a long time. Nearly perfect period ratios are often caused by a phenomenon called orbital resonance, but a nine-to-four ratio is relatively unique among planetary systems. Usually resonances take the form of ratios such as two-to-one or three-to-two. Though unconfirmed, the rarity of this ratio could hint to the presence of a middle planet with which both the inner and outer planets revolve in synchronicity, creating a pair of three-to-two resonances. The team looked for evidence of such a mystery third planet, with no results. However, that could be because the planet is too small to see or at an orbital tilt that makes it impossible to find using Kepler's transit method. Either way, this system provides yet another example of an Earth-size planet in the habitable zone of a red dwarf star. These small and dim stars require planets to orbit extremely close to be within that zone -- not too warm and not too cold -- for life as we know it to potentially exist. Though this single example is only one among many, there is increasing evidence that such planets are common around red dwarfs.
STAR ORBITING BLACK HOLE PROVES EINSTEIN RIGHT
ESO
Observations made with the Very Large Telescope (VLT) have revealed for the first time that a star orbiting the supermassive black hole at the centre of the Milky Way moves just as predicted by Einstein's general theory of relativity. Its orbit is shaped like a rosette and not like an ellipse as predicted by Newton's theory of gravity. This long-sought-after result was made possible by increasingly precise measurements over nearly 30 years, which have enabled scientists to unlock the mysteries of the behemoth lurking at the heart of our galaxy. Located 26,000 light-years from the Sun, Sagittarius A* and the dense cluster of stars around it provide a unique laboratory for testing physics in an otherwise unexplored and extreme regime of gravity. One of these stars, S2, sweeps in towards the supermassive black hole to a closest distance less than 20 billion kilometres (one hundred and twenty times the distance between the Sun and Earth), making it one of the closest stars ever found in orbit around the massive giant. At its closest approach to the black hole, S2 is hurtling through space at almost three percent of the speed of light, completing an orbit once every 16 years. Most stars and planets have a non-circular orbit and therefore move closer to and further away from the object they are rotating around. S2's orbit precesses, meaning that the location of its closest point to the supermassive black hole changes with each turn, such that the next orbit is rotated with regard to the previous one, creating a rosette shape. General Relativity provides a precise prediction of how much its orbit changes and the latest measurements from this research exactly match the theory. This effect, known as Schwarzschild precession, had never before been measured for a star around a supermassive black hole.
The study with ESO's VLT also helps scientists learn more about the vicinity of the supermassive black hole at the centre of our galaxy. Because the S2 measurements follow General Relativity so well, astronomers can set stringent limits on how much invisible material, such as distributed dark matter or possible smaller black holes, is present around Sagittarius A*. This is of great interest for understanding the formation and evolution of supermassive black holes. This result is the culmination of 27 years of observations of the S2 star using, for the best part of this time, a fleet of instruments at ESO's VLT, located in the Atacama Desert in Chile. The number of data points marking the star's position and velocity attests to the thoroughness and accuracy of the new research: the team made over 330 measurements in total, using the GRAVITY, SINFONI and NACO instruments. Because S2 takes years to orbit the supermassive black hole, it was crucial to follow the star for close to three decades, to unravel the intricacies of its orbital movement.
Topic: Early May Astronomy Bulletin (Read 2240 times)