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Late October Astronomy Bulletin
« on: October 31, 2021, 08:02 »
LUCY MISSION TO PLANET FOSSILS
NASA

NASA's Lucy mission, the agency's first to Jupiter's Trojan asteroids, has launched from Cape Canaveral Space Force Station in Florida. Over the next 12 years, Lucy will fly by one main-belt asteroid and seven Trojan asteroids to explore so many different asteroids. Lucy will investigate these "fossils" of planetary formation up close during its journey. Named after the fossilized skeleton of one of our earliest known hominid ancestors, the Lucy mission will allow scientists to explore two swarms of Trojan asteroids that share an orbit around the Sun with Jupiter. Scientific evidence indicates that Trojan asteroids are remnants of the material that formed giant planets. Studying them can reveal previously unknown information about their formation and our solar system's evolution in the same way the fossilized skeleton of Lucy revolutionized our understanding of human evolution. Lucy's Trojan destinations are trapped near Jupiter's Lagrange points -- gravitationally stable locations in space associated with a planet's orbit where smaller masses can be trapped. One swarm of Trojans is ahead of the gas giant planet, and another is behind it. The asteroids in Jupiter's Trojan swarms are as far away from Jupiter as they are from the Sun. The spacecraft's first Earth gravity assist in 2022 will accelerate and direct Lucy's trajectory beyond the orbit of Mars. The spacecraft will then swing back toward Earth for another gravity assist in 2024, which will propel Lucy toward the Donaldjohanson asteroid -- located within the solar system's main asteroid belt -- in 2025. Lucy will then journey toward its first Trojan asteroid encounter in the swarm ahead of Jupiter for a 2027 arrival. After completing its first four targeted flybys, the spacecraft will travel back to Earth for a third gravity boost in 2031, which will catapult it to the trailing swarm of Trojans for a 2033 encounter.

The launch has not gone seamlessly as after the deployment of its two large solar arrays, one of them failed to latch properly. Combined, the two solar arrays have a collecting area of 51 square meters. Such large arrays are necessary because the spacecraft will spend much of its 12-year journey about five times the distance of the Earth from the Sun. Lucy's solar panels can only generate about 3 percent of the energy at a Jovian distance than they can at Earth's orbit around the Sun. According to NASA, both arrays were providing power to Lucy and charging batteries on the spacecraft. Engineers from NASA and the spacecraft's primary contractor, Lockheed Martin, are actively working on the issue. NASA’s Lucy Mission is safe and stable the two solar arrays have deployed, but the team is analyzing data to determine next steps.


EARLY SOLAR SYSTEM HARBOURED GAP
Massachusetts Institute of Technology

In the early solar system, a "protoplanetary disk" of dust and gas rotated around the Sun and eventually coalesced into the planets we know today. A new analysis of ancient meteorites suggests that a mysterious gap existed within this disk around 4.567 billion years ago, near the location where the asteroid belt resides today. Over the last decade, observations have shown that cavities, gaps, and rings are common in disks around other young stars. These are important but poorly understood signatures of the physical processes by which gas and dust transform into the young Sun and planets. Likewise the cause of such a gap in our own solar system remains a mystery. One possibility is that Jupiter may have been an influence. As the gas giant took shape, its immense gravitational pull could have pushed gas and dust toward the outskirts, leaving behind a gap in the developing disk. Another explanation may have to do with winds emerging from the surface of the disk. Early planetary systems are governed by strong magnetic fields. When these fields interact with a rotating disk of gas and dust, they can produce winds powerful enough to blow material out, leaving behind a gap in the disk. Regardless of its origins, a gap in the early solar system likely served as a cosmic boundary, keeping material on either side of it from interacting. This physical separation could have shaped the composition of the solar system's planets. For instance, on the inner side of the gap, gas and dust coalesced as terrestrial planets, including the Earth and Mars, while gas and dust relegated to the farther side of the gap formed in icier regions, as Jupiter and its neighbouring gas giants.

Over the last decade, scientists have observed a curious split in the composition of meteorites that have made their way to Earth. These space rocks originally formed at different times and locations as the solar system was taking shape. Those that have been analyzed exhibit one of two isotope combinations. Rarely have meteorites been found to exhibit both -- a conundrum known as the "isotopic dichotomy. Scientists have proposed that this dichotomy may be the result of a gap in the early solar system's disk, but such a gap has not been directly confirmed. The group analyzes meteorites for signs of ancient magnetic fields. As a young planetary system takes shape, it carries with it a magnetic field, the strength and direction of which can change depending on various processes within the evolving disk. As ancient dust gathered into grains known as chondrules, electrons within chondrules aligned with the magnetic field in which they formed. Chondrules can be smaller than the diameter of a human hair, and are found in meteorites today. Weiss' group specializes in measuring chondrules to identify the ancient magnetic fields in which they originally formed. In previous work, the group analyzed samples from one of the two isotopic groups of meteorites, known as the noncarbonaceous meteorites. These rocks are thought to have originated in a "reservoir," or region of the early solar system, relatively close to the Sun. In their new study, the researchers wondered whether the magnetic field would be the same in the second isotopic, "carbonaceous" group of meteorites, which, judging from their isotopic composition, are thought to have originated farther out in the solar system. They analyzed chondrules, each measuring about 100 microns, from two carbonaceous meteorites that were discovered in Antarctica.

Using the superconducting quantum interference device, or SQUID, a high-precision microscope, the team determined each chondrule's original, ancient magnetic field. Surprisingly, they found that their field strength was stronger than that of the closer-in noncarbonaceous meteorites they previously measured. As young planetary systems are taking shape, scientists expect that the strength of the magnetic field should decay with distance from the Sun. In contrast, it was found the far-out chondrules had a stronger magnetic field, of about 100 microteslas, compared to a field of 50 microteslas in the closer chondrules. For reference, the Earth's magnetic field today is around 50 microteslas. A planetary system's magnetic field is a measure of its accretion rate, or the amount of gas and dust it can draw into its centre over time. Based on the carbonaceous chondrules' magnetic field, the solar system's outer region must have been accreting much more mass than the inner region. Using models to simulate various scenarios, the team concluded that the most likely explanation for the mismatch in accretion rates is the existence of a gap between the inner and outer regions, which could have reduced the amount of gas and dust flowing toward the sun from the outer regions.


INFANT PLANET DISCOVERED
University of Hawaii at Manoa

One of the youngest planets ever found around a distant infant star has been discovered. Thousands of planets have been discovered around other stars, but what sets this one apart is that it is newly-formed and can be directly observed. The planet, named 2M0437b, joins a handful of objects advancing our understanding of how planets form and change with time, helping shed new light on the origin of the Solar System and Earth. The researchers estimate that the planet is a few times more massive than Jupiter, and that it formed with its star several million years ago, around the time the main Hawaiian Islands first emerged above the ocean. The planet is so young that it is still hot from the energy released during its formation, with a temperature similar to the lava erupting from Kilauea Volcano. In 2018, 2M0437b was first seen with the Subaru Telescope on Maunakea. For the past several years, it has been studied carefully utilizing other telescopes on the mauna. The team used the Keck Observatory on Maunakea to monitor the position of the host star as it moved across the sky, confirming that planet 2M0437b was truly a companion to the star, and not a more distant object. The observations required three years because the star moves slowly across the sky. The planet and its parent star lie in a stellar "nursery" called the Taurus Cloud. 2M0437b is on a much wider orbit than the planets in the Solar System; its current separation is about one hundred times the Earth-Sun distance, making it easier to observe. However, sophisticated "adaptive" optics are still needed to compensate for the image distortion caused by Earth's atmosphere. Gathering more in-depth research about the newly-discovered planet may not be too far away. Observations with space telescopes could identify gases in its atmosphere and reveal whether the planet has a moon-forming disk. The star that 2M0437b orbits is too faint to be seen with the unaided eye, but currently from Hawaii, the young planet and other infant stars in the Taurus Cloud are almost directly overhead in the pre-dawn hours, north of the bright star Hokuʻula (Aldebaran) and east of the Makaliʻi (Pleiades) star cluster.


UPSIDE-DOWN ORBITS OF PLANETARY SYSTEM
Université de Genève

When planets form, they usually continue their orbital evolution in the equatorial plane of their star. However, an international team, led by astronomers from the University of Geneva (UNIGE), Switzerland, has discovered that the exoplanets of a star in the constellation Pisces orbit in planes perpendicular to each other, with the innermost planet the only one still orbiting in the equatorial plane. Why so? This radically different configuration from our solar system could be due to the influence of a distant companion of the star that is still unknown. This study was made possible by ESPRESSO and CHEOPS, two instruments whose development was led by Switzerland. Theories of the origin of planetary systems predict that planets form in the equatorial plane of their star and continue to evolve there, unless disturbed by special events. This is not the case in the solar system, where our planets lie close to the solar equatorial plane. In this case, the planets are said to be aligned with their star. However, a study showed in 2019 that two of the three planets around the star HD3167 are not aligned with it. HD3167c and HD3167d, two mini-Neptunes that orbit in 8.5 and 29.8 days, actually pass over the star's poles, nearly 90 degrees from its equatorial plane. By re-observing this system with more efficient instruments, a team led by astronomers from UNIGE was able to measure the orientation of the third planet's orbital plane, the super earth HD3167b, which orbits in less than a day (23 hours exactly). When a planet transits its star, the orientation of its orbit can be determined with a spectrograph, which allows measuring the motion of the stellar regions occulted by the planet and thus deducing its trajectory. The smaller the planet, the more difficult this motion is to detect. It is therefore with ESPRESSO on one of the four 8.2m telescopes of the VLT in Chile that the researchers were able to determine the orbit of HD3167b, which happens to be aligned with the star and perpendicular to the orbital plane of its two siblings.

This result could not have been obtained without a precise knowledge of when HD3167b transits its star, which was not possible with the time predicted by the literature with a precision of 20 minutes -- an eternity for a transit that lasts 97 minutes. The researchers therefore turned to the CHEOPS satellite consortium, whose main mission is precisely to measure transits with very high precision. CHEOPS allowed them to know the time of transit with a precision better than one minute. These new measurements seem to confirm the prediction made in 2019 on the presence of a fourth body orbiting HD3167. In this scenario, HD3167b's proximity to the star kept it under its influence, forcing the small planet to orbit in the plane in which it formed. On the contrary, the two more distant mini-Neptunes were able to free themselves from the star only to fall under the influence of this fourth body, which would have gradually misaligned their orbits. The path is therefore clear for the researchers, who are now setting out in search of this elusive companion.


NEUTRON STARS ARE 'GOLDMINE' OF HEAVY ELEMENTS
Massachusetts Institute of Technology

Most elements lighter than iron are forged in the cores of stars. A star's white-hot centre fuels the fusion of protons, squeezing them together to build progressively heavier elements. But beyond iron, scientists have puzzled over what could give rise to gold, platinum, and the rest of the universe's heavy elements, whose formation requires more energy than a star can muster. A new study finds that of two long-suspected sources of heavy metals, one is more of a goldmine than the other. In the last 2.5 billion years, more heavy metals were produced in binary neutron star mergers, or collisions between two neutron stars, than in mergers between a neutron star and a black hole. The study is the first to compare the two merger types in terms of their heavy metal output, and suggests that binary neutron stars are a likely cosmic source for the gold, platinum, and other heavy metals we see today. The findings could also help scientists determine the rate at which heavy metals are produced across the Universe. As stars undergo nuclear fusion, they require energy to fuse protons to form heavier elements. Stars are efficient in churning out lighter elements, from hydrogen to iron. Fusing more than the 26 protons in iron, however, becomes energetically inefficient. Scientists have suspected supernovae might be an answer. When a massive star collapses in a supernova, the iron at its centre could conceivably combine with lighter elements in the extreme fallout to generate heavier elements. In 2017, however, a promising candidate was confirmed, in the form a binary neutron star merger, detected for the first time by LIGO and Virgo, the gravitational-wave observatories in the United States and in Italy, respectively. The detectors picked up gravitational waves, or ripples through space-time, that originated 130 million light years from Earth, from a collision between two neutron stars -- collapsed cores of massive stars, that are packed with neutrons and are among the densest objects in the universe. The cosmic merger emitted a flash of light, which contained signatures of heavy metals.

Scientists wondered how might neutron star mergers compare to collisions between a neutron star and a black hole? This is another merger type that has been detected by LIGO and Virgo and could potentially be a heavy metal factory. Under certain conditions, scientists suspect, a black hole could disrupt a neutron star such that it would spark and spew heavy metals before the black hole completely swallowed the star. The team set out to determine the amount of gold and other heavy metals each type of merger could typically produce. For their analysis, they focused on LIGO and Virgo's detections to date of two binary neutron star mergers and two neutron star -- black hole mergers. The researchers first estimated the mass of each object in each merger, as well as the rotational speed of each black hole, reasoning that if a black hole is too massive or slow, it would swallow a neutron star before it had a chance to produce heavy elements. They also determined each neutron star's resistance to being disrupted. The more resistant a star, the less likely it is to churn out heavy elements. They also estimated how often one merger occurs compared to the other, based on observations by LIGO, Virgo, and other observatories. Finally, the team used numerical simulations to calculate the average amount of gold and other heavy metals each merger would produce, given varying combinations of the objects' mass, rotation, degree of disruption, and rate of occurrence. On average, the researchers found that binary neutron star mergers could generate two to 100 times more heavy metals than mergers between neutron stars and black holes. The four mergers on which they based their analysis are estimated to have occurred within the last 2.5 billion years. They conclude then, that during this period, at least, more heavy elements were produced by binary neutron star mergers than by collisions between neutron stars and black holes. The scales could tip in favor of neutron star-black hole mergers if the black holes had high spins, and low masses. However, scientists have not yet observed these kinds of black holes in the two mergers detected to date.


FIRST EXTRAGALACTIC PLANET DISCOVERED?
NASA

Astronomers have found evidence for a possible planet candidate in the M51 ("Whirlpool") galaxy, potentially representing what would be the first planet seen to transit a star outside of the Milky Way. Researchers used NASA's Chandra X-ray Observatory to detect the dimming of X-rays from an "X-ray binary,'' a system where a Sun-like star is in orbit around a neutron star or black hole. Exoplanets are defined as planets outside of our Solar System. Until now, astronomers have found all other known exoplanets and exoplanet candidates in the Milky Way galaxy, almost all of them less than about 3,000 light-years from Earth. An exoplanet in M51 would be about 28 million light-years away, meaning it would be thousands of times farther away than those in the Milky Way. This new result is based on transits, events in which the passage of a planet in front of a star blocks some of the star's light and produces a characteristic dip. Astronomers using both ground-based and space-based telescopes – like those on NASA's Kepler and TESS missions – have searched for dips in optical light, electromagnetic radiation humans can see, enabling the discovery of thousands of planets. Researchers have instead searched for dips in the brightness of X-rays received from X-ray bright binaries. These luminous systems typically contain a neutron star or black hole pulling in gas from a closely orbiting companion star. The material near the neutron star or black hole becomes superheated and glows in X-rays. Because the region producing bright X-rays is small, a planet passing in front of it could block most or all of the X-rays, making the transit easier to spot because the X-rays can completely disappear. This could allow exoplanets to be detected at much greater distances than current optical light transit studies, which must be able to detect tiny decreases in light because the planet only blocks a tiny fraction of the star.

The team used this method to detect the exoplanet candidate in a binary system called M51-ULS-1, located in M51. This binary system contains a black hole or neutron star orbiting a companion star with a mass about 20 times that of the Sun. The X-ray transit they found using Chandra data lasted about three hours, during which the X-ray emission decreased to zero. Based on this and other information, the researchers estimate the exoplanet candidate in M51-ULS-1 would be roughly the size of Saturn, and orbit the neutron star or black hole at about twice the distance of Saturn from the Sun. While this is a tantalizing study, more data would be needed to verify the interpretation as an extragalactic exoplanet. One challenge is that the planet candidate’s large orbit means it would not cross in front of its binary partner again for about 70 years, thwarting any attempts for a confirming observation for decades. Can the dimming have been caused by a cloud of gas and dust passing in front of the X-ray source? The researchers consider this to be an unlikely explanation, as the characteristics of the event observed in M51-ULS-1 are not consistent with the passage of such a cloud. The model of a planet candidate is, however, consistent with the data. If a planet exists in this system, it likely had a tumultuous history and violent past. An exoplanet in the system would have had to survive a supernova explosion that created the neutron star or black hole. The future may also be dangerous. At some point the companion star could also explode as a supernova and blast the planet once again with extremely high levels of radiation. The team looked for X-ray transits in three galaxies beyond the Milky Way galaxy, using both Chandra and the European Space Agency’s XMM-Newton. Their search covered 55 systems in M51, 64 systems in Messier 101 (the “Pinwheel” galaxy), and 119 systems in Messier 104 (the “Sombrero” galaxy), resulting in the single exoplanet candidate described here. The authors will search the archives of both Chandra and XMM-Newton for more exoplanet candidates in other galaxies. Substantial Chandra datasets are available for at least 20 galaxies, including some like M31 and M33 that are much closer than M51, allowing shorter transits to be detectable. Another interesting line of research is to search for X-ray transits in Milky Way X-ray sources to discover new nearby planets in unusual environments.


FITFUL START TO UNIVERSE
University of Nottingham

Astronomers from have used data from the Hubble Space Telescope (HST) and the Gran Telescopio Canarias (GTC), the so-called Frontier Fields, to locate and study some of the smallest faintest galaxies in the nearby Universe. This has revealed the formation of the galaxy was likely to be fitful. One of the most interesting questions that astronomers have been trying to answer for decades is how and when the first galaxies formed. Concerning the how, one possibility is that the formation of the first stars within galaxies started at a steady pace, slowly building up a more and more massive system. Another possibility is that the formation was more violent and discontinuous, with intense, but short lived bursts of star formation triggered by events such as mergers and enhanced gas accretion. Using the gravitational lensing power of some of the Universe's most massive galaxy clusters with the GTC data coming from a project entitled the Survey for high-z Red and Dead Sources (SHARDS) the astronomers searched for nearby analogues of the very first galaxies formed in the Universe, so that they could be studied in much more detail. The researchers combined the power of the most advanced telescopes, such as HST and GTC, with the aid of "natural telescopes." Some galaxies live in large groups, what we call clusters, which contain huge amounts of mass in the form of stars, but also gas and dark matter. Their mass is so large that they bend space-time, and act as natural telescopes. They are called gravitational lenses and they allow astronomers to see faint and distant galaxies with enhanced brightness and at a higher spatial resolution. Observations of some of these massive clusters acting as gravitational telescopes is the base of the Frontier Field survey. The study showed that the formation of the galaxy was likely to be stop-start with bursts of activity followed by lulls. The main result is that the start of galaxy formation is fitful, like a jerky car engine, with periods of enhanced star formation followed by sleepy intervals. It is unlikely that galaxy mergers have played a substantial role in the triggering of these bursts of star formation and it is more likely due to alternative causes that enhance gas accretion, we need to search for those alternatives.



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