Topic: Early June Astronomy Bulletin

[Clive]

WHY URANUS AND NEPTUNE ARE DIFFERENT COLOURS

Association of Universities for Research in Astronomy (AURA)

Astronomers may now understand why the similar planets Uranus and Neptune are different colours. Using observations from the Gemini North telescope, the NASA Infrared Telescope Facility, and the Hubble Space Telescope, researchers have developed a single atmospheric model that matches observations of both planets. The model reveals that excess haze on Uranus builds up in the planet's stagnant, sluggish atmosphere and makes it appear a lighter tone than Neptune. Neptune and Uranus have much in common -- they have similar masses, sizes, and atmospheric compositions -- yet their appearances are notably different. At visible wavelengths Neptune has a distinctly bluer colour whereas Uranus is a pale shade of cyan. Astronomers now have an explanation for why the two planets are different colours. New research suggests that a layer of concentrated haze that exists on both planets is thicker on Uranus than a similar layer on Neptune and 'whitens' Uranus's appearance more than Neptune's . If there were no haze in the atmospheres of Neptune and Uranus, both would appear almost equally blue. This conclusion comes from a model that an international team developed to describe aerosol layers in the atmospheres of Neptune and Uranus. Previous investigations of these planets' upper atmospheres had focused on the appearance of the atmosphere at only specific wavelengths. However, this new model, consisting of multiple atmospheric layers, matches observations from both planets across a wide range of wavelengths. The new model also includes haze particles within deeper layers that had previously been thought to contain only clouds of methane and hydrogen sulphide ices.

The team's model consists of three layers of aerosols at different heights. The key layer that affects the colours is the middle layer, which is a layer of haze particles (referred to in the paper as the Aerosol-2 layer) that is thicker on Uranus than on Neptune. The team suspects that, on both planets, methane ice condenses onto the particles in this layer, pulling the particles deeper into the atmosphere in a shower of methane snow. Because Neptune has a more active, turbulent atmosphere than Uranus does, the team believes Neptune's atmosphere is more efficient at churning up methane particles into the haze layer and producing this snow. This removes more of the haze and keeps Neptune's haze layer thinner than it is on Uranus, meaning the blue colour of Neptune looks stronger. To create this model, the team analysed a set of observations of the planets encompassing ultraviolet, visible, and near-infrared wavelengths (from 0.3 to 2.5 micrometres) taken with the Near-Infrared Integral Field Spectrometer (NIFS) on the Gemini North telescope near the summit of Maunakea in Hawai'i -- which is part of the international Gemini Observatory, a Program of NSF's NOIRLab -- as well as archival data from the NASA Infrared Telescope Facility, also located in Hawai'i, and the NASA/ESA Hubble Space Telescope. The NIFS instrument on Gemini North was particularly important to this result as it is able to provide spectra -- measurements of how bright an object is at different wavelengths -- for every point in its field of view. This provided the team with detailed measurements of how reflective both planets' atmospheres are across both the full disk of the planet and across a range of near-infrared wavelengths. The model also helps explain the dark spots that are occasionally visible on Neptune and less commonly detected on Uranus. While astronomers were already aware of the presence of dark spots in the atmospheres of both planets, they didn't know which aerosol layer was causing these dark spots or why the aerosols at those layers were less reflective. The team's research sheds light on these questions by showing that a darkening of the deepest layer of their model would produce dark spots similar to those seen on Neptune and perhaps Uranus.

PLANETS OF BINARY STARS MIGHT SUPPORT LIFE FORMS

University of Copenhagen - Faculty of Science

Nearly half of Sun-size stars are binary. According to University of Copenhagen research, planetary systems around binary stars may be very different from those around single stars. This points to new targets in the search for extraterrestrial life forms. Since the only known planet with life, the Earth, orbits the Sun, planetary systems around stars of similar size are obvious targets for astronomers trying to locate extraterrestrial life. Nearly every second star in that category is a binary star. New research indicates that planetary systems are formed in a very different way around binary stars than around single stars such as the Sun. The new discovery has been made based on observations made by the ALMA telescopes in Chile of a young binary star about 1,000 lightyears from Earth. The binary star system, NGC 1333-IRAS2A, is surrounded by a disc consisting of gas and dust. The observations can only provide researchers with a snapshot from a point in the evolution of the binary star system. However, the team has complemented the observations with computer simulations reaching both backwards and forwards in time. Notably, the movement of gas and dust does not follow a continuous pattern. At some points in time -- typically for relatively shorts periods of ten to one hundred years every thousand years -- the movement becomes very strong. The binary star becomes ten to one hundred times brighter, until it returns to its regular state. Presumably, the cyclic pattern can be explained by the duality of the binary star. The two stars encircle each other, and at given intervals their joint gravity will affect the surrounding gas and dust disc in a way which causes huge amounts of material to fall towards the star. The observed stellar system is still too young for planets to have formed. The team hopes to obtain more observational time at ALMA, allowing to investigate the formation of

Very soon the new James Webb Space Telescope (JWST) will join the search for extraterrestrial life. Near the end of the decade, JWST will be complemented by the ELT (European Large Telescope) and the extremely powerful SKA (Square Kilometer Array) both planned to begin observing in 2027. The ELT will with its 39-meter mirror be the biggest optical telescope in the world and will be poised to observe the atmospheric conditions of exoplanets (planets outside the Solar System, ed.). SKA will consist of thousands of telescopes in South Africa and in Australia working in coordination and will have longer wavelengths than ALMA. The team has had observation time on the ALMA telescopes in Chile to observe the binary star system NGC 1333-IRAS2A in the Perseus molecular cloud. The distance from Earth to the binary star is about 1,000 lightyears which is a quite short distance in an astronomical context. Formed some 10,000 years ago, it is a very young star. The two stars of the binary system are 200 astronomical units (AUs) apart. An AU equals the distance from Earth to the Sun. In comparison, the furthest planet of the Solar System, Neptune, is 30 AUs from the Sun.



JAMES WEBB TARGETS ANNOUNCED

NASA

Over the past five months, the James Webb Space Telescope and the joint NASA, European Space Agency, and Canadian Space Agency teams behind the project have been working towards the completion of the observatory’s six-month-long commissioning phase at the Sun-Earth Lagrange point 2 (L2). With the observatory’s mirrors recently completing alignment, Webb and its teams are preparing for the all-important and historic first image from the observatory. As teams continue to work towards completing commissioning, some of the first scientific research targets of Webb’s operational phase have been announced, including two strange and intriguing exoplanets that exhibit unique characteristics. After confirming that all of the optical and structural systems are operating as planned, James Webb’s commissioning phase will be complete and the observatory will assume operational status. Webb and its science teams have already lined up a plethora of research targets for the first few weeks of the observatory’s operational phase, some of which may produce images as we’ve never seen before from other telescopes. Among the many research targets outlined for Webb’s first few weeks of operation are two exoplanets that exhibit unique characteristics: 55 Cancri e and LHS 3844 b. 55 Cancri e is an extremely hot super-Earth exoplanet located 41 light-years away in the constellation Cancer, where it orbits its Sun-like parent star 55 Cancri A. The exoplanet is around double the diameter of Earth and is around 8.63 Earth masses. At the time of its discovery in August 2004, it was the first super-Earth to be found. Orbiting just 1.5 million miles from 55 Cancri A, the exoplanet completes one orbit around its parent star in just 18 hours and is thought to feature oceans of lava on its dayside due to its extremely hot surface temperatures. What’s more, 55 Cancri e is likely tidally locked due to its close proximity to 55 Cancri A. This would be expected to ensure that the parts of the surface facing the star most directly would be the hottest region of the planet. Telescope data from NASA’s Spitzer Space Telescope, however, suggests otherwise – showing that the exoplanet’s hottest region is offset from this position. Spitzer data also indicates that the total amount of heat on the dayside of the planet varies.

One of the leading theories behind the Spitzer observations is that 55 Cancri e may have a thick and dynamic atmosphere that moves heat around the planet from various regions. Another theory behind the occurrence could be that 55 Cancri e isn’t actually tidally locked, and is instead more like Mercury: orbiting on its axis three times for every two orbits it completes around 55 Cancri A in what is called a 3:2 resonance. The 3:2 resonance scenario would cause 55 Cancri e’s surface to heat up, melt, and vaporise into the exoplanet’s atmosphere. When the vapour eventually cools in the evening, it could condense and form droplets of lava that would subsequently rain onto the exoplanet’s surface. To truly determine the cause of 55 Cancri e’s heat distribution, Webb teams will use the observatory’s Near-Infrared Camera (NIRCam) and Mid-Infrared Instrument (MIRI) in different ways to test each theory. When testing the first theory involving a thick atmosphere, teams will use NIRCam and MIRI to view the thermal emission spectrum from the dayside. To test the second theory wherein the exoplanet isn’t tidally locked, teams will use NIRCam to observe and measure the heat emitted from the dayside over four orbits, allowing teams to measure each hemisphere of the exoplanet twice if the exoplanet truly has a 3:2 resonance. This allows teams to observe whether there are any differences between the planet’s two hemispheres.

Another research target for Webb is another extremely hot super-Earth exoplanet named LHS 3844 b. Located 49 light-years away orbiting red dwarf LHS 3488, LHS 3488 b – like 55 Cancri e – orbits extremely close to its parent star, completing one orbit around it in just 11 hours. LHS 3488 b’s parent star is a red dwarf, meaning it is much cooler and smaller than Sun-like stars such as 55 Cancri A. Due to its parent star’s small size, LHS 3488 b is not hot enough to have a molten surface like 55 Cancri e and Spitzer data suggests it likely does not have an atmosphere surrounding it. The lack of an atmosphere around LHS 3488 b gives scientists the chance to carry out a thorough examination of possible rock formations and other surface characteristics using telescopes. Although Webb is not capable of imaging the exoplanet’s surface directly, teams can use spectroscopy to study it. Webb’s observations of 55 Cancri e and LHS 3488 b are part of the observatory’s Cycle 1 General Observer program. Under the General Observers program, scientists can submit research targets for Webb to observe for their research. This same process is also used with the Hubble telescope to allocate time for scientists.

HIDDEN TROVE OF MASSIVE BLACK HOLES

University of North Carolina at Chapel Hill

A team of researchers has found a previously overlooked treasure trove of massive black holes in dwarf galaxies. The newly discovered black holes offer a glimpse into the life story of the supermassive black hole at the centre of our own Milky Way galaxy. As a giant spiral galaxy, the Milky Way is believed to have been built up from mergers of many smaller dwarf galaxies. For example, the Magellanic Clouds seen in the southern sky are dwarf galaxies that will merge into the Milky Way. Each dwarf that falls in may bring with it a central massive black hole, tens or hundreds of thousands of times the mass of our Sun, potentially destined to be swallowed by the Milky Way's central supermassive black hole. But how often dwarf galaxies contain a massive black hole is unknown, leaving a key gap in our understanding of how black holes and galaxies grow together. New research helps to fill in this gap by revealing that massive black holes are many times more common in dwarf galaxies than previously thought. Black holes are typically detected when they are actively growing by ingesting gas and stardust swirling around them, which makes them glow intensely. The problem is, while growing black holes glow with distinctive high-energy radiation, young newborn stars can too. Traditionally, astronomers have differentiated growing black holes from new star formation using diagnostic tests that rely on detailed features of each galaxy's visible light when spread out into a spectrum like a rainbow. The path to discovery began when researchers tried to apply these traditional tests to galaxy survey data. The team realized that some of the galaxies were sending mixed messages -- two tests would indicate growing black holes, but a third would indicate only star formation.

Scientists took on the challenge of constructing a new census of growing black holes, with attention to both traditional and mixed-message types. They obtained published measurements of visible light spectral features to test for black holes in thousands of galaxies found in two surveys, RESOLVE and ECO. These surveys include ultraviolet and radio data ideal for studying star formation, and they have an unusual design: Whereas most astronomical surveys select samples that favor big and bright galaxies, RESOLVE and ECO are complete inventories of huge volumes of the present-day universe in which dwarf galaxies are abundant. More than 80 percent of all growing black holes found in dwarf galaxies belonged to the new type. The group led an exhaustive search for alternative explanations involving star formation, modelling uncertainties, or exotic astrophysics. In the end, the team was forced to conclude that the newly identified black holes were real.



STARS IN DISTANT GALAXIES MORE MASSIVE THAN EXPECTED

University of Copenhagen - Faculty of Science

Astrophysicists have arrived at a major result regarding star populations beyond the Milky Way. The result could change our understanding of a wide range of astronomical phenomena, including the formation of black holes, supernovae and why galaxies die. Since 1955, it has been assumed that the composition of stars in the Universe's other galaxies is similar to that of the hundreds of billions of stars within our own -- a mixture of massive, medium mass and low mass stars. But with the help of observations from 140,000 galaxies across the Universe and a wide range of advanced models, the team has tested whether the same distribution of stars apparent in the Milky Way applies elsewhere. The answer is no. Stars in distant galaxies are typically more massive than those in our "local neighbourhood." The finding has a major impact on what we think we know about the Universe. The mass of stars tells astronomers a lot. If you change mass, you also change the number of supernovae and black holes that arise out of massive stars. As such, this result means that we'll have to revise many of the things we once presumed, because distant galaxies look quite different from our own. Researchers assumed that the size and weight of stars in other galaxies was similar to our own for more than fifty years, for the simple reason that they were unable to observe them through a telescope, as they could with the stars of our own galaxy. Distant galaxies are billions of light-years away.

As a result, only light from their most powerful stars ever reaches Earth. This has been a headache for researchers around the world for years, as they could never accurately clarify how stars in other galaxies were distributed, an uncertainty that forced them to believe that they were distributed much like the stars in our Milky Way. We've only been able to see the tip of the iceberg and known for a long time that expecting other galaxies to look like our own was not a particularly good assumption to make. However, no one has ever been able to prove that other galaxies form different populations of stars. This study has allowed us to do just that, which may open the door for a deeper understanding of galaxy formation and evolution. In the study, the researchers analyzed light from 140,000 galaxies using the COSMOS catalogue, a large international database of more than one million observations of light from other galaxies. These galaxies are distributed from the nearest to farthest reaches of the Universe, from which light has travelled a full twelve billion years before being observable on Earth.



'COSMIC TELESCOPE' STUDIES HEART OF YOUNG UNIVERSE

North Carolina State University

A unique new instrument, coupled with a powerful telescope and a little help from nature, has given researchers the ability to peer into galactic nurseries at the heart of the young Universe. After the big bang some 13.8 billion years ago, the early Universe was filled with enormous clouds of neutral diffuse gas, known as Damped Lyman-α systems, or DLAs. These DLAs served as galactic nurseries, as the gasses within slowly condensed to fuel the formation of stars and galaxies. They can still be observed today, but it isn't easy. Currently, astrophysicists use quasars -- supermassive black holes that emit light -- as "backlight" to detect the DLA clouds. And while this method does allow researchers to pinpoint DLA locations, the light from the quasars only acts as small skewers through a massive cloud, hampering efforts to measure their total size and mass. But astronomers at the W.M. Keck Observatory in Hawaii, found a way around the problem by using a gravitationally lensed galaxy and integral field spectroscopy to observe two DLAs -- and the host galaxies within -- that formed around 11 billion years ago, not long after the big bang. The advantage to this is twofold: One, the background object is extended across the sky and bright, so it is easy to take spectrum readings on different parts of the object. Two, because lensing extends the object, you can probe very small scales. For example, if the object is one light year across, we can study small bits in very high fidelity.

Spectrum readings allow astrophysicists to "see" elements in deep space that are not visible to the naked eye, such as diffuse gaseous DLAs and the potential galaxies within them. Normally, gathering the readings is a long and painstaking process. But the team solved that issue by performing integral field spectroscopy with the Keck Cosmic Web Imager. Integral field spectroscopy allowed the researchers to obtain a spectrum at every single pixel on the part of the sky it targeted, making spectroscopy of an extended object on the sky very efficient. This innovation combined with the stretched and brightened gravitationally lensed galaxy allowed the team to map out the diffuse DLA gas in the sky at high fidelity. Through this method the researchers were able to determine not only the size of the two DLAs, but also that they both contained host galaxies. The DLAs are huge, by the way. With diameters greater than 17.4 kiloparsecs, they're more than two thirds the size of the Milky Way galaxy today. For comparison, 13 billion years ago, a typical galaxy would have a diameter of less than 5 kiloparsecs. A parsec is 3.26 light years, and a kiloparsec is 1,000 parsecs, so it would take light about 56,723 years to travel across each DLA.

NEW MILESTONE IN UNIVERSE’S EXPANSION RATE

NASA/Goddard

Completing a nearly 30-year marathon, NASA's Hubble Space Telescope has calibrated more than 40 "milepost markers" of space and time to help scientists precisely measure the expansion rate of the Universe -- a quest with a plot twist. Pursuit of the Universe's expansion rate began in the 1920s with measurements by astronomers Edwin P. Hubble and Georges Lemaître. In 1998, this led to the discovery of "dark energy," a mysterious repulsive force accelerating the universe's expansion. In recent years, thanks to data from Hubble and other telescopes, astronomers found another twist: a discrepancy between the expansion rate as measured in the local universe compared to independent observations from right after the big bang, which predict a different expansion value. The cause of this discrepancy remains a mystery. But Hubble data, encompassing a variety of cosmic objects that serve as distance markers, support the idea that something weird is going on, possibly involving brand new physics. The new results more than double the prior sample of cosmic distance markers. The team also reanalyzed all of the prior data, with the whole dataset now including over 1,000 Hubble orbits. When NASA conceived of a large space telescope in the 1970s, one of the primary justifications for the expense and extraordinary technical effort was to be able to resolve Cepheids, stars that brighten and dim periodically, seen inside our Milky Way and external galaxies. Cepheids have long been the gold standard of cosmic mile markers since their utility was discovered by astronomer Henrietta Swan Leavitt in 1912. To calculate much greater distances, astronomers use exploding stars called Type Ia supernovae. Combined, these objects built a "cosmic distance ladder" across the universe and are essential to measuring the expansion rate of the universe, called the Hubble constant after Edwin Hubble. That value is critical to estimating the age of the universe and provides a basic test of our understanding of the Universe. Starting right after Hubble's launch in 1990, the first set of observations of Cepheid stars to refine the Hubble constant was undertaken by the HST Key Project that used Cepheids as milepost markers to refine the distance measurement to nearby galaxies. By the early 2000s the teams declared "mission accomplished" by reaching an accuracy of 10 percent for the Hubble constant, 72 plus or minus 8 kilometres per second per megaparsec.

In 2005 and again in 2009, the addition of powerful new cameras onboard the Hubble telescope launched "Generation 2" of the Hubble constant research as teams set out to refine the value to an accuracy of just one percent. This was inaugurated by the SH0ES program. Several teams of astronomers using Hubble, including SH0ES, have converged on a Hubble constant value of 73 plus or minus 1 kilometre per second per megaparsec. While other approaches have been used to investigate the Hubble constant question, different teams have come up with values close to the same number. The team measured 42 of the supernova milepost markers with Hubble. Because they are seen exploding at a rate of about one per year, Hubble has, for all practical purposes, logged as many supernovae as possible for measuring the Universe's expansion. The expansion rate of the Universe was predicted to be slower than what Hubble actually sees. By combining the Standard Cosmological Model of the Universe and measurements by the European Space Agency's Planck mission (which observed the relic cosmic microwave background from 13.8 billion years ago), astronomers predict a lower value for the Hubble constant: 67.5 plus or minus 0.5 kilometres per second per megaparsec, compared to the SH0ES team's estimate of 73. Given the large Hubble sample size, there is only a one-in-a-million chance astronomers are wrong due to an unlucky draw -a common threshold for taking a problem seriously in physics. This finding is untangling what was becoming a nice and tidy picture of the universe's dynamical evolution. Astronomers are at a loss for an explanation of the disconnect between the expansion rate of the local Universe versus the primeval Universe, but the answer might involve additional physics of the Universe. Such confounding findings have made life more exciting for cosmologists. Thirty years ago they started out to measure the Hubble constant to benchmark the Universe, but now it has become something even more interesting.