An astronomical waltz reveals a sextuplet of planets

An international collaboration between astronomers using the CHEOPS and TESS space satellites, including NCCR PlanetS members from the University of Bern and the University of Geneva, have found a key new system of six transiting planets orbiting a bright star in a harmonic rhythm. This rare property enabled the team to determine the planetary orbits which initially appeared as an unsolvable riddle.

CHEOPS is a joint mission by ESA and Switzerland, under the leadership of the University of Bern in collaboration with the University of Geneva. Thanks to a collaboration with scientists working with data from NASA's satellite TESS, the international team could uncover the planetary system orbiting the nearby star HD110067. A very distinctive feature of this system is its chain of resonances: the planets orbit their host star in perfect harmony. Part of the research team are researchers from the University of Bern and the University of Geneva who are also members of the National Center of Competence in Research (NCCR) PlanetS. The findings have just been published in Nature.

The planets in the HD110067 system revolve around the star in a very precise waltz. When the closest planet to the star makes three full revolutions around it, the second one makes exactly two during the same time. This is called a 3:2 resonance. "Amongst the over 5000 exoplanets discovered orbiting other stars than our Sun, resonances are not rare, nor are systems with several planets. What is extremely rare though, is to find systems where the resonances span such a long chain of six planets" points out Dr. Hugh Osborn, CHEOPS fellow at the University of Bern, leader of CHEOPS observation programme involved in the study, and co-author of the publication. This is precisely the case of HD110067 whose planets form a so-called "resonant chain" in successive pairs of 3:2, 3:2, 3:2, 4:3, and 4:3 resonances, resulting in the closest planet completing six orbits while the outer-most planet does one.

A seemingly unsolvable puzzle

Although multiple planets were initially detected thanks to their transits, the exact arrangement of the planets was unclear at first. However, the precise gravitational dance enabled the scientists' team to solve the puzzle of HD110067. Prof. Adrien Leleu from the University of Geneva, in charge of analysing the orbital resonances, and co-author of the study, explains: "A transit occurs when a planet, from our point of view, passes in front of its host star, blocking a minute fraction of the starlight, creating an apparent dip of its brightness." From the first observations carried out by NASA's TESS satellite, it was possible to determine that the two inner planets called 'b' and 'c' have orbital periods of 9 and 14 days respectively. However, no conclusions could be drawn for the other four detected planets as two were seen to transit once in 2020 and once in 2022 with a large 2-year gap in the data, and the other two transited only once in 2022.

The solution to the puzzle for those four additional planets finally began to emerge thanks to observations with the CHEOPS space telescope. While TESS aims at scanning all of the sky bit by bit to find short-period exoplanets, CHEOPS is a targeted mission, focusing on a single star at a time with exquisite precision. "Our CHEOPS observations enabled us to find that the period of planet 'd' is 20.5 days. Also, it ruled out multiple possibilities for the remaining three outer planets, 'e', 'f' and 'g'," reveals Osborn.

Predicting the precise waltz of the planets

That is when the team realized that the three inner planets of HD110067 are dancing in a precise 3:2, 3:2 chain of resonances: when the innermost planet revolves nine times around the star, the second revolves six times and the third planet four times.

The team then considered the possibility that the three other planets could also be part of the chain of resonances. "This led to dozens of possibilities for their orbital period," explains Leleu, "but combining existing observational data from TESS and CHEOPS, with our model of the gravitational interactions between the planets, we could exclude all solutions but one: the 3:2, 3:2, 3:2, 4:3, 4:3 chain." The scientists could therefore predict that the outer three planets ('e', 'f' and 'g') have orbital periods of 31, 41 days, and 55 days.

This prediction allowed to schedule observations with a variety of ground-based telescopes. Further transits of planet 'f' were observed, revealing it was precisely where theory predicted it based on the resonant-chain. Finally, reanalysis of the data from TESS revealed two hidden transits, one from each of planets 'f' and 'g', exactly at the times expected by the predictions, confirming the periods of the six planets. Additional CHEOPS observations of each planet, and in particular planet 'e' are scheduled in the near future.

A key system for the future

From the handful of resonant-chain systems found so far, CHEOPS has highly contributed to the understanding of not only HD110067, but also of TOI-178. Another well-known example of a resonant-chain system is the TRAPPIST-1 system which hosts seven rocky planets. However, TRAPPIST-1 is a small and incredibly faint star which makes any additional observations very difficult. HD110067, on the other hand, is more than 50 times brighter than TRAPPIST-1.

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New exoplanet discovered

An international research team led by UTSA Associate Professor of Astrophysics Thayne Currie has made a breakthrough in accelerating the search for new planets.

In a paper slated for publication April 14 in Science, Currie reports the first exoplanet jointly discovered through direct imaging and precision astrometry, a new indirect method that identifies a planet by measuring the position of the star it orbits. Data from the Subaru Telescope in Hawai`i and space telescopes from the European Space Agency (ESA) were integral to the team's discovery.

An exoplanet -- also called an extrasolar planet -- is a planet outside a solar system that orbits another star. With direct imaging, astronomers can see an exoplanet's light in a telescope and study its atmosphere. However, only about 20 have been directly imaged over the past 15 years.

By contrast, indirect planet detection methods determine a planet's existence through its effect on the star it orbits. This approach can provide detailed measurements of the planet's mass and orbit.

Combining direct and indirect methods to examine a planet's position provides a more complete understanding of an exoplanet, Current says.

"Indirect planet detection methods are responsible for most exoplanet discoveries thus far. Using one of these methods, precision astrometry, told us where to look to try to image planets. And, as we found out, we can now see planets a lot easier," said Currie.

The newly discovered exoplanet, called HIP 99770 b, is about 14 to 16 times the mass of Jupiter and orbits a star that is nearly twice as massive as the Sun. The planetary system also shares similarities with the outer regions of our solar system. HIP 99770 b receives about as much light as Jupiter, our solar system's most massive planet, receives from the Sun. Its host star is surrounded by icy debris left over from planet formation, similar to our solar system's Kuiper belt, the ring of icy objects observed around the Sun.

Currie and team used the Hipparcos-Gaia Catalogue of Accelerations to advance their discovery of HIP 99770 b. The catalogue consists of data from ESA's Gaia mission and Hipparcos, Gaia's predecessor, providing a 25-year record of accurate star positions and motions. It revealed that the star HIP 99770 is likely being accelerated by the gravitational pull of an unseen planet.

The team then used the Subaru Coronagraphic Extreme Adaptive Optics (SCExAO) instrument, which is permanently installed at the focus of the Subaru Telescope in Hawai`i, to image and confirm the existence of HIP 99770 b.

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Deep dive into the dusty Milky Way

An animated dive into the dusty Milky Way reveals the outlines of our galaxy taking shape as we look out further and further from Earth.

Based on new data from an interactive tool that exploits data from the European Space Agency's Gaia mission and other space science data sets, astronomers have created an animation to model dust in the Milky Way. The work was presented this week at the National Astronomy Meeting (NAM 2022) at the University of Warwick.

The animation shows the cumulative build-up of dust looking from Earth's local neighbourhood to ~13000 lightyears towards the galactic centre -- around 10% of the overall distance across the Milky Way. Close by, dust swirls all around but, further out, the concentration of dust along the galactic plane becomes clear. Two 'windows', one above and one below the galactic plane, are also revealed.

"Dust clouds are related to the formation and death of stars, so their distribution tells a story of how structures formed in the galaxy and how the galaxy evolves," said Nick Cox, coordinator of the EXPLORE project which is developing the tools. "The maps are also important for cosmologists in revealing regions where there is no dust and we can have a clear, unobstructed view out of the Milky Way to study the Universe beyond, such as to make Deep Field observations with Hubble or the new James Webb Space Telescope."

The tools used to create the animation combine data from the Gaia mission and the 2MASS All Sky Survey. The tools are part of a suite of applications designed to support studies of stars and galaxies, as well as lunar exploration, and have been developed through funding from the European Union's Horizon 2020 Programme.

"State-of-the-art machine learning and visual analytics have the power to greatly enhance scientific return and discovery for space science missions, but their use is still relatively novel in the field of astronomy," said Albert Zijlstra, of the University of Manchester and the EXPLORE project. "With a constant stream of new data, such as the recent third release of Gaia data in June 2022, we have an increasing wealth of information to mine -- beyond the scope of what humans could process in a lifetime. We need tools like the ones we are developing for EXPLORE to support scientific discovery, such as by helping us to characterise properties within the data, or to pick out the most interesting or unusual features and structures."

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Comet 67P’s abundant oxygen more of an illusion, new study suggests

When the European Space Agency's Rosetta spacecraft discovered abundant molecular oxygen bursting from comet 67P/Churyumov-Gerasimenko (67P) in 2015, it puzzled scientists. They had never seen a comet emit oxygen, let alone in such abundance. But most alarming were the deeper implications: that researchers had to account for so much oxygen, which meant reconsidering everything they thought they already knew about the chemistry of the early solar system and how it formed.

A new analysis, however, led by planetary scientist Adrienn Luspay-Kuti at the Johns Hopkins Applied Physics Laboratory (APL) in Laurel, Maryland, shows Rosetta's discovery may not be as strange as scientists first imagined. Instead, it suggests the comet has two internal reservoirs that make it seem like there's more oxygen than is actually there.

"It's kind of an illusion," Luspay-Kuti said. "In reality, the comet doesn't have this high oxygen abundance, at least not as far as its formation goes, but it has accumulated oxygen that gets trapped in the upper layers of the comet, which then gets released all at once."

While common on Earth, molecular oxygen (two oxygen atoms doubly linked to each other) is markedly uncommon throughout the universe. It quickly binds to other atoms and molecules, especially the universally abundant atoms hydrogen and carbon, so oxygen appears only in small amounts in just a few molecular clouds. That fact led many researchers to conclude any oxygen in the protosolar nebula that formed our solar system likely had been similarly scooped up.

When Rosetta found oxygen pouring out of comet 67P, however, everything turned on its head. Nobody had seen oxygen in a comet before, and as the fourth most abundant molecule in the comet's bright coma (after water, carbon dioxide and carbon monoxide), it needed some explanation. The oxygen seemed to come off the comet with water, causing many researchers to suspect the oxygen was either primordial -- meaning it got tied up with water at the birth of the solar system and amassed in the comet when it later formed -- or formed from water after the comet had formed.

But Luspay-Kuti and her team were skeptical. As the comet's dumbbell shape gradually rotates, each "bell" (or hemisphere) faces the Sun at various points, meaning the comet has seasons so the oxygen-water connection might not be present all the time. On short time frames, volatiles could potentially turn on and off as they thaw and refreeze with the seasons.

Now You See It, Now You Don't

Taking advantage of these seasons, the team examined the molecular data on short- and long-time periods just before the comet's southern hemisphere entered summer and then again just as its summer ended. As reported in their study, published March 10 in Nature Astronomy, the team found that as the southern hemisphere turned away and was sufficiently far from the Sun, the link between oxygen and water disappeared. The amount of water coming off the comet dropped precipitously, so instead the oxygen seemed strongly linked to carbon dioxide and carbon monoxide, which the comet was still emitting.

"There's no way that should be possible under the previous explanations suggested," Luspay-Kuti said. "If oxygen were primordial and tied to water in its formation, there shouldn't be any time that oxygen strongly correlates with carbon monoxide and carbon dioxide but not water."

The team instead proposed the comet's oxygen doesn't come from water but from two reservoirs: one made of oxygen, carbon monoxide and carbon dioxide deep inside the comet's rocky nucleus, and a shallower pocket closer to the surface where oxygen chemically combines with water ice molecules.

The idea goes like this: A deep reservoir of oxygen, carbon monoxide and carbon dioxide ice is constantly emitting gases because oxygen, carbon dioxide and carbon monoxide all vaporize at very low temperatures. As oxygen traverses from the comet's interior toward the surface, however, some chemically inserts into water ice (a major constituent of the comet's nucleus) to form a second, shallower oxygen reservoir. But water ice vaporizes at a much higher temperature than oxygen, so until the Sun sufficiently heats the surface and vaporizes the water ice, the oxygen is stuck.

The consequence is that oxygen can accumulate in this shallow reservoir for long periods until the comet surface is finally warmed enough for water ice to vaporize, releasing a plume far richer in oxygen than was actually present in the comet.

"Put another way, the oxygen abundances measured in the comet's coma aren't necessarily reflecting its abundances in the comet's nucleus," Luspay-Kuti explained.

The comet would consequently also vacillate with the seasons between strongly associating with water (when the Sun heats the surface) and strongly associating with carbon dioxide and carbon monoxide (when that surface faces away from the Sun and the comet is sufficiently far) -- exactly what Rosetta observed.

"This isn't just one explanation: It's the explanation because there is no other possibility," said Olivier Mousis, a planetary scientist from France's Aix-Marseille Université and a study co-author. "If oxygen were just coming from the surface, you wouldn't see these trends observed by Rosetta."

The major implication, he said, is that it means comet 67P's oxygen is, in fact, oxygen that accreted at the beginning of the solar system. It's just that it's only a fraction of what people had thought.

Luspay-Kuti said she wants to probe the topic more deeply by examining the comet's minor molecular species, such as methane and ethane, and their correlation with molecular oxygen and other major species. She suspects this will help researchers get a better idea of the type of ice that the oxygen was incorporated into.

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Liquid water beneath Martian south polar cap?

A Southwest Research Institute scientist measured the properties of ice-brine mixtures as cold as -145 degrees Fahrenheit to help confirm that salty water likely exists between grains of ice or sediment under the ice cap at Mars' south pole. Laboratory measurements conducted by SwRI geophysicist Dr. David Stillman support oddly bright reflections detected by the MARSIS subsurface sounding radar aboard ESA's Mars Express orbiter.

With a 130-foot antenna, MARSIS flies over the planet, bouncing radio waves over a selected area and then receiving and analyzing the echoes or reflections. Any near-surface liquid water should send a strong bright signal, whereas the radar signal for ice and rock would be much smaller.

Because conventional models assume the Mars south polar cap experiences temperatures much lower than the melting point of water, many scientists have questioned the presence of liquid water. Clay, hydrated salts and saline ices have been proposed as potential explanations for the source of the bright basal reflections. The Italian-led team investigating the proposed phenomena used previously published data, simulations and new laboratory measurements.

"Lakes of liquid water actually exist beneath glaciers in Arctic and Antarctic regions, so we have Earth analogs for finding liquid water below ice," said Stillman, a specialist in detecting water in any format -- liquid, ice or absorbed -- on planetary bodies and co-author of a paper describing these findings. "The exotic salts that we know exist on Mars have amazing 'antifreeze' properties allowing brines to remain liquid down to -103 degrees Fahrenheit. We studied these salts in our lab to understand how they would respond to radar."

Stillman has over a decade of experience measuring the properties of materials at cold temperatures to detect and characterize subsurface ice, unfrozen water and the potential for life throughout the solar system. For this project, Stillman measured the properties of perchlorate brines in an SwRI environmental chamber that produces near-liquid-nitrogen temperatures at Mars-like pressures.

"My Italian colleagues reached out to see if my laboratory experiment data would support the presence of liquid water beneath the Martian ice cap," Stillman said. "The research showed that we don't have to have lakes of perchlorate and chloride brines, but that these brines could exist between the grains of ice or sediments and are enough to exhibit a strong dielectric response. This is similar to how seawater saturates grains of sand at the shoreline or how flavoring permeates a slushie, but at -103 degrees Fahrenheit below a mile of ice near the South Pole of Mars."

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Astronomers release new all-sky map of Milky Way's outer reaches

Astronomers using data from NASA and ESA (European Space Agency) telescopes have released a new all-sky map of the outermost region of our galaxy. [Editor's note: See Related Multimedia link below.] Known as the galactic halo, this area lies outside the swirling spiral arms that form the Milky Way's recognizable central disk and is sparsely populated with stars. Though the halo may appear mostly empty, it is also predicted to contain a massive reservoir of dark matter, a mysterious and invisible substance thought to make up the bulk of all the mass in the universe.

The data for the new map comes from ESA's Gaia mission and NASA's Near Earth Object Wide Field Infrared Survey Explorer, or NEOWISE, which operated from 2009 to 2013 under the moniker WISE. The study makes use of data collected by the spacecraft between 2009 and 2018.

The new map reveals how a small galaxy called the Large Magellanic Cloud (LMC) -- so named because it is the larger of two dwarf galaxies orbiting the Milky Way -- has sailed through the Milky Way's galactic halo like a ship through water, its gravity creating a wake in the stars behind it. The LMC is located about 160,000 light-years from Earth and is less than one-quarter the mass of the Milky Way.

Though the inner portions of the halo have been mapped with a high level of accuracy, this is the first map to provide a similar picture of the halo's outer regions, where the wake is found -- about 200,000 light-years to 325,000 light-years from the galactic center. Previous studies have hinted at the wake's existence, but the all-sky map confirms its presence and offers a detailed view of its shape, size, and location.

This disturbance in the halo also provides astronomers with an opportunity to study something they can't observe directly: dark matter. While it doesn't emit, reflect, or absorb light, the gravitational influence of dark matter has been observed across the universe. It is thought to create a scaffolding on which galaxies are built, such that without it, galaxies would fly apart as they spin. Dark matter is estimated to be five times more common in the universe than all the matter that emits and/or interacts with light, from stars to planets to gas clouds.

Although there are multiple theories about the nature of dark matter, all of them indicate that it should be present in the Milky Way's halo. If that's the case, then as the LMC sails through this region, it should leave a wake in the dark matter as well. The wake observed in the new star map is thought to be the outline of this dark matter wake; the stars are like leaves on the surface of this invisible ocean, their position shifting with the dark matter.

The interaction between the dark matter and the Large Magellanic Cloud has big implications for our galaxy. As the LMC orbits the Milky Way, the dark matter's gravity drags on the LMC and slows it down. This will cause the dwarf galaxy's orbit to get smaller and smaller, until the galaxy finally collides with the Milky Way in about 2 billion years. These types of mergers might be a key driver in the growth of massive galaxies across the universe. In fact, astronomers think the Milky Way merged with another small galaxy about 10 billion years ago.

"This robbing of a smaller galaxy's energy is not only why the LMC is merging with the Milky Way, but also why all galaxy mergers happen," said Rohan Naidu, a doctoral student in astronomy at Harvard University and a co-author of the new paper. "The wake in our map is a really neat confirmation that our basic picture for how galaxies merge is on point!"

A Rare Opportunity

The authors of the paper also think the new map -- along with additional data and theoretical analyses -- may provide a test for different theories about the nature of dark matter, such as whether it consists of particles, like regular matter, and what the properties of those particles are.

"You can imagine that the wake behind a boat will be different if the boat is sailing through water or through honey," said Charlie Conroy, a professor at Harvard University and an astronomer at the Center for Astrophysics | Harvard & Smithsonian, who coauthored the study. "In this case, the properties of the wake are determined by which dark matter theory we apply."

Conroy led the team that mapped the positions of over 1,300 stars in the halo. The challenge arose in trying to measure the exact distance from Earth to a large portion of those stars: It's often impossible to figure out whether a star is faint and close by or bright and far away. The team used data from ESA's Gaia mission, which provides the location of many stars in the sky but cannot measure distances to the stars in the Milky Way's outer regions.

After identifying stars most likely located in the halo (because they were not obviously inside our galaxy or the LMC), the team looked for stars belonging to a class of giant stars with a specific light "signature" detectable by NEOWISE. Knowing the basic properties of the selected stars enabled the team to figure out their distance from Earth and create the new map. It charts a region starting about 200,000 light-years from the Milky Way's center, or about where the LMC's wake was predicted to begin, and extends about 125,000 light-years beyond that.

Conroy and his colleagues were inspired to hunt for LMC's wake after learning about a team of astrophysicists at the University of Arizona in Tucson that makes computer models predicting what dark matter in the galactic halo should look like. The two groups worked together on the new study.

One model by the Arizona team, included in the new study, predicted the general structure and specific location of the star wake revealed in the new map. Once the data had confirmed that the model was correct, the team could confirm what other investigations have also hinted at: that the LMC is likely on its first orbit around the Milky Way. If the smaller galaxy had already made multiple orbits, the shape and location of the wake would be significantly different from what has been observed. Astronomers think the LMC formed in the same environment as the Milky Way and another nearby galaxy, M31, and that it is close to completing a long first orbit around our galaxy (about 13 billion years). Its next orbit will be much shorter due to its interaction with the Milky Way.

"Confirming our theoretical prediction with observational data tells us that our understanding of the interaction between these two galaxies, including the dark matter, is on the right track," said University of Arizona doctoral student in astronomy Nicolás Garavito-Camargo, who led work on the model used in the paper.

The new map also provides astronomers with a rare opportunity to test the properties of the dark matter (the notional water or honey) in our own galaxy. In the new study, Garavito-Camargo and colleagues used a popular dark matter theory called cold dark matter that fits the observed star map relatively well. Now the University of Arizona team is running simulations that use different dark matter theories to see which one best matches the wake observed in the stars.

"It's a really special set of circumstances that came together to create this scenario that lets us test our dark matter theories," said Gurtina Besla, a co-author of the study and an associate professor at the University of Arizona. "But we can only realize that test with the combination of this new map and the dark matter simulations that we built."

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Telescopes in space for even sharper images of black holes

In space, the EHI has a resolution more than five times that of the EHT on Earth, and images can be reconstructed with higher fidelity. Top left: Model of Sagittarius A* at an observation frequency of 230 GHz. Top left: Simulation of an image of this model with the EHT. Bottom left: Model of Sagittarius A* at an observation frequency of 690 GHz. Bottom right: Simulation of an image of this model with the EHI.

Astronomers have just managed to take the first image of a black hole, and now the next challenge facing them is how to take even sharper images, so that Einstein's Theory of General Relativity can be tested. Radboud University astronomers, along with the European Space Agency (ESA) and others, are putting forward a concept for achieving this by launching radio telescopes into space. They publish their plans in the scientific journal Astronomy & Astrophysics.

The idea is to place two or three satellites in circular orbit around the Earth to observe black holes. The concept goes by the name Event Horizon Imager (EHI). In their new study, the scientists present simulations of what images of the black hole Sagittarius A* would look if they were taken by satellites like these.

More than five times as sharp

"There are lots of advantages to using satellites instead of permanent radio telescopes on Earth, as with the Event Horizon Telescope (EHT)," says Freek Roelofs, a PhD candidate at Radboud University and the lead author of the article. "In space, you can make observations at higher radio frequencies, because the frequencies from Earth are filtered out by the atmosphere. The distances between the telescopes in space are also larger. This allows us to take a big step forward. We would be able to take images with a resolution more than five times what is possible with the EHT."

Sharper images of a black hole will lead to better information that could be used to test Einstein's Theory of General Relativity in greater detail. "The fact that the satellites are moving round the Earth makes for considerable advantages," Radio Astronomy Professor Heino Falcke says. "With them, you can take near perfect images to see the real details of black holes. If small deviations from Einstein's theory occur, we should be able to see them."

The EHI will also be able to image about five additional black holes that are smaller than the black holes that the EHT is currently focussing on. The latter are Sagittarius A* at the centre of our Milky Way and M87* at the centre of Messier 87, a massive galaxy in the Virgo Cluster.

Technological challenges

The researchers have simulated what they would be able to see with different versions of the technology under different circumstances. For this they made use of models of plasma behaviour around the black hole and the resulting radiation. "The simulations look promising from a scientific aspect, but there are difficulties to overcome at a technical level," Roelofs says.

The astronomers collaborated with scientists from ESA/ESTEC to investigate the technical feasibility of the project. "The concept demands that you must be able to ascertain the position and speed of the satellites very accurately," according to Volodymyr Kudriashov, a researcher at the Radboud Radio Lab who also works at ESA/ESTEC. "But we really believe that the project is feasible."

Consideration also has to be given to how the satellites exchange data. "With the EHT, hard drives with data are transported to the processing centre by airplane. That's of course not possible in space." In this concept, the satellites will exchange data via a laser link, with the data being partially processed on board before being sent back to Earth for further analysis. "There are already laser links in space," Kudriashov notes.

Hybrid system

The idea is that the satellites will initially function independently of the EHT telescopes. But consideration is also being given to a hybrid system, with the orbiting telescopes combined with the ones on Earth. Falcke: "Using a hybrid like this could provide the possibility of creating moving images of a black hole, and you might be able to observe even more and also weaker sources."

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Pinpointing Gaia to map the Milky Way

This image, a composite of several observations captured by ESO's VLT Survey Telescope (VST), shows the space observatory Gaia as a faint trail of dots across the lower half of the star-filled field of view. These observations were taken as part of an ongoing collaborative effort to measure Gaia's orbit and improve the accuracy of its unprecedented star map.

Gaia, operated by the European Space Agency (ESA, surveys the sky from orbit to create the largest, most precise, three-dimensional map of our Galaxy. One year ago, the Gaia mission produced its much-awaited second data release, which included high-precision measurements -- positions, distance and proper motions -- of more than one billion stars in our Milky Way galaxy. This catalogue has enabled transformational studies in many fields of astronomy, addressing the structure, origin and evolution the Milky Way and generating more than 1700 scientific publications since its launch in 2013.

In order to reach the accuracy necessary for Gaia's sky maps, it is crucial to pinpoint the position of the spacecraft from Earth. Therefore, while Gaia scans the sky, gathering data for its stellar census, astronomers regularly monitor its position using a global network of optical telescopes, including the VST at ESO's Paranal Observatory. The VST is currently the largest survey telescope observing the sky in visible light, and records Gaia's position in the sky every second night throughout the year.

"Gaia observations require a special observing procedure," explained Monika Petr-Gotzens, who has coordinated the execution of ESO's observations of Gaia since 2013. "The spacecraft is what we call a 'moving target', as it is moving quickly relative to background stars -- tracking Gaia is quite the challenge!"

"The VST is the perfect tool for picking out the motion of Gaia," elaborated Ferdinando Patat, head of the ESO's Observing Programmes Office. "Using one of ESO's first-rate ground-based facilities to bolster cutting-edge space observations is a fine example of scientific cooperation."

"This is an exciting ground-space collaboration, using one of ESO's world-class telescopes to anchor the trailblazing observations of ESA's billion star surveyor," commented Timo Prusti, Gaia project scientist at ESA.

The VST observations are used by ESA's flight dynamics experts to track Gaia and refine the knowledge of the spacecraft's orbit. Painstaking calibration is required to transform the observations, in which Gaia is just a speck of light among the bright stars, into meaningful orbital information. Data from Gaia's second release was used to identify each of the stars in the field of view, and allowed the position of the spacecraft to be calculated with astonishing precision -- up to 20 milliarcseconds.

"This is a challenging process: we are using Gaia's measurements of the stars to calibrate the position of the Gaia spacecraft and ultimately improve its measurements of the stars," explains Timo Prusti.

"After careful and lengthy data processing, we have now achieved the accuracy required for the ground-based observations of Gaia to be implemented as part of the orbit determination," says Martin Altmann, lead of the Ground Based Optical Tracking (GBOT) campaign at the Centre for Astronomy of Heidelberg University, Germany.

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Hubble celebrates its 29th birthday with unrivaled view of the Southern Crab Nebula

This is the Southern Crab Nebula -- Hubble's 29th anniversary image.

This incredible image of the hourglass-shaped Southern Crab Nebula was taken to mark the NASA/ESA Hubble Space Telescope's 29th anniversary in space. The nebula, created by a binary star system, is one of the many objects that Hubble has demystified throughout its productive life. This new image adds to our understanding of the nebula and demonstrates the telescope's continued capabilities.

On 24 April 1990, the NASA/ESA Hubble Space Telescope was launched on the space shuttle Discovery. It has since revolutionised how astronomers and the general public see the Universe. The images it provides are spectacular from both a scientific and a purely aesthetic point of view.

Each year the telescope dedicates a small portion of its precious observing time to take a special anniversary image, focused on capturing particularly beautiful and meaningful objects. This year's image is the Southern Crab Nebula, and it is no exception [1].

This peculiar nebula, which exhibits nested hourglass-shaped structures, has been created by the interaction between a pair of stars at its centre. The unequal pair consists of a red giant and a white dwarf. The red giant is shedding its outer layers in the last phase of its life before it too lives out its final years as a white dwarf. Some of the red giant's ejected material is attracted by the gravity of its companion.

When enough of this cast-off material is pulled onto the white dwarf, it too ejects the material outwards in an eruption, creating the structures we see in the nebula. Eventually, the red giant will finish throwing off its outer layers, and stop feeding its white dwarf companion. Prior to this, there may also be more eruptions, creating even more intricate structures.

Astronomers did not always know this, however. The object was first written about in 1967, but was assumed to be an ordinary star until 1989, when it was observed using telescopes at the European Southern Observatory's La Silla Observatory. The resulting image showed a roughly crab-shaped extended nebula, formed by symmetrical bubbles of gas and dust.

These observations only showed the outer hourglass emanating from a bright central region that could not be resolved. It was not until Hubble observed the Southern Crab in 1998 that the entire structure came into view. This image revealed the inner nested structures, suggesting that the phenomenon that created the outer bubbles had occurred twice in the (astronomically) recent past.

It is fitting that Hubble has returned to this object twenty years after its first observation. This new image adds to the story of an active and evolving object and contributes to the story of Hubble's role in our evolving understanding of the Universe.

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Hubble peers at cosmic blue bauble

Messier 3: Containing an incredible half-million stars, this 8-billion-year-old cosmic bauble is one of the largest and brightest globular clusters ever discovered.

Globular clusters are inherently beautiful objects, but the subject of this NASA/ESA Hubble Space Telescope image, Messier 3, is commonly acknowledged to be one of the most beautiful of them all.

Containing an incredible half-million stars, this 8-billion-year-old cosmic bauble is one of the largest and brightest globular clusters ever discovered. However, what makes Messier 3 extra special is its unusually large population of variable stars -- stars that fluctuate in brightness over time. New variable stars continue to be discovered in this sparkling stellar nest to this day, but so far we know of 274, the highest number found in any globular cluster by far. At least 170 of these are of a special variety called RR Lyrae variables, which pulse with a period directly related to their intrinsic brightness. If astronomers know how bright a star truly is based on its mass and classification, and they know how bright it appears to be from our viewpoint here on Earth, they can thus work out its distance from us. For this reason, RR Lyrae stars are known as standard candles -- objects of known luminosity whose distance and position can be used to help us understand more about vast celestial distances and the scale of the cosmos.

Messier 3 also contains a relatively high number of so-called blue stragglers, which are shown quite clearly in this Hubble image. These are blue main sequence stars that appear to be young because they are bluer and more luminous than other stars in the cluster. As all stars in globular clusters are believed to have formed together and thus to be roughly the same age, only a difference in mass can give these stars a different color. A red, old star can appear bluer when it acquires more mass, for instance by stripping it from a nearby star. The extra mass changes it into a bluer star, which makes us think it is younger than it really is.

From Science Daily