Topic: Late February Astronomy Bulletin

[Clive]

DOUBT CAST ON EVOLUTION OF MARTIAN LIFE
The University of Hong Kong

Mars has long been of interest as a place to search for evidence of life beyond the Earth, because the surface has numerous features that appear to be dried-up river channels and dried lake beds that hint at a warmer, wetter, more-Earthlike climate in the past. However, new research has cast doubt on the idea of surface life evolving on Mars. For the last 2.5 billion years, surface life on Earth has thrived largely through the evolution of photosynthesis. Surface life is abundant and very successful because of the availability of sunlight, surface water, generally moderate climate conditions, and the protection of our magnetic field. But Mars
would never have experienced such habitable conditions at the surface. Now scientists show that the climate of Mars has probably been extremely cold and dry most of the time. They argue that the familiar aqueous features on Mars include widespread, weathered soil horizons that could have formed in 'geologically' short climate 'excursions'. In other words, Mars has been cold and dry almost throughout its history and has only had abundant liquid water on its surface during relatively short episodes of climate change.
However, all hope for life on Mars is not lost. The scientists point out that the prospects for surface life on Mars might be dim, but the possibilities for sub-surface life are promising. Life on Earth probably began in hydrothermal systems (environments where hot water reacts with rocks), and there is abundant evidence for many locations where hydrothermal environments existed on Mars at the time when life might have originated in similar environments on Earth. The scientists argue that, in order to understand how life formed on Earth, we should ignore the surface environments on Mars and focus exploration on hydrothermal deposits.
Using infrared data on Mars collected by spacecraft, astronomers can interpret which minerals are there and describe the 'geology' of ancient hydrothermal systems. That type of work is based on laboratory measurements, which provide the necessary
mineralogical background with which to interpret spectroscopic data from Mars.


COMPOSITION OF TRAPPIST-1 PLANETS
ESO

The seven Earth-size planets of TRAPPIST-1 are all mostly made of rock, with some having the potential to hold more water than the Earth. The planets' densities, now known much more precisely than before, suggest that some of them could have up to 5 per cent of their mass in water -- which is 250 times more than the oceans on Earth. The form that water would take on TRAPPIST-1 planets would depend on the amount of heat they receive from their star, which is a mere 9 per cent as massive as our Sun. Planets closest to the star are more likely to have water in the form of atmospheric vapour, while those farther away may have water frozen on their surfaces as ice. TRAPPIST-1e is the rockiest planet of them all, but still is believed potentially to have some liquid water. Astronomers now know more about TRAPPIST-1 than about any other planetary system apart from our own. Since the extent of the TRAPPIST system was recognized in 2017 February, researchers have been working hard to characterize the planets better and to collect more information about them. The new study offers better estimates than were available previously for the planets' densities. The TRAPPIST name comes from the Transiting Planets and Planetesimals Small Telescope in Chile, which discovered two of the seven planets we know of today --announced in 2016. The Spitzer space telescope, in collaboration with ground-based telescopes, confirmed those planets and discovered the other five in the system. Since then, the Kepler space telescope has also observed the TRAPPIST-1 system, and Spitzer began a new programme of 500 additional hours of TRAPPIST-1 observations, which will conclude in March. The new body of data has helped the authors of the study to paint a clearer picture of the system than ever before -- although there is still much more to learn about TRAPPIST-1.

The TRAPPIST-1 planets huddle so close to one another that a person standing on the surface of one of them would have a spectacular view of the neighbouring planets in the sky. Those planets would sometimes appear larger than the Moon looks to an observer on Earth. They may also be tidally locked, meaning that the same side of the planet is always facing the star, with each side in perpetual day or night. Although the planets are all closer to their star than Mercury is to the Sun, TRAPPIST-1 is such a cool
star that some of its planets could still, in theory, hold liquid water. It is impossible to know exactly how each planet looks, because they are so far away. In our own Solar System, the Moon and Mars have nearly the same density, yet their surfaces appear entirely different. TRAPPIST-1b, the innermost planet, is likely to have a rocky core, surrounded by an atmosphere much thicker than the Earth's. TRAPPIST-1c also probably has a rocky interior, but with a thinner atmosphere than planet b. TRAPPIST-1d is the lightest of the planets -- about three-tenths the mass of the Earth.  Scientists are uncertain whether it has a large atmosphere, an ocean or an
ice layer -- all three of those would give the planet an 'envelope' of volatile substances, which would make sense for a planet of its density.  Scientists were surprised that TRAPPIST-1e is the only planet in the system slightly denser than the Earth, suggesting that it may have a relatively larger iron core than our home planet. Like TRAPPIST-1c, it does not necessarily have a thick atmosphere, ocean or ice layer -- making those two planets distinct in the system. It is a mystery why TRAPPIST-1e has a much rockier composition than the rest of the planets. In terms of size, density and the amount of radiation it receives from its star, it is the planet that is most similar to the Earth. TRAPPIST-1f, g and h are far enough from the host star that water could be frozen as ice on their surfaces. If they have thin atmospheres, they would be unlikely to contain heavy molecules such as carbon dioxide.

Scientists can calculate the densities of the planets because their orbits happen to be oriented in such a way that they transit in front of their star, causing a slight dimming of the starlight. The amount of dimming is related to the radius of the planet. To get the density, scientists take advantage of what are called 'transit timing variations'. If there were no other gravitational forces on a transiting planet, it would always cross in front of its host star in the same amount of time -- for example, the Earth orbits the Sun every 365 days. But because the TRAPPIST-1 planets are packed so close together, they change the timing of one another's 'years'
ever so slightly. Those variations in orbital timing are used to estimate the planets' masses. Then, mass and radius are used to calculate density.  The next step in exploring TRAPPIST-1 will be with the James Webb space telescope, which may be able to determine whether these planets have atmospheres and, if so, what those atmospheres are like. A recent study using the Hubble telescope found no detection of hydrogen-dominated atmospheres on planets TRAPPIST-1d, e and f -- another piece of evidence for rocky composition -- although a hydrogen-dominated atmosphere cannot be ruled out for g.


MASSIVE, DENSE SUPER-EARTH DETECTED
Carnegie Institution for Science

A star about 100 light-years away in the constellation Pisces, GJ 9827, has what may be one of the most massive and dense super-Earth planets detected to date, according to new research. That new information provides evidence to help astronomers understand the process by which such planets form. The GJ 9827 star actually hosts three planets, discovered by the exoplanet-hunting Kepler/K2 mission, and all three are slightly larger than the Earth. That is the size that the Kepler mission found to be most common in the Galaxy with periods between a few and several hundred days. Intriguingly, no planets of that size exist in the Solar System. That makes scientists curious about the conditions under which they form and evolve. One important key to understanding a planet's history is its composition. Are these super-Earths rocky like our own planet? Or do they have solid cores surrounded by large, gassy atmospheres? To try to understand what an exo-planet is made of, scientists need to measure both its mass and its radius, which allow them to determine its bulk density. When quantifying planets in that way, astronomers have noticed a trend. It turns out that planets with radii greater than about 1.7 times that of the Earth have a gassy envelopes, like Neptune's, and those with radii smaller than that are rocky, like ours. Some researchers have proposed that the difference is caused by photo-evaporation, which strips planets of their envelopes of so-called volatiles -- substances like water and carbon dioxide that have low boiling points -- creating smaller-radius planets. But more information is needed to test that theory. That is why GJ 9827's three planets are special -- with radii of 1.64 (planet b), 1.29 (planet c) and 2.08 (planet d), they span the dividing line between super-Earth (rocky) and sub-Neptune (somewhat gassy) planets.

Luckily, scientists have been monitoring GJ 9827 with their Planet-Finding Spectrograph (PFS), so they were able to constrain the masses of the three planets with data in hand, rather than having to scramble to get many new observations of GJ 9827. Usually, if a transiting planet is detected, it takes months if not a year or more to gather enough observations to measure its mass. Because GJ 9827 is a fairly bright (tenth-magnitude) star, the team already had it in the catalogue of stars that it been monitoring for planets since 2010. The spectrograph was mounted on the Magellan Clay Telescopes at Las Campanas Observatory. The PFS observations indicate that planet b is roughly eight times the mass of the Earth, which would make it one of the most-massive and dense super-Earths yet discovered. The masses for planets c and d are estimated to be about two and a half and four times that of the Earth respectively, although the uncertainty in those two determinations is very high. That information suggests that planet d has a significant volatile envelope, and leaves open the question of whether planet c has a volatile envelope or not. But the better constraint on the mass of planet b suggests that that it is roughly 50% iron. More observations are needed to pin down the compositions of the three planets, but they do seem to be some of the best candidates to test our ideas about how super-Earths form and evolve.

PLANETS FOUND IN ANOTHER GALAXY
University of Oklahoma

A team of astronomers has discovered for the first time a population of planets beyond the Milky Way galaxy. Using microlensing, researchers were able to detect objects in extragalactic systems that range from the mass of the Moon to the mass of Jupiter. The discovery was made using data from the Chandra X-ray Observatory, a telescope in space that is controlled by the Smithsonian Astrophysical Observatory. The small planets are the best candidates for observations by the microlensing technique. While planets are often discovered within the Milky Way using microlensing, the gravitational effect of even small objects can create high magnification, leading to a signature that can be modelled and explained in external galaxies. Until this study, there had been no evidence of planets in other galaxies. The galaxy concerned is 3800 million light-years away, and there is not the slightest chance of observing the planets directly. However, microlensing makes it possible to study them, unveil their presence and even obtain an idea of their masses.


DWARF SATELLITES ORBITING CENTAURUS A
University of California - Irvine

An international team of astronomers has foundd that Centaurus A, a massive elliptical galaxy 13 million light-years away, is accompanied by a number of dwarf satellite galaxies orbiting the main body in a narrow disc. The researchers note that this is the first time such a galactic arrangement has been observed outside the Local Group, home to the Milky Way. The significance of this finding is that it calls into question the validity of certain cosmological models and simulations as explanations for the distribution of host and satellite galaxies in the Universe. The team says that under the 'lambda cold dark matter' model, smaller systems of stars should be more or less randomly scattered around their anchoring galaxies and should move in all directions. Yet Centaurus A is the third documented example, after the Milky Way and Andromeda, of a 'vast polar structure' in which satellite dwarfs co-rotate around a central galactic mass in what the team calls 'preferentially oriented alignment'. The difficulty of studying the movements of dwarf satellites around their hosts varies according to the target galaxy group. It is relatively easy for the Milky Way, where we can
get proper motions. You take a picture now, wait a few years, and then take another picture to see how the stars have moved; that gives you the tangential velocity. Using that technique, scientists have measurements for 11 Milky Way satellite galaxies, eight of which are orbiting in a tight disc perpendicular to the spiral galaxy's plane. There are probably other satellites in the system that can't be seen from here because they are blocked by the Milky Way's dusty disc.

Andromeda provides observers on Earth with a view of the full distribution of satellites around the galaxy's sprawling spiral. An earlier study found 27 dwarf galaxies, 15 arranged in a narrow plane. And Andromeda offers another advantage: because we see the galaxy almost edge-on, we can look at the line-of-sight velocities of its satellites to see which are approaching and which are receding, so it very clearly presents as a rotating disc. Centaurus A is much further away, and its satellite companions are faint, making it more difficult to measure distances and velocities to determine movements and distributions. But 'sleeping in the archives' were data on 16 of Centaurus A's satellites. We could do the same game as with Andromeda, where we look at the line-of-sight velocities and again we see that half of them are red-shifted, meaning that they are receding from us, and the other half are blue-shifted, which tells us that they are approaching. The researchers were able to demonstrate that 14 of the 16 Centaurus A satellite galaxies follow a common motion pattern and rotate along the plane around the main galaxy -- contradicting frequently used cosmological models and simulations suggesting that only about 0.5 per cent of satellite galaxy systems in the nearby Universe should exhibit such a pattern. That seems to mean that we are missing something: either the simulations lack some important ingredient, or the underlying model is wrong. This research may be seen as support for looking into alternative models.


GRAVITATIONAL LENS SETS NEW RECORD
University of Hawaii at Manoa

Extremely distant galaxies are mostly too faint to be seen, even by the largest telescopes. But nature has a solution: gravitational lensing, predicted by Einstein and observed many times by astronomers. Now, an international team of astronomers has discovered one of the most extreme instances of magnification by gravitational lensing. Using the Hubble Telescope to survey a sample of huge clusters of galaxies, the team found a distant galaxy, eMACSJ1341-QG-1, that is magnified 30 times thanks to the distortion of space-time created by the massive 'foreground' galaxy cluster dubbed eMACSJ1341.9-2441. The underlying physical effect of gravitational lensing was first confirmed during the solar eclipse of 1919, and can dramatically magnify images of distant celestial sources if a sufficiently massive object lies between the background source and the observers. Galaxy clusters, enormous concentrations of dark matter and hot gas surrounding hundreds or thousands of individual galaxies, all bound by the force of gravity, are valued by astronomers as powerful gravitational lenses. By magnifying the galaxies situated behind them, massive clusters act as natural telescopes that allow scientists to study faint and distant sources that would otherwise be beyond the reach of even the most powerful man-made telescopes. In the present instance, The high magnification of the image provides astronomers with a rare opportunity to investigate the stellar populations of that distant object and, ultimately, to reconstruct its undistorted shape and properties. Although similarly extreme magnifications have been observed before, the discovery sets a record for the magnification of a rare 'quiescent' background galaxy -- one that, unlike our Milky Way, is not forming new stars in giant clouds of cool gas. The team specializes in finding extremely massive clusters that act as natural telescopes and has already discovered many exciting cases of gravitational lensing. This discovery stands out, though, as the huge magnification provided by eMACSJ1341 allows us to study in detail a rare type of galaxy.


VLT WORKS AS 16-METRE TELESCOPE
ESO

One of the original design goals of the Very Large Telescope (VLT) was for its four Unit Telescopes (UTs) to work together as a single giant telescope.  With the first light of the ESPRESSO spectrograph using the four-Unit-Telescope mode of the VLT, that goal has now been reached. When all four 8.2-metre Unit Telescopes combine their light-collecting power to feed a single instrument, the VLT effectively becomes the largest optical telescope in the world in terms of collecting area. Two of the main scientific goals
of ESPRESSO are the discovery and characterization of Earth-like planets and the search for possible variability of the fundamental constants of physics. The latter experiments in particular require the observation of distant and faint quasars, and that scientific goal will benefit the most from combining the light from all four Unit Telescopes in ESPRESSO. Both rely on the ultra-high stability of the instrument and an extremely stable reference light source. Owing to the complexity involved, the combination of light from all four Unit Telescopes in this way, at what is known as an 'incoherent focus', had not been implemented until now. However, space for it was built into the telescopes and the underground structure of the mountaintop from the start. A system of mirrors, prisms and lenses transmits the light from each Unit Telescope to the ESPRESSO spectrograph up to 69 metres away. Thanks to those complex optics, ESPRESSO can either collect the light from up to all four Unit Telescopes together, increasing its light-gathering power, or alternatively receive light from any one of the Unit Telescopes independently, allowing for more flexible usage of observing time.

Light from the four Unit Telescopes is routinely brought together in the VLT Interferometer for the study of extremely fine detail in comparatively bright objects. But interferometry, which combines the beams 'coherently', cannot exploit the huge light-gathering potential of the combined telescopes to study faint objects. Feeding the combined light into a single instrument will give astronomers access to information never previously available.  That new facility is a game changer for astronomy with high-resolution spectrographs. It makes use of novel concepts, such as wavelength calibration aided by a laser frequency comb, providing unprecedented precision and repeatability, and now the ability to join together the light-collecting power of the four individual Unit Telescopes.


JODRELL BANK NOMINATED FOR UNESCO
Jodrell Bank Observatory

Jodrell Bank Observatory, home to the iconic Lovell Telescope, has been selected as the next UK candidate to become a World Heritage Site. The Jodrell Bank Observatory is the earliest large radio-astronomy observatory in the world that is still in existence. It is now the one remaining site, worldwide, that includes evidence of every stage of the post-1945 emergence of radio astronomy. Its importance in that field was recognized by the Government's statutory advisor on the historic environment, Historic England, which listed six structures at the site in 2017 August. Before the observatory was built, astronomy was largely conducted through the use of optical telescopes. It was only in the 1950s and '60s that the astrophysical world took another big leap with the introduction of radio astronomy.  With the advent of radio astronomy, astronomers were no longer limited to observing optical objects, but could also study celestial objects at radio frequencies. That greatly expanded the scope and range of objects that could be observed, and in the process significantly expanded our knowledge of the Universe. Jodrell Bank is perhaps most famous in the UK, and in the
North of England in particular, for its Lovell Telescope. According to Historic England, both the Lovell Telescope and its
accompanying Mark II Telescope have already gained Grade I listing owing to the roles that they have played in the development of radio astronomy. The Lovell Telescope was the world's first large steerable radio telescope, and is still surpassed by only two others.