The SOCIETY for POPULAR ASTRONOMY Electronic News Bulletin No. 451 August 27

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    The SOCIETY for POPULAR ASTRONOMY  Electronic News Bulletin No. 451 2017 August 27
    Here is the latest round-up of news from the Society for Popular society, with members all over the world. We accept subscription payments online at our secure site and can take credit and debit cards. You can join or renew via a secure server or just see how much we have to offer by visiting http://www.popastro.com/

    PREDICTION OF EARTH-LIKE PLANET ONLY 16 LIGHT-YEARS AWAY
    University of Texas at Arlington

    Astrophysicists have predicted that an Earth-like planet may be
    lurking in a star system just 16 light-years away. The team
    investigated the star system Gliese 832 for additional exo-planets
    residing between the two currently known ones in that system. Their
    computations revealed that an additional Earth-like planet with a
    dynamically stable configuration may be residing at a distance ranging
    from 0.25 to 2.0 AU from the star. According to calculations, the
    hypothetical planet would probably have a mass between 1 and 15 Earth
    masses. Gliese 832 is a red dwarf and has just under half the mass
    and radius of the Sun. The star is orbited by a giant Jupiter-like
    gas planet designated Gliese 832b and by a super-Earth planet Gliese
    832c. The gas giant with 0.64 Jupiter masses is orbiting the star at a
    distance of 3.53 AU, while the other planet is potentially a rocky
    one, around five times more massive than the Earth, and orbiting very
    close to its host star — about 0.16 AU. For this research, the team
    analyzed the simulated data with an injected Earth-mass planet on this
    nearby planetary system, hoping to find a stable orbital configuration
    for the planet that may be located in the vast space between the two
    known planets. Gliese 832b and Gliese 832c were discovered by the
    radial-velocity technique, which detects variations in the velocity of
    the central star, arising from the gravitational pull of an unseen
    exo-planet as it orbits the star. By regularly measuring the velocity
    of the star with sufficient accuracy one can see if it moves periodic-
    ally owing to the influence of a companion.
    Astronomers also used the integrated data from the time evolution of
    orbital parameters to generate the synthetic radial-velocity curves
    of the known and the Earth-like planets in the system and obtained
    several radial-velocity curves for varying masses and distances
    indicating a possible new middle planet. For instance, if the new
    planet is located around 1 AU from the star, it has an upper mass
    limit of 10 Earth masses and generates a radial-velocity signal of
    1.4 metres per second. A planet with about the mass of the Earth at
    the same location would give radial-velocity signal of only 0.14 m/s,
    thus much smaller and very hard to detect with current technology.
    The existence of this possible planet is supported by the long-term
    orbital stability of the system, orbital dynamics, and the synthetic
    radial-velocity signal analysis. At the same time, a significantly
    large number of radial-velocity observations, and transit-method
    studies, as well as direct imaging, are still needed to confirm the
    presence of possible new planets in the Gliese 832 system.

    TRAPPIST-1 OLDER THAN THE SOLAR SYSTEM
    NASA

    If we want to know more about whether life could survive on a planet
    outside our Solar System, it is important to know the age of its star.
    Young stars have frequent releases of high-energy radiation called
    flares that can zap their planets' surfaces. If the planets are newly
    formed, their orbits may also be unstable. On the other hand, planets
    orbiting older stars have survived the spate of youthful flares, but
    have also been exposed to the ravages of stellar radiation for a
    longer period of time. Scientists now have a good estimate for the
    age of one of the most intriguing planetary systems discovered to date
    — TRAPPIST-1, a system of seven Earth-size planets orbiting an ultra-
    cool dwarf star about 40 light-years away. Researchers say in a new
    study that the TRAPPIST-1 star is quite old, between 5.4 and 9.8
    billion years. That is up to twice as old as our own Solar System,
    which formed some 4.5 billion years ago. Three of the TRAPPIST-1
    planets reside in the star's 'habitable zone', the orbital distance
    where a rocky planet with an atmosphere could have liquid water on its
    surface. All seven planets are probably tidally locked to their star,
    each with a perpetual day side and night side.
    At the time of its discovery, scientists believed the TRAPPIST-1
    system to be at least 500 million years old, since it takes a star of
    TRAPPIST-1's low mass (roughly 8% that of the Sun) roughly that long
    to contract to its minimum size, just a bit larger than the planet
    Jupiter. However, even that lower age limit was uncertain; in theory,
    the star could be almost as old as the Universe itself. Are the
    orbits of this compact system of planets stable? Might life have had
    enough time to evolve on any of those worlds? It is unclear what the
    older age means for the planets' habitability. On the one hand, older
    stars flare less than younger stars and TRAPPIST-1 is relatively quiet
    compared to other ultra-cool dwarf stars. On the other hand, since
    the planets are so close to the star, they have soaked up billions of
    years of high-energy radiation, which could have boiled off any
    atmospheres and large amounts of water. In fact, the equivalent of an
    Earth ocean may have evaporated from each TRAPPIST-1 planet except for
    the two most distant from the host star, planets g and h. In our own
    Solar System, Mars is an example of a planet that probably had liquid
    water on its surface in the past, but lost most of its water and
    atmosphere to the Sun's high-energy radiation over billions of years.
    However, old age does not necessarily mean that a planet's atmosphere
    has been eroded. Given that the TRAPPIST-1 planets have lower
    densities than the Earth, it is possible that large reservoirs of
    volatile molecules such as water could produce thick atmospheres that
    would shield the planetary surfaces from harmful radiation. A thick
    atmosphere could also help redistribute heat to the dark sides of
    those tidally locked planets, increasing habitable real estate. But
    that could also backfire in a 'runaway greenhouse' process, in which
    the atmosphere becomes so thick that the planet's surface overheats
    — as on Venus. Fortunately, low-mass stars like TRAPPIST-1 have
    temperatures and brightnesses that remain relatively constant over
    billions of years, punctuated by occasional magnetic flaring events.
    The lifetimes of tiny stars like TRAPPIST-1 are predicted to be much,
    much longer than the 13.7-billion-year age of the Universe (the Sun,
    by comparison, has an expected lifetime of about 10 billion years).
    Some of the clues used to measure the age of TRAPPIST-1 included how
    fast the star is moving in its orbit around the Milky Way (speedier
    stars tend to be older), its atmosphere's chemical composition, and
    how many flares TRAPPIST-1 had during observational periods. Those
    variables all pointed to a star that is substantially older than the
    Sun. Future observations with the Hubble Space Telescope and upcoming
    James Webb Space Telescope may reveal whether the planets have
    atmospheres, and whether such atmospheres are like the Earth's.

    TIDALLY LOCKED EXO-PLANETS VERY COMMON
    University of Washington

    Many exoplanets to be found by upcoming high-powered telescopes will
    probably be tidally locked — with one side permanently facing the
    host star. Astronomers arrived at that finding by questioning the
    long-held assumption that only those stars that are much smaller and
    dimmer than the Sun could host orbiting planets that were in
    synchronous orbit, or tidally locked, as the Moon is with the Earth.
    Tidal locking results when there is no side-to-side momentum between a
    body in space and its gravitational partner and they become fixed in
    their embrace. Tidally locked bodies such as the Moon are in
    synchronous rotation, meaning that each takes exactly as long to
    rotate around its own axis as it does to revolve around its host star
    or gravitational partner. The Moon takes 27 days to rotate once on
    its axis, and 27 days to orbit the Earth. The Moon is thought to have
    been created by a Mars-sized celestial body slamming into the young
    Earth at an angle that set the world spinning initially with approx-
    imately 12-hour days. In the past, researchers tended to use that
    12-hour estimation of the Earth's rotation period to model exo-planet
    behaviour, asking, for example, how long an Earth-like exo-planet with
    a similar orbital spin might take to become tidally locked.
    Being tidally locked was once thought to lead to such extremes of
    climate as to eliminate any possibility of life, but astronomers have
    since reasoned that the presence of an atmosphere with winds blowing
    across a planet's surface could mitigate such effects and allow for
    moderate climates and life. Astronomers also considered the planets
    that may be discovered by NASA's next planet-hunting satellite, the
    Transiting Exoplanet Survey Satellite or TESS, and found that every
    potentially habitable planet that it will detect will probably be
    tidally locked. Even if astronomers discover the long-sought Earth
    'twin' orbiting a virtual twin of the Sun, that planet may be tidally
    locked.

    GALAXIES AT HEART OF GIANT CLUSTER
    Heidelberg University

    Our Solar System is located in an enormous galaxy composed of billions
    of stars, the Milky Way. About 3,000 stars can be seen with the naked eye.
    However, if the Earth were located in an ultra-diffuse galaxy, only a
    few dozen stars and a 'trace' of a galaxy would be visible in the
    night sky. That special class of galaxy, so named for their extremely
    diffuse appearance, apparently produced far fewer stars than other
    galaxies, or else were stripped of them long ago by galactic tidal
    forces. Astronomers began to search the Universe systematically for
    such ultra-diffuse galaxies just three years ago. Aided by large
    telescopes and new technologies, they found them, especially in
    large clusters of galaxies. Much to their surprise, the researchers
    identified about 90 such galaxies in the core of the Perseus galaxy
    cluster. The Perseus cluster is a dense collection of hundreds of
    large and small galaxies located 240 million light-years away.
    Amazingly, most of the ultra-diffuse galaxies appear intact, with
    only very few candidates showing signs of ongoing disruption in spite
    of the strong tidal field. The research was based on long-exposure
    images of the Perseus cluster obtained in 2012 with the 4.2-m William
    Herschel Telescope on La Palma in the Canary Islands;. The research
    group is now hoping to obtain data of similar quality on the outskirts
    of the Perseus cluster, where the environmental influence would have
    been less strong, preserving more of the original appearance of the
    galaxies.

    BLACK HOLES PERVADE THE UNIVERSE
    University of California, Irvine

    After conducting a sort of cosmic inventory to calculate and
    categorize stellar-remnant black holes, astronomers have concluded
    that there are probably tens of millions of such enigmatic dark
    objects in the Milky Way — far more than expected. UCI's celestial
    census began more than a year and a half ago, shortly after the news
    that the Laser Interferometer Gravitational-Wave Observatory, or LIGO,
    had detected ripples in the space-time continuum created by the
    distant collision of two black holes, each the mass of 30 Suns.
    Scientists assume that most stellar-remnant black holes — which
    result from the collapse of massive stars at the ends of their lives
    — will be about the same mass as our Sun. To see evidence of two
    black holes of such epic proportions finally coming together in a
    cataclysmic collision had some astronomers scratching their heads.
    On the basis on what we know about star formation in galaxies of
    different types, we can infer when and how many black holes formed in
    each galaxy. Big galaxies are home to older stars, and they host
    older black holes too. The number of black holes of a given mass per
    galaxy will depend on the size of the galaxy. The reason is that
    larger galaxies have many metal-rich stars, and smaller dwarf galaxies
    are dominated by big stars of low metallicity. Stars that contain a
    lot of heavier elements, like our Sun, shed a lot of that mass over
    their lives. When it comes time for one to end it all as a supernova,
    there isn't as much matter left to collapse in on itself, resulting in
    a lower-mass black hole. Big stars with low metal content do not shed
    as much of their mass over time, so when one of them dies, almost all
    of its mass winds up in the black hole.
    We have a fairly good understanding of the overall population of stars
    in the Universe and their mass distribution as they are born, so we
    can tell how many black holes should have formed with 100 solar masses
    versus 10 solar masses. The team was able to work out how many big
    black holes should exist, and it turned out to be in the millions —
    far more than anticipated. In addition, to shed light on subsequent
    phenomena, researchers sought to determine how often black holes occur
    in pairs, how often they merge, and how long it takes. They wondered
    whether the 30-solar-mass black holes detected by LIGO were born
    billions of years ago and took a long time to merge, or came into being
    more recently (within the past 100 million years) and merged soon
    after. They show that only 0.1 to 1 per cent of the black holes formed
    have to merge to explain what LIGO saw. Of course, the black holes
    have to get close enough to merge in a reasonable time, which is an
    open problem. Many more gravitational-wave detections are expected,
    so astronomers can determine if black holes collide mostly in giant
    galaxies. That would tell them something important about the physics
    that drive them to coalesce. Astronomers may not have to wait too
    long, relatively speaking. If the current ideas about stellar
    evolution are right, then calculations indicate that mergers of even
    50-solar-mass black holes will be detected in a few years.

    Bulletin compiled by Clive Down
    (c) 2017 The Society for Popular Astronomy
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