NEW THEORY ON MOON FORMATION
American Technion Society
The Moon, and the question of how it was formed, has long been a
source of fascination and wonder. Now, a team of Israeli researchers
suggests that the Moon we see now is not the Earth's first moon, but
rather the last in a series of moons that orbited the Earth in the
past. The new proposal runs counter to the commonly held 'giant
impact' paradigm that the Moon is a single object that was formed
following a single great collision between a small Mars-like planet
and the ancient Earth. The new model is consistent with science's
current understanding of the formation of the Earth. In its last
stages of growth, the Earth experienced many giant impacts with other
bodies. Each of those impacts contributed more material to the
proto-Earth, until it reached its current size. The new theory
suggests that the Earth had many previous moons, and a previously
formed moon could therefore already exist when another moon-forming
giant impact occurred. The tidal forces from the Earth could cause
moons to migrate slowly outwards (the current Moon is slowly doing
that, at a rate of about 1 cm a year). A pre-existing moon would
slowly move out by the time another moon forms. However, their mutual
gravitational attraction would eventually cause the moons to affect
one another and change their orbits. That model suggests that the
ancient Earth once hosted a series of moons, each one formed from a
different collision with the proto-Earth. It is likely that such
moonlets were later ejected, or collided with the Earth or with one
another to form bigger moons.
VLT TO SEARCH FOR PLANETS IN ALPHA CENTAURI SYSTEM
ESO
ESO has signed an agreement with 'Breakthrough Initiatives' to adapt
the Very Large Telescope instrumentation in Chile to conduct a search
for planets in the 'nearby' star system Alpha Centauri. Such planets
could be the targets for an eventual launch of miniature space probes
by the 'Breakthrough Starshot' initiative. The discovery in 2016 of a
planet, Proxima b, around Proxima Centauri, faintest of the three
stars of the Alpha Centauri system, adds further impetus to the
search. Knowing where the nearest exo-planets are is of paramount
interest for 'Breakthrough Starshot', a research and engineering
programme launched in 2016 April, which aims to demonstrate proof of
concept for ultra-fast light-driven 'nanocraft', laying the
foundation for the first launch to Alpha Centauri within a generation.
Detecting a habitable planet is an enormous challenge, owing to the
brightness of the planetary system's host star, which tends to
overwhelm the relatively dim planets. One way to make that easier is
to observe in the mid-infrared wavelength range, where the thermal
glow from an orbiting planet greatly reduces the brightness gap
between it and its host star. But even in the mid-infrared, the star
remains millions of times brighter than the planets to be detected,
which calls for a dedicated technique to reduce the blinding stellar
light.
The existing mid-infrared instrument VISIR on the VLT would provide
such performance if it were enhanced to improve the image quality by
adaptive optics, and adapted to employ coronagraphy to reduce the
stellar light and thereby reveal the possible signals of potential
terrestrial planets. 'Breakthrough Initiatives' will pay for a large
fraction of the necessary technology and development costs for such an
experiment, and ESO will provide the required observing capabilities
and time. Detecting and studying potentially habitable planets
orbiting other stars will be among the main scientific goals of the
upcoming European Extremely Large Telescope (E-ELT). Although the
larger size of the E-ELT will be essential to obtaining an image of
a planet at greater distances in the Milky Way, the light-collecting
power of the VLT is just sufficient to image a planet around the
nearest star, Alpha Centauri.
ORBIT OF PROXIMA CENTAURI DETERMINED
ESO
Interest in our neighbouring Alpha Centauri star system has been
particularly high since the recent discovery of an Earth-mass planet,
known as Proxima b, orbiting the system's third star -- the closest
star to the Sun -- Proxima Centauri. While the system's larger
stellar pair, Alpha Centauri A and B, appears to have a proper motion
on the sky that is very similar to that of the smaller, fainter
Proxima Centauri, it has not been possible to demonstrate that the
three stars do actually form a single, gravitationally bound, triple
system. Now astronomers have concluded that the three stars do
indeed form a bound system. In the century since Proxima was
discovered, its faintness has made it extremely difficult to measure
its radial velocity reliably. But now ESO's planet-hunting HARPS
instrument has provided an extremely precise measurement of Proxima's
radial velocity, and even greater accuracy has been achieved by
accounting for other subtle effects. As a result, the astronomers
have been able to deduce very similar values for the radial velocities
of the Alpha Centauri pair and Proxima Centauri, lending credence to
the idea that they form a bound system. Taking account of the new
measurements, calculations of the orbits of the three stars indicate
that the relative velocity between Proxima Centauri and the Alpha
Centauri pair is well below the threshold above which the three stars
would not be bound together by gravity.
That result has significant implications for our understanding of the
Alpha Centauri system and the formation of planets there. It strongly
suggests that Proxima Centauri and the Alpha Centauri pair are the
same age (about 6,000 million years), and that in turn provides a good
estimate of the age of the orbiting planet, Proxima b. The astronomers
speculate that the planet may have formed around Proxima Centauri on
a more extended orbit and then been brought to its current position,
very close to its parent star, as a result of a close passage of
Proxima to its cousins in the Alpha Centauri pair. Alternatively, the
planet may have formed around the Alpha Centauri pair, and was later
captured by the gravity of Proxima. If one of those hypotheses is
correct, it is possible that the planet was once an icy world that
underwent a meltdown and now may have liquid water on its surface.
TWO MISSIONS TO DETECT EARLY SOLAR SYSTEM
Science Daily
NASA has selected two missions that have the potential to open new
windows on one of the earliest eras in the history of the Solar System
-- a time less than 10 million years after the birth of the Sun. The
missions, known as Lucy and Psyche, were chosen from five finalists
and will proceed to mission formulation, with the goal of launches
in 2021 and 2023, respectively. Lucy will visit the environment of
Jupiter's Trojan asteroids, while Psyche will study a unique metallic
asteroid that has not been visited before.
Lucy is scheduled to be launched in 2021 October. It is intended to
arrive at its first destination, a main-belt asteroid, in 2025. From
2027 to 2033, Lucy will explore six Trojan asteroids. Those asteroids
are trapped by Jupiter's gravity in two swarms that share the planet's
orbit, one leading and one trailing Jupiter by 60 degrees in its
12-year circuit round the Sun. The Trojans are thought to be relics
of a much earlier era in the history of the Solar System, and may have
been formed far beyond Jupiter's current orbit.
The Psyche mission will explore one of the most intriguing objects in
the main asteroid belt -- the big metal asteroid 16 Psyche, about
three times further away from the Sun than the Earth is. It is about
210 kilometres in diameter and, unlike most other asteroids, that are
rocky or icy bodies, is thought to be composed mostly of metallic iron
and nickel, like the Earth's core. Scientists wonder whether Psyche
could be the exposed core of an early planet that could have been as
large as Mars, but which lost its rocky outer layers owing to a number
of violent collisions millions of years ago. The mission will help
scientists to understand how planets and other bodies separated into
their layers -- including cores, mantles and crusts -- early in their
histories. Psyche is intended to be launched in 2023 October,
arriving at the asteroid in 2030, following an Earth-gravity-assist
manoeuvre in 2024 and a Mars-flyby in 2025.
STAR PREDICTED TO EXPLODE IN 2022
Earth and Sky
Astronomers have made an unprecedented prediction of a star explosion
due in the year 2022 or thereabouts; they say that it will be visible
from the Earth, even to those without telescopes. The star system is
an eclipsing binary known as KIC 9832227. New evidence suggests that
the two very close stars are getting closer and soon will merge
explosively. The prediction, originally made in 2015, is that the two
stars in the KIC 9832227 system will get closer and closer and finally
merge and explode in 2022, give or take a year. When that happens,
the star will increase its brightness ten thousandfold, temporarily
becoming a bright star. The star will be visible in the constellation
Cygnus, adding a star as bright as 2nd magnitude to the familiar
pattern.
The team's exploration of the KIC 9832227 system began in 2013 when
it looked at how the colour of the star correlated with brightness and
determined that it was definitely a binary. In fact, it was actually
a contact binary, in which the two stars share a common atmosphere,
like two peanuts sharing a single shell. A precise orbital period
(just under 11 hours) was determined from Kepler satellite data, and
astronomers were surprised that the period was slightly shorter than
that shown by earlier data. That result brought to mind work published
by astronomer Romuald Tylenda, who had studied the observational
archives to see how another star (V1309 Scorpii) had behaved before
it exploded unexpectedly in 2008 and produced a red nova (a type of
stellar explosion only recently recognized as distinct from other
types). The pre-explosion record showed a contact binary with an
orbital period decreasing at an accelerating rate. That pattern of
orbital change was a 'Rosetta stone' for interpreting the new data.
In the past two years, the team has been performing observational
tests, which ruled out other explanations and strengthened the belief
that KIC 9832227 will soon explode. The team believes that the
merging-star hypothesis should be taken seriously, and astronomers
should be using the next few years to study the system intensely, so
that if it does blow up we will know what led to that explosion.
To that end, the team will be observing KIC 9832227 in the next year
over the full range of wavelengths, using the Very Large Array, the
Infrared Telescope Facility, and the XMM-Newton spacecraft, to study
the star's radio, infrared and X-ray emission, respectively. The
orbital timing can be checked by amateur astronomers. They can
measure the brightness variations of this 12th-magnitude star as it
eclipses, and see for themselves if it is continuing on the schedule
that is predicted or not.
MISSING LINK NEUTRON STAR
NASA
Astronomers have found that a misfit 'skeleton' of a star may link two
different kinds of stellar remains. The object, called PSR J1119-6127,
has been caught behaving like two distinct objects -- a radio pulsar
and a magnetar -- and could be important to understanding their
evolution. A radio pulsar is type of a neutron star -- the extremely
dense remnant of an exploded star -- that emits radio waves in
predictable pulses owing to its fast rotation. Magnetars, by contrast,
have violent, high-energy outbursts of X-ray and gamma-ray light, and
their magnetic fields are the strongest known in the Universe. Since
the 1970s, scientists have treated pulsars and magnetars as two
distinct populations of objects, but in the last decade evidence has
emerged that they could be stages in the evolution of a single object.
The new study, combined with other observations of the object,
suggests that J1119 could be in a never-before-seen transition state
between radio pulsar and magnetar. When the object was discovered in
2000, it appeared to be a radio pulsar. It was mostly quiet and
predictable until last July, when the Fermi and Swift space
observatories observed two X-ray bursts and 10 additional bursts of
light at lower energies coming from the object. When the outbursts
happened, astronomers used the Deep Space Network 70-m radio telescope
in Canberra -- the largest dish in the southern hemisphere -- to see
what was going on. It is believed that the X-ray bursts happened
because the object's enormous magnetic field became twisted as the
object was spinning. The stress of a twisting magnetic field is so
great that it causes the outer crust of the neutron star to break --
analogous to tectonic plates on the Earth causing earthquakes. That
causes an abrupt change in rotation, which has been measured by
NuSTAR.
Neutron stars are so dense that one teaspoonful has the mass of a
mountain. The star's crust, roughly 1 kilometre thick, with higher
pressure and density at greater depths, is a neutron-rich lattice.
This particular neutron star is thought to have one of the strongest
magnetic fields among the population of known pulsars: a few times 10
to the 12 times stronger than the magnetic field of the Sun. Two
weeks after the X-ray outburst, astronomers tracked the object's
emissions at radio frequencies. The radio emissions had sharp
increases and decreases in intensity, allowing scientists to quantify
how the emission evolved. Researchers used an instrument, which they
informally call a 'pulsar machine', that was recently installed at the
same DSN dish in Australia. Within 10 days, something completely
changed in the pulsar. It started behaving like a normal radio pulsar
again. The question remains: which came first, the pulsar or the
magnetar? Some scientists argue that objects like J1119 begin as
magnetars and gradually stop outbursting X-rays and gamma rays over
time. But others propose the opposite theory: that the radio pulsar
comes first and, over time, its magnetic field emerges from the
supernova's rubble, and then the magnetar-like outbursts begin. To
help solve the problem, astronomers want to find more 'missing link'
objects like J1119. That particular object was probably formed by
a supernova 1,600 years ago. Monitoring similar objects may shed
light on whether the phenomenon is specific to J1119, or whether such
behaviour is common in that class of objects.
HUBBLE PROVIDES ROAD MAP FOR VOYAGERS' GALACTIC TREK
Space Telescope Science Institute (STScI)
The two Voyager spacecraft are hurtling through unexplored territory
on their trip beyond the Solar System. Along the way, they are
measuring the interstellar medium. The Hubble telescope is providing
the road map, by measuring the material along the probes' future
trajectories as they move through space. Even after the Voyagers run
out of electrical power and are unable to send back new data, which
may happen in about a decade, astronomers can use Hubble observations
to characterize their environment. A preliminary analysis of the
Hubble observations reveals a rich, complex interstellar ecology,
containing multiple clouds of hydrogen laced with other elements.
Hubble data, combined with the Voyagers, have also provided new
insights into how the Sun travels through interstellar space. The
Voyagers are sampling tiny regions as they travel through space at
roughly 38,000 miles per hour. But we have no idea if these small
areas are typical or rare. NASA launched the twin Voyager 1 and 2
spacecraft in 1977. Both explored the outer planets Jupiter and
Saturn, and Voyager 2 went on to visit Uranus and Neptune.
The pioneering Voyager spacecraft are currently exploring the
outermost edge of the Sun's domain. Voyager 1 is now in interstellar
space, the region between the stars that is filled with gas, dust, and
material recycled from dying stars. Voyager 1 is 13,000 million miles
from the Earth. In about 40,000 years (long after it ceases to be
operational, of course), it will pass within 1.6 light-years of the
star Gliese 445, in the constellation Camelopardus. Its twin, Voyager
2, is 10,500 million miles away, and will pass 1.7 light-years from
the star Ross 248 in about 40,000 years. For the next 10 years, the
Voyagers will be making measurements of interstellar material,
magnetic fields, and cosmic rays along their trajectories. Hubble
complements the Voyagers' observations by looking at two sight-lines
along each spacecraft's path to map interstellar structure along their
routes. Each sight-line stretches several light-years to nearby
stars. Sampling the light from those stars, Hubble's 'Imaging
Spectrograph' measured how interstellar material absorbed some of the
starlight, leaving telltale spectral fingerprints. Hubble found that
Voyager 2 will move out of the interstellar cloud that surrounds the
Solar System in a couple of thousand years.
FARTHEST STARS IN MILKY WAY
Harvard-Smithsonian Center for Astrophysics
The 11 farthest known stars in our galaxy are located about 300,000
light-years away, well outside the Milky Way's spiral disc. New
research by Harvard astronomers shows that half of those stars might
have come from another galaxy, the Sagittarius dwarf. Moreover, they
are members of a long stream of stars extending for a million light-
years across space -- 10 times the diameter of our Galaxy. The
Sagittarius dwarf is one of dozens of mini-galaxies that surround the
Milky Way. Over the age of the Universe it has made several loops
around our Galaxy. On each passage, the Milky Way has raised
gravitational tides on the smaller galaxy, pulling it apart.
Researchers used computer models to simulate the movements of the
Sagittarius dwarf over the past 8 billion years. They varied the
initial velocity and angle of approach to the Milky Way to determine
what best matched current observations. At the beginning of the
simulation, the Sagittarius dwarf had a mass of about 10 to the 10
times the mass of our Sun, or about one per cent of the Milky Way's
mass. The calculations showed that, over time, the dwarf galaxy lost
about a third of its stars and a full nine-tenths of its dark matter.
That resulted in three distinct streams of stars that reach as far as
a million light-years from the Milky Way's centre. They stretch all
the way out to the edge of the Milky Way halo and display one of the
largest structures observable on the sky. Moreover, five of the 11
most distant stars in our Galaxy have positions and velocities that
match what would be expected of stars stripped from the Sagittarius
dwarf. The other six do not appear to be from Sagittarius, but might
have been removed from a different dwarf galaxy. Mapping projects
like the Sloan Digital Sky Survey have charted one of the three
streams predicted by the simulations, but not to the full extent that
the models suggest. Future instruments like the Large Synoptic Survey
Telescope, which will detect much fainter stars across the sky, may be
able to identify the other streams.
PHOTONS STRUGGLE TO ESCAPE DISTANT GALAXIES
RAS
Astronomers have discovered giant haloes around early Milky-Way-type
galaxies, made of photons that have struggled to escape them. In
order to understand how our own Milky Way galaxy formed and evolved,
astronomers rely on observing distant galaxies. With the most distant
galaxies, only one spectral feature typically stands out, the Lyman-
alpha line associated with hydrogen gas. Newly born stars in very
distant galaxies are hot enough to ionize hydrogen in surrounding
clouds of gas, which then shine brightly in Lyman-alpha light, in
theory the strongest such feature observable in a distant galaxy. Yet
in practice, Lyman-alpha photons struggle to escape galaxies, as gas
and dust block and deviate their travel paths, making it a complex
process to understand. Using the Isaac Newton telescope on La Palma,
astronomers developed a unique experiment to study almost 1000 distant
galaxies. They surveyed the sky with the Wide-Field Camera and
custom-made filters, in order to measure where the Lyman-alpha is
produced, how much of it there is, and where it comes out of galaxies.
The results show that only 1-2% of those photons escape from the
centres of galaxies like the Milky Way. Even if we account for all the
photons at a large distance from the centre, fewer than 10% escape.
Galaxies forming stars in the distant Universe seem to be surrounded
by an impressively large, faint halo of Lyman-alpha photons that had
to travel for hundreds of thousands of light-years in an almost
endless series of absorption and re-emission events, until they were
finally free. Scientists now need to understand exactly how and why
that happens. When the James Webb space telescope begins operation in
2018, astronomers expect to be able to look even further back in time,
opening up a new window on the first galaxies and stars. Studying how
the escape fraction evolves over time can tell us about the kind of
stars producing the Lyman-alpha photons, and the properties of inter-
stellar and intergalactic gas.
Topic: Late January Astronomy Bulletin (Read 1893 times)