SOME ASTRONOMERS ANTICIPATE 100 BILLION EARTH-LIKE PLANETS
RAS
Researchers have proposed a new method for finding Earth-like planets,
and they imagine that the number could be of the order of 100 billion.
The strategy uses a technique grandly called MOA (Microlensing
Observations in Astrophysics) at the Mt. John Observatory in New
Zealand.
The Kepler spacecraft finds Earth-sized planets that are quite close
to parent stars, and there have been wild estimates that there are 17
billion such planets in the Milky Way. They are generally much hotter
than the Earth, although some could be of a similar temperature if
they are orbiting red dwarfs. The proposal is to use the MOA method
to estimate the number of Earth-mass planets orbiting stars at
distances typically twice the Sun-Earth distance. Those planets will
therefore be cooler than the Earth. By interpolating between the
Kepler and MOA results, astronomers might get an estimate of the
number of Earth-like planets in the Galaxy. Of course, that number
will refer merely to planets whose temperatures are reasonable and
will still not tell us about any of the other characteristics that
make the Earth habitable, so it is not at all the same thing as
finding planets that could support life, let alone finding inhabited
planets.
The first planet (other than our own) orbiting a Sun-like star was not
found until 1995. That reflects the difficulty of detecting from a
distance a tiny non-luminous object like the Earth orbiting a bright
object like the Sun. Whereas Kepler measures the loss of light from a
star when a planet passes between us and the star, microlensing
depends on the deflection of light from a distant star that passes
close to a planet en route to Earth -- an effect predicted by Einstein
in 1936. In recent years, microlensing has detected several planets
as large as Neptune and Jupiter. Astronomers have proposed a new
microlensing strategy for detecting the tiny deflections caused by
Earth-sized planets. Simulations indicated that Earth-sized planets
were more likely to be detectable if a worldwide network of moderate-
sized, robotic telescopes were available to monitor the potential host
stars. Coincidentally, just such a network of 1-m and 2-m telescopes
is now being deployed by the Las Cumbres Observatory Global Telescope
Network (LCOGT). That network is used to study microlensing events in
conjunction with the Liverpool Telescope in the Canary Islands. It is
expected that the data from those telescopes will be supplemented by
measurements made with the existing 1.8-m MOA telescope at Mt John,
the 1.3-m Polish telescope in Chile known as OGLE, and the recently
opened 1.3-m Harlingten telescope in Tasmania.
WHITE DWARF WARPS LIGHT OF RED-DWARF COMPANION
Cornell University
Astronomers using the Kepler space telescope, and also making
measurements of stars' ultraviolet activity, have observed the effects
of a white-dwarf star bending the light of its companion red star.
The white dwarf (the burnt-out core of what used to be a star like our
Sun) is in orbit with a small red dwarf star. The white dwarf is
tiny, only about the size of the Earth, but it has the mass of the
Sun. When it passes in front of its companion, its gravity bends the
path of the latter's light and causes the system to brighten
perceptibly. The research team used ultraviolet measurements of a
star called KOI (Kepler Object of Interest) 256, taken by the Galaxy
Evolution Explorer (GALEX), which measures ultraviolet activity in all
the stars in the Kepler field of view. The orbital period is just 1.4
days, so short that the stars must previously have undergone a
'common-envelope' phase in which the red dwarf orbited within the
outer layers of the star that later formed the white dwarf.
Kepler's primary task is to scan stars in search of orbiting planets.
As the planets pass by, they block the starlight by minuscule amounts,
which Kepler's detectors can see. So far, Kepler has identified more
than 2,700 planet candidates. Still ongoing is the mission's search
for planets that are similar to the Earth in size and temperature and
orbit a star like our Sun. Ultimately, Kepler may enable us to learn
how common Earth-size planets are in the Milky Way galaxy. To learn
more about KOI 256, the team also used the Hale telescope at Palomar
Observatory. Radial-velocity changes of the red star proved to be too
large to arise from a planet and required the companion object to be a
white dwarf. It is massive enough to give rise to a phenomenon called
gravitational lensing, which can cause temporary increases in the
apparent brightness of an object that passes exactly behind the
lensing one. In the new study, scientists were able to use gravita-
tional lensing to infer the mass of the white dwarf, and from that to
estimate also the mass of the red dwarf and the physical sizes of both
stars.
EVOLVED STAR FOUND WITH PLANETS AND DEBRIS DISC
RAS
The Herschel space observatory has provided the first images of a dust
belt produced by colliding comets or asteroids orbiting a subgiant
star known to host a planetary system. After billions of years
steadily burning hydrogen in their cores, stars like our Sun exhaust
that central fuel reserves and start burning them in shells round the
core. They swell to become subgiant stars and then red giants. At
least during the subgiant phase, planets, asteroids and comet belts
around such evolving stars are expected to survive. Discs of dust,
thought to be generated by collisions between populations of asteroids
or comets, have been found around some stars. Thanks to the far-
infrared detection capabilities of the Herschel space observatory,
astronomers have been able to resolve bright emission around the star
Kappa Coronae Borealis, indicating the presence of a dusty debris
disc. The star has a mass of 1.5 solar masses, is around 2.5 billion
years old and is 100 light-years distant. From ground-based
observations, it is known to host one giant planet roughly twice the
mass of Jupiter orbiting at a distance equivalent to the asteroid belt
in our own Solar System; a second planet is suspected.
This is the first subgiant to be found with a debris disc and one or
more planets. The disc is presumed to have survived for the star's
entire lifetime without being destroyed. That is very different from
our own Solar System, where most of the debris was cleared away in a
phase called the Late Heavy Bombardment era, around 600 million years
after the Sun formed. There are ambiguities, not so far resolved, in
models of the configurations for the disc and planet that fit
Herschel's observations of Kappa CrB.
BLACK-HOLE BINARY ORBITING IN 2.4 HOURS
NASA
ESA's XMM-Newton space telescope has helped to identify a star and a
black hole that orbit each other once every 2.4 hours. The black hole
in this compact pairing, known as MAXI J1659-152, is at least three
times more massive than the Sun, while its red-dwarf companion star
has a mass only 20% that of the Sun. The pair is separated by roughly
a million kilometres. It was discovered in 2010 by the Swift space
telescope and was initially thought to be a gamma-ray burst. Later
the same day, Japan's MAXI telescope on the International Space
Station found a bright X-ray source at the same place. More
observations from ground and space telescopes, including XMM-Newton,
showed that the X-rays came from a black hole feeding from material
derived from a tiny companion. Several regularly-spaced dips in the
emission were seen in an uninterrupted 14.5-hour observation with
XMM-Newton, caused by the uneven rim of the black hole's accretion
disc briefly obscuring the X-rays as the system rotates, its disc
almost edge-on to the line of sight. From the dips, an orbital
period of just 2.4 hours was measured, the shortest known for black-
hole X-ray binary systems. The previous shortest, Swift J1753.5-0127,
has a period of 3.2 hours.
The black hole and the star orbit round their common centre of mass.
Because the star is the less-massive object, it lies further from that
point and travels in its larger orbit at a speed of more than 500 km/s
-- the fastest-moving star known in an X-ray binary system; the black
hole orbits at 'only' 40 km/s. The pair lies high above the Galactic
plane, out of the main disc of our spiral Galaxy, an unusual
characteristic shared only by two other black-hole binary systems,
including Swift J1753.5-0127. The high Galactic latitudes and short
orbital periods may represent signatures of a potential new class of
binary system, objects that may have been kicked out of the Galactic
plane during the explosive formation of the black holes themselves.
'POST-MORTEM' YIELDS INSIGHT INTO KEPLER'S SUPERNOVA
NASA
A 'type Ia' supernova observed in 1604 by the German astronomer
Johannes Kepler held a greater fraction of heavy elements than the
Sun, according to an analysis of X-ray observations from the Japan-led
Suzaku satellite. The supernova left a remnant in the form of a shell
of hot, rapidly expanding gas. Spectroscopic analysis of the remnant
can give a picture of the composition of the star before it blew up.
The Suzaku astronomers observed the remnant in 2009 and 2011. With a
total effective exposure of more than a fortnight, the X-ray spectrum
reveals several faint emission features from highly ionized chromium,
manganese and nickel in addition to a bright emission line from iron.
Cosmologists regard type-Ia supernovae as 'standard candles' because
they all release similar amounts of energy. By comparing this
standard to the observed peak brightness of a type-Ia supernova,
astronomers can estimate its distance. Their similarity stems from
the fact that the exploding star is always a white dwarf. Although a
white dwarf is perfectly stable on its own, in a binary system with
another white dwarf or a normal star an instability may occur. The
normal star may transfer material onto the white dwarf, where it
gradually accumulates, or the orbits of binary white dwarfs may shrink
until the two objects merge. Either way, once a white dwarf reaches
about 1.4 times the Sun's mass, a supernova explosion soon follows.
Somewhere within the white dwarf, carbon nuclei begin merging
together, forming heavier elements and releasing a vast amount of
energy. A wave of nuclear fusion rapidly propagates throughout the
star, shattering it in a brilliant explosion that can be detected
billions of light-years away.
Astronomers can track some details of the white dwarf's composition by
determining the abundance of certain trace elements, such as
manganese, that formed during the explosion. Specifically, the ratio
of manganese to chromium produced by the explosion turns out to be
sensitive to the presence of a neutron-rich isotope of neon, neon-22.
The star's neon-22 content is a guide to the abundance of all other
elements heavier than helium, which astronomers lump together as
'metals'. The findings provide strong evidence that the original
white dwarf possessed roughly three times the metallic content of the
Sun. Progressive stellar generations seed interstellar gas with
increasing proportions of metals. The remnant, which lies about
23,000 light-years away toward the constellation Ophiuchus, lies much
closer to our Galaxy's crowded central region than we do. There, star
formation was probably more rapid and efficient. As a result, the
star that blazed up as Kepler's supernova probably formed out of
material that was already enriched with a higher fraction of metals.
While the Suzaku results do not directly address which type of binary
system triggered the supernova, they indicate that the white dwarf was
probably no more than a billion years old when it exploded, or less
than a quarter of the Sun's current age. Theories indicate that the
star's age and metal content affect the peak luminosity of type-Ia
supernovae. Younger stars are likely to produce brighter explosions
than older ones, which is why understanding the spread of ages among
type-Ia supernovae is important.
TESS PROJECT TO SEARCH FOR TRANSITING EXOPLANETS
Massachusetts Institute of Technology
Following a three-year competition, NASA has selected the Transiting
Exoplanet Survey Satellite (TESS) project at MIT for a planned launch
in 2017. The project will use an array of wide-field cameras to
perform an all-sky survey to discover transiting exoplanets, ranging
from Earth-sized planets to gas giants, in orbit around the brightest
stars in the Sun's neighbourhood. TESS will carry out the first
space-borne all-sky transit survey, covering 400 times as much sky as
any previous mission. It is expected to identify thousands of new
planets in the solar neighbourhood, with a special focus on planets
comparable in size to the Earth. TESS relies upon a number of
innovations developed by the MIT team over the past seven years such
as devising a clever orbit for the spacecraft -- one which is not too
close, and not too far, from both the Earth and the Moon. As a
result, once a fortnight TESS will approach close enough to the Earth
for high downlink data-rates, while it remains above the harmful
radiation belts. The special orbit will remain stable for decades,
keeping TESS's cameras in a very stable temperature range. With TESS,
it should be possible to study the masses, sizes, densities, orbits
and atmospheres of a large number of small planets.
NASA MARS ORBITER IMAGES MAY SHOW 1971 SOVIET LANDER
NASA
While following news about Mars and the Curiosity rover, Russian
citizen enthusiasts found four features in a five-year-old image from
the Mars Reconnaissance Orbiter (MRO) that resemble four pieces of
hardware from the Soviet Mars 3 mission: the parachute, heat shield,
terminal retro-rocket and lander. A follow-up image by the orbiter
from last month shows the same features. The Mars 3 lander
transmitted for several seconds after landing on 1971 Dec. 21, the
first spacecraft to survive a Mars landing long enough to transmit
anything. Images of the possible Mars 3 features are available at
http://uahirise.org/ESP_031036_1345 and
http://photojournal.jpl.nasa.gov/catalog/PIA16920 . Together, this
set of features and their layout on the ground provide a remarkable
match to what is expected from the Mars 3 landing, but alternative
explanations for the features cannot be ruled out. In 1971, the
former Soviet Union launched the Mars 2 and Mars 3 missions to Mars.
Each consisted of an orbiter plus a lander. Both orbiter missions
succeeded, although the surface of Mars was obscured by a planet-
encircling dust storm. The Mars 2 lander crashed. Mars 3 became the
first successful soft landing on the planet, but stopped transmitting
for unknown reasons after just 14.5 seconds. The predicted landing
site was at latitude 45 degrees south, longitude 202 degrees east, in
Ptolemaeus Crater. MRO obtained a large image at that location in
2007 November; it contains 1.8 billion pixels of data, so about 2,500
typical computer screens would be needed to view the entire image at
full resolution. Promising candidates for the hardware from Mars 3
were found on 2012 Dec. 31. The candidate parachute is the most
distinctive feature in the images, spread out over the surface. In a
second image, the parachute appears to have brightened over much of
its surface, probably owing to its better illumination over the
sloping surface, but it is also possible that the parachute did
actually brighten in the intervening years because dust was removed.
The descent module, or retro-rocket, was attached to the lander
container by a chain, and the candidate feature has the right size and
even shows a linear extension that could be a chain. Near the
candidate descent module is a feature with the right size and shape to
be the actual lander, with four open petals. The image of the
candidate heat shield matches a shield-shaped object of the right size
if partly buried.
Topic: Late April Astronomy Bulletin (Read 2173 times)