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.
Topic: Late August Astronomy Bulletin (Read 2636 times)