FRIENDS OF THE PLANETARIUM NEWSLETTER – JUNE 2014
Astronomy is a science pursued at a distance. Most of the light we see from distant stars and galaxies
takes thousands to millions of years to reach us. That makes our solar neighbourhood a valuable place
for detailed observations. The closest companions to the Sun are benchmarks, because they are the
easiest stars to study in detail. While the census of the solar neighbourhood has tallied more stellar
citizens over time, most of the newly discovered neighbours have been relatively distant, usually at least
30 to 60 light-years away. Recently however, an astronomer from Penn State discovered a solar
neighbour about 7 light-years away, and it’s a
"cool" result in more ways than one. Using data
from NASA's Wide-field Infrared Survey Explorer
(WISE) and Spitzer Space Telescope, Kevin
Luhman recently discovered an object known as
WISE J085510.83−071442.5. The object is special
for quite a few reasons. First, it is right next door,
in astronomical terms. At 7.2 light-years away, this
is likely the fourth closest stellar system ever
detected, farther only than the Alpha Centauri
triple system (4.2 light-years), Barnard’s star (5.9
light-years) and the brown dwarf binary WISE
J104915.57−531906.1 (6.6 light-years). It
displaces Wolf 359 (known to all Star Trek fans),
which lies 7.8 light-years away. Second, it’s moving fast. Using infrared images obtained by WISE and
Spitzer, Luhman noticed the object was traveling extremely quickly across the sky in between images. .
The object is traversing 8.1 arcseconds per year, the third largest proper motion of any object outside
the solar system (second only to Barnard’s star and Kapteyn’s star). In comparison, most of the
brightest stars have a proper motion of a few tenths of an arcsecond per year or less. The other thing
that makes WISE J0855−0714 "cool" is that it really is cold! Using images of the object taken in different
filters, Luhman estimated its temperature to be about 250 Kelvin, or about 23 degrees below zero on
the Celsius scale. This makes WISE J0855−0714 not only the coldest neighbour to the Sun but also the
coldest brown dwarf ever discovered. This combination of close, fast, and cold makes WISE J0855−0714
unique among all of the solar neighbourhood members.
Stars like our Sun form in groups. We see evidence for this throughout the Milky Way, most famously in
the Trapezium Cluster in the Orion Nebula. But after stars begin to emit their nascent light, their
gravitational interactions with other nearby "siblings" send them out from their birth cluster and into
the expanses of our galaxy. Even though they travel thousands of light-years from where they first
formed, stars carry the signature of their birthplace in the detailed chemical composition contained in
their atmospheres and their motions through space. Astronomers have been searching for the lost
siblings of the Sun for some time, and they’ve found several candidates. Now, using these lines of
evidence, they think they’ve confirmed one. After extensive tests, which looked at the comparative
abundance of various elements, only two of the 30 candidates showed a match to the Sun's chemical
composition, and only one, HD 162826, was close to the Sun at the time it formed. This therefore makes
HD 162826 the best "solar sibling" candidate to date. This star is about 15 percent more massive than
the Sun, making it one of the Sun's big brothers. At 110 light-years it is also relatively nearby, shining at
magnitude 6.7 in the constellation Hercules. Although HD 162826 is slightly more massive than the Sun,
many of the solar siblings are likely to be low-mass M dwarfs, since these are the most common type of
star made during star formation.
Using NASA's Kepler Space Telescope, astronomers have discovered the first Earth-size planet orbiting
in the "habitable zone" of another star. The planet, named "Kepler-186f" orbits an M dwarf, or red
dwarf, a class of stars that makes up 70 percent
of the stars in the Milky Way galaxy. The
discovery of Kepler-186f confirms that planets
the size of Earth exist in the habitable zone of
stars other than our sun. The "habitable zone" is
defined as the range of distances from a star
where liquid water might pool on the surface of
an orbiting planet. While planets have
previously been found in the habitable zone, the
previous finds are all at least 40 percent larger
in size than Earth and understanding their
makeup is challenging. Kepler-186f is more
reminiscent of Earth. It orbits its parent M dwarf
star once every 130-days and receives one-third the energy that Earth gets from the sun, placing it
nearer the outer edge of the habitable zone. On the surface of Kepler-186f, the brightness of its star at
high noon is only as bright as our sun appears to us about an hour before sunset. It does however have
many properties that resemble Earth. It can be thought of as an Earth-cousin rather than an Earth-twin.
Kepler-186f resides in the Kepler-186 system, about 500 light-years from Earth in the constellation
Cygnus. The system is also home to four companion planets: Kepler-186b, Kepler-186c, Kepler-186d,
and Kepler-186e, whiz around their sun every four, seven, 13, and 22 days, respectively, making them
too hot for life as we know it. These four inner planets all measure less than 1.5 times the size of Earth.
A global network that listens for telltale traces of nuclear weapons tests detected 26 explosions
between 2000 and 2013 caused not by nuclear explosions but by asteroids blasting apart in the
atmosphere. Only one impact, the 2013 strike over Chelyabinsk, Russia, caused widespread injuries and
damages on the ground, but the study is a somber reminder that Earth's lucky days may be numbered.
The B612 Foundation, named after a fictional planet in the book The Little Prince by French author and
pilot Antoine de Saint-Exupery, plans to launch a telescope in 2018 to scope out potentially dangerous
asteroids, such as the one that exploded over Chelyabinsk with the force of 600 kilotons of energy.
Flying glass and debris injured more than 1000 people. In comparison, the atomic bomb dropped on
Hiroshima in 1945 had an energy impact of 15 kilotons. During its initial five and a half years of
operation, B612's telescope, called Sentinel, should be able to find 90 per cent of near-Earth asteroids
140 meters in diameter or larger, and about 50 per cent of asteroids 40 meters in diameter.
In an unrelated sky survey, astronomers have found the first known asteroid with rings. About 248 km
across, 10199 Chariklo is the largest known Centaur, one of a class of objects orbiting primarily
between Saturn and Neptune. While Centaurs share the characteristics of both comets and asteroids,
Chariklo itself behaves more like an asteroid. It likely orbited well beyond Neptune and was scattered
into its current location after a close encounter with one of the outer planets less than 10 million years
ago. The ring system consists of two sharply confined rings. The first is about 265 km from Chariklo’s
surface and only 7 km wide. There’s a 9-km gap before reaching the outer ring, which is only 3 km wide.
The new discovery resolves previous puzzling observations. In 2008, Chariklo mysteriously dimmed to
nearly half its brightness and its spectral signature due to water gradually disappeared. But now it’s
clear why: the ice-rich ring system became edge on during that time, almost disappearing entirely from
view. Chariklo's ring particles (while significantly darker than most of Saturn's rings but brighter than
those around Uranus) must therefore constitute a substantial fraction of the system's total light. As this
is the first small solar-system body detected with rings, astronomers remain unsure if the rings form
from some generic yet unknown process or are simply exceptional features circling few objects.
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