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January Night Sky

January 2010 Night Sky

—Charlie Christmann, Signpost

Extra-Solar Systems

It seems that exoplanets are being discovered every few days. We know of more than four hundred planets orbiting nearby stars. As of December 18, 2009, the Extrasolar Planets Encyclopedia lists 415 of them ( Many of the exoplanets discovered so far orbit stars that are relatively close to us, and therefore are visible to the naked eye or with binoculars. This month’s star chart points out the eight closest star systems visible in January with planets and points out where to look for these planetary systems.

Epsilon Eridani is only 10.5 light-years away from Earth and has one known planet orbiting it. This is the closest star to us in the constellation Eridanus and the third closest star visible to the naked eye. Its estimated age is around one billion years old and it is highly active. The planet, Epsilon Eridani b, was discovered in 2000 using the radial velocity method. It is around 0.86 times the mass of Jupiter. The planet’s orbit is highly elliptical, ranging from the equivalent of Jupiter to Venus.

In the constellation Geminus (the twins), Pollux has an orbiting planet. This giant orange star is thirty-four light-years from Earth. Its planet was discovered in 2006 and is 2.3 times larger than Jupiter. The orbit is nearly circular, taking 590 days to complete each circuit.

Gamma Cephei (Alrai), 38.5 light-years away, has a planet 1.6 times larger than Jupiter in a near circular orbit taking 902 days to circle the star. It was first noticed in 1988, but could not be confirmed until 2002. Gamma Cephei is the naked-eye star that will succeed Polaris as the Earth’s northern pole star around 3000 AD.

Discovered in 1996, 47 Ursae Majoris b orbits its parent star in a 1,095 day orbit, ranging from the equivalent of Mars to Jupiter. Its mass is about 2.6 times that of Jupiter. 47 Ursae Majoris is a solar twin. This yellow dwarf star is approximately forty-six light-years away from Earth in the constellation of Ursa Major. As of 2001, a second planet, “c,” was discovered. This second planet is 0.76 times the size of Jupiter.

Upsilon Andromedae is a double star system located forty-four light-years distant from Earth. The primary star, Upsilon Andromedae A, is a yellow-white dwarf star that is somewhat younger than the Sun. Its companion, Upsilon Andromedae B, is a red dwarf located in a wide orbit. As of 2008, three known planets orbit the primary star. The inner planet is seventy-one percent the mass of Jupiter, the middle planet is 2.1 times Jupiter’s mass, and the outer planet is 4.6 times more massive then Jupiter.

In the constellation Cancer is the star 55 Cancri, forty-one light-years away from Earth. The system consists of a yellow dwarf star and a smaller red dwarf star. This star has a family of five known planets orbiting the yellow dwarf in the twin star system. Masses range from eighteen times that of Earth to four times Jupiter. The outer planet, “f,” is half the mass of Jupiter in a 260-day orbit, right in 55 Cancri A’s habitable zone.

51 Pegasi is a Sun-like star located 50.9 light-years from us. Its planet was discovered in 1995. At 7.5 billion years old, somewhat older than the Sun, and a bit more massive, it is starting to run low on its hydrogen fuel. The planet 51 Pegasi b orbits very close to its parent star and has an estimated surface temperature of 3800° F. Its mass is about half of Jupiter’s.

Gleise 777 is a yellow sub-giant, 6700 million-year-old Sun-like star that is ceasing fusing hydrogen in its core. The star is fifty-two light-years away. Its inner planet is 1.5 times the mass of Jupiter, orbiting the star every 2,900 days. A second planet was discovered with a mass about eighteen times Earth, circling in a close seventeen-day orbit.

The Planets and the Moon

Mid-month, look for Mercury low in the east thirty minutes before sunrise. On the 13th, find Mercury six degrees to the upper left of the Moon.

Venus is very close to the sun at sunset in the west and will be dangerous to view.

Earth reaches its closest approach to the Sun on the 2nd at 91.4 million miles at 2:00 p.m.

Mars will be rising in the east three hours after sunset. On the 2nd, look for the Moon to join Mars two hours after sunset. Look for a second Moon-Mars conjunction on the 29th. Castor and Pollux will be directly above the Moon both days. Also on the 29th, the Sun, Earth, and Mars are all in a line in what is called “opposition.”

Jupiter sets between 8:45 p.m. early in the month to 7:15 p.m. late in the month. Look in the west for this bright planet. The Moon-Jupiter conjunction will be on the 17th; look for it ninety minutes after sunset.

Saturn is rising in the east at 11:40 p.m. early in the month to 9:40 p.m. late in the month. Saturn will be eight degrees above the waxing gibbous Moon on the 6th.

On New Year’s Day, the Moon is at Perigee (closest to Earth in its orbit), and again on the 30th. Lunar apogee (farthest from Earth) is on the 16th. The New Moon is on the 15th and the Full Moon is on the 30th.





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