SKY REPORTER: The (not-so lonely) Star of Autumn

Fomalhaut System, with primary star at the center of dark masking disk preventing its dazzle from overexposing other parts of the image. Also shown are the system’s major dust ring and a recent orbital position of Fomalhaut b.
Credit: NASA

It was tempting to add the adjective “lonely” to my description of Fomalhaut as “The Star of Autumn”. Watching it on October evenings, the closest stellar object we see rivaling Fomalhaut’s first magnitude brightness is Altair in Aquila, fully 60 degrees of arc to the northwest. However, from a cozy telescopic perspective, Fomalhaut has close and controversial company. The star has two gravitationally tethered stellar companions, but it’s an elusive object called Fomalhaut b that prompts our attention.

Bright Fomalhaut that we may see with unaided eyes on autumn nights is telescopically revealed to be surrounded by a dusty slightly off-center disk of debris. In 2006 this lack of symmetry was considered by University of Rochester astrophysicist Alice Quillen; who speculated gravitational influences of a then unknown planet orbiting the star caused the disk’s displacement. Such a “planet” was soon discovered.

The mystery planet appeared on visual light images provided by the Hubble Space Telescope’s Advanced Camera for Surveys, and noted in a study led by Paul Kalas, adjunct professor of astronomy at U.C. Berkeley. Kalas’ announcement in 2008 suggested Fomalhaut b was indeed an extrasolar planet, with mass no more than three times that of Jupiter, orbiting in the vicinity of the system’s dust disk.

Astrophysicist Sara Seager of MIT, a 2013 MacArthur Fellow, and major innovator in the now burgeoning field of extra-solar planet atmospheric study; in 2010 described Fomalhaut b as one of the very few generally accepted examples of a directly imaged extra-solar planet. The object appears as a point of light rather than being some unseen, inferred instigator of radial velocity wobbles, or invisible cause of minute dips in stellar brightness. However, subsequent studies during the following several years, using various observing systems and techniques, provided conflicting interpretations of this supposed planet’s nature.

Questions arose as to whether Fomalhaut b’s light was merely dust scattered starlight or an actual indicator of dust and atmospheric gas surrounding a solid planet. Some seemed to favor the notion object “b” was a temporary swarm of orbiting particles, lumped in a relatively small volume of space. Another concept suggested Fomalhaut b comprised fragments from a collision, perhaps between a comet and something similar to objects in the Solar System’s Kuiper Belt.

However, the tide of evidence and opinion has again turned. Studies reported this past year provide renewed support for the original notion Fomalhaut b is indeed an extra-solar planet. Thayne Currie, a postdoctoral fellow at the University of Toronto, used fresh observational data and also re-examined earlier Hubble images to conclude elusive Fomalhaut b is not a transient patch of dust but instead, in his carefully chosen words: “a planet identified from direct imaging”. It’s described as a world with a mass less than twice that of Jupiter, surrounded by a cloud of dust particles scattering starlight. With this reassertion of planetary identity some have now nicknamed Fomalhaut b “The Phoenix Planet”!

The Moon

Planets

Venus is in the southwestern sky as twilight fades, setting two hours after the Sun at midmonth. Mercury may be seen during the first half of October low in the west during evening twilight. This fastest orbiting planet joins Saturn near the Sun’s direction by month’s end.

Mars rises around three a.m. at the middle of October, but is still rather unremarkable at visual magnitude 1.5. This month Jupiter arrives in the late evening sky, rising by midnight at the beginning of this month, and around 10:20 pm by Halloween.

During a half hour period after sunset Sunday October 6th a very thin two day old crescent moon may be seen two degrees of arc to the upper right of Mercury, low in the west-southwest. Saturn is then four degrees above and to the left of the moon. The next evening, around those same twilight times, a slightly wider lunar crescent will have moved halfway between Saturn and the much brighter planet Venus.

October 9th Mercury will be at its greatest eastern elongation from the sun, 25 degrees of arc measured along the ecliptic. However, due to the low angle of the zodiac around sunset at this time of the year, Mercury will be only seven degrees above the horizon when the sun sets that night. The next evening, Mercury will pass about 5 degrees from Saturn, both close to the horizon in fading civil twilight.

The night of Tuesday October 15th, a bright waxing gibbous moon is positioned just south of the Great Square of Pegasus; with Fomalhaut about three fist lengths (seen with arm fully extended) south of the moon

October’s Full Moon occurs this year on Friday the 18th. It’s traditionally known as the Hunter’s Moon and that night will be located among stars of the constellation Pisces, the fish.

After midnight on Friday and Saturday nights of the 25th and 26th, the moon is near Jupiter which shines brightly at magnitude –2.39 among stars of Gemini.

The early mornings of Tuesday and Wednesday October 29th and 30th present a waning crescent moon adjacent to Mars, then in the constellation Leo.

Comet ISON (C/2012 S1)

It’s not brightening to the degree anticipated earlier in the year. On the first of October comet ISON moved within the orbit of Mars, as it races toward an encounter with the Sun November 28th.