Around the Sky in 28 Days – Day 11

The Moon is traversing  a region of the sky devoid of many bright stars. If it weren’t for brilliant Jupiter, many folks in bright cities might not see much in this part of the sky. Luckily there is a single bright star located to the south and east (lower left) of the Moon. Its isolation has led it to be referred to as the “lonely star of autumn”.

Fomalhaut (Arabic for “mouth of the fish”) is located in the constellation Piscis Austrinus (the “southern fish”). It is the 18th brightest star in the sky at magnitude +1.2. Shining with a solid white light, Fomalhaut is a young A-type star twice as massive as the Sun and 18 times as bright. It is also one of the closer stars only being located at a distance of 25 light-years.

Recent observations by the Hubble Space Telescope has revealed much about Fomalhaut’s “solar system”. As the image below shows, Fomalhaut possesses a bright dust disk, probably produced by an equivalent of our solar system’s Kuiper belt. Unlike most exoplanetary systems where planets are indirectly deduced, Fomalhaut has one of the few planets ever directly seen in an exosolar system.

Image of the Fomalhaut system taken with the Hubble Space Telescope. Credit: NASA, STSCI.


The planet, now tagged Fomalhaut b, orbits Fomalhaut in 872 at an average distance of 115 AU. Its mass is loosely constrained to be between 0.05 and 3 Jupiter masses. An infrared excess at the position of ‘b’ suggests that 20-40 Jupiter masses worth of material is orbiting the planet so it is likely it has or is forming a collection of moons not unlike Jupiter’s. Since the orbit of the ‘b’ does not exactly match the shape of the dust disk, it has been suggested that additional unseen planets are also involved in sculpting the dust disk. Of all the stellar systems out there, this may be the funniest one to watch as new telescopes and imaging technologies come online.