The Fomalhaut System For Dummies
What is the most interesting planetary system in the sky? Some will say Alpha Centauri’s three stars and three planets, which are three light years away. Others will say it’s Epsilon Eridani’s darling dust rings, sporting a gas giant. Many will say it’s the full solar systems like Mu Arae, 55 Cancri, or even our very own sun. However, in the trenches of the southern skies lies a record-breaking gem in the constellation Piscis Austrinus. Fomalhaut, Alpha Piscis Austrini: A system with so many quirks and surprises that an astronomer could likely spend their entire life marveling at it.
The eighteenth brightest light in the night sky, Fomalhaut is a blue main-sequence star (spectral class A3V) lying just twenty-five lightyears away from Earth. However, Fomalhaut’s main claim to fame is its luscious dust rings, which are easily visible using NASA’s high-resolution telescopes. Fomalhaut’s layers of rings range from 0.08–150 Astronomical Units (AU) away from the star, with the distance between the Earth and the Sun being one mere astronomical unit. In order to put the size of these rings into a better perspective, they extend three to five times farther out than the Kuiper Belt and are about 80,000 times the size of Saturn’s rings. Consequently, if Saturn’s rings were just one inch long, Fomalhaut’s rings would be a bit longer than the Kentucky Derby. Fomalhaut’s dust ring is also far from circular, with many people comparing images of it to the infamous “Eye of Sauron”. (see the image at the top of the article for reference)
With Fomalhaut’s rings being so large, one can only expect them to be a host for planets. In fact, in 2008, the planet Fomalhaut b (properly named “Dagon” by the IAU) was discovered orbiting the planet at the epicenter of its dust rings using direct imaging. Not only was Fomalhaut b the first exoplanet ever discovered using this technique, but it was also the first exoplanet ever detected around an A-type star. Upon discovering Fomalhaut b, astronomer Paul Kalas stated, “It’s a profound and overwhelming experience to lay eyes on a planet never before seen. I nearly had a heart attack at the end of May when I confirmed that Fomalhaut b orbits its parent star.” The planet’s existence couldn’t be “confirmed” until the year 2012, with the main suspicion regarding it being its massively elliptic orbit.
Fomalhaut b had an orbital period of 1,700 years (it took 1,700 years to orbit the host star), and its mass was assumed to be 1–3 times as large as Jupiter’s. It was heavily researched in its time, with various astronomers attempting to find out why Fomalhaut’s dust ring was so oblately shaped using it as a vessel.
Pause. Rewind. Why am I using past tense to talk about this planet?
Simple: Fomalhaut fooled us through and through.
In 2011, when attempting to sort out Fomalhaut b’s mass, Kalas and his team realized that if Fomalhaut b was, in fact, 1–3 times Jupiter’s size, it would be detectable in the infrared. However, when the infrared Spitzer telescope attempted to scout out Fomalhaut b, it didn’t detect a thing. In spite of this, Fomalhaut b was yet again rediscovered in 2012, and the idea of its existence was revived. Shortly after this, Fomalhaut b began to fade in telescopic images over the years.
And so, after a bit of research, astronomers came to the conclusion that Fomalhaut b was not an exoplanet, but a rapidly expanding cloud of dust orbiting the Fomalhaut system. Some astronomers believe that Fomalhaut b’s disappearance was the result of two exoplanets colliding with each other, although these kinematics are very rare and only happen once every ~300,000 years. In 2020, NASA removed Fomalhaut b from all of its exoplanet catalogs, and it was no longer detectable by any means. Fomalhaut b was no more.
But just because Fomalhaut was stripped of its planet, it isn’t exactly companionless. Just under a light year away from our main companion is the variable star TW Piscis Austrini, an orange dwarf orbiting Fomalhaut with a period of just under 30,000 days (82 years). For reference, that’s around the same amount of time it takes for Uranus to orbit the sun. Although it orbits such a prominent star, TW Piscis Austrini is special enough to hold its own nevertheless. TW Piscis Austrini (Which I’ll be abbreviating TW PsA from now on) is a variable star or a star that fluctuates in brightness due to unusual activity in its layers (most commonly the photosphere, AKA the surface layer of a star). TW PsA is classified as a BY Draconis variable, or a star that changes its brightness thanks to activity in its chromosphere (the last layer of a star’s atmosphere before reaching the surface) along with the star’s rotation. This variable type is only seen in K and M-type stars (usually dwarves), and other famous examples are Epsilon Eridani and Kapteyn’s Star. Despite Fomalhaut and TW PsA being a binary pair, the distance between them on Earth is equivalent to four full moons. Despite TW PsA’s grandeur, one companion wasn’t enough for the Fomalhaut system, and you’ll C why in a second.
2.5 Light-years away from our Fomalhaut is companion Fomalhaut C, a red dwarf which is apparently 11 full moons away from the primary. In fact, the reason Fomalhaut C isn’t cataloged in Piscis Austirnus is that the star is actually in Aquarius. Fomalhaut C is so far from Fomalhaut, that the two actually hold the current record for the most distant binary pair (as of April 2023). Fomalhaut C is also cataloged as a high proper-motion star, meaning it travels across the Earth’s celestial globe at a fast rate.
Peering past its binary systems, Fomalhaut has even more companions which are far more famous than the others: Vega (Alpha Lyrae), Castor (Alpha Geminorum), Alderamin (Alpha Cephei), and Zubenelgenubi (Alpha Librae).
But how?
Simple.
Fomalhaut is a member of the Castor Moving Group, a group of stars with a common proper motion through space together. Whilst it is unlikely that the stars within the moving group have a common origin, especially not its famous members, the stars move through space together (at different velocities). The moving group has a small density of stars, making up around 0.5% of our local galactic disk, and its members are mainly main-sequence A-type stars. A more well-researched moving group similar to the Castor Moving Group is the Ursa Major moving group (For reference, the UMa Moving Group’s five brightest members are all A-type stars: Alioth, Menkalinan, Mizar, Alphecca, and Merak).
In the deep south of the sky, Fomalhaut is the keeper of a thousand surprises beneath its starry surface, from dead exoplanets to companionship with other famous stars. Outside of Fomalhaut’s intimidating appearance, all it truly wants is to be appreciated for all of its darling attributes.
