The Eye of Sauron Script (Patreon)

It’s well known that after his defeat at the end of the Third Age, the dark lord Sauron fled Mordor into the sky, and traveled about 25 light years towards the constellation Piscis Austrinus. We see his terrible flaming Eye gazing down at us, biding his time. The incredible Hubble Space Telescope picture of the star Fomalhaut does excite the imagination, doesn’t it? 

Today I want to set your mind at ease. This isn’t really the Eye of Sauron. Probably. Fomalhaut is an A-type star about twice as massive and much hotter and brighter than the sun. At only 25 light years away it’s the 18th brightest star in the sky. But even as a non-dark-lord, it’s a fascinating object. Let’s look at pictures. That dot in the middle mark’s the location of the star, but you can’t actually see Fomalhaut because a mask was placed directly in front of it for this picture. Such a mask is called a coronograph, and its purpose is to block the bright light of Fomalhaut so that its faint surrounding structures can be studied. The slitted pupil is the shadow of that coronograph. Sadly it’s not a real part of the structure, but the surrounding ring is very real.

Not the colour though. That’s also false. Hubble took this photo at 500 nanometers--turquoise. But that fiery redness does have some significance. Fomalhaut itself is brightest in the ultraviolet – to our eyes it twinkles bluish-white on the night sky. But the ring is brightest in the infrared. Here are infrared pics from the Herschel space telescope and from ALMA. In the Herschel image the star doesn’t even need to be masked. We’re seeing the heat-glow of a giant ring of ice and dust 35 billion kilometers in diameter. It’s the remnant of Fomalhaut’s protoplanetary disk – the whirlpool of debris left over after the star formed and from which a planetary system may be forming. That makes Fomalhaut an incredible laboratory for us to study how our own solar system may have formed.

Here’s how we think that goes. Start with a dense core in a molecular cloud. Cold and adrift, the mostly-hydrogen gas yields to gravity once it reaches the critical Jean’s mass. As the gas collapses it builds up heat and density, until the pressure of the gas dramatically slows  collapse and you have a hot protostar surrounded by a great cloud of dust and gas. The protostar continues to collapse more slowly, until eventually the core gets hot and dense enough for to ignite in fusion. The star is born and the outflowing energy resists any further collapse.

Conservation of angular momentum has the central star spinning rapidly. The surrounding debris also rotates, and that rotation allows it to resist falling towards the central axis. But its gravity can still pull the debris together in the up-down direction, so the debris flattens into a disk.

Eventually, winds from the newborn star in the center disperse the extra gas and dust, revealing whatever planets managed to form from the debris in that time. The formation of planets itself is a fascinating process all on its own. The leading formation model is core accretion, in which tiny granules are drawn together and clump by static electricity. They grow in size until they’re large enough that they can start collecting more material through collisions. These collisions must be very gentle, or else the pieces will shatter on impact. Eventually the growing rock has enough mass to attract material through its own gravity. At this point it’s a planetesimal, at about a kilometer in diameter. By the time it’s at least 10% of Earth’s mass it will have cleared its orbit of other planetesimals and will have rounded out into a planet.

Our own solar system went through this process four and a half billion years ago, and the solar wind has long since dispersed what was left of our own protoplanetary disk. This is believed to have happened some tens of millions of years after the Sun first formed. Fomalhaut is a strange case. It’s going through an unusually long puberty. The star is 440 million years old. That’s still very young, but it’s surprising that it’s holding on to part of its protoplanetary disk for so long.

The Herschel and ALMA images show very clearly that the ring has sharp edges. The leading idea is that newly-formed planets carved out that ring with their gravity. Check out Saturn’s F-ring. It’s clean lines are carved by what we call shepherd moons. [Enya reference?] In Fomalhaut’s case it may be shepherd planets, guiding the dust into a well-defined tract. Just like Strider guiding a pack of hobbits through the wild.

Every dust grain of the Fomalhaut system is in orbit about the star. The closer an object orbits a given star the faster moves through space; the farther out, the slower. Kepler’s third law tells us this. So a planet passing along the inside of the dust ring overtakes each grain and tugs in gently forward. Infused with extra energy, the dust orbit shifts outward. A planet passing along the outside of the dust ring pulls back on the dust, so that it slows down. Having lost energy, the dust spirals closer in towards the star. Pushed outward and inward by nearby planets, the dust ring boundaries sharpen between them.

But are their really planets there? A wise man once said “There is nothing like looking, if you want to find something. You certainly usually find something, if you look, but it is not always quite the something you were after.” In fact it’s very possible that we’ve actually found the planet that carves out the inner edge of the ring. Fomalhaut b shows up as a bright, orbiting body in Hubble images. It’s bright enough to be a few times Jupiter’s mass.

However it seems to be completely invisible in the infrared. It may actually be a much smaller planet still wreathed in the dust of its formation, and the light that Hubble is seeing just reflected starlight from Fomalhaut. Even cooler, it may be a planet with a giant, reflective ring system. If so, Fomalhaut really does seem to have a thing for rings. Suspicious.

By the way, Fomalhaut b has a cooler name. Dagon, selected by the International Astronomical Union from public submissions. We’re mixing our mythologies a bit, but I have a feeling that Dagon the evil Lovecraftian fish-person-demon-god would have got along just fine with Sauron. Well, originally Dagon was an ancient semitic deity, but Cthulhu mythos is more fun. In fact the entire planet may be a myth – instead it may be a collection of debris from a recent collision. But the smart money is on it being a real planet.

Dagon’s highly eccentric, 1700 Earth-year orbit is larger than Neptune’s. That far out, there shouldn’t really have been enough resources to build a planet of even the lower end of Dagon’s mass estimate - around 3 earth masses. One possibility is that it formed closer in. At a critical point, it may have interacted with another massive planet in the system, sending that planet spiraling closer to Fomalhaut to become a so-called hot Jupiter while Dagon was flung way out. In fact a similar model may explain the locations of our four gas giant planets. Jupiter and its cousins may have formed close together, until a close encounter destabilized the whole solar system and flung the giant planets into their current, more spread-out positions. This is the Nice model, which we discuss in our episode on Planet Nine.

Fomalhaut gets even more interesting. It has two stellar companions, making it a trinary star system. One companion is a red dwarf that may actually have its own protoplanetary disk, which makes sense if they formed at the same time. Stars typically form in groups, as very large clouds of molecular hydrogen collapse and break apart into separate pieces. The stars don’t always stay in the same groups - for example our Sun has long since left its birth siblings behind - but it’s more likely to catch loosely bound stellar systems early in their lives. By the way, the third star in this system is a flare star – violently fluctuating due to magnetic storms on its surface. In fact it’s exact type is a BY Draconis variable. Smaug found his way to Sauron’s side.

The fiery Eye of Sauron, wielding rings within rings. Was our solar system once so Mordor-like? We’re going to need more observations to figure out whether Fomalhaut is typical of stars in their youth, or is actually truly unusual. We’ll need to figure out if this planet is real, and what causes its strange mismatch in visible and infrared light. Only then can we start to use it to learn about our solar system’s early years. Lots of questions still. But one does not simply cease questioning on Space Time.