Why all the fuss over a star called Fomalhaut?

Los Angeles, CA – Fomalhaut is a young A-type star approximately twice the mass of our sun, located 25 light years from Earth in the constellation Piscis Austrinus. If you wanted to find bright Fomalhaut in the night’s sky, it shouldn’t be too hard if you live in the southern hemisphere. Sadly for the northern half of our planet, it can only be spotted slightly above the horizon during early winter. But this isn’t “just another” star in the sky. If you could travel the 25 light years and look at it yourself, you’d be greeted by a very bright star surrounded by a belt of gas, dust and the debris of shredded comets. Orbiting around this belt, exoplanets also lurk. Fomalhaut is a hothouse of mystery and intrigue that is proving a challenge to theories of how star systems should behave.

In recent years, Fomalhaut was thrust to new-found fame when the Hubble Space Telescope spotted something hiding in the “Eye Of Sauron“-like ring of dust, approximately 115AU (around three times the average distance between the Sun and Pluto) from the star. In November 2008, Hubble astronomers published their work in the journal Science; they had directly imaged an exoplanet in visible light for the first time. It was duly named “Fomalhaut b” in keeping with exoplanetary discovery protocol. The discovery was no mean feat – being able to block the comparatively bright light of a star 25 light years away and resolve the faint glow of reflected starlight from a (possibly) Saturn-sized alien world is nothing short of mind-boggling.

Along with the discovery announced on the same day by the ground-based Keck and Gemini infrared observatories, of a system of worlds orbiting the star HR 8799, the double-whammy of exoplanetary news stunned the world in the winter of 2008. Be under no illusion, the direct imaging of a lone planet orbiting any star was nothing short of historic – it marked the beginning of an era that put exoplanetary studies on the world stage as one of the most profound endeavours mankind has ever embarked upon. During a 2009 Discovery News reader’s poll, Hubble’s famous Fomalhaut b image was voted the #1 “Space Story of the Decade“, beating the detection of dark matter, the demotion of Pluto and the discovery of water on Mars. Fomalhaut b had not only fascinated scientists, it had captured the imagination of the world.

Planet or phantom?

Despite all the accolades of being the “first exoplanet imaged”, Fomalhaut b hasn’t been independently confirmed by another telescope. It is floating in a strange “has yet to be confirmed” limbo. However, there is supporting evidence for the presence of a world in the approximate location of Fomalhaut b. As reported by the Planetary Society, back in 2004, astronomers using Hubble data to study Fomalhaut’s impressive belt of dust noticed that the ring of debris was slightly off-centre. Also, the inner edge of the belt appeared to have a sharp cut-off, rather than gradually thinning as it approached the star. This was circumstantial evidence that there may be something orbiting the star; the ring was offset, indicating an eccentric orbit of an as-yet discovered world, and the sharp cut off may indicate a large world’s gravity accumulating matter from the belt. Using this as their lead, astronomers then looked for the exoplanet itself, and on analysing photographs from 2004 to 2006, they noticed a small bright object move inside the disk. This was the discovery of Fomalhaut b.

The Herschel far-infrared observation of Fomalhaut’s dusty ring composted of comet material [ESA/Herschel/PACS]

But the discovery came with some controversy. In 2011, it was announced that follow-up observations by Hubble tracked the signal of Fomalhaut b and detected a misstep in its motion. Basically, the planet had shifted in position in a way that wasn’t in agreement with its predicted orbit around the star. This oddity, according to University of Toronto astronomer Ray Jayawardhana, was enough to call Fomalhaut b’s existence into question. “They continue to call it a directly imaged planet,” he said during an exoplanetary conference that year. “I think it’s time to stop doing that.”

Paul Kalas, of the University of California, Berkeley, and lead author of the 2008 Science discovery paper, went on the defensive. “You have one scientist trying to create a controversy out of nothing,” he said. As pointed out by Nature News on September 23, 2011, very few astronomers have directly imaged any exoplanets, so “for the lucky astronomers who have bagged one of these, bragging rights have accrued – but so has the backbiting”. It’s little wonder, then, that a few terse words are exchanged by astronomers making these groundbreaking discoveries. As it turns out, the misstep may be due to instrumental error – a different Hubble instrument was used to track Fomalhaut b in 2009, so this may account for the small, unexpected shift in position.

But the orbital misstep isn’t the only strange thing about Fomalhaut b. Infrared telescopes cannot see the exoplanet, which either means the exoplanet is cooler (and therefore older) or smaller than thought – or it’s an exoplanetary phantom. Also, Hubble found that it’s a lot brighter than it should be for its mass. The working theory is that it may have a system of rings, much like Saturn – a factor that would boost its reflected light, giving the impression that the world is a lot larger than predicted. Regardless of whether or not Fomalhut b is confirmed, the Fomalhaut system will be keeping astronomers busy for a long time to come. 

Comet armageddon

Yet another mystery was uncovered this month when European Space Agency’s Herschel Space Observatory scientists announced the results of a far-infrared study of the dusty belt surrounding the star. The analysis of Herschel data appears to show that dust in the belt has the thermal properties of tiny solid particles measuring only a few millionths of a metre across. But, according to ESA scientists, this has created a paradox.

Hubble also made an estimate of the size of the particles held in the belt and determined that, by analysing the scattered starlight, these dust particles must be ten times larger than the Herschel results. Was there an error in one of the observations? Apparently not, says Bram Acke, of the University of Leuven in Belgium, lead author of the Herschel study. The dust particles can have both the thermal properties of smaller particles, yet appear large in Hubble data, if the dust particles are in fact large fluffy aggregates more commonly associated with the particles originating from comets. This would explain the conflicting Hubble and Herschel results.

So the belt seems to be comprised of a huge quantity of comet dust, but how did it get there? The Fomalhaut dust belt is a real oddity, as the comet particles should be blown away by the powerful radiation being generated by the star – put simply, the belt shouldn’t exist at all. That is, it shouldn’t exist unless it is being replenished with vast quantities of new comet dust continuously. The answer, says Acke, is a comet “massacre”.

It would appear that these new data of the Fomalhaut system demonstrate that the star has a huge number of comets buzzing around the star. Also, many of these comets are pummeling planetary bodies, getting destroyed in the process. To maintain the dust in the belt, 260 billion to 83 trillion comets must be in orbit, and they are being destroyed at an astonishing rate. Every single day, the equivalent mass of “two 10 kilometre-sized comets or 2,000 one kilometre-sized comets must be completely crushed into small fluffy, dust particles”, says the ESA press release. This is a cometary armageddon of epic proportions; comets are diving into the inner system, smashing into as-yet to be discovered exoplanets, kicking huge quantities of dust into space and replenishing the dusty belt we see today.

Interestingly, this not only provides further evidence for the existence of exoplanets embedded in the dust, it also gives us a tantalising glimpse as to how our solar system may have looked when the sun was a fraction of the age it is now. Comets from the Oort Cloud – a shell of billions of comet embryos that is hypothesised to surround the sun approximately a light-year away – would have pummelled the inner solar system, potentially forming the dust belt we see surrounding Fomalhaut.

The ALMA submillimeter observations of Fomalhaut’s belt superimposed over the Hubble optical light observations – two exoplanets probably shepherd the belt (shown in orange) [ESO/NAOJ/NRAO and NASA/ESA]

Another exoplanet?

Now that a new radio telescope in the Atacama Desert in northern Chile is nearing completion, many of Fomalhaut’s mysteries may be unravelled. On February 13, researchers announced early results from their survey of the star using data from 15 of the planned 66 antennae of the Atacama Large Millimeter/submillimeter Array (ALMA). It is hoped that ALMA will be as revolutionary for radio astronomy as Hubble was to optical astronomy. When using ALMA data for Fomalhaut, astronomers from the University of Florida not only confirmed that the inner edge of the star’s dusty belt was likely being shepherded by an exoplanet – further evidence for the presence of Fomalhaut b – but the outer edge was also being shaped by another world. But in typical Fomalhaut style, the very existence of these two worlds is a conundrum.

The exoplanets, if they are confirmed to be real, orbit the star at a very large distance. Both are more than 100AU from their star, begging the question as to how they underwent planetary formation. Remember, Pluto orbits the Sun with a semi-major axis of 39AU. Perhaps they evolved with more compact orbits and then migrated to higher orbits to the locations they are observed today? For now, this is one mystery that may take some time to solve.

ALMA also put tight limits on the width of the dusty belt and the masses of the two worlds. The belt measures only 16AU wide, and the exoplanets are smaller than previously believed. Although the 2008 study indicated Fomalhaut b was a gas giant (of Saturn-like proportions), to maintain the dusty belt in its current form, the two shepherding worlds must be much smaller – between the mass of Mars and several Earths. By definition, the pair are likely “super-Earths”.

Fomalhaut has always fascinated stargazers, and now the star is causing intrigue on a whole new level. As we train our most advanced ground- and space-based observatories on Fomalhaut, each new discovery challenges planetary formation theories, and provides us with a small part of the puzzle as to how our solar system may have looked in its formative years. Putting the exciting hunt for exoplanets to one side for now, I believe one of the most profound discoveries is that of a belt of comet dust surrounding Fomalhaut. One of the biggest mysteries on our planet is why Earth is abundant in water, and astrophysicists will often point to comets as the source. Perhaps Fomalhaut is showing us how comets were processed more than four billion years ago, potentially supporting this hypothesis.

In a recent interview with Discovery News, Kalas summarised why the fuss over Fomalhaut is so well placed: “What we’re really looking at is an analogue to our early solar system, an epoch when a planetary system is very dynamically active. Comets and asteroids are everywhere. Young planets have formed. You have impacts on the surfaces, perhaps delivering water to terrestrial planets.”

A young star surrounded by exoplanets being pummelled by comets; perhaps the Sun and Fomalhaut have more in common than we’d ever dreamed.

Ian O’Neill is Space Science Producer for Discovery News. He is also the founder and editor of space blog Astroengine.

Follow him on Twitter: @astroengine