Fomalhaut b remains a fascinating mystery

The large dusty ring surrounding the nearby young main-sequence star Fomalhaut was first discovered by the Infrared Astronomical Satellite (IRAS) in 1985, which found an excess of 60 µm emission above the stellar photosphere around a number of nearby stars. The excess infrared emission arises from a ring of dust that surrounds the star. The dust absorbs the stellar light and re-emits in the infrared.  When we look at the spectrum of the star (with is a measure of its brightness as a function of wavelength), stars hosting dusty disks will have more emission in the infrared than can be explained by the stellar photosphere.

Infrared excess from a dusty disk and a dusty ring surrounding a star. The top panel shows the spectrum of a star. The second panel shows the spectrum of a star surrounded by a dusty disk, while the third panel shows the spectrum of a star surrounded by a ring of dust. In both cases, the surrounding dust is seen as an excess of infrared emission above that of the stellar photosphere. A “bump” of excess infrared emission indicates the presence of a dust ring. (Credit: NASA/JPL-Caltech/T. Pyle, SSC)

In 2005, Paul Kalas and collaborators presented the first HST/ACS image of the debris disk around Fomalhaut, finally revealing the structure of the thin ring of dust.  Previous observations with the sub-millimetre telescope Jame Clerk Maxwell Telescope by Wayne Holland and collaborators showed a cavity in the dust emission, clearly demonstrating that the dust was a ring rather than a disk, but the resolution was not high enough to reveal the dusty ring. The HST observations, with a resolution of just 0.5 AU, clearly show that the dust belt in 25 AU wide, with a very sharp inner edge at a distance 133 AU from the star. The Kalas et al. results also found that the centre of the disk was off-set from star by about 15 AU. They suggest that the sharp inner edge of the dust belt and the centre of symmetry offset from the star can both be explained by an unseen planet on an eccentric orbit between the star and the inner edge of the belt.

Dust belt around Fomalhaut as seen by HST. The centre of the dust belt is offset from the star, and the inner edge of the belt is very sharp. These two facts point to sculpting of the dust belt by an unseen planet. (Credit: NASA, ESA, Kalas & Graham (UC Berkeley) & Clampin (GSFC))

Thus the hunt for the unseen planet began. Using the Keck and Gemini telescopes, along with HST’s ACS, observations were made of Fomalhaut over several years looking for common proper motion sources (basically to hunt for a blob just interior to the dust belt moving with the same proper motion across the sky as Fomalhaut).  In 2008, Paul Kalas and collaborators announced the discovery of Fomalhaut b from HST/ACS observations made in 2004 and 2006.  A small blob, situated about 18 AU from the inner edge of the dust belt, is seen to move over the course of two years and the authors suggested that they had discovered the unseen planet.  However, the HST observations detected the source at 600 and 800 nm, but no detections of the source were made in the near-infrared with either Keck or Gemini and nor with the Spitzer Space Telescope.

HST discovery image of Fomalhaut b. Is this the planet sculpting the Fomalhaut dust belt? (Credit: NASA, ESA & Kalas, UC Berkeley)

However, the fact that Fomalhaut b was seen in scattered optical light and not in the near-infrared is a mystery, as planets are expected to be detected by their thermal emission in the near-infrared.  The lack of near-infrared emission rules out a warm, massive planet and constrains the mass to be less than 3 Mjup. So what is Fomalhaut b?  Whatever it is, it must contain dust. Perhaps the HST observations are scattered light from a large circumplanetary disk around a massive planet, or a dusty disk around the planet caused by collisions between irregular satellites, or perhaps it is not a planet at all and what we are seeing in dust created by a recent collision between two Kuiper Belt-like objects.

In January this year, Paul Kalas and his team presented new HST/STIS observations which at the very least confirm the existence of Fomalhaut b.  They present four epochs of HST data over 8 years. With this new data, the team have been able to contain the orbit of Fomalhaut b, which is highly eccentric (e = 0.8 ± 0.1).

New HST/STIS observations in 2012 confirm the presence of Fomalhaut b, but its nature remains elusive. (Credit: NASA, ESA, & Kalas, UC Berkeley)

Last week’s Transformational Science with ALMA conference in Hawaii, Paul Kalas told us that while he still doesn’t know what Fomalhaut b is, he does now know that its orbit will appear to cross the dust belt around Fomalhaut in the few decades, which could undoubtedly lead to some very interesting dynamics.  With such an eccentric orbit, Fomalhaut b’s periastron may be as small as 10 AU. This information allows Kalas to determine the lower limit on the mass and size of Fomalhaut b to about 1.5e21 kg (about 1/10th that of Pluto) and 1000 km in diameter (about half the size of Pluto) to ensure that it withstands tidal shearing at periastron passage.

The new data also confirms that Fomalhaut b is not the planet that is sculpting the inner edge of the dust belt and causing the ring to be offset from the star – which means there must still be an unseen planet hiding somewhere in the Fomalhaut system.  Paul suggest that (the as yet unseen) Fomalhaut c is sculpting the dust belt, and that perhaps (the as yet undiscovered) Fomalhaut d has scattered Fomalhaut b onto its highly eccentric orbit, a bit like Centaurs in our own Solar System.  Stay tuned for more details….!

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One Response to Fomalhaut b remains a fascinating mystery

  1. The new paper by Kalas et al. (2013) is now available on astro-ph:
    http://lanl.arxiv.org/abs/1305.2222

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