Soapbox Science

Fomalhaut b: the Phoenix Planet

Dr. Thayne Currie is currently a postdoctoral fellow in the Department of Astronomy and Astrophysics at the University of Toronto. His research focuses on detecting and characterizing massive planets via direct imaging. The main goals of his research are to understand the formation and evolution of planetary systems, to determine how the solar system fits within the range of planet formation outcomes and how the properties of planets around other stars compare to those in our solar system.


In November 2008, two teams announced what were then described as the first directly imaged exoplanetsplanets outside a Solar System, around stars HR 8799 and Fomalhaut. The object called Fomalhaut b (reported by Kalas et al. 2008) was imaged just inside the debris ring and was invoked to explain the ring’s offset from the star. From comparing the ring’s thickness and location to models for ring sculpting by planets, the authors concluded that Fomalhaut b is likely jovian mass. They found Fomalhaut b to be variable, a feature they attribute to gas accretion onto the planet.

Fomalhaut system

But studies focused on Fomalhaut published after the discovery paper cast serious doubt on Fomalhaut b’s status as a planet. There were a couple of problems. First, one of the strange things in the original paper is that Fomalhaut b was apparently variable, a feature that the authors attributed to gas accretion onto the planet. Moreover, even though Fomalhaut b was invoked to explain the debris ring, its apparent motion seemed off compared to the orbit of a ring-sculpting planet: it was moving too fast.

It was also not detected in the infrared with the Spitzer Space Telescope, even though the nominal model Kalas et al. used to model Fomalhaut b’s emission implied it should have been detectable. The study presenting the Spitzer non-detections went further, proposing an alternate model: that Fomalhaut b is a semi-transient dust cloud and Fomalhaut b’s variable brightness is evidence in support of this idea.

So our interest was to re-reduce the original Hubble Space Telescope (HST) data to see for ourselves whether Fomalhaut b is even there and, if so, whether we can better understand its orbit, whether it is variable, why it isn’t detected in the infrared, and thus what exactly this might be.

We definitely can confirm that Fomalhaut b is a real astrophysical object orbiting the star, as we detect it at three different wavelengths (and at a very high significance in one of the three).

However, we find key differences with the original paper, but these actually make interpreting the object a bit easier. We do not find that Fomalhaut b is variable. Instead its brightness appears to be constant, and thus we find no clear evidence (from variability) for gas accretion nor for a transient dust cloud.

We also get a different orbit from the original Kalas et al. paper, but one that is more in line with the speed we would expect a ring-sculpting planet to have. Furthermore, our analysis shows that Fomalhaut b was unlikely to be detectable in the infrared to begin with. So a non-detection does not rule it out as a planet.

Finally, we examined this alternate model, that Fomalhaut b is an unbound semi-transient dust cloud, in more detail. Based on what we understand about the behavior of small dust (responsible for Fomalhaut b’s emission) and the circumstellar environment of Fomalhaut, we do not think that Fomalhaut b can be an unbound dust cloud. Such a cloud would be exceptionally short-lived were it to form at all, and it would be more likely to form within the debris ring, not interior to it. Rather, it most likely identifies a massive body that is enshrouded with dust.

This dust is the reason why Fomalhaut b is visible to us, not the atmosphere of a planet. Thus, though not yet proven, we prefer to consider Fomalhaut b as likely a “planet identified from direct imaging” instead of a “directly imaged planet”.

Further Reading

Direct Imaging Confirmation and Characterization of a Dust-Enshrouded Candidate Exoplanet Orbiting Fomalhaut

RedOrbit Exclusive Interview: Dr. Thayne Currie, University of Toronto’s Department of Astronomy and Astrophysics


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