Fomalhaut b: An Exoplanet Redeemed

New analysis suggests that Fomalhaut b — an exoplanet discovered in 2008 and disputed ever since — really does exist.

Fomalhaut, one of the brightest stars in the sky, has been a tantalizing target for astronomers these past few years. In 2008 Hubble Space Telescope images revealed a purported planet nestled inside a massive dust ring surrounding the young star, the first “directly imaged” exoplanet. But later studies called its nature — and even its existence — into question. Now an international team of astronomers has published a new analysis reinstating the elusive Fomalhaut b to planet status.

When Paul Kalas (University of California, Berkeley) announced the planet’s discovery, astronomers hastened to double-check the result with other telescopes. Controversy was quick to follow. Planets emit most of their energy as infrared light, not optical, so infrared telescopes should have picked up a clear signal from the reported exoplanet. But when Markus Janson (Princeton University) turned the Spitzer Space Telescope toward Fomalhaut, he and his colleagues found nothing where Fomalhaut b should have been. The team concluded that the Hubble detection could be explained away by starlight reflecting off a transient dust cloud kicked up during a collision between planetesimals.

Debate flourished with every new observation or simulation, but the newest argument in the years-long dispute over Fomalhaut b’s existence looks back at the original images. Thayne Currie (NASA GSFC) and his colleagues reviewed the Hubble observations published in 2008, comparing and combining several new image analysis techniques. After carefully subtracting bright Fomalhaut from the image, the team detected the dust ring and potential planet with twice the sensitivity of earlier analyses. Not only did the team extract a cleaner detection of Fomalhaut b at the two wavelengths where the supposed planet had already been seen, they also detected the planet at another, even shorter wavelength.

Currie’s group also obtained new infrared images of the planet's location, but they had no better luck than Janson — the planet remains undetected in infrared observations. Any planet must weigh less than two Jupiters to emit so little infrared light. (Good thing — dynamic modeling of the disk shows that any planet more than 3 Jupiter-masses would already have torn the disk apart.)

Even without infrared detections, Currie’s re-analysis rules out the possibility of a transient dust cloud. His new analysis shows that Fomalhaut b doesn’t flicker as the original authors had thought. So if there is a cloud of dust, it’s not dispersing over time. The dust must be gravitationally bound — i.e., clinging to a planet — if it is to survive a significant length of time.

Moreover, by measuring the intensity of light emitted in each waveband, Currie and his team show that the tiny dot in the Hubble images couldn’t have been emitted from the planet itself and must instead have been come from starlight reflected off dust enshrouding a roughly Jupiter-mass planet. The scattered starlight is too dim to be detected in infrared observations; it only shows up via Hubble’s sensitive optics.

So astronomers may not have directly imaged the exoplanet, but it was still the first detected by imaging. And unraveling its mysterious nature is a beautiful example of the scientific process in action.