Fomalhaut b: An Exoplanet Redeemed

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

Fomalhaut b
An artist's illustration of the disputed exoplanet Fomalhaut b. The planet might be sweeping out the edge of a massive ring of dust around the young star.
ESA / NASA / L. Calcada
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.

Exoplanets, News
John Bochanski

About John Bochanski

John Bochanski is a physics professor at Rider University in Lawrenceville, NJ. John uses large surveys of the sky to study its coolest members, from nearby low-mass stars and brown dwarfs, to some of the most distant red giant stars ever discovered.

5 thoughts on “Fomalhaut b: An Exoplanet Redeemed

  1. Rod site shows 31 imaged exoplanets. Fomalhaut b is one of them. Most listed are quite massive with average mass 13.15 Jupiters and average semi-major axis 335.49 AU. Perhaps there is a larger population of these exoplanets that can be imaged but current efforts focus on close in bodies that transit their host stars with short periods, e.g. NASA Kepler mission. Explaining the origin of massive exoplanets forming at large distances from their host stars seems difficult.

  2. Peter

    Look at star birth regions–the Orion nebula, the Pillars of Creation–and they appear highly chaotic. While there are tendencies and limits in the star formation process, it seems almost anything can result given the wide spectrum of possible initial conditions. If binary stars can form with hundreds of AU separation, why not a star and a massive planet?

  3. Rod

    Peter et al. Ref – Rapid growth of gas-giant cores by pebble accretion,…544A..32L

    My observation – this report focused on growing 10 earth mass cores at large distances from their host stars in an effort to explain observations of imaged exoplanet studies of gas giants orbiting at large distances from their host stars. It did not show that the 10 earth mass cores could continue to grow into gas giants perhaps 2-15 Jupiters in mass at large distances from their host stars. The solution could be rapid formation of gas giants at large distances from their host stars vs the 1-10 million time scales used in the computer models. The report also started the simulations with cm size objects and not microscopic dust grains. Yes the difficulty is real.

  4. Peter

    Rod, in their abstract they write, “We conclude that pebble accretion can resolve the long-standing core accretion timescale conflict.” Maybe I am misunderstanding that, but I interpret it to mean the difficulty has been “resolved.”

  5. SlackerSlayer

    The search for exoplanets with the light changes of the host star, will only find the close in orbiting planets. How long would it take anohter civilization out there to find Earth with the same method. It takes us a year to do that one orbit dance, and how many observations of the light changes does it take to confirm that the planet really exists? Then you have the fact that the percentage of solar systems in the same plane as the star from our perspective is makes the list of findable planets even smaller.

    I suggest we use infrared telescopes in favor over this keplar method.

    The brown dwarfs would also be found with infrared easier than visible light or any effect they have on the light from a back ground star.

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