Astronomers brings giant exoplanet Fomalhaut b back from the grave

At this time of year, there's always a stretch to try find some kind of Halloween connection for a story, but in this case you don't have to stretch too far to see how a team led by University of Toronto astronomer Thayne Currie has brought an exoplanet back from the dead.

Around four years ago, astronomers used the Hubble telescope to snap a very clear picture of Fomalhaut, which is a bright star in the southern constellation Piscis Australis (the Southern Fish) that has a wide belt of dust encircling it. The presence of this dust belt made Fomalhaut a great candidate for finding exoplanets, since astronomers have long known that planets form from the disks of dust that surround stars. Examining the image, they were amazed to find what they said was the first visible image of a planet in another star system, orbiting along the inner edge of the dust belt. The planet, which they named 'Fomalhaut b', was estimated to have a maximum size of about three times the size of Jupiter, and it was orbiting a a distance of about 10 times the distance that Saturn orbits our Sun.

Due to the age of its star system, Fomalhaut b should have shown up very brightly in the infrared spectrum, but attempts to confirm the planet by using infrared spectroscopy failed. This, along with the fact that the brightness of the object varied by a factor of 2, cast doubt upon whether or not the planet actually existed, suggesting that the sighting was simply scattered light from a short-lived dust cloud.

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Subsequent studies of the system have turned up two planets, one orbiting along the inner edge of the dust belt, and one orbiting along the outer edge of the dust belt. However, these studies estimated that both planets have a lower size limit of bigger than Mars, and an upper size limit of perhaps two or three times bigger than Earth. These studies also emphasized how a planet as large as the original discovery would tear apart the dust belt. So, the original idea of Fomalhaut b being a massive planet, and the first exoplanet ever directly imaged, was still dead.

However, Currie and his team of astronomers from the United States and Japan went back to the original Hubble image from 2004 and 2006, to examine them more closely.

The Hubble telescope gathers light from the entire visible spectrum, plus ultraviolet and near-infrared. It has filters installed so that separate images can be made from the different wavelengths of the light it receives. Currie and his team were able to easily see Fomalhaut b in images that showed yellow-orange and near-infrared light (near 600 nm and 800 nm respectively) and also found it in another image that showed violet light (near 400 nm).

They found that the planet's brightness remained constant, challenging the previous finding that the brightness varied, so it was unlikely that the discovery was simply a dust cloud. They were still unable to detect the planet with an infrared telescope, but they said that only limited the size of the planet, to just under twice the size of Jupiter, which was small enough leave the dust belt intact.

"Although our results seriously challenge the original discovery paper, they do so in a way that actually makes the object's interpretation much cleaner and leaves intact the core conclusion, that Fomalhaut b is indeed a massive planet," said Currie, who worked at NASA's Goddard Space Flight Center before joining the Department of Astronomy and Astrophysics at the University of Toronto.

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Also, their findings showed that the shape and speed of the planet's orbit, both of which came under dispute in studies since the discovery, remained consistent with the original discovery — that the planet was responsible for sculpting the distinct inner edge of the dust belt.

"What we've seen from our analysis is that the object's minimum distance from the disk has hardly changed at all in two years, which is a good sign that it's in a nice ring-sculpting orbit," said team member Timothy J. Rodigas, who is a grad student at the University of Arizona.

Their study also found that the 'dust cloud' interpretation of Fomalhaut b couldn't be correct, because a dust cloud of that size would have been spread out or dissipated by interactions between the cloud and the dust belt long ago.

"Given what we know about the behavior of dust and the environment where the planet is located, we think that we're seeing a planetary object that is completely embedded in dust rather than a free-floating dust cloud," said John Debes, a member of the team from the Space Telescope Science Institute in Baltimore, Md.

So, Currie and his team have brought the original idea of Fomalhaut b — that of a massive gas-giant planet — back to life. The only part of the original discovery that remains to be revived is Fomalhaut b's status as the first directly-imaged exoplanet. The light and heat from the planet is seen only from the dust that surrounds it, rather than directly from the planet itself, so it fails to meet that criteria. However, the definition of 'directly imaged' isn't so strict that it only includes being able to see the planet itself. Precedent has already been set with a protoplanet called 'LkCa 15 b' to be considered a 'planet identified from direct imaging' because the effect it has on the debris ring surrounding its star was directly imaged.

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"Thus," their study says "Fomalhaut b is very plausibly (even 'likely') 'a planet identified from direct imaging' even if images of it, strictly speaking, do not show thermal emission from a directly imaged planet."

In the spirit of the season, allow me to add: It's alive! Mwahahahaha!

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