Taking a second look at data from NASA's Hubble Space Telescope, a team of astronomers including University of Arizona graduate student Timothy Rodigas has reanimated the claim that the nearby star Fomalhaut hosts a massive exoplanet. Their findings suggest that the planet, named Fomalhaut b, is a rare and possibly unique object that is completely shrouded by dust.
Fomalhaut is the brightest star in the constellation Piscis Austrinus and lies 25 light years away, close enough to be considered in the neighborhood of our solar system.
In November 2008, Hubble astronomers announced the exoplanet, named Fomalhaut b, as the first one ever directly imaged in visible light around another star. The object was imaged just inside a vast ring of debris surrounding but offset from the host star. The planet's location and mass – no more than three times Jupiter's – seemed just right for its gravity to explain the ring's appearance.
Recent studies have claimed that this planetary interpretation is incorrect. Based on the object's apparent motion and the lack of an infrared detection by NASA's Spitzer Space Telescope
, they argue that the object is a short-lived dust cloud unrelated to any planet.
The new analysis, however, brings the planet conclusion back to life.
"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 Thayne Currie, who led the study. Formerly an astronomer at NASA's Goddard Space Flight Center
in Greenbelt, Md., Currie is now at the University of Toronto.
The discovery study reported that Fomalhaut b's brightness varied between observations made about two years apart – something very strange and not easily explainable, Rodigas said.
That observation, together with follow-up studies, then interpreted this variability as evidence that the object was actually a transient dust cloud instead.
In the new study, Currie and his team reanalyzed Hubble observations of the star from 2004 and 2006. They easily recovered the planet in observations taken at visible wavelengths and made a new detection in violet light.
“And we found there is no variability in brightness at all,” said Rodigas, who is in the final year of his doctoral studies in the UA department of astronomy
. “We don’t know what this planet is like, but we can say that it is likely less than two Jupiter masses, and we think it is orbited by a dust cloud, scattering the light from its host star, Fomalhaut. This is the light we detected.”
The team attempted to detect Fomalhaut b in the infrared using the Subaru Telescope
in Hawaii, but was unable to do so. The non-detections with Subaru and Spitzer imply that Fomalhaut b must have less than twice the mass of Jupiter.
Another contentious issue has been the object's orbit. If Fomalhaut b is responsible for the ring's offset and sharp interior edge, then it must follow an orbit aligned with the ring and must now be moving at its slowest speed. The speed implied by the original study appeared to be too fast. Additionally, some researchers argued that Fomalhaut b’s orbit would cause the object to cross through the debris ring.
Rodigas took a closer look and studied the object’s orbit inside the debris disk very closely.
“One way to do this is to check if and how the distance between the planet and the disk changes over time. If the planet is not in a nice orbit that is sculpting the disk, if it's instead moving through the disk, then its distance from the disk should change dramatically over time.”
Rodigas explained that “sculpting the disk” is an astronomer’s way of saying the planet grooms the inner edge of the debris disk around the star, maintaining a sharp and defined edge by disrupting, ejecting or swallowing debris with its gravitational field.
"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.”
Using the Hubble data, Currie's team established that Fomalhaut b is moving with a speed and direction consistent with the original idea that the planet's gravity is modifying the ring.
Currie's team also addressed studies that interpret Fomalhaut b as a compact dust cloud not gravitationally bound to a planet. Near Fomalhaut's ring, orbital dynamics would spread out or completely dissipate such a cloud in as little as 60,000 years. The dust grains experience additional forces, which operate on much faster timescales, as they interact with the star's light.
"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 team member John Debes, an astronomer at the Space Telescope Science Institute in Baltimore, Md.
Because astronomers detect Fomalhaut b by the light of surrounding dust and not by light or heat emitted by its atmosphere, it no longer ranks as a "directly imaged exoplanet." But because it's the right mass and in the right place to sculpt the ring, Currie's team thinks it should be considered a "planet identified from direct imaging."
Fomalhaut was targeted with Hubble most recently in May by another team. Those observations are currently under scientific analysis and are expected to be published soon.