Astronomers Confirm Planet Fomalhaut B

An international team of astronomers using data from NASA’s Hubble Space Telescope has confirmed the presence of a massive planet around the nearby star Fomalhaut.

This is a Hubble image of Fomalhaut b obtained through the F606W filter (Thayne Currie et al)

Fomalhaut, a class A star twice as massive as the Sun and 20 times brighter, is located in the constellation Piscis Austrinus about 25 light-years away from us. The star is surrounded by a ring of dust and debris, making it a favorite system for astronomers to study.

In 2008, Hubble astronomers announced the discovery of a planet called Fomalhaut b, as the first extrasolar planet to be directly detected in visible light.

Astronomers believed it to be a giant planet, akin to Jupiter or Saturn, but later infrared images failed to detect it. Based on the object’s apparent motion and the lack of an infrared detection, they argue that the object is a short-lived dust cloud unrelated to any planet.

A new study, accepted for publication in the Astrophysical Journal Letters ( version), suggests that Fomalhaut b is a rare and possibly unique exoplanet that is completely shrouded by dust.

“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 study lead author Dr Thayne Currie of the University of Toronto.

The 2008 study reported that Fomalhaut b’s brightness varied by about a factor of two and cited this as evidence that the planet was accreting gas. Follow-up studies then interpreted this variability as evidence that the object actually was a transient dust cloud instead.

In the new study, Dr Currie’s team reanalyzed Hubble observations of the star from 2004 and 2006, and easily recovered the planet in observations taken at visible wavelengths near 600 and 800 nanometers, and made a new detection in violet light near 400 nanometers. In contrast to the earlier research, the team found that the planet remained at constant brightness.

The team also 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 follows a tilted orbit that passes through the ring plane.

This visible-light image from the Hubble Space Telescope shows the vicinity of the star Fomalhaut, including the location of its dust ring and Fomalhaut b (NASA / ESA / Thayne Currie / U. Toronto)

The 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.

Because astronomers detect the planet by the light of surrounding dust and not by light or heat emitted by its atmosphere, Fomalhaut b no longer ranks as a ‘directly imaged exoplanet,’ instead it should be considered a ‘planet identified from direct imaging.’


Bibliographic information: Thayne Currie et al. 2012. Direct Imaging Confirmation and Characterization of a Dust-Enshrouded Candidate Exoplanet Orbiting Fomalhaut. ApJ Letters in press; arXiv: 1210.6620