Tuesday, 24 January 2012

Fomalhaut b; not a planet after all?

Fomalhaut is a young, hot star in the Piscis Australis system, 25 light years from Earth; it is sometimes referred to as α Piscis Australis, since it is the brightest star in the constellation. Fomalhaut has a mass of about 2.1 times that of the Sun's, and a diameter 1.8 times the Sun's, but it emits roughly 18 times as much light and radiation as the sun does. The star is thought to be at very most 300 million years old, with the potential to live for about a billion years (the larger and hotter a star, the shorter its potential lifespan).

In 2005 a paper in the journal Nature by Paul Kalas and James Graham of the Astronomy Department at the University of California, Berkeley and Mark Clampin of the Goddard Space Flight Center detailing the discovery of a debris disk (astronomers refer to any disk of unknown material or origin as a 'debris disk', it does not necessarily imply that the material has a catastrophic origin) around Fomalhaut by the Infrared Astronomical Satellite, and its subsequent imaging by the Hubble Space Telescope. The disk is circular in shape, with a sharp inner radius of 133 AU, and an outer radius of 158 AU. The center is offset from the star by 15 AU. To give an idea of the scale involved 1 AU is the distance between Earth and the Sun, Neptune is 30 AU from the Sun and the outer limit of the Kuiper Belt is thought to be about 100 AU from the Sun. Given the output of Fomalhaut a protoplanetary disk far out from the star is not altogether surprising, as radiation does excerpt a gentle push (hence 'solar wind') that will move molecules and dust particles away from a star. However this cannot account for the elliptical shape of the ring, nor it's sharp inner edge. Therefore Kalas et al. theorized that a planetary body must be present in the system, shepherding the inner edge of the disk.

Hubble image of the Fomalhaut System (above) and the same image with annotation (bellow). The elliptical shape of the ring is caused by the angle at which we are viewing the system. Image from NASA/Goddard Space Flight Center.

An artist's impression of the Fomalhaut System. NASA/European Space Agency.

In 2008 another paper by a team lead by Paul Kalas, this time in the journal Science, unveiled further Hubble images of the Fomalhaut system, apparently showing a planet at the inner edge of the debris disk, fitting their previous prediction. The planet, given the name Fomalhaut b, was the first planet to be directly imaged in another star system. It apparently orbited the star at a distance of 115 AU, 18 AU (slightly less than the distance from the Sun to Uranus) from the inner edge of the debris disk, apparently to far away to be responsible for the sharp inner edge of the disk, leaving open the possibility that a second planet might exist. The planet was calculated to have a volume roughly the same as that of Jupiter, and a mass somewhere between 0.5 an 3.0 (probably less than 2.0) times Jupiter's.

2008 image of the Fomalhaut System. The scale bar is 100 AU (100 times the distance at which the Earth orbits the Sun) or 13" (13 arc-seconds' the whole sky - including the but behind the Earth - is divided into 360 degrees, each degree into 60 minutes and each minute into 60 seconds). East is reversed with regard to a ground map, as this is a map of the sky, i.e. looking up, not down. Image from NASA.

From here, however, things have gone downhill for Fomalhaut b. No other telescope managed to detect the planet, even though they now knew where to look. The in 2010 Kalas's team reported at the Extreme Solar Systems II conference in Moran, Wyoming, that they had once again imaged Fomalhaut b using Hubble, but that it was in an unexpected place. The only way that a planet could move between the two points was if it was on a highly elliptical orbit that took it straight through the debris ring. Such a large planet moving through the ring would severely disrupt it, something we would be able to detect, yet there were no signs of any such disturbance. So something was clearly wrong with Fomalhaut b.

On 20 January 2012 a team lead by Markus Janson of the Department of Astrophysical Sciences at Princeton University published a paper on the online arXiv database at Cornell University Library, detailing the results of a search for Fomalhaut b using the Spitzer Space Telescope. Spitzer is able to examine objects in the infra-red part of the spectrum from outside of the Earth's atmosphere. It is considered a vital tool in studies of exoplanets (amongst other things) as it is able to directly image their temperature. Despite this, and having a good idea where Fomalhaut b should have been, Spitzer was unable to image the exoplanet, causing Janson et al. to doubt its existence.

They concluded that it was more likely that Hubble had been imaging some sort of short lived dust clouds, though they were charitable enough to suggest that some of these may have formed around a planetary nucleus, albeit one much smaller than the proposed size of Fomalhaut b. They also noted that given the young age of the Fomalhaut system it is difficult to see how such a large planet could have formed in the available time.

Janson et al. consider that the proposed shepherding planet at the inner limit of the debris ring may well exist, as there is no other reasonable explanation for this phenomenon at this time, and do not, therefore, dispute the presence of at least one planet in the Formalhaut System, but they do not accept the claims for direct imaging of a super-Jovian planet in the system by the Hubble Space Telescope.

No comments:

Post a Comment