Fomalhaut B - Physical Characteristics

Physical Characteristics

Assuming that Fomalhaut b's orbit is in the same plane as the debris disk located exterior to it, it orbits Fomalhaut at a distance of approximately 115 AU (1.72×1010 km; 1.07×1010 mi). This distance is about 18 AU (2.7×109 km; 1.7×109 mi) closer to the star than the inner edge of the debris disk. The orbital separation of Fomalhaut b is larger than that for directly imaged planets around beta Pictoris and HR 8799 (8-70 AU). Fomalhaut b appears to be moving at about 4 kilometers per second. It is unclear whether Fomalhaut b's orbit will make it cross the debris disk, cross the debris disk only in projection (i.e it is not orbiting in the same plane as the disk), or whether its orbit is completely nested within the debris disk.

At the optical wavelengths at which Fomalhaut b is detected, it is only about 2.7×10−10 times as bright as the star and is the faintest (intrinsically) extrasolar object yet imaged. The shape of its spectrum, as determined from measurements obtained at 0.4 to 0.8 µm, appears similar to that of its host star, suggesting that the emission identifying Fomalhaut b is completely due to scattered starlight. Although the initial discovery paper for Fomalhaut b suggested that its optical brightness may be variable due to planetary accretion, later reanalyses of these data fail to find convincing evidence that Fomalhaut b is indeed variable, thus eliminating evidence for planetary accretion and also for a 'transient' dust cloud.

In order for Fomalhaut b to be detectable at optical wavelengths, it must have an emitting area much larger than the physical size of a planet, a fact further strengthening the case that what we see as Fomalhaut b is not light coming from a planet atmosphere. A circumplanetary ring system is large enough to scatter enough starlight to make Fomalhaut b visible only if it has a radius between 20 to 40 times that of Jupiter's radius. A spherical cloud of dust with a radius of 0.004 AU (600,000 km; 370,000 mi) can make Fomalhaut b visible. Fomalhaut b appears as an unresolved point source in the highest-quality data (at 0.6 µm) which would suggest that its projected emitting area cannot be larger than about 0.25 AU, about 1/4th of the Earth-Sun distance. However, it may be resolved at slightly longer wavelengths, indicating that its emitting area is larger.

The mass of Fomalhaut b, if a planet, is highly uncertain. Infrared non-detections suggest that Fomalhaut b cannot be more massive than 2 times Jupiter's mass but a lower limit on the mass depends on uncertain details for the nature of Fomalhaut b, its circumplanetary environment, and the existence of other planet-mass bodies in the system. Models of Fomalhaut b sculpting Fomalhaut's debris disk identify 0.5 times Jupiter's mass as a plausible estimate. Models for Fomalhaut b assuming it is surrounded by a swarm of planetesimals imply that it could be much lower mass (10-100 times the mass of the Earth). If Fomalhaut b is instead one of two shepherding planets which together confine the debris disk into a narrow ring, it could be anywhere between several times the mass of Mars to slightly more massive than the Earth.

If Fomalhaut b is a gas giant planet like Jupiter or Saturn, it probably formed several million years after the host star itself was formed, making it roughly 450 million years old. Alternatively, if it is a transient dust cloud it must be extremely young, perhaps created within the last few centuries.