We present new optical coronagraphic data of the bright star Fomalhaut obtained with the Hubble Space Telescope in 2010 and 2012 using STIS that extend the astrometric baseline of previous 2004/2006 observations with ACS/HRC .
Fomalhaut b is recovered at both epochs to high significance .
The observations include the discoveries of tenuous nebulosity beyond the main dust belt detected to at least 209 AU projected radius and a \sim 50 AU wide azimuthal gap in the belt northward of Fomalhaut b .
The two epochs of STIS photometry exclude optical variability greater than 35 % .
The morphology of Fomalhaut b appears elliptical in the 2010 and 2012 detections .
We show that residual noise in the processed data can plausibly result in point sources appearing extended .
A Markov chain Monte Carlo analysis demonstrates that irrespective of any assumption regarding inclination to the line of sight , the orbit of Fomalhaut b is highly eccentric , with e = 0.8 \pm 0.1 , a = 177 \pm 68 AU , and q = 32 \pm 24 AU .
Fomalhaut b is apsidally aligned with the belt and 90 % of allowed orbits have mutual inclination \leq 36 \arcdeg .
Fomalhaut b ’ s orbit is belt-crossing in the sky plane projection , but only 12 % of possible orbits have ascending or descending nodes within a 25 AU wide belt annulus ( 133 -158 AU ) .
The high eccentricity invokes a dynamical history where Fomalhaut b may have experienced a significant dynamical interaction with a hypothetical planet Fomalhaut c , and the current orbital configuration may be relatively short-lived .
The Tisserand parameter with respect to a hypothetical Fomalhaut planet at 30 AU or at 120 AU lies in the range 2 - 3 , similar to highly eccentric dwarf planets in our solar system .
The new value for the periastron distance diminishes the Hill radius of Fomalhaut b and any weakly bound satellite system surrounding a planet would be sheared and dynamically heated at periapse .
We argue that Fomalhaut b ’ s minimum mass is that of a dwarf planet in order for a circumplanetary satellite system to remain bound to a sufficient radius from the planet to be consistent with the dust scattered light hypothesis .
Fomalhaut b may be optically bright because the recent passage through periapse and/or the ascending node has increased the erosion rates of planetary satellites .
In the coplanar case , Fomalhaut b will collide with the main belt around 2032 , and the subsequent emergent phenomena may help determine its physical nature .
We show that if Fomalhaut b has a bound dust cloud , then the cloud survives hundreds of belt crossings .
If Fomalhaut b has the mass of a gas giant planet , then belt crossings will erode the belt edges after \sim 10 ^ { 2 } orbits .