Fomalhaut b Has a VERY Eccrentric Orbit, May Have Significant Impacts in 2032

STIS CORONAGRAPHIC IMAGING OF FOMALHAUT: MAIN BELT STRUCTURE AND THE ORBIT OF FOMALHAUT b

Authors:

1. Paul Kalas (a,b)
2. James R. Graham (a,c)
3. Michael P. Fitzgerald (d)
4. Mark Clampin (e)

Affiliations:

a. Astronomy Department, University of California, Berkeley, CA 94720, USA

b. SETI Institute, Mountain View, CA 94043, USA

c. Dunlap Institute for Astronomy and Astrophysics, University of Toronto, Toronto, Canada

d. Department of Physics and Astronomy, UCLA, Los Angeles, CA, USA

e. NASA Goddard Space Flight Center, Greenbelt, MD, USA

Abstract:

We present new optical coronagraphic data of Fomalhaut obtained with HST/STIS in 2010 and 2012. 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 ~50 AU wide azimuthal gap in the belt northward of Fomalhaut b. The two epochs of Space Telescope Imaging Spectrograph (STIS) photometry exclude optical variability greater than 35%. A Markov chain Monte Carlo analysis demonstrates that the orbit of Fomalhaut b is highly eccentric, with e = 0.8 ± 0.1, a = 177 ± 68 AU, and q = 32 ± 24 AU. Fomalhaut b is apsidally aligned with the belt and 90% of allowed orbits have mutual inclination ≤36°. 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. 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 120 AU lies in the range 2-3, similar to highly eccentric dwarf planets in our solar system. 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. 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.