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New HST Data and Modeling Reveal a Massive Planetesimal Collision Around Fomalhaut

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New HST Data and Modeling Reveal a Massive Planetesimal Collision Around Fomalhaut New HST data and modeling reveal a massive planetesimal collision around Fomalhaut Andras´ Gasp´ ar´ a,1 and George H. Riekea aSteward Observatory, The University of Arizona, Tucson, AZ 85719 Edited by Neta A. Bahcall, Princeton University, Princeton, NJ, and approved January 15, 2020 (received for review July 19, 2019) The apparent detection of an exoplanet orbiting Fomalhaut the constraints from assuming that the phenomenon has per- was announced in 2008. However, subsequent observations of sisted for the life of the star and that the image is point-like. Fomalhaut b raised questions about its status: Unlike other exo- Ref. 20 found that satellite swarms around planets of mass 10 planets, it is bright in the optical and nondetected in the infrared, to 100 MEarth that have evolved for 100 to 400 My could match and its orbit appears to cross the debris ring around the star the properties of Fomalhaut b. Ref. 17 suggested that the object without the expected gravitational perturbations. We revisit pre- could consist of dust created in the collision of two modest-sized viously published data and analyze additional Hubble Space (50 km) planetesimals similar to members of the Kuiper Belt. Telescope (HST) data, finding that the source is likely on a radial Ref. 18 analyzed three possible origins for the required dust trajectory and has faded and become extended. Dynamical and cloud: 1) a giant planetesimal impact, 2) material captured from collisional modeling of a recently produced dust cloud yields the prominent debris disk of the star, or 3) dust generated in results consistent with the observations. Fomalhaut b appears a collisional cascade from a massive cloud of satellites around to be a directly imaged catastrophic collision between two large a recently formed planet. The first possibility was judged to be planetesimals in an extrasolar planetary system. Similar events unlikely to reproduce the observations (21). The other two pos- should be very rare in quiescent planetary systems of the age sibilities are constrained significantly by the assumption that the of Fomalhaut, suggesting that we are possibly witnessing the dust system should persist in a state similar to its present one effects of gravitational stirring due to the orbital evolution of for the main sequence lifetime of the star, i.e., ∼400 My. Ref. hypothetical planet(s) around the star. 19 suggested a possible solution to the lifetime issues by attribut- ing the source to a transient dust cloud produced by a collision extrasolar planets j circumstellar disks j directly imaged planets between planetesimals interior to the main debris belt around the star. We report that Fomalhaut b has grown in extent and faded he simultaneous announcements of images of massive plan- since its discovery in Hubble Space Telescope (HST) images Tets around Fomalhaut (1) and HR 8799 (2) were a bench- from 2004, with motion consistent with an escaping trajectory. mark in our exploration of exoplanets. The HR 8799 system This behavior is consistent with expectations for a dust cloud pro- has indeed become the prototype for complex systems of very duced in a planetesimal collision and dispersing dynamically. As massive planets and has been the subject of many studies of the cloud disperses, its surface brightness has dropped, making it such objects (e.g., refs. 3–7). However, Fomalhaut b has been less prominent in the most recent images. enigmatic. An issue with the massive planet hypothesis for Fomalhaut b was the nondetection with the Infrared Array Camera (IRAC) Significance onboard the Spitzer Space Telescope (8), which placed an upper limit of 3 Jupiter masses (MJup) on its mass assuming an age Although originally thought to be a massive exoplanet, the for Fomalhaut of 200 My. A similar limit was derived from the faintness of Fomalhaut b in the infrared and its failure to per- lack of apparent perturbations in the Fomalhaut debris ring; this turb Fomalhaut’s debris ring indicate a low mass. We use all work favored a significantly smaller mass, e.g., 0.5 MJup (9). An available data to reveal that it has faded in brightness and upward revision of the system age to 440 ± 40 My (10) relaxed grown in extent, with motion consistent with an escaping the limit from the infrared data, but a much deeper limit was orbit. This behavior confirms suggestions that the source is obtained with Spitzer IRAC that pushed the mass limit lower. In a dispersing cloud of dust, produced by a massive collision fact, this limit appeared to be incompatible with a massive planet between two planetesimals. The visible signature appears to accounting for the visible brightness of the object (11, 12). With be very fine dust escaping under the influence of radiation determination of an orbit for Fomalhaut b, it became apparent pressure. Such events should be rare in quiescent plane- that the object would cross the ring at least in projection (13). tary systems at the age of Fomalhaut, suggesting increased The implications on the ring structure placed a tentative limit on dynamical activity within the system possibly due to orbital the mass of Fomalhaut b as low as an Earth mass (14). migration of hypothetical planets. Given that the bright optical signature of Fomalhaut b seems not to be light scattered from a giant planet, a number of alter- Author contributions: A.G. and G.H.R. designed research; A.G. performed research; A.G. native hypotheses have been proposed, e.g., light scattered by contributed analytic tools; A.G. analyzed data; and A.G. and G.H.R. wrote the paper.y a circumplanetary ring system (1) or by a dust cloud associated The authors declare no competing interest.y with a relatively low-mass planet (e.g., refs. 11, 15, and 16). A ten- This article is a PNAS Direct Submission.y tative finding that the image of Fomalhaut b might be extended Published under the PNAS license.y was interpreted to support the dust cloud hypothesis (17); how- Data deposition: The raw observational data can be downloaded from the Mikulski ever, it was suggested that this effect instead might be due to Archive for Space Telescopes (MAST) website maintained by the Space Telescope speckle and other noise sources (13, 18). Nonetheless, a dust Science Institute (STScI). Our modeling code, DiskDyn, can be downloaded from cloud appears to be the most plausible hypothesis. https://github.com/merope82/DiskDyn.y A number of papers have addressed the origin of this hypo- 1 To whom correspondence may be addressed. Email: [email protected] thetical dust cloud (15, 17–19). Ref. 15 suggested that collisions This article contains supporting information online at https://www.pnas.org/lookup/suppl/ among a swarm of satellites around a planet could be respon- doi:10.1073/pnas.1912506117/-/DCSupplemental.y sible and showed that certain models of this process lie within First published April 20, 2020. 9712–9722 j PNAS j May 5, 2020 j vol. 117 j no. 18 www.pnas.org/cgi/doi/10.1073/pnas.1912506117 Downloaded by guest on September 26, 2021 Archival Data Mikulski Archive for Space Telescopes (MAST) website. We Fomalhaut b has been observed only in scattered light and only downloaded corrected ( drz.fits) files, which rectify the non- with HST. The stable optical system of HST and the lack of square pixels of the HRC detector and also correct the field atmospheric disturbance enable high-contrast imaging at visible distortions. In our reductions, we rejected observations with wavelengths with the aid of coronagraphs. Fomalhaut b was dis- issues such as poor centering on the coronagraph and also com- covered in images taken with the Advanced Camera for Surveys bined measurements in the same filter taken sufficiently closely (ACS) coronagraph in 2004 and 2006 (1). The failure of the high- in time that the relative motion of Fomalhaut b is insignificant resolution channel (HRC) (which included the coronagraph) of (GO9862 and GO11818). ACS in 2007 led to the Space Telescope Imaging Spectrograph Over an orbit, the Hubble Space Telescope undergoes ther- (STIS) becoming the single coronagraphic instrument onboard mal expansions (a.k.a. “breathing”), which vary the PSF. Addi- HST; thereafter the monitoring of Fomalhaut b continued with tionally, the ACS coronagraph transmits a small fraction of STIS. Images of Fomalhaut b taken with STIS in 2010 and in the occulted flux; the transmitted PSF depends on the total 2012 have been published by ref. 13. count received. Therefore, the ACS observations were target- The coronagraphs in these instruments differ in design and PSF paired based on exposure length and interorbit sequence. performance. The ACS Lyot coronagraph was located in the PSF subtraction residuals can become substantial if target obser- aberrated beam of the telescope. This limited the performance vations are not registered to ≤0:1 px with their PSFs. We of the coronagraph at smaller (≤300) inner working angles. How- determined the shifts between each image and its chosen PSF ever, it was able to observe using a suite of optical filters. The by observing the subtraction residuals by eye. Using the software STIS coronagraph is located in the corrected beam; however, IDP3, we registered the images well within 0.1 px by minimizing 00 it is unfiltered and therefore detects photons from 0.4 to 1 µm. the radial streak pattern over a radius of 10 from the occul- While this yields high sensitivity, it also limits the fidelity of the ter. We also registered the shifts between the target images in a chromatically dependent point spread function (PSF) of the tele- similar manner. We found an average offset in x and y pixel coor- scope, which must be observed close in time to the target star dinates of ∆x = −0:32 ± 0:21 and ∆y = 0:35 ± 0:422, which is because of temporal drifts in the image shape.
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