Astronomers from the Harvard-Smithsonian Center for Astrophysics studied the debris disc around τ Ceti, revealing that a Jupiter-mass planet could not be present in this system – making it less than ideal as a Solar system analog.
Although thousands of exoplanets and hundreds of planetary systems (stars with multiple exoplanets) are now known, astronomers still don’t know whether our solar system is typical. The distributions of known planetary system parameters are strongly affected by observational biases that are not easy to disentangle from the true distributions. Moreover, our Solar system’s architecture (small rocky inner planets, large gaseous outer planets, and an outer debris disc made of many small objects) has not yet been seen in other systems, most likely due to these same biases. Long time baselines, for example, are required to discover planets at greater than a few astronomical units (one AU is the average distance of the Earth from the Sun) with all techniques except direct imaging, but direct imaging of planets around mature stars is difficult due to the low light from planets compared to their host stars.
Debris disks, because they are spread out over large areas, are easier to see, and structures in debris discs like rings or gaps can indicate the presence of additional planets. CfA astronomer David Wilner joined with his colleagues to search for clues of planets in the debris disc around τ Ceti, a nearby solar-type star located only ten light-years from the Sun. The infrared excess towards τ Ceti has been known for nearly three decades and has been attributed to warm dust particles in a debris disk.
The astronomers used the Herschel Space Telescope to study τ Ceti in far infrared wavelengths where the dust emission should be strongest. The carefully processed images reveal evidence for a uniform and symmetric debris disk with an inner edge about two to three AU from the star and an outer edge fifty-five AU from the star. For comparison, in our Solar system the Kuiper Belt of small objects begins at the orbit of Neptune (about thirty AU from the Sun) and extends out to about fifty AU (the colder Oort Belt of icy objects and comets extends much farther, to about fifty thousand AU).
In previous studies of τ Ceti, other astronomers had found preliminary evidence suggesting the possibility it hosted five planets. Wilner and his colleagues modeled the stellar system with their observed debris disk and including these possible planets and a range of other published observational data, and found that the system was consistent with the observations and could be stable. The putative planets would orbit between the debris disk’s inner edge and the star. The scientists conclude by noting that a Jupiter-mass planet could not be present in this system, making it less than ideal as a Solar system analog. Future observations should be able to refine the picture further.
Publication: S. M. Lawler, et al., “The Debris Disc of Solar Analogue τ Ceti: Herschel Observations and Dynamical Simulations of the Proposed Multiplanet System,” MNRAS (November 1, 2014) 444 (3): 2665-2675; doi: 10.1093/mnras/stu1641