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.
Reference(s):
"The
Debris Disc of Solar Analogue τ Ceti: Herschel Observations and
Dynamical Simulations of the Proposed Multiplanet System," S. M. Lawler,
J. Di Francesco, G. M. Kennedy, B. Sibthorpe, M. Booth, B.
Vandenbussche, B. C. Matthews, W. S. Holland, J. Greaves, D. J. Wilner,
M. Tuomi, J. A. D. L. Blommaert, B. L. de Vries, C. Dominik, M.
Fridlund, W. Gear, A. M. Heras, R. Ivison and G. Olofsson, MNRAS 444, 2665, 2014.