High-phase Saturnian imaging from Cassini
The internet is abuzz today with this beautiful image and of course also with the news that there may be planets in the tau Ceti system (see sidebar), including at least one in the habitable zone. I want to sound a note of proper scientific caution that these tiny signals have been extracted from within noise which is notoriously hard to characterize (stellar ‘jitter’), using new and highly sophisticated techniques. The work bodes extremely well for further analyses of existing radial velocity data to uncover still smaller Doppler signals of low-mass planets as yet undetected. Indeed this dataset was chosen for the study not because tau Ceti is a nearby, sunlike star with a famous place in the annals of SETI but precisely because prior to today no planets at all had been detected, making the dataset a good one to intensively model the jitter. To quote from the paper:
Future data will be of essence in determining the nature of the signals we detect. If the ﬁve periodicities can be detected independently in different datasets, their genuine nature as signals of stellar origin will be veriﬁed. While even this will not imply that they are deﬁnitely Doppler signatures of low-mass planets, it will help ruling out spurious periodicities that insufficient modelling and instrumental instability might cause.
Excellent, scientific accounts of today’s tau Ceti news can be found here, and in particular here. Meanwhile, I note in passing that there is controversy over another recent paper, this time for the Gliese 667C system (also linked in sidebar). This paper has not yet been published in peer review and it is possible the system, as newly envisaged with additional habitable zone planets, is not dynamically stable. I emphasise though that the existence and potential habitability of Gliese 667Cc itself are not in question here.