Everyone wants to know where the aliens are. In the current issue of Publications of the Astronomical Society of the Pacific (PASP), researchers released their report The Breakthrough Listen Search for Intelligent Life: No Evidence of Claimed Periodic Spectral Modulations in High Resolution Optical Spectra of Nearby Stars. That is a mouthful and a lot to unpack.
First, this work was done by the Breakthrough Initiatives organization. Their Breakthrough Listen program is tasked with combing through astronomical observations in search of intelligent life in the universe. Some of their approaches are different from the classical SETI method of analyzing radio waves looking for messages. One such approach is to look for small modulations in a star's spectrum in search of a techno-signature. A star's spectrum is basically a representation of all the colors of light coming from the object at different wavelengths (see figure 1). There are noticeable dark spots in the spectrum and these are called spectral lines. These are very important to astronomers because as the radiation leaves the star, molecules in the star's atmosphere will absorb different colors of light based on its properties. This gives clues as to what types of molecules are in the object of interest.
In addition to getting the star's composition, spectral lines contain even more information about the object for example. If the object is moving towards or away from Earth, the spectral lines with be squeezed (i.e., narrowed) or stretched (i.e., widened) accordingly. It therefore follows that this information can be used to deduce the direction of motion of the object. In 2016, Borra & Trottier proposed an even more interesting use of spectral lines: to infer the presence of aliens. Specifically, they report the possibility that certain spectral modulations could be caused by pulses of light from technological alien civilizations. The modulations are too small to be seen by eye in the raw spectra charts, but using Fourier analysis makes detection possible (see Figure 2). They came to this conclusion by analyzing 2.5 million stars' data from the Sloan Digital Sky Survey Data Release 8 (SDSS DR8). Only 234 stars analyzed displayed the modulation, so they contend that this is not simply an instrumentation effect. Furthermore, they state that while rapid pulsations in small regions of the star's atmosphere could cause these signals, the period of pulsation would be unfeasible for stars (Borra & Trottier 2016)
Isaacson et al. (2018) sought to follow up on this work by collecting high resolution spectra of the stars identified by Borra & Trottier (2016). They used the Automated Planet Finder at Lick Observatory to obtain the data. After performing a similar analysis, Isaacson et al. (2018) was not able to replicate the results (see Figure 3). As part of the Fourier analysis, different amplitudes can be chosen and thus several models can be employed in order to find the best match. As seen below, models with amplitudes of 10%, 3%, 1%, and 0.3% were tried unsuccessfully.
The researchers suggest a few explanations for the failure to replicate the results. First, the alleged signal may not be continuous and therefore may not have been present when the new observations were made. Also, there may indeed be a periodic instrumentation effect with the SDSS spectra that only affects a small amount of the data. In any event, the search for ET continues.
References
Borra, E. F. & Trottier, E. 2016, PASP, 128, 114201
Isaacson et al. 2018. PASP , v131, n995