A mysterious star that erratically dims by nearly a quarter as a swarm of unknown objects passes in front of it – leading to speculations that artificial objects could be orbiting the star – continues to refuse to give up its secrets after a search for laser pulses from the system turned up empty.
The star, nicknamed Tabby’s Star after Tabetha Boyajian of Yale University who first spotted its bizarre behaviour, hit the headlines in October 2015, sparking a variety of ideas as to what could be causing the odd dimming (see our earlier article).
Kepler discovers exoplanets by looking for them transiting – passing in front of – their star, but even the largest planets only block one or two percent of a star’s light. On the other hand, up to 22 percent of the light from Tabby’s Star is being blocked. The way the star dims implicates a cloud of objects and Boyajian’s favoured explanation is a cloud of cometary fragments after a large comet broke up.
However, the mysterious nature of the star, which is also referred to as KIC 8462852, has led to more speculative explanations. Jason Wright, a Professor of Astrophysics of Penn State University, has suggested that the swarm could be an extraterrestrial megastructure, somewhat similar to a Dyson sphere. Astronomers participating in SETI, the Search for Extraterrestrial Intelligence, have been looking for stars that look very similar to Tabby’s Star.
The SETI Institute’s Allen Telescope Array in California has attempted to detect deliberate radio signals directed our way from Tabby’s Star but has heard nothing. Now, a follow-up search for laser signals by astronomers using a 0.5-metre Newtonian telescope at the Boquete Optical SETI Observatory in Panama has also found nothing. High-powered lasers in the petawatt class and above are thought to be ideal for transmitting across interstellar distances– their shorter wavelengths and tightly collimated beams mean they do not disperse as much as radio waves, and a message sent be laser would have a much higher bit rate.
The distance of 1,480 light years to Tabby’s Star created some complications for the optical SETI search. Being so far away, any signals would be weak. Ordinarily, optical SETI experiments would split the light into two beams. Only if a laser pulse is detected in both beams would a detection be confirmed (this avoids false positives caused by cosmic ray strikes on the photometer). However at Boquete, the observatory’s director Marlin Schuetz uses a single photometer so as not to split what would already be a weak signal, before analysing the light to look for periodic pulses that could not be replicated by noise in the system.
In a press statement Douglas Vakoch of the SETI Institute and SETI International – an organisation an organisation that looks at alternative approaches to SETI such as seeking laser pulses, as well as the sending deliberate messages from Earth into space – said, “The hypothesis of an alien megastructure around KIC 8462852 is rapidly crumbling apart.”
However, the absence of intelligent signals from Tabby’s Star does not necessarily disprove the extraterrestrial megastructure hypothesis and when Astronomy Now asked Vakoch about this, he elaborated, “As we search for evidence of extraterrestrial civilisations, we take a conservative approach. While our recent SETI observations can’t rule out [the extraterrestrial hypothesis], they don’t support the hypothesis that this dimming is caused by extraterrestrial engineering.”
Tabby’s Star is no longer visible from Panama, and so future observations will have to be conducted elsewhere. “KIC 8462852 remains puzzling, so it’s worthwhile to continue to do follow-up SETI observations at larger optical observatories, as well as shifting to the infrared to rule out any signals from ET in those frequencies,” says Vakoch.
Meanwhile, efforts continue to find a natural explanation for Tabby’s Star. Last month observations of the star by Massimo Marengo of Iowa State University, who used NASA’s Spitzer Space Telescope, detected no excess infrared light coming from the system, which ruled out debris from an asteroid collision and strengthened the most likely scenario, the broken-up comet.
A paper describing the results of the optical SETI search has been submitted to Astrophysical Journal Letters, and a pre-print is available here.