It is becoming less and less likely that a swarm of comets can explain the strange twinkling of a distant star. This star (the star Tabby, named after its discoverer Tabeta Boyajian) has been worrying us since last October, when Jason Wright, an astronomer at the University of Pennsylvania, suggested that it might be surrounded by some sort of alien megastructures. A more likely idea - and less interesting, of course - is that the star is surrounded by a swarm of comets. But scientists increasingly doubt this explanation.
Let's refresh the chronology of the debate
Bradley Schaefer, an astronomer at Louisiana State University, has studied the behavior of a star over the past hundred years from old photographic plates. Not only did the star's strange twinkling begin more than a century ago, but it also gradually darkened during this period - and the second fact is already quite difficult to explain.
The first signs of the star's strange behavior were revealed by the Kepler Space Telescope, which constantly observed the star (and 100,000 other stars) from 2009 to 2013. Astronomers, civilian scientists and computers are looking for regular dims in the star's light - a sign that an exoplanet is passing in front of the star. The largest planets can block up to 1% of the star's light, but Tabby's star loses up to 20% of its brightness. And that would be strange in itself. But the periodic blackouts did not occur at regular intervals - they were sporadic. The planet will not explain such a signature.
In September, a group of scientists led by Yale University's Boyajian tried to make sense of the unusual signal. Scientists first scrutinized the data in search of errors. They even attracted Kepler's scientists to this. But everything turned out to be clean. “The data we observed with Kepler were absolutely astrophysical,” Boyajian said.
However, nothing in the observations indicated a possible cause of the interference. After considering various possible scenarios, Boyajian determined that dust from a large comet cloud would be the best explanation. But he admits that this explanation "would be a bit of a stretch, since it is difficult to find such large comets that would block so much starlight." She published the work in the hope that astronomers will find alternative solutions.
And they found it. A month later, the star blew up the public when Wright claimed that an advanced extraterrestrial civilization that built megastructures like solar panels around the star could be responsible for the signal. And Boyajian believes that this theory has an unconditional right to life.
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“We have to look at the issue from every possible angle - and this is one of the angles, no matter how crazy it seems,” she said. Blogger and astronomer Phil Plat also admits that "it is unlikely, but fits the description of what we see."
But subsequent searches for foreign signals came to nothing.
So Schaefer turned to old photographic plates from the Harvard College Observatory. Luckily for him, the star was photographed over 1,200 times as part of an ongoing sky survey from 1890 to 1989. Such an abundance of data points showed that Tabby's star behaves even more strangely: it twinkles at short intervals, as shown by the data of Kepler and Harvard, and darkens over the course of the century, as shown by the Harvard data.
“In a scenario like this, you need to use Occam's razor (the simplest explanation is probably the best),” Boyajian says. One phenomenon explains both behaviors, she adds. But which one?
Comets are not the best explanation. For a century, a huge number of comets must pass in front of the star, astronomers say.
"They must have more mass than we have in the entire Kuiper Belt (a group of icy bodies in a vast region beyond Neptune's orbit)," says Massimo Marengo, assistant professor of astronomy at Iowa State University who has worked on the comet theory in December. - This could be done if we assume that the same family of comets passes in front of the star again and again. But with a secular dimming trend, this family of comets should grow larger every time it passes a star. But this is a difficult option."
The results also changed the requirements for the alien megastructure hypothesis. Plath noted that the overall darkening could be attributed to a massive alien sphere around the star. But Plath calculated that aliens would need to rebuild at least 750 billion square kilometers of solar panels to provide a 20 percent drop in the star's brightness. This is 1500 times the area of the Earth.
Scientists continue their observations in the hope of finding explanations for the stellar oddities. “Nature could help us by creating another such event,” says Marengo. "But sometimes we are unlucky."
