A new paper from Columbia University suggests that Tabby's star -- the celestial object voted most likely to host an alien megastructure -- is acting weirdly because it recently annihilated an entire planet, and the shattered remains of that planet are now producing strange flickering effects. It's probably the best theory we've heard so far.
It isn't aliens.
At least that's the conclusion reached by a trio of astronomers from Columbia's Departments of Physics and Astronomy. Rather, the scientists say that the slow dimming of Tabby's star, along with its bizarre and dramatic dips in light -- sometimes by as much as 22 per cent -- are likely the result of a celestial collision involving the star and one of its planets. The new paper, authored by Brian Metzger, Ken Shen and Nicholas Stone, is currently undergoing peer review at the astrophysical journal Monthly Notices of the Royal Astronomical Society.
KIC 8462852, better known as Tabby's star, is a fairly standard star as far as most things go, but its strange behaviour has infuriated and confounded astronomers over the past year. Data from NASA's Kepler space telescope revealed the star's odd flickers in 2015, but a subsequent survey deepened the mystery by showing that the star's brightness faded by 14 per cent from 1890 to 1989, and another three per cent during the four years that Kepler scanned it. Such behaviour is unprecedented and, frankly, very hard to explain. A number of theories were posited, including comet swarms, an elongated star, a planetary collision and (of course) an alien megastructure.
One theory for the odd behaviour exhibited by Tabby star is that two proto-planets smashed into each other (pictured above), producing a massive debris field. The new theory suggests a collision likely occurred -- but between a large planet and the star itself. (Image: NASA/JPL-Caltech)
The trouble is, none of these theories can explain the double-whammy of anomalies, that is, the odd short-term flickers and the gradual, long-term dimming. The new theory by Metzger and his colleagues does just that.
The researchers hypothesise that a planet recently plunged into KIC 8462852. The gravitational energy produced by the collision would have boosted the star's normal nuclear energy production, causing it to brighten rapidly and then slowly decay over time. If this idea is correct, then we happen to be observing Tabby's star as it returns to its normal level of luminosity.
This theory may also help to explain the star's sudden dips in light. Following the collision, portions of the planet (and possibly bits of its moon or moons) entered into eccentric orbits. Every time this debris field moves in front of the star (from our vantage point), it appears to flicker.
As for the size of the planet, the researchers say that depends on when the impact occurred. "If the planet was Jupiter-sized, then the disruption would have taken place about 1000 years ago in order to explain the rate of dimming observed today," Metzger told Gizmodo "On the other hand, if the object was the size of the Moon, then the brightening and dimming would have taken place about a decade ago." Metzger says a collision with a moon-like object probably wouldn't produce the observed century-long dimming, but it could explain the dimming seen during four years of Kepler observations.
The researchers say they're "agnostic" about the source of the object or objects that fell into Tabby's star. "We could imagine many smaller objects in succession, or one big one a fairly long time ago," said Metzger. "It's hard to tell at this late of stage, although maybe the orbiting debris can provide some clues."
According to Stone, the collision likely didn't happen overnight. "The planet may have spent millions of years slowly decreasing its pericenter [closest approach to Tabby's star on its orbit] before it finally impacted the star, causing a brightening and then the slow dimming we have observed," he told Gizmodo. "Towards the end of this slow process... any moons around the planet would have been detached by the tidal field of Tabby's star." Given that gas giants like Jupiter and Saturn host dozens of fairly large icy moons, that's a lot of potential material to be added to the orbiting debris cloud.
"Moreover, these moons now have pericenters around the star that are closer than the orbit of Mercury around our own Sun, so if they have icy outer layers, they should be evaporating and outgassing rapidly, blowing clouds of vapour and dust off their surfaces," Stone continued. "The dust in these expanding evaporation clouds may be able to explain the irregular transits of Tabby's Star seen by Kepler." Alternately, the tidal heating of the detached moons could be creating enormous clouds of dust.
The fact that we're seeing the late stages of a collision -- an extremely transitory episode by cosmological standards -- suggests that events like these are common. Kepler has observed around 100,000 stars so far, but there are about 100 billion stars in the Milky Way galaxy. "Therefore, if we observed one star which had recently swallowed a planet in the Kepler field, that means there are one million stars our galaxy doing the same thing as Tabby's star," said Metzger. If Tabby's star is representative, the researchers estimate that a given star should suck up around 10 Jupiter-like planets over its lifetime, or about 10,000 moon-like objects (not actual moons, but objects of similar mass). If true, that's kind of incredible.
The good news about this theory is that it's testable. Metzger says the next time we see a big dip in the luminosity of Tabby's star, we should expect to see significant amounts of gas and dust released from the debris field. "This should produce a temporary flare of infrared emission (as the dust is heated) lasting for a few days to a few weeks," Metzger told Gizmodo.
Another way to prove this theory would be to actually watch a moon or planet smash into a star. Such an event should produce a temporary brightening, followed by another period of steadily decreasing luminosity.
It's an intriguing theory, and one that makes a hell of a lot of sense. It isn't an open-and-shut case, but thanks to studies like this, we're getting increasingly close to solving this fascinating mystery.