A series of bizarre readings from a star called KIC 8462852 is still baffling scientists.

Some have speculated it may be an alien 'Dyson sphere' megastructure that is causing a dramatic dip in the star's light.

More recently, Nasa scientists said that it could be a swarm of comets causing the strange signals.

Now, researchers have dismissed this theory as 'implausible' following a study of the star's history over the last 100 years.

Scroll down for video

A series of bizarre readings from a star called KIC 8462852 is still baffling scientists. Some have speculated it may be an alien 'dyson sphere' megastructure that is causing a dramatic dip in the star's light. More recently, Nasa scientists said that it could be a swarm of comets causing the strange signals

WHAT IS A DYSON SPHERE? A proposed method for harnessing the power of an entire star is known as a Dyson sphere. First proposed by theoretical physicist Freeman Dyson in 1960, this would be a swarm of satellites that surrounds a star. They could be an enclosed shell, or spacecraft spread out to gather its energy - known as a Dyson swarm. If such structures do exist, they would emit huge amounts of noticeable infrared radiation back on Earth. But as of yet, such a structure has not been detected. Source: All About Space magazine Advertisement

'Either one of our refutations has some hidden loophole, or some theorist needs to come up with some other proposal,' astronomer Bradley Schaefer from Louisiana State University told the New Scientist.

The Kepler mission monitored the star for four years, looking at two unusual incidents, in 2011 and 2013, when the star's light dimmed in dramatic, never-before-seen ways.

When a planet orbits a star, the star's brightness usually reduces by around one per cent.

But KIC 8462852 has had a reduction of around to 22 per cent, which suggests something huge may be moving past it.

In some cases, the flux dropped down to below the 20 per cent level and lasted anywhere between five and 80 days at a time.

'We'd never seen anything like this star,' said Yale University researcher Tabetha Boyajian, who first spotted the signals.

'It was really weird. We thought it might be bad data or movement on the spacecraft, but everything checked out.'

Scientists have been speculating on what could be causing such irregular dips since the paper was published.

A study using data from Nasa's Spitzer Space Telescope in November suggested it may be a swarm of comets.

Researchers said if a planetary impact, or a collision among asteroids, were behind the mystery of KIC 8462852, then there should be an excess of infrared light around the star.

Dusty, ground-up bits of rock would be at the right temperature to glow at infrared wavelengths.

Astronomers have been looking for answers about what is causing the bizarre light fluctuations around the star KIC 8462852 (pictured) for weeks. Some have suggested it is an alien megastructure such as a Dyson sphere. The strange structure was spotted by researchers from Yale

RULING OUT AN ALIEN STRUCTURE In order to explore the idea that such a structure could have been built by intelligent alien life, the Extraterrestrial Intelligence Institute, Seti, trained its Allen Telescope Array on the star for more than two weeks. Experts looked for two types of radio signal: narrow-band signals generated as a 'hailing signal' for alien societies wanting to announce their presence, and broad-band signals. These signals would be produced by 'beamed propulsion'. Seti said that if large scale alien engineering projects really are underway, the array would pick up signals made by intense microwave beams that could be used to power spacecraft. Scientists analysing the data found no clear evidence for either type of signal. They believe this rules out the presence of omnidirectional transmitters - large antenna - of approximately 100 times today's total terrestrial energy usage in the case of the narrow-band signals, and ten million times that usage for broad band emissions. So the presence of a Dyson sphere is unlikely. Seti scientists note that any society able to build such a megastructure would have access to energy at a level approaching 1027 watts, so that massive transmitters would be detected even if only a tiny percentage of this energy were used for signalling. Advertisement

'Spitzer has observed all of the hundreds of thousands of stars where Kepler hunted for planets, in the hope of finding infrared emission from circumstellar dust,' said Michael Werner, the Spitzer project scientist.

But Spitzer did not find any significant excess of infrared light from warm dust.

That made theories of rocky smashups very unlikely, and favored the idea that cold comets are responsible.

Nasa said at the time it's possible that a family of comets is traveling on a very long, eccentric orbit around the star.

At the head of the pack would be a very large comet, which would have blocked the star's light in 2011, as noted by Kepler.

Later, in 2013, the rest of the comet family, a band of varied fragments lagging behind, would have passed in front of the star and again blocked its light.

By the time Spitzer observed the star in 2015, those comets would be farther away, having continued on their long journey around the star.

In their own study, Schaefer and his team looked at data for the star's history for the past 100 years.

They noticed that the star dimmed gradually by about 20 per cent between 1890 and 1989.

'The basic effect is small and not obvious,' he told New Scientist.

Schaefer said that for the star to dim by about 20 per cent over the course of an entire century, it would require some 648,000 comets to pass in front of it.

Each of the comets would have to be around 125 miles (200km) wide, he said, and that's implausible.

As a planet passes in front of a star's light it causes the light to dim, and Kepler can capture these fluctuations. Typically this light dims in a relatively symmetrical pattern due to the nature of the orbits (examples pictured)

However, during Kepler's study into KIC 8462852 the researchers noticed it went through 'irregularly shaped, aperiodic dips in flux.' (pictured). Using data from a 2013 study, space blogger Jim Galasyn proposes that the dips in 'flux' - or brightness - are caused by the star having a lopsided disc, with two planets passing it

'The comet-family idea was reasonably put forth as the best of the proposals, even while acknowledging that they all were a poor lot,' said Schaefer.

'But now we have a refutation of the idea, and indeed, of all published ideas.'

Another theory that has got traction is that the dips are caused by an alien megastructure, similar to the Dyson sphere first proposed by theoretical physicist Freeman Dyson in 1960.

This theory suggests that a swarm of satellites or solar panels surrounding a star, known as a Dyson swarm, could harness the power of the star and this swam could be could be an enclosed shell, or spacecraft.

Other possible structures include artificial space habitats, or a planet-sized occulting object intended to provide a long-lasting signal to other galactic inhabitants.

Scientists analysing the data found no clear evidence for either type of signal between the frequencies of one and 10 GHz.

The oblate spheroidal shape of some stars cause them to be larger and darker at the equator, and lighter at the poles. As planets pass across the different regions during an orbit, the amount of flux dips in differing ways. This graphic shows five examples of different trajectories (top) and their respective flux readings (bottom)

IS AN OBLATE DISC TO BLAME FOR THE STRANGE DIPS IN FLUX? KIC 8462852, located 1,480 light-years away, was monitored by the Kepler Space Telescope for more than four years, beginning in 2009. As a planet passes in front of a star's light it causes the light to dim, and Kepler can capture these fluctuations. Typically this light dims in a symmetrical pattern. However, during Kepler's study into KIC 8462852 the researchers noticed it went through 'irregularly shaped, aperiodic dips.' Some stars don't have uniformly bright discs and spin so fast they have an oblate spheroidal shape (illustrated) with brighter poles and a darker equator In some cases, the flux dropped down to below the 20% level and lasted between 5 and 80 days at a time. Some stars don't have uniformly bright discs and spin at such a high rate that they have an spheroidal shape. This causes them to have a larger radius at the equator than at the poles. The poles, with their smaller radius, have a higher surface gravity meaning they are hotter and brighter - or 'gravity brightened.' Meanwhile, the equator is cooler and darker, which is known being 'gravity darkened.' Mr Galasyn suggests that the dips and increases in flux of KIC 8462852 are caused as planets move across these brighter and darker areas. Two of the dips, on day 1520 and 1570 of Kepler's mission, are shown having a similar shape but a different magnitude. Despite their differences, both curves follow the shape of a planet travelling across a brightened pole, as suggested by the paper. Mr Glasnyn claims that the two dips could be caused by two planets moving in front of the star. If the first planet is large it could block out around 20% of the star's disc, while a smaller planet could occlude just 8% of it. The second dip may be shorter because the smaller planet is moving faster and orbiting closer to the star. Advertisement

They believe this rules out the presence of omnidirectional transmitters - large antenna - of approximately 100 times today's total terrestrial energy usage in the case of the narrow-band signals, and ten million times that usage for broad band emissions.

So the presence of a Dyson sphere is unlikely.

Seti scientists note that any society able to build such a megastructure would have access to energy at a level approaching 1027 watts, so that massive transmitters would be detected even if only a tiny percentage of this energy were used for signalling.

Institute astronomer Seth Shostak said: 'The history of astronomy tells us that every time we thought we had found a phenomenon due to the activities of extraterrestrials, we were wrong.

'But although it's quite likely that this star's strange behaviour is due to nature, not aliens, it's only prudent to check such things out.'

Their observations will continue, but so far no evidence of deliberately produced radio signals has been found in the direction of KIC 8462852.

While the scientists have all but ruled out an intelligent alien society and comets, the truth behind KIC 8462852 continues to elude them.

Seti blieves that there is no Dyson Sphere (stock illustration). They note that any society able to build such a megastructure would have access to energy at a level approaching 1027 watts, so that massive transmitters would be detected even if only a tiny percentage of this energy were used for signalling