Volunteers spot the dips
The first people to notice that something was wrong with KIC 8462852 weren't professional astronomers. They were volunteers on Planet Hunters, a citizen science project that asks the public to look through light curves from the Kepler space telescope for the small, regular dips that mark a planet crossing its star. Most stars give clean, flat curves. This one, about about 1,450 light years away, didn't. It dimmed by about 15 percent on 5 March 2011 and by about 22 percent on 28 February 2013, 726 days later, with no repeating pattern. A Jupiter-sized planet blocks about one percent of a Sun-like star's light. Twenty-two percent means something enormous.
Volunteers flagged it, and Tabetha Boyajian led the professional follow-up. The paper's subtitle, 'where's the flux?', is where one of the star's many nicknames comes from. 'Tabby's Star' honors her first name, though some astronomers prefer 'Boyajian's Star', and the catalog name is the one the papers use.
The idea that went around the world
In October 2015, Jason Wright and colleagues pointed out that a swarm of huge artificial structures would also fit the odd light curve, and the possibility of an alien megastructure was in headlines within days. The idea was cheap to test. The SETI Institute pointed the Allen Telescope Array at the star and listened from 1 to 10 gigahertz. The result, published in 2016 (opens in a new tab), was silence: no narrow-band radio signals above the threshold they tested. Boyajian meanwhile raised more than $100,000 in a 2016 crowdfunding campaign to buy a year of continuous monitoring from the Las Cumbres Observatory network of robotic telescopes.
An ordinary star behaving oddly
During the Kepler mission, KIC 8462852 was seen to dip in brightness by up to about 20 percent, with dips lasting 5 to 80 days and no regular pattern. Follow-up observations showed it's an ordinary F3 main-sequence star with no significant infrared excess and no close companions. Boyajian's team confirmed the dips are real and not caused by the instruments, and judged that the best explanation was a family of exocomet or planetesimal fragments, from a single earlier break-up, with a mass of at least one millionth of an Earth mass, from an original rocky body more than 100 kilometers in diameter.
| When | What happened |
|---|---|
| 2015 | Boyajian and colleagues report that the star KIC 8462852 dims irregularly, by up to about 20 percent, for 5 to 80 days at a time, in Kepler data. |
| 2016 | Wright and colleagues suggest it as a target for a search for alien megastructures, while saying natural explanations have so far fit other odd signals. |
| 2016 | Schaefer reports a century-long fade of 0.164 magnitudes per century in old photographic plates (1890 to 1989). Hippke and colleagues say this can't be considered significant. |
| 2017 | Meng and colleagues find the star faded more in ultraviolet than in infrared light, a signature of dust (October 2015 to December 2016). |
| May 2017 | New dips begin: four dips of 1 to 2.5 percent, named Elsie, Celeste, Skara Brae and Angkor. |
Why dust is the leading explanation
An alien structure, or any solid object much larger than a dust grain, would block all colors equally. Dust blocks blue and ultraviolet light more than red and infrared. Meng and colleagues measured the star from the ultraviolet to the mid-infrared and found the dimming was stronger in the ultraviolet, a dimming rate of 22.1 plus or minus 9.7 millimagnitudes per year in the ultraviolet bands of the Swift satellite. In 2017 the star dipped again, and Boyajian's team found the dips were deeper in blue than in red, were inconsistent with optically thick material, and fit ordinary dust, which they said must be mostly optically thin with grains much smaller than a micrometer. The long-term fading, though, may have a separate cause, and the 1890 to 1989 plate result is disputed.
Measured: we know this
- KIC 8462852 is an ordinary F3 main-sequence star, with no close companion.
- Kepler recorded dips of up to about 20 percent, lasting 5 to 80 days.
- Four dips of 1 to 2.5 percent in 2017 were deeper in blue light than in red.
Inferred: we think this
- Most of the dimming is caused by dust, which blocks blue light more than red. NASA's illustration of a hypothetical uneven ring of dust, shown above, reflects this.
- The original Kepler dips could be fragments of exocomets from one break-up, as Boyajian's team judged the scenario most consistent with the data.
Unknown: we don't know
- What produced the dust or fragments, and why this star.
- Whether the slow fading over years and the quick dips share one cause. Boyajian's 2018 paper says its data don't tell us.
What would settle it
- Continued monitoring of the star, in several colors at once, to see how each new dip changes with wavelength.
In human terms
- A solid alien structure would dim every color equally. Dust dims blue and ultraviolet more, and the data show the second pattern.
Questions people ask
Is Tabby's star an alien megastructure?
There's no evidence for that. Measurements in several colors show the dimming is stronger in ultraviolet than in infrared, which is typical of dust and not of a solid structure.
Why does Tabby's star dim?
The best explanation is dust, perhaps from a family of fragments of an exocomet or planetesimal that broke apart. The details are still debated.
Why is it called Tabby's star?
It's named for Tabetha Boyajian, who led the team that reported its odd behavior. Its catalog name is KIC 8462852.
OumuamuaAnother puzzle object, from another star.
Fast radio burstsAnother signal astronomers can't yet explain.
The Wow! signalA 1977 radio signal that never came back.
All mysteriesEvery open question on the site.What the dust did and didn't explain
The monitoring paid off quickly. In May 2017 the star began dipping again, in real time, and observatories around the world could watch it in several colors. That's the key to the dust conclusion above. A solid object blocks every color the same. Dust, like smoke, blocks blue light more than red, and the dips were deeper in blue. Meng and colleagues then found the star's long, slow fading was stronger in the ultraviolet too.
That leaves open a harder question than whether the dust exists: where it came from. Boyajian's team judged a family of exocomet fragments from a single break-up to be the best match to the Kepler dips, and Meng's team argued that only microscopic dust could produce the color-dependent dimming. Dust explains the color of the dimming. It doesn't fully explain every feature of the pattern, and other ideas remain on the table.
How to read the numbers
A 22 percent dip looks dramatic, but two things keep it in proportion. First, the original Kepler dips come from one telescope watching for four years, and the 2017 dips come from a very different set of instruments, so comparing them is hard. Second, claims that the star has also faded over a century, which depend on old photographic plates, were challenged within weeks, and one later study of plates found no real dimming apart from a single dip in 1978.
What to watch next
The star is still watched in several colors at once, and each new dip adds to the picture. That's the same toolkit used to find planets: the transit method that found most of the confirmed exoplanets, applied to a star that breaks the rules. For another object that provoked alien speculation and resolved into something natural, see 'Oumuamua. For the one signal that never resolved, see the Wow! signal.