Tabby's star (KIC 8462852)

Kepler observed the star almost continuously for four years. Most of the time the light curve is flat; the events fall mainly in two groups, an isolated deep dip in Quarter 8, midway through the mission, and a cluster of dips in Quarters 16 and 17, the last months, reaching about 20 per cent, with complex and asymmetric shapes and durations of days⁠[1]⁠[11]. The events have no period. Wright and Sigurdsson found the timings of the deepest dips consistent with being random⁠[6].

Planet Hunters volunteers drew attention to the star, and Boyajian and colleagues reported it in 2015 as "where's the flux?". They showed the dips were in the star's light and not an artefact, that the star is an F-type main-sequence star, and that it has no infrared excess of the kind a massive dust disk would produce⁠[1]. Spitzer and ground-based infrared observations later in 2015 confirmed the absence of warm dust⁠[12]⁠[13].

After Kepler, the star was watched from the ground. In May 2017 it began a series of new dips, the first since 2013, and dipped repeatedly until it moved out of view at the end of the year⁠[7]. TESS has since observed it in seven sectors between 2019 and 2024. Nothing like the Kepler or 2017 dips has recurred, but Sector 15 holds a single symmetric dip of 1.1 per cent lasting about 21 hours on 3 September 2019. Its shape is that of a hard body crossing a limb-darkened star: a planet-transit model fits it with no excess scatter and beats any dust profile decisively, and its duration matches a circular orbit of about a thousand days around this star. A 2026 radial-velocity study reads it as a planet of about nine Jupiter masses⁠[10]⁠[14]. Every deep Kepler dip, by contrast, is skewed, with a faster ingress than egress. AAVSO observers have measured the star almost nightly since October 2015, 144,000 observations in all, and ASAS-SN has followed it since 2015: both records show the slow fade continuing to early 2018 and then stopping, with the star holding its level from 2018 to 2026, and AAVSO observers were watching through the first predicted return of the planet in October 2026.

The record is instrumental, and the accounts are the papers that interpret it. They fall into four threads.

The dips. The discovery paper is the reference description of the Kepler events⁠[1]. Bodman and Quillen fitted them with swarms of comets⁠[11]; Neslušan and Budaj with four dust-shrouded bodies on one orbit⁠[15]; Kiefer and colleagues found two near-identical dips 928 days apart⁠[16]; Ballesteros and colleagues proposed Trojan swarms on a 12-year orbit and predicted a return in 2021⁠[17]; Sacco and colleagues a 1,574-day period with a return in October 2019⁠[18]; Bourne, Gary and Plakhov a ringed brown dwarf with a repeat on 27 December 2021⁠[19]. The 2017 campaign, run mainly on the Las Cumbres network by a team of about two hundred authors, named the dips Elsie, Celeste, Skara Brae and Angkor and showed them deeper in blue than red⁠[7]; Deeg and colleagues confirmed the colour with the Gran Telescopio Canarias⁠[8]; Hitchcock and colleagues saw a month-long colour-dependent brightening after one dip⁠[20]. In 2026 three papers returned to the TESS record: Madurga-Favieres and colleagues modelled the 2019 symmetric dip with radial velocities as a planet of about nine Jupiter masses⁠[10], Zhu and colleagues fitted it with a ringed planet⁠[14], and Socas-Navarro found a Bayes factor of about 45 for recurrence of the dip families at 782 ± 80 days⁠[21].

The fade in the Kepler era and after. Montet and Simon measured it in the Kepler full-frame images⁠[4]. Meng and colleagues followed it from the ultraviolet to the mid-infrared in 2015-16 and found its colour shallower than interstellar extinction⁠[9]; Davenport and colleagues found a matching 3 per cent drop in GALEX ultraviolet between 2011 and 2012⁠[22]. Simon and colleagues showed from ASAS and ASAS-SN that the star's brightness since 2006 has wandered rather than declined steadily: brighter in 2006-07, fainter in 2009-13, brighter around 2014, fainter again at 6 mmag per year in 2015-17⁠[23]; Hippke and Angerhausen showed the year-long variations cannot be cyclic with a period under ten years⁠[24]. Schaefer and colleagues followed the decline in four colours to early 2018⁠[25]. No published measurement extends past 2018. This department has reduced two independent records that do. In the complete AAVSO record (24 observers in V, 2015-2026), the decline runs at +8.5 ± 2.6 mmag per year to early 2018 and then stops, at -0.04 ± 0.32 mmag per year from mid-2018 to late 2026. ASAS-SN image-subtraction photometry, the reduction Simon used, reproduces his 2015-2018 rate (5.8 ± 1.2 mmag per year) and then shows the g band flat at +0.19 ± 0.16 mmag per year over eight years from two cameras. The public ASAS-SN catalogue light curve shows the 2015-2018 fade only weakly, because it calibrates each image against nearby stars and absorbs slow trends; it should not be used for this. Joined on their overlaps with Simon's full ASAS table, these records give one V light curve from 2006 to 2026. On it the star faded 0.047 mag from 2010 to 2013 and recovered 0.034 by 2015, then faded 0.01 to 2018 and has been flat since. Both fades fall in the two episodes when the star was dipping.

The century fade. Schaefer measured 1,338 Harvard plate magnitudes from 1890 to 1989 and reported a fade of 0.164 mag per century with flat check stars⁠[2]. Hippke and colleagues measured the long-term stability of the same archive, found a structural break at 1962 rather than a trend, and put the systematic floor at 0.15-0.2 mag per century⁠[3]; Schaefer replied that their comparison stars were crowded and mixed emulsions⁠[26]. Lund and colleagues extended the test to 644 F stars and found the population's slopes well behaved only once a jump across the 1953-1969 "Menzel gap" in plate-taking is modelled⁠[27]. Three independent archives were then consulted: Sonneberg and Sternberg plates gave the star constant to 0.03-0.04 mag per century from 1934⁠[28]; Maria Mitchell Observatory plates gave +0.12 ± 0.02⁠[29]. A fourth archive, APPLAUSE, holds about 94,000 digitised Hamburg, Bamberg, Potsdam and other German and Baltic plates calibrated against Gaia. Its own light curve of the star shows no decade-scale variation⁠[30]. The Harvard archive was wholly reprocessed as DASCH Data Release 7 in 2024⁠[31]⁠[32], and this department's re-analysis of DR7 against matched comparison stars, with a Gaia neighbour screen and DR7's plate colour terms, is recorded under the question below. So is its re-analysis of APPLAUSE against the same comparison stars.

The explanations. Wright and Sigurdsson's survey of the families of solutions, and Wright's 2018 reassessment after the colour results, are the clearest statements of what the readings are and how each is constrained⁠[6]⁠[33]. Wyatt and colleagues showed one exocomet population can produce both dips and the Kepler-era fade, and that a century fade must have ended before 2010⁠[34]; Metzger, Shen and Stone proposed recovery from swallowing a planet⁠[35], and Martinez, Stone and Metzger an evaporating exomoon left over from such a collision⁠[36]; Foukal proposed an intrinsic change in the star's convective flux⁠[37]; Shestakova and colleagues bodies falling in and breaking up⁠[38]. The bound companion was disputed⁠[39] and then confirmed with five years of Keck astrometry⁠[40]; Young and Wyatt tested whether it could drive comets inward and found the odds about one in a thousand⁠[41].

The core data are public. The Kepler long-cadence light curves (30-minute sampling) for all 18 quarters and the full-frame images, from which the four-year fade was measured, are in the Mikulski Archive for Space Telescopes⁠[42]⁠[4]. TESS recorded the star at 2-minute cadence in Sectors 14 and 15 (2019), 41 (2021), 54 and 55 (2022), and in full-frame images in Sectors 75, 81 and 82 (2024)⁠[42]. Yang and Satarla stitched and detrended the 2019 to 2022 2-minute data with a half-day window and found no dip deeper than 0.5 per cent⁠[43]; with a three-day window the same data show the 1.1 per cent symmetric dip of 3 September 2019, and the 2024 sectors show nothing to 0.5 per cent at durations of a quarter to one day. Together Kepler and TESS cover about 1,520 days of the star's life, enough to exclude 80 per cent of the orbital periods between 100 and 3,000 days for a strictly periodic transit of the 2019 kind.

The colour information comes from the 2017 dips: multi-band photometry from Las Cumbres and other telescopes⁠[7], and spectrophotometry from the Gran Telescopio Canarias showing the dips deeper in blue than in red with an absorption coefficient that fits grains of about 0.0015 to 0.15 micron⁠[8]. One deep dip was followed by a colour-dependent brightening lasting about a month⁠[20]. The slow fade was measured from the ultraviolet to the mid-infrared from 2015 to 2016, at about 22 mmag per year with Swift and about 5 mmag per year at 3.6 to 4.5 microns⁠[9], and continued, at a varying rate, to 2018⁠[25]. The ground record from October 2015 to the present is the AAVSO International Database: 144,070 measurements, 82,000 of them in Johnson V and 33,000 in B, from about forty observers, with the comparison stars used recorded for each; the department holds the complete export in two files. ASAS-SN covers the star nightly from February 2015, in V to 2018 and in g since, and the department holds both its catalogue light curve and an image-subtraction light curve to 25 October 2026. Before 2015 the record is ASAS, whose 496 V measurements of 2006-2017 were published only in the source files of Simon and colleagues' paper; the department holds them⁠[23]. ZTF saturates on a star this bright and Gaia DR3 published no epoch photometry for it.

The long history is in the Harvard plate collection, now served only as DASCH Data Release 7, which gives 2,717 detections of the star between 1890 and 1990⁠[31]. Sonneberg (1934-1995), Sternberg (1895-1995) and Maria Mitchell (1922-1991) plates have each been measured once⁠[28]⁠[29], but their per-plate measurements are not public. The APPLAUSE archive is: it holds 238 detections of the star on Hamburg and Bamberg plates from 1915 to 1981, including the 1953-1969 years Harvard missed⁠[30]. The star has a faint companion about 2 arcseconds away, confirmed in 2021 as a bound M dwarf at about 880 au⁠[40]; Gaia DR3 shows nothing else within 15 arcseconds brighter than 1 per cent of the star. Radio and optical searches for technosignatures found nothing: no narrowband radio signal above 180 to 300 Jy in a 1 Hz channel between 1 and 10 GHz⁠[44] and no optical laser pulses⁠[45].

Circumstellar dust in clumps is the explanation the colour data support. A dip deeper in blue than red means the occulter scatters short wavelengths more, which fine dust does and solid bodies, planets or opaque panels do not⁠[7]⁠[8]. The grains implied are small enough to be pushed out of the system by starlight, so they must be renewed within days by something travelling with them⁠[8]. Candidate sources include a family of comets from a broken-up body, which can reproduce depths and durations of the later Kepler dips with tens to hundreds of large comets but not the Quarter 8 dip⁠[11]; Trojan swarms around a ringed planet, which predicted a return in 2021⁠[17]; bodies falling towards the star and breaking up⁠[38]; and debris from a planet the star has swallowed⁠[35]⁠[36]. Driving comets inward by the bound companion through the eccentric Kozai-Lidov mechanism was tested and found to have a probability of about one in a thousand of producing one such star in the Kepler field⁠[41].

The 2019 dip is a different kind of event. A limb-darkened planet transit fits it with no excess scatter and is preferred over a Gaussian dust profile by 196 in BIC and over a free asymmetric trapezoid by 27; the fitted orbit size implies a stellar density of about 460 kg per cubic metre at a thousand-day period, close to the 410 expected for the star, so the duration and a long circular orbit agree without any radial velocities. Every deep Kepler dip is skewed the other way, with ingress faster than egress. If it is a planet⁠[10]⁠[14], the system has a giant on an orbit of 1,030 to 1,370 days whose gravity the dust models will have to accommodate, and perhaps the perturber they have had to assume. The reading is testable on a calendar: the Kepler and TESS record already rules out four periods in five, AAVSO observers covered the October 2026 return of the 1,301-day window and saw no transit over three-quarters of it, and the surviving windows put the next transits in February and early March 2027 and in February 2028.

The slow fade is the harder part, and its history is now longer. Meng and colleagues found its colour dependence less steep than interstellar reddening and attributed it to circumstellar dust⁠[9]. Metzger and colleagues proposed that the star is recovering from swallowing a planet⁠[35]. Wright and Sigurdsson rated an intervening cloud in the interstellar medium, or an object with a large disk between us and the star, as the most plausible natural explanations for dips and fade together⁠[6]. The Kepler and post-Kepler fades rest on other instruments and are not in doubt⁠[4]⁠[9], but two independent records now show the decline stopping. AAVSO V and ASAS-SN image subtraction both measure 6-8 mmag per year through 2017, and both are flat to 0.3 mmag per year from mid-2018 to late 2026, in different bands, instruments and reductions. Together with the brightenings of 2006-07 and 2014⁠[23], the picture is of a screen of material whose thickness changes on timescales of a few years, not of a star running down. On one scale from 2006 to 2026 the timing is telling. The star faded by about 4 per cent from 2010 to 2013, the years of the Quarter 8 dip and the final Kepler cluster, and by about 1 per cent from 2015 to 2018, around the 2017 dips. It recovered after each, and has been flat through the eight dip-free years since. Two episodes are a small sample, but they point to one dust supply behind both the clumps and the diffuse screen, with the screen thickening while clumps are being made. That predicts a renewed fade with the next dip episode. Any model that produced a monotonic fade (a planet-swallowing relaxation, a steadily thickening disk) has to explain the recoveries.

Whether the star also faded through the twentieth century is still open, and the department's DASCH DR7 re-analysis says why. The 0.166 mag per century slope is reproduced, and against comparison stars of the same brightness and colour on the same plates the star has the steepest slope and the largest offset across the 1953-1969 gap; but it is only about 2 sigma above them, the comparison stars share a drift of +0.03 mag per century, and the star's own record is flat from 1890 to 1953. The signal is a drop of about 0.07 mag between the 1950s and the 1970s beyond what matched stars show, not a steady century-long fade⁠[2]⁠[3]⁠[27]. The two ordinary ways a plate archive can fake such a step are now excluded: Gaia DR3 shows no neighbour bright enough to blend with the star on the coarse post-1962 plates, and on plates DASCH classes as good blue the step is, if anything, larger and still above every matched star, while the colour-term dependence runs the wrong way to produce it⁠[31]. What remains is a real change in the star's light or a systematic tied to its exact position on the plates. Independent archives disagree about even that: Sonneberg sees no step⁠[28], Maria Mitchell sees a steady fade⁠[29]. The department's re-reduction of APPLAUSE against the same matched comparison stars finds the Harvard shape again: flat before 1953, flat after 1955, and about 0.15 mag fainter in between, beyond 95 per cent of the comparison stars⁠[30]. It is only about 1.4 sigma, and in APPLAUSE every change of era is also a change of telescope or emulsion. But it has the same sign as Harvard, and it puts the drop by about 1955, before most of the Harvard gap. A decade-scale drop of several per cent that then holds, which is what the plates suggest for the 1950s-60s, is the same kind of behaviour the modern record shows at the 1 per cent level.

Intrinsic variability of the star was left open by the 2017 campaign, which noted that its data might also be consistent with variations in the photosphere⁠[7]; Foukal's convective-flux model is the one worked-out version⁠[37], and no known stellar process reproduces the whole light curve⁠[4]. Artificial megastructures were proposed as a hypothesis to be tested, not a conclusion, and predicted grey (colour-independent) dips for large opaque structures⁠[5]. The chromatic dips and the null technosignature searches have removed most of the support for that reading⁠[7]⁠[44].

Spectroscopy of a deep dip across a wide wavelength range would settle what makes the dips: a comet family predicts water and carbon monoxide gas with cometary grains, debris from a destroyed planet predicts rock-forming silicates, and an intervening cloud predicts interstellar sodium lines that change during the dip. That waits on the star dipping again.

Other tests need no such luck. If the 2019 dip is a planet, a 1.1 per cent, 21-hour transit falls on 2-6 February or 28 February to 2 March 2027, or 17-23 February 2028; time series through each window, not single nightly points, are within reach of good amateur photometry. A transit establishes the planet; flat series through every remaining window retire it⁠[10]. The 2017 dips, in the AAVSO B and V record and the campaign's multi-band photometry, can be fitted both with a dust extinction law and with a cooling of the star, which predict different ratios of depth between the bands⁠[7]⁠[37]. The same record covers the windows of the claimed 782-day recurrence of dip families⁠[21]. For the century fade, Sonneberg's per-plate measurements, which are not public, reduced against the same matched stars would test the 1950s step within one telescope and emulsion⁠[28]⁠[31].

Dust gains if the band ratios follow an extinction law, and intrinsic variability if they follow a temperature drop. Dips that recur on schedule would place the dust in orbit and rule out intervening material. If fade and dips share a supply, the next dip episode should begin with a slow fade of a per cent or more, so a seasonal drop of 10 mmag in the AAVSO record is the signal to intensify monitoring; a fade that returns with the dips counts against any one-way relaxation, such as recovery from a swallowed planet.