The Ever More Puzzling, And Intriguing, “Tabby’s Star.”

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Star debris illustration
Did Tabby’s star going through periodic and deep dimmings because of dust and debris clouds that pass edbetween it and the mirror of the Kepler Space Telescope?  That was an earlier explanation for the highly unusual behavior of the star, but new research makes that answer less likely. Artist drawing by NASA/JPL-Caltech/T. Pyle

Substantial, sun-like stars are not supposed to dim.  They start with gravity and pressure induced nuclear reactions, and then they burn brighter and brighter until they either explode (go supernova) or burn all their fuel and become small, enormously dense, and not very bright “white dwarfs.”

Of course, the transit technique of searching for exoplanets looks precisely for dimmings — of stars caused by the passage of an exoplanet.  But those are tiny reductions in the star’s brightness and short-lived.  So if a star is dimming significantly over a much longer period of time, something unusual is going on.

And that is apparently exactly what is happening with the current poster child for mysterious stars — KIC 8462852 or “Tabby’s star,” named after the Yale University postdoc who, with the help of citizen scientists, discovered it,  Tabetha Boyajian.

First written up last fall, the big news was data from the Kepler Space Telescope showed that the star had experienced two major and dissimilar dips in brightness — a highly unusual and perplexing phenomenon.  The dips appeared much too large to represent the passage of an exoplanet, so explanations tended towards the baroque — a swarm of comets, a vast dust cloud, even an alien megastructure (proposed as a last possible explanation.)  The observation was first identified by citizen planet hunters working with Boyajian, making it an even more compelling finding.

Now the mystery has grown stranger still.  A paper made public last week based on a different kind of Kepler imaging (full-frame imaging) found not two but one enormous dip in the light curve, as well as a surprising and significant dimming the of star over the four year observing period of the space telescope.  The paper has been submitted for publication in American Astronomical Society journals.

Benjamin Montet of Caltech and Joshua Simon of the Observatories of the Carnegie Institution of Washington, analyzed the full-field images taken by Kepler every three months (rather than the hourly images studied by Boyajian et al,) and concluded that something strange was indeed going on.

Their conclusion: “No known or proposed stellar phenomena can fully explain all aspects of the observed light curve.”

 

Photometry of KIC 8462852 as measured from the FFI data. The four colors and shapes (green squares, black circles, red diamonds, and blue triangles) represent measurements from the four separate channels the starlight reaches as the telescope rolls. The four subpanels show ux from each particular detector individually. The main gure combines all observations together; we apply three linear osets to the data from dierent channels to minimize the scatter to a linear t to the rst 1100 days of data. In all four channels, the photometry is consistent with a linear decrease in ux for the rst three years of the mission, followed by a rapid decrease in ux of  2:5% over the next six months. The light gray curve represents one possible Kepler long cadence light curve consistent with the FFI photometry created by tting a spline to the FFI photometry as described in Section 4. The large dips observed by Boyajian et al. (2016) are visible but narrow relative to the cadence of FFI observations. The long cadence data behind this gure are available online.
Photometry of KIC 8462852 as measured from the full-frame imaging (FFI) data. The four colors and shapes (green squares, black circles, red diamonds, and blue triangles) represent measurements from the four separate channels the starlight reaches as the telescope rolls.  In all four channels, the photometry is consistent with a decrease in starlight for the first three years of the mission, followed by a rapid decrease in flux of  2:5% over the next six months. The large dips observed by Boyajian et al. (2016) are visible
but less broad relative to the FFI observations. (B. Montet and J. Simon)

Expanding a bit, Montet told Gizmodo:  “We spent a long time trying to convince ourselves this wasn’t real. We just weren’t able to.”

A paper describing the results from these full-frame observations went up recently on the prior to printing site arXiv.    The site allows members of the astronomy world to offer critiques, and so the results as now released may not be final.

But the story line does seem pretty clear — that Tabby’s star had one very large period of light dimming and had a secular decline in the light it was sending out over the four years of the Kepler mission.

Boyajian, a newly-appointed Louisiana State University researcher and professor, said that she considers the original findings to be entirely compatible with the newest results, with differences based on how the light was being captured (the once-monthly full-frame Kepler images versus the continuous imaging done of more than 100,000 stars.)

What has also become increasingly clear is that the dimming is not the result of an instrument glitch, and that the surrounding stars are not exhibiting the same unusual behavior.

“As far as we know, dimming is not something stars do; they get larger and brighter,” she said.  “Especially on these remarkably fast time scales, the dimmings are unprecedented for any kind of star.”

Boyajian had initially favored the theory that the light was being blocked by a large swarm of comets, but she said the new results make that more unlikely.  She said it is similarly unlikely that the dimmings are the result of some internal dynamics of the star.  So is it all the result of some alien megastructure, the “explanation” that initially brought a lot of attention to Tabby’s star.  I think we can assume it is not.

But given the data now available, it has become extremely difficult to find an explanation that checks all the boxes.  And that’s why Boyajian and her colleagues began a kickstarter campaign to raise $100,000 for another year of observing through the telescopes of the private Las Cumbres Observatory Global Telescope Network.

As she explained it, one of the telescopes will image the star at least two hours per night for the next year.  And if a significant dimming is observed, larger ground-based telescopes will be available to look more closely.

It’s a waiting game now, which is exciting itself,” she said. “It’s only a guess, but based on Kepler light curves, we might see something interesting next spring.”

(My earlier story on Tabby and her star can be found here:  Tabby’s Star)

Tabetha Boyajian was the driving force behind bringing the mysterious star xxxx to public attention. It had initially been identified as peculiar by the citizen scientists of xxx.
Tabetha Boyajian was the driving force behind bringing the mysterious star KIC 8462852 to public attention. It had initially been identified as peculiar by the citizen scientists of Planet Hunters, which is part of the Oxford University based “Zooniverse” Citizen Science Alliance.

 

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Rocky, Close and Potentially Habitable Planets Around a Dwarf Star

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This artist’s impression shows an imagined view from the surface one of the three planets orbiting an ultracool dwarf star just 40 light-years from Earth that were discovered using the TRAPPIST telescope at ESO’s La Silla Observatory. (M. Kornmesser/ESO)
This artist’s impression shows an imagined view from the surface one of the three planets orbiting an ultracool dwarf star just 40 light-years from Earth that were discovered using the TRAPPIST telescope at ESO’s La Silla Observatory. (M. Kornmesser/ESO)

Forty light-years away is no small distance. But an announcement of the discovery of two planets at that separation that have been determined to be rocky and Earth-sized adds a significant new twist to the ever-growing collection of relatively close-by exoplanets that just might be habitable.

The two planets in the TRAPPIST-1 system orbit what is known as a red dwarf star, a type of star that is typically much cooler than the sun, emitting radiation in the infrared rather than the visible spectrum.  While there has been much debate about whether an exoplanet around a dwarf can be deemed habitable, especially since they are all believed to be tidally locked and so only one side faces the star, a consensus appears to be growing that dwarf stars could host habitable planets.

The two new rocky exoplanets were detected using the Hubble Space Telescope and were deemed most likely rocky by the compact sizes of their atmospheres — which were not large and diffuse hydrogen/helium envelopes (like that of the Jupiter) but instead more tightly packed, more like the atmospheres of Earth, Venus, and Mars.  It was the first time scientists have been able to search for and at least partially characterize of atmospheres around a temperate, Earth-sized planet.

Having determined that the planets are rocky, principal investigator Julien de Wit of M.I.T’s Department of Earth, Atmospheric and Planetary Sciences, said the goal now is to characterize their atmospheres.

“Now the question is, what kind of atmosphere do they have?” de Wit said. “The plausible scenarios include something like Venus, where the atmosphere is dominated by carbon dioxide, or an Earth-like atmosphere with heavy clouds, or even something like Mars with a depleted atmosphere. The next step is tomtry to disentangle all these possible scenarios that exist for these terrestrial planets.”

Artist's impression of the two planets in the Trappist-1 solar system. These worlds have sizes, temperatures and potentially atmospheres similar to those of Venus and Earth. Some believe they may be the best targets found so far for the search for life outside the solar system. They are the first planets ever discovered around such a tiny and dim star. (Nasa/ESA/STScI)
Artist’s impression of the two planets in the Trappist-1 solar system. These worlds have sizes, temperatures and potentially atmospheres similar to those of Venus and Earth. Some believe they may be the best targets found so far for the search for life outside the solar system. They are the first planets ever discovered around such a tiny and dim star. (Nasa/ESA/STScI)

 

Host stars with exoplanets that are (very relatively) close to us are highly valued because they are potentially easier to observe and characterize.

There are 24 known exoplanets within 40 light-years, 14 are within 30 light-years, and only six are within 20 light-years. The closest exoplanet considered confirmed by NASA is Epsilon Eridani b, 10.5 light-years away from our solar system, while the closest known rocky planet is HD 219134 b, which is 21 light-years away..  Planetary companions have been suggested to exist in some of the nine star systems located within 10 light-years away, including in the closest system, Alpha Centauri (4.1 light-years away).

TRAPPIST-1 (planets b and c) are among the closest orbiting a red dwarf star, and they provided an unusual double transit to observe.

“The two planets actually transited their star just 12 minutes apart so we got two planets for the price of one,” said co-author Hannah Wakeford of NASA’s Goddard Space Flight Center.  “This is the first time two planets have been characterized with Hubble at the same time on purpose, and the first time such small (Earth-sized) planets have had atmospheric follow-up done.”

The researchers hope to use Hubble to conduct follow-up observations to search for thinner atmospheres, composed of elements heavier than hydrogen, like those of Earth and Venus.

“With more data, we could perhaps detect methane or see water features in the atmospheres, which would give us estimates of the depth of the atmospheres,” she said.

The results were reported in the journal Nature.

Hubble/WFC3 white-light curve for the TRAPPIST-1b and TRAPPIST-1c double transit of 4 May 2016. (NASA/SScI)
Hubble/WFC3 white-light curve for the TRAPPIST-1b and
TRAPPIST-1c double transit of 4 May 2016. (NASA/STScI)

There’s an interesting story behind their Hubble observation of the two transits.  Using their relatively small telescope at the European Southern Observatory’s La Silla facility in Chile, the TRAPPIST-1 team detected the unusual three-planet system around the small, cool star and published their discovery a little more than two months ago.

Within days, they realized that planets b and c would be orbiting the star at almost exactly the same time — an unexpected and quite valuable occurrence.  (Information about that double transit was provided via the Spitzer Space Telescope, which had also been studying the orbits of planets in the TRAPPIST-1 system.)

The upcoming double transit was confirmed but two weeks before the event. The team requested Hubble time for a quick observation, and it was granted.  The successful observation soon followed.

DeWit said that planets with the sizes and equilibrium temperatures of TRAPPIST-1b and TRAPPIST-1c could possess relatively thick atmospheres with water, carbon dioxide, nitrogen and oxygen.

The TRAPPIST (TRAnsiting Planets and PlanetesImals Small Telescope) project is the creation in large part of the Origins in Cosmology and Astrophysics group of the University of Liege in Belgium.

The TRAPPIST instrument is new kind of ground telescope designed to survey the sky in infrared. TRAPPIST was built as a 60-centimeter prototype to monitor the 70 brightest dwarf stars in the southern sky. Now, the researchers have formed a consortium, called SPECULOOS (Search for habitable Planets Eclipsing ULtra-cOOl Stars), and are building four larger versions of the telescope in Chile, to focus on the brightest ultracool dwarf stars in the skies over the southern hemisphere. The researchers are also trying to raise money to build telescopes in the northern sky.

The 60cm telescope is devoted to the detection and characterization of planets located outside our Solar System and to the study of comets and other small bodies in our solar system. (Trappist/ESO)
The 60cm telescope is devoted to the detection and characterization of planets located outside our Solar System and to the study of comets and other small bodies in our solar system. (Trappist/ESO)

According to De Wit, he TRAPPIST telescopes are inexpensive compared with their peer — about $400,000 per instrument.

He is pushing to make them a relatively affordable “prescreening tool” that scientists can use to identify planets that are potentially habitable.  The TRAPPIST observations would then be followed up by my detailed study using powerful telescopes such as Hubble and NASA’s James Webb Telescope, which is scheduled to launch in October 2018.

“With more observations using Hubble, and further down the road with James Webb, we can know not only what kind of atmosphere planets like TRAPPIST-1 have, but also what is within these atmospheres,” de Wit says. “And that’s very exciting.”

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