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.”
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)
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.”
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.
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.
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.”
Seven years ago this month the Kepler spacecraft launched into space – the first NASA mission dedicated to searching for planets around distant stars. The goal was to conduct a census of these exoplanets, to learn whether planets are common or rare. And in particular, to understand whether planets like Earth are common or rare.
With the discovery and confirmation of over 1,000 exoplanets (and thousands more exoplanet candidates that have not yet been confirmed), Kepler has taught us that planets are indeed common, and scientists have been able to make new inferences about how planetary systems form and evolve. But the planets found by Kepler are almost exclusively around distant, faint stars, and the observations needed to further study and characterize these planets are challenging. Enter TESS.
The Transiting Exoplanet Survey Satellite (TESS) is a NASA Explorer mission designed to search for new exoplanets around bright, nearby stars. The method that TESS will use is identical to that used by Kepler – it looks for planets that transit in front of their host star. Imagine that you’re looking at a star, and that star has planets around it.
If the orbit of the planet is aligned correctly, then once per “year” of the planet (i.e. once per orbit), the planet will pass in front of the star. As the planet moves in front of the star, it blocks a small fraction of the light, so the star appears to get slightly fainter. As the planet moves out of transit, the star returns to normal brightness. We can see an example of this in our own solar system on May 9, 2016, as Mercury passes in front of the Sun.
We can learn a lot from observing the transits of a planet. First, we can learn the size of a planet – the bigger the planet, the more light it will block, and the larger the “dip” in the brightness of the host star. Second, we can learn how long the planet’s year is – since it only passes in front of the star once per orbit, the time between transits is the planet’s year.
The duration of the year, in combination with the properties of the host star, also allows us to determine if a planet might be habitable. With high precision measurements, we can also infer much more about the orbit of the planet (e.g., the eccentricity of the orbit). And, in fact, in some cases, we can look at small changes in the apparent year of the planet to discover additional planets in the system that do not transit (Transit Timing Variations).
To observe these transits, TESS will use four identical, extremely precise cameras mounted behind four identical 8-inch telescopes. Each one of these cameras will be sensitive to changes in the brightness of a star as small as about 40 parts per million, allowing TESS to detect planets even smaller than our planet.
Earth, transiting the sun, would produce a dip of about 100 parts per million. Each of the four cameras has a field-of-view of 24°×24°, and the fields of the four cameras are adjacent so that TESS will instantaneously observe a 24°×96° swath of the sky (referred to as an observation sector). Within this field, TESS will collect “postage stamp” images of about 8,000 stars every two minutes – the postage stamps are small sub-images, nominally about 10×10 pixels.
TESS will stare continuously at each of these observation sectors for 27 days before moving to the next sector; over the course of one year, this will give TESS coverage of almost one entire hemisphere, with postage stamp data on approximately 100,000 stars. In the second year of the TESS mission, 13 additional sectors will cover the other hemisphere of the sky, resulting in observations of about 200,000 stars.
The method used for these postage stamp-sized observations is very similar to that used for Kepler, but the survey itself is different. While TESS is conducting an all-sky survey (about 40,000 square degrees), Kepler looked at only a relatively small patch of the sky (115 square degrees). But with a telescope seven times larger than those on TESS, Kepler was able to look much further away – TESS surveys stars within only about 200 light years, compared to 3,000 light years for Kepler.
This underscores the difference in the underlying philosophy of the two missions. The goal of Kepler was to understand the statistics of exoplanets, to conduct a census to understand the population as a whole.
TESS, on the other hand, is about finding planets around bright, nearby stars –planets that will be well-suited to follow-up observations from both the ground and from space. On average, the stars observed by TESS will be between 30 and 100 times brighter than those observed by Kepler. These brighter targets will allow for follow-up observations that will be critical for understanding the nature of the newly discovered planets – more on that in a moment.
In raw numbers, what do we expect from TESS?
Former MIT graduate student Peter Sullivan conducted detailed simulations of the mission to make a prediction on what it might discover, and these results are incredible. With TESS, we expect to find over 1,600 new exoplanets within the postage stamp data, with about 70 of those being about the size of the Earth (within 25% of the Earth’s diameter), and almost 500 “super-Earth” planets (less than twice the diameter of Earth).
Perhaps most exciting is the likelihood that TESS will discover a handful of Earth-sized planets in the habitable zones of their host stars.
In addition, while TESS obtains the postage stamp data every two minutes, it also obtains a full-frame image – a picture of the entire observing sector – every thirty minutes.
In those data, we expect to find over 20,000 additional planets. The majority of those will be large (Jupiter-size) planets, but there will also be about 1,400 additional super-Earths discovered. The sheer number of planets that will be found is amazing, but more important than the number is the fact that all of these planets will be orbiting bright, nearby stars. This is a fantastic leap relative to where we were just 25 years ago, when not a single exoplanet was known.
One of the challenges of transit measurements is that they can produce false positives. Stellar activity can cause quasi-periodic dips in the brightness of a star. An eclipsing binary star in the background could mimic the dip from a transiting planet. With careful analysis, most of these effects can be accounted for, but it remains important to follow a transit observation with a confirmation — making a secondary measurement to ensure that what was observed is, in fact, a planet.
The most straightforward way to confirm a transiting exoplanet is with a radial velocity (RV) measurement. The RV method takes advantage of the reflex motion of the star; as a planet orbits a star, the star itself doesn’t remain stationary. In fact, both the planet and the star orbit the center of mass of the system. So, if one looks at spectral lines from the host star, it is possible to measure the Doppler shift of those lines as the star does it’s little pirouette around the center of mass.
From this data, astronomers can measure the mass and the year (orbital period) of the exoplanet. This confirms the orbital period observed from the transit data, and the combination of radius (observed from the transit) and the mass (observed from the RV) gives us the bulk density of the planet. With that, we can make inferences about the composition of the planet – is it a rock, like Earth? A water-world or a ball of ice? A gas giant?
Making the RV measurement, while straightforward, is not an easy one – less than 10% of the exoplanet candidates found by Kepler have been confirmed with RV measurements, largely because the host stars themselves are faint. For TESS, however, because the host stars are nearby and bright, it will be possible to make follow-up observations on nearly all of the stars that host small planets – the only major limitation will be due to the noise from the stars themselves (i.e. flares, starspots).
Further, because these host stars are bright, they will also be excellent targets for transit spectroscopy. Imagine, for a moment, that there is a transiting planet with a very large atmosphere, and that this atmosphere is transparent in red and blue, but completely opaque in the green. Then, if you observe the planet in red light (or blue light), only the “rock” part of the planet will block light from the star. In green light, however, the rock and the atmosphere will both block light – in the green, the planet appears to be larger than at other wavelengths.
This is the core idea behind transit spectroscopy. By measuring how the apparent size of a transiting planet varies with wavelength, we can infer the composition (and potentially the structure) of the planetary atmosphere. This technique has been used successfully on a very small number of exoplanets to date, but with the large number of planets that TESS will find, and the fact that they will all be around bright, nearby stars, it will be possible to use the James Webb Space Telescope and the next generation of large ground-based telescopes to make these observations.
For the first time, astronomers will actually be able to study not only individual exoplanets, but will be able to study enough of them to make comparisons and draw conclusions about how planets form and evolve.
For me, TESS is endlessly exciting. The sheer quantity of new exoplanets is stunning. The ability to use follow-up observations to characterize these planets will create new paths for scientific investigation. And the discoveries made will help define the science that will be pursued by future missions such as WFIRST, and perhaps more ambitious missions in the future. But, perhaps most exciting, TESS is in part about making “Exoplanets for Everyone.”
In a few years, it will be possible for everyone to go outside to a dark location, point at a star that you can see with the naked eye, and say “there is a planet around that star.” And the night sky may never feel quite the same again.
This blog is being hosted by Knowinnovation Inc. and is supported by the Lunar and Planetary Institute (LPI). LPI is operated by the Universities Space Research Association (USRA) under a cooperative agreement with NASA. The purpose of this blog is to communicate the work of the Nexus for Exoplanet Systems Science (NExSS). Any opinions, findings, and conclusions or recommendations expressed on this blog or its comments are those of the author(s) and do not necessarily reflect the views of NASA.