Since Planet Hunters launched in 2010, we’ve made all sorts of interesting discoveries, but apart from the occasional blog posted here you’ve had to wait until we’ve written them up in peer reviewed papers to hear about them. Such formal publication is important – it’s how information gets written into the scientific record, and how credit is recorded – but it’s slow, and this can be frustrating. It’s especially frustrating for volunteers who think they’ve found something and who then have to wait years for us to get around to doing the work required to turn a ‘maybe’ into a candidate worth publishing.
For TESS, we wanted to do something a bit different. Once candidates have been vetted they can be posted to exoFOP, where the astronomical community coordinates follow-up, but it takes quite a lot of work to get to that point.
We want to share your discoveries as quickly as possible, so today we’ve switched on a system that analyses your classifications as they come in, and when a potential planet is detected passes this information immediately to the archive which hosts TESS data, MAST. (MAST is run by a team at the Space Telescope Science Institute in Baltimore, including original Zooniverse technical lead Arfon Smith).
This means that anyone can access the latest Planet Hunters data via the Planet Hunters Analysis Database – and from there go on to explore the rest of the data that might exist about the host stars and system. Volunteers get to see their results almost instantly (and get their name in an official NASA archive!), and scientists will be able to use this data to identify possible planets that they can follow up, and give credit to the citizen scientists when they do – so hopefully this new strategy will result in more Planet Hunters planets making it into the literature.
We can run Planet Hunters because the TESS team are being open with their data. We hope that being equally open with our data will encourage more people to get involved so we can find even more planets. Go see if your name is already up in lights at https://mast.stsci.edu/phad/, or get classifying to contribute to the results at www.planethunters.org!
PS The MAST team provide a set of Jupyter notebooks which support TESS data analysis. If you know a little Python, or are willing to learn, then you can get quite a long way analysing your own candidates! Get started at https://github.com/zooniverse/phad-notebook.
We are very excited to announce a new Planet Hunters TESS interface! Whilst the idea and the aim of the project has remained the same, we have added some additional features that will hopefully improve your overall planet hunting experience.
One of these new features is the option to zoom in on the lightcurve, giving you the opportunity to explore different parts of the data in more detail. This close up view will give you a better insight into the shape and depth of a potential transit-event and may be able to reveal more about the nature of a dip.
The new interface will be launched with data from Sector 9 – the sector that we have just finished classifying with the old interface. Not only will this give us a more accurate and careful look at this current sector, it is also important for us to have this data overlap so that we can quantify any differences in the output and allow us to fairly compare and combine our findings across all observational sectors.
We hope to have the new interface up and running later today. In order to make the switch the site will have to be down for a short while – but don’t worry, we’ll be back as soon as possible to continue our search for planets!
EDIT: The new interface is now live! It is not supported on some older browsers such as Internet Explorer 11, but will work on up-to-date versions of all major browsers.
PHT may well have found its first planets! They are not yet confirmed, but we have taken the big step of uploading the candidates to ExoFOP, the website used by the worldwide exoplanet community to contribute to the follow-up TESS planet candidates. If all goes well, the additional ground-based observations that are needed to confirm whether our candidates are really planets will be made soon.
It all started when a very exciting planet was brought up and discussed on Talk by Dolorous Edd, mhuten, davidbundy77, zbish and Vidar. This is the first of three candidates we uploaded so far, and is currently my favourite. TIC 55525572.01 is a long period planet candidate that appears in multiple observational sectors. The fact that the dips don’t repeat in any one sector is likely to be the reason why the official TESS pipeline didn’t find it (yet!). The widely separated transits suggest that the planet candidate completes an orbit every 83.4 days, making this the longest period planet found in the TESS data so far (as far as we know)!
From Talk to Telescope
It was exactly three weeks ago today when I first saw the lightcurve of TIC 55525572, a subgiant star which is potentially hosting a beautiful, distant world. Prepped with strong coffee, awesome data and many pages of code we spent the afternoon pulling together figures, parameters, models and plots in order to find out everything we could about the transit events. If this candidate was going to pass the scrutiny of the planet-jury we would need a whole file of evidence. The initial checks included looking at the plots of the background flux and stability of TESS at the time of the transits; checking whether the lightcurve extraction aperture size had an effect on the size and shape of the dips; and making sure that the brightest points in the aperture didn’t move during the transit. All these tests were passed with flying colours, which urged us to move on to modelling the transit event, to see what kind of a planet it would be, if it really is a planet. Amongst other things the models showed that all three of the observed transits have the same depth and width; and revealed that, if the planet is real, it has a radius that is approximately 7 times greater than that of the Earth. At this point we were happy to call it bona fide planet candidate and upload it to ExoFOP as a “community TESS Object of Interest”, or cTOI. The candidate is now known as cTOI 55525572.01.
Next, we wanted to gain a better understanding of the entire system, and thus we needed to obtain a spectrum of the host star in order to accurately determine properties such as its mass, radius and temperature. Due to the very Southern location of this object, we turned to our Australian friends over at Australian National University who kindly observed the star for us. We are still in the process of analysing this spectrum.
As one of the final steps in the verification process we will need to obtain images of the system that are sharper than those TESS gathered, to see if the star being obscured is really TIC 55525572 and not some fainter, neighbouring star. In an ideal world we would make these observations around a transit. In fact, there was one just this past weekend, but sadly it was only observable from Antarctica… We don’t want to wait 3 months until the next transit, so for now we will settle for just having a good, sharp image the target area. Once we have all that information in hand we should be in a position to validate the candidate statistically, and if all goes well that’s when we will be able to give it its proper, planet name: will cTOI 55525572.0 one day become PHT-1b???
This candidate got the ball rolling, and within a matter of days we found two more excellent candidates that surface on Talk. All three are now on ExoFOP and awaiting follow-up. More planet candidates can be expected to appear on there soon once they pass all our initial vetting tests!
We will soon know more about these exciting candidates and I can’t wait to share that information with you. None of this would be possible without your incredible help and dedication in finding these distant, alien worlds within our galaxy!
An earlier version of this post was briefly live at the weekend; I’m so sorry for the confusion.
By Oscar Barragán
The night has finally arrived at the Roque de Los Muchachos Observatory. The blue sky has turned into a deep ocean full of stars which eclipses the beautiful horizon that is scattered with pink clouds. The telescopes are ready to hunt for starlight. At first sight, all the stars seem static in the night sky which is victim to the Earth’s rotation. However, this is a misconception, as all the stars that shine at night are moving within our galaxy, the Milky Way. Our mission for the night is is to detect their subtle movement which may tell us about the existence of faraway worlds.
The motion of the stars manifests itself in two ways. The first one is their movement in the plane of the sky – also known as their proper motion- which slowly re-shapes the constellations. The second one, and the one that we are searching for, is the movement of the stars with respect to us. This receding and approaching velocity of the stars is known as their radial velocity. This stellar motions, however, is so small that is is imperceptible to our naked eyes, meaning that we need to use big telescopes and state-of-the-art instruments in order to detect it.
You may be familiar with the acoustic version of the Doppler effect: the change in sound as a car first moves towards and then away from you. This change in sound is caused by the compression and elongation of the car’s sound waves caused by the motion of the car. In the same vein, light travels as a wave, and the Doppler effect results in an apparent change in color. If a light-emitting astronomical object moves towards us the waves are compressed and appear redder. Conversely, if the object moves away from us the waves are elongated and appear bluer. This effect is extremely small, and thus we have to use specifically designed instruments, known as spectrographs, to measure it. These devices work by dividing starlight into all the colors of the rainbow. The resultant colourful decomposition of light -called a spectrum– is imprinted with strange dark lines which, combined, make up a signature conveying information about the building blocks of the star. This is because the dark lines are a result of the emitted light travelling through the atmosphere of the star which absorbs specific colours depending on its composition. Astronomers have been using this technique to learn about stars for centuries. Additionally, we can look at these dark spectral features to study how the star dances across the sky. The position of the lines, with respect to where we expect them to be if the star were not moving, allows us to measure the Doppler effect and therefore the radial velocity of the star. It is this effect that hints at the presence of exoplanets around stars.
Let’s picture a planet orbiting a star as a gravitational tango where one of the dancers, the planet, isinvisible. By analysing the movements of the visible dancer, we can reconstruct the choreography, the song and even the nature of the hidden companion. We, the planet hunters, search for the periodic changes in the stellar pulsations, fluctuating between red and blue, which can last anywhere from hours to years. These changes indicate perturbations in the stellar velocity, suggesting that there is a planet affecting the galactic dance of one of the stars which illuminates our night sky.
Changes in stellar radial velocity are not only useful to learn about the existence of exoplanets, but can also be used to determine the minimum mass of the planets. This is because the effect of the ‘wobble’ of the star is larger when the difference in mass of the star and the planet is higher. We can, therefore, use the the spectra of a star to understand if a planet is massive like Jupiter, or relatively light like the Earth. The problem with this method is that these changes in velocity are very small. Jupiter, for example, causes the Sun to wobble with a mere velocity of 13 m/s every 10 years, while the Earth does it with an almost insignificant 9 cm/s each year. Hence, we need instruments with extremely high levels of precision and stability if we want to be able detect the effect that exoplanets have on their stars.
We are now in the Telescopio Nazionale Galileo, which hosts one of the best exoplanet hunter instrument in the northern hemisphere: HARPS-N. This spectrograph is a copy of the original HARPS (High Accuracy Radial Velocity Planet Searcher) which is located in the Southern hemisphere, in Chile. Both of these instruments allow us to measure the stellar velocity with a mean precision of 1 m/s, which is approximately equivalent to the speed of a crawling baby. Our mission here is to follow-up exoplanets discovered by the Kepler space telescope, TESS’ predecessor. If we combine our radial velocity measurements with the transits observed by Kepler we are able to obtain the real planet mass (and not just a lower limit). This gives us a first approximation of what the planet is made of, and paves the the first step along the way of testing for habitability. Perhaps the next time we are here we will be measuring the mass of an exoplanet discovered by you via Planet Hunters…
As a matter of symmetry, the end of the night is announced on the horizon with the same colors that we saw at the beginning of the night, 9 hours ago. The vibrant colors mark the time to close to telescope before the Sun is back as a protagonist in the bright blue sky. We leave the telescope in the early hours of the day after having successfully measured the radial velocity of tens of potential planet-hosting stars. Each datum taken this night will help us to decode, step by step, a gravitational choreography, which will tell us about the existence of faraway worlds.
An Observer’s start to the Day
Our ‘day’ here in La Palma starts around 4 pm. After a quick breakfast and a much needed coffee we head up to the mountain to the telescope where the telescope operator has already started setting up the equipment for the night. The Sun is still high in the sky so the telescope dome stays closed while we carry out the calibrations of the instrument that we plan to user throughout the night.
Once all the calibrations are done we have to wait for the sunset, giving us time to have dinner, visit other telescopes, or have a quick snooze in preparation for the long night ahead. The telescopes around here are incredible, and we have bee lucky enough to get a tour of three of the most impressive ones.
MAGIC and CTA
MAGIC (Major Atmospheric Gamma Imaging Cherenkov Telescopes) and CTA (Cherenkov Telescope Array) are the first telescopes that you see when you drive up the mountain from sea-level and their impressive mirrored structures make you feel like you have entered into another world. MAGIC is a system of two Cherenkov telescoped which detect particle showers in the atmosphere released by gamma rays. The twin-telescopes each consist of a 17-m diameter dish that is covered with smaller mirrors that reflect the light into a highly sensitive camera. Next to MAGIC lies the newly built CTA which has a similar design and beautifully reflects the stars at night the sunrise at the start of the day. All three of these telescopes are sensitive to galactic and extragalactic gamma-rays, allowing us to study high energy events in the Universe such as active galactic nuclei, gamma-ray bursts, pulsars and supernova remnants. Without any domes, these telescopes proudly dominate the hillside, making the scenery look slightly surreal (or MAGICal).
Isaac Newton Telescope
The next telescope that we visited was the 2.54-m optical Isaac Newton Telescope (INT). It was initially built in 1967 at Herstmonceux Castle in Sussex, England (the initial site of the Royal Greenwhich Observatory) but was moved to La Palma in 1984, due to light pollution and the less-than-ideal British weather. The INT is the oldest telescope on the mountain, and walking into the impressive building gave me the slight feeling of going back in time. The control room is filled with computers from the 70s with a control deck that exhibits analog dials and manual knobs that control various aspects of the telescope.
The telescope is located on the third floor and sits on a warm wooden floor along with tanks of liquid nitrogen that are used to manually cool the electronics. But it’s not just the telescope that’s impressive at this observatory. Leaving the dome of the telescope we set off on a tour to explore the rest of the building, which felt like a beautiful combination of a museum and a 70s royal bunker. It is fully equipped with office spaces, sleeping rooms, rooms and cupboards filled with various electrical equipment and spare telescope parts, and an incredible library that hosts books and journals that date back to the eighteen hundreds.
We also headed to the roof of the INT, which presented us with a good view of the 4.2-m William Herschel Telescope (WHT) that is currently shining an extremely powerful orange laser into the atmosphere (see top image). Simultaneous observations of this laser and the targets throughout the night allows us to correct for the effects of turbulence in the atmosphere, transforming fuzzy observations into sharp images.
Gran Telescopio Canaria
Our final telescope tour was of the Gran Telescopio Canaria (GTC), the largest optical and infrared telescope in the world. As we walked into the silver dome we were overarched by the huge structure of the telescope – with a height of over 25 m from top to bottom it truly is gigantic! The primary mirror is made up 36 individual hexagonal segments that perfectly piece together to act as a single 10.4 m mirror. The segments, which are made of a ceramic material similar to that used for modern kitchen hobs, are polished to perfection, conforming to a 15 nanometer (millionths of a millimetre) margin of error. But not only does each mirror have to be perfectly uniform, the individual segments must also fit together smoothly with no more than 90 nanometers difference between each. To put this into perspective, if the primary mirror were scaled up to the size of Texas, the ‘unevenness’ would have to be kept to less one millimetre. Automatic sensors are used in order to ensure this degree of accuracy throughout the observations.
Its immense scale combined with the perfect location makes the GTC the ideal telescope to study the nature of black holes, the formation and evolution of stars and galaxies in the early universe, the nature of exoplanets and the mysteries of dark matter and dark energy that fills our Universe. The 400 tonne instrument was truly amazing to see.
Back to our own Telescope
At sunset we drive back up to our own telescope, slightly overwhelmed by the beauty of the sunset that is reflected off the sea of clouds that lies beneath us. So far we have been lucky and have always remained above the clouds, leaving us with beautifully clear skies that allow us to obtain spectra of Kepler and K2 targets. But what are Spectra and what can they tell us? Stay tuned for the types of observations that we are obtaining during our time here.
Oscar and I have just arrived in La Palma, one of the Spanish Canary Islands, where we will be spending the next few days taking radial velocity follow-up observations of Kepler and K2 exoplanet candidates. As this tiny island is situated north of the equator, we are not able to observe any TESS targets from here (yet!), as TESS is currently observing stars in the southern hemisphere. Even though these are not TESS targets, this is great training for when our targets are observable from here.
We will be using the Italian owned 3.58-metre Telescopio Nazionale Galileo (TNG) with the HARPS-N instrument, located at the Roque de los Muchachos Observatory. At it’s highest point of 2,400 m above sea level, the observatory lies above a beautiful ‘sea of clouds below which we find ourselves surrounded by the vast extent of the Atlantic ocean. It is this body of water that ensures that the air at the observatory is very stable, providing us with the perfect conditions to look at stars, galaxies, and our own solar system. There’s good reason as to why they call this one of the best places on Earth to observe the night sky.
We will start to use the telescope tonight, at which point I’ll be able to tell you much more about how this exciting instrument works and about the thrilling process of discovering distant worlds.
Exoplanets can help us understand our own solar system, how it formed, how it evolved and how it came to look the way it does today. Continue reading for a description of how our Solar System came to be.
The Solar System
The basic layout of our solar system has been known for centuries. We have the Sun in the center, surrounded by four rocky planets, two gas giants, and two ice giants. The planets can be seen even with small telescopes, and it was noted early in the history of astronomy that they move in orderly orbits, in near perfect circles, at a fixed distance from the Sun. Based on this observation, it is no surprise that the assumption of fixed orbits has been the bedrock of the study of the solar system since the beginning.
Over the years, this simple tale of the evolution of the solar system has progressed into a story of chaotic migration of the planets, bombardments of asteroids and comets, and potentially the exclusion of a ninth planet. Following the turn of the century, astronomers realized that their model of the formation of the solar system, which assumed that the planets formed exactly where they are now, doesn’t fully agree with what we see. Furthermore, observations of planetary systems around other stars (exoplanets) made us question the formation of our own.
A Star is Born
The story of the solar system starts 4.6 billion years ago with the collapse of a gigantic cloud of gas and dust, known as a giant molecular cloud. The majority of the collapsed cloud collected in the center to form the Sun, and the remaining material flattened out into a rotating disk around the star. It is out of this disk, that the planets, moons, asteroids and comets formed.
Jupiter Leads the Way
There are two main categories of planets in our solar system: the rocky terrestrial planets in the inner solar system (Mercury, Venus, Earth, and Mars) and the giant gassy and icy planets in the outer reaches (Jupiter Saturn, Uranus, and Neptune). Even though these two ‘types’ of planets appear so different, their formation followed the same initial recipe.
Like for any recipe, you need ingredients. The main ingredient to build planets is dust that clumps together to build larger rocks. This clumping happens as the gas and dust in the disk orbit around the Sun in swirling and random motions, resulting in many collisions between the individual particles. As the particles collide, they stick together, and eventually, given enough time, form large bodies. Similar to the rolling of a snowball across a field of snow, the larger the ball, the more efficiently it picks up more material and therefore the faster it grows.
A second ingredient is ice. Due to the heat from the Sun, ice grains can only reside beyond a certain point in the solar system, known as the ‘ice line’. As the ice helps the dust grains to stick together, planets can grow more rapidly where this ingredient is available. This is how the gas and ice giants managed to become so large. Jupiter, for example, formed just beyond the ice line. It sucked up over half the material in the disk, and quickly outgrew the other planets, making it the dominant figure in the solar system.
The third main ingredient that went into making Jupiter was gas, which formed a blanket around the planet’s core. The new-born Sun would have heated up the inner solar system and over time blast away the gas in the disk. This means that in order for Jupiter to collect the amount of gas that it did, it must have formed before the gas was ejected by the Sun, in the astronomical blink of an eye of only five million years.
Saturn was also in a favourable position to become large, and grew into a gas giant similar to Jupiter. Further out, Neptune and Uranus, grew to smaller sized and are primarily made out of ice, making them the ice giants.
Unlike the gas and ice giants, the inner-most planets had no gas to claim as their own, and are therefore made out of dust. The so called terrestrial planets, formed through the slow process of colliding particles, becoming larger with every collision until all the material was assembled into four rocky bodies. The accompanying figure shows the relative sizes of the planets.
This model of the formation of our planets can’t explain all the properties that we observe. Why is Mars so small? Why are Uranus and Neptune so far away from the Sun? Where did the craters on the moon come from? Further questions arose following the discovery of planets around other stars, known as exoplanets, which lead us to question some of our most fundamental assumptions of the solar system.
The first exoplanet was discovered in 1995, and since then we have been pointing our telescope at distant stars in order to study their solar systems. To everyone’s surprise, these other planetary systems looked nothing like our own, and we saw planets in places where they can’t have possibly formed. This was the first hint to suggest that planets are not fixed in their orbits, but take wild journeys around their stars. It was this observation that made us question the fixed nature of our own planets, and astronomers went back to the drawing board to come up with new chaotic models of the evolution of our solar system (see the image below for our theory of the evolution of the Solar System).
The Grand Tack Model
A new model was proposed, which suggests that Jupiter started to migrate towards the Sun as soon as it formed, during a time when the terrestrial planets were still in their infancy. Saturn followed Jupiter’s inwards path, resulting in the two planets getting closer and closer until Jupiter was completing exactly three orbits for every two of Saturn’s. This alignment halted the inward motion of the giants and forced them back into larger orbits. This is known as the Grand Tack model.
On this adventure, Jupiter travelled inwards to approximately the current orbit of Mars and back out to where we see it now. This movement caused large scale disruption in the solar system. First, by entering the orbit of Mars, Jupiter stole material that would have otherwise contributed to building the small red planet. This explains Mars’s smaller than expected size. Second, it is thought that Jupiter could have halted the formation of an entire other planet by scattering rocks that would have otherwise grown into a full sized planet. Instead of clumping together, a band of rocks remained to orbit the Sun, known as the Asteroid Belt. Finally, as Jupiter migrated into the inner solar system, it snowploughed gas and ice rich bodies across the ice line. These bodies would have been scattered in all directions, bombarding the young terrestrial planets and seeding them with water and other gases. Without this water brought to Earth by Jupiter, we would likely not be here.
Bombardment of the Planets
Once Jupiter and Saturn were back in larger orbits, the inner planets were able to fully grow by accumulating all the matter in their local area of the disk. Uranus and Neptune also grew to their full size, at a distance from the Sun significantly closer than where we see them now. Furthermore, contrary to where they are today, Neptune was in a smaller orbit than Uranus. Beyond the orbit of all the outermost planet resided a thick belt of icy bodies.
These icy bodies tugged at the large planets, one by one over hundreds of millions of years, once again making the orbits of the giants more and more unstable. A point came where the orbits of Jupiter and Saturn were so disrupted that they no longer remained in their stable orbits. Jupiter moved slightly inwards, pushing the other giants outwards in a violent motion that could have propelled one or even two unknown planets out of the solar system entirely. This is known as the Nice model. Neptune was thrown beyond the orbit of Uranus through the belt of ice and rocks, scattering material across the solar system. This attack, called the late heavy bombardment, left scars on the surface the planets and moons. Our Moon is no exception to this, with many of the craters that we see on it today still remaining from this violent era 4.1 to 3.8 billion years ago.
Following the late heavy bombardment, the planets settled into stable orbits around the Sun where they have remained ever since. Over the course of 20 years, our view of the formation of the solar system changed drastically. As we continue to explore solar systems around distant stars, we will learn more about our own, and continue to solve the mystery of how we got here.
With the help of the Public Engagement with Research team at the University of Oxford, we’re conducting a short survey in order to explore the impacts of Planet Hunters TESS and to better understand who takes part and why. We’d love to hear your thoughts at https://oxford.onlinesurveys.ac.uk/zooniverse-planet-hunters-feedback.
Thank you to those who have already taken the time to complete the survey.
Evaluating Planet Hunters
By Annaleise Depper
To date, thousands of volunteers worldwide have contributed their time to classify millions of light curves on Planet Hunters in the search for extrasolar planets. With the help of citizen scientists, the Planet Hunters team have been able to find out more about the diversity of planets and to understand what kind of solar systems exist.
But what we don’t know is… What impact does Planet Hunters have on its volunteer community? Who and why do people take part? What are the benefits and barriers?
I’ve been given the task of exploring these questions.
As Public Engagement with Research Evaluation Officer, my role is to support researchers at the University of Oxford to evaluate the impact of their public engagement with research activities.
I was particularly interested in collaborating with the Zooniverse Planet Hunters team as they are working on an innovative citizen science project that is constantly expanding and making scientific research more accessible. I have been inspired by the work the Planet Hunters team are doing to involve, engage and collaborate with citizen scientists worldwide in a very unique way.
Over the coming months I will be working with the TESS team to launch our evaluation survey in order to explore the views and experiences of volunteers engaging with Planet Hunters. Our aim is to find out:
- what impact does the platform have on its volunteer community?
- what are the benefits and potential challenges?
- how can Planet Hunters become even more inclusive of its growing, diverse community?
Please share your thoughts about Planet Hunters by completing our short online survey: https://oxford.onlinesurveys.ac.uk/zooniverse-planet-hunters-your-feedback-needed-
This should take no longer than 5-10 minutes to complete and your responses will be anonymised. We will share our overall findings with the Planet Hunters community on this blog and the University of Oxford webpages.
If you would like to add any additional comments or thoughts please feel free to email me firstname.lastname@example.org.
We look forward to sharing the results!
You continue to bring extremely exciting planet candidates to our attention! When we find promising targets there are many steps that we have to go through in order to determine their planetary nature – sometimes the planets pass all our tests, and sometimes they turn out to be false positives. Read on for an awesome summary of how we investigated the possible planet around τ Ceti written by Benjamin Pope from New York University.
But first, none of this would be possible without all your amazing help, your classifications and your Talk discussions. I would like to say a special thanks to the 15 volunteers who classified this target: Anchel, LarryW, JobiMine, EEZuidema, lvwarren, ElisabethB, TaxiCab1729, LAIS_IONUT_ANTONEL, Jose-Adao, DanielRA37, baconsteven, bugge, gulpfumetti, adam211 and Vidar87, and to andrey373 who brought this candidate up on Talk. We couldn’t have done it without you all!
Notes on τ Ceti
by Benjamin Pope
Outside of our solar system, the nearest solar-like stars are only a few light-years away: the two bright components of the binary system α Centauri AB (orbited by a third component Proxima Centauri, a dim red dwarf). But to find a star like the sun with no companion, you have to look a little further away to τ Ceti (tau Ceti), a G8 dwarf (which means it is a little less massive and cooler than the G2 Sun) which is the second-closest star system visible to the naked eye at a distance of only about 12 light-years.
Such a close system is one of the first targets for bold proposals for interstellar travel and contact, and for science fiction: closest to my heart, in Ursula Le Guin’s The Dispossessed, τ Ceti is home to the twin habitable worlds of Urras and Anarres; the former capitalist, the latter the home for anarchist exiles. But to astronomers since then it has become increasingly exciting as a host for real exoplanets: from radial velocity observations (measuring the red and blue shift in the star’s spectrum as it is tugged back and forth by planets) it has been suspected since 2012 that it hosts a number of exoplanets, with orbital periods of a few weeks to a few years. If one of these transits, it would be a huge discovery – both by independently confirming the existence of these planets, but also because it would open up an unprecedented opportunity to study their atmospheres as they are illuminated by the star behind during transit. τ Ceti is ten times brighter than the next-brightest transiting planet host star and the extra light would be a significant boon to photon-starved spectrographs trying to detect chemistry (and life!) in its atmosphere.
When the data from the Transiting Exoplanet Survey Satellite (TESS) covering τ Ceti came out two weeks ago, I received an email from Prof. Suzanne Aigrain at Oxford, my former DPhil supervisor, saying that the Planet Hunters team had noticed evidence of a transit in the light curve of τ Ceti and asking if I could check this – without knowing when the transit they found occurred, so that I had to replicate the result blind! One of the difficulties is that τ Ceti is very bright, a third magnitude star in a telescope that saturates (overexposes – just like in other cameras) on stars three magnitudes (fifteen times) fainter. In my DPhil, I had worked with Dr Tim White (ANU) to develop the method of ‘halo photometry’ (the code halophot) to deal with this problem for similar data obtained by the previous mission Kepler, which we used to look at the Seven Sisters and the planet-hosting red giant Aldebaran. The way it works is by discarding the unusable ‘saturated’ pixels but looking at the broader distribution of light (the ‘halo’) around the star and teasing out a signal from these many good pixels. So I used this code to look at τ Ceti (and if you want to see how it’s used and the plots below were made, check out the Jupyter notebooks on the GitHub repo!). Running halophot, it produces a huge signal that looks just like a transit (light curve on the left – standard ‘PDC’ pipeline in orange, new halo light curve in blue, halo map on the right):
When I told Suzanne, she confirmed this was exactly when they thought the transit was. So we were on: time to check if it’s real! Suzanne’s postdoc Dr Oscar Barragán modelled the transit signal in the standard PDC data, assuming the transit was equatorial, the planet was in a circular orbit and using the stellar mass and radius to try to estimate a range of valid periods. The depth of the transit signal gives you a planet to star ratio of 0.0108835, translating to a planet radius of 0.94 Earth radii. So this would be by far the closest Earth-sized planet to be known to transit. Meanwhile from the transit duration of ~ 11 hours we get that the minimum orbital period should be ~ 230 days, corresponding to an orbital radius of ~0.7 AU (though somewhat worryingly, not to the known periods of any of the planets found by radial velocity – though it could just be because it has too low a mass to detect). At this distance from the star the planet’s equilibrium temperature would be ~225 K. At nearly fifty degrees C below freezing this is quite cold, but there is a lot of uncertainty about the effects of planetary atmospheres, and to me this is quite exciting – no cool transiting Earths are known around such a nearby star!
Oscar produced this great visualization of his model:
So with such high stakes we had to be very careful. In comparison to the PDC data, the transit in the halo light curve I made was much higher signal-to-noise, but seemed much deeper (a few percent rather than a fraction of a percent). This isn’t necessarily a killer, in that neither the absolute normalization of PDC nor halo light curves of saturated stars is completely accurate, but they are usually much closer than this. First thing to check: halophot doesn’t do anything obviously wrong, and generates a model (on the right of the figure above) that looks rather like the expected pattern of light from a star as seen by TESS. The light curve you get from this has a deep and clean transit, which is maybe a bit long and deep, but looks ok.
What is immediately suspicious, though, is that it occurs just before perigee: TESS has an eccentric orbit in a 2:1 resonance with the moon, which means that twice a month it approaches very close to the Earth briefly (perigee) and then swings back out again to spend most of its orbit far away. When it is at perigee it is subject to a lot of reflected light from the Earth – Earthshine! This is why there is a gap right in the middle of the light curve. So to me it immediately raised alarm bells that this signal happened just when the telescope was most vulnerable to contamination from background light.
To figure out what is going on with the Earthshine, we produce a ‘background’ light curve for τ Ceti using only pixels far away from where the star is contributing much light. Let’s plot this with a vertical line to note the midpoint of the ‘transit’ we found earlier:
Uh-oh! There is a ‘transit’ signal in the background light, a little later than the transit. This isn’t an absolute killer – τ Ceti is very bright, and it isn’t implausible that its light could have directly contaminated the background or done so via some electronic chip effect (‘cross-talk’). It is also not quite at the same time as our putative planet. But it is pretty alarming.
Let’s look at some less highly processed data. What we have been looking at so far has been a ‘target pixel file’ (TPF) produced with a frame every 2 minutes and a lot of sophisticated calibration. Insted let’s use the TESSCut tool to grab part of the Full Frame Image (FFI) data, which has 30 minute frames and a lot less processing but of a much wider field. If you use lightkurve.interact() and look at the individual pixel time series in the FFI, they all show this dip. But in the pipeline TPF this is different: above the mid-axis of the star, they go up during ‘transit’ and below they go down. You can actually see this at a global level if you use the slider and the right scalings: it seems that as a whole the background flux shifts upward on the detector for a few hours and then shifts back down. So something funky has happened to the spatially-varying background during processing.
So let’s look at another very bright star in the field: the giant star ζ Ceti. It shows the same background dip – but as Tim White pointed out, at a slightly different time! If you look at individual pixel time series from ‘above’ and ‘below’ the star midlines, above the line they go up and below they go down – features with the same midpoint and similar duration to the ‘signal’ at τ Ceti – they are something to do with TESS and not a planet around τ Ceti. The difference between the top and the bottom, Tim realized, can be ascribed to the fact that these pixel cutouts are very elongated in that axis, so if we have a spatially-varying background but subtract only a constant background, we will find this asymmetric pattern. This poor background subtraction may therefore have contaminated all the pixels and created the appearance of a transit where in reality there is none.
So to look at the spatial detail in the background, I downloaded all TPFs on the same camera as τ Ceti, extracted their background light curves, and made a video of their background flux over time. Each point below is coloured by the logarithm of the background flux, clipped at the top and bottom to bring out the features best. τ Ceti is a blue star bang in the middle and ζ Ceti orange to the top right, and the ‘transit’ occurs at day 1394.3 or about 11 seconds into the video.
As you can probably just make out, there is a lot of spatial structure there, mostly in the lower left. Just around the transit, there is a spur through the middle towards the top right that lights up a little, and then it switches back to the lower left, and then everything gets brighter overall towards perigee. When we contacted Dr Chelsea Huang (MIT) about this, she was able to dig into the huge full frame images and make a ‘difference image’, subtracting one from the next to look for where the background might be changing. In her image below, τ Ceti is highlighted with a red arrow:
The fuzzy blobs pick out bright stars (e.g. τ Ceti itself, or ζ Ceti top right). The vertical streaks are probably ‘straps’ on the back of the detector that reflect back some of the light that passes through. There is probably also some CCD smear like you get with cheap cameras at night, and it runs up and down every column with a sufficiently bright star in it (such as τ Ceti). But more importantly are the ripply concentric rings which are lens flare from the Earth just out of shot, and you can see τ Ceti lies smack bang in the middle of one of these rings. As the Earth seems to move and get brighter this ring runs over τ Ceti and causes this apparent transit effect.
Regrettably we have been ‘dispossessed’ of this potentially very exciting planet candidate. But this is how science is: something that is too good to be true very often turns out that way, but it doesn’t make it any less worth investigating. In digging through the data on τ Ceti we were forced to fix bugs in our code and grapple with unfamiliar systematics in TESS that we didn’t see in Kepler. Personally, I am concerned that systematics which affect so many pixels in common over such a wide field are going to pose a serious problem to the approach we had been taking to bright stars in K2, and playing with the τ Ceti data has been a valuable learning experience. While we haven’t yet detected her home planet’s transits, we should keep in mind the maxim of Le Guin’s Anarresti scientist Takver:
There was process: process was all. You could go in a promising direction or you could go wrong, but you did not set out with the expectation of ever stopping anywhere.
Kepler and TESS are both amazing space telescopes that have and will revolutionise our understanding of exoplanets. But let’s have a look at how these two telescopes differ?
Kepler was launched in March 2009 and used a 1.4-m primary mirror that observed a 12×12 degree patch of sky (for reference the Moon covers half a degree on the sky). The sensitivity of Kepler was significantly better than that of any other instruments at the time, thus enabling Kepler to find exoplanets as small as half the size of the Earth.
Conversely, TESS will survey the entire sky, looking at 400 times more stars than Kepler did throughout its lifetime. TESS will do this with four identical telescopes, which, combined, observe a 24-degree patch of sky at any one point. Each 27 days, TESS changes direction and looks for planets around a different set of stars in a new ‘sector’. The entire sky has been split into 26 overlapping sectors, and TESS will visit each one over the course of the next 2 years.
The two satellites also differ in their observing strategy and the types of stars that they focus on. Whilst Kepler observed one patch of sky for a long period of time, TESS will only spend a month looking at each sector. The long exposure times of Kepler allowed it to find the dimmer and more distant stars, whereas TESS will monitor the nearby, and brightest targets. In fact, the stars observed by TESS are 10 times closer and 100 times brighter than the Kepler target stars! Observing brighter and closer stars has the advantage that any planet candidates that we find will be easier to observe using ground based telescopes.
The main Kepler mission ended in 2013, when the telescope lost its ability to change orientation without the use of fuel. Luckily, engineers and astronomers quickly realised that the pressure from the Sun could be used to steer the telescope in order to keep it pointing at one patch of the sky. This new era of observations became known as the K2 mission.
K2 ran out of fuel in mid 2018, bringing the mission to a close. Luckily, by this point NASA’s new satellite TESS had already been launched. We now have brightness measurements of around 45,000 stars from the first three sectors, and we are already finding some promising planet candidates within the TESS data!
Will you help us find the planets hidden within the TESS data? Click here to give it a go!