Dr. Annaleise Depper (Evaluation Officer, Public Engagement with Research) at the University of Oxford shares the results and findings from an evaluation of Planet Hunters conducted in 2019.
Read the full report here: http://bit.ly/Planet-Hunters-evaluation
Exploring the Impact of Planet Hunters TESS
by Dr. Annaleise Depper
To date, over 15,000 citizen scientist volunteers from over 98 different countries worldwide have contributed their time to classify millions of light curves through Planet Hunters TESS.
Still, questions remain as to why people engage in project like Planet Hunters; what, if anything, do people learn; what factors limit people’s participation; and what can researchers at the University of Oxford learn from people’s experiences in order to enhance the project?
In 2019, I carried out an evaluation of Planet Hunters TESS, which allowed me to explore these questions by engaging with the volunteers through an online survey. A total of 577 volunteers completed this survey, thereby sharing their views and experiences. While the findings are not necessarily representative of all of the volunteers, the results have provided important information about the value and barriers to taking part in Planet Hunters, and the ways in which the project could be enhanced.
Thank you to all the 577 citizen scientist volunteers who took the time to complete this survey, and shared their open and honest reflections, thoughts and recommendations.
Here’s a brief summary of what we found out:
Planet Hunters has resulted the following outcomes and impacts on volunteers:
- 74% learned about Astronomy
- 66% enjoyed learning about Astronomy through Planet Hunters
- 21% felt inspired to learn more about Astronomy beyond the project
- 19% experienced a feeling of pride and satisfaction in being a citizen scientist
- 8% experienced positive benefits to their individual wellbeing
Hearing from the volunteers:
“I learned about the actual research medium taking place and how it really works instead of viewing a graphic or reading an article.”
“I am making 3 beginners telescopes and planning to start a small Astronomy club to inspire more people in Astronomy. Planet Hunters opened my mind in many ways””
“I love what I’ve done and knowing that I am participating in a project that search for new planets makes me feel excited”
“My student really got into the project. They thought it was really neat that they would get an acknowledgement if they helped find a planet, and were excited by the fact it was really data. I asked them to do a minimum of 20 classifications and no student did less than twice that (and some completed as many as 150 classifications)”
Benefits and challenges of Planet Hunters
A key strength of Planet Hunters is its ability to bring together groups of people, including those without a background in science, to become citizen scientists and actively engaged in the exoplanet search.
At the same time, 49% of Planet Hunters volunteers highlighted a reason that limited their participation in the project, including:
- Personal circumstances
- Issues with the platform, interface and accessibility
- Limited understanding
- Tedious, fatigue and repetition
- Lack of individual feedback and recognition
- Classification anxieties
- Commitment to other Zooniverse projects
How can Planet Hunters become more inclusive of its growing, diverse community?
There is still work to be done to ensure that Planet Hunters remains rewarding, motivating and inclusive of all volunteers. Around 50% of volunteers provided a recommendation that could support the development of Planet Hunters; including:
- Providing additional information and support to volunteers
- Improving accessibility and interface usability
- Providing more feedback
Next steps for the PHT team
The Planet Hunters TESS team are currently thinking of ways to implement some of the recommended changes – here’s some of the areas they are exploring:
- Translating the project into multiple languages
- Launching a mobile app
- Offering more training and support e.g. video tutorials
- A ‘meet the researcher’ or ‘meet the volunteer’ blog
- Improving and supporting use of Planet Hunters in the classroom for educational purposes
- More regular updates e.g. to the results page; Twitter
Read the full evaluation report here: http://bit.ly/Planet-Hunters-evaluation
TESS looks at thousands of stars every night providing us with some of the most beautiful, strange and mysterious light curves. Here is my favourite one of the week: TIC 300446218.
This specific light curve was extracted from the TESS full frame images (FFIs) and it shows multiple dips of different depths. There appear to be two sets of alternating signals which have been highlighted in different colours. The signals highlighted with the red solid lines and the yellow dashed lines are equally spaces but have alternating depths (the yellow signal is deeper) and therefore look like they are caused by an eclipsing binary. Similarly, the signal highlighted by the blue dotted lines and the purple dot-dash lines are also periodic but have different depths. We are, therefore, likely seeing two sets of eclipsing binaries, which, if they are physically close together and tidally locked to one another, could be in a quadruple system!
TIC 300446218 is located close to the ecliptic pole, in the ‘continuous viewing zone’, and was continuously observed by TESS throughout the first year of operation. While the image above only shows a small segment of the light curve, you can see the entire coverage in the figure below.
The full light curve shows that all of the dips disappear after a couple of months and come back again a little while later. This could potentially be due to changes in brightness of a nearby variable star – when the nearby star is bright the dips in TIC 300446218 are diluted and disappear, whereas when the nearby star is faint the dips stand out for us to see. Alternatively, the plane in which the stars are orbiting around one another could be changing inclination by very very small amounts over time, meaning that the stars sometimes cross our line of sight and block a small amount of light, and sometimes don’t. More investigation is needed in order to untangle and understand this fun light curve.
Do you have a favourite light curve of the week?
It has been an extremely exciting year for Planter Hunters TESS and we have identified multiple potential new planetary systems. Due to the success of the project, we have decided to expand by bringing PHT to your mobile device!
To date, all of the data that we have looked at have been from the targeted bright, nearby stars that TESS observes every 2-minutes. However, in addition to these targeted stars TESS monitors the brightness of millions of other stars, both bright and faint, recording their brightness every 30-minutes. These stars are found in what are known as TESS’s Full Frame Images (or FFIs), and the time has come for us to start looking for planets there.
Due to the huge number of stars contained within these Full Frame Images, we decided to first analyse the data using a computer algorithm. This code, which was developed by Chelsea Huang at MIT pre-selects the best lightcurves for us to look at in more detail.
The automated search has two parts. First, it runs searched for periodic signals using an algorithm called Box Least Squares (BLS). This algorithm ‘folds’ the lightcurve at various trial orbital periods. If a trial period corresponds to the orbital period of an existing planet in the system the dips cause by a transiting object will overlap, suggesting the existence of a possible planet. The second part of the search uses machine learning, to look at the lightcurves that pass the BLS search, in order to assess whether the periodic signal could be the result of a transiting planet.
For many lightcurves the code is extremely good at identifying the good planet candidates without any human vetting. However, there are also a huge number of marginal cases where the machines are unsure as to whether there is a planetary signal present or not, and it is these cases that we need your help on.
The images that we show on the site have been folded at the orbital periods determined by the BLS search so that multiple dips should overlap with one another. We are showing the odd and the even (alternating) dips in red and white. This is because any difference in odd and even transit depth and shape is a tell-tale sign of the dips being the result of two stars of different sizes orbiting around one another in an eclipsing binary. Please discard these lightcurves on the app by swiping left or pressing the ‘discard’ button on your screen.
In this project we are also want to pay close attention to the shape of the dips. Transits due to eclipsing binaries tend to be very V-shaped, whereas transits due to planets are more U-shaped or have a flat bottom so look out for these!
See what you can find by downloading the Zooniverse app onto your mobile and looking for the Planet Hunters TESS project.
We hope you enjoy this new version of Planet Hunters TESS and can’t wait to see what kind of systems we can find!
It has been exactly a year since the first public TESS data release and the launch of Planet Hunters TESS! Thank you to everyone who has taken part and helped us classify all of the data so far. It has been a truly exciting year!
Since 6 December 2018, PHT has had over 14 thousand registered (and many more thousand unregistered) participants, and together you have completed almost 11 million classifications! Together, you have helped us find some exciting new planetary systems.
For example, your participation and dedication to the project over the past year have led to the detection and validation of the first PHT planet, TOI-813. TOI-813 is not only the longest period planet found in the TESS data to date, it is also in orbit around a subgiant star. Subgiant stars are stars in the later stages of their lives, meaning that studying these planets will help us understand the synergies between planetary and stellar evolution in the later stages of the stars life, in other words, it may help us understand what will happen to the Earth in the far far future.
But TOI-813 isn’t the only planet that PHT has found so far. You have brought many interesting targets to our attention and we are working hard to test whether these promising signals are indeed caused by planetary bodies. The targets that pass all of our initial vetting tests are being followed up using ground-based telescopes and we hope to validate them in the near future. This will allow us to contribute to the ever-growing population of known planets and bring us one step closer to findings a planet like Earth.
Here are some of the ones that we’re particularly excited about:
These are only some of the candidates that are currently being actively followed up using telescopes found across the globe, including Chile, France, Australia and the USA. We will be sharing the results of our findings soon!
In addition to the exciting planets that are being found by the project, we have also come across lightcurves of some puzzling stars. These are often brought to our attention via the talk discussion boards and I would like to thank you everyone for using this tool to post and highlight interesting signals and patterns in lightcurves there. Here are some of the ones that we haven’t been able to explain.
There appear to be more dips here than we would expect for a simple eclipsing binary?
This one appears to be two binaries, but could they be locked together making this a quadruple?
This is a beautifully long eclipsing binary!
Thank you so much for your participation over the past year! Here at Zooniverse we are celebrating Planet Hunter TESS’s first birthday with a sparkly cake!
Like many of you, I am extremely eager to find those tiny, elusive dips in the TESS lightcurves that reveal the existence of a distant, undiscovered, alien world. However, even though planets are my main focus, they are not the only interesting objects that we are able to find using TESS.
I have recently been talking to astronomers at the KU Leuven, in Belgium, who use the TESS lightcurves to study stars, their spots and pulsations as well as the architecture and behaviour of multiple star systems.
The lighcurves of these systems often boast beautiful patterns! Cole Johnston from the KU Leuven explains some of the systems behind these lightcurves:
Gamma Doradus variables are stars slightly more massive than the Sun with temperatures between 6,500K and 7,500K (the Sun is around 5,777K). You can recognise their lightcurves due their characteristic fluctuations in brightness that last from a few hours to several days. The fluctuations are caused by pulsations known as gravity-mode pulsations, which are waves that behave in the same way as surface waves in the ocean. These waves affect the surface temperature of the stars, changing the brightness that we observe.
Delta Scuti (also known as dwarf Cepheid variables) stars are hot, young (A-F type) stars that are ~1.5 to ~2.5 times the mass of our Sun. These stars can have a single strong pulsation or potentially hundreds of smaller pulsations with periods ranging from a couple of minutes to a few hours. The pulsations are caused by pressure waves, which behave identically to sound waves, that propagate near the surface of the star.
Cepheid Variables and RR Lyrae stars the older and more evolved cousins of delta scuti stars that also pulsate due to pressure waves. These stars are very exciting as they exhibit a pulsation period – luminosity (brightness) relationship, also known as the Leavitt law. This means that we can calculate the distances to these objects by studying the pulsations that we see in their lightcurves. Historically, these stars have helped us to calculate the distance to the Large Magellanic Cloud as well as to the centre of our Galaxy.
Slowly Pulsating B-stars
Slowly Pulsating B-stars (SPB stars) are very similar to Gamma Dor variables but are much more massive (2.5 to ~8 time as massive as the Sun). They are extremely hot with effective temperatures between ~11,000K and ~30,000K and typically show multiple gravity-mode pulsations that range from lasting a couple of hours to several days.
Beta Cephei stars are the high mass (8 to 20 times more massive than the Sun) stars that oscillate due to pressure waves, similar to Delta Scuti stars. The iron interior of these stars reaches extremely high temperatures of 200,000 K. At this temperature, the metal starts to behave strangely, resulting in a build up energy deep within the interior of the star. This causes the star to expand, resulting in an increase in surface area and thus an increase in observed brightness. The expansion of the star, however, uses up the stored energy and eventually it runs out of ‘expansion fuel’. At this point, the star begins to contract again due to gravity, resulting in a decrease in surface area and a decrease in brightness. This cycle repeats resulting in pulsations on time-scales of a couple of hours.
Stars are not simple, and many of them exhibit both pressure and gravity mode pulsations. Those lower mass stars which exhibit both are hybrid delta scuti / gamma dor pulsators, while the higher mass stars which exhibit both are hybrid SPB-Beta Cep pulsators.
Many stars are in binary (double) systems. Heartbeat stars are a class of binary stars that are so eccentric (not circular) that at the point when the stars are closest together, their gravity is so strong that the spherical stars morph into rugby-ball shapes. This increases their visible surface area, and hence increases the total amount of light that we see. Depending on the orientation of the system, we might see either a single brightening, a dip then a brightening, visa versa, or a brightening and eclipse.
We often like to visualise different types of stars on a plot of brightness (luminosity) versus their temperature, an example of which is shown in the figure below.
I love looking through the TESS data and coming across these beautiful light curves of fascinating stars. Maybe some of them even host a planet or two…
We have some exciting news! We have validated the first Planet Hunters TESS planet, TOI-813b, where validated means that we can say, beyond reasonable doubt, that it is a planet! TOI-813b is around 7 times larger than the Earth, on an 84 days orbit around a 3.7 billion year old star. The paper has been submitted to the Monthly Notices of the Royal Astronomical Society (MNRAS) journal and you can find a version of it on arXiv at: https://arxiv.org/abs/1909.09094
So how did we go from detection to validation?
You initially spotted the transit events that occurred in sector 5 and brought it to our attention via Talk. This put the candidate on our ‘to watch’ list until a second transit was discovered in sector 8 a couple of months later. Additional transits were also identified in sectors 2 and 11. With multiple transit events we could be much more certain that the signal was real and, therefore, began to invest more time looking into studying it.
Initial validation checks
We carried out a number of vetting tests on the TESS data in order to validate it as a planet. First we made sure that the signal wasn’t caused by a jolt in the satellite or a background event. Next, we verified that it wasn’t an ‘astrophysical false positive’ – signals caused by other astrophysical phenomena such as an eclipsing binary (two stars orbiting around one another). To do this, we first compared all four transit events to make sure that the shape and depth of the signals were consistent with one another, as alternating transit depths are characteristic of eclipsing binaries. Next, made sure that when extracting the lightcurve with different aperture sizes the depth of the transits didn’t change. A change in depth could indicate that the signal is caused by an eclipsing binary in the background. Furthermore, to make sure that the light wasn’t coming from a background object, we subtracted images from when the star was in transit to when it was out of transit in order to make sure that the change in light (causing the dip in the light curve) is centred on the star. Finally, we looked at nearby stars in order to make sure that their light curves did not have transit-like dips at the same time as TOI-813.
Spectroscopy. In order to determine whether TOI-813b really is a planet, we had to find out as much as possible about the host star. This can be done using spectroscopy, whereby we split light up into its individual wavelengths, much like a prism splits light into a rainbow. From this we can derive properties such as temperature and composition of the host star. We obtained these observations using the Wide Field Spectrograph instrument on the 2.3-m Australian National University telescope and the High Accuracy Radial velocity Planet Searcher (HARPS) spectrograph on the ESO 3.6-m telescope in Chile.
High Resolution Imaging. We also wanted to check for nearby stars that could contaminate the light curve. This was done using a technique known as ‘speckle’ imaging, which takes thousands of consecutive images with extremely short exposure times. When the images are combined in a particular way, we are able to essentially ‘freeze’ our the effects of the atmosphere (which usually makes our images blurry) and obtain high resolution images. This was done using the Zorro instrument on the 8.1-m Gemini South telescope in Chile.
With this data in hand, we were able to run a program, known as VESPA, to statistically validate the planet. This clever program models the data assuming a number of different astrophysical scenarios and returns the likelihood of each one being true. Based on this analysis, we can be 99.7% sure that he signal is caused by a planet! Yay!
Why is TOI-813b so great?
TOI-813b is interesting for many reasons. First because it is orbiting around an evolved, subgiant star. The subgiant phase of a star occurs when a star runs out of its nuclear fuel source and, in a desperate attempt to find another source of energy, expands its outer layers and contracts its core. Our Sun has not yet reached this stage of its life – it still has around half of its fuel source left – but it will undoubtedly one day also become a subgiant star. There is a noticeable lack of well-studied planets around these types of stars, however, they may be able to help us predict what will happen to the planets within our own solar system in the (very very distant) future. Do planets survive this stage of a star’s life? And if so, how do their characteristics change? We investigated this further for TOI-813 by modelling the size of the star over time. This analysis showed that our newly discovered 7 Earth radii planet will sadly be engulfed by its evolving host in approximately 780 million years (mark your calendars)!
TOI-813 is also interesting as it currently has the longest orbital period (84 days) of any validated TESS planet. This is largely because most of the TESS targets are only observed for ~30 days, making the discovery of longer period planets challenging. Nonetheless, the PHT project has already shown to be extremely good at finding these longer period planets. Even though TOI-813b is currently the longest period planet found by TESS, this will likely change very soon as TESS continues to observe thousands of stars every night.
Finally it’s a great planet because it was discovered by you. The discovery and validation of TOI-813b has shown that we are able to find planets that the pipelines miss! This is the first validated PHT planet, but we are actively following up more targets that you have helped us identify! Thank you so much to everyone for helping us search for unique and exciting new systems that will help us understand the Universe that we live in. Also a special thank you to Frank Barnet, Stewart J. Bean, David M. Bundy, Zbigniew Chetnik, Jamie L Dawson, Judy Garstone, Andrés Guillermo Stenner, Marc Huten, Scott Larish, Larry D. Melanson, Thomas Mitchell, Christopher Moore, Klaus Peltsch, David John Rogers, Claudia Schuster, Dean J. Simister, Daniel Shane Smith, Christopher Tanner, Ivan Terentev and Alexander Tsymbal, the PHT volunteers who helped us find TOI-813b and are now co-authors of the validation paper.
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.