In this blog post, we explain the meaning of, and how to use, the different values and info given in the subject metadata on Planet Hunters NGTS.
On Planet Hunters NGTS, once you classify a subject you can choose ‘Done’ to move to the next subject and keep classifying, or you can choose ‘Done & Talk’ if you want to start a discussion with the Planet Hunters community about anything interesting you may have found. When you are looking at a subject image in the Talk discussion boards, you’ll be able to access the subject metadata by clicking the ‘i in a circle’ button. This metadata is additional info about the subject, some of which will be interesting to Zooniverse users wanting to delve deeper into their classifications.
Once you click the metadata icon, you’ll find a varying number of information fields, depending on whether the subject has linked images or is a known planet, but more on that later.
The first field is ‘sde’ which stands for ‘Signal Detection Efficiency.’ This value can be interpreted as how “strong” the signal is, according to the computer algorithm. The calculation of this value is described in Kovács et al. (2002) which describes the Box-fitting Least Squares (BLS) algorithm that forms the basis for the NGTS exoplanet transit searches.
As a demonstration, consider a full light curve (not phase-folded) such as the image below (which uses artificial data). I’ve injected a transit with a 3-day period into the light curve and we’re going to use a BLS algorithm to search for this.
I’ve used the AstroPy package’s BLS periodogram tool. This calculates the ‘power’ for a range of periods, this ‘power’ is equivalent to our SDE value mentioned above. The resulting ‘periodogram’ is plotted in the figure below. This shows the power value plotted against period with peaks in the periodogram corresponding to where the algorithm believes there is a strong periodic signal in the light curve.
As we can see, the algorithm has picked out the strongest signal at a period of 3 days, just as we expected as this is the true period of the signal I injected. We also see peaks (of decreasing power) around 1.5 days, 6 days, 9 days and 1 day. You may notice that these are all multiples or fractions of the true period. We refer to these as period aliases and it’s expected that they will produce peaks in the BLS periodogram. Sometimes these can even be the true period of a signal, which leads us to our next metadata field.
‘peak’ is the strength of the peak according to the BLS algorithm, ranked by SDE. 1 is the strongest peak (our 3-day period in the example above), down to 5 for the 5th strongest. Anything less significant than this doesn’t have a phase-folded image generated for Planet Hunters NGTS. Currently only Peak 1 objects are on the Planet Hunters NGTS site as these are most likely to correspond to the true period of a transiting object but we plan to include plots for other peaks in future.
‘prod_id’ and ‘obj_id’ are internal identifiers that are used as labels for the stars. These values therefore only have meaning to the NGTS science team.
‘plot_type’ simply refers to which workflow this subject is from:
- ‘primary_phased’ is for subjects in the Exoplanet Transit Search. These plots show where we think the primary eclipse of the phase-folded light curve is. See Figure 1.
- ‘secondary_phased’ is for subjects in the Secondary Eclipse Check. These plots show where we think the secondary eclipse could be, around phase=0.5. See Figure 5 below
- ‘odd_even’ is for subjects in the Odd Even Transit Check. These plots show the odd (1st, 3rd, 5th, etc.) transits in green and the even (2nd, 4th, 6th, etc.) transits in magenta. If the depths of the odd and even transits don’t match then that’s a clear indicator that we’re looking at an eclipsing binary rather than an exoplanet transit. See Figure 6 below for the Odd/Even image corresponding to the subject shown in Figure 1.
‘stellar_rad’ is the radius of the host star, expressed in units of Solar radii (i.e. a star with stellar_rad equal to 2.0 has a radius twice the size of our Sun). This can be used to estimate the radius of the transiting object. First, we estimate the depth of the transit. For the subject shown in Figure 1 (which is confirmed planet NGTS-5b), the transit depth is around 0.025 (or 2.5%), since the flux drops from 1.0 to 0.975 (typo corrected). We can then use this, along with the stellar radius of 0.75 Solar radii, in the equation:
to calculate an estimated planetary radius of 1.15 Jupiter radii. You can read more about how planet radius, stellar radius and depth are related here. The equation I use here is a quicker method as the multiplication factor of 9.73 is such that if you use R* in solar radii, as given in the metadata, then your answer will be in Jupiter radii. You can use this method to work out whether a transiting object is plausibly a planet, typically anything above 1.5 Jupiter radii is unlikely to be a planet.
(Note sometimes the stellar radius value will say ‘nan’ or ‘unavailable,’ this just means we don’t have a good measurement of the star’s radius.)
‘primary_phased,’ ‘secondary_phased’ and ‘odd_even’ provide links to the different plot types for subjects that have passed into the secondary and odd/even workflows. This makes it easier to vet candidates as you can view all the available data quickly and easily.
Finally, we have the ‘known_planet’ field. This will only appear for previously known exoplanets (such as NGTS-5b above) and means that the subject image you are viewing corresponds to a planet that appears in the NASA Exoplanet Archive. Finding known exoplanets is a useful test of the detection efficiencies of the project as a whole, and it’s always exciting to know if you’re looking at data from a real exoplanet system too! Some subjects on the Planet Hunters NGTS site are still undergoing follow-up by the NGTS science team so might be strong planet candidates but won’t be confirmed and published exoplanets. This means that there will be subjects not marked as known planets in their metadata, but the NGTS team may already be aware of it and have put in the work to begin characterising the system further.
We hope this blog post can serve as a useful reference in future as you get involved with Planet Hunters NGTS.
When searching for exoplanets, the shape of the transit can tell us a lot about what object we could be looking at. For the Planet Hunters NGTS exoplanet transit search, we ask you to identify if a transit is U-shaped or V-shaped, as well as whether there’s stellar variability, data gaps or no significant dip in the flux at all. An exoplanet transiting a star will typically produce a U-shaped dip, but there are situations where that isn’t the case (more on that below). Meanwhile an eclipsing binary (two stars orbiting each other) will produce a V-shaped dip most of the time.
The first plot (Figure 1) shows a clear V-shape produced by an eclipsing binary system. In this case, the transiting star only partially eclipses the target star, meaning that it passes across the edge of the disk of the target star but never passes fully in front. This means that the point of minimum flux doesn’t last long before the flux starts to increase again.
The defining difference between U- and V-shaped dips is the angle of the sides of the transit, or ingress and egress to give them their scientific names. Ingress is when the transit begins and the flux is decreasing to the minimum (position 1 to position 2 in Figure 2), while egress is when the transit is ending and flux starts to increase back to the normal level (position 3 to position 4 in Figure 2).
V-shaped dips have sides that are at an angle whereas U-shaped transits will have a steeper decrease and increase in the flux, so much so that the sides of the transit will be almost vertical. The reason V-shaped dips have angled sides is because the object blocking out the light is typically (but not always!) another star. The eclipsing star is large (compared to a planet) so takes more time to pass fully in front of the target star, therefore the decrease in flux happens over a significant time period and we get an angled ingress (likewise for the egress as the star stops blocking light).
The angled sides are more pronounced in Figure 1, but don’t be fooled by dips with a curved base like Figure 3 below! If the transit has angled sides then it’s still a V-shape! The curved base of the transit is caused by a phenomenon called ‘limb darkening,’ where the central disk of a star appears brighter than the edge. The eclipsing star in this system is not just grazing the limb of the target star either, which is why the minimum flux of the transit is sustained for a range of phases.
How vertical is vertical? Sadly there isn’t a clear answer to this, which is why we use human vetting rather than just a computer to check these light curves. The example below (Figure 4) is the light curve for confirmed exoplanet HATS-43b, which was classified as U-shaped by all 20 volunteers who viewed it. This is a clear example of the near vertical drop in flux for the sides of the transit. The small radius of the planet compared to its host star means that it almost instantly passes through the ingress and egress phases, compared to the time taken by a larger star in an eclipsing binary system.
But wait! V-shaped dips can still be exoplanets too! Just like the partial eclipse that produced the sharp, V-shaped dip in Figure 1, an exoplanet can perform a grazing transit where it just crosses the limb of the star and doesn’t go over the centre of its host star’s disk. This will produce a very shallow V-shaped dip, therefore we will get round to searching these classifications for potential exoplanets too! The sides of the dip appear more angled due to the shorter total duration of a grazing transit; it’s very likely that the scale of the x-axis on the plots (the phase) will show a much smaller range of numbers due to how short these transits will be. The ingress and egress times will be similar to a regular transit but the central dip is much shorter. The limb darkening effect also has a more obvious effect on the shape of the dip, which we can see in the light curves below for a near-grazing transit by WASP-174b (Figure 5). The dip has angled sides due to the zoomed in x-axis and has a curved base due to limb darkening. This is a classic curvy V-shape, but it’s also a real exoplanet!
There isn’t a definitive answer for when a curvy V becomes a regular U-shape, but as always your intuition and best guess is what we want! We hope this blog post makes it easier to spot the differences between U- and V-shaped dips when you’re classifying light curves on the Planet Hunters NGTS site, and remember you can always check the Field Guide or ‘Need some help with this task?’ for more help. There’s also the team of researchers and moderators on the ‘Talk’ forums who will be happy to help!
Sean & the Planet Hunters NGTS Team
We’re extremely happy to welcome a new member of the PlanetHunters family. Planet Hunters: NGTS is our first project using data from a ground based survey – the Next Generation Transit Search based at Paranal in the Atacama Desert in Chile.The twelve telescopes of NGTS aim to find planets around the brightest stars; the hope is that, unlike the large number of planets found with NASA’s Kepler, which provided data for the original Planet Hunters, these will be targets that can be followed up with further observations designed to characterise their mass, composition and atmospheres.
As with the Exoplanet Explorers project we ran on Zooniverse a while back, the aim of this new project is to review candidates that have been selected by automatic searches. The hope is that this will make our hit rate higher, but it does mean the task is a little different. The project lives on its own webpage as ngts.planethunters.org, and is led by Meg Schwamb, Chris Watson and Sean O’Brien at Queen’s University Belfast, along with their colleagues. They’ll be responsible for looking at your data, with the help of the Planet Hunters:TESS team.
With this new project there should be enough data to keep you all searching for more than the few days each month TESS data is available. Happy hunting!
(Image: the NGTS telescopes in Paranal, with ESO’s Very Large Telescope visible in the background).
Planet Hunters TESS finds an exciting two-planet system
We have some exciting news – you helped discover another exciting planet system: TIC 349488688 (also known as HD 152843). This exciting discovery follows on from our validation of the long-period planet around an evolved (old) star, TOI-813, and from our recent paper outlining the discovery of 90 Planet Hunters TESS planet candidates, which give us encouragement that there are a lot more exciting systems to be found with your help!
The new exoplanetary system, TIC 349488688, consists of two planets that are similar in size to Neptune and Saturn in our own solar system, orbiting around a bright star that is similar to our own Sun. Planet b is around 3.4 times the size of the Earth, and takes around 12 days to complete an orbit around the star. The outer planet, planet c, is around 5.8 times the size of the Earth and has an orbital period somewhere in the range of 19 to 35 days. The paper has been published by 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/2106.04603
Figure 1: the arXiv version of the published paper.
Multi-planet systems, like this one, are very exciting as they offer a wealth of information. In particular, they allow for comparative planetology: the study of two planets that necessarily formed at the same time and out of the same material, but which have evolved in different ways over time resulting in different planet properties that we observe today. Studying these two planets together, therefore allows us to test theories of planet formation and evolution.
Figure 2: TESS lightcurve showing the transits of planet b in blue and the single transit of planet c in pink.
Detection and Validation of the planets
The target was observed in Sector 25 of the TESS data only and the light curve displayed three transit events belonging to the two different planets (see Figure 2). These events were flagged on the talk discussion forums and brought to the attention of the PHT science team. Once it was flagged, we ran a large number of vetting tests to validate it as a planet. First, we made sure that the signal wasn’t caused by a jolt in the TESS satellite or a background event. Next, we ruled out ‘astrophysical’ false positives – signals caused by other astrophysical phenomena such as two stars orbiting around one another, known as an eclipsing binary.
After ruling out a large number of astrophysical and instrumental false positive scenarios, we were confident that the signals were real! However, in order to truly confirm a planet you have to measure its mass. One of the ways to do that is to use what is known as the radial velocity method. As a planet orbits around it’s host star, the gravitational pull between the two bodies causes the star to ‘wobble’ back and forth, meaning that the star is sometimes moving towards us and sometimes moving away from us. As the star moves towards us, the light that it gives off is ‘squished’ and appears more blue, whereas when it’s moving away from us the light is ‘stretched’ and appears more red. The amount of these red and blue shift scales with the mass of the planet.
In order to measure these red and blue shifts we used two ground-based telescopes: HARPS-N located in La Palma, Canary Islands; and EXPRES located at Lowell Observatory, Flagstaff, Arizona. These two telescopes allowed us to obtain spectroscopic observations – observations that split the light of the star up into its individual wavelengths, similar to how a prism splits light into a rainbow. Careful analysis of this split light allowed us to detect the tiny shifts from red to blue and back to red, which were caused by the two planets orbiting around TIC 349488688. We obtained enough racial velocity measurements to estimate the mass of planet b to be around 12 times more massive than the Earth, and to place an upper mass limit of 28 times the mass of the Earth on planet c.
Why is this system so interesting?
Even though there are now hundreds of confirmed multi-planet systems, the number of multi-planet systems with stars that are close enough such that we can observe and study them using ground-based telescopes remains exceedingly small. The proximity and brightness of HD 152843 is one of the properties that makes this new system stand out. To date we have been able to constrain the masses of the two planets and we are currently continuing to monitor the system to confirm them.
The masses that have already been derived suggest that both planets have low densities, and therefore are likely to have extended gaseous atmospheres. Combined with the brightness of the stars these properties offer exciting prospects for probing the atmospheres and chemical composition of both planets in the future, for example with upcoming space telescopes such as NASA’s James Webb Space Telescope.
Last but not least this system is interesting because it was discovered by you! With this find you have once again shown that with visual vetting we are able to detect exciting planet systems that the automated computer algorithms struggled to find. Thank you to everyone who helps out with the search for distant worlds on Planet Hunters TESS and who help to further our understanding of our Galaxy. A special thanks also to Safaa Alhassan, Elisabeth M. L. Baeten, Stewart J. Bean, David M. Bundy, Vitaly Efremov, Richard Ferstenou, Brian L. Goodwin, Michelle Hof, Tony Hoffman, Alexander Hubert, Lily Lau, Sam Lee, David Maetschke, Klaus Peltsch, Cesar Rubio-Alfaro, Gary M. Wilson who are now coauthors of the discovery paper.
We have some very exciting news: our paper summarising the results from the first two years of Planet Hunters TESS has been accepted for publication! Check out the paper here.
The paper outlines the ins and outs of planet Hunters TESS project and presents 90 new planet candidates from the first two years of the TESS mission (sectors 1 to 26). These planets wouldn’t have been found without the help of all of the citizen scientists taking part in the Planet Hunters TESS project. The paper includes a link to a site that lists all of the citizen scientists who identified each of these 90 planet candidates mentioned in the paper. This page can also be found here.
The majority (81%) of the planet candidates outlined in the paper only exhibit a single transit event in the TESS lightcurve, meaning that they tend to have longer orbital periods (where the orbital period corresponds to the duration of a ‘year’ on this planet) than the average duration of the planets found by the TESS automated algorithms. This is because automated pipelines often require two or more transit events in order to be able to detect the signal. However, with visual vetting, we are equally sensitive to a single transit event as we are to planets that transit multiple times within the duration of one light curve.
You can see that in the figure below, where the orange and pink points show the PHT candidates, and the blue points show the automated pipeline found Tess Objects of Interest (TOIs). The figure highlights that the planets identified with PHT tend to have longer orbital periods than the TOIs, and therefore allowing us to study the characteristics of a different ‘set’ of planets, and maybe even of planets that are more similar to the planets within our own solar system.
Even though the majority of the planet candidates outlined in the paper are not yet confirmed planets, we are following them up using ground-based telescopes which are situated around the world, including in Australia, Chile, USA and the Canary Islands. Hopefully these observations, including both photometric and radial velocity observations, will allow us to confirm the planetary nature of these objects, and even derive masses for some of them which will allow us to infer their densities and therefore bulk compositions. This is ongoing work and we hope to share some of it with you in the near future.
In addition to the 90 new planet candidates, the paper presents some of the most interesting stellar systems that have been discussed on the Planet Hunters TESS Talk discussion forum. An example of a potential multi stellar system is shown below.
Multi stellar systems not only provide very interesting and pretty lightcurves, they also allow us to probe stellar evolution theories in more detail, as all the stars in one system must have necessarily formed at the same time and out of the same material. This highlights some of the other exciting science that results from Planet Hunters TESS and from the continued work of so many citizen scientists.
Since the launch of the Planet Hunters TESS project, almost exactly 2 years ago, we have had over 25.5 million classifications completed by over 25 thousand citizen scientists from around the word. This huge global effort can help us understand what kind of planets exist within our galaxy, how planets form and evolve over time, as well as bring to light some of the other interesting and bizarre astrophysical phenomena that TESS observed over the last two years.
The above light curve, with its periodic increase and decrease in brightness, has the clear signature of an RR Lyrae stars. The pulsations, caused by increases and decreases in the radii and temperatures of these types of stars, typically vary on times-scales ranging from a few hours to and a couple of days. These stars are also evolved stars, meaning that they tend to be older than the Sun with typical ages of around 10 billion years.
RR Lyraes are not only nice to look at, they are also very important for the field of astronomy, as they allow us calibrate the ‘distance ladder’ and thus help us determine the distance to far away objects. They can do that because the time between the pulsations depends on the mass, temperature and intrinsic brightness (the brightness if you were right next to the star) of the star. When we compare the intrinsic brightness to the brightness that we see from Earth, we can calculate how far away the RR Lyrae star is using the inverse-square law.
This target was discussed on the PHT discussion forums at: https://www.zooniverse.org/projects/nora-dot-eisner/planet-hunters-tess/talk/2107/1550146
This week we have an exotic EB, explained to us by Dr. Cole Johnston, where the primary star is a subdwarf which is the stripped helium-burning core of a star. The temperature of this star is so high that it illuminates the much cooler secondary star, causing the surface of the secondary star that is facing the primary to heat up and appear much brighter than the side that is facing away. This causes a dramatic increase in brightness approaching and receding from the secondary eclipse (the small dip at the top of the ‘wave’ in the above lightcurve). The two stars are so close together that they complete one orbit in just a few hours! The above light curve is phase folded to emphasise the brightening which is known as the ‘reflection effect’.
Studying these systems is important because these primary stars are thought to be the tracers of a very strange evolutionary path, whereby the entire hydrogen envelope of an evolving star is stripped away by some mechanism (probably by a binary or high mass planetary companion), just at the point were helium burning starts in the core of the star.
Planet Hunters TESS users have identified two very interesting lightcurves of M-dwarf stars that show repeating patterns that are difficult to explain with well-known astrophysical phenomena such as star spots, eclipsing binaries or stellar pulsations.
Both of these lightcurves exhibit two dips, a shallow one and a deeper one, which could be explained as the primary and secondary eclipses of an eclipsing binary. However, in addition to these ‘eclipses’, we can also see a w-shaped dip after or after the deeper eclipse. Both lightcurves repeat with a period of less than 1 day and the patterns remain stable over the course of the available TESS observations, as shown in the phase folded figure below. The target stars are both M-dwarfs, with temperatures of around 3000 K and radii of around 0.3 Solar radii.
Similar lightcurves had previously been seen in the Kepler data, see Stauffer et al. 2017 (https://iopscience.iop.org/article/10.3847/1538-3881/aa5eb9/pdf) for more detail. This paper suggests that the pattern could be caused by clouds of dust orbiting around one of the stars. Could that also explain the w-shaped dips in the TESS data, or are we seeing an entirely different phenomenon here?
In order to study these systems in more detail we want to see if we can identify more of them in the TESS data.
If you see any of these please tag a member of the researcher team or use the hashtag #wstars to help us find and study these elusive systems.
A new paper released this week and accepted by the Astronomical Journal (https://arxiv.org/abs/2004.07783) announced the discovery of TOI-1338, a planet in an eclipsing binary system. An eclipsing binary consists of two stars in orbit around each other, and this new planet orbits both stars. The paper was led by Veselin Kostov and his team at NASA’s Goddard Space Flight Center and elsewhere, but they were first tipped off to the presence of this interesting system by reading comments on Planet Hunters Talk. Several citizen scientists appear as authors on the paper – congratulations to everyone involved!
This isn’t the first time Planet Hunters have found a circumbinary system – PH1b, discovered way back in 2014, was found in Kepler data by volunteers, and is still the only planet known in a four-star system.
Many stars are not alone, but instead form part of a multi-stellar system of two or more stars that are gravitationally bound together. Even though the prime science goal of TESS is to find exoplanets, it also observes a plethora of eclipsing binaries that allow us to study these systems in more detail.
The light curve of TIC 8153514, observed in Sector 21, shows a sharp eclipse superimposed on top of the signal of a variable star.
If you see eclipsing binaries on PHT you can tag it with #EB or #eclipsingbinary to let others known what you have found!