Planet Hunters NGTS: Metadata
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.
U- or V-shaped dip? How to spot the difference?
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
The Next Generation Transit Survey needs you
Today we a guest blog by Sam Gill. Sam Gill is a research assistant at the University of Warwick. He studies long-period planets discovered with TESS and leads the monotransit working group within NGTS. He also researches red dwarfs to empirically calibrate the physical properties of the latest-type stars and is a keen spectroscopist and binary star enthusiast. In his free time, Sam enjoys Brewing and hiking with his beagle, Bruce.
In 2015, more than half of all known exoplanets with masses determined to better than 20 per cent were found from ground based surveys for transiting exoplanets. One such survey which you may know from the successful Zooniverse project searching for variable stars is the Wide Angle Search for Planets (WASP), with installations in Tenerife and South Africa. Both camera arrays have taken over 430 billion measurements of 30 million stars and has found over 150 new planets with many more candidates.
Typically, WASP found planets spanning the masses of Saturn to a few times of Jupiter, along with many low-mass stars which have radii similar to Jupiter. If we look at the mass-radius diagram for planets (with masses known to better than 20%) found around other stars, we see that there is a dearth of those with masses below Saturn. These planets are notoriously hard to identify around stars like our Sun because of their small comparative size. However, these planets should be detectable around much smaller stars with ground-based telescopes which is where the Next Generation Transit Survey (NGTS) comes in.
NGTS is located at the Paranal observatory and consists of 12 fully-robotic telescopes operating at red-optical wavelengths (520-890 nm). This maximises sensitivity to bright and relatively cool and small stars enabling us to find planets less massive than Saturn. With a field of view of 2 degrees per telescope and an average cadence of 13 seconds, NGTS can take over 200 GB of images each night which include many thousands of stars.
Data from NGTS is processed so that the relative brightness of each star can be measured with the influence of the atmosphere mitigated. These data can then be searched using a box-fitting algorithm which searches for periodic dips in a stars brightness which is a characteristic trait of transiting exoplanets. The box-fitting algorithm is powerful and can find many potential planet candidates in our data. The NGTS consortium tries their best to find as many planet candidates as they can but some of these candidates turn out to be false positives. We are successfully finding planets in NGTS data, but the size of our datasets are so large the we may have overlooked some of the most promising planet candidates. That is were you come in.
The aim of Planet Hunters NGTS is to find planets in NGTS data. To do this, we have created a variety of plots (described in a future blog post) embedded in multiple workflows which will help identify promising candidates and many other interesting systems. Today, we just launched two new workflows that you can check out the Odd Even Transit Check and the Secondary Eclipse Check to help vet the best candidates identified by the Exoplanet Transit Search workflow. With your help, we can focus our efforts on the most promising systems and work towards completing a broader consensus of exoplanets. We look forward to you classifications!
More about Planet Hunters NGTS
We were excited to announce yesterday a new chapter in the Planet Hunters project with Planet Hunters NGTS. Here’s a bit more background for those of you who are new to the exoplanet hunt.
The Next-Generation Transit Survey (NGTS) has been searching for extrasolar planets (exoplanets) around other stars for over 5 years but we can’t be sure we’ve found them all without your help. Exoplanets are planets outside our own Solar System. We know of their existence from various techniques, most popularly the exoplanet transit method. This method works by observing a star and watching for regular dips in the amount of light emitted by the star. These dips can be caused by other stars (“binary companions”), exoplanets and other strange objects but we can use the size and shape of the dips to pick the most promising planet candidates. Smaller dips typically correspond to exoplanets, for example Jupiter causes a 1% dip in our Sun’s light for any aliens looking our way. The best planet candidates will be chosen for follow-up observations with different techniques to hopefully confirm whether they really are planets!
Planet Hunters and Planet Hunters TESS used data from NASA space telescopes to search for these dips. This project differs slightly as we’re using the NGTS facility which sits atop a mountain in Chile, surveying the sky every night with its 12 telescopes. Computers have searched the NGTS observations to find these dips but the NGTS team can’t check everything the computer finds as there are just too many candidates. Not every dip is caused by a planet but our handy tutorials will guide you through and turn you into an exoplanet spotting expert.
Finding more exoplanets will allow us to learn more about how planets are formed and how they evolve, which in turn will teach us more about our own Solar System and the Earth we live on. If we can understand the architecture of our own Solar System and other exo-planetary systems, then we can better approach the search for life beyond Earth.
We look forward to working together with you and hope you enjoy the project!
Sean & the Planet Hunters NGTS team