On this weblog submit, we clarify the which means of, and find out how to use, the totally different values and data given within the topic metadata on Planet Hunters NGTS.
On Planet Hunters NGTS, when you classify a topic you may select ‘Completed’ to maneuver to the subsequent topic and preserve classifying, or you may select ‘Completed & Speak’ if you wish to begin a dialogue with the Planet Hunters neighborhood about something attention-grabbing you will have discovered. If you find yourself a topic picture within the Speak dialogue boards, you’ll have the ability to entry the topic metadata by clicking the ‘i in a circle’ button. This metadata is more information concerning the topic, a few of which might be attention-grabbing to Zooniverse customers desirous to delve deeper into their classifications.
When you click on the metadata icon, you’ll discover a various variety of info fields, relying on whether or not the topic has linked pictures or is a identified planet, however extra on that later.
The primary area is ‘sde’ which stands for ‘Sign Detection Effectivity.’ This worth may be interpreted as how “sturdy” the sign is, in response to the pc algorithm. The calculation of this worth is described in Kovács et al. (2002) which describes the Field-fitting Least Squares (BLS) algorithm that types the idea for the NGTS exoplanet transit searches.
As an indication, contemplate a full gentle curve (not phase-folded) such because the picture beneath (which makes use of synthetic information). I’ve injected a transit with a 3-day interval into the sunshine curve and we’re going to make use of a BLS algorithm to seek for this.
I’ve used the AstroPy bundle’s BLS periodogram software. This calculates the ‘energy’ for a variety of durations, this ‘energy’ is equal to our SDE worth talked about above. The ensuing ‘periodogram’ is plotted within the determine beneath. This reveals the ability worth plotted towards interval with peaks within the periodogram comparable to the place the algorithm believes there’s a sturdy periodic sign within the gentle curve.
As we are able to see, the algorithm has picked out the strongest sign at a interval of three days, simply as we anticipated as that is the true interval of the sign I injected. We additionally see peaks (of lowering energy) round 1.5 days, 6 days, 9 days and 1 day. You might discover that these are all multiples or fractions of the true interval. We refer to those as interval aliases and it’s anticipated that they’ll produce peaks within the BLS periodogram. Typically these may even be the true interval of a sign, which leads us to our subsequent metadata area.
‘peak’ is the power of the height in response to the BLS algorithm, ranked by SDE. 1 is the strongest peak (our 3-day interval within the instance above), down to five for the fifth strongest. Something much less vital than this doesn’t have a phase-folded picture generated for Planet Hunters NGTS. At the moment solely Peak 1 objects are on the Planet Hunters NGTS website as these are almost certainly to correspond to the true interval of a transiting object however we plan to incorporate plots for different peaks in future.
‘prod_id’ and ‘obj_id’ are inside identifiers which can be used as labels for the celebrities. These values subsequently solely have which means to the NGTS science group.
‘plot_type’ merely refers to which workflow this topic is from:
- ‘primary_phased’ is for topics within the Exoplanet Transit Search. These plots present the place we expect the first eclipse of the phase-folded gentle curve is. See Determine 1.
- ‘secondary_phased’ is for topics within the Secondary Eclipse Test. These plots present the place we expect the secondary eclipse might be, round section=0.5. See Determine 5 beneath
- ‘odd_even’ is for topics within the Odd Even Transit Test. These plots present the odd (1st, third, fifth, and many others.) transits in inexperienced and the even (2nd, 4th, sixth, and many others.) transits in magenta. If the depths of the odd and even transits don’t match then that’s a transparent indicator that we’re an eclipsing binary reasonably than an exoplanet transit. See Determine 6 beneath for the Odd/Even picture comparable to the topic proven in Determine 1.
‘stellar_rad’ is the radius of the host star, expressed in models of Photo voltaic radii (i.e. a star with stellar_rad equal to 2.0 has a radius twice the dimensions of our Solar). This can be utilized to estimate the radius of the transiting object. First, we estimate the depth of the transit. For the topic proven in Determine 1 (which is confirmed planet NGTS-5b), the transit depth is round 0.025 (or 2.5%), for the reason that flux drops from 1.0 to 0.975 (typo corrected). We will then use this, together with the stellar radius of 0.75 Photo voltaic radii, within the equation:
to calculate an estimated planetary radius of 1.15 Jupiter radii. You may learn extra about how planet radius, stellar radius and depth are associated right here. The equation I take advantage of here’s a faster technique because the multiplication issue of 9.73 is such that when you use R* in photo voltaic radii, as given within the metadata, then your reply might be in Jupiter radii. You should utilize this technique to work out whether or not a transiting object is plausibly a planet, usually something above 1.5 Jupiter radii is unlikely to be a planet.
(Word generally the stellar radius worth will say ‘nan’ or ‘unavailable,’ this simply means we don’t have an excellent measurement of the star’s radius.)
‘primary_phased,’ ‘secondary_phased’ and ‘odd_even’ present hyperlinks to the totally different plot sorts for topics which have handed into the secondary and odd/even workflows. This makes it simpler to vet candidates as you may view all of the out there information shortly and simply.
Lastly, we’ve got the ‘known_planet’ area. This can solely seem for beforehand identified exoplanets (equivalent to NGTS-5b above) and implies that the topic picture you’re viewing corresponds to a planet that seems within the NASA Exoplanet Archive. Discovering identified exoplanets is a helpful check of the detection efficiencies of the undertaking as a complete, and it’s all the time thrilling to know when you’re information from an actual exoplanet system too! Some topics on the Planet Hunters NGTS website are nonetheless present process follow-up by the NGTS science group so may be sturdy planet candidates however received’t be confirmed and printed exoplanets. Because of this there might be topics not marked as identified planets of their metadata, however the NGTS group could already concentrate on it and have put within the work to start characterising the system additional.
We hope this weblog submit can function a helpful reference in future as you get entangled with Planet Hunters NGTS.