On this weblog submit, we clarify the that means of, and use, the totally different values and information given within the topic metadata on Planet Hunters NGTS.
On Planet Hunters NGTS, when you classify a topic you’ll be able to select ‘Achieved’ to maneuver to the following topic and maintain classifying, or you’ll be able to select ‘Achieved & Speak’ if you wish to begin a dialogue with the Planet Hunters group about something attention-grabbing you’ll have discovered. If you end up taking a look at 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 additional information concerning the topic, a few of which shall 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 photographs or is a identified planet, however extra on that later.
The primary area is ‘sde’ which stands for ‘Sign Detection Effectivity.’ This worth might be interpreted as how “robust” the sign is, in line with 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 illustration, contemplate a full mild curve (not phase-folded) such because the picture under (which makes use of synthetic knowledge). 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 under. This reveals the ability worth plotted in opposition to interval with peaks within the periodogram similar to the place the algorithm believes there’s a robust periodic sign within the mild curve.
As we will 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 reducing energy) round 1.5 days, 6 days, 9 days and 1 day. Chances are you’ll 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. Generally 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 line with 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 important than this doesn’t have a phase-folded picture generated for Planet Hunters NGTS. Presently solely Peak 1 objects are on the Planet Hunters NGTS website as these are most definitely 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 might be used as labels for the celebs. These values due to this fact solely have that means to the NGTS science workforce.
‘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 predict the first eclipse of the phase-folded mild curve is. See Determine 1.
- ‘secondary_phased’ is for topics within the Secondary Eclipse Verify. These plots present the place we predict the secondary eclipse could possibly be, round section=0.5. See Determine 5 under
- ‘odd_even’ is for topics within the Odd Even Transit Verify. 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 taking a look at an eclipsing binary somewhat than an exoplanet transit. See Determine 6 under for the Odd/Even picture similar 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 scale 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%), because the 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 exploit 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 shall be in Jupiter radii. You should use 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.
(Be aware typically the stellar radius worth will say ‘nan’ or ‘unavailable,’ this simply means we don’t have a very good 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’ll be able to view all of the accessible knowledge rapidly and simply.
Lastly, we’ve got the ‘known_planet’ area. It will solely seem for beforehand identified exoplanets (comparable 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 take a look at of the detection efficiencies of the undertaking as an entire, and it’s all the time thrilling to know when you’re taking a look at knowledge from an actual exoplanet system too! Some topics on the Planet Hunters NGTS website are nonetheless present process follow-up by the NGTS science workforce so may be robust planet candidates however gained’t be confirmed and revealed exoplanets. Which means there shall be topics not marked as identified planets of their metadata, however the NGTS workforce might already pay attention to 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.
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